Design And Implementation Of Focus Drift Detection System For High Power Pulsed Laser

High-power pulsed lasers are widely applied in the science,the industry and the military fields.However,its focus position drift is ineluctable caused by many sensitive factors,and hence its working accuracy is limited.In this dissertation,a set of solution scheme is proposed and implemented for accurately measuring the laser focus drift in three-dimensional space.The details of the study work are as follows:Firstly,the sensitive factors that cause the focus drift are theoretically analyzed,including the thermal lens effect,the laser amplitude fluctuation,the plasma randomness,the auxiliary gas and the ambient temperature,and then the formula of the laser focus drift due to the lens thermal deformation is derived.Secondly,a structure of semi-insulating(SI)photosensitive surface sensor is proposed for positioning the focus and measuring the focus drift of the power pulsed laser spot.A planar sensor with the novel structure is manufactured,which,is mainly composed of a self-compensated SI GaAs:EL2 substrate and two Au semicircle electrodes.In order to guide the vertical incidence and reduce the thermal effect caused by the high power laser,a double-layer cross slit structure is designed and optimized above the planar sensor.On this basis,a x-y plane focus drift measurement system is built,and then tested by a laser with a pulse width of 14 ns,a wavelength of 532 nm and a repetition frequency of 1 kHz.The experimental results show that,the sensor is able to withstand a single-shot peak-power density of greater than 72 kW/cm2,and the positioning sensitivities in the x and the y directions are higher than 500 mV/mm and 111mV/mm,respectively.Finally,based on the strong negative correlation between the defocus amount and the ultraviolet radiation intensity of the laser-induced plasma from a metal target board,a high-power pulsed laser z-axial focusing system is designed and built using a wide bandgap ultraviolet photodetector,which system is suitable for measuring the ray-axial focus drift.The relationship between the laser sensitive parameters and the ray-axial focal drift is studied experimentally based on the system,and a semi-empirical formula of the ray-axial focal drift on the laser average power and the pulse repetition frequency is fitted according to the experimental data.