Stress Measurement System Based On Double Photoelastic Modulation

Stress is an important performance parameter of optical components.The distribution of stress will affect the properties of optical components,which may result in the performance degradation of optical systems.Therefore,it is of great significance to measure the size and distribution of the stress of optical components.Nowadays,the stress of the optical element is described by the optical path difference generated by the birefringence effect,which can be measured by three main methods: simple polarizer-based method,compensation method,and modulation method.Among three methods,the simple polarizer-based method and the compensation method have simple structures and are easy to implement,unfortunately,these methods require mechanical rotating polarization components and have limitation of the measurement accuracy.The modulation method is fast and has high accuracy when measures the single-point stress,nevertheless,this method cannot measure the distribution of the singlepoint stress.In order to solve the above problems,in this paper,a stress measurement system based on double photoelastic modulation is proposed.Based on the establishment of the system,the main research works of this paper are summarized as follows:1 、 Based on the principle of birefringence effect,the basic theory of photoelastic modulation technology is analyzed in detail by combining with Stokes vector and Muller matrix.A stress measurement method based on the double photoelastic modulation technology is also proposed.2、The overall scheme of the stress measurement system is proposed.According to the working principle of the photoelastic modulator,the photoelastic modulator and the driving module are designed,and the frequency of the photoelastic modulator is tested.In order to satisfy the displacement demand of the sample which needs to be tested,a two-dimensional scanning platform is designed from three aspects: driving motor,driving control scheme,and grating ruler.Finally,the overall hardware structure is designed,which provides the hardware for the measurement of stress distribution.3、On the Win Form platform in Visual Studio,the upper computer software of the stress measurement system is developed,which includes the program design of: 1)the communication software module,2)the data acquisition software module,and 3)the data processing and display module.The software is tested and the feasibility of the software is verified.4 、 The performance of the stress measurement system is verified by experiments.According to the results of displacement accuracy experiment,the maximum displacement error of the system is 0.072 mm,which shows that the system has good displacement accuracy.According to the results of optical path difference measurement experiment,the maximum standard deviation of different wave plates is 0.035 nm,and the maximum standard deviation of compensator is 0.032 nm,which proves that the system has good repeatability and measurement accuracy.In the tests of the stress distribution of BK7 glass samples and wave plates,the accuracy and reliability of the system for stress distribution measurement are verified,which fulfil the requirements of designed system.