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									Optical Communications
     Semester 2/2005

      Lecture 1

      What is lightwave technology?

• Lightwave technology uses light as the
  primary medium to
  carry information.

• The light often is guided through
  optical fibers (fiberoptic technology).

• Most applications use invisible
  (infrared) light.
         Why lightwave technology?

• Most cost-effective way to move
  huge amounts of information (voice,
  data) quickly and reliably.

• Light is insensitive to
  electrical interference.

• Fiberoptic cables have less weight
  and consume less space than           (HP)

  equivalent electrical links.
     Use Of Lightwave Technology

• Majority applications:
  – Telephone networks
  – Data communication systems
  – Cable TV distribution

• Niche applications:
  – Optical sensors
  – Medical equipment
        Basic Fiber-Optic System

• Transmitter (laser diode or LED).
• Fiber-optic cable.
• Receiver (PIN diode or avalanche
• Most fiber systems are digital but analog is
  also used.
            Basic Link Design

     Transmitter    Connector          Cable

Splice      Cable           Receiver
             Typical Long-haul System
                     Two pairs of single-mode

 Terminal              Amplifier                Amplifier
 Equipment               Unit                     Unit

     Regenerator              Amplifier                 Terminal
        Unit                    Unit                    Equipment

Amplifier spans:     30 to 120 km
Regenerator spans:   50 to 600 km
Terminal spans:             up to 600 km (without regenerators)
                     up to 9000 km (with regenerators)
         Typical Regenerator Unit

            Pulse re-shaping & re-timing
                                       Modulation & bit
                                       rate dependent!

Power                                      Telemetry &
Supply                                     Remote Control
    Typical Amplifier Unit

         Optical Amplifiers
                         Modulation & bit
                         rate independent!

Power                         Telemetry &
Supply                        Remote Control
                  How “fast” is fiber optics?
(Light travels in fibers at about 2/3 the speed of light, but so do electrical signals in wire!)
   • Copper wire (twisted pair) up to ~ 100 Mb/sec (short distances)
          – 1,500 phone calls
          – 2 TV channels
          – 2 Bibles/sec

   • Coaxial cable (also copper) Up to ~1 Gb/sec (short distances)
          – 15,000 phone calls
          – 20 TV channels (> 200 with “data compression”)
          – 20 bibles/second

   • Optical Fiber up to 50 Tb/s (50,000 Gb/s) (long distances)
          – 0.78 billion phone calls
          – 1 million TV channels
          – 1 million Bibles/second
                    Company Types
• Component Manufacturers
   – Lasers/LEDs, photodetectors,
     couplers, multiplexers, isolators,             Port 1

     fibers, connectors                    DWDM     Port 2

• Subsystem Manufacturers
                                                    Port 3

                                                    Port 4

   – Transmitters, receivers, amplifiers          COMMON

     (EDFA), repeaters
• System Manufacturers
   – Point-to-point, SONET/SDH, WDM
• Installers & Service Providers
   – Link signature, fault location
    Physical Basics

LW Technology
                 The Carrier - Light

 Particles                Waves          Rays

Conduction band
       Bandgap                          n1

Valence band

Absorption              Interference   Refraction
Emission                               Reflection
            Light Properties - Wavelength

    Strength                       

     Wavelength : distance to complete one sine wave

1000 pm (picometer)   = 1 nm (nanometer)    1000 m         = 1 mm
1000 nm (nanometer)   = 1 m (micrometer)   1000 mm = 1 m (meter)
               Electromagnetic Spectrum

                      1 kHz   1 MHz   1 GHz   1 THz   1 YHz   1 ZHz
        Wavelength    1 Mm    1 km    1 m     1 mm    1 nm    1 pm

c =f •  • n
    c:   Speed of light ( 2.9979 m/µs )
    f:   Frequency
    :   Wavelength
    n:   Refractive index
         (vacuum: 1.0000; standard air: 1.0003; silica fiber: 1.44 to
           LW Transmission Bands

                           193     229           353     461                THz
                           Near Infrared                              UV
(vacuum)           1.8   1.6   1.4   1.2   1.0    0.8     0.6   0.4   0.2   µm

