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The described system measures the flow velocity without directional ambiguity. It does not need an optical frequency offset. Polarized illumination beams and two optical detection channels with appropriate polarizing elements are used to determine the flow direction. The Doppler signals from the two channels are identical but have a phase offset that changes sign if the flow direction is reversed. The method has been verified experimentally for low and high concentrations of scattering particles.
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Dändliker et al. (1974) studied this question.
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