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Reference: Fiber Optic Technology
Contents
A brief history of Fiber Optic Technology ... 5
The Nineteenth Century ................................. 5
The Twentieth Century .................................. 5
Applications in the Real World ......................... 7
The Twenty-First Century and Beyond .................... 8
Fiber Optic Components .................... 10
Optical Fiber & Cable ................................... 11
Determining Fiber Size ................................ 11
Background .......................................... 11
Fiber Preparation ................................... 11
Cladding size ............................ 11
Core Size ........................................... 11
Fiber Dispersion ...................................... 13
Introduction ........................................ 13
Chromatic Dispersion ................................ 13
Polarization Mode Dispersion ........................ 14
Calculating Dispersion .............................. 14
Dispersion Power Penalty ............................ 15
Fiber Types ......................................... 19
Laser Types ......................................... 20
Countermeasures ..................................... 21
Fiber Nonlinearities .................................. 22
Introduction ........................................ 22
Causes of Nonlinearities ............................ 22
Stimulated Brillouin Scattering ..................... 23
Stimulated Raman Scattering ......................... 26
Four Wave Mixing .................................... 27
Modulation .......................................... 29
Handling Fragile Optical Fibers and Fiber Pigtail
Assemblies ............................................ 30
Background .......................................... 30
Safe Fiber Assembly Handling ....................... 30
Inserting the Optical Device into a Circuit Board ... 31
Types of Optical Fiber ................................ 31
Introduction ........................................ 31
Multimode Fiber ..................................... 32
Single-mode Fiber ................................... 33
Fiber Optic Connectors .................................. 34
Fiber Optic Connectors ................................ 34
Background .......................................... 34
Installing Fiber Optic Connectors ................... 36
Cleaving ............................................ 36
Polishing ........................................... 37
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Reference: Fiber Optic Technology
Care and Handling of Fiber Optic Connectors ......... 38
Effects on Fiber Optic Connectors ................... 38
Cleaning Technique .................................. 39
Handling ............................................ 39
Standards For Fiber Optic Connectors ................ 40
Connector Loss Test Measurements ...................... 41
Interconnection Loss Measurements ................... 41
Measurement System Components ....................... 41
Insertion Loss Test ................................. 44
NA Mismatch Loss Test ............................... 44
Core/Cladding Diameter Mismatch Tests ............... 44
Alignment Loss Tests ................................ 45
Fresnel Reflection Loss ............................. 46
System Related Losses ............................... 46
Fiber Optic Light Emitters & Detectors .................. 47
Light-emitting Diode (LED) ............................ 47
Background .......................................... 47
Light Emitter Performance Characteristics ........... 48
LED TYPES ............................................ 49
LED Drive Circuits .................................. 50
ENERGY GAPS IN LEDs ................................... 54
Laser Diode ........................................... 55
Background .......................................... 55
Laser Diode Performance Characteristics ............. 56
LASER TYPES ......................................... 58
BACKREFLECTIONS ..................................... 59
LASER DRIVE CIRCUITS ................................ 60
PACKAGING CHARACTERISTICS ........................... 62
Laser Backreflection - The Bane of Good Performance ... 62
Introduction ........................................ 62
Explanation of Backreflections ...................... 62
Optical Isolators ................................... 63
Conclusion .......................................... 66
Fiber Optic Detectors ................................. 66
Introduction ........................................ 66
Important Detector Parameters ....................... 67
PIN Photodiode ...................................... 67
Avalanche Photodiode (APD) .......................... 68
Light Emitters As Detectors ......................... 69
Transmitters, Receivers, & Transceivers ................. 70
Troubleshooting Transmitters & Receivers .............. 70
Background .......................................... 70
Problems and Comments ............................... 70
Parts of A Fiber Optic Link ........................... 71
Couplers, Splitters, Switches, & WDM .................... 73
Couplers & Splitters .................................. 73
Background .......................................... 73
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Reference: Fiber Optic Technology
Couplers ............................................ 73
Splitters ........................................... 73
Coupler and Splitter Applications ................... 74
Switches .............................................. 75
Opto-mechanical Switches ............................ 75
Thermo-optic Switches ............................... 76
Electro-optic Switches .............................. 76
Wavelength-division Multiplexing ...................... 76
Background .......................................... 76
WDM Applications .................................... 76
Coarse Wavelength-division Multiplexing ............... 77
Background .......................................... 77
Unidirectional Applications ......................... 78
Dense Wavelength-division Multiplexing ................ 79
System Growth with DWDM ............................. 79
DWDM System Considerations .......................... 79
Optical Amplifiers & External Modulators ................ 83
Optical Amplifiers .................................... 83
Improving Long-Haul Network Performance ............. 83
Semiconductor Optical Amplifiers .................... 83
EDFAs ............................................... 84
Raman Optical Amplifiers ............................ 88
External Modulators ................................... 90
Background .......................................... 90
Theory of Operation ................................. 90
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Part 1: Hisory of Fiber Optic Technology
A brief history of Fiber Optic Technology
The Nineteenth Century
Figure 1 - John Tyndall’s Experiment
In 1870, John Tyndall, using a jet of water that
flowed from one container to another and a beam
of light, demonstrated that light used internal
reflection to follow a specific path. As water
poured out through the spout of the first container,
Tyndall directed a beam of sunlight at the path of
the water. The light, as seen by the audience,
followed a zigzag path inside the curved path of
the water. This simple experiment, illustrated in
Figure 1, marked the first research into the guided
transmission of light.
William Wheeling, in 1880, patented a method of light transfer called “piping light.”
Wheeling believed that by using mirrored pipes branching off from a single source of
illumination, i.e. a bright electric arc, he could send the light to many different rooms in the
same way that water, through plumbing, is carried throughout buildings today. Due to the
ineffectiveness of Wheeling’s idea and to the concurrent introduction of Edison’s highly
successful incandescent light bulb, the concept of piping light never took off.
That same year, Alexander Graham Bell developed an optical voice transmission system he
called a photophone. The photophone used free-space light to carry the human voice 200
meters. Specially placed mirrors reflected sunlight onto a diaphragm attached within the
mouthpiece of the photophone. At the other end, mounted within a parabolic reflector, was a
light-sensitive selenium resistor. This resistor was connected to a battery that was, in turn,
wired to a telephone receiver. As one spoke into the photophone, the illuminated diaphragm
vibrated, casting various intensities of light onto the selenium resistor. The changing
intensity of light altered the current that passed through the telephone receiver which then
converted the light back into speech. Bell believed this invention was superior to the
telephone because it did not need wires to connect the transmitter and receiver. Today,
“free-space” optical links find extensive use in metropolitan applications.
The Twentieth Century
Fiber optic technology experienced a phenomenal Figure 2 - Optical Fiber with
rate of progress in the second half of the twentieth Cladding
century. Early success came during the 1950’s with
the development of the fiberscope. This image-
transmitting device, which used the first practical
all-glass fiber, was concurrently devised by Brian
O’Brien at the American Optical Company and
Narinder Kapany (who first coined the term “fiber
optics” in 1956) and colleagues at the Imperial
College of Science and Technology in London.
Early all-glass fibers experienced excessive optical
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Part 1: Hisory of Fiber Optic Technology
loss, the loss of the light signal as it traveled the
fiber, limiting transmission distances.
This motivated scientists to develop glass fibers that included a separate glass coating. The
innermost region of the fiber, or core, was used to transmit the light, while the glass
coating, or cladding, prevented the light from leaking out of the core by reflecting the light
within the boundaries of the core. This concept is explained by Snell’s Law which states
that the angle at which light is reflected is dependent on the refractive indices of the two
materials — in this case, the core and the cladding. The lower refractive index of the
cladding (with respect to the core) causes the light to be angled back into the core as
illustrated in Figure 2.
The fiberscope quickly found application inspecting welds inside reactor vessels and
combustion chambers of jet aircraft engines as well as in the medical field. Fiberscope
technology has evolved over the years to make laparoscopic surgery one of the great
medical advances of the twentieth century.
The development of laser technology was the next important step in the establishment of
the industry of fiber optics. Only the laser diode (LD) or its lower-power cousin, the light-
emitting diode (LED), had the potential to generate large amounts of light in a spot tiny
enough to be useful for fiber optics. In 1957, Gordon Gould popularized the idea of using
lasers when, as a graduate student at Columbia University, he described the laser as an
intense light source. Shortly after, Charles Townes and Arthur Schawlow at Bell
Laboratories supported the laser in scientific circles. Lasers went through several
generations including the development of the ruby laser and the helium-neon laser in 1960.
Semiconductor lasers were first realized in 1962; these lasers are the type most widely used
in fiber optics today.
Because of their higher modulation frequency capability, the importance of lasers as a
means of carrying information did not go unnoticed by communications engineers. Light
has an information-carrying capacity 10,000 times that of the highest radio frequencies
being used. However, the laser is unsuited for open-air transmission because it is adversely
affected by environmental conditions such as rain, snow, hail, and smog. Faced with the
challenge of finding a transmission medium other than air, Charles Kao and Charles
Hockham, working at the Standard Telecommunication Laboratory in England in 1966,
published a landmark paper proposing that optical fiber might be a suitable transmission
medium if its attenuation could be kept under 20 decibels per kilometer (dB/km). At the
time of this proposal, optical fibers exhibited losses of 1,000 dB/ km or more. At a loss of
only 20 dB/km, 99% of the light would be lost over only 3,300 feet. In other words, only
1/100th of the optical power that was transmitted reached the receiver. Intuitively,
researchers postulated that the current, higher optical losses were the result of impurities in
the glass and not the glass itself. An optical loss of 20 dB/km was within the capability of
the electronics and opto-electronic components of the day.
Intrigued by Kao and Hockham’s proposal, glass researchers began to work on the problem
of purifying glass. In 1970, Drs. Robert Maurer, Donald Keck, and Peter Schultz of
Corning succeeded in developing a glass fiber that exhibited attenuation at less than 20
dB/km, the threshold for making fiber optics a viable technology. It was the purest glass
ever made.
The early work on fiber optic light source and detector was slow and often had to borrow
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Part 1: Hisory of Fiber Optic Technology
technology developed for other reasons. For example, the first fiber optic light sources were
derived from visible indicator LEDs. As demand grew, light sources were developed for
fiber optics that offered higher switching speed, more appropriate wavelengths, and higher
output power. For more information on light emitters see “Laser Diodes” and “LEDs.”
Figure 3 - Four Wavelength Regions of Fiber optics developed over the years in a series
Optical Fiber of generations that can be closely tied to
wavelength. Figure 3 shows three curves. The
top, dashed, curve corresponds to early 1980’s
fiber, the middle, dotted, curve corresponds to
late 1980’s fiber, and the bottom, solid, curve
corresponds to modern optical fiber. The
earliest fiber optic systems were developed at
an operating wavelength of about 850 nm. This
wavelength corresponds to the so-called “first
window” in a silica-based optical fiber. This
window refers to a wavelength region that
offers low optical loss. It sits between several
large absorption peaks caused primarily by
moisture in the fiber and Rayleigh scattering.
The 850 nm region was initially attractive because the technology for light emitters at this
wavelength had already been perfected in visible indicator LEDs. Low-cost silicon
detectors could also be used at the 850 nm wavelength. As technology progressed, the first
window became less attractive because of its relatively high 3 dB/km loss limit.
Most companies jumped to the “second window” at 1310 nm with lower attenuation of
about 0.5 dB/km. In late 1977, Nippon Telegraph and Telephone (NTT) developed the
“third window” at 1550 nm. It offered the theoretical minimum optical loss for silica-based
fibers, about 0.2 dB/km.
Today, 850 nm, 1310 nm, and 1550 nm systems are all manufactured and deployed along
with very low-end, short distance, systems using visible wavelengths near 660 nm. Each
wavelength has its advantage. Longer wavelengths offer higher performance, but always
come with higher cost. The shortest link lengths can be handled with wavelengths of 660
nm or 850 nm. The longest link lengths require 1550 nm wavelength systems. A “fourth
window,” near 1625 nm, is being developed. While it is not lower loss than the 1550 nm
window, the loss is comparable, and it might simplify some of the complexities of long-
length, multiple-wavelength communications systems.
Applications in the Real World
The U.S. military moved quickly to use fiber optics for improved communications and
tactical systems. In the early 1970’s, the U.S. Navy installed a fiber optic telephone link
aboard the U.S.S. Little Rock. The Air Force followed suit by developing its Airborne Light
Optical Fiber Technology (ALOFT) program in 1976. Encouraged by the success of these
applications, military R&D programs were funded to develop stronger fibers, tactical
cables, ruggedized, high-performance components, and numerous demonstration systems
ranging from aircraft to undersea applications.
Commercial applications followed soon after. In 1977, both AT&T and GTE installed fiber
optic telephone systems in Chicago and Boston respectively. These successful applications
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Part 1: Hisory of Fiber Optic Technology
led to the increase of fiber optic telephone networks. By the early 1980’s, single-mode fiber
operating in the 1310 nm and later the 1550 nm wavelength windows became the standard
fiber installed for these networks. Initially, computers, information networks, and data
communications were slower to embrace fiber, but today they too find use for a
transmission system that has lighter weight cable, resists lightning strikes, and carries more
information faster and over longer distances.
The broadcast industry also embraced fiber optic transmission. In 1980, broadcasters of the
Winter Olympics, in Lake Placid, New York, requested a fiber optic video transmission
system for backup video feeds. The fiber optic feed, because of its quality and reliability,
soon became the primary video feed, making the 1980 Winter Olympics the first fiber optic
television transmission. Later, at the 1994 Winter Olympics in Lillehammer, Norway, fiber
optics transmitted the first ever digital video signal, an application that continues to evolve
today.
In the mid-1980’s the United States government deregulated telephone service, allowing
small telephone companies to compete with the giant, AT&T. Companies like MCI and
Sprint quickly went to work installing regional fiber optic telecommunications networks
throughout the world. Taking advantage of railroad lines, gas pipes, and other natural rights
of way, these companies laid miles fiber optic cable, allowing the deployment of these
networks to continue throughout the 1980’s. However, this created the need to expand
fiber’s transmission capabilities.
In 1990, Bell Labs transmitted a 2.5 Gb/s signal over 7,500 km without regeneration. The
system used a soliton laser and an erbium-doped fiber amplifier (EDFA) that allowed the
light wave to maintain its shape and density. In 1998, They went one better as researchers
transmitted 100 simultaneous optical signals, each at a data rate of 10 gigabits (giga means
billion) per second for a distance of nearly 250 miles (400 km). In this experiment, dense
wavelength-division multiplexing (DWDM technology, which allows multiple wavelengths
to be combined into one optical signal, increased the total data rate on one fiber to one
terabit per second (1012 bits per second).
For more information on fiber optic applications see Fiber Optic Transport Solutions
The Twenty-First Century and Beyond
Today, DWDM technology continues to
Figure 4 - Projected Internet Traffic Increases
develop. As the demand for data
bandwidth increases, driven by the
phenomenal growth of the Internet, the
move to optical networking is the focus
of new technologies. At this writing,
nearly half a billion people have Internet
access and use it regularly. Some 40
million or more households are “wired.”
The world wide web already hosts over
2 billion web pages, and according to
estimates people upload more than 3.5
million new web pages everyday.
The important factor in these Figure 5 - The Growth of Optical Fiber Transmission
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Part 1: Hisory of Fiber Optic Technology
developments is the increase in Capacity
fiber transmission capacity, which
has grown by a factor of 200 in
the last decade. Figure 5 illustrates
this trend.
Because of fiber optic
technology’s immense potential
bandwidth, 50 THz or greater,
there are extraordinary
possibilities for future fiber optic
applications. Already, the push to
bring broadband services,
including data, audio, and
especially video, into the home is
well underway.
Broadband service available to a mass market opens up a wide variety of interactive
communications for both consumers and businesses, bringing to reality interactive video
networks, interactive banking and shopping from the home, and interactive distance
learning. The “last mile” for optical fiber goes from the curb to the television set top,
allowing video on demand to become a reality.
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Part 2: Fiber Optic Components
Fiber Optic Components
Optical Fiber & Cable Fiber Optic Connectors
Determining Fiber Size Fiber Optic Connectors
Fiber Dispersion Connector Loss Test
Fiber Nonlinearities Measurements
Handling Fragile Optical Fiber
and Fiber Pigtail Assemblies
Types of Optical Fiber
Fiber Optic Light Emitters & Transmitters, Receivers, &
Detectors Transceivers
Light-emitting Diode (LED) Fiber Optic Troubleshooting:
Laser Diode (LD) Problems and Comments
Laser Backreflection - The Bane Parts of a Fiber Optic Link
of Good Performance
Fiber Optic Detectors
Couplers, Splitters, Switches, & WDM Optical Amplifiers & External
Modulators
Couplers & Splitters
Switches Optical Amplifiers
WDM External Modulators
CWDM
DWDM
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Part 2: Fiber Optic Components
Optical Fiber & Cable
Determining Fiber Size
Background
Quick identification of the exact size and type of a given piece of optical fiber is a routine
but necessary task. This article will examine several approaches to determining fiber size.
Fiber Preparation
If one has access to the fiber itself, the first step in identification is to remove any outer
jacket material that may exist and carefully remove the plastic buffer from the fiber. (Note:
If you do not have access to the fiber itself and can only view the fiber end, then proceed to
the section on Core Size.) To do this, use fiber strippers designed for the task, or use a razor
blade. (It takes practice to remove the plastic with a razor blade, but it can be mastered after
a few repetitions.) Always cut along the fiber axis towards the cut end of the fiber. Fiber has
tremendous strength in tension but is very weak in all other directions. Always stroke the
razor blade away from your body. Use the razor blade to remove a sliver of plastic, then
rotate the fiber 90° and repeat the process until the fiber cladding is fully exposed. Once the
bulk of the plastic coating is removed, carefully clean the bare fiber with a tissue soaked in
alcohol. (Note: Use only industrial grade 99% pure isopropyl alcohol. Commercially
available isopropyl alcohol, for medicinal use, is diluted with water and a light mineral oil.
Industrial grade isopropyl alcohol should be used exclusively.) Always wipe along the fiber
axis with continuous strokes to the end of the fiber.
Cladding Size
Once the fiber is clean, take a clean machinist micrometer, such as the one in Figure 1, and
carefully measure the outer diameter of the fiber. This outer diameter is the cladding
diameter of the fiber. Be certain that the metal faces of the micrometer are clean. Do not
over tighten the micrometer as the fiber will fracture. Table 1 shows the possible results for
the most common fiber sizes and the interpretation of the results.
Figure 1 – Micrometer
Table 1 - Common Fiber Cladding Size & Corresponding Micrometer Reading
Nominal High
Low Micrometer
Fiber Type Cladding Tolerance Micrometer
Reading
Diameter Reading
Single-mode 125 µm ±1 µm 4.88 mils 4.96 mils
Multimode 125 µm ±4 µm 4.76 mils 5.08 mils
Multimode 140 µm ±4 µm 5.35 mils 5.67 mils
Multimode 230 µm ±5 µm 8.86 mils 9.25 mils
Thus, if the fiber cladding diameter measures 5.38 mils, then the fiber is almost certainly
multimode with a 140 µm cladding diameter. Cladding diameter is the most important
parameter when selecting the fiber optic connector size. The cladding diameter determines
the size of the hole in the fiber optic connector.
Core Size
Once the cladding diameter is determined, one must find the core size, unless the cladding
diameter uniquely identified the fiber size. This step requires a microscope capable of about
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Part 2: Fiber Optic Components
50X magnification. A high intensity light or penlight is useful to light the fiber end. The
idea is to get a good look at the end of the fiber, and judge the fiber size from what is seen.
This technique works best if the fiber is in a connector so that the fiber end is polished and
flat. If that is not the case, it may be necessary to cleave the fiber so that the end can be
examined. Clamp the fiber end or fiber optic connector at the focal plane of the microscope
and shine the light onto the fiber end. It is sometimes useful to light the far end of the fiber
as well if it is accessible. Once focus is established, compare the view with the five
drawings shown in Figure 2.
Figure 2 - Relative Core/Cladding Size
This figure represents scale drawings of the relative size of the core and cladding on the six
most popular fiber types for fiber optic communication. Of these six, the last two, PM
single-mode 9/125 µm and single-mode 9/125 µm, are the most common and 110/125 µm is
the least common of the six types.
With a little practice, it is easy to quickly and accurately identify the fiber types. When
looking through the microscope, the fiber core will be very dark if the fiber is illuminated
only at the microscope side. If the distant end of the fiber is illuminated, then the fiber core
will appear brightly lit. Simply compare the image in the microscope to the scale drawing to
the left, and match the relative size of the core to cladding. With basically only five sizes of
fiber to worry about, it becomes a relatively simple matter to judge the correct size. There is
also a range of much larger fibers, but these have limited use in communications
applications. Other large fiber sizes include 200/230 µm, 400/430 µm and 1000/1050 µm.
The latter fiber is very nearly a glass rod.
There are other imaginative ways of determining fiber size. For instance, if one has a
multimode LED light source available (e.g. a fiber optic video transmitter), a simple light
injection test can be performed to quickly determine the fiber size. A surface-emitting LED
is best for this purpose since its light injection level varies most dramatically with fiber size.
