Phase transition theory for passive mode locking of lasers

Summary from only given. Passive mode locking (PML) is an intrinsically nonlinear effect. Its nonlinear core is a saturable absorber, which favors pulses, since it attenuates them the least. Nevertheless, pulsation is achieved only when optical power reaches a certain threshold, so that the absorbers´ nonlinearity is felt strongly enough. This suggests that there is a mechanism in the laser that opposes mode locking, which needs to be overcome. In this work we present a novel theory for PML with a fast saturable absorber. It is based on statistical mechanics and phase transition theory on one hand, and on known rules of the dynamics in a laser undergoing PML on the other hand. We claim that effort is needed in order to achieve mode locking since it is an ordered configuration of the modes and thus rare among all possible configuration. The quantitative measure for this rareness is entropy, and it is the balance between (generalized) energy and entropy which shapes the behavior of passively mode-locked lasers.