Laser Doppler anemometry permits, in principle, the measurement of both magnitude and direction of components of a particle’s velocity vector. Most exiting anemometers, however, permit measurements only with a directional ambiguity of 180°, resulting in errors in certain flow fields. Available methods of eliminating the directional ambiguity of Laser Doppler anemometers are reviewed, covering frequency shifting of the incident and scattered light beams, the use of beams with different polarization properties, and employment of multicolor laser beams. The advantages and disadvantages of existing methods are summarized, and suggestions for alterations are made. Different techniques used to remove the pedestal of laser Doppler anemometer signals are also reviewed. Optical techniques should be employed in any advanced optical anemometer system to avoid dynamic range limitations by electronic bandpass filters. Suggestions are made for advanced optical anemometers employing multielement avalanche photodiodes that can be used for simultaneous measurements of two velocity components. These anemometers incorporate devices to sense the direction of the velocity components and to eliminate optically the pedestal of laser Doppler signals.
F. Durst and M. Zaré, "Removal of Pedestals and Directional Ambiguity of Optical Anemometer Signals," Appl. Opt. 13, 2562-2579 (1974)