Effect of the Raman part of the nonlinear refractive index on propagation of ultrashort optical pulses in fibers

For ultrashort optical pulses propagating in silica optical fibers, the Raman contribution to the effective nonlinear refractive index decreases for pulse widths less than ~100 fsec. By numerical simulation, we show that this contribution not only decreases, but becomes negative, for pulse widths less than ~30 fs. This negative contribution can be understood in both time and frequency domains and comes from the finite time delay inherent in the Raman response. We expect that these features of the Raman contribution to the nonlinear refractive index will be especially important in high-nonlinearity glasses.