Research On Chirped Pulse Stacking Technique In High Power Laser Front-end System

The research work within this dissertation is supported by 863-804 project "The Simulation and Design of High Power Laser Front-end System" and the National Natural Science Foundation of China "Nonlinear Transmission Characteristics of High Power Chirped Stacked Pulses in Single-mode Fibers".The novel high power laser front-end system,based on the technique of Chirped Pulse Stacking(CPS),becomes an important part of the new high power laser system,which aims for the Inertial Confinement Fusion(ICF).It provides for the subsequent power amplifier system several pulses with the ability of arbitrary shaping, high energy(mili-joule level),bandwidth,high Signal-to-Noise Ratio(SNR) and high beam quality.In this dissertation,we systematically study the physical and engineering problems in the technique of CPS,including the temporal fluctuation, spectral characteristics and the stability of the stacked pulse.We also develop a set of simulation software for the front-end system(SGFSSim),which can communicate with the software for power amplifier system(SG99).Moreover,we propose a new scheme for generating square pulses theoretically,which can eliminate the coherent noise of CPS.The main research works and results are as followings:1.We present a systematic analysis on intensity fluctuation for the technique of Linearly Chirped Gaussian Pulse Stacking(LCGPS) based on spectral analysis.The results show that there are several separated frequency sidebands in the intensity spectrum of the stacked pulse,which is responsible for the intensity fluctuation.The fluctuation frequencies are discussed in detail in multiple LCGPS.In the case of multiple LCGPS with same amplitude and same timing delay,the fluctuations have a fundamental frequency sidebands centered at the frequency which is the same as that of two LCGPS,and high order frequency sidebands which are multiples of the fundamental frequency.We also find that the contributions to intensity fluctuation from high frequency sidebands would be small because of the amplitude overlapping.Generally the fluctuation frequencies increase with the increase of chirp factor and timing delay,and with the decrease of pulse width of the initial Gaussian pulse.2.Spectral characteristics of the stacked pulse in the technique of LCGPS are analyzed systematically.The results show that there are modulation structures in the spectrum of the stacked pulse.Modulation frequencies are discussed in detail.The analysis matches with the experiment results. Our spectral analysis reveals that the temporal intensity fluctuation of the stacked pulse in LCGPS cannot be reduced by applying an optical amplitude filter,but can be reduced by applying an intensity filter.3.We study the LPR in the ICF system and analyze the stability of both LCGPS and NCGPS in the LPR.The LPR can be treated as the process that the stacked pulse is first intensity filtered and then induces the plasma, due to the thermalization time of the plasma.We also discuss and compare the stability of both LCGPS and NCGPS with the thermalization time of the plasma,timing delay and intensity attenuation of the stacked pulse in the LPR.Our results show that the LCGPS is more stable than NCGPS in the LPR with the environment change in the ICF system.We believe this would be helpful for experiments of the ICF front-end systems.4.We propose a novel method for square pulse generation.We prove theoretically the power of the generated square pulse is steady and flat. We point out there would be a stable region around the certain steady power.Shapes and temporal width of the square pulse are analyzed. Multi-square pulse output is mentioned.We also study the power tunability of the square pulse and give a restriction for optical components in order to generate the square pulse.5.A new software(SGFSSim) is developed for studying the physical and engineering problems in the CPS.The design,physical models and functions of SGFSSim are fully introduced.The most important function of the software is to communicate with the software of power amplifier system(SG99).It also has the ability of setting damage threshold for every device in the high power laser front-end system,and showing SNR, waveform,and spectrum information of the lasing pulses.Highlights of the dissertation are as following: 1.The intensity fluctuation and spectral characteristics of the stacked pulse in LCGPS are analyzed systematically.The intensity fluctuation of the stacked pulse after intensity filtering is also discussed based on the spectral analysis.2.Stability of the LCGPS in LPR is studied in detail.Our results show that the LCGPS is more stable than NCGPS in the LPR process.3.A new square pulse generator is proposed based on the Nonlinear Amplifying Loop Mirror(NALM).The figure-eight square pulse generator can eliminate the coherent noise in the technique of CPS.4.SGFSSim is developed for communicating with SG99,which provides a simulation platform for the high power laser system.