STEERABLE OPTICAL REFLECTOR AND OPTICAL SWITCH

BACKGROUND OF THE INVENTION

The present invention relates to optical switching, including optical switches and optical reflector devices which may be used to implement optical switches.

An optical space switch is an apparatus that enables an optical signal entering any one of a plurality of input ports of the switch to be routed to any one of a plurality of output ports of the switch. Figure 1 shows a highly schematic representation of a generic example of an optical switch. The switch 1 has M optical input ports 2, each of which can receive an optical signal for routing, and N optical output ports 3, to each of which an optical signal can be routed from any of the input ports 2. M and N may or may not be equal. The routing is performed by a switching apparatus 4 though which the optical signals pass from an input port 2 to an output port 3. The switching apparatus 4 is operated by generating and applying electronic control signals 5 to the switching apparatus 4 to drive the switch 1 so as to determine or set the path taken from input port 2 to output port 3 by any received optical signal. The switch 1 may be a free space optical (FSO) switch, in which the optical signals travel in free space through the switching apparatus 4, typically being delivered to the switch 1 and collected from the switch 1 by optical waveguides such as optical fibres.

A FSO switch can be implemented using micro-electro-mechanical switches (MEMS). Figure 2 shows a simplified perspective view of an example MEMS-based FSO switch 1a. The input ports 2 comprise a two-dimensional array of optical collimators 2c to collimate received optical signals 2b delivered to the switch 1a by M optical fibres 2a each associated with a collimator 2c. After passing through the switching apparatus 4 as a beam 6 propagating in free space, the optical signal reaches the output ports 3, comprising a two-dimensional array of lenses 3c for coupling the optical signal into an associated optical fibre 3a, such that the optical signal becomes an output optical signal 3b. The switching apparatus 4 comprises an input steering array 7 of mechanically actuated mirrors 8 arranged in a two dimensional array, each of which is arranged to receive light from one optical fibre 2a of the input ports 2. Control by electronic signals adjusts the angle of each mirror 8 to steer the beam 6 to a mechanically actuated mirror 8 in an output steering array 9, which is in turn controlled to steer the beam to a corresponding fibre 3b in the output ports 3. A number of problems are associated with MEMS-based optical switches. These include: i) limited reliability owing to mechanical fatigue in the materials of the moving parts; ii) manufacturing and maintenance costs for the micro-assembly processes required for MEMS; iii) low operational speed caused by the time required for mechanical movement of the mirrors; and iv) limited functionality, since the mirrors passively reflect all incident light without any wavelength discrimination.

US 2020/0393737 describes an alternative design for an FSO switch which uses optical phased arrays (OPA) to steer the beams, instead of MEMS. Fig. 3A shows a schematic representation of an OPA 100. An incoming optical wave 101 is received at a waveguide and is split (so that its power is divided) into, in this example, four portions, by passing through a cascade or tree 102 of 1x2 optical splitters 104 (formed as waveguides). Each portion is passed to a phase shifter or phase control element 106, which outputs its portion to an antenna or emitting element 108 which emits that portion 101a, 101b, 101c, 101 d into free space, the antenna elements 108 being arranged in an array. The phase shifters 106 are dynamically controlled by electronic control signals to modulate the respective portions of the optical signal to have different phase shifts, so that the emitted portions 101a, 101b, 101c, 101 d combine by constructive interference at a particular propagation angle, thereby transmitting an output beam 110 which can be steered by adjustment of the phase shifts. The OPA 100 can also operate in reverse to receive an incoming beam at any angle, the antenna elements becoming receivers instead of emitters, and the optical splitter tree becoming an optical combiner tree. Use of solid-state OPAs such as this in a FSO switch addresses the problems i), ii) and iii) noted above for MEMS-based switches. An OPA-based switch can be photonically integrated, and operate by ultra-fast electro-optic phenomena.

