Theoretical And Experimental Study On A Hybrid Femtosecond Fiber Laser Amplification System

Femtosecond laser features short pulse duration,broad spectral range,high peak power etc.,which make it an ideal tool for scientific research and industrial machining.With high average power,good beam quality and compact design,femtosecond fiber lasers have a rapid development.But due to the limitation of small core areas and limited gain bandwidth,it's very hard to generate high energy pulses with high quality and short duration.This thesis is dedicated to obtaining short pulse duration,high quality and high energy pulses from fiber systems.Based on the dynamics of fiber amplifiers,chirped-pulse amplification and nonlinear amplification(self-similar amplification)are studied theoretically and experimentally.Through combining advantages of both techniques,a new hybrid femtosecond fiber laser amplification system is demonstrated.This system delivers few cycle pulses with micro-Joule-level energy and high pulse quality,which could act as a good femtosecond laser source for generating ultra-high intensity(10¹² W-10¹⁵ W)femtosecond laser used in strong-filed physics.The dissertation is summarized as below:1.Based on nonlinear Schr?dinger equation with gain,a numerical model which describes actual propagation of pulses in amplifiers is built.In this model,the gain process is established by absorption/emission cross sections of Yb³⁺-doped fiber to make sure real gain shaping effect is considerd in amplification,which gives a more comprehensive and accurate amplification process in fibers.2.Self-similar amplification of picosecond pulses in short gain fiber is systematically studied.Influences of amplifier-and seed-pulse-parameters on self-similar amplification of picosecond pulses are studied theoretically and experimentally.A self-similar amplification system based on a picosecond laser source is designed,which achieves compression of picosecond pulses and deliveres<100-fs transform-limited pulses.This technique,as a method of pulse compression,not only simplifies experimental setups and improves stabilities of the system,but also provides a solution to achieve high-quality and short-duration femtosecond laser.3.A nonlinear chirped-pulse amplification system based on third-order dispersion(TOD)pre-compensation is proposed.This pre-compensation process is demonstrated by analytical derivation,and influence of different pulse shapes is also studied with numerical simulation.Furthermore,steepness and modulation of seed spectra are studied to understand their impacts on the pulses.A nonlinear chirped-pulse amplification system with negative TOD fiber is designed and demonstrated,and up to 10-?J pulses are generated.More than 90%pulse energy is concentrated at the main pulse,and the pulse duration is 280 fs.4.According to the research above,a hybrid amplification system is firstly proposed based on chirped-pulse amplification and nonlinear amplification.Pulse pre-shaping technique is used to optimize dispersion,nonliearity and gain in the system.With this hybrid structure,the system delivers micro-Joule-level pulses with few-cycle transform-limited pulse duration.The photo-damage on cells through nano-rods is studied with this system,which chould provides a basis for fiber femtosecond laser using in biological and chemical applifications.