Design of Displacement Demodulation System for Phase-modulated Self-mixing Interferometer Based on FPGA

• 1、NARI Group Co.,Ltd.(State Grid Electric Research Institute Co.,Ltd.),Nanjing,Jiangsu Province,211106
• 2、School of Physical Science and Technology,Nanjing Normal University,Nanjing,Jiangsu Province,210046

Abstract：This paper presents the design of a phase demodulation system based on FPGA for the phase-modulated self-mixing interferometer. By the processing that the modulation SIN signal and the doubling frequency modulation COS signal are respectively multiplied by the phase-modulated self-mixing signal and then low-pass filtered and shaped in the electrical domain, two orthogonal square wave signals sensitive to the direction of the object's movement and displacement are obtained for displacement Refactoring. The FPGA signal processing system controls the generation of a highly stable and highly accurate sinusoidal signal source for the modulation of electro-optic crystals. At the same time, the two orthogonal square wave signals obtained after the demodulation process are subdivided for the high-precision measurement of the displacement of a uniformly moving object. The high frequency amplifier circuit that amplify the self-mixing interference signal ,multiplier and frequency doubler circuits based on MLT04, low-pass filter circuits are designed. FPGA-based counting programs and DAC circuits & programs used for generating sine waves are also designed. In the experiment, a comparison test of displacement was performed using a PI high-precision guide rail, and high-precision displacement measurement can be achieved. The demodulation system does not rely on a PC & a high-precision acquisition card, and can realize real-time measurement of displacement, greatly improving the practicability of the system.

Keywords： FPGA phase modulation self-mixing displacement demodulation real-time