Locking of laser modes interacting with a split level in a resonant medium

An analysis is made of the interaction between modes in a laser with a nonlinear filter in the form of a two-level medium in which the lower energy level is split. It is shown that mode locking is possible under resonance conditions when the splitting frequency coincides with any one of the intermode beat frequencies. The mode locking mechanism differs from that in the case ofsaturable absorption and it imposes no conditions on the rate of relaxation of the difference between the populations.