GB2100876A - Planar optical waveguide device - Google Patents

Abstract

A planar optical waveguide device, e.g. an acousto-optic modulator using a Bragg cell, has integrated mirrors fabricated in the waveguide region to achieve optical beam shaping and direction changing. Light from a laser diode 200 is collimated by a curved mirror 202 and focussed by a second mirror 204 onto a photodiode array 205. The collimated beam 203 is traversed by surface acoustic waves launched from an interdigital transducer 206. The mirrors are formed by ion beam milling of the waveguide region to define mirror edges penetrating to the depth of the waveguide. The milled edge is then coated with dielectric or metal to raise the reflectivity. The waveguide itself is formed by photoresist etching techniques. <IMAGE>

Description

SPECIFICATION Planar optical waveguide device This invention relates to planar optical waveguide devices and to methods of fabricating the same. Such devices find application in, inter alia, acousto-optic modulators utilising Bragg cells.

Planar devices which make use of surface acoustic waves (SAW) have a greater efficiency than alternative bulk effect devices and a more compact form which make them potentially suitable for fabrication as an integrated subsystem. Lithium Niobate (LiNbO3) is a favoured material for the acousto-optic interaction, having superior piezoelectric and electro-optic properties.

Unfortunately, semiconductor lasers and photodiode arrays must be fabricated in other materials but may be combined with LiNbO3 substrates through hybrid integration. This step is, however, only meaningful if the essential optical beam forming functions can be performed by integrated structures, in place of the commonly used external lenses. The two beam shaping processes, expansion and collimation of radiation from a laser source and post-diffraction focussing onto a photodiode array, in a planar device must by performed in a waveguide mode which penetrates to a few microns only of the substrate surface.

The range of refractive indices in LiNbO3 is restricted, which rules out the possibility of Luneberg lenses, and has lead to the application elsewhere of goedesic lenses. These have been fabricated by the diamond point turning of depressions in the substrate, prior to waveguide fabrication; but although offering a technically feasible solution, this approach involves the use of very expensive and specialised equipment. In principle, Bragg type gratings formed in the surface of the substrate could perform beam expansion and 'chirp' focussing, but are not thought to be applicable to the short focal length imposed by the limited expanse of the LiNbO3 surface.

According to the present invention there is provided a planar optical waveguide device having optical waveguides formed in a surface region of a substrate wherein optical beam shaping and/or direction changing functions are performed by integrated optic mirror(s) formed in the waveguide region of the device.

The invention also provides a method of fabricating an integrated optic mirror in a planar optical waveguide device comprising the steps of forming an optical waveguide structure in a surface region of a substrate, removing substrate surface material to a depth not less than that of the waveguide to define a mirror edge to the remaining waveguide and then coating the mirror edge with a material to provide a reflective mirror surface at said edge.

Embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figs. 1 a-ic illustrate optical beam functions achieved by the use of plane mirrors.

Fig. 2 and 3 illustrate optical beam functions achieved by the use of curved mirrors.

Figs. 4a-4g illustrate the fabrication of integrated optic mirrors in planar optical waveguide devices, and Figs. 5a-5d illustrate integrated mirror beam shaping in acousto-optic modulators using Bragg cells.

The design of mirrors for integrated beam shaping in the Bragg cell and other planar optical devices is more involved than for the usual case of reflection of a TEM wave in an isotropic medium.

Since the light is combined within a planar waveguide which only permits the passage of discrete modes, polarization cannot be random, but must occur in TE orTM form. Boundary conditions at a reflecting transverse surface may introduce mode conversion on reflection, which could impair performance.

Figs. la-ic illustrate useful applications of plane mirrors, which are not trivial in the guided wave case. Changing the direction of a guided wave (Fig. 1 a) in an anisotropic medium such as LiNbO3 implies a change of phase velocity, and the angle of incidence will no longer be equal to the angle of reflection. Side stepping by means of two plane mirrors (Fig. 1 b) can be useful for traversing an optical beam between two given points whilst propagating along a major axis of the crystal.

Doubling back (Fig. 1 c) can have application when the optical input and output must be located at the same end of the substrate. Finally, plane mirrors can serve to increase optical path length; which might be desirable on account of restricted substrate dimensions.

Fig. 2 depicts two additional fuctions which are provided by concave mirrors, each of which is required for the Bragg cell. A collimated beam has to be produced for the acousto-optic interaction, from a divergent beam; which in a completely integrated Bragg cell would originate from an edge mounted laser diode. The width of the collimated beam must be sufficient for the required optical resolution after refocussing onto a photo diode array by a second curved mirror. It may be advantageous to employ a plane mirror with one or both curved mirrors to allow a greater focal length to be provided.

Single mode waveguide may prove to be essential to avoid significant mode conversion during reflection. The launching of unwanted modes in a Bragg cell could result in the production of one or more 'ghost' output signals.

