Importance

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Abstract

Reflections, having positive and negative Doppler frequency shifts, from a laser beam directed at a moving surface are passed through spatial filters, combined by a beam splitter, and focussed on an observation plane of a photodetector to provide a fringe pattern having an amplitude variation frequency responsive to the velocity of the moving surface. The laser beam may be a single beam directed substantially perpendicularly to the moving surface, or the laser beam may be divided into two beams angled toward the surface, one beam impinging with the direction of motion of the surface and the other beam impinging against the direction of motion. When two impinging beams, split from a common beam, are utilized, different polarizations and beam impingement points on the surface may be used to provide beam isolation between the beam signals. Homodyne detection with single frequency lasers is used. With multi-mode lasers emitting two separate frequencies of radiation, heterodyne detection may be used to provide better resolution at low surface velocities.

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First Claim