GB2388918A

GB2388918A - Optical waveguide switch with movable reflector in trench

Info

Publication number: GB2388918A
Application number: GB0212109A
Authority: GB
Inventors: Gerardus Johannes Burger; Remco Stoffer
Original assignee: Alcatel Optronics UK Ltd; Alcatel Optronics Netherlands BV
Current assignee: Kaiam Europe Ltd; Concept to Volume BV
Priority date: 2002-05-25
Filing date: 2002-05-25
Publication date: 2003-11-26
Also published as: GB0212109D0

Abstract

A 2 x 2 MEMS switch 1 is described comprising a chip 3 having at least two V-shaped waveguides 4,5 therein, the vertices of the V-shapes being located at a wall 8 of a trench 2 formed in the chip 3. A MEMS actuator 20 moves a micromirror 15 between first and second positions relative to the wall 8 of the trench, to switch between an express mode (Fig.3(a)) and a switched mode (Fig.3(b)) of operation. The 2 x 2 switch may be replicated to build a 2D switch matrix 40. In a modified embodiment there are three waveguides (60,62,63, fig 6) at the trench and the switch forms a 2 x 2 Add-drop switch which can be used as a building block for an ROADM (70, fig 7). In a further alternative embodiment, instead of using a mirror the actuator 20 is configured to use total internal reflection in the express and switched operation modes (fig 4).

Description

MEMS SWITCH

_ The present invention relates to micro electromechanical system (MEMS) technology and, in particular. to MEMS switches for use in 5 optical networks. More specifically, but not exclusively, the invention concerns a 2 x 2 MEMS switch suitable for use in building a two-

dimensional (2D) switch matrix.

In most MEMS switch designs the manipulation of the optical path is done in the free space field. For example, in the fiber optic switch

0 described in WO 98/12589 the light has to travel through an air gap at the ends of the optical fibers. This is true whether the input optical signal from the input fiber is switched by the movable mirror element, or passes i'unswitched" through the air gap, into the opposing optical fiber. This causes such switches to have a high insertion loss, especially in relation to 5 the 'unswitched paths" through the switch, commonly referred to as the express lines i.e. the optical path which an input optical signal takes if it is not switched. The known solution to this problem, also described in WO 00/25160 which uses waveguides instead of optical fibres, is to fill the air gap with an index-matching fluid, to minirnise spreading of the optical 20 signal beam in the switching region (the former air gap), and also to reduce reflections which would otherwise occur at the air-glass interface, and so to minimise insertion losses in the switch. Without the use of the index-

matching fluid the high insertion loss would lead to poor scalability of this switch design to a 2D switch matrix, the high losses on the express lines in 25 such a 2D matrix being hard to accept for switch designers. However, the disadvantages of using index-matching fluid are that that the packaging of such a fluid-filled package is a very complex operation, and there is the risk of leakage from the final packaged device. Moreover, the long term stability and thus performance of such index-matching fluids is not fully

known. Moreover, there is the possibility that the index-matching fluid may corrode the mirror and/or degrade the input/output fiber arrays used to input/output optical signals to/from the switch.

It is an aim of the invention to avoid or rninimise one or more of the 5 foregoing disadvantages.

According to the invention there is provided a switch comprising: a substrate having at least two waveguides provided therein, at least a first one of said waveguides comprising a V-shaped portion having its vertex at a wall of a trench formed in the substrate, and the second lo waveguide intersecting with said wall of the trench at a position spaced apart from the vertex of said first waveguide; a movable reflective surface disposed in the trench; and an actuator for moving the reflective surface between at least first and second positions relative to said wall of the trench, wherein with the 15 reflective surface in said first position an input optical signal in the first waveguide remains in the first waveguide, and with the reflective surface in said second position there is an optical path between the first and second waveguides. The first and second waveguides are preferably arranged so that with the 20 reflective surface in said first position, an optical signal input in a first arm of the V-shaped portion of the first waveguide is reflected, by the reflective surface, down the second arm of this V-shaped portion, and with the reflective surface in said second position an optical signal input in said first arm of the V-shaped portion of the first waveguide exits the first 25 waveguide into the trench, where it is reflected, by the reflective surface, into the second waveguide in the substrate. Preferably, the second waveguide comprises a portion having an optical axis parallel to the optical axis of the nearest arm thereto of the Vshaped portion of the first

waveguide, and the reflective surface is substantially perpendicular to the plane of the substrate when in its said first and second positions.

