GB1301553A - - Google Patents

Abstract

1301553 Light guides PLESSEY CO Ltd 26 May 1971 [14 July 1970] 34055/70 Heading G2J [Also in Division H4] An electric potential is applied between a strip electrode 5 and an electrically conductive substrate 3 disposed either side of an insulating electro-optic layer 4, Fig.2, whereby a region 6 of layer 4, of dimensions equal to the width and length of electrode 5 and to the thickness of layer 4, is altered in refractive index, the electric potential and consequent refractive index change being such that propagation of an electromagnetic wave within region 6 is restricted to a single mode, or to a restricted set of modes, i.e. region 6 acts as a light guide. Restriction of the propagation mode is effected by reducing the transverse dimensions of the region 6 to below the cut-off dimensions for higher modes. Thus for a wavelength 0.7 Ám, and a refractive index change of + 0.1% in region 6, the height and width of region 6 should be 4 Ám and 8Ám respectively. In the preset construction the two dimensions are controllable in separate steps. The electrode 5 also serves to eliminate modes of polarization parallel to the major surfaces of layer 4. Layers 3 and 4 may be modifications of a single material e.g. layer 4 may be formed by diffusion or epitaxial growth on a semi-conducting substrate of Zn Se or Zn Te, or alternatively they may be dissimilar e.g. KDP contented on tin oxide/glass, or a semi-conductor, and subsequently lapped to a desired thickness. With the latter construction, operation in a single mode may only occur in the plane of the substrate. A delay line or other optical processing device may be interposed between two such waveguide in series. A prism coupler may be used for entering a selected mode in a region 6. A directional coupler may be formed by disposing two such wave-guides close together. As shown in Fig. 3b, the two guides may be formed in a single layer 4. The behaviour is determined by the gap between the guides, the coupling length and the various refractive indices. Therefore by varying the potential applied to electrodes 5, 5, 3, within the limits required for single mode propagation, to modify one or more of the refractive indices in the coupling region, light passed into one guide may be selectively passed out by either guide.