Passive harmonic mode locking in soliton fiber lasers

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Abstract
We present an experimental and theoretical study of passively mode-locked fiber soliton lasers. Our theoretical analysis based on perturbation theory describes the soliton interactions that occur when pulse bunches form. Our results indicate that the nonsoliton component emitted by the propagating solitons causes small changes of the central frequency of individual solitons, and the strength and the sign of this interaction between the soliton and dispersive waves depend on their mutual phase as well as on the soliton position within the soliton bunch. For a certain phase difference between the solitons and the nonsoliton 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 into their temporal positions by acoustic effects. We also demonstrate that the laser performance could be further improved by the use of a multiple-quantum-well saturable absorber in combination with a nonlinear amplifying loop mirror. In this instance the multiple-quantum-well 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.