In the switch of the invention, the optical properties of the express line (i.e. the optical path through the first waveguide), particularly with regard s to insertion loss, are much better then the switched line (i.e. the optical path from the first waveguide to the second waveguide), since in the express line the optical signal effectively does not leave the waveguide substrate.

Therefore the scalability and relative losses (between the express and switched lines) of a switch matrix based on this 2 x 2 switch design should lo be much better than the prior art switches in which the optical signal must

travel through am air gap in both the express and switched lines.

In a first embodiment, the actuator means is configured to move the reflective surface between at least a first position in which the reflective surface is disposed opposite and adjacent said vertex of said first IS waveguide at the trench wall to complete an optical path through the first waveguide, and a second position in which the reflective surface is spaced apart from said wall of the trench to complete an optical path from the first waveguide to the second waveguide. In this embodiment the wall of the trench is preferably coated with an anti- reflection coating. This reduces any 20 possible reflection of input signal light at the wall of the trench when the reflective surface is spaced apart from the wall of the trench. Such reflection would increase the insertion loss of the switch and crosstalk into the first waveguide, when the reflective surface is in its second position spaced apart from said wall of the trench, 25 The second waveguide may comprise a V-shaped portion having its vertex in the same wall of the trench as the first waveguide, and the reflective surface may contact the vertices of the V- shaped portions of both the first and second waveguides when the actuator is in its said first position, and the two arms of the V-shaped portion of the second

waveguide are arranged such that an optical input signal input in a first arm of the V-shaped portion of the first waveguide is reflected down the second arm of this V-shaped portion when the actuator is in said first position. In this embodiment, the switch may conveniently be used as a 2 x 2 switch 5 module having two express lines and two switched lines. It will be appreciated that a multiplicity of such 2 x 2 modules may be readily employed to build up a two-dimensional matrix switch in a single substrate.

Instead of contacting the waveguide vertices, in order to avoid any possible damage to the mirror surface by such contact, it may be preferable for the 10 mirror to be located in close vicinity to, but not touching, the waveguide vertices. Alternatively, the switch may be designed for use as an add-drop switch, for use in, for example, building an Optical Add-Drop Module (OADM) or Reconfgurable Optical Add-Drop Module (ROADM). In this case the 5 second waveguide is a straight waveguide which terminates in the wall of the trench, a third waveguide is provided having an optical axis parallel to the optical axis of the second arm of the V-shaped portion of the first waveguide and also terminates in the same wall of the trench, and the second and third waveguides are disposed on opposite sides of the V 20 shaped portion of the first waveguide. The second and third waveguides are arranged so that with the reflective surface located in its second position the third waveguide and said second arm of the V-shaped portion of the first waveguide together form an Add path, and the second waveguide and the first arm of the V-shaped portion of the first waveguide together form a 25 Drop path. In this embodiment the use of an anti-reflection coating on the trench wall (containing the waveguide vertex and ends) is particularly desirable as it will have the benefit of reducing crosstalk between the first waveguide and the Drop path, as well as reducing insertion loss of the

s switch in the Add-Drop mode (as compared with the same switch where no such anti-reflection coating is used).

Preferably, the reflective surface is in the form of a mirror provided on a free end portion of the actuator. The actuator is preferably movable in a s direction parallel to the plane of the substrate and generally perpendicular to the plane of the reflective surface. The mirror may be hinged to the actuator free end portion, about an axis substantially perpendicular to the plane of the substrate. This facilitates selfalignment of the mirror with the wall of the trench. To further facilitate alignment, the substrate may lo protrude into the trench at a plurality of locations in said wall of the trench, the protrusions acting as alignment "bumps" for contacting the mirror when the mirror is in its first position in which it forms an optical path through the first waveguide. For example, the substrate may protrude into the trench at each vertex of a V-shaped waveguide portion and each location where a 5 waveguide terminates at the trench. More preferably, in order to avoid damage to the mirror surface in the areas which reflect optical signals to be switched, the substrate protrudes into the trench close to, but not at, each vertex of a V-shaped waveguide portion and close to, but not at, each location where a waveguide terminates at the trench. Additionally, or 20 alternatively, the substrate may be provided, at the opposite side of the trench to the waveguides, with alignment protrusions for contacting a rear side of the mirror, for facilitating self-alignment of the mirror when the actuator is disposed in its second position in which it is spaced from the wall of the trench containing the waveguide vertices and ends. It will be Is appreciated that the alignment bumps on the trench wall and the rear mirror surface are especially advantageous for facilitating the flip-chip bonding of a MEMS chip having the actuator provided thereon to the substrate having the waveguides provided therein: the bumps will at least partially

