Importance of the nonlinearity of the active medium in the mode locking in a solid-state laser

A theoretical analysis is made of the influence of the parametric interaction between laser modes on their frequency and phase. The threshold conditions for passive mode locking are determined. It is shown that passive mode locking is possible in principle under steady-state operating conditions. Estimates are made which indicate that the nonlinearity of the refractive index of the active-element matrix may be responsible for partial and, under favorable conditions, complete mode locking.