Research On Optical Fiber Chirped Pulse Amplification Technology Based On Spectral Shaping Of Liquid Crystal Spatial Light Modulator

With the continuous development of femtosecond lasers and related technologies,femtosecond lasers play an important role in many fields such as medicine,fine processing,and cutting-edge scientific research.Among them,the femtosecond laser has extremely high peak power and low thermal effect,which has brought revolutionary changes to the field of fine processing.At present,the industrialized femtosecond laser mainly includes three types: fiber laser,slat laser and disc laser.Femtosecond fiber laser has been widely used in fine material processing industry due to its good beam quality and stability.Industrialized fiber femtosecond laser mainly realizes its high power through chirped pulse amplification system.However,the extremely high fiber amplification gain will lead to serious gain narrowing effect and limit the final compression pulse width.Therefore,it is of great significance to study the gain narrowing problem and its suppression technology for increasing the peak power of laser and achieving better processing effect.In order to solve the gain narrowing problem,shorten the pulse width,improve the peak power,and achieve better "cold" machining effect,many solutions have been proposed to eliminate the gain narrowing effect,including self-phase modulation,new amplification gain medium,polarized light modulation,dielectric film filter,liquid crystal spatial light modulator spectral modulation and so on.Compared with other solutions,liquid crystal spatial light modulator has the advantages of simple structure,strong programmable ability,dynamic optimization gain narrowing of multiple amplification systems or under different output power of the same amplification system.At the same time,it also has a high precision of adjustment,which can be adjusted at any position of the spectrum to achieve arbitrary filtering.Based on the above-mentioned advantages of the liquid crystal spatial light modulator,this paper proposes a fiber femtosecond laser spectral shaping scheme based on the liquid crystal spatial light modulator to solve the problem of gain narrowing.The required grayscale image is obtained by theoretical simulation of the chirped pulse amplification system of the femtosecond fiber laser,and the grayscale image obtained by the theoretical calculation is loaded on the spatial light modulator to generate the central depression,flat top,and so on.The special spectrum shape matched by the subsequent amplification system makes the amplified spectrum width consistent with the seed source spectrum width,or even wider,to achieve the purpose of increasing the peak power after compression.The main progress and research results of the paper are as follows: 1.By calculating the one-to-one correspondence between the different positions of the gray image and the spectral wavelength,the initial spectrum can be shaped into multiple spectra such as central depression,triangle,flat top,and multi-wavelength tuning.From the shape,it can be seen that this spectral shaping system has good programmability and adjustment accuracy.2.Couple the signal light after spectral shaping into the subsequent amplification and compression system,and compare the spectral width and pulse width before and after the shaping.Experimental results show: for the existing two-stage all-fiber amplifier system,the length is 1m,the core diameter is 10?m,the length is 2m,the core diameter is 25?m,and the spectral width is increased from 7nm to 9.5nm,which is compared with the system without spectrum modulation,the limit pulse width is reduced from 222 fs to about 164 fs.Finally,through transmission grating compression,a femtosecond laser output with an average power of 1.2W is obtained.After the autocorrelator performs Gaussian fitting,the pulse width is 170 fs,which is close to the transformation limit pulse.Finally,by adjusting the current of the semiconductor pump laser,the entire system can obtain different output powers,simulating different the amplification system,obtaining different degrees of gain narrowing,and changing the parameters to obtain different grayscale images for spectral shaping.The final experimental results show that the pulse width approximating to the transformation limit can be obtained by Gaussian fitting.