AU9339801A - Integrated optical switch array - Google Patents

Description

P/00/01i1 Regulation 3.2

AUSTRALIA

Patents Act 1990 ORI GINAL COMPLETE SPECIFICATION STANDARD PATENT Invention tidle: Integrated optical swvitch array The following statement is a full description of this invention, including the best method of performing it known to us: dxbm M01 1 1044397v 1999995 INTEGRATED OPTICAL SWITCH ARRAY Field and background of the invention The present invention is divided from Australian patent application number 38980/99 and relates to optical switch arrays and, more particularly, to an optical switch array, of particularly compact geometry, in which arbitrary combinations of the inputs and outputs are explicitly addressable.

In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date: part of the common general knowledge, or known to be relevant to an attempt to solve any problem with which this specification is concerned.

Integrated optical switches are well-known. For an early review of the art, see Lars Thylen, "Integrated optics in LiNbO 3 recent developments in devices for telecommunications", Journal of Lightwave Technology vol. 6 no. 6 (June 1988), pp. 847- 861. Waveguides are created in a lithium niobate substrate by processing the substrate locally to increase the index of refraction. For example, the index of refraction of lithium niobate may be increased locally by diffusing titanium into the substrate. To divert light 20 from one waveguide to another, the waveguides are coupled by local optoelectrical manipulation of their indices of refraction. Well-known examples of optoelectrical switches include directional couplers, BOA couplers, digital-optical-switches and xo switches. Depending on the voltage applied to such a switch, light is thus partly or completely diverted from an input waveguide to an output waveguide.

25 By appropriately combining waveguides and switches, a switch array is formed to switch light from a plurality of input waveguides among a plurality of output waveguides.

A variety of switch array geometries are known. Figure 1A is a conceptual illustration of a switch of one such geometry: crossbar geometry. A set of input waveguides 10 crosses a set of output waveguides 12. 'At the crossing points, the waveguides are coupled by 2x2 switches 14. For simplicity, only four input waveguides 10 and four output waveguides 12 dxbm M0111044261vl 999995 22.11.2001 are shown in Figure 1A. Typically the numbers of input waveguides 10 and output waveguides 12 are equal powers of 2, up to a practical maximum of 32.

Figure 1B shows, schematically, the actual layout of the switch array of Figure 1A.

Switches 14 are shown as directional couplers, in which parallel segments of the waveguides are flanked by electrodes (not shown) to which the coupling voltages are applied. Note that input waveguide 10a leads directly into output waveguide 12a, that input waveguide 10b leads directly into output waveguide 12b, that input waveguide 1Oc leads directly into output waveguide 12c, and that input waveguide 10d leads directly into output waveguide 12d. To allow arbitrary coupling of inputs to outputs, three auxiliary waveguides 11a, lib and 11c are provided. Waveguides 10a-12a and 10b-12b are coupled in switch 14a. Waveguides 10b-12b and 10c-12c are coupled in switches 14b and 14c.

Waveguides 10c-12c and 10d-12d are coupled in switches 14d, 14e and 14f. Waveguides 10d-12d and lla are coupled in switches 14g, 14h, 14i and 14j. Waveguides 1la and lb are coupled in switches 14k, 141 and 14m. Waveguides lib and lic are coupled in switches 14n and 14o. Note that switches 14g, 14k and 14n actually are 1x2 switches, that switches 14j, 14m and 14o actually are 2x1 switches, and that there is no switch corresponding to the lowermost 2x2 switch 14 of Figure 1A. (A 1 x2 switch is a 2x2 switch with one input deactivated; a 2x1 switch is a 2x2 switch with one output deactivated.) Switch arrays based on geometries such as the crossbar geometry of Figures 1A 20 and 1B can be used to divert input signals to output channels arbitrarily. Signals from any input channels can be directed to any output channel, and even to multiple output channels, in broadcast and multicast transmission modes.

Despite the conceptual simplicity of the crossbar geometry of Figures 1A and 1B, this geometry has been found inferior, in practice, to two other geometries, the tree 25 geometry, illustrated in Figure 2, and the double crossbar geometry, illustrated in Figure 3.

