GB874462A - Improvements relating to diffraction gratings - Google Patents

Abstract

874,462. Diffraction gratings. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. May 17, 1960 [May 21, 1959], No. 17344/59. Drawings to Specification. Class 97(1) A transmission phase diffraction grating comprises a plane parallel transparent plate having several parallel-sided strips of silicon dioxide on one surface, the thickness of the strips being such that the retardation of a ray of light travelling in a direction normal to the grating and passing through the plate and a strip is one half of a wave length of the light compared with the' ray of light passing normally through the plate only. This grating may be made by first coating a plane parallel glass plate with a layer of photosensitive resist such as bichromated glue or gelatin and photographically reproducing thereon the required diffraction grating pattern by exposure to ultra-violet or other exciting radiation, so that on subsequent development in an appropriate solvent a negative image of the grating in hardened resist material remains upon the surface of the plate. The plate is then mounted in a vacuum chamber and those portions of the glass plate which are not covered by the resist material are cleaned by a glow discharge. Oxygen is then fed into the vacuum chamber and silicon monoxide, contained in a tantalum foil capsule, is slowly vaporised within the chamber by passing a current through the foil. In this way a layer of silicon dioxide is deposited over the surface of the glass block and the layer of resist material. After removal of the glass plate from the vacuum chamber, it is rubbed with a pad soaked in hydrogen peroxide or nitric acid which removes the resist material and the overlying layer of silicon dioxide leaving the required strips of silicon dioxide on the glass plate to form the diffraction grating. Another method is described in which a silver layer is first coated on the glass plate and is then overcoated with a layer of photo-sensitive resist on which a master negative of the required grating is contact printed. A negative image in the resist material is then formed as in the first method whereupon the uncovered portions of the silver layer are etched away with an alcoholic ferric nitrate solution. The hardened resist material covering the remaining portions of the silver layer is then removed by soaking in a weak acetic acid followed by immersion in a trypsin suspension. The block is then cleaned by a glow discharge and coated with a layer of silicon dioxide by the vacuum process. Hydrogen peroxide or nitric acid is then used to remove the silver layer and the overlying silicon dioxide layer so as to leave only the silicon dioxide strips forming the diffraction grating. If desired, the. strips may be arranged in. a plurality of concentric circles so as to form a circular diffraction grating.