Observation of dynamics of harmonic order self-adaptation in optoacoustically mode-locked fiber laser

High-harmonic mode-locking based on strong optoacoustic interactions in solid-core photonic crystal fiber (PCF) has been an established mechanism to achieve a stable GHz repetition rate in a conventional soliton fiber laser, in which a uniform pulse sequence is self-locked to the acoustic core-resonance of the PCF with a specific harmonic order. However, due to the finite bandwidth of the acoustic core-resonance, there may be multiple choices of harmonic order within the resonance bandwidth that could lead to uncertainties in the resultant harmonic order. In this work, we report observations of dynamic self-adaptation of the pulses in the laser cavity towards a stable harmonic order when the cavity is initially set at a neighboring less stable harmonic order. We observed that self-adaptation can occur spontaneously towards either higher or lower harmonic order depending on the initial condition. We observed rich dynamics during the self-adaptation process namely transient pulse sliding, random pulse collisions, and pulse drop-outs. This work provides experimental insight into the robustness of the self-locked repetition rate in optoacoustic mode-locking, and we believe will be useful in understanding and controlling harmonically mode-locked lasers.