Figure 2 shows the tree geometry, for four input waveguides 20 and four output waveguides 22. Waveguides 20 lead into a binary tree of 1 x 2 switches 24. Waveguides 22 emerge from a complementary binary tree of 2 xl switches 26. The highest order branches of the binary trees are connected by intermediate waveguides 28. Figure 3 shows the double crossbar geometry, for four input waveguides 30 and four output waveguides 32. Each input waveguide 30 traverses four 1 x 2 switches 34a, 34b, 34c and 34d. Each output waveguide 32 traverses four 2 x 1 switches 36a, 36b, 36c and 36d. The remaining outputs of switches 34 are connected to respective inputs of switches 36 by intermediate waveguides 38. Note that, in principle, switches 34d and 36a are not needed, because input waveguides 30 could lead directly to switches 36d and output waveguides 32 could emerge directly from switches 36a; but, in practice, the illustrated configuration has been found to reduce cross-talk.

The tree and double crossbar geometries require larger numbers of switches than the equivalent crossbar geometry. Nevertheless, the tree and double crossbar geometries have certain advantages over the crossbar geometry: 1. The tree and double crossbar geometries have lower worst-case crosstalk than the crossbar geometry.

2. In general, the path from a particular input waveguide to a particular output waveguide through a crossbar switch array is not unique. Therefore, computational resources must be devoted to reconfiguring a crossbar switch array in real time. In a tree switch array or in a double crossbar switch array, the path from any particular input waveguide to any particular output waveguide is unique, so it is trivial to compute how to reconfigure such a switch array in real time.

3. To prevent loss of optical power by radiation, the intermediate waveguides of an optical switch array must have gentle curvature. In the case of the crossbar geometry, this requires that the switches be arranged in a diamond pattern, as illustrated in Figures 1A 20 and IB. This is a less efficient packing of the switches than, for example, the rectangular matrix pattern of the double crossbar switch as illustrated in Figure 3.

SUMMARY OF THE INVENTION According to the present invention there is provided an optical switch array 25 comprising: a plurality of input waveguides; a plurality of output waveguides; and a plurality of switches coupling said input waveguides to said output waveguides; all said waveguides being arranged substantially in a common plane, all said waveguides being substantially curved and concentric.

Preferably the optical switch array comprises said input and output waveguides having substantially a single common center of curvature. The input waveguides may preferably be substantially radially inward from said output waveguides.

Preferably the optical switch array may have at least 16 said input waveguides and at least 16 said output waveguides, more preferably the optical switch may have 32 said input waveguides and 32 said output waveguides.

Also according to the present invention there is provided an optical switch array including: at least three input waveguides; a first group of at least three output waveguides; for each of the output waveguides of the first group: for each of the input waveguides, a switching element coupling the each input waveguide only to the each output waveguide; and for each of the output waveguides of the first group, a combining mechanism for coupling all of the input waveguides to the each output waveguide; the input waveguides, the output waveguides, the switching elements and the combining mechanism all being arranged substantially in a common plane; all of the input waveguides traversing successively respective the switching elements in a common order relative to the output waveguides of the first group.

According to the present invention there is provided a method for switching signals from at least one of at least three input channels to at least one of at least three output channels, each output channel receiving signals from only one input channel, including the steps of: providing an optical switch array including: at least three input waveguides, each of the input waveguides corresponding uniquely to one of the input channels, (ii) at least three output waveguides, each of the output waveguides corresponding uniquely to 20 one of the output channels, (iii) for each of the output waveguides: for each of the input waveguides, a switching element coupling the each input waveguide only to the each output waveguide, and (iv) for each of the output waveguides, a combining mechanism for coupling all of the input waveguides to the each output waveguide, the input waveguides, the output waveguides, the switching elements and the combining mechanism all being arranged substantially in a common plane, all of the input waveguides traversing successively respective the switching elements in a common order relative to the output waveguides; and for each of the output waveguides: setting the switching element, that couples the each output waveguide to the input waveguide that corresponds to the input channel wherefrom a signal is to be switched to the output channel corresponding to the each output waveguide, to divert at least a portion of the signal to the each output waveguide.

