NONLINEAR EFFECTS IN COHERENT MULTICHANNEL TRANSMISSION THROUGH OPTICAL FIBERS

The strength of nonlinear interactions in optical materials depends on the nonlinear susceptibility, the intensity of the incident light, and the interaction length. Although the nonlinear susceptibility in single mode fibers is very low, their small core size and their low losses lead to high intensities and long interaction lengths. Consequently, strong nonlinear interactions may occur in single mode fiber, and these set an upper limit to the power levels that can be transmitted before affecting the operation of optical transmission systems. The dominant nonlinear processes in single mode optical fibers are four wave mixing (FWM), stimulated Bril-louin scattering (SBS), and stimulated Raman scattering (SRS). In coherent frequency-division-multiplexed (FDM) communication systems, these nonlinearities lead to crosstalk between channels, power losses, and deleterious phase fluctuations, which in turn limit the power of the transmitted light and the number of allowed channels, and dictate the channel allocations. Our results indicate that for long haul transmission systems with fiber lengths exceeding 100 km, typical channel separation of 10 CHz and few channels, the maximum allowed input power per channel, Pmai, is limited by SBS to about 5 dBm. As the number of channels increases, FWM becomes the limiting process with Pmax of about - 5 dBm, whereas above several hundred channels SRS becomes dominant with Pmax of about -5 dBm. For local area networks with shorter lengths, the results are similar, except that the values of Pmax are uniformly higher by about 5 dB. © 1990 IEEE