The Boeing Company

Battelle Development Corporation

Alan P Waggoner

Robert J Charlson

Norman C Ahlquist

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.

CW doppler lidar

An improved integrating nephelometer having superior sensitivity and long-term stability to previous designs includes a container defining a measurement chamber. A continuously energized light source, such as a tungsten filament incandescent bulb, illuminates the measurement chamber through an opal glass diffuser. The intensity of the light source is regulated in one embodiment by a simple optical-electric feedback system, and in another embodiment by a voltage regulated supply. A cone of observation of the chamber is defined by a plurality of spaced, apertured plates located in the chamber, at right angles to the light source. A measuring photomultiplier tube views the cone of observation and provides an output signal including components resulting from detected photoelectrons, noise, and thermally emitted 'dark current' electrons. A photon-detecting apparatus removes the noise component and provides an output pulse for each photoelectron and 'dark current' electron. The output pulses are counted, and converted into an analog signal representing a desired extinction coefficient b.sub.scat by any one of a number of apparatus, including an averaging circuit, or a second counter and a D-to-A converter. Dark current compensation, if necessary, is effected by either subtracting a fixed value, or a value dependent on some environmental variable, from the measured value of b.sub.scat, or by utilizing a reversible counter apparatus and light source apparatus. Variations of this nephelometer are also described, including an instrument for automatically compensating for the component of b.sub.scat due to Rayleigh scatter by gases, an instrument which is sensitive to a plurality of wavelengths, and a self-calibrating instrument which is adapted for the measurement of total light scatter and back scatter at a plurality of wavelengths.

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Seattle, WA, United States of America

Alan P Waggoner