Transient Raman gain stimulated by a noise pump

Abstract

Abstract We predict a strong transient Raman gain stimulated by a Gaussian noise field in dispersive media. Our theory is based on the method of successive approximations, the first of which is the Markovian approximation. It is shown that the transient behavior will take place near critical pump intensity both for “short” ( τ p > T 2 ) and “ultrashort” ( τ p T 2 ) laser pulses (τ p is the pump pulse duration, T 2 is the relaxation time of optical phonons). Solutions for average intensities of the Stokes wave and optical phonons are obtained for arbitrary form of the given pump pulse. We show that transient Raman gain allows to form a very steep leading edge of the Stokes pulse. The steady-state increments of the second approximation describe the effects of a strong noise field. We show that after the intensity of the pump has exceeded a value beyond which the framework of validity of the Markovian approximation does not hold anymore, the steady-state Raman gain is saturated. We give a review of experimental results and make estimations for conditions of observing transient scattering in the field of a noise pump. Methods of obtaining the powerful femtosecond pulses based on stimulated Raman scattering of broad-band excimer lasers and supercontinuum emission are suggested.