Research On Laser Beam Homogenization In Dynamic Calibration Of Temperature Sensor

In dynamic calibration of temperature sensor,the laser heating method is widely used due to its advantages of high temperature step,fast rise time and high energy utilization rate.However,the laser power distribution is Gaussian,and the energy distribution in the laser spot is not uniform,which will result in low accuracy of sensor calibration.In order to improve the laser quality and the calibration accuracy of the temperature sensor,the microlens arrays were used to homogenize the laser in this paper.The homogenization principles of imaging microlens arrays and diffractive microlens arrays were analyzed.The relationship between the size of the uniform spot and the system parameters was analyzed by using geometric optics,and it was proved that the size of the spot is proportional to the size of the sublens in the microlens arrays and the focal length of the Fourier lens.In the imaging microlens arrays,the spot size is also inversely proportional to the distance between the two microlenses within a certain range.The distribution of the light field was analyzed by using the wave theory of light,and it was theoretically shown that the light field distribution in the focal plane of the main lens is proportional to the Fourier transform of the mathematical function of the incident light beam and the microlens arrays.The imaging and diffractive microlens homogenization systems were simulated and analyzed by optical design software,ZEMAX.The laser beam size,beam divergence angle,sublens size and the distance between two microlenses were simulated and analyzed,and the diffractive homogenization system and the imaging homogenization system were compared.The effect of different parameters in the two homogenization systems of laser beam was examined.On the basis of simulation,the imaging laser beam homogenization system was built using the microlens arrays.The attenuation system based on wedge-shaped prism was designed to attenuate the power of the high-power laser to about 0.6 W,and the degree ofhomogenization was tested by laser beam analyzer.The results show that with the homogenization system,the laser unevenness is about 9.3%,and the spot utilization is about87.5%.A thermocouple with diameter of 0.25 mm was placed on the focal plane of the Fourier lens and its output temperature was measured.The results show that before the homogenization,the output temperature of the thermocouple in the center is greatly different,which will have a great influence on sensor calibration.However,after homogenization,the output temperatures of the thermocouple at the two locations are almost equal.According to the sensor calibration theory,the calibration accuracy of temperature sensor can be improved by homogenization of laser heat source.