GB2186386A - Optical channel waveguides - Google Patents

Abstract

A film 1 of waveguide material is deposited on a substrate 2 of silica. A laser L is focussed on the film to produce local vapourisation. The substrate is moved relative to the laser to form grooves 3 and 4 so that the film material 1 between the grooves provides the channel waveguide. The relative laser and substrate position, laser scan speed and laser power are controlled by system 7,9,8. The waveguide material 1, may be germanium-doped silica and with this a CO2 laser would be used. It is also possible to form direction couplers with varying degrees of field overlap, see Figures 4 and 5. It may be desirable to add a cladding layer of lower refractive index material, see Figure 3. <IMAGE>

Description

SPECIFICATION Optical channel waveguides This invention relates two the formation of optical channel waveguides in integrated optical com- ponents, such waveguides consisting of a path of relatively high refractive index material on a base material having a lower refractive index.

Several methods of forming optical channel waveguides have been proposed which make use of laser heating, this involving focussing the output of a laser on to a surface so as to effect some change in the surface, and producing a relative movement between the surface and the laser output. Most of these previously proposed methods rely on localised refractive index changes induced in the surface of a substrate, either by inducing thermal stresses or compositional changes in the substrate material. In both cases the induced change in refractive index is generally very small and the resultantwaveguides are, therefore, only weakly guiding. Accordingly they tend to suffer from large bending losses, high crosstalk, low packing density and high sensitivity to environmental changes.

An alternative method invoives the deposition of a porous layer of SiO2-TiO2 glass on a supporting surface, and then heating the deposited layer by a focussed CO2 laser beam so as to cause localised shrinkage of the layer material, a relative movement between the support and the laser output producing a desired pattern of grooves in the layer. The porous glass deposit remaining is then consolidated into a transparent glass by further heating in a furnace, a pattern of waveguides being formed between pairs of grooves. This method overcomes many of the weaknesses of the other techniques referred to above by producing a larger change in the refractive index; however, because it involvesthreestages of processing it is both time consuming and expensive.

An object of the present invention is to provide another method of forming optical channel waveguides which has the advantages of the last-mentioned technique compared with the previusly proposed processes but which is less expensive.

According to the invention a method of forming an optical channel waveguide comprises depositing a film of waveguide material on a substrate surface of lower refractive index, forming a pair of spaced grooves along predetermined paths in the film by laser heating, such that the region of the film between the grooves forms the channel waveguide. It will be understood that the expression "waveguide material" means a material capable of acting as an optical waveguide.

If necessary (e.g. to reduce waveguide attenuation orto make the waveguide more compatible with optical fibres) a cladding layer of a predetermined lower refractive index material can be deposited on the waveguide structure.

By utilising a computer controlled XYtranslation stage for producing the relative movement between the coated substrate and the laser the invention enables complexwaveguide structures, such as branching networks, Mach-Zehnder interferometers, ring-resonators, directional couplers and so on, to be readily fabricated. By adjusting the dimensions and/ or the refractive index of the guiding channel both multimode and monomode waveguides can be fabricated.

In order to form strongly guiding waveguides the depth of the grooves is preferably such that they extend completely th rough the thickness ofthe deposited film, and in some instances into the surface of the substrate itself. On the other hand weakly guiding waveguides, e.g for providing directional couplers between adjacentwaveguide sections, can be formed by the use of relatively shallow grooves.

The deposited film is conveniently of germania doped silica, and the substrate is of undoped silica or doped silica with a suitably lower refractive index than the deposited film. The laser employed in such a case is preferably a CO2 laser.

However the fabrication technique in accordance with the invention is not limited to germania doped silica asthewaveguide material, but can be used with otherwaveguide materials capable of being heated by laser radiation sufficiently to cause local vaporisation ofthe deposited film. Furthermore otherforms of lasers producing a sufficiently high output power and emission wavelength(s) which isl are absorbed by the waveguide material can be used instead of CO2 lasers.

In order to minimise waveguide scattering it is necessary to ensure that the grooves have smooth walls with minimum roughness. The main cause of wall roughness is likely to be silica dust formed during vaporisation as this can re-deposit on the walls ofthe grooves and so create a rough surface, this problem becoming more acute if the laser power is increased and/orthescanning speed of the laser beam across the deposited waveguide material is reduced. Whilst this can be removed by suction or other means, we have found that by choosing a suitable combination of scanning speed and laser power density there is virtually no silica power re-deposited in the grooves, and very smooth groove walls are produced.

Thus, wherethe deposited film consists of germania doped silica, it is possible to obtain a substantially smooth sided groove with a scanning speed of 2.5cm/second, and a laser power of 17W focussed through a 1.5. inch focal length lens.

However the most satisfactory scanning speed and laser power for any particular application of the invention may readily be found by trial.

