Influence of two-photon absorption on modulational instability

The instability of a plane wave in an optical medium with two-photon absorp tion is studied. The analysis is based on the modified nonlinear Schrodinge r equation. The linearized equation for the modulation is shown to have an exact solution in terms of confluent hypergeometric functions. It is found that the gain spectrum varies with position. This may result in a change of the wave dynamics and in a decrease of the repetition rate of the pulse tr ain developed from the plane wave. The application of the results to the op tical pulse propagation in semiconductor gratings and fiber gratings is dis cussed. (C) 2001 Optical Society of America.