However, each OPA can only operate as either a transmitter or a receiver for a given transmit or receive angle, in which case each requires its own power dividing or combining tree. Therefore, in order to receive an optical signal and redirect it, in other words to provide the function of a mirror as in a MEMS-based switch, it is necessary to use two OPAs connected back-to-back, one of which receives the optical signal at its antenna and combines the received portions, and one of which divides the combined beam and retransmits it from its antenna, directed as required. Fig. 3B shows an example of two OPAs connected in this way, a receiver OPA 100a being coupled via the output of its power combining tree 102a to the input of the power splitting tree 102b of a transmitter OPA 100b, so that an incoming received beam 112 at a first angle or propagation direction can be steered to a provide a transmitted beam 110 at a second angle or propagation direction. The requirement for multiple division trees, back-to-back connections and phase shifters for each branch of every division tree makes for significant complexity of each port in a switch, dramatically increases the optical losses of the switch, and leads to increased device size and fabrication costs. Also, the switch as configured according to US 2020/0393737 corresponds to the operation of MEMS- based switches in that it does not provide wavelength-dependent switching, there being no capability to spectrally separate optical signals.

Accordingly, developments in optical switches are of interest.

SUMMARY OF THE INVENTION

Aspects and embodiments are set out in the appended claims.

According to a first aspect of certain embodiments described herein, there is provided an optical reflector device comprising: two or more device elements, each device element comprising: an antenna element configured to receive and re-emit incident light; a half-length phase shifter optically coupled to the antenna element and configured to apply a first half of a phase shift to light received by the antenna element; and a reflector arranged to receive the light from the phase shifter and redirect it back through the phase shifter for the phase shifter to apply a second half of the phase shift to the light before the light is re-emitted by the antenna element; wherein: the antenna elements are arranged in a matrix such that each antenna element receives a portion of a light beam incident on the matrix, the portions combining into a reflected light beam when re-emitted from the antenna elements; and the phase shifters are configured to be controllable such that the phase shifter of each device element applies a different phase shift to the portion of the light beam in the device element, the phase shifts selectable to provide an angle of propagation for the reflected light beam defined by constructive interference of the re-emitted portions of the light beam.

According to a second aspect of certain embodiments described herein, there is provided a beam steering array comprising a plurality of optical reflector devices according to the first aspect, the optical reflector devices arranged on a planar substrate in a one-dimensional array or two-dimensional array.

According to a third aspect of certain embodiments described herein, there is provided an optical switch comprising: an input beam steering array according to the second aspect, the plurality of optical reflector devices arranged in the array such that each can be addressed by and receive an input light beam from one port in a plurality of optical input ports; and an output beam steering array comprising a plurality of adjustable optical reflectors arranged in the array such that each can address and reflect an output light beam to one port in a plurality of optical input ports; wherein: each optical reflector device in the input beam steering array is configured, by control of its phase shifters, to reflect a received input light beam from its optical input port to any of the adjustable optical reflectors in the output beam steering array; and each adjustable optical reflector in the output beam steering array is configured and adjustable to reflect a light beam from any of the plurality of optical reflector devices in the input beam steering array as an output light beam to its optical output port, such that the optical switch can route a light beam from any optical input port to any optical output port.

These and further aspects of certain embodiments are set out in the appended independent and dependent claims. It will be appreciated that features of the dependent claims may be combined with each other and features of the independent claims in combinations other than those explicitly set out in the claims. Furthermore, the approach described herein is not restricted to specific embodiments such as set out below, but includes and contemplates any appropriate combinations of features presented herein. For example, devices and apparatus may be provided in accordance with approaches described herein which includes any one or more of the various features described below as appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which:

Figure 1 shows a highly schematic representation of an example generic multi-port optical switch;

Figure 2 shows a simplified perspective view of an example of a conventional free space optical switch implemented using micro-electro-mechanical switches;

Figure 3A shows a schematic representation of an example optical phased array and Figure 3B shows a schematic representation of an example “mirror” formed from two optical phased arrays, suitable for implementing a known configuration of a free space optical switch;

