Patent GB-1313844-A (GB)
Title: Interferometer
IPC: G01B9/02
Filed: 1969-04-23
Abstract: 1313844 Photo-electric interferometer VICKERS Ltd 9 April 1970 [23 April 1969] 20848/69 Heading G1A [Also in Division G2] An interferometer has a first beam splitter, for dividing an incident beam of light 12 into a plurality of beams of coherent light and a second beam splitter for recombining two of the component beams into a single composite beam, one of the beam splitters being a rotating radial diffraction grating 14. The zero order diffracted beam 12a and the plus first order beam 12b are rendered parallel by a lens 15, polarized by polarizer 16, then double refracted by a doublet plate 17 (e.g. a Savart plate) such as to bring one doublerefracted continuation of beam 12a into coincidence with the other double-refracted continuations of beam 12b, thus producing a composite beam comprising two mutually perpendicular plane polarized components whose phase difference varies cyclically at a rate proportional to the rate of rotation of the grating 14. The composite beam passes through a field diaphragm 18 and through lenses 19 and 20, between which is a refracting scanning system which deflects the beam into a master pattern which is imaged by objective 22 on to the specimen to be examined, thus allowing the rapid scanning of a specimen, such as a living cell, which is liable to change. The objective 22 is the first element of a double-refracting interferometer microscope system also comprising double refracting plates 23 and 25, half-wave plate 24 and condenser lens 26. The plate 23 divides the composite beam into its components, one of which is passed through the specimen, and the plate 25 recombines them. An analyser 27 renders the polarization planes of the components parallel to allow interference, the result being a phasedisplaced version of the modulation introduced by the rotating grating 14. This is converted to an A.C. voltage by photo-detector 28, and this is compared with a second voltage produced by a second photo-detector 32 which receives light from a source 30 after passage through a slit 31 and through the transparent slots of the grating 14 so as to be modulated by the rotation of the grating. Alternatively the rotating grating may constitute the second beam splitter, and the doublet 17 the first. In a modification, Fig. 2, the first beam splitter may be a rotating phase grating 36 receiving polarized light from source 11 via a polarizer 34. The plus first order diffracted beam 12a and the minus first order beam 12b are rendered parallel by lens 15 and mutually orthogonally plane polarized by polarizers 37. A slot-form diaphragm 37a placed in any position where the diffracted orders of the grating 36 are spatially separated eliminates orders higher than unity. The beams 12a and 12b are focussed on a stationary phase grating 39 where they are combined. For accurate recombination the two gratings must be of identical shape and pitch; it is possible for the second grating to be rotated instead of the first. The composite beam may be directed to a double refracting interferometer microscope as before, the reference voltage being generated as before; alternatively the reference voltage may be provided from a photodetector 42 receiving part of the composite beam from a beam splitter 40. In either arrangement either an amplitude grating or a phase grating may be used. In the modification of Fig. 2 however a wide spectral range may be used, whereas this is not so when a doublet is used. In either arrangement the polarizer 16 or 34 may be omitted and the light source 11 be a laser emitting polarized light.