Passive harmonic mode-locking in soliton fibre lasers

Abstract

We present an experimental and theoretical study of passively mode-locked fibre soliton lasers. Our theoretical analysis based on perturbation theory describes the soliton interactions which occur when pulse bunches form. Our results indicate that the non-soliton component emitted by the propagating solitons causes small changes of the central frequency of individual solitons and the strength and sign of this interaction between the soliton and dispersive waves depends on their mutual phase as well as on the soliton position within the soliton bunch. For a certain phase difference between the solitons and non-soliton component the interaction force becomes repulsive for all solitons within a soliton bunch and results in an almost uniform distribution of the pulses inside the laser cavity. The pulses are then locked in their temporal positions by acoustic effects. We also demonstrate that the laser performance could be further improved by the use of a MQW saturable absorber in combination with the nonlinear amplifying loop mirror. In this instance the MQW sample acts not only as a fast saturable absorber but also as a passive phase modulator. We experimentally demonstrate that such a laser is capable of generating 500 fs pulses at repetition rates exceeding 2 GHz.

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