The LED should be powered at a fairly constant current and then attached to a few known
size fiber optic cables. For more information on LEDs see “Light-emitting Diodes.” In each
case, note the relationship between fiber size and launched power. Once a small database of
results is available, then proceed to attach the unknown fiber optic cables. It is generally
very easy to distinguish between fiber sizes. One possible drawback of this method is the
possibility of high loss due to a bad fiber optic connector or a stressed or broken optical
fiber. Launching the LED into both ends of the fiber optic cable will usually improve the
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Part 2: Fiber Optic Components
chances of a correct result.
Fiber Dispersion
Introduction
Once upon a time, the world assumed that fiber possessed infinite bandwidth and would
meet mankind’s communication needs into the foreseeable future. As the need arose to send
information over longer and longer distances, the fiber optic community developed
additional wavelength “windows” that allowed longer transmission. The 1550 nm region,
with a loss of only 0.2 dB/km, seemed like the answer. Millions of kilometers of fiber were
installed around the world creating a high-speed communication network. However, as the
data rates increased and fiber lengths increased, limitations due to dispersion in the fiber
became impossible to avoid.
Dispersion was initially a problem when the first optical fibers, multimode step-index fiber,
were introduced. Multimode graded-index fiber improved the situation a bit, but it was
single-mode fiber that eliminated severe multimode fiber related dispersion and left only
chromatic dispersion and polarization mode dispersion to be dealt with by engineers. This
article describes the sources of dispersion in optical fiber and the strategies for getting
around this limitation.
Chromatic Dispersion
Chromatic dispersion represents the fact that
different colors or wavelengths travel at
different speeds, even within the same mode.
Chromatic dispersion is the result of material Figure 1 - Chromatic Dispersion
dispersion, waveguide dispersion, or profile
dispersion. Figure 1 below shows chromatic
dispersion along with key component
waveguide dispersion and material dispersion.
The example shows chromatic dispersion
going to zero at the wavelength near 1550 nm.
This is characteristic of bandwidth dispersion-
shifted fiber. Standard fiber, single-mode, and
multimode has zero dispersion at a wavelength
of 1310 nm.
Every laser has a range of optical wavelengths,
and the speed of light in fused silica (fiber)
varies with the wavelength of the light. Figure
2 illustrates the refractive index of fused silica
as it changes with wavelength. Since a pulse of
light from the laser usually contains several
wavelengths, these wavelengths tend to get
spread out in time after traveling some distance
in the fiber. The refractive index of fiber
decreases as wavelength increases, so longer
wavelengths travel faster. The net result is that Figure 2 - Refractive Index of Fused Silica
the received pulse is wider than the transmitted
one, or more precisely, is a superposition of
the variously delayed pulses at the different
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Part 2: Fiber Optic Components
wavelengths.
A further complication is that lasers, when they are being turned on, have a tendency to shift
slightly in wavelength, effectively adding some Frequency Modulation(FM) to the signal.
This effect, called “chirp,” causes the laser to have an even wider optical line width. The
effect on transmission is most significant at 1550 nm using non-dispersion-shifted fiber
because that fiber has the highest dispersion usually encountered in any real-world
installation.
Polarization Mode Dispersion
Polarization mode dispersion (PMD) is
another complex optical effect that can occur
in single-mode optical fibers. Single-mode
fibers support two perpendicular polarizations
of the original transmitted signal. If a were
perfectly round and free from all stresses,
Figure 3 - Polarization Mode Dispersion both polarization modes would propagate at
exactly the same speed, resulting in zero
PMD. However, practical fibers are not
perfect, thus, the two perpendicular
polarizations may travel at different speeds
and, consequently, arrive at the end of the
fiber at different times. Figure 3 illustrates
this condition. The fiber is said to have a fast
axis, and a slow axis. The difference in
arrival times, normalized with length, is
known as PMD (ps/km0.5).
Excessive levels of PMD, combined with laser chirp and chromatic dispersion, can produce
time-varying composite second order(CSO) distortion in amplitude modulated (AM) video
systems. This results in a picture that may show a rolling or intermittent diagonal line across
the television screen.
Like chromatic dispersion, PMD causes digital transmitted pulses to spread out as the
polarization modes arrive at their destination at different times. For digital high bit rate
transmission, this can lead to bit errors at the receiver or limit receiver sensitivity.
Calculating Dispersion
Computing PMD is quite difficult unless specific measurements are made on the particular
fiber span of interest. Because of this difficulty, and because PMD is generally a much
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Part 2: Fiber Optic Components
smaller effect at any given data rate, we will not go into details of PMD computation. We
will focus on computing the effects of chromatic dispersion.
Let’s first consider non dispersion-shifted single-mode fiber, such as Corning SMF-28
CPC3 single-mode fiber. This fiber type makes up the largest percentage of the installed
fiber base. Its zero-dispersion wavelength lies between
1301 nm and 1321 nm. At the zero-dispersion wavelength, the fiber bandwidth is very high.
However, the fiber attenuation in this range is about 0.5 dB/km. This attenuation limits
transmission distances to perhaps 60 km. It would be more desirable to operate in the 1550
nm band where attenuation is about 0.2 dB/km. This attenuation would allow transmission
to about 150 km as long as dispersion does not limit performance.
Equation 1 can be used to compute the dispersion of Corning SMF-28 single-mode fiber.
Figure 4 shows the behavior of Equation 1 over the wavelength range from 1250 nm to
1650 nm. As expected, the dispersion goes to zero at a wavelength of 1311 nm. At the
window of greatest interest, near 1550 nm, the dispersion is about 17 ps/nm/km. If a laser
has a spectral width of 1 nm, then the dispersion will be 17 ps/km/nm.
Figure 4 - SM Fiber Dispersion
Dispersion Power Penalty
Now that we know the dispersion of the fiber, we can compute the effect on our
transmission link. When a fiber optic transmitter is connected to a fiber optic receiver
through a short length of fiber and an optical attenuator, the attenuation can be increased to
determine the receiver sensitivity. Usually the receiver sensitivity limit is defined at a given
bit error rate (BER). Usually a BER of 10-9 or 10-12 is used. Figures 5 and 6 illustrate test
setup for receiver sensitivity with and without fiber dispersion.
Figure 5 - Receiver Sensitivity with no Fiber Dispersion
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Part 2: Fiber Optic Components
Figure 6 - Receiver Sensitivity with Fiber Dispersion
The expected power penalty due to dispersion is given by a parabolic function of the ratio of
symbol rate to dispersion limited bandwidth times a coefficient, “c,” which relates to the
roll-off of a raised cosine receiver response. If we wish to examine this in terms of
dispersion power penalty versus total dispersion.
First we need to know the spectral width of the laser. For multilongitudinal mode (MLM)
laser, usually Fabry-Perot (FP) type, the spectral width is the root-mean-square spectral
width, and for single-longitudinal mode (SLM) lasers, usually distributed feedback (DFB)
lasers, the spectral width is the width at the 20 dB down points divided by 6.07. This is the
Gaussian spectral width at the 20 dB down point.
Figure 7 shows the typical optical output spectrum of an MLM laser and the corresponding
RMS spectral width.
Figure 7 - MLM Laser Spectral Output
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Figure 8 shows the typical optical output spectrum of an SLM laser and the spectral width.
Figure 8 - SLM Laser Spectral Output
Equation 2 describes the calculations for dispersion power penalties in fiber optic systems.
Figure 9 - Dispersion with a Normal DFB Laser
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Figure 9 shows the dispersion penalty for a data link operating at three data rates. The laser
is 0.1 nm wide with a center wavelength of 1550 nm and a fiber dispersion of 17 ps/nm/km.
The maximum acceptable dispersion penalty is usually 2 dB, though it is possible for a
system to tolerate a larger dispersion penalty if the optical attenuation is low. For the
example shown in Figure 9, the maximum usable fiber length at a data rate of 3.11 Gb/s
would be 85 km. At a wavelength of 1550 nm, the optical attenuation would be about 20 dB
for that distance, much less than the 30 dB loss budget provided by many high-speed links.
In this case, the fiber optic link would be considered dispersion-limited.
Figure 10 shows a second example with a much more narrow line width laser. In this case,
all conditions are the same except the laser spectral width is 0.05 nm.
Figure 10 - Dispersion with a Narrow DFB Laser
The dispersion penalty has dropped more than a factor of two compared to Figure 9. In this
case, the dispersion penalty at a data rate of 3.11 Gb/s never reaches 2 dB, even at 130 km.
The fiber optic link will, however, reach its optical attenuation limit near this distance. In
this case, the fiber optic link is said to be attenuation-limited.
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Part 2: Fiber Optic Components
To show that severe impact a laser spectral width has on the dispersion power penalty,
Figure 11 shows the dispersion power penalty of a FP MQW laser with a spectral width of 2
nm. The fiber dispersion is still 17 ps/nm/km and the operating wavelength is 1550 nm.
Figure 11 - Dispersion with an FP Laser
In Figure 11, we have chosen three lower data rates. Even so, the FP laser hits the
dispersion penalty limit of 2 dB at a distance of 17 km at 780 Mb/s and 50 km at a data rate
of 270 Mb/s. At both of these data rates, the data link is dispersion-limited. At a data rate of
100 Mb/s, the link is likely attenuation-limited.
Fiber Types
Thus far, we have only considered the most prevalent fiber type, nondispersion-shifted SM
fiber. There are a number of more modern fiber designs available. All offer lower dispersion
than the first example, but that may make little difference if the fiber is already in the
ground.
Figure 12 - Dispersion of SM Fiber Types
Figure 12 shows the dispersion characteristics of the four key types of optical fiber being
deployed at this time in the 1550 nm window. The shaded area, known as the Erbium-
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Part 2: Fiber Optic Components
Doped Fiber Amplifier (EDFA) window, represents the wavelengths used in DWDM (dense
wavelength-division multiplexing) systems.
Non-DSF: Nondispersion-shifted fiber has zero dispersion near 1310 nm.
DSF: Dispersion-shifted fiber works well in single channel 1550 nm systems, but in
DWDM systems, fiber nonlinearities near to the zero-dispersion wavelength cause
problems.
(+D) NZ-DSF: Similar to DSF, except that the zero-dispersion wavelength is intentionally
placed outside of the 1550 nm window. The fiber has a positive dispersion slope versus
wavelength.
(-D) NZ-DSF: Almost identical to the (+D) NZ-DSF type, except that the dispersion slope
is negative versus wavelength.
Laser Types
Based on what we saw in Figures 9, 10 and 11, the line width of the laser is critical in
limiting the magnitude of the dispersion power penalty. The key laser classes are:
Fabry-Perot/MQW: This is the lowest cost laser type available. It also has the worst
dispersion power penalty because of its wide optical line width, typically 1-4 nm. (Note that
laser line width is often referred to in MHz or GHz, rather than nm. The conversion factor is
1 nm = 125 GHz. So, an FP laser has a line width of 125-500 GHz).
Standard DFB: Standard DFB lasers have optical line widths on the order of 0.1 nm, or 12
GHz. At gigabit data rates, this can be a serious limitation for distances over 50 km.
Screened DFB: This is basically the same laser design as the standard DFB, however it has
been selected for very narrow line width, typically in the 0.01 to 0.05 nm range, 1-5 GHz.
This allows the link to reach much longer distances at gigabit data rates.
External Modulator/DFB: A very narrow line width laser (1-2 MHz or 0.000008-
0.000016 nm) operates in a CW (continuous wave), eliminating any chirp effects that
increase the laser line width even further. An external modulator is then used to turn the
light on and off. The external modulator acts as an electronic shutter. External modulators
are available for digital and analog applications and are capable of data rates to 40 Gb/s and
analog bandwidths of 20 GHz or more. The downside of this approach is that very narrow
line width sources can stimulate a host of additional fiber nonlinear effects, especially SBS
or stimulated brillouin scattering.
VCSELs: The vertical cavity surface-emitting laser is the newest laser structure. The
VCSELs emit light vertically, as the name suggests, and has a vertical laser cavity. These
are typically multi-quantum well (MQW) devices with lasing occurring in layer only 20-30
atoms thick. Bragg-reflectors with as many as 120 mirror layers form the laser reflectors.
Because the VCSELs are small and the high efficiency of the mirrors, the threshold current
is very low, below 1 mA. VCSELs also exhibit a high efficiency slope. Because of the way
they are manufactured, the VCSELs are ideal for applications the require an array of
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Part 2: Fiber Optic Components
devices.
Countermeasures
We have now learned about the major types of dispersion in single-mode fiber, the major
types of single-mode fiber and techniques for calculating the impact of dispersion on link
performance. Now we need to learn about other components that will allow us to minimize
the effects of dispersion. There are several passive components that can be used to reduce
the effects of dispersion. Generally they consist of introducing an element that has the
opposite dispersion of that in the fiber. These are usually referred to as dispersion
compensating modules (DCM). They are usually nothing more than a long spool of fiber
with the opposite dispersion characteristics. These can be purchased with specified amounts
of dispersion, e.g. -1000 ps/nm. There drawback is that they introduce considerable loss in
the system, often 8 dB or more.
Sometimes, DCMs are used in conjunction with circulators. Circulators are interesting 3-
port devices. An example is shown in the Figure 13.
Figure 13 - Use of Circulator to Compensate for Dispersion
In this example light enters the circulator in port 1. Light that enters port 1 is output to port
2 only. Now the light travels through the DCM, reflects off of the reflector and reenters port
2. The light that enters port 2 is output on port 3 only. The net effect is that the light has
now traveled through the DCM twice allowing us to use half as much fiber to get the same
compensating effect.
Circulators are also used in conjunction with devices called Bragg grating reflectors. These
devices connect to port 2 of the circulator. They do not require the use of a separate
reflector. The Bragg grating reflectors again introduce the opposite dispersion to clean up
the signal. Currently they only operate over a very narrow range of wavelengths, perhaps a
few nanometers. Thus they can be used to correct for a single channel in a DWDM system,
not the entire band.
An elegant solution to dispersion compensation consists of alternating lengths of (+D) NZ-
DSF and (-D) NZ-DSF fiber types. This would yield very low overall dispersion and could
be readily used for DWDM applications. The correction is never perfect over the entire
band, but does reduce overall dispersion. The dispersion and transmission distance for
alternating lengths of these fiber types is illustrated in Figure 14.
Figure 14 - Alternating (+ D) NZ-DSF Fiber and (-D) NZ-DSF Fiber
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Part 2: Fiber Optic Components
Fiber Nonlinearities
Introduction
Nonlinearity effects arose as optical fiber data rates, transmission lengths, number of wavelengths,
and optical power levels increased. The only worries that plagued optical fiber in the early day
were fiber attenuation and, sometimes, fiber dispersion; however, these issues are easily dealt with
using a variety of dispersion avoidance and cancellation techniques. Fiber nonlinearities present a
new realm of obstacle that must be overcome. These nonlinearities previously appeared in
specialized applications such as undersea installations. However, the new nonlinearities that need
special attention when designing state-of-the-art fiber optic systems include stimulated Brillouin
scattering (SBS), stimulated Raman scattering (SRS), four wave mixing (FWM), self-phase
modulation (SPM), cross-phase modulation (XPM), and intermodulation (mixing).
Fiber nonlinearities represent the fundamental limiting mechanisms to the amount of data that can
be transmitted on a single optic fiber. System designers must be aware of these limitations and the
steps that can be taken to minimize the detrimental effects of fiber nonlinearities.
Causes of Nonlinearities
Fiber nonlinearities arise from two basic mechanisms. The most detrimental mechanism arises from
the refractive index of glass being dependent on the optical power going through the material. The
general equation for the refractive index of the core in an optical fiber is:
Where:
n0 = The refractive index of the fiber core at low optical power
levels.
n2 = The nonlinear refractive index coefficient (2.35 x 10-20
m2/W for silica).
P = The optical power in Watts.
Aeff = The effective area of the fiber core in square meters.
The equation shows that minimizing the amount of power, P, launched and maximizing the
effective area of the fiber, Aeff, eliminates the nonlinearities produced by refractive index power
dependence. Minimizing the power goes against the current approach to eliminating the detrimental
effects; however, maximizing the effective area remains the most common approach in the latest
fiber designs.
Figure 1 depicts the relationship of the refractive index of silica versus optical power. The
magnitude of the change in the refractive index is relatively small; this only becomes important
because the interaction length in a real fiber optic system can be hundreds of kilometers. SPM,
XPM, FWM, and intermodulation nonlinearities result from the power dependent refractive index
of silica.
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Figure 1 - The Refractive Index of Silica versus Optical
Power
The second mechanism that cause fiber nonlinearities is the scattering phenomena, which product
SBS and SRS.
Stimulated Brillouin Scattering
Stimulated Brillouin Scattering (SBS) sets an upper limit on
the amount of optical power that can be usefully launched
into an optical fiber. When the SBS threshold for optical
power is exceeded, a significant amount of the transmitted
light is redirected back to the transmitters. In addition to
causing a saturation of optical power in the receiver,
problems also arise with backreflection in the optical
signal, and noise that degrades the BER performance. It is
particularly important to control SBS in high-speed
transmission systems the use external modulators and
continuous wave (CW) laser sources. It should be noted that
1550 nm CATV signals in the high-speed transmitters often
possess the characteristics that trigger the SBS effect.
Brillouin scattering is the time-varying electric fields
within a fiber interacting with the acoustic vibrational
modes of the fiber material which in turn scatter the
incident light. Stimulated Brillouin Scattering is when the
source of the high intensity electric fields is the incident
lightwave. The periodic variation in in the refractive index,
caused by the high power incident lightwave, cases
backreflection similar to the effect of Bragg gratings. An
increasing portion of light is backscattered because of the
increasing optical level beyond the SBS threshold. This
creates an upper limit to the power levels that can be
carried over the fiber. Figure 2 illustrates this phenomenon.
As the launch power is increased above the threshold, there
is a dramatic increase in the amount of backscattered light.
Wavelength (the threshold is lower at 1550 nm than 1310 nm)
and the linewidth of the transmitter, among other parameters,
govern the precise threshold for the onset of the SBS effect.
Values of +8 to +10 dBm are typical for direct modulated
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optical sources operating at 1550 nm over standard single-
mode fiber.
Figure 2 - SBS Threshold Effects
The SBS threshold is strongly dependent on the linewidth of the optical source with narrow
linewidth sources having considerably lower SBS thresholds. Extremely narrow linewidth lasers
(e.g. less than 10 MHz wide), often used in conjunction with external modulators, can have SBS
thresholds of +4 to +6 dBm at 1550 nm. Figure 3 illustrates how the SBS threshold increases
proportionally as the optical linewidth increases. Broadening the effective spectral width of the
optical source minimizes SBS.
Externally modulating the transmitter provides one way to broadening the linewidth. This involves
adding a small AC modulation signal to the DC current source used drive to laser. This broadens the
spectral linewidth of the transmitter and increases the threshold for the onset of SBS. This option
also increases the dispersion susceptibility of the transmitter, primarily a concern when operating at
1550 nm over non dispersion-shifted single-mode fiber. Practical implementations of SBS
suppression circuitry based on laser drive dithering can increase the SBS threshold by 5 dB.
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Part 2: Fiber Optic Components
Figure 3 - SBS Threshold versus Source Linewidth
Phase dithering the output of the external modulator provide
another common means of increasing the SBS threshold. In this
case, a high frequency signal, usually twice that of the
highest frequency being transmitted, is imposed onto both
output legs of the external modulator. This modulates the
phase of the light, effectively spreading out the spectral
width. Figure 4 shows the optical spectra of an VSB/AM
transmitter without phase dithering. The central carrier
exceeds the SBS threshold, causing serious system
degradation.
Figure 4 - Optical Spectrum without Phase Modulation
In Figure 5, a high frequency dither signal has been applied
to the phase modulation input of the external modulator. It
can be seen that all of the lines are now comfortably below
the SBS threshold. This technique can raise the SBS threshold
optical power by about 10 dB.
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Part 2: Fiber Optic Components
Figure 5 - Optical Spectrum with Phase Modulation
Adding EDFAs to a signal path greatly decrease the SBS threshold. The SBS threshold for a system
containing N amplifiers is the threshold without amplifiers in mW divided by N. This can result in
very low SBS thresholds that can seriously impair system performance.
Stimulated Raman Scattering
Stimulated Raman Scattering (SRS) is much less of a problem
than SBS. Its threshold is close to 1 Watt, nearly a thousand
times higher than SBS. But real systems are being deployed
with EDFAs having optical output powers of 500 mW (+27 dBm),
and this will only go higher. A fiber optic link that
includes three such optical amplifiers will reach this limit
since the limit drops proportionally by the number of optical
amplifiers in series. SRS can cause scattering like SBS, but
usually the effect first seen is that the shorter wavelength
channels are robbed of power, and that power feeds the longer
wavelength channels. This is similar to the operation of
EDFAs where a 980 nm pump wavelength provides the energy that
amplifies the signals in the longer wavelength, 1550 nm,
region.
Figure 6 - Six Channel DWDM Transmitted Optical Spectrum
Figure 6 shows the typical transmit spectrum or a six-channel
DWDM system. Note that all of the six wavelengths have
identical amplitudes. These signals are all in the 1550 nm
window. Figure 7 illustrates the SRS effect. It can be seen
that the short wavelength channels have a much smaller
amplitude compared to the longer wavelength channels. Plain
silica fiber can provide similar gain using the Raman gain
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Part 2: Fiber Optic Components
mechanism. Raman amplifiers are only now becoming mainstream
additions to long-haul telecommunications system.