According to the present invention there is provided a method for multicasting from at least one of at least three input channel to at least two of at least three output channels, each output channel receiving input from only one input channel, including the steps of: (a) providing an optical switch array including: at least three input waveguides, each of the input waveguides corresponding uniquely to one of the input channels, (ii) at least three output waveguides, each of the output waveguides corresponding uniquely to one of the output channels, (iii) for each of the output waveguides: for each of the input waveguides, a switching element coupling the each input waveguide only to the each output waveguide, thereby coupling the input channel corresponding to the each input waveguide to the output channel corresponding to the output waveguide, and (iv) for each of the output waveguides, a combining mechanism for coupling all of the input waveguides to the each output waveguide, the input waveguides, the output waveguides, the switching elements and the combining mechanisms all being arranged substantially in a common plane, all of the input waveguides traversing successively respective the switching elements in a common order relative to the output waveguides; and for each output channel: setting the switching element, that couples the each output channel to the input channel wherefrom a signal is to be switched to the each output channel, to divert at least a portion of the signal to the each output channel, at least one of the switching elements being set to divert Sonly a portion of the signal.

20 We have discovered that, by rearranging the connections of the double crossbar geometry of Figure 3, a new geometry is obtained that allows a spatially more compact *e configuration of switches and interconnecting waveguides. Compactness is an important consideration, because it allows a larger switch array- (more inputs and outputs) to be fabricated on a substrate of a given size. One substrate suffices for a switch array of the present invention that is functionally equivalent to a prior art switch array that may require two (double crossbar geometry) or three (tree geometry) substrates.

Figure 4 shows the geometry of a switch array of the present invention, in the case of four input waveguides 40 and four output waveguides 42. As in the double crossbar geometry of Figure 3, each input waveguide 40 traverses four 1 x 2 switches 44, each output waveguide 42 traverses four 2 x 1 switches 46, and the remaining outputs of switches 44 are connected to respective inputs of switches 46 by intermediate waveguides 48. Unlike the double crossbar geometry of Figure 3, switches 46a all are traversed by the same output waveguide 42a, switches 46b all are traversed by the same output waveguide 42b, switches 46c all are traversed by the same output waveguide 42c, and switches 46d all are traversed by the same output waveguide 42d, so that all input waveguides 40 are coupled to output waveguides 42 in the same order: first to output waveguide 42a, then to output waveguide 42b, then to output waveguide 42c, and finally to output waveguide 42d.

This allows intermediate waveguides 48 that lead to a particular output waveguide 42 to be geometrically adjacent, with a corresponding increase in the compactness of a switch array of the present invention as compared to an equivalent double crossbar switch array.

As in the double crossbar geometry of Figure 3, strictly speaking, 1 x 2 switches 44d and the first 2 x 1 switches 46 traversed by output waveguides 42 are not necessary, and are present only to reduce cross-talk. Co-pending US Patent Application 09/085,369 teaches a similar switch array geometry, in which these switches are in fact not present.

In the days before integrated optics, Fulenwider, in US Patent No. 3,871,743, described an optical switch array having a topology similar to that of the present invention.

Unlike the present invention, the particular embodiment described by Fulenwider is not well-suited to fabrication as an integrated optical device. By contrast, a switch array of the present invention is easily fabricated, essentially in a single plane, as an integrated optical device, for example on a Z-cut lithium niobate substrate.

1 x 2 switches 44 and 2 x 1 switches 46 are indicated on Figure 4 for illustrative S 20 purposes only. More generally, the scope of the present invention includes any suitable switching element in the role of 1 x 2 switch 44 and any suitable coupling element in the role of 2 x 1 switch 46. In particular, passive y-junction combiners may be substituted for 2 x 1 switches 46.

To switch signals from an input channel, associated uniquely with a corresponding 25 input waveguide, to one or more output channels, each output channel associated uniquely with a corresponding output waveguide, the output waveguides are considered in turn. For each output waveguide, the switching element that couples the input waveguide associated with the desired input channel is set to divert the appropriate portion of the input signals of S"that channel to the target output waveguide. If signals from other input channels are to be switched to other output waveguides, then the corresponding other switching elements associated with the target output waveguide are set to pass those signals without diversion.

The optical switch array may be fabricated on a substantially circular substrate having a diameter of at most about four inches. Preferably the optical switch substrate is a face of a Z-cut lithium niobate crystal.

