ABSTRACT
Fiber nonlinearities fall into two general categories. The first category of nonlinearities arises from modulation of the refractive index of silica by intensity changes in the signal. The second category of nonlinearities corresponds to stimulated scattering processes, such as stimulated Brillouin scattering and stimulated Raman scattering, which are interactions between optical signals and acoustic or molecular vibrations in the fibers. Waves with different wavelengths propagating in the same fiber can interact with each other since the refractive index that a wave experiences depends on the intensities of all other waves. The efficiency of the four-wave mixing (FWM) process depends on the relative phase among the interacting optical waves. In the quantum mechanical description, FWM occurs when photons from one or more waves are annihilated and new photons are created at different frequencies. The Raman gain depends on the relative state of polarization of the pump and signal fields.
