Patent GB-1529451-A (GB)
Title: Optical waveguide couplers
IPC: G02F1/313
Filed: 1975-02-27
Abstract: 1529451 Controlling light STANDARD TELE PHONES & CABLES Ltd 6 Jan 1976 [27 Feb 1975] 8244/75 Addition to 1437067 Heading G2F In a modification of the optical waveguide coupler of the parent Specification, lateral optical confinement is produced by providing protruding ribs on one surface of the coupler rather than by changes in free carrier concentration, since high free carrier concentrations produce undesirable optical loss, and the depletion region extending into or through one of the guides or the region therebetween may be that occurring on either side of a p-n junction, but preferably the n-type side. In Fig. 1, a GaAS n-type layer 10 is disposed between GaAlAS n-type layers 11 and 12 of lower refractive index. A layer of p-type GaAlAS having a relatively high carrier concentration is disposed on layer 12 and masked and selectively anodized to leave an oxide layer 13 through which protrude two ribs 14 providing lateral confinement for guide regions in layer 10. With heating the zinc p-type dopant diffuses into layer 12 but is arrested at layer 10 due to the higher doping level thereof, so that a junction region 16 is formed at or in each guide. Electrodes 17, 17 and one (not shown) on the substrate 11 are provided to complete a structure similar to Fig. 4 (not shown) of the parent Specification. In Fig. 2 (not shown), layer 12 is omitted and the anodization of the p-type layer is limited so as to leave a portion between the ribs. A channel between the ribs, extending down into the substrate, is rendered semi-insulating by proton bombardment, and like channels may be provided adjacent the outer sides of the ribs to reduce capacitance effects. A diffusion step is not required and hence the carrier concentration in layer 10 may be reduced. Fig. 3 is similar to (Fig. 2) but the ribs are proton bombarded and the intervening region masked. The p-type layer does not require anodization and the p-n junction is here formed between the guides as in Fig. 1 of the parent Specification. The embodiments of (Fig. 4) and Fig. 5 are similar to Fig. 1, but the ribs 14 are doped with (non-mobile) germanium, there being no diffusion step, so that the p-n junctions are spaced from the guiding layer 10 and the layer 12" is lightly doped to ensure that the high field associated with reverse biasing of the junction extends to layer 10". In addition, in Fig. 5 only, which depends on the refractive index change induced by the electric field rather than the change in carrier concentration, the layer 10" is relatively lightly doped, and a further lightly doped layer 50 of the same composition as heavily doped substrate 11" is provided between the substrate and the guiding layer. Due to the low doping of layer 10" the refractive index change thereof due to carrier removal is low and the associated change in optical absorption is also small, preventing attenuation mismatch between the guides and enabling either output port of the coupler to be substantially isolated. The layer 50 and modified layer 10" may also be provided in (Fig. 2). In the embodiment of Figs. 1-3, the doping and thickness of layer 10 may be such that free carriers are substantially removed at a reverse bias just below breakdown value. Preferably however, for a greater change in carrier concentration, the layer has a composite refractive index and comprises a highly doped region adjacent the p-n junction and an underlying lightly doped region (of the same material).