Theoretical And Experimental Researches Of Yb-doped Mode-Locked Fiber Laser And Nonlinear Fiber Amplifier

Ultrashort pulses have the characteristics of high peak power,short pulse duration and broad spectrum,which have been applied in many fields such as material processing,biological detection,medical treatment,optical communication,national defense and aerospace,etc.With the constant progress of ultrafast laser technique,ultrashort pulses are gradually becoming more and more important as light sources.Fiber lasers have been developing rapidly for its small volume,low energy consumption,high beam quality,good heat dissipation,low cost and maintenance-free.In the research about ultrafast fiber laser,how to design and build mode-locked fibers and ultrashort fiber amplifiers is always the core topic.Mode-locked fiber lasers could generate stable pulse trains whose single pulse energy and peak power are usually low while its working wavelength and spectrum width are limited by the gian bandwidth.Ultrashort pulse amplifiers are used for improving the pulse energy and peak power of the pulse trains to make them suitable for applications.During the process of amplification,if the nonlinear efftcts have no evident impact on the pulse spectrum,then the amplification can be called “linear”,and the amplified pulse could be recovered through dispersion compensation;on the other hand,if the spectrum is changed or even new spectral components are generated beacused of the nonlinear effects,then the amplification is “nonlinear”,under this circumsatance,ultrashort pulses with exotic wavelengths or supercontinuum might be obtained.This thesis focuses on the theoretical and experimental researches of Yb-doped mode-locked fibers and ultrashort pulse nonlinear amplifications.The main content includes five parts listed below.1.Introduction for the pulse propagation equation in fiber and the theoretical models of the optical elments used in ultrafast fiber lasers.The generalized nonlinear Schr?dinger equation and its variations are introduced.The mathematic descriptions for both polarization dependent and independent elements are introduced repectively.The spectrum method for solving the nonlinear Schr?dinger equation is briefly introduced.2.Mathematic simualtions of all-normal-dispersion mode-locked fiber lasers.The effects of cavity length,gain level,spectrum filtering and saturable absorber have on the mode-locking characteristics are studied.How dissipative soliton,dissipation soliton resonance and noise-like pulse are generated in the mode-locked fiber laser is simulated.3.Experimental research and theoretical analysis for Yb-doped fiber lasers mode locked by nonlinear polarization rotation(NPR)technique.The experiments include common NPR mode locking and NPR mode locking in a simplified cavity that has no wave plates and spectrum filter.The experiments of common NPR mode locking can be devided into two groups.Dissipative solitons with pulse durations of 4.6 ps and 11.7 ps are achieved respectively in the two experiment groups.In the simplified cavity NPR mode-locked fiber laser,four different cavity lengths are tested and stable self-start mode locking are all achieved.The maximum single pulse energy is about 16 nJ.Theoretical analysis for the simplified mode locking is carried out,which shows a good accordance with the experiments.4.Simulations for pulsed fiber amplifiers.A comnined model of the rate equation and the nonlinear Schr?dinger equation is introduced.Based on this model,the effects of pump method,gain fiber length and repetition rate of pulse on the output characteristics are researched.The chirp accumulation and how it affects the output characteristics are also discussed for different pump method.At last,a comparison between the chirped pulse amplification and common amplification is made.5.Experimental research and theoretical analysis of ultrashort pulse nonlinear amplification.The ampliers are seeded by the short-cavity mode-locking and the minddle-long-cavity mode-locking respectively.While the short-cavity mode locking works as seed,a broadband spectrum ranging from 1 ?m to 1.2 ?m is obtained from the nonlinear fiber amplifier.By appling a spectrum filter to the baroadband spectrum,wavelength tunable ultrashort pulses are achieved.When the middle-long-cavity modelocking works seed,a wider barodband spectrum that covers 1.05-1.57 ?m is acquired.With further experiments and simulations,the generation of the broadband spectrum is researched and the characteristics of the amplified pulse are discussed.It is proved that noise-like pulses can be generated in an Yb-doped nonlinear fiber amplifier,and by compressing the noise-like pulse,a double-scale intensity autocorrelation trace with a 3.70 ps pedestal and 14.5 fs spike is achieved.