BEAM RESHAPING AND SELF-MODE-LOCKING IN NONLINEAR LASER RESONATORS

The modal structures of near-concentric laser resonators with a saturable gaussian gain and a Kerr nonlinearity are investigated numerically. As a consequence of self-focussing, the output power from unstable resonators tends to increase as a function of on-axis nonlinear phase-shift, and their beams are more concentrated at the center of the gain medium; on the other hand, stable resonators lead to larger beams and, in most cases, lower output power. Self-mode-locking should take place in unstable resonators and in some stable resonators with small beam waists, as they favor the guild-up of intense signals. Bistable behaviour is predicted when the pumping rate is modulated about the threshold value for laser operation.