THEORY OF SMALL-SCALE SELF-FOCUSING OF INTENSE LASER BEAMS IN MEDIA WITH GAIN AN D LOSS

Abstract

The Bespalov-Talanov theory is extended to include the effects of gain and loss of the medium. An analytic expression for the growth rate of the ripples is obta ined and it reveals some new properties of small-scale self-focusing. For a give n input power, the cutoff spatial frequency, the fastest growing frequency and t he maximum perturbation growth, in an amplifying medium, increase with the incre ase of the gain of the medium and propagation distance. For a given output power , as the medium gain increases, the maximum perturbation growth, and thus the B- integral, decreases, while the fastest growing frequency shifts to lower frequen cy. Also, the expression for the distance at which the filaments are formed is o btained by means of a phenomenological approach, and it is found that the gain ( loss) of the medium extends (reduces) the distance. It is thus concluded that it is an effective way to suppress the small-scale self-focusing by increasing the gain of the medium.