Laser Doppler velocimeter based on dual-longitudinal-mode He-Ne laser

In order to make the measurement for high speed and super speed, a novel structure of Doppler velocimeter based on the dual-longitudinal-mode He-Ne laser was proposed. The principle of the dual-longitudinal-mode laser Doppler velocimeter was expounded while the optical schematic of the system was designed. And the Doppler signal was processed using the digital filtering and autocorrelation. Results of the theory analysis and experiment show that the dual-longitudinal-mode laser Doppler velocimeter reduced the scale factor highly using the beat frequency of the adjacent two longitudinal-modes as the basic frequency and solved the problem of high speed and super speed measurement; basement and part of noise are get rid of with the help of the digital filter; noise is restrained further with the method of digital autocorrelation so that the signal-to-noise ratio is raised and the Doppler frequency is extracted easily. The tangential velocity of the measured point in the rotating disc was obtained and the repeatability of the results was good to 0.8%.