GB2377762A

GB2377762A - Divergent beam steerer formed in a substrate

Info

Publication number: GB2377762A
Application number: GB0117340A
Authority: GB
Inventors: Fabrice Franck Mauric Maillaud; Arnold Peter Roscoe Harpin
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-07-17
Filing date: 2001-07-17
Publication date: 2003-01-22
Also published as: GB0117340D0

Abstract

An integrated optical beam steerer comprising: an optically conductive substrate 2; a beam steerer 1 formed in the substrate 2 and a light input 3 directing a divergent beam of light through the beam steerer 1. The beam steerer is formed by a portion of the substrate 2 the refractive index of which is adjustable, the portion being shaped such as to produce a substantially constant phase shift across the beam of light passing therethrough whereby the output of the portion is a beam of light diverging from a virtual point 3' spaced laterally from the light input 3. The beam steerer may be combined with a mirror for focussing the beam (Fig 2). The steerer may alternatively collimate or focus the beam. The adjustment to refractive index may be made by thermo-, electro- or acousto- optical effects or by free carrier injection.

Description

BEAM STEERER This invention relates to an integrated optical beam steerer for steering a beam of light through a selected angle.

Various forms of beam steerers have been previously proposed. One type comprises an array of waveguides into which the beam is directed, each waveguide being provided with adjustment means for adjusting its effective optical path length. The outputs of the array of waveguides interfere with each other to form an output beam. By appropriate control of the individual adjustment means, the output beam can be arranged to be focussed at selected points. Such arrangements suffer from phase errors due to optical coupling between the waveguides in the array. Moreover, as each waveguide is provided with individually tuneable adjustment means, e. g. heaters, the device tends to be relatively large.

Another form of beam steerer has been proposed in PCT application No. PCT/GB01/00880 the disclosure of which is incorporated herein. In this case, a collimated beam is directed through a portion of a substrate the refractive index of which is adjustable, this portion being shaped to imitate an optical component, e. g. a prism, so as to deviate the beam through a selected angle.

The present invention aims to provide a further version of this type of beam steerer which is capable of being used in further applications and/or which enables devices to be fabricated with fewer components and hence with lower optical losses.

According to a first aspect of the present invention, there is provided an integrated optical beam steerer comprising: an optically conductive substrate; beam steering means formed in the substrate and at least one light input arranged to direct a divergent beam of light through the beam steering means; the beam steering means comprising a portion of the substrate the refractive index of which is adjustable, the said portion being shaped such as to produce a substantially constant phase shift across the beam of light passing therethrough whereby the output of the portion is a beam of light diverging from a virtual point spaced laterally from said light input.

According to a second aspect of the invention, there is provided an optical switching device comprising a beam steerer as described above, and focussing means for focussing the beam of light output from the beam steering means whereby the beam of light output from the beam steering means can be focussed at different positions as the refractive index of said portion is adjusted.

According to a third aspect of the invention, there is provided an integrated optical beam steerer for refracting a beam of light comprising: an optically conductive substrate; beam steering means formed in the substrate comprising a portion of the substrate through which the beam passes and the refractive index of which is adjustable, the said portion being shaped both to collimate or focus the beam and to deviate the beam so the beam leaving the steering means can be directed in two or more selected directions.

The invention will now be further described, merely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of an embodiment of beam steerer according to the first aspect of the invention; and Figure 2 is a schematic plan view of two such beam steerers used in an optical switching device.

Figure 1 shows a beam steerer formed in an optical substrate arranged to receive a divergent beam of light from a light input 3. The beam steerer 1 comprises a portion of the substrate 2 the refractive index of which is adjustable and has an input side 1 A and an output side 1 B. The refractive index may be adjusted, for instance, by heating the portion 1, e. g. by means of heating elements (not shown) positioned above and/or below said portion. Such heating elements may, for example, comprise a layer of aluminium, or other electrically conductive material, having substantially the same shape as the portion 1 or a series of heater strips covering an area the same shape as the portion 1, arranged to form a resistance heater. When the heater is switched on, it increases the refractive index of a portion of the substrate 2 having a shape substantially similar to that covered by the heating element (s). Light travelling through that portion of the substrate thus experiences a change in refractive index as it enters the portion and, again, when it leaves the portion.

The shape of the beam steerer 1 is designed so as to produce a substantially constant phase shift across the divergent beam of light passing therethrough so that the divergent beam leaving the beam steerer appears to emanate from a virtual light input 3', as indicated by dashed lines in Figure 1.

A convenient way of achieving this is to shape the beam steerer 1 so that its input side 1A has a curved shape which is substantially centred on the light input 3. The wave-front from the light input 3 is thus incident on the input side 1 A in phase across the width of the beam. The shape of the output side 1B of the beam steerer is designed so that the distance d between input and output points of each part of the beam on the two sides 1A and 1B increases substantially linearly from one side of the beam to the other, i. e. the distance d is proportional to the angle A shown in Figure 1. The out put side 1 B thus also has a curved shape and, typically, would be part of a spiral centred on light input 3.