                                                          HeNe Lasers
Longhaul Telecom                                          633 nm
                         1550 nm
                                                        CD Players
Regional Telecom
                               1310 nm                  780 nm
Local Area Networks
                                             850 nm
               Wavelength and “Color” Names                                      *not visible to
     violet*           blue           green           red             Infra-red* human eyes

               400nm          500nm           600nm         700nm   850nm        1300nm     1550nm

•   Wavelength (and “color”) can be controlled by type and amount of                         850, 1300
                                                                                             and 1550
    “dopants” (alloy materials) used to make the P and N sides of the light                  nm are
    emitting diode.                                                                          local minima
          • Light emitting diodes (LEDs) with visible light output are used                  in the fiber
            for indicator lights, etc.                                                       transmission
          • LEDs with infra-red output used as electro-optic (EO)                            wavelengths
            converters for step or graded index fibers                                       often used
     – Construction of two parallel semi-reflecting surfaces on the diode                    for fiber
        with proper spacing relative to desired wavelength produces
        enhancement of one wavelength, yielding almost monochromatic
        LASER radiation (laser diode -- LD), used for single-mode fiber
     – Proper efficient coupling of light into the fiber core is a major
        design consideration as well (not discussed here)
                         Optical Power

• Power (P):
   – Transmitter: typ. -6 to +17 dBm (0.25 to 50 mW)
   – Receiver: typ. -3 to -35 dBm (500 down to 0.3 µW)
   – Optical Amplifier: typ. +3 to +20 dBm (2 to 100 mW)

• Laser safety
   – International standard: IEC 825-1
   – United States (FDA): 21 CFR 1040.10
   – Both standards consider class I safe under reasonable forseeable
     conditions of operation (e.g., without using optical instruments, such
     as lenses or microscopes)
                                Snell’s “Law”
      • Demonstration with glass of water
no=1/co=oo :vacuum (or air)
n1=1/c1=1o :lower index medium
n2=1/c2=2o :higher index medium

Snell’s “law”:                                              Incident ray power
n2•Sin(D) = n1•Sin(F)                                       is partly in reflected
                                                            ray, partly in refracted
                                             Angle of
                                             Ray R       Angle of Refraction F

             R=D and
             Sin(R)=Sin(D)                                            Line perpendicular to interface at
                                 Angle of                             point where ray intersects interface.
                                 Incident Ray

                                                          Material with lower
                             Material with higher         dielectric constant ,
                             dielectric constant ,      faster wave speed, c1,
                             slower wave speed, c2,       smaller index n1.
                             larger index n2.
         Total Internal Reflection
• When angle of incidence is beyond B, ~100% of optical
  power is reflected internally
   – some sources measure angle from the perpendicular
      line rather than from the interface, so inequality is
      stated differently
• When you (or a fish) go under a smooth water surface
  (e.g., a swimming pool), you can see up to the air only
  inside of a circle. Outside that circle, you see only
  reflections from the surface.


                                  Location of your (underwater) eye
          What is an optical fiber?
  • It‟s basically, a highly transparent “light pipe”
                                                 High index

Light                                         Low index
                     “Total internal          cladding

                      up to many kilometers
                      The Logarithmic Scale

dB          = 10 • log10 (P1 / P0) dBm   = 10 • log10 (P / 1 mW)

     0 dB         = 1                 0 dBm     = 1 mW

     +   0.1 dB   =   1.023 (+2.3%)   3 dBm     =   2 mW
     +   3 dB     =   2               5 dBm     =   3 mW
     +   5 dB     =   3               10 dBm    =   10 mW
     +   10 dB    =   10              20 dBm    =   100 mW

     -3 dB        =   0.5             -3 dBm    =   0.5 mW
     -10 dB       =   0.1             -10 dBm   =   100 W
     -20 dB       =   0.01            -30 dBm   =   1 W
     -30 dB       =   0.001           -60 dBm   =   1 nW

• Incoherent light adds up optical power

• Coherent light adds electromagnetic fields

• Zero phase shift:
                                   +   =
  constructive interference

• 180º phase shift:
                               +       =
  destructive interference

• Coherent light
  Photons have fixed phase
  relationship (laser light)

• Incoherent light
  Photons with random phase
  (sun, light bulb)

