GB2182158A - Electro-optical waveguide - Google Patents

Abstract

An electro-optical waveguide comprises a substrate 1 which embodies at least one waveguide channel 2, 3 and associated electrodes 5, 6 to which signal voltages are applied during operation of the waveguide to produce modulation or switching of light propagating through the waveguide channel. A conductive layer 11 is provided on the surface of the substrate remote from the electrodes with or without the interposition of an insulating buffer layer 12 so that the flux lines of the electric field generated by voltages applied to the electrodes are directed vertically through the waveguide channel so that the overlap between the optical and electrical field is complete. <IMAGE>

Description

SPECIFICATION Improvements relating to electro-optical waveguides This invention relates to electro-optical waveguides which utilisethe reactive interaction of light with an applied electric field to produce modulation or switching of lightthrough the electro-optic effect.

Such electro-optical waveguides may comprise a substrate of lithium niobate, for example, into which a pattern of waveg uide channels or stripes may be formed by thermally diffusing dopant material, such as titanium, for example, into the substrate in order to produce therein light propagation channels of re latively high refractive index near the surface ofthe substrate.

Alternativeforms of such electro-optical wavegui- des may comprise substrates of PLZT (lanthanumdoped lead-zirconate-titanate, gallium arsenide or other llltoVcompounds in which thewaveguiding action may be performed by a Schottky barrier formed on the substrate surface.

More specifically, the present invention is concerned with electro-optical waveguides ofthe kind in which the flux lines of an electric field generated by signal voltages applied to electrodes substantially co-planar with one or more waveguide channels, or the equivalent, in orion the surface of a substrate (e.g. lithium niobate) are generallyvertically directed through the waveguide channels orthe equivalent.

The operation of such waveguides (e.g. modulation) is dependent upon the degree of overlap between the propagating optical field due to the passage of light along the waveguide channels or equivalent and the electric field applied from the electrodes.

Due to the substantially co-planarelectrode arrangementthe overlap referred to is often substantially incomplete. It is possible to vary the depth ofthewave- guide structure, or otherwise vary the geometry of the structure, in order to improve the field overlap but this in turn produces other disadvantages (e.g.

small guided mode width). This is particularly true in cases where the waveg uide structure is to be buttcoupled to optical fibres, since any difference in the mode shapes ofthe optical fibre and the waveguide structure results in loss of coupling efficiency.

According to the present invention the problem of incomplete overlap between the interactive optical and electrical fields in a waveguide justabovedescri- bed is alleviated by providing a conductive (e.g.

metal) layer on the opposite surface of the substrate of predetermined mode depth remote from the substrate surface in or on which the waveguide channels or equivalent are formed together with the metal electrodes, with or without the interposition of an insulating buffer layer, whereby the flux lines of an electric field generated by voltages applied to the electrodes are directed vertically through the waveguide channels or equivalent so that the overlap between the optical and electrical field is complete.

In carrying outthe present invention the metal or other conductive layer referred to which is preferably grounded may extend over the entire surface of the substrate remote from the metal electrodes orthe conductive layer may be restricted to the general area directly opposite the metal electrodes.

By way of example the present invention will now be described with reference to the accompanying drawings in which Figure 1 (a) (b) (c) and (d) illustrates certain steps in the fabrication of one waveguide structure according to the present invention; Figure2 shows an alternative embodiment ofthe invention to that depicted in Figure (c); and, Figure 3(a) (b) and (c) depicts certain steps in the fabrication of a different form ofwaveguide to that shown in Figures 1 and 2.

Referring firstly to Figure 1(a), a substrate of lithium niobate 1 has a pair of waveguide channels 2 and 3 of titanium diffused therein by well-known diffusion techniques. An insulating buffer layer or diffusion barrier layer4 (e.g. silicon oxynitride) is superimposed upon the upper surface ofthe substrate 1 and metal electrodes 5 and 6 locate directly overthe waveguides 2 and 3 are formed on the insulating buffer layer4 by conventional metal deposition and etching techniques, after which connecting wires 7 and 8 are bonded thereto. The upper part ofthe waveguide structure, including the metal electrodes 5 and 6 having connecting wires 7 and 8 bonded thereto, is encapsulated in a suitable cement 9 (e.g.

epoxy resin) and bonded to a back plate 10 which may be of lithium niobate or any other suitable material.

The backing plate 10 serves as a structural support means for the waveguide structure and the lower partofthestructure is then removed by lapping and polishing to producethestructureshown in Figure 1 (b) the depth or thickness ofthe substrate 1 being dictated primarily by the particularwaveguide mode depth, typically 10 m.

The polished lower surface of the substrate 1 is then coated with a conducting film or layer 11 (e.g.

evaporated metal) with an intervening buffer layer 12 being provided, or, alternatively, a metal plate (not shown) may be bonded directly to the substrate 1.

In operation ofthe waveguide illustrated the metal film 11 or plate will preferably be grounded and signal (e.g. modulation) voltages will be applied to the encapsulated metal electrodes Sand 6 as shown in Figure 1 (d). These voltages give rise to vertically directed flux lines which, as shown, substantially completely overlap the guided optical fields propagating in thewaveguide channels 2 and 3, dueto the presence of the metal film 11 on the lower surface of the substrate.

