Fast Nonlinear Growth And Control In Picosecond Laser Amplification

The small-scale self-focusing(SSSF)effect is one of the main factors limiting the output capability of high-power laser systems,therefore,it has important significance to study how to effectively suppress the SSSF effect of the laser beams.How to effectively suppress the fast nonlinear growth and improve the near-field quality of the output beam in picosecond laser systems has become a hot topic of interest.Traditionally,low-pass spatial filtering techniques have been used to suppress the SSSF effects,which mainly use 4f spatial filters and volume Bragg gratings(VBGs),to remove the medium-high frequencies(MHFs)in the laser beam.Previous research has shown that the MHFs of the beam with the fastest nonlinear gain will cause the start of SSSF effects.In recent years,a band-stop filtering technology based on VBGs has been proposed,which can effectively filter out the specific MHFs in the laser beam,thereby suppressing the SSSF effect.In this paper,a band stop filtering technique based on VBGs is used to suppress the SSSF generated in picosecond laser systems,and improve the performance,beam quality and reliable of laser systems,this method proposes a new approach for picosecond laser amplification.The main research contents and results of this paper are as follows:(1)Analyzing the diffraction characteristics of VBGs.Based on coupled wave theory,the effects of different grating parameters on the peak diffraction efficiency,angular selectivity and spectral selectivity of VBGs are analyzed,introducing the grating parameters design method and the band-stop filtering principle of high-power laser systems.(2)Simulating the transmission of picosecond laser in nonlinear media.Firstly,based on B-T theory,studying the transmission characteristics of mid and high spatial frequency phase distortion in the laser beam,the results show that the highest gain of the gain spectrum curve increases significantly as the number of amplification pieces increases;Based on the nonlinear Schrodinger equation,the laser propagation amplification model is established by considering the three processes of diffraction,amplification and nonlinear self-focusing,the simulation results are analyzed using beam evaluation parameters.The results show that when the beam passes directly four pieces of neodymium glass,the spatial frequency components in the range of 0.4 mm-1～1.3 mm-1 in the output beam has the fastest nonlinear growth rate,the beam near-field modulation is increased from 1.14 to 1.26,and the accumulated B-integral of the beam reached 1.44 μrad.(3)Proposing a multi-stage laser amplification system suitable for picosecond laser amplification and simulated the transmission process of the laser beam.In order to suppress the SSSF effect in the process of picosecond laser amplification,the VBGs for high-power picosecond laser system is designed according to the determined spatial frequency range and grating parameters design method,the peak diffraction efficiency of the grating is 10%,50%and 93.6%,respectively.The effect of band-stop filtering for pulse widths of 10 picoseconds and 100 picoseconds,and the safe amplification scheme for picosecond laser are discussed respectively.