Research On Q-switched Nd:YAG Laser With Thermal Effect

Diode pumped solid state laser(DPSSL)is widely used in the national economy because of its compact structure,light weight,high efficiency and long life.Among them,Q-switched laser has an urgent demand in the fields of laser ranging,laser detection,laser imaging,laser guidance and other military applications.In these applications,the laser is required to have large output pulse peak power,good output beam quality,compact structure and high reliability.Traditional Q-switched methods,such as electro-optic Q-switched,acousto-optic Q-switched and passively Q-switched,are difficult to realize the high peak power laser pulse.The main problems faced by high peak power Q-switch are:the higher the peak power is,the higher the requirement of Q-switch is,the more complex the Q-switch is,and the reliability becomes poor;the high-order mode is difficult to turn off in the Q-switch process,which seriously affects the beam quality;under the condition of high peak power,the Q-switch is easy to be damaged and the stability is reduced.In order to solve the above problems,a new Q-switching technique,thermal effect Q-switching technique,is developed.By using the inherent thermal lens effect in the gain medium of solid-state laser,the resonator is transformed from unstable to stable,and the loss in the resonator is reduced from large to small,so that the Q value of the resonator can be adjusted.In this method,the inherent thermal lens effect of the laser resonator is turned into a treasure,which is directly used to control the output state of the laser resonator.Without adding additional Q-switching elements in the resonator,the high peak power Q-switching pulse laser output can be realized.By using this Q-switching method,not only the compactness of the laser structure and the reliability of the laser are improved,but also the generation of higher-order modes in the resonant cavity is effectively suppressed,and the laser output pulse with higher beam quality is obtained.Based on the heat conduction equation of the solid-state laser,the temperature field distribution and its changing trend of the laser medium under the condition of end pumping are analyzed by numerical calculation method;the temperature distribution inside the laser medium caused by the thermal lens effect is calculated by establishing the model;the focal length calculation formula of the lens like laser medium caused by the thermal lens effect is obtained;according to the stability condition of the resonator and the rate equation of the solid-state laser,the process of adjusting the loss of the resonator by using the thermal lens effect of the medium in the laser and adjusting the Q value of the laser is simulated and calculated.On the basis of simulation,the output characteristics of the laser based on thermal lens effect Q-switching and their key influence factors are analyzed.At the same time,the paper carried out the experimental research of Q-switching based on the thermal lens effect.In the experiment,the circular rod-shaped Nd:YAG crystal is used as the laser medium.The laser medium adopts the end pumped mode.The pump end adopts a concave surface with a certain curvature radius and is coated with a corresponding film system.The output end adopts a plane mirror with a certain transmittance.According to the conditions given in the simulation,the experiment of laser diode(LD)end pumped circular rod concave Nd:YAG laser is carried out,and the final experimental results are compared with the simulation results.After many experiments,when the injection power of the pump source is 113W,the repetition frequency is 100Hz and the pump pulse width is 200?s,we get the Q-switched laser pulse output with pulse width of about 40ns,peak power of about 88KW and beam quality factor M2=1.38.The output pulse waveform is stable.The experimental results verify the feasibility of using thermal lens effect to adjust the Q value in the laser.Using this technique can greatly simplify the resonator structure of Q-switched laser,and provide a new scheme for the performance optimization of high-power all solid-state laser,which is of great significance.