High reflectivity of the mirrors is desirable in order to maintain an acceptable system efficiency and it is important that the reflection be specular.

Any diffuse reflection of light could provide spurious optical output at the photo diode array, and hence limit the dynamic range. Smith et al, report a reflection of 50% with silvered cleaved ends to an optical waveguide, and 90% with bonded dielectric coated mirrors (Applied Physics Letter, 34(1), 1 January, 1969, p. 62).

In a practical device, where reflected light has to traverse a prescribed path, the orientation of mirrors must be carefully considered to allow for the direction sensitive law of reflection. For a reflection in which the refractive index is unaltered, the obvious shape for a curved mirror is a parabola, as shown in Fig. 3. This condition applied when the light is polarised normal to the optic axis, e.g. TM on Y cut LiNbO3. A second case occurs when the normal to the mirror at the centre of its arc is parallel to a principal crystal axis. The incident and reflected beams are then symetrically orientated about the axis. In general, however, the shape of a curved mirror must be modified from a parabola, to take account of the orientation.

The shape and size of a collimating mirror may be calculated after specifying the aperture, focal length, direction of propagation of the incident and reflected beams, and the optical polarization.

Calculation of the shape of the focussing mirror for Bragg cell is more complicated in that parallel beams propagating in a number of different direction must be focussed on a number of defined points. The design would involve establishing anyalytical solutions and a computer model of the mirror. The effect of the mirror on parallel beams in a number of specified directions would then be computed by optical ray tracing. The analysis would also include the variation of the refractive index of the substrate with crystal direction and variation in reflection phase shift with orientation of the mirror element and optical rays.

The steps in fabricating integrated mirrors in planar optical waveguides will now be described referring to Figs. 4a-4g.

An attractive approach to the fabrication of waveguide mirrors is to use ion beam milling to define a mirror edge penetrating to the depth (~4 ym) of the waveguide. The milled areas may extend from the mirror position to the substrate edge, or may be localized to form a groove.

Coating of the milled edge with a dielectric or metal is then required to raise mirror reflectivity.

The use of ion beam milling offers the following advantages: (i) The mirror position and shape is defined with the precision of a lithographic step.

(ii) The area of the substrate adjacent to the milled area (e.g. the optical waveguide surface) is protected by a mask layer.

(iii) The process variables (ion energy, beam intensity and angle) allow precise control of the depth contour of the mirror edge.

(iv) The etch rate is insensitive to the variations in crystal orientation, allowing smooth edges to be formed off the crystal axis.

(v) The ion beam angle of impact is independently controllable allowing, if required, non-vertical edges.

Required for the mirror fabrication is a precision photomask providing good edge definition over relatively large areas. For this purpose the photomask can be produced on an Electromask pattern generator. This will allow geometries to be composed from rectangles of 5 micron minimum dimension, orientated in steps of 0.1 degree, and positioned in steps of 0.1 micron over an area of 100 mm square. A curved mirror, therefore, would be quantized into a number of linear elements.

Taking for example a 3 mm mirror, it will be possible to use at least 600 independent elements to create the profile. Further, this number could be increased to approximately 3000 by overlapping elements, and to more than 12000 by employing a photoreduction step.

The photomask is used to define an ion milling mask on the surface of the substrate. The choice of material and thickness of this mask involves a number of considerations. Firstly, the mask has to protect the waveguide areas from damage, and since it is also eroded during an ion exposure it limits the edge depth. For this purpose then, photoresist is a suitable material since it can be spun on as a thick layer and it has an etch rate comparable to that of LiNbO3. Another consideration, however, is the smoothness of the etched wall and it is known for example that manganese or chromium are superior masks in this respect. Use of these metals could, however, risk modification of the waveguide and an alternative for consideration must be titanium, which is already present.

The fabrication of integrated mirrors in a Bragg cell follows the indiffusion of titanium to form the waveguide, and precedes the production of SAW transducers. An example of a possible chain of process steps is depicted in Figs. 4a-4g and summarised here. Photoresist 100 is spun onto a thin film of titanium 101 which has been evaporated onto the surface of a titanium diffused waveguide 1 02a. After exposure through a photomask (not shown) and development of the photoresist pattern, several fabrication stages may be performed within a single evacuation of an ion milling equipment. Initial milling in a low pressure argon rapidly removes the exposed titanium right through the substrate 102 before removing all the photoresist. Areas 1 04 of LiNbO3 for further erosion are now defined solely by the titanium mask 101.A low concentration of oxygen is allowed to enter, which reduces the milling rate of the titanium considerably whilst allowing the lithium niobate to be removed relatively quickly. A high differential milling rate is essential at this stage for the production of a well shaped edge.

Metallisation of the edge 104 may be accomplished by angled evaporation 105 onto a photoresist pattern 106 which covers the unmilled waveguide surface, followed by a solvent lift off operation.

