Patent GB-1065868-A (GB)
Title: Improvements in the measurement of distance
Inventor: ROWLEY WILLIAM RICHARD CHARLTO
IPC: G01S17/32
Filed: 1964-05-26
Abstract: 1,065,868. Photo-electric distance measuring. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Aug. 5, 1965 [May 26, 1964], No. 21820/64. Heading H4D. A method of determining whether a reflecting object is in a desired position comprises directing two beams of coherent electromagnetic radiation at the object from a first known position, examining at a second known position the phase relationships between direct and reflected beams received there, adjusting the frequency of each radiation such that the difference in length of the paths of the said reflected and direct beams contains a whole number plus a fraction of wavelengths, the fraction being the same for each radiation, determining the differences between said whole numbers and between the frequencies of the radiations, and deducing therefrom the distance between the first known position and the reflecting object. As described, two continuous wave gas lasers 12, 13 provide beams 14, 15 of coherent light, which are plane polarized at 90 degrees to one another, and which intersect at 90 degrees at a half-silvered mirror 20, from which one composite beam 22 proceeds directly towards a halfsilvered mirror 23 whilst another composite beam 21 proceeds towards a reflector 11, from which it is returned as a beam 24 to the mirror 23. The beams 22 and 24 are mixed on intersection at the mirror 23 and composite beams 26, 27 are directed to polarization analysers 28, 32 each of which separates out components originating from the lasers 13, 12 respectively, and directs them to associated photo-detectors 30, 31, and 33, 34. As.the beam which traverses the longer path 21, 24 experiences a different number of reflections from that which traverses the shorter path 22, the components of each in the composite beams 26, 27 are distinguishable by polarization differences. Thus, the analyser 28 may be arranged to transmit the beam component from the laser 13 which has traversed the longer path and the analyser 32 may be arranged to transmit the beam component from the laser 13 which has traversed the shorter path. These transmitted beams illuminate the photo-detectors 30, 33, whose outputs are connected to the two inputs of a difference amplifier 35. Similarly, the photo-detectors 31, 34 whose outputs are connected to the two inputs of a difference amplifier 36, may be made responsive to beam components from the laser 12 which have traversed, respectively, the longer path and the shorter path. The outputs of the amplifiers 35, 36 are connected to automatic controls 37, 38 which adjust the frequencies of the lasers 13, 12 so that each amplifier gives minimum output, indicating that the two signals from the photo-detectors 30, 33. are substantially equal and likewise those from the photo-detectors 31, 34 so that there is nominally a whole number of wavelengths of the light from each laser in the path difference. A half-silvered mirror 41 extracts a portion of the beam 22 and directs it through an analyser 42 to a photo-multiplier 40, from which an output is obtained with a beat frequency f indicative of the frequency difference between the beams 14, 15. The control 38 has a manual adjustment for the frequency of the laser 12 and, in use, the two lasers are first brought to the same frequency and then laser 12 is adjusted to obtain the next minimum using the said manual control, when there will be one more wavelength of its radiation in the path difference. The length of the path difference can then be obtained by dividing the known value for the velocity of light by the observed beat frequency. When the apparatus has been set up, any movement of the reflector 11 may be measured from observation of the corresponding change in the beat frequency f.