Figure 7 - SRS Effect on Six Channel DWDM Transmitted Optical Spectrum
Four Wave Mixing
Usually only systems that carry a number of simultaneous wavelengths, such as DWDM systems,
exhibit four wave mixing (FWM). Caused by the nonlinear nature of the refractive index of the
optical fiber itself, the FWM effect is similar to composite triple beat (CTB) distortion observed in
CATV systems. CTB is also caused by nonlinearity, this time in the electrical amplifier chain or one
of the optical components, usually the laser. CTB, like FWM, is classified as a third-order distortion
phenomenon. Third-order distortion mechanisms generate third-order harmonics in systems with
one channel. In multichannel systems, third-order mechanisms generate third-order harmonics and a
gamut of cross products. These cross products cause the most problems since they often fall near or
on top of the desired signals.
Consider a simple three-wavelength (1, 2, and 3) system that is experiencing FWM distortion. In
this simple system, nine cross products are generated near 1, 2, and 3 that involve two or more of
the original wavelengths. Note that there are additional products generated, but they fall well away
from the original input wavelengths. Let us assume that the input wavelengths are 1 = 1551.72 nm,
2 = 1552.52 nm, and 3 = 1553.32 nm. The interfering wavelengths that are of most concern in our
hypothetical three wavelength system are:
1 + 2-3= 1550.92
nm 1-2+ 3=1552.52
2+ 3-1= 1554.12 nm
1-2 +3= 1552.52 nm
22- 1= 1553.32 nm
nm 21-3= 1550.12 nm
23 -2= 1554.12 nm
2+31= 1554.12 23- 11554.92 nm
nm
It can be seen that three of the interfering products fall right on top of the original three signals. The
remaining six products fall outside of the original three signals. These six can be optically filtered
out. Because the three interfering products that fall on top of the original signals are mixed together,
they cannot be removed by any means. Figure 8 shows the results graphically. The three tall solid
bars are the three original signals. The shorter cross-hatched bars represent the nine interfering
products. The number of interfering products increases as ½ • (N3-N2) where N is the number of
signals.
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Part 2: Fiber Optic Components
Figure 8 - FWM Products for a Three Wavelength System
Figure 9 shows that the number of interfering products
rapidly becomes a very large number. Since there is no way to
eliminate products that fall on top of the original signals,
the only hope is to prevent them from forming in the first
place.
Figure 9 - FWM Products versus Channel Count
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Part 2: Fiber Optic Components
Two factors strongly influence the magnitude of the FWM products, referred to as the FWM
mixing efficiency. The first factor is the channel spacing; mixing efficiency increases dramatically
as the channel spacing becomes closer. Fiber dispersion is the second factor, and mixing efficiency
is inversely proportional to the fiber dispersion, being strongest at the zero-dispersion point. In all
cases, the FWM mixing efficiency is expressed in dB, and more negative values are better since
they indicate a lower mixing efficiency.
Figure 10 shows the magnitude of FWM mixing efficiency versus fiber dispersion and channel
spacing. If a system design uses NDSF with dispersion of 17 ps/nm/km and the minimum
recommended International Telecommunication Union (ITU) DWDM spacing of 0.8 nm, then the
mixing efficiency is about -48 dB and will have little impact. On the other hand, if a system design
uses DSF with a dispersion of 1 ps/nm/km and a non-standard spacing of 0.4 nm, then the mixing
efficiency becomes -12 dB and will have a severe impact on system performance, perhaps making
recovery of the transmitted signal impossible. The data presented in Figure 3.34 is for a given
optical power level, fiber length, wavelength and so on. The magnitude of the mixing efficiency
will vary widely as these parameters vary. The data presented is intended to illustrate the principles
only.
Figure 10 - FWM Mixing Efficiency in Single-mode Fibers
Modulation
Self-phase Modulation
Figure 11 illustrates self-phase modulation. Like FWM, self-phase modulation (SPM) is due to the
power dependency of the refractive index of the fiber core. It interacts with the chromatic dispersion
in the fiber to change the rate at which the pulse broadens as it travels down the fiber. Whereas
increasing the fiber dispersion will reduce the impact of FWM, it will increase the impact of SPM.
As an optical pulse travels down the fiber, the leading edge of the pulse causes the refractive index
of the fiber to rise, resulting in a blue shift. The falling edge of the pulse decreases the refractive
index of the fiber causing a red shift. These red and blue shifts introduce a frequency chirp on each
edge which interacts with the fiber’s dispersion to broaden the pulse.
Figure 11 - Effects of SPM on a Pulse
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Part 2: Fiber Optic Components
Cross-phase Modulation
Cross-phase modulation (XPM) is very similar to SPM except that it involves two pulses of light,
whereas SPM needs only one pulse. In XPM, two pulses travel down the fiber, each changing the
refractive index as the optical power varies. If these two pulses happen to overlap, they will
introduce distortion into the other pulses through XPM. Unlike, SPM, fiber dispersion has little
impact on XPM. Increasing the fiber effective area will reduce XPM and all other fiber
nonlinearities.
Intermodulation (Mixing)
Intermodulation is fairly similar to SPM and XPM. Consider the case where two laser light sources
are transmitting light through the fiber. Again as the optical power in each light wave peaks and
drops, the refractive index of the fiber changes accordingly. Now the two different light sources
have different frequencies f1 and f2. As the refractive index changes in concert with frequencies f1
and f2, new frequencies, 2 • f1 - f2 and 2 • f2 - f1, appear. This is similar in many ways to the FWM
nonlinearity.
Handling Fragile Optical Fibers and Fiber Pigtail Assemblies
Background
Many fiber optic products incorporate fiber pigtail interfaces between the optical device and the
optical connector. These fiber pigtails are extremely fragile and must be handled carefully to avoid
breakage. Knots, kinks, twists and bends in the optical fiber will ruin the fiber’s ability to transmit
light. In many cases, a bend or break in an optical fiber will completely disrupt the system’s
performance. Dangling even an insubstantial weight from the end of a fiber can break or irreparably
damage the fiber pigtail. This article outlines the proper procedure for handling these fragile optical
fiber assemblies.
Safe Fiber Assembly Handling
1. Always read and comply with the handling instructions on
the shipping container.
2. Remember, optical fiber is made of glass. It should be
treated with the same care.
3. Never use the fiber to pick up or support the weight of
the device to which it is attached. Instead, always keep both
the device and the optical connector together in your hands.
4. Do not allow kinks or knots to develop in the fiber.
Carefully work out any tangles. Pulling on the fiber will
cause any kinks or curls to tighten and exceed the minimum
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Part 2: Fiber Optic Components
bend radius.
5. Do not force the fiber into any retaining clips that may
be used. First open the clips, slip the fiber carefully
inside, and close the clips, making sure the fiber will not
get caught in the clip latch.
6. To remove pigtailed devices from the shipping container,
use one hand to lift out the connector. Use the other hand to
carefully uncoil the fiber. Once the fiber is uncoiled,
carefully lift the fiber optic unit out of the shipping box.
Do not dangle the box from the fiber as shown in Figure 1.
Figure 1 - Improper way to Figure 2 - Letting the Fiber
Handle Fragile Fiber Pigtails Dangle Over Sharp Corners will
Damage the Fiber
Inserting the Optical Device into a Circuit Board
1. Follow all integrated circuit standard insertion practices including ESD prevention measures.
2. During insertion and soldering, take care not to place a thumb or heavy object on the fiber end of
the device.
3. A pigtailed device can be damaged easily if the fiber is touched by a hot solder iron. While this
may not immediately break the fiber, it will increase loss due to local stresses, and it will
compromise the physical strength of the fiber. A fiber damaged by a hot soldering iron is no longer
a reliable device and should be replaced.
4. After insertion, the fiber pigtail should be protected from damage.
5. Never use the pigtail to support the weight of the circuit board.
6. For best results, follow all ESD precautions and approved fiber cleaning procedures. For details
on the proper fiber cleaning procedures, see article, “Fiber Optic Connectors.”
7. When securing the fiber to the PCB, do not use tight string, clamps or any mechanical mean to
tightly bind the fiber. Local stress on the fiber increases loss and may break the fiber. Hard epoxies
should also be avoided when securing fibers on a PCB. A pliable RTV is acceptable. Velcro® dots
are by far the best way to secure fibers on a PCB.
8. Be sure to secure fibers in units that include a ventilation fan so that the fiber does not get pulled
into the fan.
Types of Optical Fiber
Introduction
Understanding the characteristics of different fiber types aides in understanding the
applications for which they are used. Operating a fiber optic system properly relies on
knowing what type of fiber is being used and why. There are two basic types of fiber:
multimode fiber and single-mode fiber. Multimode fiber is best designed for short
transmission distances, and is suited for use in LAN systems and video surveillance.
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Part 2: Fiber Optic Components
Single-mode fiber is best designed for longer transmission distances, making it suitable for
long-distance telephony and multichannel television broadcast systems.
Multimode Fiber
Multimode fiber, the first to be manufactured and commercialized, simply refers to the fact
that numerous modes or light rays are carried simultaneously through the waveguide.
Modes result from the fact that light will only propagate in the fiber core at discrete angles
within the cone of acceptance. This fiber type has a much larger core diameter, compared to
single-mode fiber, allowing for the larger number of modes, and multimode fiber is easier to
couple than single-mode optical fiber. Multimode fiber may be categorized as step-index or
graded-index fiber.
Multimode Step-index Fiber
Figure 2 shows how the principle of total internal reflection applies to multimode step-index
fiber. Because the core’s index of refraction is higher than the cladding’s index of
refraction, the light that enters at less than the critical angle is guided along the fiber.
Three different lightwaves travel down the
fiber. One mode travels straight down the Figure 2 - Total Internal Reflection in
center of the core. A second mode travels at Multimode Step-index fiber
a steep angle and bounces back and forth by
total internal reflection. The third mode
exceeds the critical angle and refracts into
the cladding. Intuitively, it can be seen that
the second mode travels a longer distance
than the first mode, causing the two modes
to arrive at separate times.
This disparity between arrival times of the different light rays is known as dispersion, and
the result is a muddied signal at the receiving end. For a more detailed discussion of
dispersion, see “"Dispersion in Fiber Optic Systems;" however, it is important to note that
high dispersion is an unavoidable characteristic of multimode step-index fiber.
Multimode Graded-index Fiber
Graded-index refers to the fact that the refractive index of the core gradually decreases
farther from the center of the core. The increased refraction in the center of the core slows
the speed of some light rays, allowing all the light rays to reach the receiving end at
approximately the same time, reducing dispersion.
Figure 3 shows the principle of multimode Figure 3 - Multimode Graded-index Fiber
graded-index fiber. The core’s central
refractive index, nA, is greater than that of
the outer core’s refractive index, nB. As
discussed earlier, the core’s refractive index
is parabolic, being higher at the center.
As Figure 3 shows, the light rays no longer follow straight lines; they follow a serpentine
path being gradually bent back toward the center by the continuously declining refractive
index. This reduces the arrival time disparity because all modes arrive at about the same
time. The modes traveling in a straight line are in a higher refractive index, so they travel
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Part 2: Fiber Optic Components
slower than the serpentine modes. These travel farther but move faster in the lower
refractive index of the outer core region.
Single-mode Fiber
Single-mode fiber allows for a higher capacity to transmit information because it can retain
the fidelity of each light pulse over longer distances, and it exhibits no dispersion caused by
multiple modes. Single-mode fiber also enjoys lower fiber attenuation than multimode
fiber. Thus, more information can be transmitted per unit of time. Like multimode fiber,
early single-mode fiber was generally characterized as step-index fiber meaning the
refractive index of the fiber core is a step above that of the cladding rather than graduated as
it is in graded-index fiber. Modern single-mode fibers have evolved into more complex
designs such as matched clad, depressed clad and other exotic structures.
Single-mode fiber has disadvantages. The Figure 4 - Single-mode Fiber
smaller core diameter makes coupling light
into the core more difficult. The tolerances
for single-mode connectors and splices are
also much more demanding.
Single-mode fiber has gone through a continuing evolution for several decades now. As a
result, there are three basic classes of single-mode fiber used in modern telecommunications
systems. The oldest and most widely deployed type is non dispersion-shifted fiber (NDSF).
These fibers were initially intended for use near 1310 nm. Later, 1550 nm systems made
NDSF fiber undesirable due to its very high dispersion at the 1550 nm wavelength. To
address this shortcoming, fiber manufacturers developed, dispersion-shifted fiber (DSF),
that moved the zero-dispersion point to the 1550 nm region. Years later, scientists would
discover that while DSF worked extremely well with a single 1550 nm wavelength, it
exhibits serious nonlinearities when multiple, closely-spaced wavelengths in the 1550 nm
were transmitted in DWDM systems. Recently, to address the problem of nonlinearities, a
new class of fibers were introduced. These are classified as non zero-dispersion-shifted
fibers (NZ-DSF). The fiber is available in both positive and negative dispersion varieties
and is rapidly becoming the fiber of choice in new fiber deployment. For more information
on this loss mechanism, see the article “Fiber Dispersion.”
Figure 6 - Dispersion for Alternating 20 km Lengths of (+D) NZ-DSF and (-D) NZ-DSF
Fiber
One additional important variety of single-
mode fiber is polarization-maintaining (PM) Figure 7 - Cross-section of Polarization-
fiber. All other single-mode fibers discussed maintaining Fiber
so far have been capable of carrying
randomly polarized light. PM fiber is
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Part 2: Fiber Optic Components
designed to propagate only one polarization
of the input light. This is important for
components such as external modulators that
require a polarized light input.
Figure 7 shows the cross-section of a type of PM fiber. This fiber contains a feature not seen
in other fiber types. Besides the core, there are two additional circles called stress rods. As
their name implies, these stress rods create stress in the core of the fiber such that the
transmission of only one polarization plane of light is favored.
Single-mode fibers experience nonlinearities that can greatly affect system performance. For
complete information, see “Fiber Nonlinearities.”
Fiber Optic Connectors
Fiber Optic Connectors
Background
Fiber optic connectors have traditionally been the biggest concern in using fiber optic
systems. While connectors were once unwieldy and difficult to use, connector
manufacturers have standardized and simplified connectors greatly. This increasing user-
friendliness has contributed to the increase in the use of fiber optic systems; it has also taken
the emphasis off the proper care and handling of optical connectors.
This article covers connector basics including the parts of a fiber optic connector, installing
fiber optic connectors, and the cleaning and handling of installed connectors. For
information on connector loss, see “Connector Loss Test Measurement.”
Figure 1 - Parts of a Fiber Optic Connector
Fiber-to-fiber interconnection can consist of a splice, a permanent connection, or a
connector, which differs from the splice in its ability to be disconnected and reconnected.
Fiber optic connector types are as various as the applications for which they were
developed. Different connector types have different characteristics, different advantages and
disadvantages, and different performance parameters. But all connectors have the same four
basic components.
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The fiber is mounted in a long, thin cylinder, the ferrule, which acts as a fiber
alignment mechanism. The ferrule is bored through the center at a diameter that is
The Ferrule: slightly larger than the diameter of the fiber cladding. The end of the fiber is
located at the end of the ferrule. Ferrules are typically made of metal or ceramic,
but they may also be constructed of plastic.
Also called the connector housing, the connector body holds the ferrule. It is
usually constructed of metal or plastic and includes one or more assembled pieces
The Connector which hold the fiber in place. The details of these connector body assemblies vary
Body: among connectors, but bonding and/or crimping is commonly used to attach
strength members and cable jackets to the connector body. The ferrule extends
past the connector body to slip into the coupling device.
The cable is attached to the connector body. It acts as the point of entry for the
The Cable: fiber. Typically, a strain-relief boot is added over the junction between the cable
and the connector body, providing extra strength to the junction.
Most fiber optic connectors do not use the male-female configuration common to
electronic connectors. Instead, a coupling device such as an alignment sleeve is
The Coupling
used to mate the connectors. Similar devices may be installed in fiber optic
Device:
transmitters and receivers to allow these devices to be mated via a connector.
These devices are also known as feed-through bulkhead adapters.
Table 1 illustrates some types of optical connectors and lists some specifications. Each
connector type has strong points. For example, ST connectors are a good choice for easy
field installations; the FC connector has a floating ferrule that provides good mechanical
isolation; the SC connector offers excellent packing density, and its push-pull design resists
fiber end face contact damage during unmating and remating cycles.
Table 1- Types Of Optical Connectors
Connector Insertion Loss Repeatability Fiber Type Applications
0.50-1.00 dB 0.20 dB SM, MM Datacom, Telecommunications
FC
0.20-0.70 dB 0.20 dB SM, MM Fiber Optic Network
FDDI
0.15 db (SM)
0.2 dB SM, MM High Density Interconnection
LC 0.10 dB (MM)
0.30-1.00 dB 0.25 dB SM, MM High Density Interconnection
MT Array
0.20-0.45 dB 0.10 dB SM, MM Datacom
SC
0.20-0.45 dB 0.10 dB SM, MM Datacom
SC Duplex
Typ. 0.40 dB
Typ. 0.40 dB (SM) (SM) Inter-/Intra-Building, Security,
SM, MM
ST Typ. 0.50 dB (MM) Typ. 0.20 dB Navy
(MM)
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Part 2: Fiber Optic Components
Installing Fiber Optic Connectors
The method for attaching fiber optic connectors to optical fibers varies among connector
types. While not intended to be a definitive guide, the following steps are given as a
reference for the basics of optical fiber interconnection.
1. Cut the cable one inch longer than the required finished length.
2. Carefully strip the outer jacket of the fiber with “no nick” fiber strippers. Cut the exposed
strength members, and remove the fiber coating. The fiber coating may be removed two
ways: by soaking the fiber for two minutes in paint thinner and wiping the fiber clean with a
soft, lint-free cloth, or by carefully stripping the fiber with fiber stripper. Be sure to use
strippers made specifically for use with fiber rather than metal wire strippers as damage can
occur, weakening the fiber.
3. Thoroughly clean the bared fiber with isopropyl alcohol poured onto a soft, lint-free cloth
such as Kimwipes®. NEVER clean the fiber with a dry tissue. Note: Use only industrial
grade 99% pure isopropyl alcohol. Commercially available isopropyl alcohol is for
medicinal use and is diluted with water and a light mineral oil. Industrial grade isopropyl
alcohol should be used exclusively.
4. The connector may be connected by applying epoxy or by crimping. If using epoxy, fill
the connector with enough epoxy to allow a small bead of epoxy to form at the tip of the
connector. Insert the clean, stripped fiber into the connector. Cure the epoxy according to
the instructions provided by the epoxy manufacturer.
5. Anchor the cable strength members to the connector body. This prevents direct stress on
the fiber. Slide the back end of the connector into place (where applicable).
6. Prepare the fiber face to achieve a good optical finish by cleaving and polishing the fiber
end. Before connection is made, the end of each fiber must have a smooth finish that is free
of defects such as hackles, lips, and fractures. These defects, as well as other impurities and
dirt change the geometrical propagation patterns of light and cause scattering.
Figure 2 - Fiber End Face Defects: Hackle (left), Lip (right)
Cleaving
Cleaving involves cutting the fiber end flush with the end of the ferrule. Cleaving, also
called the scribe-and-break method of fiber end face preparation, takes some skill to achieve
optimum results. Properly done, the cleave produces a perpendicular, mirror-like finish.
Incorrect cleaving will result in lips and hackles as seen in Figure 2. While cleaving may be
done by hand, a cleaver tool, available from such manufacturers as Fujikura, allows for a
more consistent finish and reduces the overall skill required. The steps listed below outline
one procedure for producing good, consistent cleaves such as the one shown in Figure 3.
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Part 2: Fiber Optic Components
1. Place the blade of the cleaver tool at the tip of the ferrule.
2. Gently score the fiber across the cladding region in one direction. If the scoring is not
done lightly, the fiber may break, making it necessary to reterminate the fiber.
3. Pull the excess, cleaved fiber up and away from the ferrule.
4. Carefully dress the nub of the fiber with a piece of 12-micron alumina-oxide paper.
5. Do the final polishing. (See Figure 4.)
Figure 3 - A Well-cleaved Multimode Fiber
Polishing
After a clean cleave has been achieved, the fiber end face is attached to a polishing brush,
and the fiber is ground and polished. The proper finish is achieved by rubbing the
connectorized fiber end against polishing paper in a figure-eight pattern approximately sixty
times.
Figure 4 - Polishing Technique
To increase the ease and repeatability of connector installation, some companies offer
connector kits. Some kits are specific to the type of connector to be installed while others
supply the user with general tools and information for connecting different types of
connectors. Some connectors require the use of an alignment sleeve, also called an
interconnection sleeve. This sleeve serves to increase repeatability from connection to
connection.
