Unveiling dissipative soliton spectral-temporal dynamics in mode-locked fiber lasers

Abstract

Solitons have attracted numerous research attention in biology, fluid dynamics, plasma physics, and photonics, which as localized wave packets in time and space. In the original studies, solitons were regarded as the solutions to integrable Hamiltonian systems, Then, the initial concept has been extended to nonlinear dissipative systems, in which the dissipative soliton arises from the balance between dispersion and nonlinearity, and between gain and loss. Mode-locked fiber lasers, as an absolutely dissipative system and efficient pulse source, have been applied to many fields, e.g., material processing, medical surgery, and optical data storage. Beyond the very evident practical importance, mode-locked lasers also exhibit complex instabilities when deviating from a steady-state or reaching stable mode-locking develops from noise. These instabilities are of particular interest in passively mode-locked fiber lasers, as they disclose a rich landscape of dissipative soliton dynamics. Further, investigating dissipative soliton dynamics can shed new insights into fundamental physics of nonlinear systems, benefiting the design and optimization of ultrafast mode-locked lasers for practical applications. In this thesis, benefiting from the powerful single-shot time-stretch dispersive Fourier transform (TS-DFT) technique, we first explore the buildup and dissociation dynamics of dissipative soliton molecules (DSM) in a net-normal dispersion mode-locked fiber laser. We experimentally observed short-time soliton molecule buildup (~ 21 μs) and an exotic soliton molecule dissociation process, further corroborated by the simulation. Moreover, a long-time soliton molecule buildup of 900 ms is resolved with single soliton splitting and soliton pair attraction. Secondly, we explore the diversified breathing soliton dynamics in a bidirectional ultrafast fiber laser, such as breathing soliton explosions, breathing soliton molecule (BSM) switching, and nonequilibrium dynamics of breathing soliton pairs (BSP). Breathing soliton versatile dynamic process exists high behavior similarity that attributes to the common gain and loss modulation. Besides the stationary DSM and breathing solitons, we also explore the collision dynamics of dissipative soliton molecules in a dual-wavelength ultrafast fiber laser. The soliton molecules with central wavelengths of 1532.8 nm and 1561 nm exhibit markedly different evolution characteristics, which are attributed to the difference in gain spectral intensity and trapping potential. The different oscillating solutions coexisting in dual-wavelength soliton molecules involving oscillating and sliding phase evolution confirm the multistability of the dissipative system. In addition, the reconfigurable dynamics of optical soliton molecular complexes (SMC) was explored in a 2 μm ultrafast thulium fiber laser. We observe a periodic switching of SMC with dual-stability. Furthermore, the single and multiple switching of SMCs can be triggered by the collision of drifting solitons or the control of saturable absorption parameters. We also demonstrate continuous reversible switching with high fidelity in SMCs with periodic pump modulation. Moreover, we combined real-time coherent homodyne detection methodology and nonlinear Fourier transform (NFT) as a signal processing tool to reveal full-field dynamic evolution of multiple solitons, which as a more advanced tool than TS-DFT. With the approach of inverse NFT, the corresponding various pure solitons buildup and collision are reconstructed. Further, the controllable multiple solitons drifting is achieved and characterized by using all-optical methods.