Research On Visual Measurement And Optimization Of Refractive Index Of Gas-wedge Based On Schlieren Technology

The index distribution ouccurs a wedge shaped distrubition in the surface of thin-disk crystal caused by the density reduced during laser heating and thermal convection called gaswedge which leads the deflection of the reflected light from pump region of the disk crystal.Such effect seriously influences the stability of the output power and beam quality.In recent years,with the development of high power thin-disk laser,gas-wedge effct on performance parameters of disk laser becomes a research hotspot.This thesis focuses on the formation and elimination of the gas-wedge effect,and the results shows a reference value for improving the performance of high-power thin-disk lasers.Based on the light deflection when passing through flow field,schlieren technology with high sensitivity is used to transform the invisible index distrubition to visible light intensity in real time.Background schlieren technology measures the movement of spots on the background image and visualizes the flow field through image data processing.Based on schlieren technology and background schlieren technology,a visual experimental platform for measuring the refractive index of gas-wedge is built,the gas-wedge schlieren image is obtained,and the data of the image is processed,Select the region of interest(ROI)in the schlieren image,using the algorithms of image binarization,minimum circumscribed circle,dichotomy and five point difference method,the program for measuring the refractive index and temperature distribution of gas-wedge is developed,and the quantitative calculation of the refractive index distribution of gas-wedge is realized.The correctness of the measurement results of refractive index distribution of gas-wedge is verified by measuring the temperature distribution of gas-wedge.The experimental data show that without external wind field,the temperature of gaswedge is between 22.3 ～ 113.0 ℃,the displacement of light passing through gas-wedge is 0～ 0.5494 mm,and the refractive index is 1.000222 ～ 1.00029.In order to suppress and eliminate the gas-wedge,an external wind field is used to blow from bottom to top to the gas-wedge,which has the best suppression effect on the gas-wedge.With the increase of wind speed,the thickness of the gas-wedge gradually decreases and the refractive index distribution gradually becomes uniform.There is a threshold wind speed to minimize the thickness of the gas-wedge.At this time,the most uniform refractive index distribution is1.000285 ～ 1.000293,After exceeding the threshold wind speed,the thickness of the gaswedge will no longer decrease,the refractive index will increase,and the wind field will become another factor affecting the uniformity of the refractive index.