• Coherence length (CL)     1
  Average distance over which
  photons lose their phase
• Reflections: root cause for many problems
  Return loss definition:

• RL = 10 * log    P   incident
                   P   reflected



                                     SOP: linear
• Most lasers are highly polarized   vertical          z

• Degree of polarization (DOP):
  DOP = P polarized / P total y

• State of polarization (SOP):
  describes the orientation                     SOP: linear
  and rotation of the                           horizontal
  polarized light                           x
Brief quantum description of gain process
Optical Resonator
Focusing to overcome diffraction
Why use Guided Waves?
Optical Waveguides
Optical Waveguide Properties
               Waveguide Principles
➤ Waves propagating in a waveguide are called MODES

➤ Perpendicular Polarised Wave
 ➤ Electric Field Transverse to the direction of Propagation

➤ Parallel Polarised Wave
 ➤ Electric Field Parallel to the direction of Propagation
A History of Fiber Optic Technology
               The Nineteenth Century
• John Tyndall, 1870
   – water and light experiment
   – demonstrated light used
      internal reflection to follow a
      specific path
• William Wheeling, 1880                Light
   – “piping light” patent
   – never took off
• Alexander Graham Bell, 1880
   – optical voice transmission
   – called a photophone
   – free light space carried voice
      200 meters
• Fiber-scope, 1950‟s
               The Twentieth Century
• Glass coated fibers developed to reduce optical
  loss                                                  core
• Inner fiber - core
• Glass coating - cladding
• Development of laser technology was important to
  fiber optics
• Large amounts of light in a tiny spot needed
• 1960, ruby and helium-neon laser developed
• 1962, semiconductor laser introduced - most
  popular type of laser in fiber optics

      The Twentieth Century (continued)
•   1966, Charles Kao and Charles Hockman proposed optical fiber could be
    used to transmit laser light if attenuation could be kept under 20dB/km
    (optical fiber loss at the time was over 1,000dB/km)
•   1970, Researchers at Corning developed a glass fiber with less than a
    20dB/km loss
•   Attenuation depends on the wavelength of light
      The Twentieth Century /Present
• Late 1970s, early 1980s:         • Present:

– Second-generation                – Fourth generation technology
   technology                      » 1550 nm operation to use
» Sources/receivers: visible and      fiber amplifiers
near-IR (600 to 920 nm)            » Several wavelengths per fiber
» Fibers: individual multi-mode       (WDM)
   fiber                           • – Wavelength addressable
• Mid -1980s to present::

– Third generation technology
» Sources/receivers: near-IR
(1300, 1550 nm)
» Fibers: individual single-mode
                 Real World Applications

• Military
   – 1970‟s, Fiber optic telephone link installed aboard the U.S.S. Little Rock
   – 1976, Air Force developed Airborne Light Fiber Technology (ALOF)

• Commercial
   – 1977, AT&T and GTE installed the first fiber optic telephone system
   – Fiber optic telephone networks are common today
   – Research continues to increase the capabilities of fiber optic
                         The Future
• Fiber Optics have immense potential bandwidth (over 1
  teraHertz, 1012 Hz)

• Fiber optics is predicted to bring broadband services to the
   – interactive video
   – interactive banking and shopping
   – distance learning
   – security and surveillance
   – high-speed data communication
   – digitized video
       Advantages of Fiber Optics

• Immunity from          •   Better Signal Quality
  Electromagnetic (EM)   •   Lower Cost
  Radiation and          •   Easily Upgraded
                         •   Ease of Installation
• Lighter Weight
• Higher Bandwidth
       Advantages of Fiber Optics
Why are fiber-optic systems revolutionizing telecommunications?

Compared to conventional metal wire (copper wire), optical fibers are:

        •   Less expensive
        •   Higher carrying capacity
        •   Less signal degradation.
        •   Less interference
        •   Low power losses
        •   Safer
        •   Lightweight
        •   Flexible
             Why Not Fibers?

