Research On The Generation And Amplification Characteristics Of Picosecond High Energy Flat-Top Beam

High-energy,high-power picosecond lasers are widely used in strong field physics,laser satellite ranging,biomedical,industrial material processing,nonlinear optics,and driving X-ray sources.The high-energy picosecond laser is also an ideal pump source for the Optical Parametric Chirped Amplification System?OPCPA?.In order to more effectively improve laser processing efficiency and laser ranging accuracy,distance,and acquire more energy of higher harmonics,a higher energy and power picosecond laser system is required.Limited by the damage threshold of optical components and the self-focusing effect of the crystal,it is often necessary to reshape the beam intensity distribution in a high-power and high-energy picosecond laser system.In this paper,by studying beam shaping theory and flat-top beam amplification experiment,the 39.5mJ single-pulse energy of 11ps pulse width and 94.8mJ pulse burst energy of 110ps pulse width were obtained respectively at 1 kHz.The content mainly includes the following parts:1.The background and significance of high-energy high-power picosecond laser are introduced.The research status of all-solid-state high-power picosecond laser and Gaussian beam shaping methods are studied.2.The theoretical model of flattop beam is introduced.The principle and shaping characteristics of the circular aperture,aspheric lens and variable reflectivity Gaussian mirror are described.The energy calculation expression of the traveling wave amplification is obtained by using the pulse amplification theory.3.Using a circular aperture to shape the Gaussian distribution beam.The spatial filter-image relaying system is used to optimize the shaped beam,and the effective transmission range of flat-top beam is improved.The 8^th-order super Gaussian with a filling factor of 0.78 is obtained.Shaping efficiency is greater than 32%.A two-stage LD side pumped Nd:YAG amplifier is designed to amplify the power of the flat-top beam.The effect of using the module's thermal lens instead of lens in the 4F image relaying system on the amplification is studied.By changing the module pump current,the depolarization compensation capability between the two amplification modules with inconsistent gain distribution is improved.Finally,through two-stage amplifier,a flat-top beam with 39.5W average power and 11ps pulse width is obtained at 1kHz.4.Using Plano-aspheric lens pair to shape the Gaussian profile beam.A 10^th-order super-Gaussian beam with a filling factor of 0.68 is obtained.The shaping efficiency is96%.The average power of the pulse burst was amplified to 94.8W with 11ps pulse width and 1 kHz repetition rate by two-stage single-pass LD side pump Nd:YAG module amplifier,and there are 5 pulses in a burst.During the amplification process,the uniformity of the intensity of the final amplified beam is improved by reasonably designing the object and the image plan of image relaying system.By adding a half wave plate and a polarizer among two-stage amplifier,the depolarization ratio of the system is reduced.