Study On Thermal Effects & Thermal Management In High Power Solid State Lasers

With the increase of the demands of inertial fusion driver and military weapon, high power & large energy diode-pumped solid-state laser(DPSSL) has become one of the main objectives of current research and development activities within the laser community.DPSSL has become one of the most promising approaches in the high-power laser engineering.Research of DPSSL has mainly concentrated on scaling of output power,while remaining of a high spatial beam quality.The main problem hindering scaling of diode-pumped solid-state lasers is heat deposition within the laser medium.A detailed investigation of the theoretical design and thermal aspect for the high average power and large energy DPSSL and the thermal management technology is reported in this dissertation,including four parts contents.The first part is theoretical design and thermal management for the pulsed energy-storage quasi-three-level Yb-ion DPSSL.We begin with the discussion on the quasi-three-level laser oscillator performance model,which predicts both intensity and spectrum evolutions of the laser output.Followed by the numerical simulation of the spatio-temporal dynamics of the pulse formation in the diode-pumped Q-switched Yb:YAG laser.We then study in details the optimal design for the pulsed energy-storage Yb:YAG amplifiers considering the criteria of amplified spontaneous emission(ASE) and the capability of forced convection cooling in a narrow passage.Finally,based on our model,the baseline parameters are presented for a 100J-class diode-pumped Yb:YAG disk amplifiers.Efficient thermal management is the key technology for average power Yb lasers. The thesis investigates the temperature distributions and liquid flow of the cooling system utilizing the concept of forced convection to cool the Yb:YAG disk laser.The numerical simulation platform was built based on coupling thermal exchange model and turbulent flow model.Based on this numerical platform,the water cooling system is presented for V-shape Yb:YAG laser.Finally,the numerical calculation model of thermal contact was built and the simulation was carried out by ANSYS finite element program.Then the Sapphire cooling at both faces of high-power cryogenic Yb:YAG disk laser is optimized according to the model.The second part is about thermal aspect and design for the high average power Pockels cell.Firstly,the numerical calculation model for thermo-mechanicss of nonlinear crystals such as KDP,DKDP and BBO which used in high average power laser systems was built and the simulation was carried out by ANSYS finite element program.We then investigate in detail the machining or assembly errors and performance of a thermal compensation pockels cell(TCPE).Based on these analyses, the large aperture TCPE based on DKDP has been developed successfully.Finally,we discuss the theoretical design of repetitively plasma-electrodee pockels cell,then we have completed thermal modeling of a scaleable,face-cooled repetitively plasma-electrodee pockels cell.The third part is for thermal effects and laser characteristics of solid state heat capacity lasers(SSHCL).An unsteady-state thermomechanics model was established in the condition of the practical distribution of gain in the laser medium according to the characteristics of waste heat accumulate persistent in laser medium when working on the mode of heat capacity.Based on this model the thermomechanics optimization was done. Furthermore a theoretical model of the optical transmission was established to study the optical distortions which result from the dynamic thermal effect.The numerical simulation research shows that the main optical distortion is result from the bulging of the end face because it is not pumped fully aperture.The research also included lots of experimental study.A new type of a high-efficiency V-shape active-mirror laser was proposed,which was composed of four modules with two large-aperture Nd:YAG slabs in each module.The experiments verifies that the output energy has a linear scaling law with the module number,which implies that this kind of multi-module large-aperture active-mirror laser would be suitable for high-average-power laser operation.The final part is the investigation of pump-induced wavefront distortion and the thermal recovery of the large aperture Nd:glass amplifiers.The main innovation points are summarized as following:1.Quasi-three-level laser oscillator performance modelThe model to describe the spatio-temporal dynamics of the pulse formation in the diode-pumped Q-switched Yb:YAG laser is established.This model,based on the rate equations of quasi-three-level and the theory of scalar diffraction,predicts the value of the beam-quality factor M~2 during the pulse evolution.The model to describe the spectral dynamic behavior of quasi-three-level laser resonators operated in gain-switched conditions is also established.Numerical simulations of our analytical model are useful to optimal design quasi-three-level laser resonators.2.Water cooled & conduct cooled Yb:YAG disk for the 10Hz,100J-class amplifierThe feasibility of cooling 10Hz/100J amplifier with "thermal equilibrium" cooling configuration have been demonstrated:Back pumping and bank cooling with water while the front face is cooled by air.The numerical calculation model of thermal contact was built and the simulation was carried out by ANSYS finite element program.Then the Sapphire cooling at both faces of cryogenic Yb:YAG disk for the 10Hz,100J-class amplifier is optimized according to the model.We compare the peak temperature of Yb:YAG for water cooled and conduct cooled scheme.The results show that temperature rise achievable with conduct cooled is quite low to those obtained with water cooled.3.Thermal compensation Pockels cell and rep-frequency plasma-electrodee Pockels cell for high average powerWe study in details the machining or assembly errors and the performance of thermal compensation pockels cell(TCPE) for the first time.Based on these basic analysis,the large aperture(64mm×35mm) TCPE based on DKDP has been developed successfully.Experiments have validated the feasibility of TCPE for high average power applications.The optimal design of rep-frequency plasma-electrodee pockels cell were proposed and realized for the first time.By optimizing the configuration of the Pockels cell,the DKDP Pockels cell can be operated only by switching pulse thanks to the share voltage capacitance added.4.A new type of a high-efficiency V-shape active-mirror laser was proposed, which was composed of four modules with two large-aperture Nd:YAG slabs in each module.The experimental results show that the maximum output energy is about 47J with an overall efficiency of 1.2%and slope efficiency of 1.3%.Also,the experiments verifies that the output energy has a linear scaling law with the module number,which implies that this kind of multi-module large-aperture active-mirror laser would be suitable for high-average-power laser operation.The experiments of the retro-reflector array to improve the quality of SSHCL were carried out in this paper.The experiment results show that the action of retro-reflector array to reduce beam's focal spot size was testified visibly and≤10TDL spatial beam quality can be reached.