Novel figure-eight fiber laser scheme including a power-symmetric nonlinear optical loop mirror with adjustable switching power

Abstract

We propose and study experimentally a novel passively mode-locked figure-eight fiber laser scheme based on a polarization-imbalanced Nonlinear Optical Loop Mirror (NOLM). In contrast to conventional power-imbalanced structures, the NOLM used in the proposed laser relies on a difference of nonlinear polarization rotation between the counter-propagating beams to provide switching. In this experiment, the polarization state at the NOLM input is set to linear. By controlling the polarization orientation at the NOLM input through a half-wave retarder plate, it is possible to adjust the NOLM switching power. This property of the NOLM is attractive in the frame of a figure-eight laser. Firstly, the switching power can be readily set to a value ensuring stable mode-locking operation, without having to cut into the loop and modify the NOLM length. On the other hand, we observed that stable pulsed operation is maintained over a certain range of the NOLM input polarization angle, whereas the pulse properties vary over that range. In particular, the spectral width varies from 16 to 52 nm over that range. This spectral variation is associated with a variation of the pulse temporal properties. This result can be explained by the fact that the input polarization angle allows controlling the critical power of the NOLM, which in turn affects the pulses characteristics. The proposed device thus offers a convenient way to adjust the pulses properties (in particular their spectral bandwidth and duration), simply by controlling the angle of a wave retarder, a property which is attractive for some applications.