• Lack of bandwidth demand
–HDTV requires high bandwidth

• Lack of standards
»Telecomm industry
»Computer industry

• Radiation darkening
–Depends on dose, exposure, glass materials, impurity
types and levels
–Clears with time
     Fiber Optic Components - Fiber
• Extremely thin strands of ultra-pure glass
• Three main regions
   – center: core (9 to 100 microns)
   – middle: cladding (125 or 140 microns)
   – outside: coating or buffer (250, 500 and 900 microns)
             Fiber Structure

• Core and cladding are both transparent,
  usually glass, sometimes plastic.
• Core has higher index of refraction.
• Light propagates down the core, reflecting
  from cladding.
Fiber Communication
  Fiber Optic Components - Light Emitters
• Two types
   – Light-emitting diodes
       • Surface-emitting
         (SLED): difficult to
         focus, low cost
       • Edge-emitting
         (ELED): easier to
         focus, faster
   – Laser Diodes (LD‟s)
       • narrow beam
       • fastest
Communications Diode Laser &



                                                n-InP substrate

          InGaAsP                         Frequency Stability~10-5
     Multiquantum                         Lifetime >> 25 years
      Well Layers

    Maximum modulation speed ~ 40 GHz ( 25 psec ber bit)
  (hard to do) - but fibers can carry more information than this
              Laser light and LED light compared
• LED are an extended source; light appears as many
  independent light modes
   – each small element of the LED is spatially incoherent
   – minimum focused size is an image of the LED and this is much larger
     than the core of a single-mode fibre and hence coupling efficiency is
   – Multimode fibre is normally used with LED
                                                         Multi-mode fibre

   • Ideal laser light is a single ordered light beam
      – It is spatially and temporally coherent
      – Laser light can be focused to a very small spot
                                                       Single-mode fibre
   Fiber Optic Components - Detectors
• Two types
   – Avalanche photodiode
      • internal gain
      • more expensive
      • extensive support electronics required
   – PIN photodiode
      • very economical
      • does not require additional support circuitry
      • used more often
Refraction and reflection
            Meaning of refractive index

• Refractive index, n defined by:   Speed of light,    V

                                      n1 sin 1  n2 sin  2
               1 1

       n2                              Here n1 < n2
                Typical Fiber Construction

Core - Thin glass center of the fiber where the light travels
Cladding - Outer optical material surrounding the core that reflects the light back
into the core
Buffer coating - Plastic coating that protects the fiber from damage and moisture

Hundreds or thousands of these optical fibers are arranged in bundles in optical
cables. The bundles are protected by the cable's outer covering, called a jacket.
                   Typical Fiber Structure
     • Many fibers may be gathered in a protective
       covered cable, with steel or kevlar plastic “rope”
       (not shown) incorporated for pulling strength.

  Lower index glass cladding


                                           Plastic protective jacket, prevents mechanical
                   High index glass core   damage to outside surface of fiber. Can be removed
                                           for splicing by cutting or dissolving. Typically
                                           color coded for identification of each fiber.
   Principles of Operation - Refraction

 • Light entering an optical fiber bends in towards the
   center of the fiber – refraction


LED or
     Principles of Operation - Reflection

  • Light inside an optical fiber bounces off the cladding -


LED or
Principles of Operation - Critical Angle
• If light inside an optical fiber strikes the cladding too
  steeply, the light refracts into the cladding - determined
  by the critical angle

                        Critical Angle
   Principles of Operation - Angle of
• Also incident angle
• Measured from perpendicular

                     Incident Angles
   Principles of Operation - Angle of
• Also reflection angle
• Measured from perpendicular

               Reflection Angle
    Principles of Operation - Angle of
• Also refraction angle
• Measured from perpendicular

                   Refraction Angle
Principles of Operation - Angle Summary
 • Three important angles
 • The reflection angle always equals the incident angle
         Refraction Angle

    Incident Angles

                      Reflection Angle
Meridional ray representation
    Principles of Operation - Index of
• n = c/v
    – c = velocity of light in a vacuum            Light bends
    – v = velocity of light in a specific medium   away from
                                                   normal - higher
• light bends as it passes from one medium         n to lower n
  to another with a different index of
    – air, n is about 1
    – glass, n is about 1.4

