Patent GB-1165553-A (GB)
Title: Improvements in or relating to Apparatus for Combining Radiations.
IPC: G02B5/30
Filed: 1966-11-23
Abstract: 1,165,553. Optical apparatus. WESTERN ELECTRIC CO. Inc. Nov. 23, 1966 [Nov. 30, 1965], No. 52368/66. Heading G2J. In a device for combining two coherent monochromatic input radiant energy beams with like electric field polarisation and optical wavelength from sources 32, 33, Fig. 3, to provide the sum and difference of the amplitude-phase vectors of the two input beams, and particularly for use in an infrared communications system, a metallic grid 34 is disposed at the intersection of the beams, the grid 34 having parallel elongated elements 31 of width d parallel or perpendicular to the electric field polarisation to provide a grid which is respectively inductive or capacitive, the normal to the plane of the grid being at an angle # to the direction of each beam to provide identical propagation directions for the transmitted component of one beam and the reflected component of the other, the angle # lying in a plane, orthogonal to the direction of elongation of the elements, the ratio of each wavelength # in the medium surrounding the grid to the element centre-to-centre spacing, a, lying in the range 1 + sin # <#/a < 9 cos #, and the ratio of a to the grid parameter D being appropriate for splitting the beam into equal transmitted and reflected components, where D is d when the grid is inductive and D is (a - d) when the grid is capacitive. As shown the sources 32, 33, are water vapour lasers operating at 118-65Á with electric field polarisation parallel to the grid elements 31 so that the grid is inductive with D = d = 3À93 Á and a = 43À4Á, the grid thus being able to be self supporting between end members such as 39. The sources 32, 33 which are locked in phase by phase lock circuit 35, are so spaced from the grid 34 that the beam from source 33 lags 90 degrees behind that from source 32 as indicated at A and B. Since the reflected component of the beam from source 32 suffers a phase lag of 180 degrees - 45 degrees leaving the grid 34, as indicated at C, and the transmitted component of the beam from source 33 suffers a phase lag of 45 degrees, as indicated at D, the two components are in phase, their amplitudes add arithmetically and, since the amplitude of each component is proportional to the input amplitude, the sum of the component amplitudes is proportional to the sum of the input amplitudes. The phase relationship of the transmitted component from source 32 is shown at E and the reflected component from source 33 at F one component thus being subtracted from the other and the resulting component amplitude difference being proportional to the amplitude difference of the input amplitudes. In the general case in which the input beams have a relative phase difference other than 90 degrees, a vector sum and a vector difference will result at the two outputs and any two of the three input parameters, viz. the two amplitudes and their relative phase, may be calculated if any one of them and the output amplitudes are reasured. In the case of a capacitive grid 34, (i. e. the electric field polarisation orthogonal to the elements 31, a = 43À4 and D = a - d = 3À93Á so that d = 39À5Á) the respective transmitted or reflected components suffer phase leads equal to the corresponding phase lags for an inductive grid. When using a smaller wavelength, and correspondingly smaller grid constants, the grid elements (51, Fig. 5, not shown), may be deposited on the broad face of a right angle silicon prism (55), a similar prism (54) then being pressed firmly against the grid elements (51). The entry and exit faces may be coated with antireflection coatings (56, 57), or in an alternative arrangement, Figs. 6 and 7 (not shown) the entry and exit faces are arranged at the Brewster angle. If the two sources 92, 93, Fig. 8, are aligned with one another, circulators 94, 95, are interposed between the sources and the grid 91, the outputs being obtained from the circulators as shown.