Adjustable noiselike pulses from a figure-eight fiber laser

We propose and study experimentally and numerically a passively mode-locked figure-eight fiber laser scheme generating noiselike optical pulses, or subns wave packets with a fine inner structure of subps pulses presenting random amplitudes and durations. The particular design of the nonlinear optical loop mirror (NOLM) used in this laser, relying on nonlinear polarization rotation, allows adjusting the switching power through input polarization control. Experimental results show stable pulsed operation over a limited range of the NOLM input polarization angle. Interestingly, the spectral and temporal characteristics of these pulses are observed to be widely variable over that range. In particular, the spectral width varies from 16 to 52 nm and this spectral variation is associated with an inverse evolution in the durations of the bunch and of the inner ultrashort pulses. Simulation results are in good agreement with the experiment. They confirm the strong dependence of the pulse properties on the value of the NOLM switching power, although NOLM switching is not alone responsible for the appearance of the noiselike pulsing mode. (C) 2011 Optical Society of America