Influence of the symmetry rules for Raman susceptibility on the accuracy of nonlinear index measurements in optical fibers

A general expression for a nonlinear polarization in fibers, in which the different symmetry rules for the electronic and Raman contributions to the susceptibility are taken into account, is derived. The contribution of the Raman effect to the total nonlinearity is calculated for different polarizations of a pump and the Stokes wave in polarization-maintaining fibers as well as in fibers with randomly varying birefringence. The error in the measurements of the nz coefficient caused by the different symmetry rules for the Raman and electronic contributions is shown to depend on germanium concentration and a frequency shift varying from 3.3% in polarization-maintaining fibers to 10% in fibers with random birefringence. The ratio between cross-phase modulation and self-modulation strengths strongly depends on a frequency shift and the polarization state of the interacting waves and varies from 1.2 to 2 in fibers with germanium concentration from 0 to 18 mol.%. (C) 2001 Optical Society of America.