Study On Several Key Issues Of Laser-Diode Pumped Solid-State Lasers

Several key issues of laser-diode-pumped solid-state laser (DPL) are discussed in this paper. A part of pumping energy absorbed by laser crystal is changed into heat and the thermal effects are caused by the heat consumption. The thermal effects of laser crystal are the important factors which affect the output power, spatial and time characteristics, frequency characteristics of DPL. The key issues of this paper are to study the spatial and time varying thermal effects of the laser crystal and the influences of thermal effects on laser performances. The generation and output characteristics of laser pulse output by acousto-optic Q-switched Nd:YAG lasers are also discussed in this paper; the laser design method is studied and a software is developed, it is suitable for the 100-kW-class compact laser-diode (LD) end-pumped acousto-optic Q-switched Nd:YAG laser. The key issues discussed in this paper are divided into four parts:In the first part of this paper, the spatial non-uniform distribution of heat consumption and heat dissipation conditions of the laser crystal are studied synthetically, the analytical method and finite element method are used to analyze the influences of the pumping beam distribution on temperature and the thermal induced distortion of laser crystal. In the end-pumped solid-state lasers, the spherical aberration of crystal thermal lens is studied, and the influences of the eccentricity pumping on thermal effects and thermal induced distortion are also discussed. In the side-pumped lasers, the spatial distribution of pumping beam in the laser crystal and the thermal induced distortions of crystal are studied. The results show that, the thermal lens caused by the temperature gradient can not be equivalent by an ideal lens, and the focal length of thermal lens is varied with the radial coordinate. If the light rays transmitted parallel to the optical axis, they will not converged to one point after across the thermal lens, the dispersion speckle of output beam will be observed at different axial position. The thermal induced diffractive loss is influenced by the ratio of pumping beam and the oscillating beam radius, the pumping power and the order of super-Gaussian. In the eccentric pumping condition, the thermal induced distortion is shifted, the maximum temperature in the crystal is decreased gradually and the position of highest temperature is shifted when the eccentricity of pumping beam is increased.The laser crystal cooling technology is studied. Theoretical and experimental researches of heat conduction in heat sinks are made, such as the material, structure and the heat dissipation method. Focuses on the problem of thermal conduction between the laser crystal and the heat sink, many experiments are made, the indium foil, silver foil, aluminum foil and carbon nanotubes is chosen as the filling material between the crystal and the heat sink respectively, due to the thermal conductivity and ductility of the material, the best results are obtained when using the silver foil.In the second part of this paper, the time varying thermal effects in the pulsed laser-diode-pumped solid-state lasers are studied. The time varying thermal effects in the short-pulse, long-period lasers are mainly researched. The thermal relaxation time of the crystal is introduced and its influencing factors are analyzed. The influences of thermal fluctuation in the laser crystal on the resonator are researched and several Q-switch methods based on the thermal effect are presented. The results show that, the time varying temperature in the crystal is influenced by pumping beam, crystal properties and cooling conditions, the temperatures rising process is mainly affected by the pumping beam and the crystal doping concentration; the temperature decreasing process is mainly affected by the crystal size, thermal properties, and the cooling conditions. In the periodic pulse pumping conditions, the temperature of crystal and the temperature gradient periodically change with time and they are influenced by the thermal relaxation time. The temperature, refractive index, thermal lens and the resonant cavity are all changed rapidly because of the strong thermal relaxation effect, which induce the resonant cavity structure influenced by the thermal lens changed with time and can not reached steady state during the pulse process. The phenomenon is even more significant in the short-pulse, long-period (the low-duty-cycle) pulse laser.In the third part of this paper, many issues about the intracavity frequency doubled acousto-optic Q-switched Nd:YAG lasers are discussed, including the generation process of laser pulse and its influencing factors. The influence of the KTP length and the cavity parameters on the output power under the completely phase-matched condition; the influences of KTP thermal effects on the phase matching; because of achieving the phase match is difficult in the actual condition, the influences of phase mismatch degree on the laser output performances are discussed in this paper. The results show that, the output laser pulse non-symmetry in time-domain is deterioration in the intracavity frequency doubled acousto-optic Q-switched lasers. The higher the basic frequency laser power, the higher the temperature of KTP, refractive index changed with the temperature so the phase mismatch condition is much more serious, which cause the efficiency of frequency doubling reducing.In the fourth part of this paper, based on the adaptive method, an LD end-pumped 100-kW-class compact acousto-optic Q-switched Nd:YAG laser design software is completed. The reverse direction design function of this software: input the laser performance parameters such as peak power, pulse width, repetition rate and laser beam quality factor into the user window, the laser parameters such as pumping power, cavity length, pumping light radius, transparent rate of the output mirror and the curvature radius can be displayed in the output window by running the software, the analyses of crystal thermal effects and drawing the laser pulse waveform can also be down by running this software. The positive direction design function: enter the laser parameters above into the user window, the corresponding laser performance parameters can be displayed by running the software.