compensate for any flip-chip bonding misalignment between the MEMS substrate and the waveguide substrate.

In a second and alternative embodiment of the invention, instead of the reflective surface being a mirror on the free end portion of the actuator, the 5 switch may utilise total internal reflection. The reflective surface is preferably provided in the form of an interface between two different materials, each material having a different refractive index. In one embodiment, the reflective surface may be formed by a dielectric/air interface. (conveniently, the free end portion of the actuator may be 10 provided with a slab of dielectric material for contacting the wall of the trench, and the actuator is provided with an aperture at the rear side of the dielectric slab, providing a dielectric- air interface. In this embodiment the waveguides are preferably arranged such that when the actuator is in its second position the dielectric slab is in contact with the vertex of the first 5 waveguide and an optical signal entering the dielectric slab from a first arm of the V-shaped portion of the first waveguide in the substrate is totally internally reflected at the dielectric-air interface in the actuator, back out of the dielectric slab and into the second waveguide in the substrate (this is the "switch line"), and when the actuator is in its first position the dielectric 20 slab is spaced apart from said wall of the trench and an optical signal in said first arm of the V-shaped portion of the first waveguide is totally internally reflected at the substrate-trench interface, into the second arm of the V-shaped portion of the first waveguide (this is the "express line"). In a modified version of this embodiment, instead of a using a dielectric/air 25 interface the free end of the actuator may be in the form of a slab comprising two layers of dielectric material, the front layer, for contacting the wall of the trench, having a higher refractive index than the rear layer.

Preferred embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Fig. 1 is a plan view of a 2 x 2 switch according to a first embodiment 5 of the invention; Fig.2 is a side view of the switch of Fig. 1; Figs.3(a) and (b) are plan views of a portion of the switch of Fig. 1, in "express" and "switched" modes respectively; Figs.4(a) and (b) show a 2 x 2 switch according to an alternative 0 embodiment, in express and switched modes respectively; Fig.5 is a schematic plan view of a 2D matrix switch built using a multiplicity of the 2 x 2 switches of Fig. 1; Figs.6(a) and (b) illustrate a modified version of the embodiment of Figs.3(a) and (b), which operates as an add-drop switch, Figs.(a) and (b) 5 showing the switch in express mode and switched mode respectively; and Fig.7 is a plan schematic view of an ROADM incorporating a plurality of the add-drop switches of Fig.6.

Fig. 1 shows a 2 x 2 switch 1 fabricated using hybrid MEMS/PLC 20 (Planar Lightguide Circuit) technology. A silica-on-silicon PLC substrate or "chip" 3 is provided having a trench 2 formed therein, for example by etching. Two V-shaped waveguides 4,5 formed in the substrate are arranged side-by-side and so that the vertices of both V-shapes lie in a wall 8 of the trench. A MEMS chip 10 is flip-chip bonded to the PLC chip 3.

z5 The MEMS chip incorporates an actuator 20 which is located in a well 6 fonned in the PLC chip 3, the well 6 being in communication with the trench 8 via a connecting well passage 7. The actuator has a free end portion 23 which extends through the well passage 7 into the trench 8 and which has a mirror 15 attached at the free end thereof. The mirror 15 is

hinged to the actuator by a hinge point 16 so at to be tillable about an axis substantially perpendicular to the plane of the PLC chip 3. The reflective surface 17 of the mirror 15 faces the wall 8 of the trench and lies in a plane generally perpendicular to the plane of the PLC chip 3. The actuator and 5 mirror are formed in known manner by micromachining from a silicon-on-

insulator (SOI) wafer using Deep Reactive Ion Etching (DRIE), whereby the mirror is formed vertically in the depth of the substrate, as is already well known in the art.