   Light bends in towards normal -
   lower n to higher n
 Principles of Operation - Snell’s Law
• The amount light is bent by refraction is given by Snell‟s
                 n1sin1 = n2sin2
• Light is always refracted into a fiber (although there will
  be a certain amount of Fresnel reflection)
• Light can either bounce off the cladding or refract into
  the cladding
 Principles of Operation - Snell’s Law
               Example 1
• Calculate the angle of refraction at the air/core interface
• Solution - use Snell‟s law: n1sin1 = n2sin2
   – 1×sin(30°) = 1.47×sin(refraction)
   – refraction = sin-1(sin(30°)/1.47)
   – refraction = 19.89°

        nair = 1
        ncore = 1.47
        ncladding = 1.45
        incident = 30°
  Principles of Operation - Snell’s Law
                Example 2
• Calculate the angle of refraction at the core/cladding
• Solution - use Snell‟s law and the refraction angle from
  Example 1
   – 1.47sin(90° - 19.89°) = 1.45sin(refraction)
   – refraction = sin-1(1.47sin(70.11°)/1.45)
   – refraction = 72.42°

nair = 1
ncore = 1.47
ncladding =
incident = 30°
 Principles of Operation - Critical Angle
• The angle of incidence that produces an angle of
  refraction of 90° is the critical angle
   – n1sin(c) = n2sin(90°)
                                n1 = Refractive index of the core
   – n1sin(c) = n2             n2 = Refractive index of the cladding
   – c = sin-1(n2 /n1)
• Light at incident angles
  greater than the critical
  angle will reflect back
  into the core
                          Critical Angle, c
 Principles of Operation - Acceptance
             Angle and NA
• The angle of light entering a fiber which follows the
  critical angle is called the acceptance angle, a

  a = sin-1[(n12-n22)1/2]
                                   n1 = Refractive index of the core
                                   n2 = Refractive index of the cladding
• Numerical Aperture (NA)
  describes the light- gathering                    Acceptance Angle, a
  ability of a fiber

       NA = sina
                                                        Critical Angle, c
       Numerical Aperture (NA)

Acceptance /     Emission Cone

    NA      =    sin     =      n2core -
 Principles of Operation - Acceptance
• There is an imaginary cone of acceptance with an angle
• The light that enters the fiber at angles within the
  acceptance cone are guided down the fiber core

     Acceptance Angle, a

  Acceptance Cone
• For example, a typical silica fibre has n1=1.48 and n2 =1.45
  giving an NA of 0.3.
• For a „large‟ (extended) source, such as an LED, which also
  emit light over a wide range of angles, the product of the NA
  and the fibre entrance aperture area determines the fraction of
  the LED output light that can be coupled into the LED. Normally
  this fraction is small.


• A laser is effectively a very small source (it is said to be
  spatially coherent) and can be matched to the fibre to give high
  power coupling efficiency
    Principles of Operation - Formula
• Index of Refraction   n
  Snell‟s Law           n1 sin 1  n2 sin  2

                                   n2 
  Critical Angle         c  sin  
                                  n 
                                   1

  Acceptance Angle                 
                        a  sin 1 n12  n2
  Numerical Aperture     NA  sin a  n12  n2
   Basic Step-Index (SI) Fiber Design
• Most common designs:         100/140 or 200/280 m
• Plastic optical fiber (POF): 0.1 - 3 mm , core 80 to


Index (n)                                    Primary coating
                  100 m
   1.480                                     (e.g., soft plasti
                  140 m
                              Diameter (r)
Representative Fiber Parameter Values
                Fiber Types

SM step index    MM step index MM graded index
       Multi-mode (Step-index), Graded
             Index, Single Mode
• Cross sectional views ( should be circles*)
  Multi-mode        Graded Index         Single Mode

             ~80m                                                                ~10m

  Accurate alignment less needed      Accurate alignment less needed
  for splicing. Higher loss. Major    for splicing. Higher loss. Reduced       Accurate alignment needed
  time dispersion of short optical    dispersion due to lower wave speed       for splicing. Best low loss.
  pulses due to different geometric   in central rays, higher wave speed       Most widely used fiber type
  paths. Less used today, but         (lower index) in outer part of core.     for long spans.
  historically important.             Used for “last mile” and service drops
                                      with single mode for long runs.
                                                                               *non-circularity is an
                                                                               artifact of computer
                                                                               artwork software    .
  Mechanical structure of single-mode
and multimode step/graded index fibers

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