Study Of Spectrum Breathing In Femtosecond Fiber Lasers

For the recent ten years, due to its compact structure，high stability, low cost,good beam quality, rich mode-locking mechanisms and so on, the fiber femtosecondlaser has been studied a lot and applied to micro-machining, biological imaging,medical surgeries and other cutting edges. However, as it is used in more and morefields, the requirement for quality is also being increased, such as pulse duration,spectrum width and single pulse energy. To achieve shorter pulses, wider spectrumand higher energy, researchers have studied and proposed different mode-lockingmechanisms to meet distinct needs. This dissertation is not only to explore themechanism of spectrum breathing in Yb-fiber femtosecond lasers, and its influence topulse duration, spectrum width and energy, but also to discuss the mode-lockingdynamics intra this system’s cavity, and make prospects about future work in thestudy of ultra-short pulse fiber lasers, based on numerical simulations and systematicexperiments. The main contents of this thesis include:1. The development of femtosecond lasers and the latest progress about how toobtain ultra-short pulses in fiber lasers are briefly introduced. The method forfurther pulse shortening, which is to break through the gain bandwidth limitation,is summarized. This is also the major work of this dissertation.2. Based on the nonlinear propagation of ultra-short pulses in fibers, gain as well asloss, group velocity dispersion, self-phase modulation, and the saturable absorber,the fundamental theory of spectrum breathing is established, which combinesproperties of dispersion managed soliton, self-similariton and dissipative soliton.3. The numerical model is established through Matlab, the initial parameter isdecided, and the intra-cavity dynamics mechanism is simulated. Sub-25fs pulseduration and over120nm spectrum bandwidth are obtained in stablemode-locking. Meanwhile, the evolution of nonlinear spectrum narrowing,self-similar amplifying, nonlinear spectrum broadening is analyzed, and theimpact of dissipation intensity is discussed, to further the study of spectrumbreathing mechanism.4. Based on the spectrum breathing model, a mode-locked Yb-fiber laser is built,pumped by580mW power, which outputs pulses with135mW power,58.9MHz repetition frequency,2.3nJ single pulse energy, over80dB SNR,115nmspectrum width from PBS,120nm spectrum before output, and23.5fs pulseduration after compensation by a pair of prisms. When using a20nm band-passfilter replacing the initial30nm band-pass filter, dechirped pulses with100nmspectrum and25fs duration are obtained. Nevertheless, the basement of dechirpedpulses is smaller. After comparison, it is concluded that a relatively broader filterwould contribute to shorter pulses, while the pulse quality would be influenceddue to more nonlinear chirp. This result shows excellent agreement with what thesimulation predicts, proving the feasibility of spectrum breathing mechanism.