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where N is the number of signals. So if a DWDM system is transporting 16 channels, the total number of potentially destructive harmonics created would be 1,920. Four-wave mixing is directly related to DWDM. In DWDM fiber systems, multiple simultaneous optical signals are transmitted across an optical span. They are separated on an ITU-blessed standard transmission grid by as much as 100 GHz (although most manufacturers today have reduced that to 50 GHz or better). This separation ensures that they do not interfere with each other. Consider now the effect of dispersion-shifted fiber on DWDM systems. In DSF, signal transmission is moved to the 1,550-nm band to ensure that dispersion and loss are both minimized within the same window. However, minimal dispersion has a rather severe, unintended consequence when it occurs in concert with DWDM: because it reduces dispersion to near zero, it also prevents multichannel systems from existing because it does not provide proper channel spacing. Four-wave mixing, then, becomes a serious problem. Several things can reduce the impact of FWM. As the dispersion in the fiber drops, the degree of four-wave mixing increases dramatically. In fact, it is worst at the zero dispersion point. Thus, the intentional inclusion of a small amount of chromatic dispersion actually helps to reduce the effects of FWM. For this reason, fiber manufacturers sell non-zero dispersion-shifted fiber, which moves the dispersion point to a point near the zero point, thus ensuring that a small amount of dispersion creeps in to protect against FWM problems. Another factor that can minimize the impact of FWM is to widen the spacing between DWDM channels. This, of course, reduces the efficiency of the fiber by reducing the total number of available channels and is therefore not a popular solution, particularly because the trend in the industry is to move toward narrower channel spacing as a way to increase the total number of available channels. Already several vendors have announced spacing as narrow as 5 GHz. Finally, large effective area fibers tend to suffer less from the effects of FWM. Intermodulation Effects In the same way that cross-phase modulation results from interference between multiple simultaneous signals, intermodulation causes secondary frequencies to be created that are crossproducts of the original signals being transmitted. Large effective area fibers can alleviate the symptoms of intermodulation.
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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Scattering within the silica matrix causes the second major impairment phenomenon. Two significant nonlinearities result: Stimulated Brillouin Scattering (SBS) and Stimulated Raman Scattering (SRS). Stimulated Brillouin Scattering (SBS) SBS is a power-related phenomenon. As long as the power level of a transmitted optical signal remains below a certain threshold, usually on the order of 3 milliwatts, SBS is not a problem. The threshold is directly proportional to the fiber s effective area, and because dispersion-shifted fibers typically have smaller effective areas, they have lower thresholds. The threshold is also proportional to the width of the originating laser pulse: as the pulse gets wider, the threshold goes up. Thus, steps are often taken through a variety of techniques to artificially broaden the laser pulse. This can raise the threshold significantly, to as high as 40 milliwatts. SBS is caused by the interaction of the optical signal moving down the fiber with the acoustic vibration of the silica matrix that makes up the fiber. As the silica matrix resonates, it causes some of the signal to be reflected back toward the source of the signal, resulting in noise, signal degradation, and a reduction of overall bit rate in the system. As the power of the signal increases beyond the threshold, more of the signal is reflected, resulting in a multiplication of the initial problem. It is interesting to note that two forms of Brillouin Scattering are actually available. When (sorry, a little more physics) electric fields that oscillate in time within an optical fiber interact with the natural acoustic resonance of the fiber material itself, the result is a tendency to backscatter light as it passes through the material; this is called Brillouin Scattering. If, however, the electric fields are caused by the optical signal itself, the signal is seen to cause the phenomenon; this is called Stimulated Brillouin Scattering. To summarize: because of backscattering, SBS reduces the amount of light that actually reaches the receiver and causes noise impairments. The problem increases quickly above the threshold and has a more deleterious impact on longer wavelengths of light. One additional fact: in-line optical amplifiers such as erbium-doped fiber amplifiers (EDFAs) add to the problem significantly. If four optical amplifiers are present along an optical span, the threshold will drop by a factor of four. Solutions to SBS include the use of wider-pulse lasers and larger effective area fibers.
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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