Experimental analysis on quasi-mode-locked regimes of fiber laser

Abstract

In this dissertation, quasi-mode-locked (QML) regimes of fiber laser are experimentally investigated. The study focuses on four subjects: (1)The shot-to-shot coherence, (2)the wave-packet formation, i.e., noise-like pulse (NLP) packet formation, (3)the cascaded pulse interaction and (4)the super rogue wave triggering via dispersive wave synchronization. It is important to note that the subject (1) and the subject (2) are strongly correlated by the nonlinear phase shift accumulated and the degree of soliton interaction. The subject (3) triggers the super rogue wave in the subject (4) by accumulation of extreme events through the cascaded pulse interaction explained in the subject. In the introduction, I explain the principles and brief history of fiber optics, fiber laser, mode-locking, passively mode-locked fiber laser and QML fiber laser as groundwork for research presented in later sections. I provide motivation and scope of the dissertation to emphasize the novelty and set boundaries of my research. In the first part of the dissertation, I carry out extensive experimental analysis on the shot-to-shot coherence and the wave-packet formation in QML regimes, including NLP, symbiotic, and multi-soliton (MS) regimes in an anomalous-dispersion fiber ring cavity. In order to overcome the limitation of classical techniques in real-time measurement, I utilize the spatio-temporal analysis for shot-to-shot measurement of stochastically natured QML regimes. Each QML regime exhibits significantly distinct coherence characteristics, depending not only on the amount of nonlinear phase shift (NPS) accumulated per roundtrip but also on the degree of soliton interaction, the latter of which crucially governs the bunching (i.e., the wave-packet formation) or anti-bunching mechanisms in the corresponding QML regimes. It is clear that solitons with higher intensities tend to undergo higher NPS and stronger soliton interactions. Subsequently, the intensified soliton interactions among the individual solitons in the MS regime cavity trigger them to form a bunched soliton-group, i.e., a wave packet, thereby resulting in QML pulses in the noise-like pulse or symbiotic regime. The complicated nonlinear process, in turn, causes a severe degradation in the shot-to-shot coherence of the resultant QML pulses and shows clearly distinct characteristics among each regime. The shot-to-shot coherence trends observed in the experiment are in good agreement with numerical analysis verifying the strong correlation between the shot-to-shot coherence of QML pulses and the corresponding NPS accumulated per roundtrip. In addition, such trend and formation process suggests that cavity formation into NLP, symbiotic, MS or other QML regimes are governed by the degree of soliton interaction among soliton pulses in case of anomalous dispersion regime. In the second part of the dissertation, I report intermittent burst of a super rogue wave in the breathing multi-soliton regime of an anomalous fiber ring cavity. I utilize the spatio-temporal shot-to-shot measurement technique to capture the dynamics of extreme wave formation in the cavity and intensity probability density function of pulse events in the cavity for more than 800 roundtrips to assist my analysis of every pulse event. In MS regime, where hundreds of solitary waves and dispersive waves can exhibit nonlinear dynamics due to complex pulse-to-pulse interactions, dispersive waves can synchronize, constructively interfering, to trigger localized extreme waves nearly 10 times higher than the average solitons observed in the cavity. I trigger the process by increasing the soliton population inside of the cavity to stimulate the pulse interaction probability. The synchronization of dispersive waves intensifies and localizes extreme events until it reaches the maximum point and burst into super rogue wave, which quickly breaks and returns to the initial state with the least number of extreme events. Without any cavity modification or external control, the process naturally repeats in ten seconds order, roughly analogous to sub-hundred years in the open ocean considering the speed of tidal waves. The phenomenon demonstrates the optical analogy of the hundred-year wave observed in the ocean. Experimental analysis presented here regarding the shot-to-shot coherence, wave-packet formation, and intermittent super rogue wave triggering are pioneering work in the realm of the QML regimes of fiber laser. I believe the study carried out in this dissertation should function as a cornerstone for further study of pulse dynamics in QML fiber lasers and extreme wave generation not only in the optical domain but also in other disciplines such as oceanography.