Weak mode locking dynamics in a thulium-doped fiber laser

In this work, we study partial mode locking dynamics in a figure-eight thulium-doped fiber laser. With this particular laser design, which includes a long, highly power-imbalanced NOLM, two regimes are successively identified as pump power is raised, both characterized by a narrow-linewidth emission consisting of broad periodic pulses emerging from a dominant background radiation. In each case, peculiar dynamics are identified. In the first regime, the pulses undergo dynamical evolution at two different time scales: at the scale of hundreds of cycles, a slow quasi-periodic modulation affects both the pulse intensity and temporal position, whereas at the scale of the pulse duration, the inner details of its complex profile are found to drift under the pulse envelope. These phenomena are interpreted in terms of slow and fast gain dynamics, respectively. In the second regime, the waveform displays large Q-switched-like oscillations whose details reveal both the peak power clamping effect of the NOLM nonlinear transmission and competition between the pulse and the spurious background radiation. Finally, reducing pump power yields a quasi-continuous-wave regime in which the temporal waveform displays a self-imaging effect, as complex features in the temporal map repeat with a periodicity of 4.81 cavity cycles. Such behavior strongly contrasts with the stochasticity expected for this type of regime. These results illustrate the complex operation of long thulium-doped fiber lasers producing pulses in the 2-micron region, which are attractive for fundamental research, and also for applications such as surgery.