37
Part 2: Fiber Optic Components
Care and Handling of Fiber Optic Connectors
A number of events can damage fiber optic connectors. Unprotected connector ends can
experience damage by impact, airborne dust particles, or excess humidity or moisture. The
increased optical output power of modern lasers also have the potential to damage a
connector, an often overlooked factor in discussions about handling and caring for optical
fibers and connectors. Most designers tend to think of the power levels in optical fibers as
relatively insignificant. However, a few milliwatts at 850 nm will do permanent damage to
a retina. Today, optical amplifiers can generate optical powers of 1 Watt of more into a
single-mode fiber. This becomes quite significant when one considers that the optical power
is confined in the optical core only a few microns in diameter. Power densities in a single-
mode fiber carrying an optical power of 1 Watt (+30 dBm) can reach 3 megawatts/cm2 or
30 gigawatts/m2! To put it in everyday terms, sunlight at the surface of the Earth has a
power density of about 1,000 Watts/m2. Most organic materials will combust when exposed
to radiant energies of 100 kilowatts/m2. Clearly, power densities of 30 gigawatts/m2
deserve attention.
Effects on Fiber Optic Connectors
One should never clean an optical connector attached to a fiber that is carrying light.
Optical power levels as low as +15 dBm, or 32 milliwatts, may cause an explosive ignition
of the cleaning material when it contacts the end of the optical connector, destroying the
connector. Typical cleaning materials, such as tissues saturated with alcohol, will combust
almost instantaneously when exposed to optical power levels of +15 dBm or higher. The
micro-explosions at the tip of the connector can leave pits in the end of the connector and
crack the connector’s surface, destroying its ability to carry light with low loss. Figure 5
shows an optical connector that has been heavily damaged by high optical power levels.
Usually the damage is limited to less severe pitting.
Figure 5 - Connector Damaged by High Optical Power (Photo courtesy of Dr. D.D. Davis.)
CLEANING
Another important thing to remember in handling fiber optic connectors is that the fiber
end face and ferrule must be absolutely clean before it is inserted into a transmitter or
receiver. Dust, lint, oil (from touching the fiber end face), or other foreign particles obscure
the end face, compromising the integrity of the optical signal being sent over the fiber.
From the optical signal’s point-of-view, dirty connections are like dirty windows. Less light
gets through a dirty window than a clean one.
It is hard to conceive of the size of a fiber optic connector core. Single-mode fibers have
cores that are only 8-9 µm in diameter. As a point of reference, a typical human hair is 50-
75 µm in diameter, approximately 6-9 times larger! Dust particles can be 20 µm or larger in
diameter. Dust particles smaller than 1 µm can be suspended almost indefinitely in the air.
A 1 µm dust particle landing on the core of a single-mode fiber can cause up to 1 dB of
38
Part 2: Fiber Optic Components
loss. Larger dust particles (9 µm or larger) can completely obscure the core of a single-
mode fiber. Fiber optic connectors need to be cleaned every time they are mated and
unmated; it is essential that fiber optics users develop the necessary discipline to always
clean the connectors before they are mated.
It is also important to cover a fiber optic connector when it is not in use. Unprotected
connector ends are most often damaged by impact, such as hitting the floor. Most connector
manufacturers provide some sort of protection boot. The best protectors cover the entire
connector end, but they are generally simple closed-end plastic tubes that fit snugly over the
ferrule only. These boots will protect the connector's polished ferrule end from impact
damage that might crack or chip the polished surface. Many of the tight fitting plastic tubes
contain jelly-like contamination (most likely mold release) that adheres to the sides of the
ferrule. A blast of cleaning air or a quick dunk in alcohol will not remove this residue. This
jelly-like residue can combine with common dirt to form a sticky mess that causes the
connector ferrule to stick in the mating adapter. Often, the stuck ferrule will break off as
one attempts to remove it. The moral of the story is always thoroughly clean the connector
before mating, even if it was cleaned previously before the protection boot was installed.
Cleaning Technique
Required Equipment:
• Kimwipes® or any lens-grade, lint-free tissue. The type sold for eyeglasses work quite
well.
• Denatured alcohol. Note: Use only industrial grade 99% pure isopropyl alcohol.
Commercially available isopropyl alcohol is for medicinal use and is diluted with water and
a light mineral oil. Industrial grade isopropyl alcohol should be used exclusively.
• 30X microscope.
• Canned dry air.
1. Fold the tissue twice so it is four layers thick.
2. Saturate the tissue with alcohol.
3. First clean the sides of the connector ferrule. Place the connector ferrule in the tissue, and
apply pressure to the sides of the ferrule. Rotate the ferrule several times to remove all
contamination from the ferrule sides.
4. Now move to a clean part of the tissue. Be sure it is still saturated with alcohol and that it
is still four layers thick. Put the tissue against the end of the connector ferrule. Put your
fingernail against the tissue so that it is directly over the ferrule. Now scrape the end of the
connector until it squeaks. It will sound like a crystal glass that has been rubbed when it is
wet.
5. Use the microscope to verify the quality of the cleaning. If it isn’t completely clean,
repeat the steps with a clean tissue. Repeat until you have a cleaning technique that yields
good, reproducible results.
6. Mate the connector immediately! Don’t let the connector lie around and collect dust
before mating.
7. Air can be used to remove lint or loose dust from the port of a transmitter or receiver to
be mated with the connector. Never insert any liquid into the ports.
Handling
39
Part 2: Fiber Optic Components
1. Never touch the fiber end face of the connector.
2. Connectors not in use should be covered over the ferrule by a plastic dust cap. It is
important to note that inside of the ferrule dust caps contain a sticky residue that is a by-
product of making the dust cap. This residue will remain on the ferrule end after the cap is
removed.
3. The use of index-matching gel, a gelatinous substance that has a refractive index close to
that of the optical fiber, is a point of contention between connector manufacturers. Glycerin,
available in any drug store, is a low-cost, effective index-matching gel. Using glycerin will
reduce connector loss and backreflection, often dramatically. See article “"Backreflection –
The Bane of Good Performance" for more information. However, the index-matching gel
may collect dust or abrasives that can damage the fiber end faces. It may also leak out over
time, causing backreflections to increase.
Standards For Fiber Optic Connectors
Industrial Standards
Informative Test Methods for Fiber Optic Fibers, Cable, Opto-Electronic Sources
TSB-62 and Detectors, Sensors, Connecting and Terminating Devices, and Other Fiber
Optic Components
EIA-440-A Fiber Optic Connector Terminology
Standard Test Procedure for Fiber Optic Fibers, Cables, Transducers, Sensors,
EIA-455-A
Connecting and Terminating Devices, and Other Components
EIA-455-1A Cable Flexing for Fiber Optic Interconnecting Devices
EIA/TIA-455-
Cable Retention Test Procedure for Fiber Optic Cable Interconnecting Devices
6B
EIA-455-9 Fiber Optic Test Procedure for Bundle Connector
EIA/TIA-455- Visual and Mechanical Inspection of Fibers, Cables, Connectors and/or Other
13 Fiber Optic Devices
EIA-455-17A Maintenance Aging of Fiber Optic Connectors and Terminated Cable Assemblies
EIA-455-21A Mating Durability for Fiber Optic Interconnecting Devices
EIA-455-26A Crush Resistance of Fiber Optic Interconnecting Devices
EIA-455-34A Interconnection Device Insertion loss Test
EIA-455-36A Twist Test for Fiber Optic Connecting Devices
TIA/EIA-455- Measurement of Breakaway Frictional Force in Fiber Optic Connector Alignment
158 Sleeves
EIA-455-172 Flame Resistance of Fire wall Connector
EIA/TIA-455-
Engagement and Separation for Fiber Optic Connector Sets
187
EIA/TIA-
Generic Specification for Fiber Optic Connectors
4750000-B
EIA/TIA-
Sectional Specification for Type FSMA Connectors
475C000
TIA/EIA- Blank Detail Specification for Connector Set for Optical Fiber and Cables, Type
475EA BFOC/2.5, Environmental Category I
Blank Detail Specification for Connector Set for Optical Fiber and Cables, Type
TIA/EIA-475EB
BFOC/2.5, Environmental Category II
TIA/EIA- Blank Detail Specification for Connector Set for Optical Fiber and Cables, Type
475EC00 BFOC/2.5, Environmental Category III
40
Part 2: Fiber Optic Components
TIA/EIA-604 Fiber Optic Connector Intermateability Standards
Bellcore Standards
GR-326 Generic Requirements for Single-Mode Optical Fiber Connectors
GR-1081 Generic Requirements for Field Mountable Optical Fiber Connectors
GR-1435 Generic Requirements for Multi-fiber Optical Connectors
SR-ARH-
Single-mode Fiber Connectors Technology
002744
SR-4226 Fiber Optic Connector Certification
TR-73536 Technical Requirements for Optical Connectors
Connector Loss Test Measurements
Interconnection Loss Measurements
The ideal interconnection of one fiber to another would have two fibers that are optically and
physically identical held by a connector or splice that squarely aligns them on their center axes.
However, in the real world, system loss due to fiber interconnection is a factor. Insertion loss is the
primary consideration for connector performance. There are three types of insertion loss: fiber-
related loss, connector-related loss, and system factors that contribute to loss. Because of the
discrepancy between insertion-loss testing and connector performance, users must understand the
test methods used to measure insertion loss. The best test results are obtained when lengths of fibers
are attached to the source and detector as permanent parts of the test setup. This avoids variations in
results that are caused by source and detector interconnection losses from test to test.
Measurement System Components
There are several components required to test interconnection losses.
1. Light Source: Light sources include lasers, LEDs, broadband sources, or monochromators.
A) Lasers and light-emitting diodes (LEDs) are widely used as sources. Important characteristics
include output power, speed, output pattern or numerical aperture (NA), spectral width, fiber-type
compatibility, ease of use, lifetime, and cost. Figure 1 illustrates various methods for interfacing a
light source to an optical fiber.
Figure 1 - Methods of Interfacing a Source to a Fiber
B) Broadband Source: Once popular but now seldom used,
typical incandescent sources include quartz, halogen, or
41
Part 2: Fiber Optic Components
xenon arc lamps with interference filters. If possible, the
filter should have a bandpass that approximates the output of
the source to be used in the proposed system to better
account for wavelength-dependent fiber characteristics such
as NA, attenuation, and dispersion.
C) Monochromator: This device isolates narrow portions of
light by dispersing light into its component wavelengths.
Most commercial monochromators exhibit very low energy on the
output side, and they select a very narrow bandwidth.
2. Mode Scramblers: Mode scramblers mix light to excite every
possible mode of transmission within the fiber. The easiest
to make is a 15-cm tube at least 7 cm in diameter filled with
1-mm lead shot through which the bare fiber passes. Another
type uses a row of one-eighth inch diameter brass pins
through which the fiber zigzags. The resulting bends in
either type cause mode coupling that fills the fiber. A more
complex scrambler is a butt-welded (fusion-spliced) length of
alternating graded-index, step-index, graded index fibers.
The step-index fiber generally has a length of one meter. The
discontinuities that result mix the light; however, butt-
welded scramblers are difficult to fabricate and are weak,
exhibiting less than 20% of the original fiber’s mechanical
strength.
Figure 2 - Mode Scramblers
3. Core Mode Filter: Mandrel wrap core
mode filters allow high-order mode
signals from the core to be removed.
High-order modes traveling through
several hundred meters of fiber leak into
the cladding and are lost. This results
Figure 3 - Mandrel Wrap Core
in an exit numerical aperture less than
Mode Filter
the material NA of the fiber. A fiber
that has reached modal equilibrium, along
with the reduced NA, is said to exhibit
long-launch conditions. Rather than
testing connector loss over several
42
Part 2: Fiber Optic Components
hundred meters of fiber, core mode
filters simulate this distance. The
standard recommended core mode filter for
smaller fibers is 12.5-mm diameter
mandrel around which the fiber is wrapped
five times under zero tensions. The
mandrel wrap reduces the exit NA to about
50% of the fiber’s material NA. The
mandrel wrap also reduces the light-
emitting area of the core of a graded-
index fiber by about 50%. This reduction
in the emitting area affects the
performance of a connector or a splice
during loss measurements.
4. Cladding Mode Stripper: The use of the mandrel wrap
described above scrambles the modes or strips the high-order
modes. This stripped light has nowhere to go except the
cladding. In short fiber runs or in setups where the mandrel
wrap occurs at the end of the fiber, this light deflected to
the cladding can be substantial. It is necessary to use to
remove these modes, a cladding mode stripper, which
incorporates a fiber, stripped of its cladding buffer, and
covered in Corona Dope (available from TV repair suppliers)
or some other liquid with a refractive index higher than the
cladding. Corona Dope has advantages: it is low-cost, it has
a high refractive index, and the coating is black. Figure 4
illustrates a cladding mode stripper.
Figure 4 - Cladding Mode Stripper
5. Detector System: Optical multimeters, also called optical
power meters, read optical power levels. The meter is
completely electronic with sensors that plug into the unit.
Different sensors are available for use at different power
levels and operating wavelengths. Adapters permit bare fibers
or a variety of popular connectors to be connected to a
sensor. A drawback of the multimeter is that in many
applications both ends of the fiber must be available. An
optical time-domain reflectometer allows testing when only
one end of the fiber is available. This device relies on the
43
Part 2: Fiber Optic Components
backscattering of light that occurs in an optical fiber for
detection.
Insertion Loss Test
Insertion loss tests will reduce the influence of fiber-
related losses. A general test should be reproducible and
provide applicable results. Most tests measure the output
power (P1) of a length of fiber. The fiber is then cut in the
middle and terminated with a connector or splice. The output
power (P2) is measured again. Insertion loss is given by: Loss
(dB) = 10 • log10 (P1/P2)
The length of fiber must be broken perfectly in the middle to produce an identical fiber on each side
of the splice. This method purposely eliminates fiber-induced losses in order to evaluate connector
performance independently of fiber-related variations. Three sets of launch conditions are of
interest.
1. Short-launch, short-receive: Represented by short fibers with no mandrel wrap on the
transmitting or receiving ends. Short-launch, short-receive conditions exhibit losses that increase
with the slightest mechanical offset of the connection. Lateral misalignment is a critical parameter
under these conditions.
2. Long-launch, short-receive: A mandrel wrap is on the transmitting end but not on the receiving
end. This condition reduces the exit numerical aperture of the transmitting fiber, and end-separation
losses are smaller. Since all of the receiving core can be used, greater separation of the fibers can be
tolerated.
3. Long-launch, long-receive: This condition is created by using a mandrel wrap at both the
transmit and receive end and shows greater sensitivity to lateral misalignment than the other two
conditions. Because the effective core area of both fibers is reduced, any offset increases loss more
significantly.
The insertion loss test assumed that two pieces of identical fiber were used. However, if two
different types of fibers are connected, then NA mismatch loss and diameter mismatch loss must be
accounted for.
NA Mismatch Loss Test
NA mismatch loss occurs when the numerical aperture of the
transmitting fiber (t) is larger than that of the receiving
fiber (r). NA mismatch loss is illustrated in Figure 5.
Figure 5 - NA-Mismatch
The calculated loss for numerical aperture mismatch is approximated by:
LossNA = 10 • log10(NAr/NAt)
Core/Cladding Diameter Mismatch Tests
As illustrated in Figure 6, core diameter mismatch occurs
when the core diameter of the transmitting fiber (t) is
larger than the core diameter of the fiber at the receiving
end (r). Cladding diameter mismatch is similar to core
44
Part 2: Fiber Optic Components
diameter mismatch loss except the cladding of the
transmitting fiber differs in diameter from the cladding of
the receiving fiber. Either mismatch prevents the cores from
aligning.
Figure 6 - Core-Diameter Mismatch
Both types of diameter mismatch loss are approximated by:
Lossdia = 10 • log10 (diar/diat)2
This equation is only accurate if all of the modes in the fiber are excited. When only low-order
modes are excited, the loss is greatly reduced and may not be present at all.;
Alignment Loss Tests
Concentricity occurs because the core may not be perfectly
centered in the cladding. Ellipticity or ovality describes
the fact that the core or cladding may be elliptical rather
than circular. The alignment of the two elliptical cores will
vary depending on how the fibers are brought together. These
forms of connector loss are illustrated in Figure 7.
Figure 7 - Concentricity and Ellipticity
Connector-related loss can also result from the mechanical
misalignment of the optical fiber cores. There are several
types of misalignment loss: lateral displacement, angular
misalignment, and end separation.
A connector should align the fibers on their center axes, but when one fiber’s axis does not coincide
with the other fiber’s axis, lateral displacement occurs. A displacement of only 10% of the core axis
diameter results in a loss of about 0.5 dB. The ends of mated fibers should be perpendicular to the
fibers’ axes and to each other. Failure to be perpendicular is called angular misalignment. Figure 8
illustrates angular misalignment, and Figure 9 illustrates lateral misalignment.
Figure 8 - Angular Misalignment
45
Part 2: Fiber Optic Components
Figure 9 - Lateral Misalignment
Fresnel Reflection Loss
Some connectors hold the two fibers slightly apart to prevent the fibers from rubbing against each
other and damaging their end polishes. Fresnel reflection loss or end separation loss is caused by the
difference in the refractive indices of the two fibers and the air that fills the gap between the two
fibers. Some connector manufacturers believe the use of index-matching gel in the gap reduces
Fresnel reflection loss, but others do not recommend using index-matching gel. This gap may
collect small flecks of abrasive contaminates that will damage the end finishes, and the addition of
index-matching gel could compound this contamination.
In a single-mode interconnection with a flat end finish, Fresnel reflection loss can be as much as -
11 dB, a level sufficient to disrupt the operation of most lasers. This loss can be reduced by
rounding the fiber end of one fiber during polishing (called a PC or physical contact finish). While
it would seem practical to use a flat finish and butt the ends, getting two perfectly smooth, flat
finishes is nearly impossible. With a rounded finish, fibers always touch on a high point near the
light-carrying fiber core.
Figure 10 - Fiber End Face Finishes
System Related Losses
System-related factors in connector loss involve the launch
and receive conditions. These conditions result from the mode
distribution in the fibers. The performance of the connector
depends on modal conditions and the connector’s position in
the system.
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Part 2: Fiber Optic Components
These launch conditions must be controlled in order to
provide repeatable measurements. Long-launch conditions are
generally preferred. Long-launch or receive conditions mean
that equilibrium mode distribution (EMD), illustrated in
Figure 11, exists in the fiber. The Electronic Industry
Association (EIA) recommends a 70/70 launch: 70% of the fiber
core diameter and 70% of the fiber NA should be filled. This
recommendation corresponds to the EMD in a graded-index
fiber. EMD can be reached three ways: by the optical
approach, filtering, or long fiber length. In general,
connector losses under long-launch conditions range from 0.4-
0.5 dB. Under short-launch conditions, losses are in the 1.3-
1.4 dB range.
Figure 11 - Equilibrium Mode Distribution
Fiber Optic Light Emitters & Detectors
Light-emitting Diode (LED)
Background
Light emitters are a key element in any fiber Figure 1 - LEDs Convert an Electrical Signal
optic system. This component converts the to Light
electrical signal into a corresponding light
signal that can be injected into the fiber. The
light emitter is an important element because
it is often the most costly element in the
system, and its characteristics often strongly
influence the final performance limits of a
given link.
LEDs are complex semiconductors that convert an electrical current into light. The
conversion process is fairly efficient in that it generates little heat compared to incandescent
lights. LEDs are of interest for fiber optics because of five inherent characteristics:
1. They are small.
2. They possess high radiance (i.e., They emit lots of light in a small area).
3. The emitting area is small, comparable to the dimensions of optical fibers.
4. They have a very long life, offering high reliability.
5. They can be modulated (turned off and on) at high speeds.
Table 1 offers a quick comparison of some of the characteristics for lasers and LEDs. These
characteristics are discussed in greater detail throughout this article and the article on laser
diodes.
Table 1 – Comparison of LEDs and Lasers
47
Part 2: Fiber Optic Components
Characteristics LEDs Lasers
Linearly proportional to drive
Output Power Proportional to current above the threshold
current
Drive Current: 50 to 100 mA
Current Threshold Current: 5 to 40 mA
Peak
Coupled Power Moderate High
Speed Slower Faster
Output Pattern Higher Lower
Bandwidth Moderate High
Wavelengths
0.66 to 1.65 µm 0.78 to 1.65 µm
Available
Spectral Width Wider (40-190 nm FWHM) Narrower (0.00001 nm to 10 nm FWHM)
Fiber Type Multimode Only SM, MM
Ease of Use Easier Harder
Lifetime Longer Long
Cost Low ($5-$300) High ($100-$10,000)
Light-emitting diodes use GaAlAs (gallium aluminum arsenide) for short-wavelength devices.
Long-wavelength devices generally incorporate InGaAsP (indium gallium arsenide phosphide).
Light Emitter Performance Characteristics
Several key characteristics of LEDs determine their usefulness in a given application. These
are:
Peak Wavelength: This is the wavelength at which the source emits the most power. It should
be matched to the wavelengths that are transmitted with the least attenuation through optical
fiber. The most common peak wavelength are 780, 850, and 1310 nm.
Spectral Width: Ideally, all the light emitted from an LED would be at the peak wavelength,
but in practice the light is emitted in a range of wavelengths centered at the peak wavelength.
This range is called the spectral width of the source.
Emission Pattern: The pattern of emitted light affects the amount of light that can be coupled
into the optical fiber. The size of the emitting region should be similar to the diameter of the
fiber core.
Power: The best results are usually achieved by coupling as much of a source’s power into the
fiber as possible. The key requirement is that the output power of the source be strong enough
to provide sufficient power to the detector at the receiving end, considering fiber attenuation,
coupling losses and other system constraints. In general, LEDs are less powerful than lasers.
