Research On Birefringence Measurement Based On Dual Photoelastic Modulation

Birefringence refers to the phenomenon that when a beam of natural light is refracted by anisotropic crystal,two beams of linearly polarized light with different vibration directions are produced,while isotropic optical materials will also produce birefringence under the action of stress.Accurate measurement of birefringence can not only obtain the optical constants of materials,but also realize the analysis of optical anisotropy of materials.It is widely used in the measurement of optical constants of optical crystals,glass and other materials,the defect analysis of optical materials,and the defect detection of optical components due to cutting,polishing and clamping.At present,according to different measurement principles,many birefringence measurement methods have been developed,including methods of polarization interference,compensation and modulation.Methods of polarization interference and compensation are simple and easy to realize,but the measurement process is time-consuming and the accuracy is limited.According to the characteristics of fast modulation rate and high frequency of modulator,modulation method can achieve fast and high sensitivity measurement,therefore it has become the mainstream method of birefringence measurement compared with other measurement methods.Taking advantage of the application advantages of photoelastic modulation technology,such as high modulation efficiency,high frequency and wide spectral range,this paper studies a birefringence measurement method based on dual photoelastic modulation.The main research contents are as follows:First of all,based on the principle of polarization,combined with the principle of birefringence and photoelastic modulation,the theoretical model of birefringence measurement by using Stokes vector and Mueller matrix is established.The preliminary design of birefringence measurement scheme by dual photoelastic modulation is completed based on the theoretical model.Additionally,according to the scheme,the photoelastic modulators(PEM)of different frequency are designed,and the photoelastic crystal and piezoelectric crystal are designed,simulated and processed.The resonant frequency of the processed piezoelectric actuator is tested and matched.The PEMs of different frequency are manufactured and the phase delay modulation range of two PEMs is tested.The results show that the phase delay modulation of both PEMs can verify in the range of 0~5 ?.Moreover,the research on precise calibration method of PEM is completed.A new structure is adopted,and the calibration elements are miniaturized and integrated on both sides of PEM.The phase delay of PEM is calibrated in real time,thus completing the construction of the microsystem.The high-voltage drive circuit based on LC circuit and the adaptive drive control circuit with FPGA(Field programmable gate array)as the control core are selected as the hardware control system.Digital phase-locked technology is used to complete high-precision extraction of signal amplitude and data processing.Through the above methods,the stable control of the elastic-optical modulator can be realized.Finally,a birefringence measurement system based on photoelastic modulation is set up,and a quarter wave plate is used to test and calibrate the system.The experimental results show that the measured average value of the phase delay of the wave plate is 1.569 rad,and the standard deviation is less than 1.1% of the average value.Soleil-Babinet compensator with adjustable phase delay is used for further experiments,and the feasibility of the experimental scheme is verified in the range of 0 ? /2.