Optics: measuring and testing – Velocity or velocity/height measuring – With light detector
Patent
1992-01-17
1992-11-17
Hellner, Mark
Optics: measuring and testing
Velocity or velocity/height measuring
With light detector
356 28, G01P 336
Patent
active
051647842
ABSTRACT:
A continuous wave Doppler LIDAR with an enhanced signal-to-noise ratio (SNR) that greatly enhances its ability to determine relative fluid velocity. A laser source (12, 50, 102) produces coherent light that is split between a reference beam and a test beam by a beam splitter (16, 106). A quarter-wavelength plate (26, 108) changes the linear polarization of the test beam to a circular polarization. The circularly polarized test beam is focused on a target cell (30) having a very small volume. Any particle in a fluid moving relative to the CW Doppler LIDAR system that passes through the target cell causes a Doppler shift in the frequency of the coherent light reflected from the particle and reverses the rotational direction of circular polarization of the reflected beam. The light reflected from the particle is combined with the reference beam, creating an interference pattern (or difference signal) incident on a photodetector (44, 118). A level detection trigger signal is developed from a corresponding electrical signal produced by the photodetector in response to this difference signal. This level detection trigger signal is used to enable fast Fourier transform processing of the electrical signal only when a particle is within the target cell. The enhanced SNR resulting from the relatively high ratio of the volume per particle to the sensed volume of the target cell thus is not averaged with the low SNR evident when a particle is not in the sensed volume. Consequently, the enhanced SNR provides a much more robust measurement of the particle's relative velocity (and thus the relative fluid velocity) as a function of the Doppler frequency shift in the reflected coherent light caused by the particle as it passes through the target cell.
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Hellner Mark
The Boeing Company
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