Abstract

The Doppler shift of lidar signals backscattered from windborne particulates can be measured with heterodyne techniques in the infrared, and with optical techniques in and near the visible. So-called “direct”, i.e., optical, detection is generally implemented with a high resolution Faby-Perot interferometer, with an imaging detector to measure the displacement of the interference fringes indicating the frequency of the laser backscatter¹. An alternative method for measuring the Doppler frequency shift is based on the “edge” detector^2,3, in which the signal is passed through a high resolution filter off the passband center, so that the amplitude of the signal is strongly modulated by the filter transmittance versus frequency function. Then a measurement of the signal level before and after the filter yields, via the known filter function, the signal frequency. An advantage of this approach is the replacement of the expensive and technically challenging fringe imaging detector with a pair of simple, nonimaging detectors. Because the edge filter detectors can have significantly higher quantum efficiency than an imaging detector, it may be possible for this technique to yield not only a simplification of the system instrumentation but higher Doppler measurement performance.

© 1997 Optical Society of America