Self-aligned heterodyne laser radar system for surface displacement monitoring

摘要

A novel configuration for a reference-beam, continuous-wave, heterodyne low-power laser radar prototype is presented. It measures both magnitude and sign of the radial component of the displacement velocity. The baisc set-up includes a low power (approx 10 mW) commerical HeNe laser, a beam-splitter, an acousto-optic modulator, and a two-lens system that both focuses the transmitted beam on teh target surface and collects the scattered light. Both the reference beam and the radiation collected are focussed onto a Si avalanche photo-detector. The self-aligned configuration of the receiver makes possible, theoretically, to perform optimal mixing between the received scattered radiation and the reference beam. The resulting electrical signal is fed to a transimpedance amplifier and displayed on a spectrum analyser. Laboratory experiments employing as a target the rim of a 50 cm-diameter rotating wheel placed at several distances have been performed. Results concerning detected signal-to-noise ratio, detected-signal spectral width, accuracy of the radial component of the velocity under measurement, system working range, and system tolerance in focus-adjustment distance will be presented and discussed. Compared to a previous homodyne prototype presented by the authors, the present system shows a shorter working range (approx 12 m compared to nearly approx 16 m in the homodyne prototype). We attribute this smaller range to the additional losses in the acousto-optic modulator.