Investigation On The Sub-nanosecond Fourth Harmonic Generation At 266nm With SBS

UV laser at 266 nm can react with many materials in a special way.The main researches based on this feature include laser-induced fluorescence spectroscopy,fingerprint detection and DNA processing.Due to high maintenance costs,low conversion efficiency and other reasons,traditional gas-state UV lasers have gradually been replaced by solid-state lasers which is based on nonlinear frequency conversion.The subnanosecond 266 nm laser studied in this paper with a special pulse duration and high peak power can be a reliable light source for the strong interaction between light and matter.Stimulated Brillouin Scattering(SBS)technology is an attractive method to obtain sub-nanosecond laser pulses.It has the advantages of simple structure,high conversion efficiency,tunable outputs and integrability.In recent years,high-quality nonlinear optical crystals contribute to the development of harmonic generation.In this paper,the frequency conversion technology and SBS are combined to achieve the conversion from nanosecond fundamental laser at 1064 nm to sub-nanosecond 266 nm.The main research are as follows:Theoretically,the main factors affecting frequency doubling are analyzed according to the coupling wave equations.Based on phase match theory,the nonlinear crystals that may be used in the experiment are numerically simulated.The simulation includes phase match angle,the effective nonlinear coefficient and the walkoff angle.And the cut angle and the optimal length of each crystal are determined.In terms of SBS,the single-cell compression structure is numerically proved to generated nanosecond pulses with proper pump energy.Experimentally,the single-longitudinal mode was output by a oscillator based on three mirror resonance,the 1064 nm laser pulse has a energy range of 3.5~4m J,with pulse duration of 8ns.The repetition is 1Hz.Then the nanosecond pulse was injected into the SBS cell.1064 nm fundamental light with a pulse duration of 580 ps and energy from 0 to 200 m J was obtained by using FC-770 as SBS medium.Also a Gaussian distribution of energy was obtained.The 10×10×6mm3 KTP crystal(XY plane,θ=90°,φ=23.5° cut)output the green laser with energy from 0 to 50 m J corresponding to a maximum conversion efficiency of 52.5%;The fourth harmonic generation is through a BBO crystal(θ=47.7°,φ=0°),a maximum conversion efficiency of 48% from 532 nm to 266 nm was achieved.The 266 nm UV laser was output with a pulse duration of 400 ps and peak power of 59.3MW.The limitation of conversion efficiency is analyzed,the main factors are the absorption of BBO at 266 nm and the two photon absorption at 266 nm.