Laser Doppler Velocity Measurement Without Directional Ambiguity by Using Frequency Shifted Incident Beams

A simple laser Doppler heterodyning system has been developed where velocities in any set of orthogonal axes can be measured without the directional ambiguity of 180°. The method employs a two-incident-beam heterodyning system where the two incident beams used are of different frequencies. This is accomplished by using a rotating diffraction grating or by employing a Bragg cell. The Doppler signal from the moving scatterer is no longer an ``even'' function of velocity and the frequency bias can be adjusted depending on the maximum value of the velocity to be measured.