New Optical Technique for the Generation of Quadrature Signals on Laser Doppler Velocimeter.

Abstract

A novel laser Doppler velocimeter (LDV) technique which uses two single-frequency diode lasers, one for each LDV beam, is presented. The existing techniques using the optical frequency difference between two lasers for directional discrimination was enhanced by the technique discribed here. The Doppler frequency extracted by an optical superposition technique and a pair of quadrature signals in the base band was generated without any broadband electronic heterodyning. The electronic heterodyning on the conventional systems was implemented by an electronic mixing technique of the beat signal of the two lasers and the measuring signal generated by the scattered light of the LDV beams. This paper describes a new optical technique in contrast to these electronic heterodyning systems. In order to extract the Doppler frequency optically, undesired mixing products must be avoided. The realization of this concept is based on the superposition of four light waves on one photodetector, where only two pairs of light waves are made to forcedly interfere by polarization or by optical adjustment. Thus, the photodetector acts as an optoelectronic element that superimposes two optical beat signals, so that the Doppler frequency is obtained from the envelope of the resulting output signal directly. In order to obtain the desired directional information, a second photodetector for quadrature signal generation is required. The phase delay between the detected signals was fixed by positioning in the expanded wavefront of the reference beams. A prototype of the directional LDV, which uses two lasers and the described optical mixing, has been realized.