Optical Amplifiers V3.1

Document Sample
Optical Amplifiers V3.1 Powered By Docstoc
					   Katherine Griffin
   Jeremy Kaznocha
   Karam Saab
   Optical Amplifiers
I) Introduction
        Fiber optic communication systems fall into two broad categories; those that cover a local area, and
those that span vast distances. With respect to 3R (Reshaping, Retransmission, Regeneration), different
strategies for transmission and amplification are required depending which is the case. This report will
concentrate on the latter case of amplification over long distances in juxtaposition with the requirements of
smaller networks. More specifically this study will focus on the technology specifically behind amplification.
Within this field, this study will investigate why fully optical networks are the preferred choice for long haul
systems, but hybrid systems are a better choice for short-range systems.
   A) Thesis Statement
        This study will focus on the technology specifically behind amplification. Within this field, this study
will investigate why fully optical networks are the preferred choice for long haul systems, but hybrid systems
are a better choice for short-range systems.
   B) Overview of fiber optic networks
        The fundamental medium in a fiber optic network is the optical fibers themselves. These fibers are long
thin glass core surrounded by a cladding, which is used to reflect the light back into the center of the core. This
system is then surrounded by a plastic buffer to protect it from the environment and bundled in groups of
hundreds. It is the cladding that keeps all the light inside the core while it is traveling the length of the fiber,
without this total internal reflection the signal intensity would degrade very quickly. However, a 100% index of
refraction is not physically possible. This, combined with optical absorption, is the reason that the signals need
to be periodically amplified, which is most commonly done using Erbium Dope Fiber-Optic Amplifiers
(EDFAs) or Raman Amplifiers.

Figure 1
This figure illustrates the principle of
Total internal reflection, which is
Accomplished by coating a glass
Core with optical cladding
Available online at:

       1) How a semiconductor optical amplifier works
        Although EDFAs and Raman Amplifiers are the major fiber optic amplifiers, there are other
amplification technologies that need to be acknowledged. Praseodymium Doped Fluoride Fiber Amplifiers
(PDFFAs) provide operation in the 1.3m telecom window. Even though EDFAs have shifted the interest to the
1.55m telecom window, there is interest in operating in the 1.3 range because about 50Mkm of the network
worldwide was designed for operation at the 1.31m window [1].
        An alternative to fiber amplifiers is optical amplifiers. Semiconductor Optical Amplifiers (SOAs) have
many features that make them advantageous to use in all-optical functions. This would be necessary if optical
networks become developed with large-traffic-handling capabilities; however, SOAs compare unfavorably with
EDFAs when being considered for fiber optic networks. SOAs experience polarization sensitivity and exhibit
nonlinear distortions that prevent them from currently being used in practical systems. At this time there are no
optical amplifiers that compare favorably with fiber amplifiers but there is potential for Planar Waveguied
Optical Amplifiers to provide an inexpensive alternative in the future.

Figure 1
Simplified Drawing of a Semi-
Conductor Optical Amplifier
Available online at:

               (a) Description of laser diode
II) System Amplification Overviews
   A) Long haul system requirements
          1) Introduction to EDFA Amplifiers
               (a) Erbium Dope Fiber-Optic Amplifiers
        With the huge growth in dense wavelength-division multiplexing, which is the transmission of a large
number of wavelengths located in a small region of bandwidth, a device called an erbium doped fiber amplifiers
(EDFAs) have been used for amplification over the long haul. One of the benefits to EDFAs is the lack of a
need for conventional repeaters, which can be very costly. The basic workings of an EDFA consist of a section
of fiber being doped with erbium, a length of roughly three meters. Erbium has atomic energy levels, which are
excellent for amplifying light (around 1550 nanometers). A pump laser is used to inject energy into the doped
section of fiber. As the un-amplified light enters the doped region of the fiber, the erbium atoms are excited.
This causes the stored energy to be released as amplified light at the same wavelength as the un-amplified light.
This happens continually down the entire doped region. Here is a simple diagram illustrating the process:

Figure 3
Simplified illustration of
Erbium Doped Fiber Optic
Available online at:

        This technology has flourished in underwater systems due to the aforementioned lack of need for
conventional repeaters. More recently, this technology has been used on land. Some other advantages of
erbium doped fiber amplifiers are that they can be used as either in-line amplifiers or pre-amplifiers, having
gain of over 50 dB. They also have very good noise performance, and are capable of transmitting over a wide
bandwidth, as much as 80 nm. Finally, and most astonishingly, laboratory experiments have produced data
rates over five terabits per second.
                 (i) Low Noise
        The main introduction of noise in an EDFA from amplified spontaneous emissions (ASE). These are
when atoms that have been pumped into an excited state fall back into their ground state by emitting a photon
even though it has not been driven by a signal. This turns out to not be a major problem since the amount of
noise created by ASEs is very small in comparison with the intensity of the amplified signal.
                 (ii) No Cross Talk
        Cross talk occurs when two signals become coupled because the optical fibers are too close together.
The result of cross talk is that changes in the state of one signal will effect the state of signals in another fiber.
Because the EDFAs can be built into the same shielding as the optical fiber, this type of amplification is
resilient to cross talk with neighboring signals.
         2) Introduction of Raman Amplifiers
       The other major optical amplifier on the market today is called Raman optical amplifiers. This type of
amplifier differs on the fundamental level from EDFAs. Raman amplifiers employ stimulated Raman scattering
(SRS) to achieve gain. In SRS, power is taken from shorter wavelengths and cause gain in longer wavelengths.
The below diagram illustrates the effect.