Speed: A source should turn on and off fast enough to meet the bandwidth limits of the system.
The speed is given according to a source’s Rise or fall time, the time required to go from 10%
to 90% of peak power. LEDs have slower rise and fall times than lasers.
Linearity is another important characteristic for some applications. Linearity represents the
degree to which the optical output is directly proportional to the electrical current input. Most
48
Part 2: Fiber Optic Components
light sources give little or no attention to linearity, making them usable only for digital
applications. Analog applications require close attention to linearity. Nonlinearity in LEDs
causes harmonic distortion in the analog signal that is transmitted over an analog fiber optic
link.
LEDs are generally more reliable than lasers, but both sources will degrade over time. This
degradation can be caused by heat generated by the source and uneven current densities. In
addition, LEDs are easier to use than lasers.
LEDs are found in a wide variety of consumer electronics products. LEDs are used as visible
indicators in most electronics equipment, and laser diodes are most widely used in compact
disk (CD)players. The LEDs used in fiber optics differ from the more common indicator LEDs
in two ways:
1. The wavelength is generally in the near infrared (because the optical loss of fiber is lowest at
these wavelengths).
2. The LED emitting area is generally much smaller in order to allow the highest possible
modulation bandwidth and improve the coupling efficiency with small core optical fibers.
LEDs and laser diodes are very similar devices. In fact, when operating below their threshold
current, all laser diodes act as LEDs.
Figure 2 - Emitter Characteristics,
(a) LED,
(b) Laser
Figure 2a shows the behavior of an LED, and Figure 2b shows the behavior of a laser diode.
The plots show the relative amount of light output versus electrical drive current. The LED
outputs light that is approximately linear with the drive current. Nearly all LEDs exhibit a
“droop” in the curve as shown in Figure 2b. This nonlinearity in the LED limits its usefulness
in analog applications. The droop can be caused by a number of factors in the LED
semiconductor physics but is often largely due to self-heating of the LED chip. All LEDs drop
in efficiency as their operating temperature increases. Thus, as the LED is driven to higher
currents, the LED chip gets hotter causing a drop in conversion efficiency and the droop
apparent in Figure 2a. LEDs are typically operated at currents to about 100 mA peak. Only
specialized devices operate at higher current levels.
LED TYPES
There are two basic types of LED structures: edge emitters and surface emitters.
Figure 3 - LED Structures
Edge emitters are more complex and expensive devices, but offer high output power levels and
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high speed performance. The output power is high because the emitting spot is very small,
typically 30-50 µm, allowing good coupling efficiency to similarly sized optical fibers. Edge
emitters also have relatively narrow emission spectra. The full-width, half-maximum (FWHM)
is typically about 7% of the central wavelength. Another variant of the edge emitter is the
superradiant LED. These devices are a cross between a conventional LED and a laser. They
usually have a very high power density and possess some internal optical gain like a laser, but
the optical output is still incoherent, unlike a laser. Superradiant LEDs have very narrow
emission spectra, typically 1-2% of the central wavelength and offer power levels rivaling a
laser diode. These devices are popular for fiber optic gyroscope applications.
The second type of LED is the surface emitter. Surface emitters have a comparatively simple
structure, are relatively inexpensive, offer low-to-moderate output power levels, and are
capable of low-to-moderate operating speeds. The total LED chip optical output power is as
high or higher than the edge-emitting LED, but the emitting area is large, causing poor
coupling efficiency to the optical fiber. Adding to the coupling efficiency deficit is the fact that
surface-emitting LEDs are almost perfect Lambertian emitters. This means that they emit light
in all directions. Thus very little of the total light goes in the required direction for injection
into an optical fiber.
LED Drive Circuits
LED optical output is approximately proportional to drive current. Other factors, such as
temperature, also affect the optical output. Figure 4 shows in greater detail the typical behavior
of an LED. Two curves are shown. The top curve represents a 0.1% duty cycle with the peak
current as shown on the horizontal axis. The bottom curve shows the output with 100% duty
cycle. Note the light versus current curve droops below the linear curve.
Figure 4 - Optical Output vs. Current in a LED
LEDs are usually driven with either a digital signal or an analog signal.
Analog LED Driver Circuits
Figure 5 shows three configurations for analog LED drive circuits.
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Figure 5 - Analog LED Drive Circuits
Circuit 5a illustrates the simplest of the three configurations. It uses a transistor, Q1, and a
limited amount of resistors to convert an analog input voltage into a proportional current
flowing through the LED, D1. Also referred to as a transconductance amplifier, this
configuration converts a voltage into a current. In LEDs, the light output equates proportionally
to the drive current, not the drive voltage. While the drive current varies, this circuit illustrates
the voltage dropping across that LED and remaining constant. LEDs exhibit a peak drive current
at about 100 mA, and the voltage drop is typically 1.5 Volts.
Circuit 5a works as follows: the small resistor, R1, prevents oscillations in Q1. The input
voltage, VIN, appears on the base of Q1. VR2 is the voltage at the emitter of Q1, and it equals the
base voltage minus 0.6 Volts. Since these base and emitter voltages only differ by a DC offset
voltage, the AC portion of the base equals that of the emitter. The emitter voltage VR2 causes a
current equal to VR2/R2 to flow through R2. Due to the nature of transistors, the Q1 collector
current approximately equals the Q1 emitter current. (To be precise, the collector current equals
/( +1) times the emitter current. The transistor current gain, , is usually 10 to 100.)
Collectively, we find that the LED current, and thus the output light, relates to the input voltage
VIN as follows:
A drawback of the simple circuit is that the base capacitance varies with the base voltage, which
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introduces nonlinearities that limit the circuits linearity.
However, the linearized, low frequency circuit shown in Figure 5b eliminates most of the
nonlinearities associated with Q1. In this case, U1 forms a feedback loop that drives the base of
Q1 in such a way that assures that VR2 equals VIN. In this case, LED current, and thus the output
light, relates to the input voltage VIN as follows:
The circuit shown in Figure 5b still experiences some lesser nonlinearities associated with Q1,
but these do not represent the limiting factor. The circuit is limited by the delay associated with
the feedback signal in the servo loop formed by U1, allowing the circuit to only achieve a
bandwidth of about 10-100 MHz. This limitation makes the circuit in Figure 5b work well in
application transmitting DC coupled analog signals.
Figure 5c shows the highest performance analog LED drive circuit. In this case, resistor, R1
supplies the DC current through D1. Sometimes, a constant current source or a network that
includes temperature compensation replaces R11. A wideband RF amplifier, U1, serves two
purposes. First it amplifies VIN to allow the use of a small input signal. Second, it isolates the
LED from the input circuit, allowing precise impedance matching at the input, VIN, which
reduces reflections.
The output of U1 is usually 50 Ohms or 75 Ohms. A typical LED may have an input impedance
ranging from 5 Ohms to 10 Ohms. An impedance matching network is inserted between the
amplifier and D1. Furthermore, capacitor, C1, serves to block any DC level associated with the
output of the matching network. This circuit will drive LEDs to their highest possible
frequencies. Circuit 5c usually delivers the highest possible linearity. In this case, the LED, D1,
usually limits performance.
Digital LED Drive Circuits
When the drive signal is digital, as illustrated in Figure 6, there is no concern about LED
linearity. The LED is either on or off. There are special problems that need to be addressed
when designing an LED driver. The key concern is driving the LED so that the maximum speed
is achieved. Figures 6a, 6b, and 6c show three popular digital LED driver circuits. The first
circuit, shown in Figure 6a, is a simple series driver circuit. The input voltage is applied to the
base of transistor Q1 through resistor R1. The transistor will either be off or on. When transistor
Q1 is off, no current will flow through the LED, and no light will be emitted. When transistor
Q1 is on, the cathode (bottom) of the LED will be pulled low. Transistor Q1 will pull its
collector down to about 0.25 Volts. The current is equal to the voltage across resistor R2
divided by the resistance of R2. The voltage across R2 is equal to the power supply voltage less
the LED forward voltage drop and the saturation voltage of the drive transistor. The key
advantage of the series driver shown in Figure 6a is its low average power supply current. If one
defines the peak LED drive current as ILEDmax and assumes that the LED duty cycle is 50%, then
the average power supply current is only ILEDmax/2. Further, the power dissipated is
(ILEDmax/2)•VSUPPLY where VSUPPLY is the power supply voltage. The power dissipated by the
individual components, the LED, transistor and resistor R1, is equal to the voltage drop across
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each component multiplied by (ILEDmax/2).
The key disadvantage of the circuit shown in Figure 6a is low speed. This type of driver circuit
is rarely used at data rates above 30-50 Mb/s. In general, there are two ways to design an LED
drive circuit for low power dissipation. The first is to use a high-efficiency LED and reduce
ILEDmax to the lowest possible value. The second is to reduce the duty cycle of the LED to a low
value. Usually larger gains can be made with the second method.
Figure 6 - Digital LED Drive Circuits
The second LED driver circuit, shown in Figure 6b, offers much higher speed capability. It uses
transistor Q1 to quickly discharge the LED to turn it off. This circuit will drive the LED several
times faster than the series drive circuit shown in Figure 6a. The key advantage of the shunt
drive circuit is that it gives much better drive symmetry. LED’s are easy to turn on quickly, but
are difficult to turn off because of the relatively long carrier lifetime. In the shunt driver circuit
in Figure 6b, resistor R2 provides a positive current to turn on the LED. Typically, R2 would be
in the 40 Ohm range. This makes the turn-on current about 100 mA peak. Transistor Q1
provides the turnoff current. When saturated, transistor Q1 will have an impedance of a few
Ohms. This provides a much larger discharging current allowing the LED to turn off quickly.
The key disadvantage of the shunt driver is the power dissipation. It is typically more than
double that of the series driver. In fact, the circuit draws more current and power when the LED
is off than when the LED is on! The exact power dissipation can be computed by first analyzing
the off and on state currents and then combining the two values using information about the
operating duty cycle.
The last driver circuit, shown in Figure 6c, is a variation on the shunt driver shown in Figure
6b.Two additional resistors and two capacitors have been added to the basic circuit. The
purpose of these additional components is to further improve the operating speed. Capacitor C1
serves to improve the turn-on and turnoff characteristics of transistor Q1 itself. One has to be
careful that C1 is not made too large. If this occurs, the transistor base may be overdriven and
damaged. The additional components, resistors R3 and R4 and capacitor C2, provide overdrive
when the LED is turned on and underdrive when the transistor is turned off. The overdrive and
underdrive accelerates the LED transitions. Typically, the RC time constant of R3 and C2 is
made approximately equal to the rise or fall time of the LED itself when driven with a square
wave.
Figure 7 - LED Response to Digital Modulation
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Figure 7 shows the response of an LED to a digital modulation signal. The electrical signal
shown is the type generated by more sophisticated LED driver circuits such as that shown in
Figure 6c. Starting at time zero, we first see the digital signal go to a logic level 1. The most
remarkable part of this event is the strong overshoot seen on the electrical drive signal. This
overshoot may be two times the steady state logic 1 drive current. This overshoot accelerates the
turn-on time or rise time of the LED. Even so, we see that the optical output lags behind the
electrical signal. Typical values for very high-performance LED’s and driver circuits would be
0.7 ns rise time of the electrical signal and 1.5 ns optical rise time. Later, when the digital signal
goes back to a logic 0, we see the same process repeated. The electrical signal has a strong
undershoot component which acts to accelerate the turn-off of the LED. The undershoot serves
to reverse bias the LED, sweeping out the carriers. Even so, the turn-off time of most LED’s is
always slower than the turn-on time. Typical values for turn-off times are 0.7 ns for the
electrical signal and 2.5 ns for the optical signal. Note that while in a logic 0 state, the drive
current does not quite go to zero. It is common to provide a small amount of pre-bias current,
typically a few percent of the peak drive current, to keep the LED forward biased and improve
dynamic response.
All of these tricks together can increase the operating speed of the LED and driver circuit to
about 270 Mb/s. There have been numerous laboratory tests and prototype circuits that have
achieved rates to 500-1000 Mb/s, but none of these have ever made it into mass production.
Typically these levels of performance require a great deal of custom tweaking on each part to
achieve the high data rates.
ENERGY GAPS IN LEDs
When turned on, the LED will have a forward voltage drop of about 1.1 to 1.5 Volts. Shorter
wavelength diodes (e.g. 850 nm) have the largest voltage drops. As the wavelength increases,
the voltage drop decreases. This phenomenon can be related to the bandgap energy Eg of the
LED.
Equation 1 defines the bandgap energy Eg:
Where:
h = Plank's Constant = 4.13 x 10-15 eV•s
Eg -
c = speed of light = 2.998 x 108 m/s
in nm
Using equation 1, we can predict the energy gap of an LED based on its emission wavelength.
Table 2 - Common Light Emitter Materials & Characteristics
Material Formula Energy Gap Wavelength
Gallium Phosphide GaP 2.24 eV 550 nm
Aluminum Arsenide AIAs 2.09 eV 590 nm
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Gallium Arsenide GaAs 1.42 eV 870 nm
Indium Phosphide InP 1.33 eV 930 nm
Aluminum-Gallium
AIGaAs 1.42-1.61 eV 770-870 nm
Arsenide
Indium-Gallium-
InGaAsP 0.74-1.13 eV 1100-1670 nm
Arsenide-Phosphide
Table 2 lists some common light emitter materials, the emission wavelength and
corresponding energy gap. The first materials, GaP and AlAs, are used to make emitters in
the visible portions of the spectrum. The next three materials, GaAs, InP, and AlGaAs, are
used to make emitters in the near infrared portion spectrum generally referred to as the
“first window” in optical fiber. The last material, InGaAsP is used to make emitters in the
infrared portion spectrum referred to as the “second and third windows” in optical fibers.
The energy gap corresponds to the energy of the emitted photons and also is indicative of
the voltage drop associated with a forward biased LED. Knowing the voltage drop of the
LED and the saturation voltage of the transistor we can compute the LED current. Equation
2 below shows the general form of the calculation.
Where:
VPOWER = DC power supply voltage.
VLED = forward voltage drop of the LED.
ILED= VPower-VLED-VSAT/R3
VSAT = drive transistor saturation voltage
R3 = series LED current limiting resistor
ILED = peak LED current
Another common use of LEDs is to simply use their large forward voltage drop in some part
of a circuit. In this case, the fact that the LED emits light is incidental. For instance, if one
needed a 2.3 Volt drop in a circuit, then one could use three 1N4148 diodes in series or a
single green LED. Obviously, only inexpensive indicator LEDs are candidates for this
application. One important consideration for this usage is that all light emitters will also
function as detectors. If the LED is in a sensitive portion of the circuit, then the circuit may
become sensitive to ambient light conditions. It may be necessary to shield the LED or coat
it with an opaque paint. It is also useful to note that many ordinary glass diodes, such as the
1N4148, also function as light detectors. Keep this in mind when using diodes in circuits
that have high gains. One possibility pursued in the past was using ultra-low cost
germanium diodes as long wavelength detectors. They in fact work very well, but are
somewhat inconsistent from part to part.
Laser Diode
Background
Light emitters are a key element in any
fiber optic system. This component Figure 1 - Laser Diodes Convert an Electrical
converts the electrical signal into a Signal to Light
corresponding light signal that can be
injected into the fiber. The light emitter
is an important element because it is
often the most costly element in the
system, and its characteristics often
strongly influence the final
performance limits of a given link.
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Part 2: Fiber Optic Components
Laser Diodes are complex semiconductors that convert an electrical current into light. The
conversion process is fairly efficient in that it generates little heat compared to incandescent
lights. Five inherent properties make lasers attractive for use in fiber optics.
1. They are small.
2. They possess high radiance (i.e., They emit lots of light in a small area).
3. The emitting area is small, comparable to the dimensions of optical fibers.
4. They have a very long life, offering high reliability.
5. They can be modulated (turned off and on) at high speeds.
Table 1 offers a quick comparison of some of the characteristics for lasers and LEDs. These
characteristics are discussed in greater detail throughout this article and in the article on light-
emitting diodes.
Table 1 - Comparison of LEDs and Lasers
Characteristic LEDs Lasers
Linearly proportional to Proportional to current above the
Output Power
drive current threshold
Drive Current: 50 to 100
Current Threshold Current: 5 to 40 mA
mA Peak
Coupled Power Moderate High
Speed Slower Faster
Output Pattern Higher Lower
Bandwidth Moderate High
Wavelengths
0.66 to 1.65 µm 0.78 to 1.65 µm
Available
Narrower (0.00001 nm to 10 nm
Spectral Width Wider (40-190 nm FWHM)
FWHM)
Fiber Type Multimode Only SM, MM
Ease of Use Easier Harder
Lifetime Longer Long
Cost Low ($5-$300) High ($100-$10,000)
Laser diodes are typically constructed of GaAlAs (gallium aluminum arsenide) for short-
wavelength devices. Long-wavelength devices generally incorporate InGaAsP (indium gallium
arsenide phosphide).
Laser Diode Performance Characteristics
Several key characteristics lasers determine their usefulness in a given application. These are:
Peak Wavelength: This is the wavelength at which the source emits the most power. It should
be matched to the wavelengths that are transmitted with the least attenuation through optical
fiber. The most common peak wavelengths are 1310, 1550, and 1625 nm.
Spectral Width: Ideally, all the light emitted from a laser would be at the peak wavelength, but
in practice the light is emitted in a range of wavelengths centered at the peak wavelength. This
range is called the spectral width of the source.
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Emission Pattern: The pattern of emitted light affects the amount of light that can be coupled
into the optical fiber. The size of the emitting region should be similar to the diameter of the
fiber core. Figure 2 illustrates the emission pattern of a laser.
Power: The best results are usually achieved by coupling as much of a source's power into the
fiber as possible. The key requirement is that the output power of the source be strong enough
to provide sufficient power to the detector at the receiving end, considering fiber attenuation,
coupling losses and other system constraints. In general, lasers are more powerful than LEDs.
Figure 2 - Laser Emission Pattern
Speed: A source should turn on and off fast enough to meet the bandwidth limits of the system.
The speed is given according to a source's rise or fall time, the time required to go from 10%
to 90% of peak power. Lasers have faster rise and fall times than LEDs.
Linearity is another important characteristic to light sources for some applications. Linearity
represents the degree to which the optical output is directly proportional to the electrical
current input. Most light sources give little or no attention to linearity, making them usable
only for digital applications. Analog applications require close attention to linearity.
Nonlinearity in lasers causes harmonic distortion in the analog signal that is transmitted over
an analog fiber optic link.
Lasers are temperature sensitive; the lasing threshold will change with the temperature. Figure
3 shows the typical behavior of a laser diode. As operating temperature changes, several
effects can occur. First, the threshold current changes. The threshold current is always lower at
lower temperatures and vice versa. The second change that can be important is the slope
efficiency. The slope efficiency is the number of milliwatts or microwatts of light output per
milliampere of increased drive current above threshold. Most lasers show a drop in slope
efficiency as temperature increases. Thus, lasers require a method of stabilizing the threshold
to achieve maximum performance. Often, a photodiode is used to monitor the light output on
the rear facet of the laser. The current from the photodiode changes with variations in light
output and provides feedback to adjust the laser drive current.
Figure 3 - Temperature Effects on Laser Optical Output Power
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Figure 4 - Emitters Characteristics
a) LED
b) Laser
Figure 4a shows the behavior of an LED, and Figure 4b shows the behavior of a laser diode.
The plots show the relative amount of light output versus electrical drive current. The LED
outputs light that is approximately linear with the drive current. Nearly all LED's exhibit a
"droop" in the curve as shown in Figure 4b. This nonlinearity in the LED limits its usefulness
in analog applications. The droop can be caused by a number of factors in the LED
semiconductor physics but is often largely due to self-heating of the LED chip. All LED's drop
in efficiency as their operating temperature increases. Thus, as the LED is driven to higher
currents, the LED chip gets hotter causing a drop in conversion efficiency and the droop
apparent in Figure 4a. LED's are typically operated at currents to about 100 mA peak. Only
specialized devices operate at higher current levels.
LASER TYPES
There are two basic types of laser diode structures: Fabry-Perot (FP) and distributed feedback
(DFB). Of the two types of lasers, Fabry-Perot lasers are the most economical, but they are
generally noisy, slower devices. DFB lasers are quieter devices (e.g., high signal-to-noise),
have narrower spectral widths, and are usually faster devices.
DFB lasers offer the highest performance levels and also the highest cost of the two types.
They are nearly monochromatic (i.e. they emit a very pure single color of light.) while FP
lasers emit light at a number of discrete wavelengths. DFB lasers tend to be used for the
highest speed digital applications and for most analog applications because of their faster
speed, lower noise, and superior linearity. Fabry-Perot lasers further break down into buried
hetero (BH) and multi-quantum well (MQW) types. BH and related styles ruled for many
years, but now MQW types are becoming very widespread. MQW lasers offer significant
advantages over all former types of Fabry-Perot lasers. They offer lower threshold current,
higher slope efficiency, lower noise, better linearity, and much greater stability over
temperature. As a bonus, the performance margins of MQW lasers are so great, laser
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manufacturers get better yields, so laser cost is reduced. One disadvantage of MQW lasers is
their tendency to be more susceptible to backreflections. See article "Laser Backreflection -
The Bane of Good Performance" for more information.
Figure 5 - Laser Construction
VCSELs are a new laser structure that emits laser light vertically from its surface and has
vertical laser cavity. Figure 6 illustrates the structure of a VCSEL.
Figure 6 - Basic VCSEL Structure
The VCSEL's principles of operation closely resembles those of conventional edge-emitting
semiconductor lasers. The heart of the VCSEL is an electrically pumped gain region, also
called the active region, emits light. Layers of varying semiconductor materials above and
below the gain region create mirrors. Each mirror reflects a narrow range of wavelengths back
into the cavity causing light emission at a single wavelength.
VCSELs are typically multi-quantum well (MQW) devices with lasing occurring in layers
only 20-30 atoms thick. Bragg-reflectors with as many as 120 mirror layers form the laser
reflectors.
There are many advantages to VCSELs. Their small size and high efficiency mirrors produce a
low threshold current, below 1 mA. The transfer function allows stability over a wide
temperature range, a feature that is unique to this type of laser diode. These features make the
VCSEL ideal for applications that require an array of devices.
BACKREFLECTIONS
Actually, all lasers are susceptible to backreflections. Backreflections disturb the standing-
wave oscillation in the laser cavity, and the net effect is an increase in the effective noise floor
of the laser. A strong backreflection can cause some lasers to become wildly unstable and
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Part 2: Fiber Optic Components
completely unusable in some applications. It can also generate nonlinearities, called kinks, in
the laser response. Most analog applications and some digital ones cannot tolerate these
degradations.
The importance of controlling backreflection depends on the type of information being sent
and the particular laser. Some lasers are very susceptible to backreflections due to the design
of the laser chip itself. Most often the determining factor is how tightly the fiber is coupled to
the laser chip. A low-power laser generally has weak coupling to the fiber. Perhaps only 5-
10% of the laser power is coupled into the fiber. This means that only 5-10% of the
backreflection would be coupled into the laser cavity, making the laser relatively immune to
backreflections. On the other hand, a high-power laser may have 50-70% of the laser chip
output coupled to the fiber. This also means that 50-70% of the backreflection will be coupled
back into the laser cavity. This makes high-power lasers more susceptible to backreflections.
LASER DRIVE CIRCUITS
Analog Laser Drive Circuits
Figure 7 illustrates two common circuit configurations used to drive lasers for analog
applications. The simpler of the two, shown in figure 7a, offers moderate linearity and good
performance in frequencies up to 500 MHz. The analog signal path only involves C1, R1, Q1,
R2, and D1, the laser diode. Q1 acts as a transconductant stage in which voltage flows in and
current flows out. C1 passes only the AC portion of the analog input signal. R1, usually only a
few tens of Ohms, squelches any possible oscillations in Q1. The AC portion of analog input
voltage VIN appears at the base of Q1 and also at the emitter of Q1. VIN, the AC voltage at the
emitter of Q1, imposes across R2 to create a modulation current VIN/R2. U1 supplies DC
current to the laser through R3 and R1. U1 creates a servo loop that maintains a constant
photodiode current through the rear facet monitor PIN diode.
Figure 7 - Analog Laser Drive Circuits
The circuit illustrated in Figure 7a indirectly maintains constant laser optical output. The rear
facet monitor PIN diode receives light from one end of the laser chip while the other end of the
chip illuminates the optical fiber. While the light in the fiber correlates to light in the monitor
PIN diode, it never matches exactly at all output and environmental conditions, an
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Part 2: Fiber Optic Components
phenomenon called tracking error.
Figure 7b shows a more advanced analog laser circuit, offering good to excellent linearity at
very high frequencies (GHz). The signal path of this circuit only involves U2, Z1, C1, and the
laser diode, D1. Amplifier U2 provides input matching, gain and isolates the laser from outside
conditions. The block labeled Z1 can take on many functions. At a minimum, it interfaces the
output of the amplifier U2, usually 50 or 75Ohms, to the laser that has an impedance ranging
from 5 Ohms to 25 Ohms. As shown, sometimes the laser package incorporates this
impedance matching.
Digital Laser Drive Circuits
Figure 8 illustrates two common discrete component circuit configurations that function to
drive lasers for digital applications. However, a wide variety of highly integrated ICs exist
because of the high demand for digital laser drivers. The discrete component circuit
configurations illustrate the most commonly used principles in commercially available laser
driver ICs.
Figure 8 - Digital Laser Circuits
Figure 8a illustrates a simple circuit that is utilized at frequencies to several hundred
megahertz. "Digital data in" takes a relatively simple path. The NAND gate, U2, buffers the
signal and provides fast and consistent edges. Potentiometer, R3, adjusts the amplitude of the
laser's oncoming digital signal, usually referred to as a modulation depth adjustment.
Capacitor, C2, block any DC component, allowing the AC component of the "digital data in"
to pass. Incidentally, nearly all digital laser drive circuits cannot handle a DC component in the
"digital data in" signal, meaning that the "digital data in" signal must always have transitions
present. Resistor, R5, provides impedance matching into the laser, and feeds directly into the
cathode of the laser, D1. Inductor, L1, allows the AC component of the "digital data in" signal
to reach the laser, as well as a DC signal. The rear facet monitor photodiode, D2, outputs a
current proportional to the laser output. The current out of D2 goes to a servo loop, ensuring
that the average optical output of D1 remains constant. U1 forms the heart of the servo loop.
Capacitor, C1, configures U1 as an integrator. The +input of U1 remains at a positive voltage,
VREF. The value of VREF usually lies midway between ground and +Power.
Potentiometer, R4, adjusts the average optical output power of the laser D1 by sinking a
current out of the -input of U1. This negative current causes the output of U1, referred to as
V2, to increase. As V2 increases, transistor Q1 turns on. This causes an increasing current to
flow through both L1 and D1. As the current through D1 increases, the average optical output
of D1 increases, which causes the current from D2, the rear facet monitor photodiode, to
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Part 2: Fiber Optic Components
increase. This continues until the current out of D2 matches the current being sinked by
potentiometer, R4. R4, usually referred to as the "power adjust" in digital laser drive circuits,
sets the rear facet monitor photodiode current. The average optical output power and the rear
facet monitor photodiode current are nearly equal, differing only by tracking error. Three
components in the circuit, C2, L1, and C1, function to limit the low-frequency, and thus
limiting low data rate operations. Normally, a digital laser driver circuit should handle
frequencies as low as 1/100th of the design data rate. Therefore, a laser driver designed to
handle a 622 Mb/s data rate must also handle frequencies as low as 6.22 MHz.
The more complex circuit shown in Figure 8b allows very high, multi-gigabit speeds. With
only the omission of L1, the servo loop portion of the circuit matches the circuit in Figure 8a.
L1 is replaced in this circuit by Q4 a very fast , low capacitance transistor. To not interfere
with the modulation signal, Q4's collector will appear as a current source. Potentiometer, R4,
sets the rear facet monitor photodiode current or average optical output power. The "digital
data in" signal first goes through the NAND gate, U2, as in the first circuit. However, this
circuit incorporates a NAND gate with the differential outputs of U2 to drive a transistor-based
differential amplifier consisting of Q1 and Q2. Transistor Q3 forms a constant current source.
The potentiometer, R3, sets the current flowing in the collector of Q3. The current flowing out
of Q3 determines the amount of modulation current that is switched to the laser in response to
1's and 0's. The modulation current from the collector of Q3 oscillates between the +power line
(by Q1) and the laser, D1, (by Q2), as the outputs of U2 switch back and forth. To avoid a
circuit becoming slow, the digital laser circuit must avoid saturation. Q1, Q2 and Q3 all
operate in a linear mode in circuit 8b allowing them to operate at very high speeds.
PACKAGING CHARACTERISTICS
We have touched on the electrical and optical characteristics of laser diodes. Other factors that
are important are the thermal and packaging characteristics. Laser diodes are available
pigtailed to fiber or mounted in active device mounts (ADMs). Lasers with fiber pigtails
require special handling precautions to prevent damage to the fiber. See Handling Fragile
Optical Fibers and Fiber Pigtail Assemblies for more information.
Lasers are very sensitive to backreflection, limiting their usefulness in the ADM add/drop
multiplexer configurations. Some recent lasers mounted in ADMs incorporated a short length
of single-mode fiber that provides the interface to the fiber optic connector. This technique not
only enhances the launch stability, it also improves the backreflection problem.
Laser Backreflection - The Bane of Good Performance
Introduction
All lasers are susceptible to backreflections. Backreflections disturb the standing-wave
oscillation in the laser cavity, increasing the effective noise floor of the laser. A strong
backreflection causes certain lasers to become wildly unstable and completely unusable in
some applications. Backreflection can also generate nonlinearities in the laser response
which are often described as kinks. Most analog applications and some digital applications
cannot tolerate these degradations.
Explanation of Backreflections
The general knowledge has been that backreflections hurt the performance of a link because
the reflected light gets into the laser cavity, disturbs the standing optical wave, and creates
noise. Some lasers are very susceptible to backreflection due to the design of the laser chip
itself. Most often the determining factor is how tightly the fiber is coupled to the laser chip.
In a low power laser potentially only 5-10% of the laser power is coupled into the fiber.
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This means that only 5-10% of the backreflection would be coupled to the laser chip,
making the laser relatively immune to backreflections. On the other hand, a high-power
laser may have 50-70% of the laser chip output coupled to the fiber. This means that 50-
70% of the backreflection is coupled to the laser cavity.
Optical Isolators
One strategy to reduce backreflections places an opitcal isolator at the laser output. In some
cases, lasers incorporate dual isolators offering 50 dB or more reduction in backreflections
reaching the laser. One would think that a double isolated laser would not be bothered by
backreflections, but this is not the case. The noise that is generated by backreflections
reaching the laser is only one possible source, in many cases a minor source of noise
because of the widespread use of optical isolators.
The Faraday rotator shown in Figure 1 and based on the Faraday effect, is one example of
an optical isolator.
Figure 1 - Isolator Based on Faraday Rotator
Before entering the Faraday rotator, which is usually an yttrium-iron-garnet (YIG) material,
the light beam passes through a polarizer which is oriented parallel to the incoming state of
polarization. The Faraday rotator then rotates the polarization by 45°. At the output, the
beam passes an analyzer which is oriented at an angle of 45° relative to the first polarizer.
Of all possible reflected beams, only those with a 45° orientation of the polarization are
allowed to pass backwards. The polarization of the reflected beam is rotated by another 45°
which results in a total rotation of 90°. This way, the reflected beam is blocked by the
polarizer, reducing backreflections by 20 to 45 dB. In order not to disturb the proper
function of the isolator, all of its surfaces should be antireflection-coated.
A more significant source of noise in a modern fiber optic system can be Interfereometric
Intensity Noise (IIN). This noise is generated by Fabry-Perot cavities created between
multiple reflecting elements in the fiber plant, usually fiber optic connectors or splices.
Fabry-Perot lasers break down into buried hetero and multi-quantum well (MQW) types.
BH and related styles ruled for may years, but now MQW types dominate. MQW lasers
offer significant advantages over all former types of Fabry-Perot lasers. They offer lower
threshold current, higher slope efficiency, lower noise, better linearity, and much greater
stability over temperature. As a bonus, the performance margins of MQW lasers are so
great, laser manufacturers get better yields, so laser cost is reduced. One disadvantage of
MQW lasers is their tendency to be more susceptible to backreflections. MQW lasers also
perform poorly as detectors.
The isolator design in Figure 2 works with polarized light. In newer designs, the input and
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output polarizers are replaced with birefringent crystals which eliminate the sensitivity to
the polarization of the light.
Figure 2 - Generation of AM Noise by a Fabry-Perot Cavity
The amount of rejection offered by an optical isolator will improve problems caused by
backreflections but often will not eliminate them. The effects of these backreflections can
disrupt a fiber optic transmission system. Figure 3 shows a laser waveform with no
backreflection, and Figure 5 shows a laser waveform with a strong backreflection.
Figure 3 - Laser Optical Output With No Backreflection
First the waveform in Figure 3 needs some explanation. As seen in the figure, the rising
edge is followed by a damped oscillation. This overshoot and the subsequent oscillation is
called the relaxation oscillation. Most lasers exhibit this phenomenon. It can be understood
by looking at the frequency versus amplitude response of the laser as illustrated in Figure 4.
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Figure 4 - Frequency versus Amplitude Response of a Laser
The laser exhibits a resonance (high gain point) near 4.4 GHz in this case. This will be the
approximate frequency of the relaxation oscillation. The frequency of the resonance peak is
a factor that limits the maximum data rate a given laser can transmit. When the maximum
frequency component of the data stream gets close to the laser resonance frequency,
performance degrades quickly. The frequency of the resonance and the magnitude of the
overshoot depend on the drive levels applied to the laser. Overshoot is generally most
severe when the laser is turned completely off and then back on. This condition is avoided
in most practical data links by keeping the laser always above the threshold.
The backreflection illustrated in Figure 5 shows the same characteristics as the initial
relaxation oscillation. The time, T1, can be precisely measured to determine the distance to
the reflecting point. Often, the frequency has to be greatly reduced to observe such
reflections. One fallacy is that only close reflections matter. Closer reflections are often a bit
stronger, but at 1310 nm or 1550 nm, fiber loss is so low that reflections from a distance of
several kilometers can be significant.
Figure 5 - Laser Optical Output With Backreflection
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Unfortunately, optical isolators do not substitute for properly polished, low-backreflection
connectors. The technology for modern splices has advanced making fiber optic connectors
the main source of IIN. In order to address the backreflection problem, the fiber optic
industry first introduced PC (Physical Contact) polished connectors and later Angled
Physical Contact. The APC connectors especially go a long way towards eliminating any
concern of backreflections from the fiber optic connectors. (IIN is even generated by the
fiber itself, but that is a minor effect and beyond the scope of this discussion.) The basis for
IIN, Rayleigh scattering, is the basis for a popular piece of fiber optic test equipment, the
Optical Time Domain Reflectometer(OTDR).
Conclusion
Backreflections can be observed by monitoring the photodiode servo-loop for disturbances.
To do this, place the end of the laser pigtail in glycerin, which will eliminate virtually all
backreflections. Then note the output of the servo loop at that time. Afterwards, connect the
laser pigtail to the system. Any significant perturbations noted are backreflections of
sufficient amplitude to disturb the standing wave in the laser cavity. This directly observes
laser backreflections. A less direct test for laser backreflections involves testing at
frequencies where backreflections will occur at an exact multiple of the bit time. Basically,
this procedure calculates the round trip transit time to the potential reflection interfaces in
the system. It is generally easiest to measure the spacing between the high interference
points when using this method. Often, the laser pigtail is made very short so that the first
reflection occurs at a frequency higher than any frequency being transmitted by the system.
However, longer fiber segments in the system will yield a low fundamental interference
frequency and harmonics that will clutter the spectrum. Designing laser-based systems for
low backreflections remains the only practical strategy.
See Also: “Multichannel CATV Systems and Backreflection.”
Fiber Optic Detectors
Introduction
Detectors perform the opposite function of light emitters. They convert optical signals back
into electrical impulses that are used by the receiving end of the fiber optic data, video, or
audio link. The most common detector is the semiconductor photodiode, which produces
current in response to incident light. Detectors operate based on the principle of the p-n
junction. An incident photon striking the diode gives an electron in the valence band
sufficient energy to move to the conduction band, creating a free electron and a hole. If the
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creation of these carriers occurs in a depleted region, the carriers will quickly separate and
create a current. As they reach the edge of the depleted area, the electrical forces diminish
and current ceases. While the p-n diodes are insufficient detectors for fiber optic systems,
both PIN photodiodes and avalanche photodiode (APDs) are designed to compensate for the
drawbacks of the p-n diode.
Important Detector Parameters
Responsivity: Ratio of current output to
light input. High responsivity equals high
receiver sensitivity.
Quantum Effeiciency: Ratio of primary Figure 1– C-V Curve
electron-hole pairs created by incident
photons to the photons incident on the
detector material.
Capacitance: Dependent upon the active
area of the device and the reverse voltage
across the device. This relationship is
illustrated in Figure 1.
Response Time: Time needed for the
photodiode to respond to optical inputs
and produce and external current.
Response time can be affected by dark
current, noise, linearity, backreflection,
and edge effect (see Figure 2). Edge effect
results from the fact that detectors only
Figure 2 – Edge Effect
provide fast response in their center region.
The outer region of the detector has a
higher responsivity than the center region,
which can cause problems when aligning
the fiber to the detector. The higher
responsivity may fool one into thinking
they have aligned the fiber to the center
region. Because response is much slower
at the edge, this misalignment will reduce
the response time of the detector.
PIN Photodiode
A p-n diode’s deficiencies are related to the fact
that the depletion area (active detection area) is
small; many electron-hole pairs recombine before
they can create a current in the external circuit. In
the PIN photodiode, the depleted region is made as
Figure 3 – PIN Photodiode
large as possible. A lightly doped intrinsic layer
separates the more heavily doped p-types and n-
types. The diode’s name comes from the layering of
these materials positive, intrinsic, negative — PIN.
Figure 3 shows the cross-section and operation of a
PIN photodiode.
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Avalanche Photodiode (APD)
The avalanche photodiode (APD) operates as
the primary carriers, the free electrons and
holes created by absorbed photons, accelerate,
Figure 4 – APD
gaining several electron Volts of kinetic energy.
A collision of these fast carriers with neutral
atoms causes the accelerated carriers to use
some of their own energy to help the bound
electrons break out of the valence shell. Free
electron-hole pairs, called secondary carriers,
appear. Collision ionization is the name for the
process that creates these secondary carriers. As
primary carriers create secondary carriers, the
secondary carriers themselves accelerate and
create new carriers. Collectively, this process is
known as photomultiplication. Typical
multiplication ranges in the tens and hundreds.
For example, a multiplication factor of eighty
means that, on average, eighty external
electrons flow for every photon of light
absorbed.
APDs require high-voltage power supplies for their operation. The voltage can range from
30 or 70 Volts for InGaAs APDs to over 300 Volts for Si APDs. This adds circuit
complexity. Also, APDs are very temperature sensitive, further complicating circuit
requirements. In general, APDs are only useful for digital systems because they possess
very poor linearity. Because of the added circuit complexity and the high voltages that the
parts are subjected to, APDs are always less reliable than PIN detectors. This, added to the
fact that at lower data rates, PIN detector-based receivers can almost match the performance
of APD-based receivers, makes PIN detectors the first choice for most deployed low-speed
systems. At multigigabit data rates, however, APDs rule supreme.
Table 1 – Comparison of PIN Photodiodes and APDs
Parameter PIN Photodiodes APDs
Construction Materials Si, Ge, InGaAs Si, Ge, InGaAs
Bandwidth DC to 40+ GHz DC to 40+ GHz
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Wavelength 0.6 to 1.8 µm 0.6 to 1.8 µm
Conversion Efficiency 0.5 to 1.0 Amps/Watt 0.5 to 100 Amps/Watt
High Voltage, Temperature
Support Circuitry Required None
Stabilization
Cost (Fiber Ready) $1 to $500 $100 to $2,000
Light Emitters As Detectors
Light emitter such as LEDs and lasers, will also function as light detectors, allowing a
unique technology to evolve, using light emitters as half-duplex fiber optic communication
devices. This scheme involves using the LED or laser alternately as a light emitter, then as a
light detector, which allows the transmission of information in either direction over the
fiber. While all LEDs and lasers have the ability to act as detectors, a few perform this task
much better than most. The key parameter to look for is very efficient coupling between the
light emitter and the fiber. This allows good performance in both modes. It is also important
that the LEDs have consistent spectral characteristics. While a good InGaAs detector may
have a responsivity of 0.8 A/W at a wavelength of 1310 nm, an LED operating as a detector
may provide a responsivity of 0.08 A/W at 1310 nm.
The main reason for the much lower response is the fact that the LED operating as a
detector has a relatively narrow spectral response spectrum that does not fully overlap with
the LED emission spectrum. Figure 5 shows the spectral response of a typical InGaAs
detector as well as the emission spectrum of an InGaAsP LED and the LEDs spectral
response as a detector. It can be seen that a normal InGaAs detector has a very broad
spectral response from 800 nm to beyond 1600 nm. Because the response is so wide, the
detector responds to all photons emitted by the LED. The spectral emission of the LED is a
relatively narrow spectrum, perhaps 60 nm wide, centered around 1310 nm. Notice that the
spectral response of the same LED operating as a detector is shifted to the left. The center of
the spectral response is centered at perhaps 1270 nm. The overall response as a detector is a
bit wider than the emissions as an LED. However, note that the overlap between the LED
emissions and the LED spectral response is rather low. This accounts for poor responsivity
attributed to most LEDs operating as detectors. The problem becomes even worse when the
emitting LED and the detecting LED are at different operating temperatures. This causes the
individual spectral responses to drift with respect to each other. This will either increase or
decrease the amount of overlap. The overwhelming concern when applying full-duplex
LEDs is considering the different temperatures that the two ends will see. Laser diodes
exhibit similar characteristics to the LED shown in Figure 5.
Figure 5 - Ping-Pong (Full-Duplex) LED
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Transmitters, Receivers, & Transceivers
Troubleshooting Transmitters & Receivers
Background
Most fiber optic equipment is designed to be as simple as the application will allow.
