The Research Of Measurement Of Optical Rotation Using Photoelastic Modulation Based On FPGA Digital Phase Lock

With the development of science and technology,the optical material in food safety,drug identification,medicine,petroleum,chemical and other fields has been more and more widely used,the measurement of the optical rotation is more important.Because of the advantages of high precision,wide spectral window and large field of view,the photoelastic modulator is used to modulate the signal,and the rotation angle of the material is measured by the digital phase locked demodulation technique.This paper first analyzes the principle of rotation,Photoelastic Modulator and digital demodulation technology,and according to Stokes and Muller matrix derived the principle of measurement of optical rotation based on Photoelastic Modulator,and then build the optical system for measuring optical rotation.Then design the hardware circuit with FPGA as the core,mainly including amplification,A/D conversion,serial port,optocoupler module.Within the FPGA completed the generation of square wave drive signal based on the DDS technology,amplifier circuit amplification,A/D acquisition clock control and digital phase-locked algorithm,and build a SOPC system to achieve the Lab VIEW control of FPGA.Among them,through the serial port will be digital phase-locked multiplication data transmitted to Lab VIEW,data splicing in Lab VIEW,and ultimately in the host computer to complete the data analysis and processing.At the same time,Lab VIEW can also send data to the FPGA to control the frequency of drive square wave signal,magnification of magnification circuit and multiplication of the cumulative time.Finally,the hardware part and the software part were tested respectively.The experiment platform of optical rotation was set up,and the experiment was carried out on different concentrations of glucose solution.The experimental results show that the sensitivity of the system for measuring the optical rotation angle is 0.00041°,the maximum relative deviation is 2.78%,according to the 650 nm incident the laser,the rotation rate of glucose solution is 31.0(°ml·g^-1·dm^-1).This system has high sensitivity,stable work,convenient integration design and other advantages.