As shown in the Figs. 1 and 3, the wall 8 of the trench incorporates two 10 alignment bumps 22,24 in the form of respective protrusions from the substrate into the trench in the vicinity of the two locations where a V-

shape waveguide vertex meets the trench. The actuator 20 is electrically controllable (for example, the actuator may conveniently be an electrostatic comb actuator) so that the free end portion 23 thereof is movable between 5 two positions: a first position in which the reflective surface 17 of the mirror 15 is in contact with the two alignment bumps 22, 24 in the wall 8 of the trench (and so is in contact with the vertices of the V-shaped waveguides), and a second position in which it is spaced apart from the wall 8 of the trench. The bumps 22,24 facilitate selfalignment of the 20 hinged mirror 15 against the wall 8 of the trench 2. At least two further alignment bumps 26,28 are provided in an opposing wall 25 of the trench 2, with which bumps a rear side 27 of the mirror 15 contacts when the free end portion of the actuator is in its second position, to facilitate self-

alignment of the hinged mirror in this second position. In Figs. 1 to 4 the 25 alignment bumps 22,24 are shown located at the vertices of the two waveguides 4,5. This may be disadvantageous as the mirror could be susceptible to damage from contact with the bumps. Thus, in an alternative possible embodiment (not shown), these alignment bumps 22,24 may be located close to, but not at, the waveguide vertices in the trench wall 8, so

that the reflective surface 17 of the mirror is disposed opposite and adjacent to (but not touching) the waveguide vertices. This will leave a small gap between each waveguide vertex and the reflective surface 17 of the mirror 15 when the mirror is in contact with the bumps, but as long as the distance 5 between the waveguide vertices and the mirror is smaller than the wavelength(s) of the signal beam, this should not cause any problems in terms of operation of the switch.

A significant advantage of the alignment bumps is that they will at least partially compensate for any misalignment of the MEMS chip 10 and the lo PLC chip 3 resultant from the flip-chip bonding operation used to bond the two chips together. Without this facility for self-alignment of the mirror 15 (by virtue of the alignment bumps), it would be technically difficult to achieve perfect of alignment of the MEMS and PLC chips in the flip-chip bonding process.

5 The two arms of each V-shaped waveguide are angled appropriately relative to the trench wall 8 and also each other (Spell's law can readily be employed to do these calculations) such that: 1) with the mirror in its first position, shown in Fig.3(a), in which it is in contact with the alignment bumps 22,24 containing (or close 20 to) the vertices of the waveguides 4,5, first and second input optical signals travailing in an input arm 30,32 of each waveguide respectively are reflected by the mirror surface 17 back down an output arm 31,33 of the respective waveguide. In this configuration, the switch is said to be in its "Express mode", the 25 two optical paths provided by the two waveguides 4,5, respectively being referred to as express lines l and 2; and 2) with the mirror 15 in its second position, spaced apart from the wall 8 of the trench containing the waveguide vertices and in contact with the alignment bumps 26,28 on the rear wall 25 of the trench 8, as shown in

Fig.3(b), an input optical signal in the input atm 30 of the first waveguide 4 exits the substrate into the trench and travels to the mirror 15 which reflects the signal beam back towards the substrate, so as to enter the output arm 33 of the second waveguide 5 i.e. express line 1 has been connected to express s line 2. We call this the "Switched mode".

The wall 8 of the trench is coated with an anti-reflection (AR) coating (not shown) to minimise reflection of signal light at the waveguide/air interface when the mirror 15 is in its second position, spaced apart from the trench wall 8. The AR coating may itself comprise one or more layers of lo material, the refractive indices of the layers being chosen so that the AR coating formed thereby will minimise reflection of signal beam light at the waveguide/air interface formed by the trench wall 8. Such reflection would increase the insertion loss of the switch in the "Switched mode". It will of course be appreciated that the depth of the trench, and of the mirror in the 15 trench, and also the reflectivity of the mirror, are together designed to ensure that substantially all of the input signal beam(s) power incident upon the mirror is reflected by the mirror.

