Research On The Influence Of Gain Distribution In Ultrashort Laser Pulse Self-similar Amplification

Femtosecond laser has the advantages of short pulse duration,high peak power and wide coherent spectrum width,which plays an increasingly important role in industrial production and scientific research fields such as physics,chemistry,biology,etc.Fiber femtosecond laser technology,a crucial way to obtain high average power femtosecond laser pulses,becomes one of the representatives of the new generation of femtosecond laser pulses.Self-similar amplification,which is a unique technique for fiber femtosecond laser amplification,possesses notable characteristics and can output parabolic pulse with linear chirp that can be dechirped to transform-limited pulse.To this end,high quality femtosecond laser with high average power and short pulse duration can be generated,which will benefit varaities of practical applications.This thesis focuses on the self-similar femtosecond laser pulse amplification technology.A self-similar amplification numerical model is built to explore the influences of different gain distributions on the self-similar amplification processes and results of the gain fiber under different pumping schemes.Then,different characteristics of self-similar evolution under different gain distributions are analyzed.Finally,the optimization of self-similar amplification under different gain distributions is summarized.Based on the theory mentioned above,a fetmosecond laser self-similar amplification system is developed experimentally,where sub-100 fs pulse outputs are obtained under different pumping schemes.Besides,the differences of self-similar evolution processes and results under different gain distributions are compared.The main tasks of the thesis are as follows:1.Based on the nonlinear Schr(?)dinger equation,the self-similar amplification analytical theory is introduced.The characteristic of self-similar amplification pulse is described.Furthermore,two factors which dominate the self-similar evolution process are analyzed.2.A numerical model is proposed to study different gain distributions of the self-similar amplification process in Yb³⁺-doped fiber laser amplifier under different pumping schemes.The numerical results show that the evolution rate under the forward-pumped scheme is faster than that under the backward-pumped scheme when the same pulse evolves into the self-similar amplification process in the gain fiber.The self-similar region of the signal pulses under forward-pumped scheme is mainly distributed in the location of low-energy and long-wave region.In contrast,backward-pumped scheme is more suitable for the high-energy and short-wave signal pulses.Finally,it is easier to achieve the self-similar amplification process when the gain is low.3.A femtosecond laser self-similar amplification system is developed and built.To achieve different gain distributions in the gain fiber,different pumping schemes and different pump power are employed.The seed source is a home-made fiber femtosecond laser,which could produce stable pulse trains with average power of 120m W,repetition rate of 60 MHz,and pulse width of 300 fs.The experimental results present that the self-similar amplification process under the backward-pumped scheme could be realized.In this way,near transform-limited sub-100 fs high-quality pulses are produced.However,using the same signal pulse and pump condition,the self-similar amplification under the forward-pumped scheme can not be achieved.The compressed pulse width is wider than that of the transform-limited duration.This indicates that the nonlinear chirp component in the pulse can not be compensated,and the values of Strehl ratio are all below 0.85.The experimental results are consistent with the numerical simulation.