Harmonic Mode-Locking in All-PM Er-Doped Fiber Laser NALM Configuration

We present, to the best of our knowledge, the first demonstration of a sub-GHz harmonically mode-locked (HML) Er-doped fiber laser based on a nonlinear amplified loop mirror (NALM). This fully spliced, polarization-maintaining (PM) fiber configuration operates in a self-starting regime, delivering high-frequency pulse trains with significantly enhanced stability against environmental perturbations. The laser achieves a maximum pulse repetition rate (PRR) of ~630 MHz, corresponding to the 242nd cavity harmonic. Furthermore, we apply a supermode noise mitigation technique using continuous-wave injection from an external narrow-band source successfully stabilizing the HML operation and reducing timing jitter by more than a factor of two across the entire range of observed repetition rates. This novel design provides a robust, adjustment-free solution for high-frequency operation with exceptional stability, meeting the demands of precision and reliability required for advanced applications.