Optics: measuring and testing – Velocity or velocity/height measuring – With light detector
Patent
1983-08-25
1986-03-11
Buczinski, S. C.
Optics: measuring and testing
Velocity or velocity/height measuring
With light detector
356 28, G01P 336
Patent
active
045752389
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a laser-doppler-anemometer of the kind in which a radiation beam having a predetermined direction of polarization emitted from a laser is separated into two spatially distinct partial beams, one of which is led through a frequency displacement device, following which the separated frequency-displaced partial beams having the same direction of polarization are focussed in a measuring volume for measuring the rate of flow of a liquid or gaseous medium by interference between said partial beams, whereby an interference picture is supplied to a photoelectric receiving device.
Such a laser-doppler-anemometer is applicable for contactless, non-field disturbing measuring of flow rates of liquid or gaseous media. The measurement depends on photoelectric detection of the movement of a particle conveyed by the flowing medium through the interference pattern of the partial beams focussed in the measuring volume. As for other speed measurings applying the doppler-principle two mutually frequency-displaced coherent laser radiation beams are used to detect the direction of flow, said radiation beams having moreover the same direction of polarization and being synchronized as to their phase.
Thus, in the paths of the two partial beams elements causing different directions of polarization of the partial beams focussed in the measuring volume or influencing them differently as to their phase may not be present.
In flow measuring systems applying laser-doppler-anemometry it has therefore been considered necessary to position the comparatively bulky laser-equipment with associated frequency-displacing and focussing elements directly adjacent to the measuring volume, but this has restricted the possibilities of application.
The said deficiency is remedied according to the present invention in that the laser together with a first beamsplitter for separating the laser radiation beam into said spatially distinct partial beams and said frequency-displacing member affecting one partial beam form parts of a transmitting device which further includes a first polarization-direction-turning device positioned in the path of one of said spatially distinct partial beams for turning the direction of polarization of said partial beam by 90.degree., and another beamsplitter for spatially uniting said two frequency-displaced partial beams having directions of polarization mutually displaced by 90.degree., said transmitting device being connected with the inlet end of a polarization-direction-preserving optical "single-mode" fiber having two orthogonal principal axes adjusted to each respective of said two directions of polarization of said partial beams displaced by 90.degree., the outlet end of said fiber being connected with a measuring probe to be positioned close to said measuring volume and accommodating an additional beamsplitter for spatially separating the partial beams transmitted through the fiber into partial beams with each respective of said two directions of polarization displaced by 90.degree., and a second polarization turning member acting on one of said partial beams for turning the direction of polarization of said partial beam to the same direction of polarization as that of the other partial beam.
By using an optical "single-mode" fiber of the kind concerned as transmitting medium for spatially distinct frequency-displaced partial beams of laser radiation it is made possible to position the bulky part of the measuring equipment at a comparatively great distance from the measuring volume, as the measuring probe connected with the outlet end of the fiber may have small physical dimensions. Between the transmitting device and the measuring probe arranged at the measuring volume light is being transmitted solely in the form of said partial beams, thereby eliminating the risk of disturbances and noise when transmitting electrical supply flows and signals through cables onto an inaccesible measuring volume.
Optical "single-mode"-fibers of the kind concerned are described, inter alia, in an article by
REFERENCES:
patent: 4387993 (1983-06-01), Adrian
patent: 4470696 (1984-09-01), Ballard
Dyott, "The Fiber-Optic Doppler Anemometer," Microwaves, Optics and Acoustics, Jan. 1978, vol. 2, No. 1, pp. 13-18.
Kyuma et al., "Laser Doppler Velocimeter with a Novel Optical Fiber Probe," Applied Optics: vol. 20, No. 14, Jul. 15, 1981, pp. 2424-2427.
Knuhtsen John
Olldag Peder E. S.
Buczinski S. C.
Disa Elektronik A/S
Koltak Melissa
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