Method and apparatus for determining the sign of the direction of a

Abstract

In a method in which a heterodyne-laser-Doppler method is used for the demination of the sign of the direction of a particle stream, the beam from a laser source is directed by means of an optical arrangement used as an emitter optical system to a measuring volume in an interferometer device. A frequency difference between the emitted laser beam and the laser beam returning into the optical arrangement is measured through heterodyne reception by means of the same optical arrangement used as a receiving optical system; this returning laser beam is frequency-shifted because of scattering by particles moving with the same speed in the particle stream and because of the Doppler effect. In the method according to the invention, an additional Doppler shift with a definite sign then is alternately impressed on the laser beam in exactly defined areas of beam paths, the additional Doppler shift being preferably considerably smaller than the Doppler shift generated by the particle movement. The exact direction of the particle stream then is determined from the received scattered back laser beam by comparison of this scattered back laser beam with a laser source reference beam. The additional Doppler shift can be impressed on a reference beam or emitted beam split from the laser beam, on the laser beam scattered back by the measuring volume or on the emitted beam and signal beam flowing in opposite directions in the optical arrangement.