Higher order FM mode locking for pulse-repetition-rate enhancement in actively mode-locked lasers: Theory and experiment

We present a novel higher order FM mode-locked technique for active mode-locked lasers which utilizes the higher order sidebands generated by an intracavity phase modulator to establish the mode locking. The resulting mode-locked output exhibits an enhancement of the pulse repetition rate over the modulation frequency by an integral multiple. The higher order FM mode locking is studied theoretically in a laser with a homogeneous gain medium, and simple analytical expressions are obtained to characterize the output pulses. It is shown that the scheme not only enhances the pulse repetition rate but also ensures chirp-free pulses and is effective in eliminating the output pulse phase-state instability, which are commonly observed in conventional FM mode-locked lasers. The effect of group velocity dispersion and cavity nonlinearity is also investigated through numerical solution of the self-consistency equation applied to a fiber ring laser. Finally, detailed experimental results on repetition rate enhancement in fiber lasers are presented and shown to be in good agreement with the theoretical results.