Patent GB-1309378-A (GB)
Title: Multi-frequency laser interferometers
IPC: G01B9/02
Filed: 1971-06-15
Abstract: 1309378 Interferometers NORGES TEKNISK-NATURVITENSKAPELIGE FORSKNINGSRAD 15 June 1971 [16 June 1970] 27970/71 Heading G2J [Also in Division G4] In an interferometer using as the light source a multi-frequency laser, to ensure that the contrast of at least one interference pattern is good, the sub-beams in one of the branches of the interferometer are made to travel over different length paths, since if the laser has a resonator of length L all frequencies except those for which half the wavelength is a fraction of the resonator length L are suppressed and consequently if light beams of adjacent frequencies are in phase after travelling a particular distance they are out of phase after travelling a further distance L. In one embodiment, Fig. 5 (not shown) a movable prism (P51) and a fixed prism (P52) each receiving one of the sub-beams from the laser via a mirror (S51) split the sub-beams into two light beams one of the light beams associated with the fixed prism additionally being transmitted to and reflected from a further mirror (S'51) so that it travels an additional distance L. After recombination the light is transmitted to two separate detectors (D51, D52 and D53, D54), good contrast always being obtained at one set. In the embodiment of Fig. 6 the light from the laser is split into two parallel light beams by mirrors S62 and S62', the beams then being directed towards semi-reflecting plane mirror S61. The resulting two reflected beams are transmitted to two fixed prisms P62, P62', at different distances from mirror S61. The transmitted beams are fed to movable prism P61 After reflection the recombined light beams are received by detector sets D61, D62, D63, D64. The two parallel light beams may alternately be derived using two pentagon prisms (S72, S'72) Fig.7a (not shown) or a plane parallel transparent plate (S72) Fig.7b (not shown) with a mirror on part of the side nearest the laser and a semi-reflecting mirror on part of the side furthest from the laser. In another embodiment, Fig. 8 (not shown) light emitted from both ends of the laser is divided by mirrors (S'83, S83 and S'82, S82) into two pairs of parallel beams which are transmitted to fixed pentagon prisms (P'82, P82) and a movable prism (P81) the reflected beams after recombination at the lowest (S81, S'81) being received by detectors (D81, D82, D83, D84). In another embodiment two parallel light beams (I, II) Fig.10c (not shown) with a suitable phase difference are derived by means of two mirrors (S'102, S102) and fed via a first prism (103) having a semi-reflecting coating (S101) on the hypotenuse to movable prism (P101) and fixed prism (P102) cemented to the first prism (103). The reflected light is detected by two sets of detectors. In another embodiment, Fig.11 (not shown) light beams (I, II) from the laser are transmitted to fixed prisms (P111, P112) having semireflecting coating on two of their sides so that the light beams are fed via movable mirrors (S111, S112, S113) back via the other prism (P112, P111) to four detectors. Further prisms (P113-P116) act as complementary prisms. This apparatus is suitable for measuring displacement in a drafting machine. In the embodiment of Fig.12a a fixed mirror arrangement S125-S128 receiving light beams I, II from a laser and co-operating with a beam splitter 120 results in light beams IT, IR, IIT, and IIR being fed by a second mirror assembly S121-S124 to a movable assembly 121 comprising two triangular prisms with a semi-reflecting common base surface, the resulting light being sensed by four detectors. The beam splitter P120 comprising two triangular prisms one half of the common base surface being semi-reflecting and one half being completely reflecting, Fig.12b (not shown). Additional prisms (P121, P122) provide for the return of light reflected from the semi-reflecting section of the base surface. In another embodiment, Fig.13a (not shown) the light from a polarizing laser is split by plane parallel plates (F132, F133) into phase displaced beams (IR, IT, IIR, IIT) which are reflected by a movable prism (P132) and a fixed prism (P131) onto a plate (F131) where the beams are recombined and detected by four detectors. A pair of detectors may be selected by either (1) comparing the contrasts in the two sets of interference patterns (2) switching from one detector set to another each time a predetermined number of patterns have been counted or (3) operating a switch each time the movable member moves past sensing elements at fixed positions.