Study On Time-domain Coherent Beam Combination Of Ultra-short Pulse Based On Multi-dithering Technology

Ultrashort laser with excellent beam quality and high peak power is an important tool to explore new areas of basic science and industry.Single beam fiber amplifier can generate ultrashort laser with high output power,which has advantages of convenient thermal management and high optical-to-optical conversion efficiency.However,in further promoting peak power as well as maintaining excellent beam quality,nonlinear effects and mode instability become the limitations.By coherently combining several laser beams,coherent beam combination in time domain can produce pulses with high peak power and good beam quality,which has become a hot research area.In this thesis,the temporal coherent beam combination of ultrashort fiber lasers is studied theoretically and experimentally,which mainly focus on optical systems,phase controlling system and phase–locking algorithm of the temporal coherent beam combination.The main work of the thesis is as follows:In the first part,the background and significance of the research are introduced,the progresses of temporal coherent beam combination of ultrashort pulses at home and abroad are investigated,and the research direction is clarified.In the second part,coherent beam combination of fiber lasers is theoretically analyzed,which includes optical system and phase controlling system.In optical system,transfer characteristics of pulses in fiber and amplifier are theoretically analyzed respectively;in phase controlling system,the basic principles,composing elements and basic requirements of automatics control are analyzed.In the third part,firstly,the main active phase controlling algorithms are briefly summarized and analyzed.Based on these analyses,a mathematical model of multi-dithering algorithm for phase locking is deduced from the principle of wave optics.Secondly,the parameters that affect the stability of phase locking are studied,which take coherent beam combination with two channels as an example,and subsequently,the specific implementation operation is generalized to coherent beam combination with several channels.Finally,a method to obtain suitable parameters in multi-dithering algorithm is introduced.In the final part,the temporal coherent beam combination of ultrashort fiber lasers is experimentally analyzed,which includes optical system and phase controlling system.The optical system is consists of a mode-locked fiber oscillator,the stage of pre-amplification,the stage of pulses division,the stage of main power amplification and the stage of pulses combination.The mode-locked fiber oscillator emits ultrashort pulses with repetition rate of 14.392 MHz,which has central wavelength of 1064 nm,3-dB spectral width of 0.08 nm and pulse width of 34.5 ps respectively.The average power of the fiber oscillator is 4.25 mW.In the part of phase controlling,the phase control system based on multi-dithering algorithm is constructed,which is based on the principle of automatic control.In the time-domain coherent combining system of two-channels phase locking,adaptive parameters selection algorithm,the influence of power and power deviation on polarization state are studied.Finally,the average power of 260 mW is achieved,and the combination efficiency is 85.2%.Optimizing the factors affecting the combination efficiency can further improve the performance of the system and achieve higher average power and peak power.