Figs. 5a-5d show examples of the way in which mirrors can be integrated to perform beam shaping and direction changing in an acousticoptic modulator using a planar Bragg cell. In order to specify the focal lengths and apertures of the curved mirrors, it is necessary to anticipate the properties of the optical source and detector, and also the system requirements.

Light emitted by a typical laser diode 200 diverges into a fan shape beam which subtends 500 and 50 in two perpendicular planes. If butt coupled into the planar waveguide 201, two situations may arise according to the desired polarisation but in each case the light will diverge from a point on the substrate edge. The collimating mirror 202 will be sited with the laser at its principal focus, but the focal length will be determined by the required width of the collimated beam 203. Previous experience indicates that a beamwidth of 7 mm will give a resolution of 1 MHz. The focal length is chosen so that the mirror aperture only receives light from the centre of the laser radiation pattern, where the phasing is acceptable. Restriction in the size of available substrates limits the maximum focal length of a single curved mirror to less than 100 mm.

The second curved mirror 204 is required to focus the diffracted beam onto a photo diode array 205 at the edge of the substrate. An input frequency variation of 500 MHz would sweep the beam through a 2.40 angle, and impose a requirement on depth of focus if resolution is to be maintained. For a focal length of 76 mm, a depth of focus of 0.5 mm would be necessary. The pitch of the photo diode array 205 has implications for the focal length of the focussing mirror 204. For a typical array consisting of 1024 cells on a 16 micron pitch, it is necessary to provide a focal length of 398 mm to obtain a maximum resolution over 500 MHz of 0.5 MHz per cell. A single transit of the length of the substrate would limit the resolution of the photo diode array to 2 MHz per cell.If this is inadequate to satisfy the system requirement, one or more plane mirrors 208 may be used to increase the focussing path length.

The structure shown in Fig. 5e also includes an optical absorber 209 positioned such that at one end of the beam sweep the optical path is not completed. This in effect introduces a switching function as part of the beam sweeping function.

The acousto-optic interaction is obtained by launching, transverse to the collimated optical beam 203, surface acoustic waves from an interdigital transducer 206 fabricated on the surface of the substrate. Once the SAW perturbations have transversed the collimated beam they are absorbed by SAW absorber 207.

It will be apparent to those skilled in the art that integrated mirrors such as have been described herein are not limited in application to acoustooptic modulators but could with advantage be incorporated in other planar optical waveguide devices.

Claims (15)

1. A planar optical waveguide device having optical waveguides formed in a surface region of a substrate wherein optical beam shaping and/or direction changing functions are performed by integrated optic mirror(s) formed in the waveguide region of the device.

2. A device according to claim 1 wherein said mirrors are concave mirrors.

3. A device according to claim 1 wherein said mirrors are parabolic mirrors.

4. A device according to any preceding claim including two beam shaping mirrors arranged to collimate divergent light from a source and to focus the collimated light onto one or more photodetectors respectively.

5. A device according to claim 4 including a plane mirror introduced between a beam shaping mirror and a light source or photodetector to increase the optical path length.

6. A device according to claim 4 or 5 including a surface acoustic wave transducer adapter to launch surface acoustic waves into a path traversed by a collimated beam of light.

7. A device according to any preceding claim including a semiconductor laser light source combined with the substrate by hybrid integration.

8. A device according to any preceding claim including one or an array of semiconductor photodiodes combined with the substrate hybrid integration.

9. A device according to any preceding claim wherein the substrate is lithium niobate.

10. A device according to any preceding claim wherein said mirrors are formed by removal of a portion of the substrate surface material to a depth not less than that of the optical waveguide structure to define the required mirror edges to the remaining waveguide and subsequently coating the mirror edges to provide reflective mirror surfaces at the defined edges.

11. A method of fabricating an integrated optic mirror in a planar optical waveguide device comprising the steps of forming an optical waveguide structure in a surface region of a substrate, removing substrate surface material to a depth not less than that of the waveguide to define a mirror edge to the remaining waveguide and then coating the mirror edge with a material to provide a reflective mirror surface at said edge.

12. A method according to claim 11 wherein the substrate surface material is removed by ion beam milling.

13. A method according to claim 12 wherein a lithium niobate substrate is coated with a layer of titanium on which a film of photoresist material is formed, selectively exposed and then developed, the substrate then being placed in an ion milling equipment where initial milling is effected in an argon atmosphere to remove exposed titanium down to the substrate surface and subsequent milling is effected in an argon/oxygen atmosphere to achieve differential milling of the substrate material and the remaining titanium to form the mirror edges.

14. A method according to claim 13 wherein the formed mirror edges are coated with evaporated metal.

15. A method of fabricating an integrated optic mirror in planar optical waveguide device substantially as described with reference to Figs.

4a-4g of the accompanying drawings.

1 6. A planar optical waveguide device substantially as described with reference to the accompanying drawings.