Numerical analysis of nonlinear fiber laser systems in the ultrashort pulse regime

Abstract

In this dissertation, nonlinear fiber laser systems in the sub-ps pulse regime are numerically investigated. The main research subjects of this dissertation are categorized into two groups: (1) nonlinear effects in noncavity configurations, e.g. nonlinear fiber sections pumped by ultrashort pulses and (2) nonlinear effects in cavity configurations, e.g. mode-locked fiber ring lasers. When the systems accompany large amount of nonlinearity, the nonlinear effects result in extreme spectral broadening, called supercontinuum generation (SCG) for the former case and multi-pulse operation for the latter case. By exploiting the master equations based on the nonlinear Schrödinger equation (NLSE), I numerically simulate the ultrashort-pulse evolution in the nonlinear fiber laser systems. By analyzing the simulation results, I optimize the system performance or verify unclarified phenomenon in the investigated systems. In the first two parts of the dissertation, I focus on studying the SCG in noncavity configurations. First, I numerically study the dynamics of SCG for a variety of possible combinations of photonic crystal fibers (PCFs) and ultrafast fiber laser pulses that current technologies offer. Three types of PCFs typically used in SCG and four representative types of ultrafast fiber laser pulses are considered for this combinatorial study. I numerically model and qualitatively discuss the nonlinear evolution of the pulses for their 12 combinatorial cases. I also quantitatively analyze their output spectra and organize a performance chart for them in terms of spectral bandwidth, flatness, and degree of spectral coherence. Through these works, I suggest the most viable combinations among the given PCFs and ultrafast fiber laser pulses in order to generate a target SC spectrum for various specific cases. In the second part of this dissertation, I numerically study the dynamics of SCG in a ytterbium-doped highly nonlinear PCF (HNL-PCF) in the sub-ps regime. I disucss the enhancement of the energy spectral density and the recovery of the peak power depletion in the SCG process through the fiber in comparison with the SCG based on a passive-type fiber. As a unique application of the novel characteristics of the active HNL-PCF, I also analyze the direct amplification of a SC pulse through it, showing that the incident SC pulse can be amplified by 10 dB without undergoing significant degradations in terms of spectral bandwidth and flatness. These numerical investigations on the active HNL-PCF will be helpful for opening up new opportunities for fiber-based SCG technology in the sub-ps pulse regime. In the third part of this dissertation, I focus on the nonlinear effects in cavity configurations. I numerically investigate quasi-mode-locked (QML) multi-pulse dynamics in a fiber ring laser in the anomalous dispersion (AD) regime. I show that the laser cavity can operate in five constitutively different QML regimes, depending on the saturation power of the saturable absorber element and the length of the passive fiber section that parameterize the overall nonlinearity and dispersion characteristics of the laser cavity. I classify them into the incoherent noise-like pulse (NLP), partially-coherent NLP, symbiotic, partially-coherent multi-soliton, and coherent multi-soliton regimes, accounting for their coherence and multi-pulse formation features. In particular, I numerically clarify and confirm the symbiotic regime for the first time to the best our knowledge, in which NLP and multi-solitons coexist stably in the cavity that has recently been observed experimentally. Furthermore, I analyze the shot-to-shot coherence characteristics of the individual QML regimes relative to the amount of the nonlinear-phase shift per roundtrip, and verify a strong correlation between them. I also show that the net-cavity dispersion plays a critical role in determining the multi-pulse dynamics out of the partially-coherent NLP, symbiotic, and partially-coherent multi-soliton regimes, when the cavity bears moderate nonlinearity. I quantify and visualize all those characteristics onto contour maps, which will be very useful and helpful in discussing and clarifying the complex QML dynamics. In the fourth part of this dissertation, I extend the analysis of the AD QML cavity into the normal-dispersion (ND) QML cavity. While adjusting two chosen cavity parameters: saturation power of the SA and the length of the ND passive fiber, I verify three distinctive QML operation regimes. Accounting for the temporal dynamics in three QML regimes, I classify them into the NLP, transient, and symbiotic regimes. In each regime, I quantitatively analyze the shot-to-shot stability of the QML pulse and its relation with the cavity nonlinearity by visualizing the results on contour maps. Finally, I discuss the origin of the degradation of the shot-to-shot stability in the ND QML cavity.