GB896934A - Improvements relating to diffraction gratings - Google Patents

Abstract

A reflecting phase diffraction grating comprises a base plate with an optically flat surface, several parallel sided strips of a first material on this surface, the strips being of equal width and adjacent strips being spaced apart by a distance equal to their width and the thickness of the strips being equal to one quarter of a wavelength of the light to be used with the grating; a reflecting layer of a second material covering the strips and the uncovered portions of the surface so as to form a reflecting surface with several parallel sided steps corresponding with the strips and a transparent layer of silicone monoxide covering the reflecting surface. The diffraction grating may be made by applying to the optically flat surface of a block, a layer of photo-sensitive resist material on which a diffraction grating pattern is contact printed so that on subsequent development a negative image of the grating in hardened resist material remains upon the surface of the block. The block is then placed inside a vacuum chamber and cleaned by means of a glow discharge whereupon the chromium is vaporised to form a layer over the uncovered part of the surface of the block and over the resist material. The chromium layer is deposited until its thickness is equal to one quarter of the wavelength of the light to be used. On removal of the block from the vacuum chamber it is rubbed with a pad soaked in hydrogen peroxide or nitric acid so as to remove the resist material and its overlying chromium layer. The block is then replaced in the vacuum chamber and cleaned again before a layer of aluminium, chromium or rhodium is deposited on the pattern of chromium and the uncovered portions of the surface of the block so as to result in a reflecting surface in the form of a series of steps, each of a thickness equal to one quarter of a wavelength of the light to be used with the grating. Finally, a layer of silicon monoxide is deposited over the second layer to form an abrasion resistant coating. In a modified method the block is first given a silver coating and this is overcoated with photo-sensitive resist material in which a negative image of the grating is formed as before. The uncovered portions of the silver layer are then etched away with an alcoholic ferric nitrate solution and resist material covering the remaining portions of the silver layer is removed as by weak acetic acid and immersion in a trypsin suspension to leave silver strips on the block. After cleaning, a chromium layer is deposited by evaporation in a vacuum and the silver strips with overlying chromium layer are removed as before. Thereupon a second reflecting coating and a silicon monoxide overcoat are applied as in the first method. Apparatus for measuring the thickness of the chromium layer is also described (see Group XL(b)).