Stable all polarization-maintaining fiber mode-locked laser at 2 μm in both anomalous and normal dispersion regimes

We experimentally present a flexible mode-locked all-polarization-maintaining (PM) fiber oscillator at the 2 μm spectral region, relying on the nonlinear amplifying loop mirror (NALM). Stable mode-locked pulse trains under net anomalous and normal dispersions were obtained, respectively, by tailoring the length of dispersion compensating fiber. The oscillator with net anomalous dispersion delivers the femtosecond-level pulse with a repetition rate of ∼13 MHz at a central wavelength of ∼1951.14 nm. While in a net normal dispersion regime, picosecond-level bound-state mode-locked pulses centered at ∼1949.5 nm with a repetition rate of ∼9.71 MHz were obtained. Both mode-locked operations exhibit high stability with power fluctuations of less than ∼1 %. Our work simultaneously reveals the two kinds of stable mode-locked operations in a NALM-based all-PM fiber oscillator for the first time, to the best of our knowledge, which contributes to the generation of stable ultra-short pulse laser in 2 μm.