Figure 4 shows a schematic representation of an example wavelength selective steerable reflector device according to the present disclosure;

Figure 5 shows a schematic representation of an example steerable reflector device without wavelength selection according to the present disclosure;

Figure 6 shows a schematic representation of a part of a device element that may be comprised in a steerable reflector device according to an example of the present disclosure;

Figure 7 shows a schematic plan view of a matrix of antenna elements arranged as an antenna of a steerable reflector device according to an example of the present disclosure;

Figure 8 shows a schematic simplified perspective view of an example optical switch according to the present disclosure; Figure 9 shows a schematic plan view of an example of a steering array formed from steerable reflector devices according to the present disclosure; and

Figure 10 shows a simplified perspective view of a further example optical switch according to the present disclosure.

DETAILED DESCRIPTION

Aspects and features of certain examples and embodiments are discussed / described herein. Some aspects and features of certain examples and embodiments may be implemented conventionally and these are not discussed / described in detail in the interests of brevity. It will thus be appreciated that aspects and features of devices and apparatus discussed herein which are not described in detail may be implemented in accordance with any conventional techniques for implementing such aspects and features.

An optical switch intended to address issues with existing configurations is proposed herein. The optical switch is based on optical reflector devices which are steerable and additionally can be made wavelength selective, and which can be deployed in input and output steering arrays to provide a switching capability. The steerable reflector devices are solid state photonic devices and hence avoid the drawbacks of slow-moving non-robust moving parts in MEMS. Each steerable reflector device comprises an array of unconnected optical antenna elements, so optical splitters and combiners are not needed, and each device performs both receiving and transmitting hence avoiding the complex back-to-back connections used with OPAs. Importantly, in some configurations the devices enable individual treatment of different wavelengths or wavelength bands/ranges, thereby enabling multiwavelength switching with multiplexing and demultiplexing. This is valuable for applications such as telecommunications where frequency domain multiplexing is used to carry different channels of information as optical signals of different wavelengths.

Figure 4 shows a schematic representation of a first example of a steerable reflector device of this type. The term “reflector” indicates its ability to act as a mirror, in that it can receive an incident light beam propagating in free space along a general input direction, and retransmit (reflect) that light back to free space in a generally opposite output direction, albeit at a reflection angle different from the angle of incidence to enable routing of the light beams. The term “steerable” indicates that the reflection or retransmission angle can be varied or tuned, in other words, a light beam can be selectively or tuneably steered so that when the device is used in a switch or similar component, the routing can be directed from any input to any output. In this example, the device is also wavelength selective, in that it is configured to separate received light into different wavelengths and reflect the different wavelengths at different angles. Hence optical signals at different wavelengths can be delivered together to the device as a single multiplexed signal, and separately steered to different outputs if required.