However, due to the complexity of some of the applications, things go wrong. In many
cases, resolving a troubleshooting challenge can be as simple as properly cleaning the
optical connectors. See article “Fiber Optic Connectors” for more information on cleaning
optical connectors. In more complex scenarios, additional troubleshooting will be required.
This article describes some of the more common problems that have been encountered.
Problems and Comments
1. PROBLEM: No optical power out of the transmitter or transceiver.
A) Check the transmitter or transceiver power connection. If there is less than the specified
supply voltage between power pins, a higher current power supply may be required. Be sure
the power supply polarity is correct.
B) Be sure that data input is present. Many data links put out no light if a logic “0” is input.
Be sure that the input data is alternating between 0’s and 1’s, otherwise no output light may
be present.
2. PROBLEM: No optical power out of the fiber at the optical input port.
A) Check power at optical output port of the transmitter or transceiver. If optical power is
present at optical output port, ensure that the proper fiber is connected at optical input port.
Verify the integrity of the fiber.
3. PROBLEM: Receiver output electrical signal is noisy or intermittent.
A) Check that optical loss does not exceed the rated value between transceivers or between
the transmitter and receiver. If the loss is too high, reduce optical loss, or insert a repeater
between transceivers or the transmitter and receiver. High loss may be caused by bad
connectors, improperly seated connectors, or bad splices. See “ Fiber Optic Connectors” for
information on the proper use of connectors.
B) Check the wavelength of the transceivers or transmitter and receiver. Detectors are
typically optimized for one wavelength. Mixing 850 nm units and 1310 nm units, for
instance, may result in poor or no performance.
C) Be sure that the transmitter and receiver enclosures are grounded. It should be noted that
850 nm FM video links are generally bandwidth-limited at distances over 1.5 km. When this
occurs, the receiver output will not be usable even when sufficient optical power is received.
4. PROBLEM: No signal out of the receiver.
A) Verify signal input at transmitter. Be sure that an electrical input signal is present at the
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transmitter input. Also verify that the signal has proper amplitude, frequency, and
impedance. For instance, a video signal must be 1.0 Volt peak-to-peak. Several high-speed
data links can be configured for positive or negative ECL levels. Be sure that the power
supply voltages and connections are correct.
B) Check receiver power connection. If there is less than the specified voltage between the
power supply pins, a higher current power supply may be required. Be sure that the power
supply polarity is correct.
5. PROBLEM: Signal amplitude out of the fiber optic receiver is too large.
A) Verify that the receiver output is terminated into the proper impedance. Many data links
and most audio and video links require that a terminating resistor be added to the receiver
output. If this resistor is omitted, the amplitude will be two times too large. If the value is
incorrect, the receiver output level may be too large or too small depending on the value of
the resistor. Video links typically require a 75 Ohm terminating resistor. Audio links
typically require a 600 Ohm or 10 kOhm terminating resistor. Low-speed data links such as
RS-422 and RS-485 typically require a 120 Ohm terminating resistor and high-speed ECL
data links typically require a 50 Ohm terminating resistor.
6. PROBLEM: Signal out of the receiver is distorted.
A) Verify the input signal at the transmitter. Must be 1.0 Volt peak-to-peak or less. A larger
signal will cause distortion.
B) Verify the fiber size. See "Determining Fiber Size” details. Fibers with larger core sizes
may overload the receiver. Verify that receiver power is within specifications.
7. PROBLEM: Data errors occur.
A) Be sure that the power supply voltage is correct and clean for both the transmitter and
receiver or transceivers.
B) Be sure that the enclosures are properly grounded, especially when using a wall-mount
power supply.
C) Be sure that the data inputs and outputs are properly terminated.
D) Be sure that the input data levels are correct.
E) Be sure that the optical input level to the receiver is within valid limits.
8. PROBLEM: Signal out of diplexer/demultiplexer is noisy.
A) Check the copper or fiber optic link between the diplexer mux/demux pair. Ensure that
the losses in the optical path do not exceed the loss budget of the transmitter/receiver pair
used.
9. PROBLEM: Audio signal amplitude out of diplexer demultiplexer is too large or
distorted.
A) Verify the signal input at the multiplexer.
B) Verify that the demultiplexer audio output has been properly terminated into the required
impedance, usually 600 Ohm or 10 kOhm terminations.
C) The audio input to the multiplexer must be 1.0 Volt RMS maximum. This translates to
about 4 Volts peak-to-peak maximum.
Parts of A Fiber Optic Link
Fiber optic transmission uses the Figure 1 – Elements of a Fiber Optic Link
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same basic elements as copper-based
transmission systems: A transmitter,
a receiver, and a medium by which
the signal is passed from one to the
other, in this case, optical fiber.
Figure 1 illustrates these elements.
The transmitter uses an electrical
interface to encode the use
information through AM, FM or
digital modulation. A laser diode or
an LED do the encoding to allow an
optical output of 850 nm,1310 nm, or
1550 nm (typically).
The optical fiber connects the
transmitter and the receiver. This
fiber may be either single-mode or Figure 2 – Cross-section of an Optical Fiber
multimode. The fiber consists of
three main regions, as illustrated in
Figure 2. The core, the center of the
fiber, actually carries the light. The
cladding surround the core in a glass
with a different refractive index than
the core, allowing the light to be
confined in the fiber core. A coating
or buffer, typically plastic, provides
strength and protection to the optical
fiber.
The receiver uses either a PIN photodiode or an APD to receive the optical signal and
convert it back into an electrical signal. A data demodulator converts the data back into its
original electrical form. These elements comprise the simplest link, but other elements may
also appear in a fiber optic transmission system.
For example, the addition of WDM components allows two separate signals to be joined
into a composite signal for transmission, and then can be separated into their original signals
at the receive end. Other wavelength-division multiplexing techniques allow up to eight
signals (CWDM) or more (DWDM) to be combined onto a single fiber. These are discussed
in separate articles as linked in this paragraph.
Long distance fiber optic transmission leads to further system complexity. Many long-haul
transmission systems require signal regenerators, signal repeaters, or optical amplifiers such
as EDFAs in order to maintain signal quality. System drop/repeat/add requirements, such as
those in multichannel broadcast networks, further add to the fiber optic system,
incorporating add-drop multiplexers, couplers/splitters, signal fanouts, dispersion
management equipment, remote monitoring interfaces, and error-correction components.
See the linked articles for additional information on these components
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Couplers, Splitters, Switches, & WDM
Couplers & Splitters
Background
Fiber, connectors, and splices rank as the most important passive devices. However, closely
following are tap ports, switches, wavelength-division multiplexers, bandwidth couplers and
splitters. These devices divide, route, or combine multiple optical signals.
Some of the most common applications for couplers and splitters include:
Local monitoring of a light source output (usually for control purposes).
Distributing a common signal to several locations simultaneously. An 8-port coupler
allows a single transmitter to drive eight receivers.
Making a linear, tapped fiber optic bus. Here, each splitter would be a 95%-5%
device that allows a small portion of the energy to be tapped while the bulk of the
energy continues down the main trunk.
For more information on switches and wavelength division multiplexers see Fiber Optic
Componenents.
Couplers
Fiber optic couplers either split optical signals into multiple paths or combine multiple
signals on one path. Optical signals are more complex than electrical signals, making optical
couplers trickier to design than their electrical counterparts. Like electrical currents, a flow
of signal carriers, in this case photons, comprise the optical signal. However, an optical
signal does not flow through the receiver to the ground. Rather, at the receiver, a dectector
absorbs the signal flow. Multiple receivers, connected in a series, would receive no signal
past the first receiver which would absorb the entire signal. Thus, multiple parallel optical
output ports must divide the signal between the ports, reducing its magnitude.
The number of input and output ports, expressed as an N x M configuration, characterizes a
coupler. The letter N represents the number of input fibers, and M represents the number of
output fibers. Fused couplers can be made in any configuration, but they commonly use
multiples of two (2 x 2, 4 x 4, 8 x 8, etc.).
Splitters
The simplest couplers are fiber optic splitters. These devices possess at least three ports but
may have more than 32 for more complex devices. Figure 1 illustrates a simple 3-port
device, also called a tee coupler. It can be thought of as a directional coupler directional
coupler. One fiber is called the common fiber, while the other two fibers may be called
input or output ports. The coupler manufacturer determines the ratio of the distribution of
light between the two output legs. Popular splitting ratios include 50%-50%, 90%-10%,
95%-5% and 99%-1%; however, almost any custom value can be achieved. (These values
are sometimes specified in dB values.) For example, using a 90%-10% splitter with a 50
µW light source, the outputs would equal 45 µW and 5 µW. However, excess loss hinders
that performance. All couplers and splitters share this parameter. Excess loss assures that
the total output is never as high as the input. Loss figures range from 0.05 dB to 2 dB for
different coupler types. An interesting, and unexpected, property of splitters is that they are
symmetrical. For instance, if the same coupler injected 50 µW into the 10% output leg, only
5 µW would reach the common port. Click here to view the table of typical insertion losses
for modern single-mode couplers. Adobe Acrobat Reader Required
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Figure 1 - Typical Tee Coupler
Coupler and Splitter Applications
In applications that require links other than point-to-point links, optical couplers find the
widest use. This includes bidirectional links and local area network (LAN). In LAN
applications, either a star network topology or a bus topology incorporate couplers. Figure 2
illustrates a star topology, notice that stations branch off from a central hub, much like the
spokes on a wheel. The allows easy expansion of the number of workstations; changing
from a 4 x 4 to an 8 x 8 doubles the system capacity. The star coupler divides all outputs
allowing every station to hear every other station. Star couplers have many ports (usually a
power of two), and couplers with 32 or 64 ports are not uncommon. One use of a star
coupler creates a large party-line circuit. Many transceivers connect to the star coupler and
can communicate with all other transceivers, assuming the network adopts a protocol which
prevent two or more transceivers from communicating simultaneously. Large insertion loss,
(20 dB typically for a 64-port device) creates the biggest disadvantage of the star coupler, as
is the need for a complex collision-prevention protocol.
Figure 2 - Star Topology
Bus topology utilizes a tee coupler to connect a series of stations that listen to a single
backbone of cable. In a typical bus network, a coupler at each node splits off part of the
power from the bus and carries it to a transceiver in the attached equipment. In a system
with N terminals, a signal must pass through N-1 couplers before arriving at the receiver.
Loss increases linearly as N increases. A bus topology may operate in a single direction or a
bidirectional or duplex transmission configuration. In a one way, unidirectional setup, a
transmitter at one end of the bus communicates with a receiver at the other end. Each
terminal also contains a receiver. Duplex networks add a second fiber bus or use an
additional directional coupler at each end and at each terminal. In this way, signals flow in
both directions.
By far the most popular type of coupler in use today is a fused coupler fused fiber coupler.
In this type of coupler, two or more fibers are twisted together and melted in a flame.
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Figure 3 shows the basic construction.
Figure 3 - Fused Fiber Coupler
Switches
Many optical networks incorporate optical switches. Networks that require protection
switching (switching between redundant paths), where key attributes must operate reliably
after a long period in one position, system monitoring, and diagnosis commonly feature
these devises. Speed is not a crucial parameter for these applications, as speed as high as
tens of milliseconds are acceptable. However in the future, dynamic optical routing will
require much faster switching speeds. Figure 1 below illustrates common switch
configurations.
Figure 1 - Typical Switch Configuration
More technology exists for optical switches than any other function within the optical
network. Three main types optical switches include opto-mechanical switches, thermo-optic
switches, and electro-optic switches.
Opto-mechanical Switches
Opto-mechanical switches are the oldest type of optical switch and the most widely
deployed at the time. These devices achieve switching by moving fiber or other bulk optic
elements by means of stepper motors or relay arms. This causes them to be relatively slow
with switching times in the 10-100 ms range. They can achieve excellent reliability,
insertion loss, and crosstalk. Usually, opto-mechanical optical switches collimate the optical
beam from each input and output fiber and move these collimated beams around inside the
device. This allows for low optical loss, and allows distance between the input and output
fiber without deleterious effects. These devices have more bulk compared to other
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alternatives, although new micro-mechanical devices overcome this.
Thermo-optic Switches
Thermo-optic switches are normally based on waveguides made in polymers or silica. For
operation, they rely on the change of refractive index with temperature created by a resistive
heater placed above the waveguide. Their slowness does not limit them in current
applications.
Electro-optic Switches
These are typically semiconductor-based, and their operation depends on the change of
refractive index with electric field. This characteristic makes them intrinsically high-speed
devices with low power consumption. However, neither the electro-optic nor thermo-optic
optical switches can yet match the insertion loss, backreflection, and long-term stability of
opto-mechanical optical switches/
The latest technology incorporates all-optical switches that can cross-connect fibers without
translating the signal into the electrical domain. This greatly increases switching speed,
allowing today's telcos and networks to increase data rates. However, this technology is
only now in development, and deployed systems cost much more than systems that use
traditional opto-mechanical switches.
Wavelength-division Multiplexing
Background
The fiber optic industry first deployed single wavelength transmission links. As
requirements changed, the industry responded with wavelength-division multiplexing
(WDM), which sends two distinct signals per fiber, doubling transmission capacity. Similar
to a simple splitter, WDMs typically have a common leg and a number of input or output
legs. Unlike the splitter, however, they have very little insertion loss. They do have the same
range of excess loss. Two important considerations in a WDM device are crosstalk and
channel separation. Crosstalk, also called directivity, refers to separation of demultiplexed
channels. Each channel should appear only at its intended port. The crosstalk specification
expresses how well a coupler maintains this port-to-port separation. Channel separation
describes a coupler’s ability to distinguish wavelengths. In most couplers, the wavelengths
must be widely separated allowing light to travel in either direction without the penalty
found in splitters. WDMs allow multiple independent data streams to be sent over one fiber.
The most common WDM system uses two wavelengths, although four or more-wavelength
systems are available.
WDM Applications
Figure 1 illustrates two WDMs
permitting two streams of data to
be carried on a single fiber. The
type of data does not matter. For
example, one stream could be a
video signal and the other could be
an RS-232 data stream. Figure 1 - WDM Application
Alternatively, both signals could
be video signals or high speed data
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signals at 2.488 Gb/s. The
configuration shown is
unidirectional, but bidirectional
configurations are also available.
Figure 2 illustrates bulk optics WDM. Constructed from discrete lenses and filters, a
dichroic filter lies at the center of the WDM. Dichroic filters, based on interferometric
techniques, reflect the light that they do not transmit. Looking at the figure, image that Fiber
1 carriers two wavelengths, 850 nm and 1310 nm. Also imagine that the dichroic filter
passes wavelengths longer than 1100 nm, known as long-wave pass (LWP) filter. As the
light exits Fiber 1 it first passes through the lens which focuses the light at a point. As the
light hits the filter, the 1310 nm light passes through the filter and is collected by Fiber 3.
The 850 nm light exiting Fiber 1 on the other hand reflects off of the filter and is collected
by Fiber 2. Thus the information on the two effectively paired wavelengths can be
independently decoded. The dichroic filter can offer a great deal of isolation in the
transmission mode, but has poor isolation in the reflection mode. Usually these types of
WDMs feature both short-wave pass (SWP) and LWP filters, and combining these filters
achieves the best system performance.
Figure 2 - Bulk Optics WDM
For additional information, see: “Couplers and Splitters,” “CWDM,” and “DWDM.”
Coarse Wavelength-division Multiplexing
Background
The development of CWDM(coarse wavelength-division multiplexing), an intermediate
technology, responded to the growing fiber network demand. With a capacity greater than
WDM and smaller than DWDM, CWDM allows a modest number of channels, typically
eight or less, to be stacked in the 1550 nm region of the fiber called the C-Band. To
dramatically reduce cost, CWDMs use uncooled lasers with a relaxed tolerance of ± 3 nm.
Whereas DWDM systems use channel spacing as close to 0.4 nm, CWDM uses a spacing of
20 nm. The wide spacing accommodates the uncooled laser wavelength drifts that occurs as
the ambient temperature varies. The uncooled laser drifts about ±0.06 nm/°C. CWDM
transmission may occur at one of eight wavelengths: typically 1470 nm, 1490nm, 1510 nm,
1530 nm, 1550 nm, 1570 nm, 1590 nm, 1610 nm. Figure 1 illustrates the CWDM passband
for an eight channel device.
Figure 1 - CWDM Passband for an Eight Channel Device
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Unidirectional Applications
Insertion loss for an eight channel device is about 2 dB per end. The passband is around 13
nm wide at the -0.5 dB loss point. CWDM demultiplexers typically have higher insertion
loss and significantly better isolation loss. The multiplexer have a lower insertion loss and a
poorer isolation loss. Isolation does not matter in a unidirectional application because the
multiplexer combines several transmitter outputs. Figure 2 illustrates a unidirectional
CWDM application.
Figure 2 - Unidirectional CWDM Application
In a bidirectional application, illustrated in Figure 3, any input on either end of the fiber can
be an input or an output, requiring the higher isolation of demultiplexers to guarantee that
the system will work without interference between channels.
Figure 3 - Bidirectional CWDM Application
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For more information DWDM or WDM see Dense Wavenlength-division Multiplexing or
Wavelength-division Multiplexing.
Dense Wavelength-division Multiplexing
System Growth with DWDM
Dense wavelength-division multiplexing (DWDM) revolutionized data transmission
technology by increasing the capacity signal of embedded fiber. This increase means that
the incoming optical signals are assigned to specific wavelengths within a designated
frequency band, then multiplexed onto one fiber. This process allows for multiple video,
audio, and data channels to be transmitted over one fiber while maintaining system
performance and enhancing transport systems. This technology responds to the growing
need for efficient and capable data transmission by working with different formats, such as
SONET/SDH, while increasing bandwidth.
The fiber optic amplifier component of the DWDM system provides a cost efficient method
of taking in and amplifying optical signals without converting them into electrical signals.
In addition, DWDM amplifies a broad range of wavelengths in the 1550 nm region. For
example, with a DWDM system multiplexing 16 wavelengths on a single optical fiber,
carriers can decrease the number of amplifiers by a factor of 16 at each regenerator site.
Using fewer regenerators in long-distance networks results in fewer interruptions and
enhanced efficiency.
DWDM System Considerations
Important components for a DWDM systems are transmitters, receivers, fiber amplifiers,
DWDM multiplexers, and DWDM demultiplexer. These components, along with
conforming to ITU channel standards, allow a DWDM system to interface with other
equipment and to implement optical solutions throughout the network.
Figure 1- DWDM System Application
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Multiplexers and Demultiplexers
The recent explosion of DWDM technology forced the fiber optic manufacturers to develop
DWDM multiplexers and demultiplexers that can handle closely spaced optical
wavelengths. These designs require narrow passbands, usually 0.4 nm wide, steep roll-off to
reject adjacent channels, and stable operation over increased temperature. Recently,
multiplexers have gained versatility, moving beyond the “wideband” wavelengths and into
densely packed wavelengths that can be integrated into a multiple high frequency, 192 to
200 THz, transmission system. This type of system can maintain up to 16 channels, acting
as a 16 fiber channel cable with each frequency channel operating to serve a STM-16/OC-
48 carrier.
Demultiplexers need to eliminate crosstalk and channel interference. Couplers and dichroic
filter, both passive devices, are the most favorable demultiplexers today. The first DWDM
coupler design is based on fiber Bragg grating (FBG) filters illustrated in figure 2. Bragg
gratings are comprised of a length of optical fiber with the index of the core permanently
modified periodically usually when exposed to an ultraviolet interference pattern. As a
result, the fiber grating behaves as a wavelength dependent reflector and lends itself to
precise wavelength separation.
Figure 2 - Bragg Grating
The second design is based on cascaded dichroic filters much like those used in the WDM
system shown below in Figure 3. In a DWDM coupler, a second dichroic filter would be
placed where the fiber 2 is located, and additional dichronic filters would be cascaded until
all wavelengths have been combined or separated. At moderate cost, the dichronic filter
method assures stability and excellent isolation between channels.
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Figure 3 - Dichronic Filter
Fiber Amplifiers for DWDM
Because DWDM systems handle information optically rather than electrically, it is
imperative that long-haul applications do not suffer the effects of dispersion and attenuation.
Erbium-doped fiber amplifiers (EDFAs) counteract these problems. EDFAs are silica based
optical fibers that are doped with erbium. This rare earth element has the appropriate energy
levels in its atomic structure for amplifying light at 1550 nm. A 980 nm “pump” laser is
used to inject energy into the doped fiber. When a weak signal at 1310 nm or 1550 nm
enters the fiber, the light stimulates the rare earth atoms to release their stored energy as
additional 1310 nm or 1550 nm light. This process continues as the signal passes down the
fiber, continually growing stronger. Figure 4 illustrates an erbium-doped fiber.
Figure 4 - Erbium-doped Optical Fiber
The photons amplify the incoming signal optically, boosting the wavelength, and avoiding
almost all of the active components. The output power of the EDFA is large, and thus, fewer
amplifiers may be needed in any given system design. The amplification process is
independent of the data rate. Because of this benefit, upgrading a system means only
changing the launch/receive terminals.
