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Optical Networks - _5_6 Crosstalk_61

Optical Networks - _5_6 Crosstalk_61 - 304 Transmission...

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304 Transmission System Engineering 10% 10% 90% 90% Loop filter λ l λ l Data channels Data channels Figure 5.8 Optical automatic gain control circuit for an optical amplifier. loop is encountered with amplifiers in the loop, and the total gain in the loop is comparable to the total loss in the loop, the loop may begin to lase. The effect here is similar to the optical automatic gain control circuit that we discussed in Section 5.5.5, but in this case lasing loops can cause power to be taken away from live channels and distributed to the channel that is lasing—a highly undesirable attribute. Note that this phenomenon may occur even if the loop is closed only for a single wavelength and not closed for the other wavelengths. Lasing loops are particularly significant problems in ring networks (which are inherently closed loops!) with optical add/drop multiplexers. In this case, even the amplified spontaneous emission traveling around the ring may be sufficient to cause the ring to lase. We can deal with lasing loops in a few different ways. The preferred safe method is to ensure that the amplifier gain is always slightly lower than the loss being compensated for. The trade-off is that this would result in a small degradation of the signal-to-noise ratio. Another possibility is to ensure that closed loops never occur during operation of the system. For example, we could break a ring at a certain point and terminate all the wavelengths. Note, however, that it may not be sufficient to ensure loop freedom just under normal operation. We would not want a service person making a wrong fiber connection in the field to take down the entire network. Therefore we need to make sure that loops aren’t created even in the presence of human errors—not an easy problem to solve. 5.6 Crosstalk Crosstalk is the general term given to the effect of other signals on the desired sig- nal. Almost every component in a WDM system introduces crosstalk of some form
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5.6 Crosstalk 305 or another. The components include filters, wavelength multiplexers/demultiplexers, switches, semiconductor optical amplifiers, and the fiber itself (by way of nonlin- earities). Two forms of crosstalk arise in WDM systems: interchannel crosstalk and intrachannel crosstalk . The first case is when the crosstalk signal is at a wave- length sufficiently different from the desired signal’s wavelength that the difference is larger than the receiver’s electrical bandwidth. This form of crosstalk is called interchannel crosstalk. Interchannel crosstalk can also occur through more indirect interactions, for example, if one channel affects the gain seen by another channel, as with nonlinearities (Section 5.8). The second case is when the crosstalk signal is at the same wavelength as that of the desired signal or sufficiently close to it that the difference in wavelengths is within the receiver’s electrical bandwidth. This form of crosstalk is called intrachannel crosstalk or, sometimes, coherent crosstalk. Intra-
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Optical Networks - _5_6 Crosstalk_61 - 304 Transmission...

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