Polarization dynamics of an NPR-based Q-switched mode-locked twisted fiber ring laser

In this work, we study the passive mode-locked and Q-switched mode-locked (QSML) operations of an erbium-doped fiber ring laser with twisted fiber in the cavity. Using twisted fiber together with precisely adjusted wave retarders, this configuration allows very tight polarization control. As pump power was increased, the mode locking regime was first observed, which was spectrally centered at 1565 nm. By increasing the cavity gain (increasing pumping power), the spectrum shifts to a wavelength of 1530 nm, where a QSML regime is observed. In the QSML regime, the pulses envelopes have a global repetition period of the order of 11 μs, whereas the internal separation period remains at 98 ns, consistent with the cavity length. The transitions between multiple laser emission regimes induced by increasing pump power allows the observation of polarization dynamics at the laser output, which is rarely studied. Distinct polarization regions were identified for each of these operating regimes.