GB1463983A - Laser doppler velocimeter - Google Patents

Abstract

1463983 Laser doppler velocimetry NATIONAL RESEARCH DEVELOPMENT CORP 13 Nov 1974 [13 Nov 1973] 52708/73 Heading H4D The velocity of gas flow, e.g. in a wind tunnel 9 which may be turbulent is measured using laser doppler velocimetry and includes splitting a laser beam 2 into two convergent beams 4, 5 by prisms 3 which beams are made to converge by lens or prisms 7 to intersect at the area 8 in which measurement takes place thereby forming interference fringes 13 see Fig. 2; radiation reflected from particles passing through the fringes being detected and processed using the method described in Specification 1,450,911 at 10. The phase of one or both beams is varied to cause movement of the fringes, in phase shifter 6. The phase of both beams may be varied in opposite senses or the phase of one beam only may be varied from zero towards 2# radians by a sawtooth waveform applied to electrodes of the ammonium dihydrogen phosphate crystal arrangement Fig. 3 comprising two attached crystals 14, 15, having end faces 16, 17 coated in staggered fashion with reflective layers 18, 19 respectively to enable a beam to make three transits through the crystal as shown. In Fig. 3 the crystals are cut to produce output beams phased in opposite senses. The sawtooth waveform produces an effect of continuously advancing fringes in area 8 (see Figs. 4, 5, not shown) the moving fringe pattern enhancing measurement in turbulent flow. In the embodiment of Fig. 7 (not shown) two detached crystals are used in the phase shifter. In the Fig. 8 (not shown) embodiment the converging optics 7 are dispensed with and the detached crystals are disposed in a converging configuration. In the Fig. 9 embodiment (not shown), the phase shifter operates on one beam only. In the embodiment of Figs. 10 and 11 (not shown), in the phase detector, one movable beam (26) is directed through a glass block (27) for optical path compensation the other fixed beam (26) passing through an ADP phase shifting crystal (29). In the Fig. 12 embodiment (not shown) one beam of laser light enters the ADP phase modulator (56) plane polarized at 45 degrees, the orientation of the ADP crystal being such that the vertical beam component is advanced in phase and the horizontal beam is retarded in phase. These components are split in a polarized beam splitter (58) the horizontally polarized beam being converted to vertically polarized in a half wave plate and the beams made to converge as described. Instead of electro-optic crystals, magneto-optic crystals (Y1G) may be used. The laser light may be infra-red, visible or ultraviolet.