Analyzing the fundamental properties of Raman amplification in optical fibers

The Raman response of germanosilicate fibers is presented. This includes not only the material dependence but also the relation between the spatial-mode profile of the light and the Raman response in the time and frequency domain. From the Raman-gain spectrum, information is derived related to the nonlinear refractive index due to nuclear motions and the Raman response function in the time domain. It is demonstrated that the Raman-gain coefficient may be reduced up to 60% if the signal propagates in the fundamental mode while the pump alternates between the fundamental mode and a higher order mode. A simple model shows that the time response related to the decay of phonons is significantly larger in germanate glass relative to silica glass. From the Raman gain, it is found that the contribution to the nonlinear refractive index from nuclear motions is reduced by a factor of 2 in germanate relative to silica glass.