A particular advantage of the invention is that it enables the axial and/or the cross-sectional geometry of the waveguides to be readily altered. This can be achieved by adjusting the laser power, scanning speed orfocussing. One or more ofthese parameters can be changed either before a run (to achieve changes in the cross-section geometry} or during a run (to create axial changes in geometry). It is, therefore, possible to fabricate waveg u ides with gradual, abrupt or periodic variations in width, depth and/or profile. Such waveguides can have very useful wavelength selective properties and could find applications in wavelength division multiplexer/ demultiplexers, narrow band filters etc.

The invention will be further explained by describ ing byway of example, with reference to Figures 1 to 50ftheaccompanying schematic drawings, a method of producing a channel waveguide in accordance with the invention, andvariouswaveguide structures formed in accordance with the invention.

Thus referring to Figure 1, a film 1 ofgermania doped silica is deposited on a planar substrate 2 of silica by a low pressure gas phase reaction. Typically a plasma may be used as the activating medium for effecting the deposition of the film 1 ,for example as described in co-pending Patent Application No.

8514618 (2160226A).

Following the formation of the film, the output of a CO2 laser L is focussed on to the film 1 to produce local vaporisation of the film, and in this case an underlying part ofthe surface of the substrate.

Then by moving the coated substrate 2 relative to the output spot of the laser La groove 3 is formed, as shown in Figure 2, the scanning speed and laser output power being seiected to produce smooth sided grooves as above described. A second groove 4 is formed in a similar manner, close to the groove 3, thewaveguide material 1' between the grooves then provides the channel waveguide. However in some cases a cladding layer 5 of silica, or of doped silica having a lower refractive indexthanthewaveguide material, may be deposited on top ofthewaveguide 1 ' as shown in Figure 3.

In orderto effectthe required movement ofthe coated substrate 2 it is conveniently mounted on a support 6 which is movable parallel to the plane of the substrate in two directions at right angles to each other, by means of a computer-controlled XYtrans lator shown diagrammatically at 7. A laser control circuit as at 8 is also conveniently controlled by the computer 9. By controlling the laser power, scanning speed and/orfocussing, the cross-sectional geometry can be altered, therefo re8na bl i ng waveguides with gradual, abruptor periodic variations of width, depth and/or profile to be obtained as above described.

Thus in the waveguide structures illustrated in Figures 2 and 3 the grooves 3,4 extend completely through the film 1, and form a strongly guiding waveguide. However, this is not always necessary, and for some applications it is advantageous for them notto do so. Atypical example of such an application is a directional coupler, the operation of such a device being strongly dependent on the extent two which a field of one guide can penetrate an adjacent guide (i.e. field overlap).Transverse sections oftwo such couplers are illustrated in Figures 4 and 5 The coupler of Figure 4 is made by forming three grooves 6,7,8 in a deposited film of waveguide material, for example germania doped silica, by means of a laser, as above described, so as to form a pair of adjacentwaveguide sections 11,1 2; howeverthe central groove 7 penetrates only part of the way through the thickness of the film. By adjusting the depth of this groove 7 one can control the degree of field overlap, and therefore modify the mode of operation of the device.

Thus whilst Figure 4 shows a coupler having a re lativelyweakfield overlap, the coupler illustrated in Figure 5, which is formed in a similar manner, has a much shallower central groove 7 giving rise to a stronger field overlap.

The depth of the groove 7 required to provide a required degree field overlap will depend upon the refractive index difference between the waveguide material and the surrounding media, the crosssectional geometry of the waveguides and otherfactors, but may readily be ascertained bytrial in any particular case.

Claims (10)

1. A method of forming an optical channel waveguide comprising depositing a film ofwaveguide material on a substrate surface of lower refractive material than the waveguide material, forming a pair of spaced grooves along predetermined paths in the film by laser heating, such that the region ofthefilm between the grooves forms the channel waveguide.

2. A method as claimed in Claim 1, wherein a cladding layer of a predeterminedly lower refractive index material can be deposited on the waveguide structure.

3. A method as claimed in Claim 1 or Claim 2, wherein a computer-controlled X-Ytranslation device is utilised to produce relative movement between the coated substrate and the laser.

4. A method as claimed in any one of the preceding claims, wherein the depth of the grooves is such that they extend completely through the thickness of the deposited film.

5. A method as claimed in Claim 4, wherein the grooves extend into the surface ofthe substrate.

6. A method as claimed in any one of the preceding claims, wherein the deposited film of waveguide material is of germania doped silica.

7. A method as claimed in Claim 6, wherein the substrate is of silica which is either doped or undoped.

8. A method as claimed in Claim 7, wherein the laser is a CO2 laser which is scanned overthesubstrate and deposited film at a speed of 2.5 cm/sec,the laser operating at a power of 17W focussed through a 1.5 inch focal length lens.

9. A method offorming optical channel waveguides substantially as hereinbefore described with reference to the accompanying drawings.

10. An optical guide wavelength manufactured by the method as claimed in any one of the preceding claims.