The device 10 comprises a plurality, in this example four, of substantially identical optical device elements 12. Each device element 12 comprises an optical antenna element 14 configured to both collect or receive incident optical power, and emit or transmit output optical power. The device elements 12 are arranged so that the antenna elements 14 are disposed with a common orientation in an array or matrix which may be a line as in Fig. 4 but will more usefully be a two-dimensional matrix. For optimum operation, within the array or matrix the antenna elements should be specifically aligned and positioned with reference to one another, including the distances/spacings between them and their arrangement in space. As well as being one-dimensional or two- dimensional, the antenna elements may be regularly spaced or irregularly scattered. Algorithms may be used to optimally position the antenna elements [2] This group of antenna elements 14 forms an antenna, with the overall group of device elements 12 forming the device 10. Light incident on the matrix is able to undergo reflection from the device. Aside from this physical arrangement, the device elements 12 are unconnected to one another, in other words, there is no optical coupling between any of the device elements 12. Within each device element 12, the antenna element 14 is coupled, such as via a waveguide 16 (although the coupling may be direct), to a multiplexer/demultiplexer 18 (MUX/DMUX) which operates in the conventional manner to either separate or divide a single incoming optical signal (light beam or light wave) into two or more components according to wavelength (demultiplexing), or combine two or more incoming optical signals of different wavelengths into a single optical signal including all the different wavelengths (multiplexing). In this example, the MUX/DEMUX 18 is configured to divide an incoming signal into four different wavelength or spectral components. The MUX-DEMUX therefore has four outputs, one for each wavelength, so that each spectral component has its own optical path. Each output is coupled to a phase shifting element or phase shifter 20. In other words, the optical path for each spectral component has its own phase shifter. Each of the four phase shifters 20 is a half-length phase shifter, so that propagation of a light wave along the phase shifter 20 provides one-half of a desired change or shift in the optical phase of the light wave. Each phase shifter 20 terminates in a mirror 22, comprising a reflector or reflective element of high reflectivity preferably configured to give maximum reflection (so, as close to 100% as possible) of incident light at the relevant wavelength carried by the phase shifter 20, that is, for the spectral component in that optical path. Alternatively, any component configured to reverse the propagation direction of the light and redirect it back through the phase shifter may be used, for example a loop of waveguide. Note that the identicality of the device elements 12 is in regard to the combination of components they comprise; there may be differences such as the length of waveguides, the length or size of the phase shifters and the type of antenna.

In operation, an incident light beam (input optical signal) 24 comprising a plurality of wavelengths (or simply being spectrally broadband) is directed, via free space, onto the matrix of antenna elements 14. As an example, the beam 24 comprises four optical channels, in other words, light at four different wavelengths (or narrow wavelength bands) is included. These are represented in Figure 4 by arrows with a solid line, a dashed line, a dotted line, and a dash-dot line. The beam 24 may be the collimated output of an optical fibre or optical waveguide, for example. The antenna elements 14 are sized and arranged relative to the beam cross-sectional area such that different portions of the beam 24 are received by different antenna elements 14. Hence, the optical power is divided between the antenna elements, and owing to the Gaussian intensity pattern across the typical collimated beam, plus incidence angle and any imperfections in alignment, the distribution of power, and also phase, between the antenna elements is unequal. The multi-wavelength composition is preserved in each portion, however, so that each portion includes light from all four channels. The multi wavelength portion of the beam 24 received by an individual antenna element 14 is carried by the associated waveguide 16 to the associated MUX/DEMUX 18, where the light is separated into the four spectral components corresponding to the four channels and directed to its corresponding path. Hence, each spectral component is delivered from the MUX/DEMUX into one of the four phase shifters 20. Electrical drive signals (not shown) are applied to the phase shifters 20 (in accordance with the design of the phase shifters, which is not limiting; some examples are mentioned below) in order to induce a phase shift or phase change in each spectral component. Since the spectral components have been separated according into the individual channels, and allocated to separate physically independent optical paths, a different phase shift can be applied to each channel. As mentioned above, the phase shifters 20 are half-length only, so each spectral component receives only half of a total desired phase shift during propagation through its phase shifter 20. On reaching the end of its optical path (which is shown as the end of the phase shifter 20 but which may be remote from that point, with some intervening length of optical path between), the spectral component reaches the associated reflector 22 and is reflected for back-propagation through the device element 12. On passing back through the phase shifter 20, the second half of the required phase shift is acquired. The spectral components are then returned to the MUX/DEMUX 18, which operates to combine the four spectral components back into a single beam, which is propagated to the antenna 14 by the waveguide 16. On reaching the antenna 14, the multi-wavelength light beam portion is emitted, with the various spectral components having phases independent of one another.

