Abstract

We describe a laser Doppler velocimeter based on a single illuminating beam from a laser diode and employing a complex homodyne detection system. Conventional laser Doppler velocimeters are based on the detection of light scattered by particles that traverse a region illuminated by intersecting laser beams. Performance is generally improved, particularly in low-velocity applications, by means of heterodyne detection (the intersecting beams differ in optical frequency by the heterodyne frequency). The system we describe is compact and achieves precise and unambiguous measurements through the use of a complex homodyne technique. In this technique we make simultaneous measurements, updated at 5 MHz, of three interferometric signals whose phases differ by 120. These data are processed in real time to provide a linear output of unwrapped phase, whose derivative is proportional to velocity. The sample volume is illuminated by a single beam, and a complex spatial filter is used to generate the three, differently phased interferometric signals. We describe the theoretical basis for the measurement, the sensor design, and a variety of data obtained from both fluids and diffusely scattering surfaces.

© 1990 Optical Society of America