Measurement of linear distances

Abstract

Abstract 1,076,991. Measuring distance optically. NATIONAL RESEARCH DEVELOPMENT CORPORATION. July 19, 1965 [July 17, 1964], No. 29321/64. Heading G1A. A distance is measured by modulating the frequency of a coherent light source, directing a fuse beam from said source over a path of known length through an interferometer, directing a second beam from said source over a distance to be measured through said interferometer, adjusting one of the parameters controlling the relative frequencies of the two light beams on arrival at a detection point at the end of said paths to bring said two frequencies to equality and measuring the extent of such adjustment. The adjusted parameter may be one of the optical paths or the frequency of modulation. In the form shown the cavity of a gas laser L is bounded by end mirrors M 1 , M 2 , the former being driven by an oscillator O to modulate the emitted frequency. The light enters a two-path interferometer, D 1 being the distance to be measured, D 2 a known distance. Beams from the recombining mirror M 3 differ in phase because of phase change on reflection but will, in general, both contain the modulation frequency as a component. The two beams fall on photo-multipliers P 1 , P 2 connected to a differential amplifier A 1 the output of which drives the mirror M 1 to vary the mean laser frequency to maintain a constant phase difference between the beams traversing the paths D 1 , D 2 . The output of P 1 is fed also to a superheterodyne detector A 2 the oscillator of which is varied in step with the oscillator O which is adjusted so that a null output is obtained from the detector. By selecting successive values of the frequency of the oscillator O which give a null output a value for D 1 can be obtained in terms of D 2 .

Inventors

• 👤 Alan Hugh Cook

Bibliographic Details