Patent GB-1263550-A (GB)
Title: Laser interferometer
Inventor: PARKINSON GEOFFREY JOHN
IPC: G01B9/02
Filed: 1969-06-10
Abstract: 1,263,550. Photo-electric laser interferometer. T. I. (GROUP SERVICES) Ltd. Aug.28, 1970 [June 10, 1969], No.29206/69. Heading G1A. In an interferometer employing a multi-frequency laser adapted to give two light beams polarized in mutually perpendicular directions, the photodetector system E<SP>1</SP>, E<SP>11</SP> derives information from only one interference pattern, and in the case where two patterns are formed at the detector system, derives information only from the more distinct of the patterns. In the embodiment of Fig.2 employing a planar multi-frequency laser, one of the polarized components is monochromatic and hence produces a sharp pattern, while the other component contains two different frequencies and its interference pattern is correspondingly blurred. The components traverse together either the reference path terminating in a fixed mirror C, or the measuring path, terminating in a mirror D attached to an object whose position is to be measured, and are finally separated one from another by polarizing devices H<SP>1</SP> , H<SP>11</SP> to form an interference pattern at each detector E<SP>2</SP>, E<SP>11</SP>. At any one instant, one pattern will be sharp, the other blurred. However, any slight change in the length of the laser, caused e.g. by warming-up, causes both planes of polarization to rotate by 90 degrees, and hence the two patterns to interchange at the detectors. This changeover can be a quite frequent occurrence. To obviate this, the detector outputs are fed to a logic circuit L which switches the two detectors so that only the output signal from the one which is receiving the sharp pattern is utilized for measurement purposes. This discrimination is based on the amplitude modulation of each pattern, such modulation being greater for the sharp pattern. The two paths traversed by the polarized components may be physically separated by employing a beamsplitting arrangement (J, K) together with the two polarizers H<SP>1</SP>, H<SP>11</SP> between the laser and the main beam-splitter B Fig.3 (not shown). In an embodiment employing a single detector Fig.4 (not shown) a polarizer (P) placed upstream of the splitter B ensures that the detector receives radiation polarized in just one plane. This plane may accommodate either the mono-chromatic component, or the two-frequency component, according to the polarization of these components when they leave the laser. A Kerr cell (K) is placed in the beam upstream of the polarizer and is responsive to a logic circuit which in turn is connected to the detector output. As the interference pattern changes from sharp to blurred, the logic circuit responds and actuates the Kerr cell, which rotates the two polarization planes of the laser beam by 90 degrees, hence bringing back the sharp pattern to the detector. In another singledetector embodiment Fig. 5 (not shown), the laser is placed in a constant or pulsed magnetic field which maintains the mono-chromatic component in a fixed plane. In this case the polarizer P simply is set to pass only this plane, the logic circuit being dispensed with. In the embodiment of Fig.6 the multi-frequency laser A emits all its frequency components in a single plane, and additional polarizers G<SP>1</SP>, G<SP>11</SP> produce two perpendicularly polarized multifrequency components. These are subsequently separated by two further polarizers H<SP>1</SP>, H<SP>11</SP> to produce two interference patterns, one at each detector. Both patterns will in general be blurred, but by using two reference paths C<SP>1</SP>, C<SP>11</SP> of different length the patterns are mutually phase-shifted, so that there will be a part of one pattern which is relatively sharp, with a corresponding part of the other less sharp. The detectors are placed to take maximum advantage of these relative quality differences in the two patterns, and the logic circuit L switches the detectors so that only the output produced by the more distinct pattern is utilized, the respective quality of the patterns changing as a result of changes in the laser length. The phaseshift in the two patterns may be introduced by physically separating the paths of the two polarized components and using two mirrors on the object.