Research On Thermal Effects Of High-power And Narrow Pulse Width LD Pumped Nd:YAG Laser

In recent years,Synthetic Aperture Lidar(SAL)breaks the diffraction limit to entrance pupil diameter,improves traditional lidar spatial resolution,and becomes more and more important in coherent lidar detection system and laser sources are important for it.But there will be temperature gradient through the oscillation and amplification in a high-power Laser because of nonradiative transition and absorption increasing the optical elements' temperature.And temperature gradient will further impact the crystal's mechanical parameter and optical parameter,which will change refractive index and result in thermal effect including thermal lens and thermal depolarization.Those will decrease the Laser's property badly,so it's significant to research lasers' thermal effect.Theoretically,thermal lensing and thermal birefringence are described.The thermodynamic model of the laser crystal is set up with theories of heat transfer and the temperature distribution is solved out while boundary conditions simplified reasonably.Then the refractive index profile is deduced through the temperature gradient with theories of crystal optics.,whose first approximated solution is the thermal focal distance.Besides,the expression of depolarization ratio is inferred by crystal optics theories.At last,more accurate thermal distribution is modeled while considering asymmetrical optical field formed by five-directional pumping Laser diode,which account for the angular distortion after the light outgoing.In the experimental section for measuring the thermal effect of Laser crystal,the laws under which thermal focal distance and depolarization ratio change with the pumping currents are discussed by measuring the spatial difference of a parallel beam after passing through the laser crystal.In addition,the angular unevenness function is defined to describe the angular thermal property and measured to build the relationship with pumping currents.Apart from the spatial distribution,the time response is discussed through measuring relaxation time with different repetition frequency.In the compensation section,common scenarios to compensate thermal effect is orientated.Then,thermal lensing is compensated totally by lens system and thermal birefringence is suppressed with depolarization ratio decreased to 3.2% from 36.8%,and the angular unevenness ratio decreased to 72.5 from 120.