Patent GB-926796-A (GB)
Title: Light modulating device
Inventor: HYMAN ROBERT ANTHONY
IPC: G02F1/01
Filed: 1960-08-10
Abstract: 926,796. Terebrescence. STANDARD TELEPHONES & CABLES Ltd. Aug. 4, 1961 [Aug. 10, 1960], No. 27713/60. Class 39 (1). [Also in Groups XX, XXXVIII and XL (b)] A light-modulating device includes a light transmitting solid having a temperature dependent light absorption edge (i.e. a region of its light transmission characteristic where the absorption of transmitted light varies very rapidly with wavelength), and means for simultaneously varying the temperature of the whole of an area of the extending across the width of a surface of the solid through which light to be modulated is passed. Examples of suitable solids are germanium, silicon, selenium, indium antimonide, lead sulphide, lead selenide, or lead telluride in single crystal form. The position of the absorption edge in germanium and silicon is situated at a wavelength of the order of 1.8 microns and 1.1 microns respectively, and moves to the longer wavelengths as the temperature is raised, whilst for the lead compounds the position of the absorption edge is situated at a wavelength of the order of 0.73 microns and moves to shorter wavelengths as the temperature is raised. The temperature of the solid may be raised by a beam of radiation, but only detailed embodiments are described for raising the temperature by electric heating. In a first arrangement, Fig. 1, for use where the material is an electric conductor the material is formed as two blocks 11, 12 joined by integral bridge portion 13 through which radiation 18 to be modulated is passed. The radiation is chosen to be monochromatic and has a wavelength located at a suitable point on the absorption edge. Lowresistance electrode 14, 15 with associated connecting wires are affixed to blocks 11, 12 and permit the passage of electric current to heat the bridge portion 13. The blocks 11 and 12 are immersed in a constant temperature bath, for example a cooling liquid such as nitrogen, oxygen, neon, helium, hydrogen or an organic liquid. Alternatively, a liquid-solid (i.e. an ice-water) bath, or an electric refrigerating system may be used. The electric current is applied in pulses and modulates the intensity of radiation 18 at the frequency of the pulses, the degree of modulation depending on the temperature rise during each pulse, which may be between 1 ‹ and 10 ‹ C. In a second arrangement, Fig. 2 (not shown), only a single cooled block is employed with a thin radiationmodulating portion extending from it. In a third arrangement, Fig. 3, for use where the material is an insulating material, such as selenium or a selenium-sulphur alloy, the material is formed as a film 31 which is deposited or otherwise attached to a bridge portion 32 joining blocks 33, 34 all formed of a material, such as silicon carbide, which is transparent to the radiation to be modulated, has high thermal conductivity and which may be electrically heated. A fourth arrangement, Fig. 4 (not shown), uses the same principle but comprises a single block with a thin section extending from it and bearing the modulating film 31. In a fifth arrangement, Fig. 5 (not shown), using indium antimonide which is electrically conducting, the material is formed as a film on a light transparent block of high thermal conductivity and is heated by passage of current through the film. The block may be electrically insulating or electrically conducting with an interposed insulating layer. Other constructional arrangements are referred to in which separate electric heating films are sandwiched with a modulating film. The portions of the blocks which are immersed in a cooling bath may be made of copper. Surfaces of light transmitting solids may be bloomed. An application of the invention to a co-ordinate array of light modulators with an associated array of light-responsive devices is described, Fig. 6 (not shown, see Group XXXVIII).