Novel laser Doppler velocimeter enabling fast instantaneous recordings

A new type of a laser Doppler velocimeter based on wavelength detection with a two beam interferometer is presented. The sensitivity to small Doppler shifts of the monochromatic laser light is obtained using an optical path difference which is large as compared to the light wavelength. By means of a fast feedback system, the ratio of the optical path difference to the light wavelength is kept constant; wavelength changes are immediately answered by changes of the optical path difference generated with a Pockels cell. The Pockels cell voltage is a signal proportional to the instantaneous wavelength changes of the scattered light enabling a direct recording of the corresponding velocity component of the scattering object. With a slight seeding of the flow, a continuous recording can be obtained with a time resolution of 1 microsec or less. The technique is specially suited for local and nonintrusive velocity measurements in highly unsteady flows. Its application to shock tube experiments was demonstrated by examples