The effect of pulse walk-off on the amplitude and phase modulation in a quadratically nonlinear medium

Abstract

During the past several years it has been shown that cascading of quadratic nonlinearities, i.e. the consecutive up-and down-conversion of an optical field, may have a realistic potential in all-optical signal processing as a supplement or an alternative to cubic nonlinear effects [1,2]. Potentially reduced power requirements and interesting peculiarities have led to various device concepts [2-5] where some of them have been already implemented. Major differences to the cubic scenario consist in the unavoidable coexistence of amplitude and phase modulation where the latter one can be almost homogeneous across a short pulse [2,3]. This is advantageous in comparison to self-phase modulation in cubic materials which generates a strong chirp. The primary disadvantage of cascading is the pulse walk-off due to the different group velocities of the participating waves. In particular, this effect is intruding in fairly long waveguide configurations in which all-optical devices are most likely supposed to be realized. Until now, most experiments were carried out in thin bulk samples where the walk-off can be neglected. Thus, the continuos wave (CW) approximation with the time being a mere parameter could be applied in modelling these experiments.