Highly stable Q-switched and mode-locked pulse generation from an all-PM figure-9 fiber laser

A highly stable figure-9 Yb-doped fiber laser with all polarization-maintaining (PM) double-cladding fiber is demonstrated. Through leveraging the saturable absorption effect of a nonlinear amplifying loop mirror, both the Q-switched and mode-locked operation are realized by adjusting the pump power. With increasing the pump power from the threshold to the maxima, the repetition rate of the Q-switched pulses is linearly increased from 14.9 kHz to 138.0 kHz with the pulse duration accordingly reduced from 3.9 µs to 970 ns. The corresponding maximum average power and pulse energy are respectively 2.34 W and 17 µJ, which are more than ten times larger than the common material-based Q-switched all-fiber lasers. In addition, in the process of increasing and decreasing the pump power, an optical bistability that manifested as a significant power jumping effect is observed, while its effect on the pulse repetition rate and duration is trivial. Whereas for the single pulse mode-locked operation, a maximum output power of 56.3 mW with a fundamental repetition rate of 12.5 MHz is realized, corresponding to a pulse energy of 4.5 nJ. To the best of our knowledge, it is much higher than the most of previous works concerning figure-9 all-PM-fiber lasers of which the emitted pulse energy is generally less than 1 nJ. After being compressed by a pair of diffraction grating, a minimum pulse width of 378 fs and a maximum peak power of 9.76 kW are respectively obtained. In addition, through characterizing the spectral and temporal properties of the laser source, the excellent stability of both the Q-switched and mode-locked operations is verified.