Thus, by controlling the refractive index of the beam steerer 1, the divergent beam from light input 1 can be steered so as to appear to enamate from another point. The angle through which the beam can be steered depends primarily on the refractive index difference between the substrate 2 and the beam steerer 1. If the substrate 2 is formed of silicon, changes in temperature of several tens of degrees Centigrade, e. g. in the range of 10-100 degrees Centigrade, produce a change in the refractive index of the silicon of less than one percent but this is sufficient to cause the required refraction of the beam.

The beam steerer can be arranged to have two states i. e. switched on and switched off, so as to deviate the beam through a set angle or not at all. Alternatively, the angle of deviation can be controlled continuously by gradually increasing or decreasing the temperature of the steerer or varied between a plurality of set angles of deviation.

Figure 2 shows the use of two such beam steerers and an optical switch, one steerer being for steering the beam in one direction, i. e. left, and the other being for steering the beam in the opposite direction, i. e. right.

The optical switch comprises one or more input waveguides 10A, 10B, 10C,.... first and second beam steerers 11 and 12, a mirror 13 and one or more output waveguides 14A, 14B, 14C,... all formed in an optically conductive substrate 15.

Light from an input waveguide diverges through the substrate 15, passes through the steerers 11 and 12, is reflected by mirror 13 and focussed thereby towards an output waveguide 14. The arrangement may, for instance, be designed so that when neither of the beam steerers is activated, light from the central input waveguide 10B passes through the beam steerers 11 and 12 without deviation and is reflected and focussed by the mirror 13 towards the central output waveguide 14B. To this end, the mirror is formed as part of an ellipse (the ellipse 16 being shown by dashed lines) with the output face of waveguide 10B and the input face of waveguide 14B being positioned at the focal points of the ellipse 16.

If beam steerer 11 is switched on, it IS arranged to deviate the beam to the left (as described in relation to Figure 1) so the mirror 13 reflects and focuses the beam towards output waveguide 14A. Similarly, if beam steerer 12 is switched on, it is arranged to deviate the beam to the right so the mirror 13 reflects and focuses the beam towards the output waveguide 14C.

Light from input waveguides 10A or 1 OC can similarly be selectively directed towards at least two of the output waveguides 14. The fact that input waveguides 10A and 10C are not exactly at the focal point of the ellipse 16 may introduce some minor aberrations but in most cases these should not be of significance. The output waveguides 14 can be arranged so that they are appropriately positioned and spaced apart so that any such problems are minimised. Optical losses due to a beam being focussed slightly off centre on the input face of one of the output waveguides are small, the main requirement being sufficient angular separation between beams focussed towards different output waveguides so as to avoid or minimise cross-talk between the different channels.

The wavelength of the light has relatively little effect on the design of the beam steerers although the beam steerers may be designed for use with either the 1300nm or 1500nm wavelength bands commonly used in telecommunications applications. Nevertheless, in many cases, the same design may function satisfactorily for both of these wavelength bands.

A switching device such as that shown in Figure 2 may be used for a variety of different switching functions. Light on each input may be of a relatively narrow wavelength band or may be multiwavelength and light may be input on one or more inputs simultaneously. In a typical arrangement, each input waveguide may be provided with an attentuator (not shown) which can be used, in effect, to switch on or off each channel and only one channel used at a time. In another arrangement, a plurality of channels may be used simultaneously and the switching device used to deviate these in unison either one channel to the left or one channel to the right, i. e. so called"shuffling".

The arrangement described in relation to Figure 2 has advantages over that described in PCT/GB01/00880, in that as it is able to steer a diverging beam and so only requires the use of one mirror rather than two. Optical losses due to reflection are therefore reduced by a factor of two. The angle through which the output beam is steered in also reduced (as it is not amplified by a double reflection). This helps make the device more compact.

The arrangement described in relation to Figure 2 also has advantages over the prior art which uses an array of heaters on an array of waveguides. Not only does the arrangement described herein enable a more compact device to be fabricated, with less problems due to cross-talk and phase errors, it is also able to provide a flatter band spectrum, i. e. the response of the device (the angle of deviation) is substantially independent of wavelength.

As indicated above, the substrate in which the beam steerer is formed is preferably silicon. The beam steerer may thus use the thermo-optic effect to alter the refractive index by heating the silicon. Alternatively, the beam steerer may comprise electrodes for applying an electric field to the substrate to alter the refractive index either by free carrier injection or by free carrier depletion.

The input and output waveguides may be rib waveguides, again preferably formed in a silicon substrate. Preferably, the silicon substrate is separated by an optical confinement layer, e. g. of silicon dioxide, from a supporting substrate, e. g. also of silicon. The substrate may thus be formed from a so-called silicon-on-insulator chip.

The mirror may be formed by a deep etch in the optical substrate so a to provide an interface substantially perpendicular to the plane of the substrate at which interface total internal reflection occurs (for light incident thereon at angles greater than the critical angle).

A beam steerer such as that described above may, however, be formed in other materials besides silicon, e. g. InP or GaAs substrates. Depending on the material used, the beam steerer may alter the refractive index of the substrate using other effects, e. g. electro-optic effects or acusto-optical effects.

