Coherent and noncoherent methods in Doppler optical beat velocity measurement

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Abstract
In arrangements for measuring velocity by the optical beating of Doppler shifted scattered light, a distinction is made between two modes of operation. This is illustrated by a detailed calculation based on the fringe or differential Doppler arrangement. The signals obtained may be considered as arising from two terms: one due to the coherent beating of light scattered from different particles and another, termed noncoherent, resulting from light scattered from one particle only. Signal-to-noise ratios are calculated as functions of the detector aperture and the effective number of particles in the scattering volume, and compared with the reference beam technique. For the coherent signal, the useful detector aperture is limited by the coherence condition. The noncoherent signal is proportional to the detector aperture but depends not only upon the intensity of scattered light but also on the effective number of particles in the illuminated volume. If the number of particles is small, the best signals are generally obtained by the use of the noncoherent mode. In most other circumstances, the use of a reference beam is an advantage. The case of intermittent signals due to very low concentrations of scattering particles is considered. A suitable general criterion for the detectability of the signal is proposed.

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