The quasi mode-locked regime: glassy disorder and turbulent dynamics in fiber laser

Abstract

In this work, we investigate the quasi mode-locked (QML) regime in a Yb-doped mode-locked fiber laser, a new and largely unexplored operating phase where intensity fluctuations reveal complex collective behavior. Although this regime may be familiar to experimentalists working with mode-locked lasers, the QML has not been systematically studied in a fiber laser, and only recently has its importance begun to emerge. QML typically appears in the intermediate region between continuous-wave (CW) operation and the standard mode-locking (SLM). To characterize this regime, we turn to statistical analysis, which offers us powerful ways to uncover the underlying dynamics and emergent properties of the laser. By collecting realizations (replicas) of the laser output and examining their correlations, we identify signatures of glassy dynamics, including replica symmetry breaking (RSB). This situates the QML phase as a new optical platform — complementary to random lasers (RLs) — for connecting concepts from disordered magnetic systems to photonics. Beyond its glassy features, the QML regime also displays turbulence-like behavior. By employing a dynamical stochastic model (H-theory), we characterize Gaussian and non-Gaussian distributions on the intensity increments of the spectra, which reveal the presence of intermittent fluctuations and scaling behavior typical of turbulence-like dynamics. Principal component analysis (PCA) is also used to reduce the dimensionality of the data, allowing us to find hidden structures in the data and enabling a more robust identification of scaling regimes. The results presented in this dissertation provide experimental evidence of turbulence in a optical phase of a fiber laser supporting mode-locking operation. Understanding QML is not only important in itself, but also helps us to better comprehend the fully mode-locked regime, which is of central relevance for ultra-fast laser applications. Insights gained from studying QML can shed light on how pulses form, what determines their stability, and so on. Overall, this dissertation establishes the QML regime as a distinct and valuable optical phase for exploring complex collective phenomena in light. By bridging ideas from statistical physics, nonlinear dynamics, and photonics, we highlight the role of interaction in shaping laser dynamics, and we open new directions for understanding complexity in optical systems.