It will be readily appreciated how the 2 x 2 switch described above can be replicated so as to build up a two-dimensional (2D) switch matrix as 20 illustrated in Fig.5. In the switch matrix 40 of Fig.5 all the waveguides 50 are provided in the same PLC chip 3. The MEMS actuators may each be formed in separate respective MEMS chips which are then each flip- chip bonded to the PLC chip 3, or may all be fondled in a single MEMS chip which is then flip-chip bonded to the PLC chip.

25 Figs. 4(a) and (b) show an alternative version of the 2 x 2 switch of Figs3(a) and (b). Like parts are referenced by like reference numerals in Figs.3 and 4. In the switch of Fig.4, instead of using the reflective surface of a mirror to perform the switching, total internal reflection of the signal beams is employed. The free end portion 23 of the actuator in this case is

provided with a slab of dielectric material 50 attached thereto. An air filled aperture 52 is formed in the free end portion 23 of the actuator, immediately behind the dielectric slab. The dielectric-air interface 54 so formed is used in the switched mode, as will be described. In the express mode of the switch, the actuator is spaced apart from the wall 8 of the trench containing the vertices of the V-shaped waveguides 4,5, as shown in Fig.4(a). The two arms of each V-shaped waveguide are angled relative to each other and to the wall 8 of the trench with which they intersect (again, Snells' law can be used to do these calculations) so that: 0 1) with the dielectric spaced apart from the waveguide vertices, an input optical signal in the input arm 30,32 of each waveguide is totally internally reflected at the substrate/trench interface so as to travel down the output anns 31,33 of the respective waveguides i.e. forming express lines 1 and 2 of the switch; and 5 2) with the actuator disposed such that the dielectric slab 50 is in contact with the vertices of the two waveguides 4,5 in the trench wall 8, an input signal in the input arm 30 of the first waveguide 4 travels out of the substrate into the dielectric, where it is totally internally reflected at the dielectric-air interface 54 back into the 20 substrate where it travels down the output arm 33 of the second waveguide 5 i.e. express line 1 is connected to express line 2, giving the Switched mode.

With a small change in the switch configuration of the switch of Fig.3 or Fig.4, the 2 x 2 switch can be designed as an add-drop switch, as 25 illustrated in Figs.6(a) and (b), and can be replicated to build up an ROADM, as illustrated in Fig.7. Like reference numerals are used to refer to like parts in Figs.3 and 6. In the embodiment of Fig.6, instead of using two V-shaped waveguides, there is only one V-shaped waveguide 60. On either side of this a straight waveguide 62,63 is placed, each terminating in

- the same trench wall 8 as the vertex of the V-shaped waveguide. Each of the two straight waveguides has its optical axis parallel to the optical axis of the nearest arm 60a,60b of the V-shaped waveguide, as shown in Fig.6(a). The portions of the wall 8 containing the vertex of the Vshaped s waveguide 60, and the ends of the other two waveguides 62,63 are formed as alignment bumps 70,72,74 for self-alignment of the mirror 15 (or the dielectric slab 50, if an actuator like that of Fig.4 were to be employed).

The waveguides are arranged such that: 1) with the mirror 15 in contact with the alignment bumps 70,72,74 0 (and thus in contact with waveguide vertex and waveguide ends disposed therein) an input signal in the input arm 60a of the V shaped waveguide is reflected by the mirror, into the output arm 60b of the V-shaped waveguide. This is express line 1; and 2) with the mirror spaced apart from the trench wall 8 (containing the waveguide vertex and ends) and in contact with the rear surface 25 of the trench, in particular with alignment bumps 26,28 provided thereon, as in the Fig. l embodiment, an add path is formed via the mirror 15 between the output arm 60b of the V shaped waveguide and the one 62 of the straight waveguides 20 which is not parallel thereto, and a drop path is formed (via the mirror 15) between the other straight waveguide 63 and the input arm 60a of the V-shaped waveguide, as shown in Fig.6(b). This is called the "Add-I)rop mode" of the switch.

As previously noted, the alignment bumps 70,72,74 may alternatively 25 be located close to, but not at, the waveguide vertex and waveguide ends, in order to avoid contact damage to the crucial areas of the mirror.

Fig.7 illustrates one way in which the 2 x 2 add-drop switch of Fig.6 can be replicated to form an ROADM 70 incorporating a demultiplexer 72, a multiplexer 74, six Add paths 76, six Drop paths 78, and six 2 x 2

switches 80 like the switch of Fig.6. Of course, the ROADM could be designed to have any number of Add and Drop paths, six being chosen here only as an example.

