Research On Thermal Management Technology Of Slab Laser Amplifier

Diode Pumped Solid-State Lasers（DPSSL）because of its compact structure,high conversion efficiency,high repetition rate operation,and good beam quality,has a wide range of applications in laser fusion energy drivers,ultra-strong ultra-short laser pump source,laser shock peening and other fields.Therefore,it has become the research focus of the new generation of heavy frequency high-energy lasers and hot spot.As an important component of the laser,the laser amplifier largely determines the output energy,efficiency and beam quality of the whole laser system and other key parameters.High-efficiency thermal management is one of the key technologies to achieve high-performance output of laser amplifiers.Due to the absorption of non-uniform pumping,the laser gain medium in the laser amplifier will form a temperature gradient inside it,which not only limits the further improvement of laser output energy and repetition rate,but also causes adverse thermal effects such as thermal-induced wavefront distortion and thermal depolarization,resulting in a decrease in the performance of the output laser.Therefore,the research on efficient thermal management technology of laser amplifiers is of great significance for the design and development of high-energy solid-state lasers.This paper focuses on the thermal problems existing in the high-energy solid-state laser amplifier.In order to reduce the thermal effect in the laser gain medium and improve the output power and beam quality of the laser amplifier,the key technologies of high-efficiency thermal management of the laser amplifier are explored and developed.The scheme design,simulation and experimental research of the high-efficiency uniform pumping and high-efficiency cooling structure of the slab laser amplifier have been carried out successively,mainly including the following aspects:1.The pump homogenization system suitable for the laser amplifier with conduction cooling structure is designed,including the use of cylindrical lens to homogenize the pump light in the fast axis direction and a hollow light guide tube to homogenize the pump light in the slow-axis direction.Based on the specific parameters used in the experiments,a modular model of LD array,cylindrical lens and hollow light guide tube is established by Trace Pro ray tracing software.The influence of the curvature radius of the cylindrical lens and the distance between the cylindrical lens and the hollow light guide tube on the pumping homogenization effect is studied.The simulation model is verified and optimized by the combination of simulation and experiment.The simulation and experimental results show that when the radius of curvature of the cylindrical lens is 220 mm and the distance between the cylindrical lens and the hollow light guide tube is 46 mm,the uniform pump spot of the size is obtained,and the coupling transmission efficiency of the system is 91.3%.2.The thermal management performance of laser gain medium cooled by microchannel sink is studied.The uneven thermal distribution model in laser gain medium and the thermal-fluid-solid coupling numerical model between gain medium and cooling heat sink are established,which makes the simulation fit the actual situation.The corresponding simulation model is established by using the FLUENT module in the ANSYS finite element simulation software,and the parameters are optimized by changing the relevant parameters,the effect of the parameter change of the microchannel sink on the maximum temperature of the gain medium is analyzed systematically when the gain medium is in an uneven thermal state,and the heat transfer coefficient of the microchannel heat sink is estimated.The simulation results show that when the peak power of the pump source is 56 kW,the repetition frequency is 20 Hz,the thickness of the microchannel base is 2 mm,the height and width of a single microchannel are 4 mm and 0.4 mm,the spacing between the two microchannels is 0.3 mm,and the coolant flow rate is 2.5 m/s,the microchannel heat sink has the strongest cooling capacity,and the average equivalent heat transfer coefficient of the microchannel heat sink can reach about 50000W/m²·K^-1.3.The wavefront distortion and depolarization of the gain medium under the condition of microchannel structure cooling heat sink are calculated and analyzed.The calculation results show that the maximum wavefront distortion caused by temperature change is 0.52λ;the maximum value of wavefront distortion caused by medium deformation is 0.59λ;the total wavefront distortion is1.11λ;the depolarization loss at the four corners of the gain medium is the largest,and the maximum depolarization amount is 9.25%.