All-polarization-maintaining Figure-9 erbium-doped mode-locked fiber laser based on a bidirectionally pumped weak asymmetric NALM

This paper presents, for the first time, to the best of our knowledge, an all-polarization-maintaining Figure-9 erbium-doped mode-locked fiber laser utilizing a weakly asymmetric nonlinear amplifying loop mirror (NALM) with bidirectional pumping. In the constructed dispersion-managed Figure-9 erbium-doped mode-locked fiber laser, with a fixed splitting ratio of 0.5, a linear phase shift of 1.5 π , and an approximately symmetric distribution of fiber within the loop, we investigated the impact of the bidirectional pumping power ratio on the intracavity asymmetry. By precisely controlling the incremental bidirectional pumping power, while ensuring the self-starting single-pulse operation of the laser, we achieved a mode-locked pulse with a repetition rate of 103 MHz, a direct output pulse width of 284 fs, and an average output power of 58 mW, corresponding to a single-pulse energy of 0.56 nJ. To the best of our knowledge, this represents the highest single-pulse energy achieved in a fully polarization-maintaining erbium-doped Figure-9 mode-locked fiber laser solely by increasing the pump power, without the need for any additional adjustments. Moreover, we constructed a bidirectional pumped dissipative soliton type fully polarization-maintaining Figure-9 erbium-doped mode-locked fiber laser without an actual filter and investigated the changes in output pulse spectral characteristics as the splitting ratio and bidirectional pumping power varied. This study provides significant reference value for future efforts to achieve higher single-pulse energies in Figure-9 mode-locked fiber lasers.