Limitations of the linear Raman gain approximation in modeling broadband nonlinear propagation in optical fibers.

We consider the accuracy of modeling ultrashort pulse propagation and supercontinuum generation in optical fibers based on the assumption of a material Raman response that varies linearly with frequency. Numerical simulations in silica fiber using the linear Raman gain approximation are compared with simulations using the full Raman response, and differences in the spectral, temporal and stability characteristics are considered. A major finding is that for conditions typical of many experiments, although the input pulses may satisfy the criteria where the linear gain approximation is valid, the subsequent evolution and breakup of the input pulse can rapidly lead to a situation where the linear model leads to severe inaccuracies. Numerical artifacts within the linear model inducing unphysical pulse collapse are also identified.