Velocity measurement using the phase shift of a laser-induced grating

We have developed a simple and general technique for measuring the flow velocity over a wide range of values. It involves creating a cw light-induced grating and measuring the velocity-induced phase shift occurring in a known time period. It uses polarization spectroscopy and optical heterodyne detection with two beams from a single cw laser. Measurements are made of the two spatial phase components of the induced steady-state grating, and their ratio determines the velocity. Because a ratio is computed, the effects of many possible error sources cancel out. We have demonstrated this method for an approximately 1-m/sec flow of ethanol seeded with an absorbing, nonfluorescing dye.