Research On Birefringence Parameter Detection Technology Of Cascade Modulated Waveplates

Waveplates are widely used in various precision optical instruments and play an important role in polarization,optical precision measurement and other fields.Wave Plate,also known as phase retarder,is a planar plate made of birefringent crystal,which can change the phase of the incident polarized light at a certain wavelength,and thus change the polarization state of the incident light.The use of wave plates in combination with other optical devices can realize the conversion of various polarization states of light or the rotation of the polarization plane.With the continuous development of optical precision instruments,the high precision requirements for wave plates are becoming more and more stringent.The phase retardation and fast-axis azimuth angle of the wave plate are the main parameters of the wave plate,and their slight changes will directly or indirectly affect the quality and performance of optical precision instruments.In view of the problems of poor real-time performance and complex system of the current wave plate birefringence parameter detection method,this paper proposes a cascade modulation waveplate birefringence parameter detection method with phase compensation.Mainly from the following aspects:Firstly,the structure and working mechanism of the photo-elastic modulator,the electro-optic effect and working principle of the electro-optic modulator and the structure and working principle of the Soleil-Barbinet compensator are analyzed.The cascaded polarization modulation method and the phase compensation method are combined to establish a Theoretical model for parametric detection of birefringence of cascaded modulated waveplates.Secondly,the hardware system of the wave plate birefringence parameter detection system is designed with FPGA as the control core.By designing the high-voltage coordinated drive control circuit of the photo-elastic modulator and the electro-optical modulator,the filtering and sampling circuit of the polarization modulation signal,and the FPGA peripheral circuit design,a wave plate birefringence parameter acquisition and processing hardware platform is built;and based on the analysis of DDS principle,the drive signal design with adjustable frequency and duty cycle is completed based on FPGA,and the stable drive control of the photo-elastic modulator and the electro-optical modulator is realized;And based on the digital phase locking technology,the fundamental frequency,double frequency,quadruple frequency and other signals in the polarization signal are separated,and the FPGA-based soft core design completes the calculation and analysis of the wave plate birefringence parameters.Finally,the experimental platform for cascading modulation birefringence parameter detection with phase compensation is built.The birefringence parameters of the waveplate were measured and analyzed with and without the Soleil-Barbinet compensator,respectively.Experiments show that through the cascade modulation birefringence parameter detection system with phase compensation,the maximum relative error of the detected waveplate phase retardation is 0.362%,and the maximum relative error of fast-axis azimuth angle is 0.320%.It is proved that this method can effectively improve the speed and accuracy of the detection system.