Enhanced performance of all-PM Ho-doped fiber oscillator using NPE-SESAM hybrid mode-locking

We demonstrate a 2.08 µm all-polarization-maintaining (PM) holmium-doped fiber oscillator using a hybrid mode-locking technique with nonlinear polarization evolution (NPE) and a semiconductor saturable absorber mirror (SESAM). This oscillator features a linear structure with dual output ports. It initiates stable single-pulse mode-locking at a fundamental repetition rate of 57.86 MHz, requiring only about 400 mW of pump power. However, removing SESAM requires increasing the pump power to about 2.2 W-about a 5.8-fold increase-to initiate mode-locking, which starts in a multi-pulse state. Additionally, the oscillator can operate in two distinct states by adjusting the phase bias, ensuring that the main output port delivers high-quality, soliton-like femtosecond pulses. The other port, known as the rejection port, emits lower-quality pulses with different spectral and temporal characteristics. However, in one state, the pulse quality at the rejection port is significantly enhanced, approaching the high quality of the main output port. Theoretical analysis indicates that the pulse quality at the rejection port strongly relies on the energy distribution between the two orthogonally polarized pulses in the PM fiber. Another advantage of the hybrid mode-locking configuration is that it allows a pump power tuning range to maintain single-pulse operation as high as 220 mW. It is ∼4 times higher than that of the NPE-only configuration which is about 60 mW. These results indicate that oscillators using hybrid mode-locking exhibit much greater flexibility and reliability than those using PM-NPE alone. The hybrid mode-locking technique shows potential in addressing the self-starting challenges inherent in PM-NPE lasers, thereby advancing their applicability in practical scenarios.