Patent GB-970640-A (GB)
Title: Improvements in interferometers
IPC: G01B9/02
Filed: 1960-10-04
Abstract: 970, 640. Electric selective signalling systems. GENERAL PRECISION Inc. Oct. 4, 1960 [Oct. 7, 1959], No.34003/60. Heading G4H. [Also in Division G2]  An optical interferometer wherein an incident beam of light or equivalent radiation is split by a beamsplitter into a reference beam and a measuring beam which are subsequently recombined, the optical path length of the measuring beam being variable to create interference fringes in the recombined beam and the beamsplitter also serving as a recombiner, characterized in that the beamsplitter is part of a unitary optical member that includes at least one reflecting surface additional to and having a fixed spatial relationship with the beamsplitter whereby the reference beam is caused to follow, after splitting and before recombination, an optical path of fixed length within the boundaries of said unitary optical member.  As shown, in Fig. 2A, light from a lamp 72 is collimated at 74 and the required spectral line selected by an interference filter 76 before incidence on a prism cube 1 having an internal beamsplitting surface 2, which divides an incident light into a reference beam 1 and a measuring beam <SP>1</SP>M The reference beam is reflected on to a mirrored surface 5 and thence through a transparent plate 8 to a plane mirror 10 on one side of a plate 20 which is attached to the transparent plate 8. This mirror returns the reference beam back to the beamsplitter 2, at which it is combined with the measuring beam 1 M which has been returned to the beamsplitter after passage through a retardation plate 14, which introduces a phase difference of 8 at each pass between the two halves of the measuring beam, the plate 8 retroreflector 15 and a mirror 11 on the opposite side of the plate 20 from mirror 10. The recombined beams proceed from beamsplitting surface 2 to a mirrored surface 7 and are there reflected to lenses 80, 80<SP>1</SP> and photomultipliers 82, 82<SP>1</SP> disposed behind slits 81, 82<SP>1</SP> at the focus of each lens. Only the half of the measuring beam which has been phase retarded enters the lens 80, the other half of this beam entering the lens 80<SP>1</SP>. In this way a bi-directional fringe count can be obtained corresponding to the displacement of the retro-reflector 15 from a reference surface 83. In another form, the collimated incident light is split at 2 and the measuring beam is returned to the beam splitter after traversing a retro-reflector 23 in the form of a cube corner prism. The beams recombined at the beamsplitter 2 are then internally reflected at the surface 6 and dispersed by refraction at the surface 28, this dispersion being supplemented by an auxiliary prism 29 which may be dispersed at minimum deviation.  The incident light may, as shown, in Fig. 6A, be oblique to the surface of the beamsplitting cube so as to result in the dispersion at the surface 4. The recombined beams are additionally dispersed at the exit surface 28 which is parallel to the surface 4. Fig. 7A shows an arrangement in which the path of the reference beam 13 within the prism cube is lengthened. In this embodiment also, there is an initial dispersion at the surface 4 and a further dispersion at the surface 28.  In a further form, Fig. 8A, the collimated light is obliquely incident on the surface 21 and is split into the surface 2 on to the reference beam 1 B  and the measuring beam 1 M . In this case, the reference beam is reflected at a mirrored surface 5 to a mirrored surface 34, which returns it along the same path to the beamsplitter 2 where it is recombined with the measuring beam 1 which has been returned to the beamsplitting surface by a mirrored surface 33 after traversing a retroreflector 23. The recombined beams are again reflected at 37 and dispersed at the incident surface 21 before reaching the lenses 80, 80<SP>1</SP> and the photomultipliers. In this embodiment the prism cube 1 is made up of four prisms 38, 39, 40 and 41 which are cemented together, the beamsplitting surface 2 and the mirrored surface 37 being provided at interfaces. If desired the path length of the measuring beam may be increased by attaching to the unmirrored surface 34 a transparent block having a mirrored surface 36.