Two regimes of widely tuneable noise-like pulses from a figure-eight fibre laser

In this paper we study a dispersion-managed figure-eight fibre laser generating noise-like pulses with adjustable characteristics. Non-self-starting mode locking leads to the formation of a single noise-like pulse circulating in the cavity. Both the duration of the pulse and its spectral width can be adjusted by tuning the angle of wave retarders, in particular a half-wave retarder that controls the switching power of the polarization-imbalanced nonlinear optical loop mirror that is used as mode locker. Wave retarder tuning also allows observing an abrupt transition between two clearly distinct noise-like pulse regimes, one characterized by a long (> 1 ns) rectangular pulse envelope with a narrow spectrum and the other characterized by shorter sub-ns bell-shaped pulses whose Raman-enhanced spectrum extends far beyond the doped fibre gain spectrum. The existence of two distinct noise-like pulsing modes can be understood in terms of the periodic variation of the pulse spectrum along the cavity, which is able to shift the effective dispersion regime of the laser. By joining the tuning ranges of each regime, the noise-like pulse duration can be adjusted between 57 ps and 6.3 ns, and its bandwidth between 3.5 and 59 nm. (paper)