Referring now to Figure 2 of the drawings, this shows an alternative embodiment to that shown in Figure 1 (c), in which a slot 13 cut into the substrate enables a metal film 14 restricted in dimensions to that area generally below the waveguide channels and metal electrodes to be deposited on to the substrate with or without the interposition of a buffer layer 15. Once again, the depth of the substrate in the region ofthe channels 2 and 3 when the metal film is provided is determined primarily by the waveguide mode depth.

The slot or groove 13 may, for example, be prov ided by saw-cutting foilowed by ion-beam milling.

However, precision alignment of the slot or groove 13 relative to the waveguide channels 2 and 3 is not required.

It may here be mentioned that the backing plate 10 which serves to provide the mechanical strength and support to the waveguide structure may be dispensed with if the substrate 1, instead of having its lower part lapped away orslotted as shown in Figure 1 (c) and 2, respectively, is provided on its lowersurface with a recess of comparable depth to the groove or slot 13 of Figure 2, but not actually extending to the edges ofthe substrate. This recessing of the substrate leaves the substrate with sufficient strength and rigidity to obviate the needfora backing plate.

Such a recess into which a metal film may be deposited with orwithout an intervening buffer layer, may be provided by laser-assisted photo-chemical etching.

In other contemplated embodiments of the invention the lithium niobate substratewith waveguide channels therein may be replaced by material exhibiting useful electro-optical effects such as PLZT (lanthanum-doped lead-zirconate-titanate), gallium arsenide or other Ill to Vcompounds. In such embodiments one ofthe materials referred to may be deposited on to a support body and the supportsubsequently removed to define a substrate of the material concerned to the lower surface of which a metal film may be applied with orwithoutthe intervention of a buffer layer.

To provide a better understanding ofthesefurther contemplated embodiments Figure 3 (a) (b) and (c) shows certain steps in the fabrication of an electrooptical waveguide.

Referring to Figure 3 (a) a gallium arsenide body 20 has epitaxially grown thereon a layer 21 of aluminium gallium arsenide alloy and a gallium arsenide layer 22. A Schottky barrier layer 23 is provided on the upper surface of the gallium arsenide layer 22 and metal electrodes 24 and 25, bonded connecting wires 26 and 27, encapsulating layer 28 and backing plate 29 are provided as in thewaveguide constructions of Figure 1 and Figure 2.

The gallium arsenide body 20, followed by the aluminium gallium arsenide alloy layer 21 are removed, as shown in Figure 3 (b), by etches which have different etch rates for gallium arsenide and aluminium gallium arsenide alloy, the first etch removing gallium arsenide strongly and the second removing aluminium gallium arsenide alloy. Therqaf- ter, a metal film 30 is applied to the bottom surface of the epitaxially-grown gallium arsenide layer 22 with our without lapping or polishing of the surface and the provision of a buffer layer 31. As before, the metal film 30 may be grounded when the waveguide structure is in operation having signal voltages applied to the electrodes. The electric field in the gallium arsenide will be vertically directed through the Schottky barrier layer 23 to provide the requisite overlap between the optical field in the layer 23 and the electric field.

Electro-optical waveguides constructed in accordance with the present invention may be incorporated in switched directional couplers and interferometric modulators for example.

Claims (10)

1. An electro-optical waveguide structure ofthe kind hereinbefore described comprising a substrate supporting or embodying at least one waveguide channel or the equivalent and substantially coplanar electrodes to which signal voltages are applied during operation of the waveguide to produce by the electro-optic effect modulation or switching of light propagating through said waveguide channel or equivalent, in which a conductive (e.g. metal) layer is provided on the surface of the substrate remote from the electrodes, with or without the interposition of an insutating buffer layer, and in which the substrate has a predetermined mode depth between the conductive layer and the electrodes wherebytheflux lines of an electric field generated by voltages applied to the electrodes are directed verticallythrough the waveguide channel orequiva- lent so that the overlap between the optical and electrical field is complete.

2. An electro-optical waveguide structure as claimed in claim 1, in which the conductive layer is restricted to the general area of the substrate opposite the metal electrodes.

3. An electro-optical waveguide structure as claimed in claim 2, in which the conductive layer is located on the base surface of a recess in the substrate which has sufficient overall strength and rigidity to provide support for the waveguide struc- ture.

4. An electro-optical waveguide structure as claimed in claim 1, in which the conductive layerex- tends over the entire surface ofthe substrate remote from the metal electrodes, and in which the structure comprises a backing plate attached to the substrate.

5. An electro-optical waveguide structure as claimed in claim 4, in which the backing plate is bonded to an encapsulating layer embodying the electrodes which in turn is attached to the substrate through an insulating or barrier layer.

6. An electro-optical waveguide structure as claimed in any preceding claim, in which the substrate comprises lithium niobate having waveguide channels oftitanium diffused therein.

7. An electro-optical waveguide structure as claimed in any of claims 1 to 6, in which thesubstrate comprises gallium arsenide having a schottky barrier layer interposed between the substrate and the electrodes.

8. An electro-optical waveguide structure substantially as herein before described with reference to Figure 1 of the accompanying drawings.

9. An electro-optical waveguide structure sub stantial ly as hereinbefore described with reference to Figure 2 of the accompanying drawings.

10. An electro-optical waveguide structure substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.