Multiple noise-like pulsing of a figure-eight fibre laser

Abstract

In this work we study multiple noise-like pulse generation in a 320 m long passively mode-locked erbium-doped figure-eight fibre laser in the normal net dispersion regime. The nonlinear optical loop mirror (NOLM) that is used as a mode locker operates through polarization asymmetry, which allows us to control its switching power by birefringence adjustments at the NOLM input, using a half-wave retarder (HWR). Over some range of the HWR orientation, a single noise-like pulse is observed in the cavity. Its peak power is adjustable as it remains clamped to the variable switching power, and its duration varies inversely between ∼5 and ∼22 ps. Beyond the HWR position, corresponding to the longest duration, the pulse splits into several noise-like pulses. These multiple pulses usually present a walkoff, however they can be synchronized through slight birefringence adjustments, although they are not evenly spaced in time. Up to 12 simultaneous noise-like pulses were observed experimentally, with a duration of ∼2 ns. Multiple pulsing and synchronization of the pulses are interpreted in terms of mechanisms of interaction between pulses. Multiple pulsing appears to be indirectly related to the peak power limiting effect of the NOLM.