Evolution from Q-switch to harmonic waves in an NPR/PbS-based hybrid mode-locked fiber laser

A passively mode-locked fiber laser is constructed by combining a PbS quantum dot saturable absorber with nonlinear polarization rotation, enabling four switchable operating states. With the pump power maintained at 100 mW, four types of pulsed operations can be flexible switched via continuous adjustment of the intracavity polarization rotation angle within the laser cavity, namely Q-switching, Q-switched mode-locking, fundamental mode-locking, and harmonic mode-locking. For the fundamental mode-locked pulses, the optical spectrum is centered at 1560.4 nm with a 3 dB bandwidth of 5.1 nm. Pulse duration measurements yield a value of 0.893 ps. The harmonic mode-locking order is countable from 1 to 6, with repetition rates tunable from 21.24 MHz to 127.42 McHz. Across the 42 mW to 172 mW pump power range, the output pulse energy is tunable over 12.19 nJ to 19.19 nJ for Q-switching and 10.59 nJ to 18.17 nJ for Q-switched mode-locking. Our work utilizes polarization control to enable flexible generation of multiple pulsed states from a fiber laser, providing a new avenue for exploring different mode-locking regimes.