Recent progress in passively stabilized single-frequency Brillouin fiber lasers with doubly-resonant cavities

Single longitudinal mode (SLM) fiber lasers have attracted interest due to many applications in optical fiber sensing and fiber optical communication. Among SLM fiber lasers, Brillouin lasers with doubly-resonant cavity demonstrate low threshold, low intensity noise and perfect spectral performance. Such lasers employ a short ring cavity that has to be simultaneously resonant for the pump and Stokes waves. For stabilization of such lasers against environment noise two tasks have to be solved: 1) to lock the generated pump radiation to the frequency of one of the cavity modes; 2) to adjust the ring cavity length such that the Brillouin frequency shift for the locked pump frequency be exactly equal to an integer number of the ring cavity mode spacing.Here we report two recent and completely passive solutions enabling perfect stabilization of the Brillouin lasers with doubly-resonant cavities. In our experiments (Fig.1), the first laser configuration is stabilized through self-injection locking mechanism [1] implemented to the laser cavity with DFB laser diode used as a pump. The second configuration employs a nonlinear narrow-band mirror based on population inversion dynamical gratings induced in low-absorbed Er-doped fiber [2]. In both cases, once the pump laser gets a resonance with the ring laser cavity mode, the growing optical feedback to the DFB diode in the first case or nonlinear mirror reflectivity in the second case push the pump laser to operate the ring resonance frequency with the linewidth -500Hz. After such locking any slow detuning of the cavity resonance caused, for example, by temperature variations is compensated by the corresponding detuning of the pump operating wavelength. In order to adapt the cavity for operation in the second resonance, i.e. in resonance with the Stokes wave, the ring cavity length was adjusted employing the algorithm recently reported in Ref. [3]. After this adjustment, the pump wave radiation becomes locked to the ring cavity mode and causes efficient generation of a single Brillouin cavity mode wave.Pump-to-Stokes conversion efficiency up to -40% and Stokes linewidths <;500Hz and <; 100Hz measured through self-heterodyne technique have been successfully achieved in the experiment with first and second laser configuration, respectively. Dynamical Stokes power behaviors are similar in both configurations. Stable power generation is observed during time intervals of several seconds and could be interrupted by short-time jumping provoked by extended external perturbations. The physical mechanism behind such lasers behavior, the ways of further system stabilization and corresponding results will be discussed on the presentation. This research was supported by the European FEDER-Wallonia “Mediatic” and FP7 IRSES projects, the Interuniversity Attraction Pole program VII/35 of the Belgian Science Policy and programs of Russian Federal Agency on Science and Innovation.