Passive mode locking by dissipative four-wave mixing

A nonlinear fiber with a gain that has symmetric lateral frequency bands is found to generate a stable train of bright or dark pulses with a well-defined repetition rate. The pulse train is generated from noise; the pulses are unchirped, and their amplitude and repetition rate are determined by the parameters of the system. The underlying mechanism for this pulse formation is recognized as dissipative four-wave mixing in which only two frequencies are nonnegligible; one near the maximum gain transmits energy by wave mixing to its third harmonic, which is in the region of negative gain. An implicit analytical expression for the pulse train solution is derived. It is demonstrated that dissipative four-wave mixing can be employed to yield passive mode locking in a fiber ring laser consisting of only a filter and an active fiber. The resulting train of pulses is shown to have a high soliton content.