GB957737A - Improvements relating to spectrometers - Google Patents

Abstract

957,737. Spectrometers. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. June 8, 1962 [June 9, 1961], No. 20953/61. Heading G2J. A spectrometer comprises a beam splitter for dividing a light beam of narrow bandwidth into a plurality of plane polarized elemental beams of the same intensity, a plurality of interferometers for dividing each elemental beam into four pairs of interfering beams and for allowing these pairs to recombine so as to form, for each elemental beam, four interference patterns, and means for measuring the intensity of each interference pattern, the path difference for each pair of interfering beams being selected so that the four patterns are identical but have relative phase differences equal to a quarter of a cycle of the variation of the mean wavelength of the incident light beam. As shown in Fig. 1, a beam 1 is passed through a monochromator 2 and then split into plane polarized elemental beams at 3, each elemental beam passing to an interferometer 4 which divides it into four pairs of interfering beams forming interference patterns, the intensities of which are measured by photo-cells 5 associated with cathode ray tubes for displaying the output. Each interferometer may be, as shown in Fig. 3, a modified Michelson having a split cube with a polarizing interference filter at its interface 9. The reflected and transmitted component beams pass through quarter-wave plates 13, 14 before being reflected back to the cube whereby they are respectively transmitted and reflected, along the same path as two beams which are polarized in orthogonal planes and have a phase difference therebetween. Half of each beam then passes through a quarter-wave plate 15 so as to produce two pairs of beams, the path difference of one pair differing from that of the other. These two pairs of beams then pass through lens 16 to birefringent prism 17 whereby each beam is split into two beams polarized in planes at 90 degrees to each other and at 45 degrees to the plane of polarization of the incident beams. Thus eight beams are formed in four interfering pairs to give rise to four interference patterns. The device 3 of Fig. 1 may be formed from a number of double-image polarizing prisms in cascade. In an alternative the two component beams of which half of each passes through quarterwave plate 15 may be formed by splitting the elemental beam by a double image prism 21, Fig. 4, and passing them through lens 22 so that each strikes a part 23, 24 of a split concave mirror positioned so as to image the prism 21 back on itself. In yet another embodiment, the plane polarized elemental beam passes through a birefringent plate so as to form two component beams which are perpendicularly polarized and have a phase difference proportional to the thickness and birefringence of the plate. The birefringent plate in this embodiment may be replaced by two birefringent plates, relatively rotated through 90 degrees and having a quartz crystal therebetween, this crystal being cut normal to the optic axis and of a thickness such that the plane of polarization of light of the wavelength for which the apparatus is designed is rotated through 90 degrees.