Description of the drawings The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1A (prior art) shows the geometry of a crossbar switch array; FIG. 1B (prior art) is a schematic depiction of the layout of the crossbar switch array of FIG. 1A; FIG. 2 (prior art) shows the geometry of a tree switch array; FIG. 3 (prior art) shows the geometry of a double crossbar switch array; FIG. 4 shows the geometry of a switch array of the present invention; FIG. 5 is a schematic illustration of a first embodiment of a switch array of the present invention; FIGs. 6A and 7B show alternate combining mechanisms; FIG. 7 is a schematic illustration of a second embodiment of a switch array of the present invention; FIG. 8 is a schematic illustration of a third embodiment of a switch array of the present invention; FIG. 9 shows the preferred layout of the embodiment of Figure 5 on a surface of a ooo00 **z-cut lithium niobate crystal, representing the present invention as divided from Australian Patent Application No. 38980/99.

•DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is of an integrated optical switch array whose geometry allows it to be fabricated more compactly than similar known optical switch arrays.

S°The principles and operation of an optical switch array according to the present invention may be better understood with reference to the drawings and the accompanying description.

Referring now to the drawings, Figure 5 is a schematic illustration of an optical switch array of the present invention, for coupling input waveguides 140a, 140b, 140c and 140d to output waveguides 142a, 142b, 142c and 142d. Input waveguide 140a is coupled to output waveguide 142a by a 1 x 2 switch 144aa via an intermediate waveguide 148aa and a passive y-junction combiner 146aa, to output waveguide 142b by a 1 x 2 switch 144ab via an intermediate waveguide 148ab and a passive y-junction combiner 146ab, to output waveguide 142c by a 1 x 2 switch 144ac via an intermediate waveguide 148ac and a passive y-junction combiner 146ac, and to output waveguide 142d by a 1 x 2 switch 144ad via an intermediate waveguide 148ad and a passive y-junction combiner 146ad. Input waveguide 140b is coupled to output waveguide 142a by a 1 x 2 switch 144ba via an intermediate waveguide 148ba and a passive y-junction combiner 146ba, to output waveguide 142b by a 1 x 2 switch 144bb via an intermediate waveguide 148bb and a passive y-junction combiner 146bb, to output waveguide 142c by a 1 x 2 switch 144bc via an intermediate waveguide 148bc and a passive y-junction combiner 146bc, and to output waveguide 142d by a 1 x 2 switch 144bd via an intermediate waveguide 148bd and a passive y-junction combiner 146bd. Input waveguide 140c is coupled to output waveguide 142a by a 1 x 2 switch 144ca via an intermediate waveguide 148ca and a passive y-junction combiner 146ca, to output waveguide 142b by a 1 x 2 switch 144cb via an intermediate waveguide 148cb and a passive y-junction combiner 146cb, to output waveguide 142c by a 1 x 2 switch 144cc via an intermediate waveguide 148cc and a passive y-junction combiner 146cc, and to output waveguide 142d by a 1 x 2 switch 144cd via an intermediate waveguide 148cd and a passive y-junction combiner 146cd. Input waveguide 140d is coupled to output waveguide 142a by a 1 x 2 switch 144da via an intermediate waveguide 148da and a passive y-junction combiner 146da, to output waveguide 142b by a 1 x 2 switch 144db via an intermediate waveguide 148db and a passive y-junction combiner 146db, to output waveguide 25 142c by a 1 x 2 switch 144dc via an intermediate waveguide 148dc and a passive y-junction combiner 146dc, and to output waveguide 142d by a 1 x 2 switch 144dd via an intermediate waveguide 148dd and a passive y-junction combiner 146dd.

o.o..