As demands for wider bandwidth grow there is a call for more efficient and reliable optical
amplifiers. The usable bandwidth of an EDFA is only about 30 nm (1530 nm-1560 nm), but
the minimum attenuation is in the range of 1500 nm to 1600 nm. The dual-band fiber
amplifier (DBFA) solves the usable bandwidth problem. It is broken down into two sub-
band amplifiers. The DBFA is similar to the EDFA, but its bandwidth ranges from about
1528 nm to 1610 nm. The first range is similar to that of the EDFA and the second is known
as extended band fiber amplifier (EBFA). Some features of the EBFA include flat gain,
slow saturation, and low noise. The EBFA can achieve a flat gain over a range of 35 nm
which is comparable to the EDFAs. EBFAs have the advantage of reaching a slower
saturation keeping the output constant even though the input increases.
Channel Spacing
DWDM channel spacing governs system performance; 50 GHz and 100 GHz outline the
standards of ITU channel spacing. Currently, 100 GHz is the most commonly used and
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reliable channel spacing. This spacing allows for several channel schemes without imposing
limitations on available fiber amplifiers. However, channel spacing depends on the system’s
components.
Channel spacing is the minimum frequency separation between two multiplexed signals. An
inverse proportion of frequency versus wavelength of operation calls for different
wavelengths to be introduced at each signal. The optical amplifiers bandwidth and receivers
ability to identify two close wavelength, sets the channel spacing. Figure 5 illustrates the
typical DWDM specifications.
Figure 5 - Typical Optical Characteristics for DWDM Channels
Signal Direction
DWDM involved sending a large number of closely spaced optical signals over a single
fiber. Standards developed by the ITU (International Telecommunications Union) define the
exact optical wavelength used for DWDM applications. The center of the DWDM band lies
at 193.1 THz with standard channel spacing of 200 GHz and 100 GHz. The closest
"standard" spacing (100 GHz) allows transmission of 45 channels on one fiber. A 45
channel system spaced at 100 GHz would cover a optical span of 35 nm and require a costly
wide bandwidth, gain-flattened EDFA.
As system designers looked to pack more than the 45 channels at 100 GHz spacing, they
started to use closer spaced optical channels. The channel spacing, in GHz, relates to the
optical wavelength as follows: A spacing of 200 GHz corresponds to about 1.6 nm, 100
GHz corresponds to about 0.8 nm, and 50 GHz corresponds to about 0.4 nm channels
spacing. Most commonly 50 GHz follows 100 GHz, although attempts at 75 GHz and 37.5
GHz show up in literature. While there is nothing magical about any of these numbers, it
seems likely that 50 GHz will be the next logical step below 100 GHz. Using a channel
spacing of 50 GHz (0.4 nm) allows 45 channels to occupy only 17.5 nm of optical
bandwidth. This greatly simplifies the requirement for optical amplifiers in the system.
Fiber increases in channels per fiber would likely lead to the use of 25 GHz spacing.
Designing the optical demultiplexer to separate the signals at the receive end defines the
greatest challenge in closely spaced optical channels. Because of subtle color differences in
each of the optical channels, high performance DWDM optical demultiplexers must have
three characteristics. First, it must be very stable over time and temperature. Second, it
needs to have a relatively flat passband or region of frequencies. Third, it must reject
adjacent optical channels so that they do not interfere. Several basic types of designs can be
used in optical demultiplexers to separate the optical channels. Many of these designs have
an increasingly difficult time separating the optical channels as the spacing becomes very
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close. Some, however, such as fiber Bragg gratings actually appear better suited for closer
channel spacing. The need for close optic channel spacing is a trade-off between the
performance required of the optical amplifiers used in the system and the number of
channels to be transmitted per fiber. Figure 6 illustrates the transmission spectra of 0.4 nm
spacing DWDM FBGs.
Figure 6 - 0.4 nm Channel Spacing DWDM Fiber Bragg Grating
Red and Blue Bands
The ITU approved DWDM band extends from 1528.77 nm to 1563.86 nm, and divides into
the red band and the blue band. The red band encompasses the longer wavelengths of
1546.12 nm and higher. The blue band wavelengths fall below 1546.12 nm. This division
has a practical value because useful gain region of the lowest cast EDFAs corresponds to
the red band wavelengths. Thus, if a system only requires a limited number of DWDM
wavelengths using the red band wavelength yields the lowest overall system cost.
Optical Amplifiers & External Modulators
Optical Amplifiers
Improving Long-Haul Network Performance
With the demand for longer transmission lengths, optical amplifiers have become an
essential component in long-haul fiber optic systems. Semiconductor optical amplifiers
(SOAs), erbium doped fiber amplifiers (EDFAs), and Raman optical amplifiers lessen the
effects of dispersion and attenuation allowing improved performance of long-haul optical
systems.
Semiconductor Optical Amplifiers
Semiconductor optical amplifiers (SOAs) are essentially laser diodes, without end mirrors,
which have fiber attached to both ends. They amplify any optical signal that comes from
either fiber and transmit an amplified version of the signal out of the second fiber. SOAs are
typically constructed in a small package, and they work for 1310 nm and 1550 nm systems.
In addition, they transmit bidirectionally, making the reduced size of the device an
advantage over regenerators of EDFAs. However, the drawbacks to SOAs include high-
coupling loss, polarization dependence, and a higher noise figure. Figure 1 illustrates the
basics of a Semiconductor optical amplifier.
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Figure 1 - Semiconductor Optical Amplifier
Modern optical networks utilize SOAs in the follow ways:
Power Boosters: Many tunable laser designs output low optical power levels and must be
immediately followed by an optical amplifier. ( A power booster can use either an SOA or
EDFA.)
In-Line Amplifier: Allows signals to be amplified within the signal path.
Wavelength Conversion: Involves changing the wavelength of an optical signal.
Receiver Preamplifier: SOAs can be placed in front of detectors to enhance sensitivity.
EDFAs
The explosion of dense wavelength-division multiplexing (DWDM) applications make
these optical amplifiers an essential fiber optic system building block. EDFAs allow
information to be transmitted over longer distances without the need for conventional
repeaters. The fiber is doped with erbium, a rare earth element, that has the appropriate
energy levels in their atomic structures for amplifying light. EDFAs are designed to amplify
light at 1550 nm. The device utilizes a 980 nm or 1480nm pump laser to inject energy into
the doped fiber. When a weak signal at 1310 nm or 1550 nm enters the fiber, the light
stimulates the rare earth atoms to release their stored energy as additional 1550 nm or 1310
nm light. This process continues as the signal passes down the fiber, growing stronger and
stronger as it goes.
Figure 2 shows a fully featured, dual pump EDFA that includes all of the common
components of a modern EDFA.
Figure 2 - Block Diagram of an EDFA
The input coupler, Coupler #1, allows the microcontroller to monitor the input light via
detector #1. The input isolator, isolator #1 is almost always present. WDM #1 is always
present, and provides a means of injecting the 980 nm pump wavelength into the length of
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erbium-doped fiber. WDM #1 also allows the optical input signal to be coupled into the
erbium-doped fiber with minimal optical loss. The erbium-doped optical fiber is usually
tens of meters long. The 980 nm energy pumps the erbium atom into a slowly decaying,
excited state. When energy in the 1550 nm band travels through the fiber it causes
stimulated emission of radiation, much like in a laser, allowing the 1550 nm signal to gain
strength. The erbium fiber has relatively high optical loss, so its length is optimized to
provide maximum power output in the desired 1550 nm band. WDM #2 is present only in
dual pumped EDFAs. It couples additional 980 nm energy from Pump Laser #2 into the
other end of the erbium-doped fiber, increasing gain and output power. Isolator #3 is almost
always present. Coupler #2 is optional and may have only one of the two ports shown or
may be omitted altogether. The tap that goes to Detector #3 is used to monitor the optical
output power. The tap that goes to Detector #2 is used to monitor reflections back into the
EDFA. This feature can be used to detect if the connector on the optical output has been
disconnected. This increases the backreflected signal, and the microcontrolled can set to
disable the pump lasers in this event, providing a measure of safety for technicians working
with EDFAs.
Figure 3 shows a two-stage EDFA with mid-stage access. In this case, two single-stage
EDFAs are packaged together. The output of the first stage EDFA and the input of the
second stage EDFA are brought out the user. Mid-stage access is important in high
performance fiber optic systems. To reduce the overall dispersion of the system, dispersion
compensating fiber (DCF) can be used periodically. However, problems can arise from
using the DCF, mostly the insertion loss reaching 10 dB. Placing the DCF at the mid-stage
access point of the two-stage EDFA reduces detrimental effects on the system, and allows
the users noticeable gain.
Figure 3 - Two-stage EDFA with Mid-stage Access
The optical input first passes through optical Isolator #1. Next the light passes through
WDM #1, which provides a means of injecting the 980 nm pump wavelength into the first
length of erbium-doped fiber. WDM #1 also allows the optical input signal to be coupled
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into the erbium-doped fiber with minimal optical loss. The erbium-doped optical fiber is
usually tens of meters long. Like the fully feature, dual pumped EDFA, the 980 nm energy
pumps the erbium atoms into an excited state that decays slowly. When light in the 1550 nm
band travels through the erbium-doped fiber it causes stimulated emission of radiation. As
the optical signal gains strength, output of the erbium-doped fiber then goes into the optical
isolator #2, the output of which is available to the user. Typically, a dispersion
compensating device will be connected at the mid-stage access point. The light then travels
through isolator #3 and WDM #2, which couples additional 980 nm energy from a second
pump laser into the other end of a second length of erbium-doped fiber, increasing gain and
output power. Finally, the light travels through isolator #4.
Photons amplify the signal avoiding almost all active components, a benefit of EDFAs.
Since the output power of an EDFA can be large, any given system design can require fewer
amplifiers. Yet another benefit of EDFAs is the data rate independence means that system
upgrades only require changing the launch/receive terminals. The most basic EDFA design
amplifies light over a narrow, 12 nm, band. Adding gain equalization filters can increase the
band to more than 25 nm. Other exotic doped fibers increase the amplification band to 40
nm.
Because EDFAs greatly enhance system performance, they find use in long-haul, high data
rate fiber optic communication systems and CATV delivery systems. Long-haul systems
need amplifiers because of the lengths of fiber used. CATV applications often need to split a
signal to several fibers, and EDFAs boost the signal before and after the fiber splits. There
are four major applications that generally require optical fiber amplifiers: power
amplifier/booster, in-line amplifier, preamplifier or loss compensation for optical networks.
Below are detailed description of each application.
Power Amplifier/Booster
Figure 4 illustrates the first three application for optical amplifiers. Power amplifiers (also
referred to as booster amplifiers) are placed directly after the optical transmitter. This
application requires the EDFA to take a large signal input and provide the maximum output
level. Small signal response is not as important because the direct transmitter output is
usually -10 dBm or higher. The noise added by the amplifier at this point is also not as
critical because the incoming signal has a large signal-to-noise ratio (SNR).
Figure 4 - Three Applications for an EDFA
In-Line Amplifiers
In-line amplifiers or in-line repeaters, modify a small input signal and boost it for
retransmission down the fiber. Controlling the small signal performance and noise added by
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the EDFA reduces the risk of limiting a system’s length due to the noise produced by the
amplifying components.
Preamplifiers
Past receiver sensitivity of -30 dBm at 622 Mb/s was acceptable; however, presently, the
demands require sensitivity of -40 dBm or -45 dBm. This performance can be achieved by
placing an optical amplifier prior to the receiver. Boosting the signal at this point presents a
much larger signal into the receiver, thus easing the demands of the receiver design. This
application requires careful attention to the noise added by the EDFA; the noise added by
the amplifier must be minimal to maximize the received SNR.
Compensating for Loss in Optical Networks
Inserting an EDFA before an 8 x 1 optical splitter increases the power to almost +19 dBm
allowing each of the eight output legs to provide +9 dBm, making the output almost equal to
the original transmitter power. The optical splitter alone has a nominal optical insertion loss
of 10 dB. The transmitter has an optical output of +10 dBm, meaning that the optical splitter
outputs without an EDFA would be 0 dBm. This output power would be acceptable for most
digital applications; however, in analog CATV applications this is the minimal acceptable
received power. Therefore, inserting the EDFA before the optical splitter greatly increases
the output power.
Figure 5 - Loss Compensation in Optical Networks
Wideband EDFAs
Optical communication systems carrying 100 or more optical wavelengths require and
increase in the bandwidth of the optical amplifier to nearly 80 nm. Normally employing a
hybrid optical amplifier, consisting of two separate optical amplifiers, allows for separate
amplification, one for the lower 40 nm band and the second for the upper 40 nm band.
Figure 6 exemplifies the optical gain spectrum of a hybrid optical amplifier. The solid lines
illustrate the response of two individual amplifier sections. The dotted line, which has been
increased by 1 dB for clarity, shows the response of the combined hybrid amplifier.
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Figure 6 - Optical Gain Spectrum of a Hybrid Optical Amplifier
Raman Optical Amplifiers
Raman optical amplifiers differ in principle from EDFAs or conventional lasers in that they
utilize stimulated Raman scattering (SRS) to create optical gain. Initially, SRS was
considered too detrimental to high channel count DWDM systems. Figure 7 shows the
typical transmit spectrum of a six channel DWDM system in the 1550 nm window. Notice
that all six wavelengths have approximately the same amplitude.
Figure 7 - DWDM Transmit Spectrum with Six Wavelengths
By applying SRS the wavelengths, it is obvious that the noise background has increased,
making the amplitudes of the six wavelengths different. The lower wavelengths have a
smaller amplitude than the upper wavelengths. The SRS effectively robbed energy from the
lower wavelength and fed that energy to the upper wavelength.
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Figure 8 - Received Spectrum After SRS is on a Long Fiber
A Raman optical amplifier is little more that a high-power pump laser, and a WDM or
directional coupler. The optical amplification occurs in the transmission fiber itself,
distributed along the transmission path. Optical signals are amplified up to 10 dB in the
network optical fiber. The Raman optical amplifiers have a wide gain bandwidth (up to 10
nm). They can use any installed transmission optical fiber. Consequently, they reduce the
effective span loss to improve noise performance by boosting the optical signal in transit.
They can be combined with EDFAs to expand optical gain flattened bandwidth.
Figure 9 shows the topology of a typical Raman optical amplifier. The pump laser and
circulator comprise the two key elements of the Raman optical amplifier. The pump laser, in
this case, has a wavelength of 1535 nm. The circulator provides a convenient means of
injecting light backwards in to the transmission path with minimal optical loss.
Figure 9 - Typical Raman Amplifier Configuration
Figure 10 illustrates the optical spectrum of a forward-pumped Raman optical amplifier.
The pump laser is injected at the transmit end rather than the receive end as shown in Figure
9. The pump laser has a wavelength of 1535 nm; the amplitude is much larger than the data
signals.
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Figure 10 - Example of Raman Amplifier -- Transmitted Spectrum
As before, applying SRS makes the amplitude of the six data signals much stronger. The
energy from the 1535 nm pump laser is redistributed to the six data signals.
Figure 11 - Example of Raman Amplifier -- Received Spectrum
External Modulators
Background
When data rates were in the low gigabit range and transmission distances were less than
100 km or so, most fiber optic transmitters used directly modulated lasers. However, as data
rates and span lengths grew, waveguide chirp, caused by turning a laser on and off, limited
data rates. Dispersion problems resulted when the wavelength chirp widened the effective
spectral width of the laser. A laser source with no wavelength chirp and a narrow linewidth
provide one solution to the problem. This solution took the form of external modulation
which allows the laser to be turned on continuously; the modulation is accomplished
outside of the laser cavity.
Theory of Operation
An external modulator restrains the light, functioning like an electrically activated shutter.
As analog devices, external modulators allow the amount of light passed to vary from some
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maximum amount (PMAX) to some minimum amount (PMIN). Other key terms related to
external modulators include:
Vp: This is the voltage required to take the response function through ½ cycle or 180°.
Bias Point: The DC point around which the modulation signal swings.
Insertion Loss: The amount of loss from the light injected by the laser at the peak of the
waveform. This usually amounts to 3-5 dB. Keep in mind that operating at the usual bias
point will introduce an additional 3 dB of loss for a total insertion loss of 6 to 8 dB. (See
Figure 5 for details.)
PMIN: The minimum light output from the external modulator. Usually about 5% of the
maximum value.
PMAX: The maximum light output from the external modulator. Usually 3 to 5 dB less than
the laser input.
PAVG: The average light out of the external modulator. Usually 3 dB less than PMAX if
driven by a 50% duty cycle waveform.
Lithium Niobate Amplitude and Phase Modulators
The popularity of lithium niobate (LiNbO3) as a material used in external modulators
results from its low optical loss and high electro-optic coefficient. This coefficient refers to
the electro-optic effect, which occurs in some materials such as lithium niobate, in which
the refractive index of the material changes in response to an applied electric field. The
refractive index of the material causes light to travel at a speed inversely proportional to the
refractive index of the material. Thus, if we could suddenly increase the refractive index of
a material, we would slow the light beam down and vice versa.
Figure 1 shows the block diagram of a typical external modulator. The input light enters the
external modulator via the input fiber. The light is first splits into two fibers using an
optical splitter. The top fiber path travels through a length of LiNbO3 crystal. The light in
the bottom fiber experiences a fixed delay. After the light travels through the lithium
niobate crystal and the fixed length of fiber, an optical combiner merges the two fiber paths.
The light travels through identical path legs.
Figure 1 - Typical Lithium Niobate (LiNbO3) Optical Modulator
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By applying an electric field to the material, its refractive index changes. We now see that if
the time delay through the fixed fiber and the LiNbO3 crystal is equal, the light will be in
phase when it reaches the output optical combiner. Due to the nature of light, we see that
since the light in both legs are in phase, they will constructively add to form the maximum
possible output. The refractive index and the speed of light change as the applied voltage
changes. When the speed changes enough to delay the light by half of one wavelength, the
light will be out of phase when it reaches the output 3 dB coupler. Now the light will
destructively form, yielding a minimum possible output.
Building a waveguide in the substrate makes the device suitable for use in fiber optic
devices. As with optical fiber itself, this is accomplished by introducing dopant materials
into the area that will become the waveguide. Doping raises the refractive index of the
waveguide relative to the surrounding substrate while maintaining optical transparency.
Once accomplished, the waveguide will contain the light by the principles of total internal
reflection. If the dimensions of the waveguide remain consistent with the dimensions of the
core of a single-mode fiber, about nine microns in diameter, then light will efficiently
couple into and out of the waveguide. This basic design proves useful in a fiber optic
system.
Figure 2 illustrates the simplest type of external modulator, a phase modulator. The phase
modulator has a single optical input of polarization maintaining (PM) fiber and a single
optical output of PM or single-mode (SM) fiber. In a simple phase modulator, two
electrodes surround the waveguide. The bottom electrode is grounded while the top
electrode is driven by an outside voltage signal. As the voltage on the top electrode
changes, the refractive index of the waveguide changes accordingly, alternating the light as
the refractive index rises and falls. While this modulates the phase of the light, the output
intensity remains unchanged. This modulation overcomes stimulated brilliouin scattering,
the easiest fiber nonlinearity to trigger. The SBS threshold can increase by as much as 10
dB because phase modulating the light effectively widens the optical energy.
Figure 2 - Simple Phase Modulator
Figure 3 shows a more internally complex device. It has the same input and output fiber
setup as the simple phase modulator. However, after the light enters the lithium niobate
waveguide, it optically splits into two paths using a fiber optic coupler designed into the
substrate. These two paths travel for a distance and then recombine using another fiber
optic coupler. If the light waves are in phase, they will add constructively to produce a large
output on the output leg. If they are out of phase, destructive interference yields little or no
output. The two paths of light travel through sets of electrodes arranged so that they have
opposite effects on the two paths. By applying an external voltage, the refractive index of
one path will rise while the refractive index of the other path falls. This causes the output
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optical amplitude to vary as the light from the two paths moves from constructive addition
to destructive interference.
Figure 3 - Single Output Intensity Modulator
A third type of external modulator, illustrated in Figure 4, resembles the modulator shown
in Figure 3. However, in this case, a 3 dB coupler forms at the output, giving two output
fibers rather than one. The light amplitude of the two output legs will move opposite of
each other. When the light level of one leg increases, the light level of the other leg
decreases. The dual output modulator, which provides two out of phase outputs works best
in analog drive situations.
Figure 4 - Dual Output Intensity Modulator
Figure 5 shows the typical raised sine function response of the dual output intensity
modulator. The modulator operates around zero Volts bias. At zero Volts bias, the output
intensity of both output legs is equal. As the applied voltage increases slightly, the intensity
of output 2 increases, while the intensity of output 1 decreases. This continues until the
voltage reaches Vp/2. At that point, the intensity of output 2 will be at a maximum and the
intensity of output 1 will be at a minimum. This sine function response repeats as the
applied voltage increases or decreases. Usually, modulator designers exploit the response
nearest zero Volts bias.
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Figure 5 - Dual Output External Modulator Response
Digital Operation
In the simple applications, an external modulator transmits a digital data stream, toggling
the drive voltage between -Vp/2 and Vp/2. This causes the output intensity to swing from
maximum to minimum utilizing maximum modulation depth.
Analog Operation
External modulators may also be used to transmit analog signals. This modulation scheme
may require extensive stabilization and linearization. Stabilizing the bias point at exactly
the 50% point minimizes that second-order distortion. However, a third-order distortion
remains. A small drive signal may yield a response that does not require linearization.
CATV applications require predistortion of the signal to remove the effects of third-order
distortion when sending 80 or 110 channels.
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