Signal-to-noise ratio of laser Doppler signals

This paper presents a contribution to laser-Doppler anemometry. Theoretical investigations are performed on the maximum possible signal-to-noise ratio of laser-Doppler signals. Theoretical calculations of the light scattered by small particles are described, using a computer program for a dual-beam anemometer. The scattered light intensity and the depth of modulation of laser-Doppler signals is calculated as a function of the angle between the two beams and of the aperture angle of the collecting optical system. The influence of different types of detectors on laser-Doppler signals is described, as well as the signal-to-noise ratios, for different types of detectors as a function of light power and the electronic components of the detectors. A comparison of different types of detectors shows that photomultipliers can be used to measure the speed of small particles. New developments of avalanche photodiodes have lead to photodetectors which yield, in some ranges, qualities of laser-Doppler signal comparable to photomultipliers.