Optical System Design Of A Portable Laser Doppler Velocimeter

Laser Doppler velocimeter, which measures the Doppler frequency of the scattering light from particles in flow, can calculate velocity of flow,according to the proportional relationship between Doppler frequency and velocity of flow. Resolution of the Laser Doppler velocimeter is up to micrometers, and dynamic response of the velocimeter is very fast because of the Doppler signal propagated with light. As a non-contact velocimeter, Laser Doppler velocimeter can measure velocity of particular flow such as high temperature, poisonous and corrosive one.A portable system of laser Doppler velocimeter based on dual-beam mode is designed to measure velocity of the smoke from stationary pollution source in industrial production, and the system contains optical unit and signal acquisition and processing unit.The optical unit is used to modulate velocity of the smoke to frequency of the light, and make photoelectric conversion. In order to create interference fringe, the beam of a collimated laser diode is split by depolarization dispersion prism and isosceles right-angle prism, then two beams behind the prisms are focused by a focusing lens. A collecting lens and pinhole is used to project the useful scattering light onto an avalanche photo diode (APD) to detect Doppler signal.The signal acquisition and processing unit makes up of signal conditioning circuit, analog to digital conversion circuit, central processing unit (CPU), data storage unit and kinds of interface circuit. Signal from APD is amplified and filtered by the signal conditioning circuit, and then it is converted to digital signal by analog to digital conversion circuit. Digital signal processor (DSP), C6000 from Texas Instruments, is used as the system's CPU, and it makes fast Fourier transform to calculate Doppler frequency of the input signal. According to the linear relationship between velocity of flow and Doppler frequency, velocity of flow can be calculated.In addition, preliminary experiment is performed,the result proves the system is approved.