Many of the effects available to alter the refractive index of the optical substrate are only able to alter it by a few percent. This is sufficient to provide deviation of the beam, e. g. to switch the beam between different output waveguides. However, in

cases which enable larger changes in the refractive index, the beam steerers may be used to collimate or focus the beam as well as deviating the beam. This may be achieved by providing a first portion of the beam steerer acting to deviate the beam i. e. acting in a similar manner to a prism, and a second portion acting to collimate or focus the beam, i. e. acting as a lens. However, it may be preferable to combine these two portions into one. In this case, the beam steerer portion may have a complex shape but nevertheless is the optical equivalent to the two components described. The beam steerer if thus shaped both to collimate or focus the beam and to deviate the beam so the output thereof can be selectively directed in two or more directions.

In such cases, a first device may be used both to collimate and deviate a divergent beam and a second device to deviate the beam further and to focus the beam. Alternatively, a mirror may be used to collimate the beam and a beam steerer device used to focus and deviate the beam.

Claims

CLAIMS 1. An integrated optical beam steerer comprising: an optically conductive substrate; beam steering means formed in the substrate and at least one light input arranged to direct a divergent beam of light through the beam steering means; the beam steering means comprising a portion of the substrate the refractive index of which is adjustable, the said portion being shaped such as to produce a substantially constant phase shift across the beam of light passing therethrough whereby the output of the portion is a beam of light diverging from a virtual point spaced laterally from said light input.
2. A beam steerer as claimed in claim 1 in which said portion has an input side through which the beam of light from the light input means passes and an output side through which the beam of light leaves said portion, the input side having a curved shape centred substantially on the light input so as to substantially match the shape of the wavefront incident thereon.
3. A beam steerer as claimed in claim 2 in which the second face has a curved shape arranged such that a distance between points at which parts of the beam enter the portion and leave the portion increases substantially linearly from one edge of the beam to the opposite edge thereof.

<Desc/Clms Page number 11>
4. A beam steerer as claimed in any preceding claim in which the beam steering means comprising adjustment means for adjusting the refractive index using one or more of the following : thermo-optical effects, free carrier injection or depletion, electro-optical effects and acousto-optical effects.
5. A beam steerer as claimed in claim 4 in which the adjustment means comprise one or more electrodes and/or one or more heaters extending over an area of the substrate, said area having a shape substantially corresponding to the desired shape of the portion the refractive index of which is to be adjustable.
6. A beam steerer as claimed in claim 4 in which the adjustment means comprises a layer of electrically conductive material, or a series of strips of electrically conductive material, which forms a resistance heater for heating said portion of the substrate to adjust its refractive index by the thermo-optic effect.
7. A beam steerer as claimed in any preceding claim in which said at least one light input comprises an optical waveguide integrated on the substrate.
8. A beam steerer as claimed in any preceding claim comprising a pair of beam steering means, one arranged to steer the team of light in a first direction and the other arranged to steer the beam of light a second direction opposite to the first direction.

<Desc/Clms Page number 12>
9. An optical switching device comprising a beam steerer as claimed in any preceding claim, and focussing means for focussing the beam of light output from the beam steering means whereby the beam of light output from the beam steering means can be focussed at different positions as the refractive index of said portion is adjusted.
10. An optical switching device as claimed in claim 9 in which the focusing means comprises a mirror or other reflective device.
11. An optical switching device as claimed in claim 10 in which the mirror or other reflective device is integrated in the substrate.
12. An optical switching device as claimed in claim 10 and 11 in which the mirror or other reflective device has a substantially elliptical or part-elliptical shape.
13. An optical switching device as claimed in any of claims 9 to 12 comprising at least one output waveguide having an end face towards which the beam of light output from the beam steering means can be focussed.
14. An optical switching device as claimed in claims 12 and 13 in which the said light input means and the end face of he output waveguide are located substantially at the focal points of said ellipse.

<Desc/Clms Page number 13>
15. An optical switch as claimed In claims 13 or 14 having a plurality of input waveguides and a plurality of output waveguides, wherein light from each of the input waveguides can be selectively directed towards at least two of the light output waveguides by adjustment of the beam steering means.
16. A beam steerer or optical switching device as claimed in any preceding claim in which the substrate is silicon.
17. A beam steerer or optical switching device as claimed in claim 16 in which the silicon substrate is separated from a supporting substrate by an optical confinement layer.
18. A beam steerer or optical switching device as claimed in claim 17 in which the optical confinement layer comprises silicon dioxide.
19. An integrated optical beam steerer for refracting a beam of light comprising: an optically conductive substrate; beam steering means formed in the substrate comprising a portion of the substrate through which the beam passes and the refractive index of which is adjustable, the said portion being shaped both to collimate or focus the beam and to deviate the beam so the beam leaving the steering means can be directed in two or more selected directions.

<Desc/Clms Page number 14>
20. An integrated optical beam steerer substantially as hereinbefore described with reference to and/or as shown in one or more of the accompanying drawings.
21. An optical switching device substantially as hereinbefore described with reference to and/or as shown in one or more of the accompanying drawings.