DEPENDENCE OF CROSS-PHASE MODULATION ON CHANNEL NUMBER IN FIBER WDM SYSTEMS

The phase term appearing in the expression for cross-phase modulation due to the optical Kerr effect depends on the sum of the powers carrie d by each wavelength channel. For this reason, one might expect that t he amount of cross-phase modulation would increase with increasing cha nnel number, causing increased interference among channels and hence l imiting the total number of channels that a WDM system can support. Ho wever, computer simulations of multichannel systems have shown no chan ge in signal distortion as the number of wavelength channels is increa sed from four to eight. In a simulated three-channel system, the signa l distortion of the central channel approaches that of a single-channe l system as the wavelength separation is increased to approximately 2 nm. Thus, even a moderate amount of dispersion tends to cancel out the influence of cross-phase modulation, so that beyond a certain wavelen gth spacing, additional channels do not interfere with the channel und er consideration. From these observations, we conclude that cross-phas e modulation does not limit the number of wavelength channels that a s ingle optical fiber can support. However, self- and cross-phase modula tion are not the only nonlinear effects influencing fiber lightwave sy stems. Stimulated Raman scattering tends to transfer optical power fro m short-wavelength channels to channels operating at longer wavelength , degrading their signal-to-noise ratio. The efficiency of this proces s increases with increasing wavelength spacing. Clearly, a compromise needs to be reached between the conflicting requirements imposed by th e optical Kerr effect and by stimulated Raman scattering.