Abstract
A Laser Doppler Instrument has been developed for the measurement of vibrations on moving objects. A particular interest has been taken in rotating turbine blades. The technique used is similar to the well-known microwave Doppler radar system, but the many elements in an optical counterpart have been investigated. The analysis of the coupling network between photodetector and following amplifier, and the optimization of the optical receiver para-meters such as beam splitting and current gain are amongst the main subjects of this work. The signal and noise performance has been fully specified by means of a newly proposed Generalized System Noise Figure, Fs*, which is expressible as a function of a few simple, measurable parameters. Using this Noise Figure a comparison between different systems and photodetectors such as photomultiplier, p-i-n and avalanche photodiodes, has been carried out and in conclusion the avalanche photodiode is recommended. The minimum measurable vibration velocity depends on the Doppler broadening, and the optimum optical arrangement is calculated to minimize this effect. Very closely related to the main work is the analysis of the performance of an avalanche photodiode used as an electronic mixer. The results of measurements in the laboratory prove that it is feasible to measure vibrations on moving objects using laser Doppler techniques and that the analysis developed satisfactorily describes the performance of the system.