Patent GB-731922-A (GB)
Title: Light modulator
IPC: H04B10/50
Filed: 1951-06-29
Abstract: 731,922. Polarizing apparatus. GOODYEAR AIRCRAFT CORPORATION. June 29, 1951 [Dec. 16, 1950], No. 15467/51. Class 97 (1). [Also in Group XL (b)] Apparatus for modulating a beam of light from a source 1 comprises a polarizer 3 and analyser 4 between which are situated two crystal elements 6, 7 having a 90-degree opticalrotation plate 8 situated between them, the crystal elements 6, 7 being such that when excited electrostatically, as shown, or by the application of heat or mechanical stress thereto, they are changed from a uni-axial to a bi-axial condition thus effecting a relative phase retardation between perpendicularly polarized components of the incident light. In the embodiment illustrated the crystals employed are P-type crystals, i.e. one of the primary phosphates or arsenates of ammonium, potassium or rubidium, these having the property of changing from a uni-axial to a bi-axial condition upon the application of an electric field thereto in the direction of the Z-axis. Each crystal 6, 7 is consequently provided with a light transmitting electrode surface on each side thereof, such as for example a thincoating of gold, glycerine or stannous oxide. The polarizer and analyser are so positioned relatively to one another that in the absence of the bias retardation plate 5 and without any excitation of the crystals 6, 7 no light passes to the pick-off device 2. However, upon the application of oppositely directed electrostatic fields from the source 9 to the crystals 6, 7 positioned with their axes as shown, each is. altered to a bi-axial condition, the retardations introduced by the two crystals adding together on account of the interposition between them of the polarization rotator 90 degrees. The plane polarized light from the polarizer 3 is consequently altered to an elliptically polarized condition and light is passed by the analyser 4. The crystals may alternatively be orientated as shown in Fig. 2, in which case the electrostatic fields are made to act in the same direction. Fig. 3 illustrates the manner in which the amplitude of the light passing through the analyser 4 varies with the state of polarization of the light incident thereon, the " demodulator point " showing the state when the plate 5 is omitted and the crystals 6, 7 are unexcited. By the optional introduction of a retardation plate 5, the plane polarized light from the polarizer 4 may be converted to approximately circular polarization as indicated by the " typical modulator point " and modulation from this point will be substantially linear. The bias retardation plate comprises a sheet of bi-refringent material such as mica, quartz, selenite (crystalline calcium sulphate) or a stretched sheet of a suitable plastic material such as polyvinyl alcohol laminated between glass plates. Variable retardation may be achieved with a sheet of polyvinyl butyrate plasticized with 15 per cent butyl sebacate mounted in a clamp which can be adjusted to stretch the sheet to give the desired retardation. In alternative forms of the invention the light may be modulated in accordance with the application of heat to the crystal elements, the crystal material being selenite, or in accordance with the application of mechanical stress in which case crystals of ammonium dehydrogen phosphate, potassium dihydrogen phosphate or Rochelle salt may be used. Reference has been directed by the Comptroller to Specification 690,467.