All polarization-maintaining fiber laser architecture for robust femtosecond pulse generation

We report on a novel architecture for robust mode-locked femtosecond fiber lasers using a nonlinear optical loop mirror with all polarization-maintaining fibers. Due to a nonreciprocal phase shift, the loop mirror can be operated in a compact and efficient reflection mode, offering the possibility to reach high repetition rates and easy implementation of tuning elements. In particular, longitudinal mode spacing and carrier-envelope offset frequency may be controlled in order to operate the laser as an optical frequency comb. We demonstrate femtosecond pulse generation at three different wavelengths (1030, 1565, and 2050 nm) using Ytterbium, Erbium, and co-doped Thulium-Holmium as gain media, respectively. Robust operation is achieved for a wide range of parameters, including repetition rates from 10 to 250 MHz.