It will be appreciated that modifications and variations to the above s described embodiment are possible within the scope of the invention.

For example, the MEMS actuator need not be formed using micromachining of an Sol wafer: surface micromachining using polysilicon layers could alternatively be used, or bulk micromachining plus wafer bonding, or any other suitable technique for fabricating 0 MEMS devices.

In the PLC chip described above, the silica waveguides are buried waveguides formed on a silicon substrate using Flame Hydrolysis Deposition (FHD), together with photolithographic masking and etching, but other embodiments are possible in which different 15 techniques are used to from the waveguides, for example Chemical Vapour Deposition (CVD), plus masking and etching. Also, the waveguides need not be buried waveguides: they could, for example, be rib waveguides.

Additionally, the terms "input" and "output" as used above in 20 relation to the direction of propagation the optical signal beams shown in the drawings, and could of course be interchanged where the signal beams travel in the opposite direction in the waveguides. i.e. the waveguides described above as input waveguides may equally be used as output waveguides, and vice versa.

2 in the Fig.4 embodiment the diciectric slab may be made of, for example, a material having the same, or substantially the same, refractive index to the waveguides (so minimising any reflective losses at the waveguide- slab interface), or a material having a different refractive index to the waveguides but which is coated (on the surface

which faces the waveguides) with an anti-reflection (AR) coating. In one possibility, the dielectric slab could be a piece of silicon which is coated with a layer of silicon oxynitride (SiON) having a refractive index chosen such that reflection at the waveguide/SiON interface in the 5 Switched mode is minimised.

Also, instead of using an air-flled aperture to create the necessary interface to cause total internal reflection in the actuator, it would of course be possible to create the interface using two solid material layers having different refractive indices, the refractive index of the rear layer 10 of material (i.e. the layer furthest from the wall of the trench containing the waveguide vertices/ends) being less than that of the front layer, and the waveguides being arranged such that the input optical signal is above its critical angle to this interface when the actuator is positioned in the switched mode, so that total internal reflection at this interface is will occur.

In the Figs. 1 to 3 type embodiment, incorporating the movable mirror 15, instead of coating the trench wall 8 with an AR coating the trench 8 can be filled with index-matching fluid if desired i.e. a fluid having substantially equal refractive index to the (effective) refractive 20 index of the waveguides. This will minimise unwanted reflections at the waveguide/air interface and also beam spreading in the trench, and so may improve the overall performance of the switch. Of course, this needs to be considered against the possible disadvantages of using index- matching fluid, as already described. It will be appreciated that 25 index-matching fluid is not appropriate for use in the dielectric-slab embodiment of Fig. 4, as it would prevent the desired total internal reflection taking place in (a least) the Express mode of that switch.

Claims

- CLAIMS

1. A switch comprising: a substrate having at least two waveguides provided therein, at least a 5 first one of said waveguides comprising a V-shaped portion having its vertex at a wall of a trench formed in the substrate, and the second waveguide intersecting with said wall of the trench at a position spaced apart from the vertex of said first waveguide; a movable reflective surface disposed in the trench; and 0 an actuator for moving the reflective surface between first and second positions relative to said wall of the trench, wherein with the reflective surface in said first position an input optical signal in the first waveguide remains in the first waveguide, and with the reflective surface in said second position there is an optical path between the first and second s waveguides.

2. A switch according to claim 1, wherein the second waveguide comprises a straight waveguide portion having an optical axis parallel to the optical axis of the nearest arm thereto of the V-shaped portion of the 20 f rst waveguide, and the reflective surface is substantially perpendicular to the plane of the substrate when in its said first and second positions.

3. A switch according to claim 1 or claim 2, wherein the second waveguide comprises a V-shaped portion having its vertex in the same wall 2s of the trench as the first waveguide, and arranged such that when the actuator is its first position an input optical signal in the second waveguide remains in the second waveguide.

4. A switch according to claim 1 or claim 2, wherein the second waveguide is a straight waveguide which terminates in the wall of the trench, a third waveguide is provided which also terminates in the same wall of the trench, the second and third waveguides are disposed on 5 opposite sides of the V-shaped portion of the first waveguide, and the second and third waveguides are arranged so that with the reflective surface located in its second position the third waveguide and said second arm of the V- shaped portion of the first waveguide together form an Add path, and the second waveguide and the first arm of the V-shaped portion of the first 10 waveguide together form a Drop path.

