A bipolar, two component laser-Doppler velocimeter

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Abstract
A fringe mode laser-Doppler velocimeter is described that instantaneously measures two orthogonal velocity components that have fluctuating signs. The optical system utilizes an equiangular configuration of three intersecting beams which allows measurements of the normal velocity component to be taken in close proximity to a wall. Acousto-optic frequency shifting of two beams is used to eliminate cross channel interference and to provide detection of the velocity polarity. The instrument is applicable to flows having velocities in the range of +or-1 mm s-1 to +or-10 m s-1.