Breathers in mode-locked lasers based on saturable absorbers
Authors: HOU Liumin , HOU Yunlong , LIU Yuankai , LI Yuanhua , LIN Jia , CHEN Xianfeng ,   College of Mathematics and Physics, Shanghai University of Electric Power, Shanghai 201306, China,   State Key Laboratory of Advanced Optical Communication Systems and Networks, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China,   Shanghai Research Center for Quantum Sciences, Shanghai 201315, China,   Collaborative Innovation Center of Light Manipulation and Applications, Shandong Normal University, Jinan 250358, China
Published: December 2024

<sec>Breathing pulses, as a unique nonlinear pulse phenomenon, play a key role in optimizing laser performance, nonlinear optical processes, and complex signal transmission. Unlike stable solitons, the breathing pulses fluctuates in energy periodically with time, and both pulse frequency and amplitude exhibit periodic changes. Through appropriate nonlinear effects, lasers can generate stable breathing pulses, achieving a mode-locked state that exhibits a periodic “breathing” pattern. Based on this, a fiber laser combining a saturable absorber as the mode-locking element is designed and built, and stable breathing states are successfully observed at lower pump power levels. High-speed detection techniques and time-stretched dispersive Fourier transform (TS-DFT) technology are used to time-amplify and spectrally analyze the rapid pulses, while monitoring the evolution of the breathing pulse in both time domain and frequency domain. Experimental results indicate that the change in pump power significantly affects the periodic modulation induced by additional oscillations, thereby controlling the breathing ratio and ultimately resulting in the formation of a stable soliton. When the pump power is between 470 and 480 mW, the formation of the breathing pulse is first observed, with a breathing ratio of up to 4.5. As the pump power increases, the breathing effect gradually diminishes, and at 510 mW, it completely disappears, with the breathing ratio dropping to 1.</sec><sec>These results confirm the critical role of pump power in controlling the breathing pulse state and its transition, demonstrating the potential of controlling pump power in ultrafast laser technology and nonlinear optics. The breathing pulse phenomenon, as a periodic pulse behavior, reflects the complex dynamical characteristics between nonlinear optical effects and cavity parameters. Combined with the natural synchronization system formed between the breathing frequency and the cavity frequency (determined by the cavity length), the periodic change of the breathing pulse becomes a crucial factor for controlling laser output. By adjusting parameters such as the laser’s nonlinearity and dissipation, the characteristics of the breathing pulse and breathing ratio can be precisely controlled, thus achieving precise control of the laser output. The periodic oscillatory characteristics of the breathing pulse inside the laser cavity lead to the non-uniform distribution of pulses, a feature that demonstrates enormous potential in pulse shaping, ultrashort pulse generation, and precise frequency comb control. Additionally, the presence of the breathing pulse may affect the stability and energy conversion efficiency of the laser, providing new perspectives for designing and optimizing lasers.</sec>