Figure 2
Stimulated Raman Scattering effect
 used for singal amplification
Available online at:

              (a) Raman Amplifiers
                  (i) Any optical fiber can be amplification medium
        Raman scattering is the phenomenon when photons of light interacting with molecules in an inelastic
collision either gain or lose energy and are thereby shifted in frequency. Naturally, a Raman amplifier would
want to take advantage of the case where the photons gain energy, which is produced by exciting the particles
that are causing the photons to scatter. This is accomplished by sending some pumped laser signals along the
same fiber as the message signal. The energy in the pumped laser then gets transferred into the message signal
through the very silica fiber being used to transmit the signals over long distances.
                (ii) Work over higher frequencies than EDFA Amplifiers
        The main advantage of Raman Amplifiers over EDFAs is that an ordinary optical fiber can be used to
amplify signals ranging from 1270 nm to 1670 nm, which extends beyond the range of signals that EDFAs can
amplify. This allows for a significantly large portion of bandwidth to be harnessed.
III) Other Amplification Technologies
   A) Specific Amplifiers
         1) Nortel Mini Block Optical Amplifiers
       One specific optical amplifier is the Nortel Networks Mini Block Optical Amplifier. This amplifier was
designed for many possible amplification scenarios, ranging from ultra-long haul to short metropolitan links.
The unit itself is very small, measuring only 9x7cm in footprint, and does not consume much power. The
output power of the unit can be up to +17dBm.
             Full data sheet available at:   http://www126.nortelnetworks.com/datasheets/pdf/mb_mini_block_optical_amp.pdf

This amplifier works over the wavelength range of 1529 to 1562 nanometers. This was expected due to the
range which EDFAs work around. Also the noise on this amplifier is less than 5 dB. More specific information
about this amplifier is available at the above listed website.
   B) Short Haul system requirements
      1) Wideband vs. Narrowband EDFAs
        One of the major advantages of erbium doped fiber optic amplifiers is that they can amplifier over a
range of wavelengths simultaneously. This means that if a specific bit rate is being used for transmission,
another channel (meaning a different wavelength) can be used over the same fiber at the same bit rate. In effect,
this doubles the overall bit rate of the fiber. EDFAs can handle roughly forty different wavelengths of evenly
spaced transmitted light at once. This phenomenon is called wave division multiplexing. Although they are
very powerful devices, wideband EDFAs are very expensive, making them impractical for most metropolitan
networks, instead narrow band EDFAs are used. Narrowband amplifiers can only use between two and ten
different wavelengths, making their capability much less than the wideband versions. Due to the fact that the
requirements for a narrowband EDFA is less than that of a wideband EDFA, shortcuts can be taken in the
manufacturing process, such as the laser pump does not have to be actively cooled, and gain flattening filters,
which balance the amplification of the laser over the range of wavelengths, can be omitted completely.
Through a company called Lightwaves2020, a narrowband EDFA can be purchased for roughly 1000 U.S.
IV) Specifications for amplification
  A) How long haul amplification works
       1) Technical Specifications of EDFA
           (a) Respect to 3R
       2) Technical Specifications of Raman
           (a) Respect to 3R
   B) How short haul works (brief)
      1) Why hybrid
           (a) Cable Television Networks
   C) Spectrum utilization
V) Overcapacity
  A) How did long haul amplification contribute to is?
       1) Significance of amplification
           (a) Market Forecast
       2) Speed of development
           (a) Market Growth over the upcoming five years
           (b) Percent the American Market Comprises
       3) Speed it was deployed
           (a) EDFA vs. Raman Market share
       4) Combination of factors?
VI) Work Cited
[1] J. J. Bernard, M. Renaud, "Semiconductor Optical Amplifiers" SPIE, [Online tutorial] 2001 Sep [cited
2002 Oct 23], Available HTTP: http://www.howstuffworks.com/fiber-optic.htm
[2] W. Ciciora, J. Farmer, D. Large, Modern Cable Television Technology: Video, Voice, and Data
Communications. San Francisco, CA: Morgan Kaufmann Publishers, Inc., 1999, pp. 468-500.
[3] D. A. Francis, S. P. DiJaili, J. D. Walker, "A single-chip linear optical amplifier" Optical Fiber
Communication Conference and Exhibit, 2001 Mar. 17-22, Available HTTP:
[4] C. Freudenrich, "How fiber optics work", How Stuff Works, [Online article], [cited 2002 Oct 23], Available
HTTP: http://www.howstuffworks.com/fiber-optic.htm/
[5] Force, Inc., "Fiber Optic Components" [Online document] 2002 [cited 2002 Oct 22] Available HTTP:
[6] Light Reading, Inc., "Erbium Doped-Fiber Amplifiers (EDFAs)" Light Reading, 2001 Aug 01, [cited 2002
Oct 25], Available HTTP: http://www.lightreading.com/document.asp?doc_id=3484

Shared By: