In-situ aligned all-polarization-maintaining Er-doped fiber laser mode-locked by a nonlinear amplifying loop mirror

Despite the wide applications for high-repetition-rate mode-locked fiber lasers, challenges persist in shortening the cavity length and coupling the fiber collimators for most existing techniques. Here, we introduce a novel collimator alignment method and demonstrate an all-polarization-maintaining erbium-doped fiber laser that contains a nonlinear amplifying loop mirror with a repetition rate of 213 MHz. Compared to the conventional method, we achieve in-situ alignment of the collimators in a simplified two-step process. Besides, through a comparison of the spectra from the output ports of the laser, we assess their quality and establish the spectral evolution relationships among these ports. It is found that, in addition to the widely believed large nonlinear effects, spectral interference also plays a significant role in spectral distortion. Moreover, a transition between different stability states is observed from the power variation of the single pulse.