Suppression of Carrier-Phase Random-Walk in Harmonically Mode-Locked Fiber Laser Through Kelly Sidebands Enhancement

Harmonically mode-locking (HML) suffers a unique type of noise, known as supermode noise, which results from statistical independence of individual pulses in the cavity imposed by photon noise fluctuations [1]. Previous studies on supermode noise mainly focus on the resultant pulse timing jitter, while the instabilities of pulse carrier-phases have rarely been studied in experiments. Using HML based on optoacoustic mode-locking [2], we investigate in this work the supermode carrier-phase noise in a HML soliton fiber laser by studying the relative phases between adjacent intra-cavity pulses that evolve in the time domain. The measured temporal evolution exhibits uncorrelated random-walk trajectories, while the averaged random-walk rate decreased dramatically with enhanced Kelly sidebands (KS). The results suggested that the KS may, surprisingly, serve as a stabilizing effect for the carrier-phases of HML soliton lasers rather than a source of instabilities.