Kerr Nonlinearity and Nonreciprocity in a Ring Laser Cavity

Abstract

The nonreciprocal effect introduced by the gain media in a Kerr-lens-modelocked laser is considered. For real nonlinear optical susceptibilities in media without gain or losses phase conjugation is reciprocal[1], the same happening in a cavity where gain is neglected. Once gain is considered, the position of the laser crystal may discriminate between the clockwise and anticlockwise beams through the self-focusing effect, similar to what happens in a nonlinear amplifying loop mirror (NALM). In the present analysis this assymmetry is attributed to the gain dependence on the position z within the laser crystal, due to pump beam depletion. The laser beam that propagates in the same direction as the pump beam acquires a larger nonlinear phase shift, compared to the counterpropagating beam, and therefore suffers a greater self-focusing effect.