Waveguides 140 and 142, as well as 1x2 switches 144 and y-junction combiners 146, are fabricated by standard techniques, for example on the surface of a Z-cut lithium niobate 30 crystal, essentially in a single plane. As a result, some of the intermediate waveguides intersect all but one of the input waveguides. Specifically, intermediate waveguide 148ba intersects input waveguide 140a at intersection 150ba; intermediate waveguide 148ca intersects input waveguide 140a at intersection 150ca and input waveguide 140b at intersection 150ca'; intermediate waveguide 148da intersects input waveguide 140a at intersection 150da, input waveguide 140b at intersection 150da' and input waveguide 140c at intersection 150da"; intermediate waveguide 148bb intersects input waveguide 140a at intersection 150bb; intermediate waveguide 148cb intersects input waveguide 140a at intersection 150cb and input waveguide 140b at intersection 150cb'; intermediate waveguide 148db intersects input waveguide 140a at intersection 150db, input waveguide 140b at intersection 150db' and input waveguide 140c at intersection 150db"; intermediate waveguide 148bc intersects input waveguide 140a at intersection 150bc; intermediate waveguide 148cc intersects input waveguide 140a at intersection 150cc and input waveguide 140b at intersection 150cc'; and intermediate waveguide 148dc intersects input waveguide 140a at intersection 150dc, input waveguide 140b at intersection 150dc' and input waveguide 140c at intersection 150dc".

1x2 switches 144 are illustrative of switching elements for coupling input waveguides 140 to output waveguides 142. The scope of the present invention includes all such switching elements. The particular 1x2 switches 144 illustrated in Figure 2 are directional couplers. For simplicity, the electrodes of directional couplers 144 are not shown. As in the case of the prior art switch arrays, any suitable 1x2 switches, including BOA couplers, digital-optical-switches and x-switches, may be used as 1x2 switches 144.

Passive y-junction combiners 146 are illustrative of coupling elements for coupling input waveguides 140 to output waveguides 142. The difference between a "switching element" and a "coupling element", as these terms are used herein, is that a coupling element 20 may be either passive or active, whereas a switching element is necessarily active. In Figure 4, coupling elements 46 that couple input waveguides 40 to output waveguides 42 are active coupling elements, specifically 2 x 1 switches. As in the case of 1 x 2 switches 44 and 144, these 2 x 1 switches may be any suitable 2 x 1 switches, including directional couplers, BOA couplers, digital-optical-switches and x-switches.

25 The advantage of passive couplers 146 over active couplers 46 is that in an optical switch using passive couplers 146, fewer active elements need to be addressed than in an S• optical switch using active couplers 46. The advantages of active couplers 46 over passive couplers 146 are that a passive coupler 146 requires an elaborate design geometry to prevent loss of part of the incoming radiation to a second order mode; and that an active coupler 46 in 30 its off state reduces crosstalk by actively blocking incoming signals from the associated intermediate waveguide 48.

In the general case of N input waveguides 40 or 140 and M output waveguides 42 or 142, an optical switch array of the present invention includes NM switching elements 44 or 144, NM coupling elements 46 or 146, and N(N-1)M/2 intersections 28.

To switch optical signals from input waveguides 40 or 140 to output waveguides 42 or 142 efficiently, with minimal losses, implementations of the optical switches of Figures 4 and must obey certain geometric constraints. These constraints depend on the wavelength of the light used. For the commonly used wavelength of 1550 nm, the following constraints apply: Except where coupled in switching elements 44 or 144 or coupling elements 46 or 146, waveguides 40, 42, 140 and 142 should be at least about 0.5 mm apart. 1x2 switches 44 and 144 and 2x1 switches 46 typically are between 5 mm and 7 mm long. Parallel columns of 1x2 switches, for example the column including switches 44a and the column including switches 44b, should be at least about 1 mm apart. The intersection angle at intersections 150 should be such that input waveguides 140 and intermediate waveguides 148 are not coupled at intersections 150. The radii of curvature of the curved portions of intermediate waveguides 48 and 148, and the radii of curvature of the curved portions, if any, of input and output waveguides 40, 42, 140 and 142, should be at least 25 mm, and more preferably at least mm. Within these geometric constraints, it is possible to fit as many as 32 input waveguides or 140 and as many as 32 output waveguides 42 or 142 on the face of a Z-cut 4" diameter lithium niobate crystal.

