Research And Application Of Time-varying Thermal Effect In End-pumped Solid-state Laser

Diode pump solid state laser(DPSSL)is an important direction for the development of solid state laser with the advantages of compact structure,high efficiency,long life and so on,which is widely used in various industries in modern society.About 20%?25%of the energy pumped in laser crystal by laser diode(LD)is transferred into thermal energy which deposit in laser crystal.The thermal effect produced by the laser has a great impact on the stable operation and output characteristics of the laser.Therefore it is significant to study the thermal effect to improve the performance of laser.In this thesis the process of heat transfer in solid is analyzed and studied,the heat transfer model is established based on the finite difference solution to the heat conduction equation.The heat conduction model is used to simulate heat distribution in the gain medium when the crystal radial length,axial length and absorption coefficient are different.On the basis,the time-varying thermal effect during laser operation is studied.And changes of the equivalent thermal focal length in the laser crystal with time during a pumping cycle and long-term pumping are respectively analyzed.Several main factors affecting the thermal effect of the crystal are obtained.On the basis of studying the thermal effect of crystals,the thesis has carried out a new research on Q-switching technology:Q-switching using crystal thermal lens effect.Firstly,the variation of the resonant cavity parametersg₁ g₂ with the equivalent thermal focal length when the cavity type is different is simulated,and loss of the resonant cavity under the condition of different values of the thermal focal length is analyzed.On this basis,a specific cavity type is selected so that the cavity loss will change with the change of thermal focal length,thereby realizing the adjustment of the laser Q value.Theoretical research has found that thermal lens Q-switched lasers have a special period of time that we call effective pumping time,which refers to a period of pumping time from the beginning of the pump pulse to the output of the laser pulse.Usually,the effective pumping time and the pumping pulse width is not equal.Combined with the analysis of the change curve of the equivalent thermal lens focal length with time,the change rule of the effective pumping time in the case of thermal lens Q-switching is obtained,and on this basis,the Nd:YAG laser four-level rate equation is used to output the pulse waveform of the laser A simulation was carried out.The thesis designs and builds an experimental device using thermal lens to adjust Q.The output performance of the thermal-effect Q-switched laser was studied experimentally.In the experiment,a semiconductor laser is used as the pump source to end-pump the Nd:YAG laser.The Q-switched pulse output is obtained under the condition that one end of the resonant cavity is a crystal coated concave surface and one end is a coated output mirror.Under the conditions of 113W pump power,200?s pulse width,100Hz repetition frequency,and 52mm cavity length,the thermal Q-switched laser is used to obtain a stable laser pulse output with a peak power of 63KW and a pulse width of 52.5ns at an effective pumping time of 19.2?s.Both theoretical and experimental results show that it is feasible to use the thermal lens effect in the crystal to Q-switch the laser.This thesis adopts a new method to study the time-varying heating effect of laser medium,which provides a new idea for using the thermal effect of laser.