 50 um ², = gr=6x10-14 m/W and with Leff = 20 km, the threshold is reached for 0.666 W. Considering that optical systems are low-power systems in terms of optical power, single-channel systems are not affected by SBS. With optical amplifiers and single-channel systems one has to use equation 5.50 for Leff. The threshold given by equation 5.66 is defined as the threshold at which the power of the forward-scattered wave, the Stokes wave, equals the incident radiation (pump). The Raman gain coefficient at 1550 nm is approximately 6 X 10-14 m/W. This is of course much smaller than the gain coefficient of SBS, but the fundamental difference is in the bandwidth of the two processes. From Figure 5.26 the gain extends up to 15 THz (125 nms), and two waves 125 nm apart will still be coupled. Although SRS is of no particular importance in the performance of single-channel systems, in WDM systems it becomes very important. The reason for this is the wide bandwidth of the SRS. Let us consider a WDM system and one wavelength, say 20, that is at 10 THz in the Raman gain profile. All the channels with shorter wavelengths act as pumps and amplify the wavelength at 20, losing photons (power) in the process. The SRS process can be used for optical amplification, and the ampli- fiers using it are referred to as Raman amplifiers. In a WDM system the SRS can lead performance impairments if care is not taken. Figure 5.27 illustrates the effect of SRS in four-channel WDM system. It is clear that SRS cannot be avoided in WDM systems, and the way to handle it is to accept impairment in system performance and design with this as a constraint. If this approach is acceptable, then the loss of power to the channel of the shortest wavelength is the constraint. After some assumptions, such as that (1) the channel spacing is equal, (2) the power in each channel is the same, and (3) all channels fall within the Raman gain Nonlinear Optical Effects in Fibers Optical Optical Amplifier Amplifier Fig. 5.27 Impacts of SRS in a four-channel WDM system