Patent GB-1437067-A (GB)
Title: Optical waveguide couplers
IPC: G02F1/03
Filed: 1974-08-08
Abstract: 1437067 Controlling light STANDARD TELEPHONES & CABLES Ltd 8 Aug 1974 34979/74 Heading H4F [Also in Division H1] Two optically coupled waveguides are formed in a three layer semi-conductor structure 10- 12, the centre layer 10 having a higher refractive index for optical confinement in one direction, and a reverse bias is applied to a P-N junction in the structure to produce on the N-type side of the junction a depletion region (of increased refractive index) in at least one guide or in the coupling region between the guides, whereby the mutual coupling between the guides is controlled. The guides should be of semiinsulating or P-type material with surrounding portions of layer 10 being of N-type material for lateral optical confinement. Layers 11 and 12 may be respectively if P-type and N-type GaAlAs, and layer 10 of GaAlAs with reduced Al content, or GaAs, for greater refractivity. In Fig. 2, the guides are defined by the region common to layer 10 and two semi-insulating channels 14 formed by proton bombardment. Electrodes 16 and 17 are used to bias the P-N junction, which here lies in the coupling region between the guides. In Fig. 3 (not shown) the channels for defining the guides are formed in layers 10 and 12 by P-type diffusion of zinc, to provide a structure similar to that of Fig. 2. In Fig. 4 a layer 40 of N-type GaAs is flanked by layers 41 and 42 of N-type GaAlAs. Two P-type channels extending into but not through layer 40 are made by zinc diffusion, and electrodes 46-48 are provided. The refractive index charges at the sides of the channel provide a degree of guiding across the whole thickness of layer 40. A reverse bias between electrode 46 and one of the other electrodes produces a depletion region in layer 40 underlying the associated P-type channel, affecting the guiding properties of that guide. The degree of matching between the propagation velocities and hence the coupling of the guides is thereby effected. By providing two electrodes 47, 48, an offset voltage may be applied to one of them to compensate for any inherent mismatch in propagation velocity between the guides. A further offset voltage on both electrodes modifies the guides so that there is 100% coupling. A signal voltage on one electrode may then reduce to coupling to zero. It is said that the devices of Figs. 3 and 4 may also be made by N-type diffusion into mainly P-type material, this being particularly advantageous for Fig. 4, in which layer 42 and the lower part of layer 40 are grown in N-type material and the top part of layer 40 and layer 41 are grown in P-type material. The refractive index change due to carrier depletion is isotropic and is the only change observed for propagation in the (100) direction if the P-N junction lies in the (100) plane. For other orientations additional rerefractive index changes occur due to the normal electro-optic (locked) effect, being anisotropic and depending on the optical polarization, so that some devices exhibit a polarization dependence also. Reference is made to devices with more waveguides, e.g. a centre guide with individual controlled coupling to each of two outer guides.