5. A switch according to any preceding claim, wherein in said first position the reflective surface is disposed opposite and adjacent said vertex of said first waveguide at the trench wall to complete an optical path t5 through the first waveguide, and in said second position the reflective surface is spaced apart from said wall of the trench to complete an optical path from the first waveguide to the second waveguide.

6. A switch according to claim S. wherein in said first position the 20 reflective surface contacts said vertex of said first waveguide.

7. A switch according to claim 5 or claim 6, wherein the first and second waveguides are preferably arranged so that: with the reflective surface in said first position, an optical signal input US in a first ann of the V-shaped portion of the first waveguide is reflected, by the reflective surface, down the second arm of this V-shaped portion; and with the reflective surface in said second position an optical signal input in said first arm of the V-shaped portion of the tirst waveguide exits

the first waveguide into the trench where it is reflected, by the reflective surface, into the second waveguide in the substrate.

8. A switch according to any preceding claim, wherein the reflective s surface is in the form of a mirror provided on a free end portion of the actuator.

9. A switch according to claim 8, wherein the mirror is hinged to the actuator free end portion, about an axis substantially perpendicular to the to plane of the substrate.

10. A switch according to claim 8 or claim 9, wherein said wall of the trench is coated with an anti-reflection coating.

1S

11. A switch according to any of claims 1 to 4, wherein the reflective surface is in the form of an interface between two different materials, each material having a different refractive index.

12. A switch according to claim 1 1, wherein one of said two different 20 materials is air.

13. A switch according to claim 12, wherein the free end portion of the actuator is provided with a slab of dielectric material for contacting the wall of the trench, and the actuator is provided with an aperture at the rear 2s side of the dielectric slab.

14. A switch according to claim 13, wherein the waveguides are arranged such that:

when the actuator is in its second position the dielectric slab is in contact with the vertex of the first waveguide and an input optical signal in the first arm of the V-shaped portion of the first waveguide enters the dielectric slab from the substrate and is totally internally reflected at the s dielectric-air interface in the actuator, back out of the dielectric slab and into the second waveguide in the substrate; and when the actuator is in its first position the dielectric slab is spaced apart from said wall of the trench and an optical signal in said first arm of the V-shaped portion of the first waveguide is totally internally reflected at 0 the substrate-trench interface, into the second arm of the V-shaped portion of the first waveguide.

15. A switch according to claim 13 or claim 14, wherein the dielectric slab comprises a block of material having a refractive index substantially 5 equal to the effective refractive index of the waveguides.

16. A switch according to claim 13 or claim 14, wherein the dielectric slab comprises a block of dielectric material having an anti-reflection coating applied to a free end surface thereof which contacts said wall of the 20 trench when the actuator is in its second position.

17. A switch according to any preceding claim, wherein the actuator is movable in a direction parallel to the plane of the substrate and generally perpendicular to the plane of the reflective surface.

18. A switch according to any preceding claim, wherein the substrate protrudes into the trench to form alignment bumps at a plurality of locations in said wall of the trench.

19. A switch according to claim 18, wherein each said alignment bump is at a said vertex of a V-shaped waveguide portion or at a location where a waveguide terminates at the trench.

20. A switch according to any preceding claim' wherein at the opposite side of the trench to the waveguides the substrate protrudes into the trench at a plurality of locations to form alignment bumps for the actuator.

21. A switch according to any preceding claim, wherein the actuator is 0 formed in a separate substrate to the waveguides, and the substrate incorporating the actuator is flip-chip bonded to the substrate containing the waveguides.

22. A two-dimensional matrix switch comprising a plurality of switches is according to any of claims 1 to 3, wherein the substrate of each of the plurality of switches is the same substrate.

23. An Optical Add Drop Module (OADM) comprising a plurality of switches according to claim 4, wherein the substrate of each of the plurality 20 of switches is the same substrate.

24. A Reconfigurable Optical Add Drop Module (ROADM) comprising a plurality of switches according to claim 4, wherein the substrate of each of the plurality of switches is the same substrate.

2s