Noise-cancelling signal difference method for optical velocity measurements

Abstract

Polarized light is used to generate two phase-shifted interference fringe fields at the beam crossing point (test point) of optical velocity measurement systems (laser Doppler anemometers). Signals observed separately at two photodetectors are differenced, leaving an almost pure Doppler frequency, and eliminating practically all disturbances. The method presented improves the signal quality and signal-to-noise-ratio for further processing of the frequency content than even very complex and costly electronic processing. The beamsplitting and polarization properties of Wollaston prisms are used to advantage to obtain simple and compact systems. Single and dual photodiodes are used without preamplification.