Balanced Detection for the Dual Scatter Laser Doppler Velocimeter

Abstract
Single particles traversing the region of interest produce both a DC term and an AC term as seen by a photomultiplier. The AC term is directly proportional to the particle velocity. In order to accurately determine the frequency of this burst of information, the DC term is rejected by electronic filtering technique. At higher particle velocities the DC rejection becomes difficult, if not impossible, and an alternate technique is necessary. This report describes a balanced detection method to reject the DC term optically without electronic filters. Balanced detection rotates one of the laser beams in a dual scatter laser Doppler velocimeter such that light scattering from the probe volume has the two scattering components 90 degrees apart. A polarization- sensitive beam splitter is employed to split the scattered light into two components with one beam undergoing a 90-degree phase shift. Two photomultipliers are used to detect each signal.

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