20 Depending on the voltages applied to their electrodes, 1x2 switches 44 or 144 and 2x1 switches 46 may be placed in a straight-through state, in which the two channels of the switch p are uncoupled, a crossover state, in which the two channels exchange signals, and any state in-between, for partial exchange of signals. In general, it is straightforward to select switch configurations to achieve any desired switching pattern of signals from input waveguides 40 or 25 140 to output waveguides 42 or 142. Switch configurations are selected by successive consideration of the desired output waveguides 42 or 142, taking advantage of the fact that each output channel receives input from only one input channel. For each output waveguide 42 or 142, switch 44 or 144 that couples the desired input waveguide 40 or 140 to the target output waveguide 42 or 142 is set to the state that diverts the desired portion of the input signal 30 to the target output waveguide 142, and, if necessary, some or all of the rest of switches 44 or p p 144 that couple to the target output waveguide 42 or 142 are set to the straight-through state.

This applies both to ordinary switching, in which signals from each input channel is switched to only one output channel, and to multicasting, in which signals from one of the input channels are split among two or more output channels. An important special case of multicasting is broadcasting, in which signals from only one input channel are distributed among all the output channels.

For example, using the embodiment of Figure 5, and associating channel a with waveguides 140a and 142a, channel b with waveguides 140b and 142b, and channel c with waveguides 140c and 142c, suppose that it is desired to direct input signals from channel a to output on channel b, input signals from channel b to output on channel c, and input signals from channel c to output on channel a. In the leftmost column of switches 144, that couples to output waveguide 142a, switch 144ca is set to the crossover state, while switches 144aa and 144ba are set to the straight-through state. In the next column of switches 144, that couples to output waveguide 142b, switch 144ab is set to the crossover state, while switch 144bb is set to the straight-through state. The state of switch 144cb is arbitrary, because the entire incoming signal on channel c was diverted to channel a by switch 144ca. Finally, in the next column of switches 144, that couples to output waveguide 142c, switch 144bc is set to the crossover state.

The states of the remaining switches 144 is arbitrary.

Similarly, to broadcast equally from channel a to all three output channels, switch 144aa is set to divert 1/3 of the incoming signal, switch 144ab is set to divert 1/2 of the incoming signal, and switch 144ac is set to the full crossover state. The states of the remaining switches 144 is arbitrary.

In this context, it should be noted that the switches used by Fulenwider, which consist o* 20 of input gratings and acoustic beam steerers, can assume only the straight-through state and the S crossover state. Partial diversion of a signal from one channel to another, as is necessary for multicasting, requires the use of more modem switches, such as the integrated optic switches used in the present invention.

Active couplers 46a collectively constitute a combining mechanism for coupling input

S

waveguides 40 into output waveguide 42a. Likewise; active couplers 146aa, 146ba, 146ca and 146da collectively constitute a combining mechanism for coupling input waveguides 140 into output waveguide 142a. Similarly, active couplers 46b collectively constitute a combining o mechanism for coupling input waveguides 40 into output waveguide 42b; active couplers 46c V*.00 collectively constitute a combining mechanism for coupling input waveguides 40 into output 30 waveguide 42c; and active couplers 46d collectively constitute a combining mechanism for coupling input waveguides 40 into output waveguide 42d. Likewise, active couplers 146ab, 146bb, 146cb and 146db collectively constitute a combining mechanism for coupling input waveguides 140 into output waveguide 142b; active couplers 146ac, 146bc, 146cc and 146dc collectively constitute a combining mechanism for coupling input waveguides 140 into output waveguide 142c; and active couplers 146ad, 146bd, 146cd and 146dd collectively constitute a combining mechanism for coupling input waveguides 140 into output waveguide 142d.

Figures 6A and 6B show alternative combining mechanisms.

Figure 6A shows four intermediate waveguides 148 merging into a passive funnel structure 152 at an input end 143 of an output waveguide 142. Funnel structures 152 must be designed geometrically to minimize losses due to generation of high order modes at the funnel necks.

Figure 6B shows four intermediate waveguides 148 coupled into input end 143 of output waveguide 144 by a planar lens 154. Planar lens 154 may be fabricated in a lithium mobate substrate by proton exchange, to locally increase the index of refraction of the lithium niobate. Planar lens 154 is shown as a refractive lens. Alternatively, planar lens 154 may be a Fresnel lens.

Figure 5 illustrates another feature of the present invention that increases the compactness of an optical switch of the present invention, particularly when many more than the only four input waveguides 140 shown in Figure 5 are coupled to many more than the only four output waveguides 142 shown in Figure 5. Specifically, switching elements 144 that couple input waveguides 140 into a particular output waveguide 142 are displaced relative to each other along input waveguides 140. As drawn in Figure 5, switching element 144aa is displaced rightward of switching element 144ab, switching element 144ab is displaced rightward of switching element 144ac, and switching element 144ac is displaced rightward of switching element 144ad. Switching elements 144ab, 144bb, 144cb and 144db that couple input waveguides 140 into output waveguide 142b, switching elements 144ac, 144bc, 144cc and 144dc that couple input waveguides 140 into output waveguide 142c, and switching elements 144ad, 144bd, 144cd and 144dd that couple input waveguides 140 into output :i waveguide 142d are mutually displaced along their respective input waveguides 140 in a similar manner. In the case of a large number of input waveguides 140 and output waveguides 142, this mutual displacement allows an intermediate waveguide 150, that couples a last (bottommost in figure 5) input waveguide 140 to one of output waveguides 142 (for example, output waveguide 140a), to avoid intersecting intermediate waveguides 150 that couple first (topmost in Figure 5) input waveguides (for example, input waveguides 140a and 140b) to the next output waveguide (for example, output waveguide 142b). It will be appreciated that limiting the number of waveguide intersections, to the N(N-1)M/2 minimum number of 13 intersections required by the geometry of the present invention, minimizes the cross-talk between input and output channels. Of course, the mutual displacement shown for switching elements 144ad, 144bd, 144cd and 144dd is not strictly necessary, because there is not "next output waveguide" following last output waveguide 142d.

Inspection of Figure 5 also shows that the mutual displacement of switching elements 144 that couple into the same output waveguide 142 also allows intermediate waveguides 148 that lead to that waveguide 142 to be positioned closer to each other than would otherwise be possible. The upper bound on the mutual displacement of those switching elements 144 is set by the constraint that, just as parallel waveguides 140 or 142 must be separated by a minimum distance in order to prevent crosstalk, so parallel waveguides 148, that lead to the same output waveguide 142, must be separated by a minimum distance in order to prevent crosstalk.

Figure 7 is a schematic illustration of a second embodiment of the optical switch array of the present invention, for coupling three input waveguides 240 to six output waveguides 242. Each input waveguide 240 is coupled to a corresponding, parallel auxiliary waveguide 241 by a splitting switch 243. Input waveguides 240 are coupled to output waveguide 242a by switching elements 244a via intermediate waveguides 248a and coupling elements 246a; to output waveguide 242b by switching elements 244b via intermediate waveguides 248b and coupling elements 246b; and to output waveguide 242c by switching elements 244c via intermediate waveguides 248c and coupling elements 246c. Auxiliary waveguides 241 are coupled to output waveguide 242d by switching elements 244d via intermediate waveguides 248d and coupling elements 246d; to output waveguide 242e by switching elements 244e via intermediate waveguides 248e and coupling elements 246e; and to output waveguide 242f by switching elements 244f via intermediate waveguides 248f and coupling elements 246f. As in the embodiment of Figure 5, switching elements 244a are mutually displaced along input waveguides 240, as are switching elements 244b and switching elements 244c. Similarly, switching elements 244d are mutually displaced along auxiliary waveguides 241, as are switching elements 244e and switching elements 244f. The configuration of Figure 7 allows increased compactness because the two groups of intermediate waveguides 248 and output waveguides 242 branch away from input waveguides 240 on opposite sides of input 30 waveguides 240. Because some intermediate waveguides 248d, 248e and 248f intersect some intermediate waveguides 248a, 248b and 248c, there are more intersections in the embodiment of Figure 7 than in an equivalent embodiment configured according to Figure 5; but this does not add appreciably to the cross-coupling because the angles of mutual intersection of intermediate waveguides 248 is about twice the angles of intersection of intermediate waveguides 248 with input waveguides 240 and auxiliary waveguides 241.

Figure 8 is a partial schematic illustration of a third embodiment of the optical switch array of the present invention, for coupling three input waveguides 340 to six output waveguides (not shown). Each input waveguide 340 is coupled to a corresponding, parallel auxiliary waveguide 341 by a 50% coupler 343 and a 100% reflector 350. In effect, each auxiliary waveguide 341 is an extension of the corresponding input waveguide 340 in the opposite direction, because half the light entering an input waveguide 340 and reaching the corresponding 50% coupler 343 is coupled into the corresponding auxiliary waveguide 341, and the remaining half of the light is coupled into the corresponding auxiliary waveguide 341 by the corresponding 50% coupler 343 after reflecting off of reflector 350. Input waveguides 340 are coupled to a first output waveguide by switching elements 344a via intermediate waveguides 348a, to a second output waveguide by switching elements 344b via intermediate waveguides 348b, and to a third output waveguide by switching elements 344c via intermediate waveguides 348c. Auxiliary waveguides 341 are coupled to a fourth output waveguide by switching elements 344d via intermediate waveguides 348d, to a fifth output waveguide by switching elements 344e via intermediate waveguides 348e, and to a sixth output waveguide by switching elements 344f via intermediate waveguides 348f. As in the embodiments of Figures 5 and 7, switching elements 344a are mutually displaced along input waveguides 340, as are switching elements 344b and switching elements 344c. As in the embodiment of Figure 7, switching elements 344d are mutually displaced along auxiliary waveguides 341, as are switching elements 344e and switching elements 344f. For illustrational simplicity, the output waveguides and the coupling elements that couple intermediate waveguides 348 thereto are not shown. The configuration of Figure 8 allows 25 increased compactness because the two groups of intermediate waveguides 348 and corresponding output waveguides branch away from input waveguides 340 in opposite directions.

If the embodiment of Figure 8 is fabricated on the surface of a z-cut lithium niobate crystal, then reflector 350 is formed by depositing a metal coating on a flattened and polished 30 surface perpendicular to waveguides 340 and 341, or by depositing a series of dielectric layers, appropriate to the wavelength of the light being switched, on that flattened and polished surface, or by mechanically attaching a mirror to that flattened and polished surface.

°o Figure 9 shows the preferred layout of a switch array of the embodiment of Figure representing the present invention as decided from Australian Patent Application No. 38980/99. On a surface 202 of a z-cut lithium niobate crystal 200, for coupling twelve input waveguides 140 to twelve output waveguides 142. Light enters input waveguides 140 via a surface 204 that is etched perpendicular to input waveguides 140. Light exits output waveguides 142 via a surface 206 that is etched perpendicular to output waveguides 142.

Waveguides 140 and 142 are curved, with a radius of curvature of about 35 mm. If waveguides 140 and 142 are straight, as drawn in Figure 5, then input waveguides must be separated by about 0.7 mm to make sure that intermediate waveguides 148 cross input waveguides 140 at intersections 150 at at least a minimum angle 0 of 11.50 to minimize cross-talk. Curving waveguides 140 and 142 as in Figure 9 allows input waveguides 140 to be separated by only about 0.35 mm while still intersecting intermediate waveguides 148 at an angle 0 of at least 11.50.

The illustrative geometric parameters given above are for an optical switch array of the present invention that is fabricated in a lithium niobate substrate. It will be clear to those skilled in the art how to apply the present invention to other substrates, for example, polymer substrates and silica/Si substrates. In particular, the geometric constraints relevant to these other substrates will be clear to those skilled in the art.

While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

The word comprising and forms of the word comprising as used in this description and in the claims does not limit the invention claimed to exclude any variants or additions.

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Claims (6)

1. An optical switch array comprising: a plurality of input waveguides; a plurality of output waveguides; and a plurality of switches coupling said input waveguides to said output waveguides; all said waveguides being arranged substantially in a common plane, all said waveguides being substantially curved and concentric.

2. The optical switch array of claim 1, wherein all said input and output waveguides have substantially a single common center of curvature.

3. The optical switch array of claim 1 or 2, wherein said input waveguides are substantially radially inward from said output waveguides.

4. The optical switch array according to any one of claims 1 to 3, having at least 16 said input waveguides and at least 16 said output waveguides, the optical switch array being fabricated on a substantially circular substrate having a diameter of at most about four inches.

The optical switch array according to any one of claims 1 to 4, wherein said substrate is a face of a Z-cut lithium niobate crystal.

6. The optical switch array according to any one of claims 1 to 5, having 32 said input 20 waveguides and 32 said output waveguides. **L Lynx Photonic Networks Inc 23 November 2001 *4 eeoc* *loo