The same thing has meanwhile taken place in all of the device elements 12, but with different phase shifts applied to the various portions of each spectral component. Hence, when the four portions of one of the spectral components are emitted from the four antennae back into free space, they have different phases and constructively interfere when they combine. Appropriate selection of the different phase shifts can be made to control the propagation direction of the final combination of the portions, to give a reflection angle for the spectral component from the device 10. Hence, adjustment of the phase shifts applied by the phase shifters 20 (by control of the drive signals) will steer the relevant spectral component of the incident beam 24 to a desired emission or output angle. Different phase shifts can be applied to the other spectral components via the corresponding phase shifters, to achieve a desired output angle and propagation direction for each spectral component via different constructive interference effects. Hence, each spectral component can be steered in free space independently of the other spectral components, and the output of the device 10 comprises four separate output beams 26a, 26b, 26c, 26d corresponding to the four spectral components or optical channels, each at its own propagation angle/direction relative to the device and the angle of incidence of the incident light beam 24.

Hence, the steerable reflector device is operable to receive as an input an incident multiwavelength optical beam (such as a beam formed from several multiplexed optical signals at different wavelengths, such as optical channels), separate the beam spectrally into components of different wavelengths (such as the various optical signals or optical channels), and reflect each component independently to a different direction/angle selectable by control of the phase shifters.

While the wavelength selection functionality of the device of Figure 4 is particularly useful and beneficial, especially when compared to the lack of wavelength discrimination in a MEMS or OPA, it is possible to omit the wavelength selectivity to provide a device configured for beam steering only, in other words, directing an incident beam (which may be a single wavelength, a group of wavelengths, or broadband) along a chosen reflection direction by control of the phase shifter drive signals. Such a device may be referred to a colourless or monochromatic steerable reflector device, since it has no individual wavelength treatment capability. Figure 5 shows a schematic representation of an example steerable reflector device configured in this way. As in the Figure 4 example, the device 10 comprises a plurality of device elements 12 (four shown for the purpose of illustration), with each optical element 12 comprising an antenna element 14 for receiving a portion of a light beam 24, and re-emitting the portion after propagation through the device element 12 and reflection from a reflector 22 at the opposite end of the device element 12. The antenna elements 14 are arranged in a matrix to form an antenna onto which the incident light beam 24 is directed. However, in this example, the MUX-DEMUX are omitted from each device element 12; instead each antenna element 14 is coupled directly to a single half-length phase shifter 20 (via an intermediate waveguide 16, of which the length may vary between the device elements, and which is not essential in this or other examples) which terminates in a reflector 22. Hence, in each device element 12, the received portion of the incident beam 24 undergoes no spectral separation; instead, the whole of the portion, regardless of wavelength, follows the same optical path through the whole of the device element, and experiences the same phase shift during its double-pass through the phase shifter 20. As before, control of the drive signals for the phase shifters 20 so as to apply different phase shifts to the different portions in the various device elements 12 modifies the constructive interference which occurs when the portions are emitted by the antenna elements 14 back into free space and combine, so that the direction of the output beam 26 can be adjusted and the beam steered. Hence, in this example, there is a single output beam 26 which is a reflected version of the input beam with the same spectral composition, but with a propagation angle Q (being the angle at which the output beam is emitted or reflected from the device) which is selectable by controlling the phase shifters.

As with optical and photonic devices in general, the devices described above will typically be subject to some level of loss of optical power, such as happens generally by power dissipation, scattering and absorption during optical propagation and interaction of the light with individual components. Hence, the total optical power emitted from a device may well be somewhat less that the total amount of incident power. Depending on the application to which the device is put, this loss may be more or less problematic or undesirable. For example, when routing within an optical telecommunications network, it is desirable to maintain the power of each channel as much as possible to maximise the signal-to-noise ratio and ensure correction reception of the signals at the intended destinations. Accordingly, in some embodiments, it is proposed to include optical gain within the device. Figure 6 shows a schematic representation of part of an example device configured in this way. The reflector or mirror element 22, arranged at the end of the phase shifter 20, is provided with an amplification stage or gain block 30. The portion of the optical signal (or spectral component of the optical signal, if demultiplexing is included) propagating in the device element passes through the gain block 30 in the process of being redirected back to the phase shifter 20 by the reflector 22. In this way, the optical signal can be boosted in power to that the overall output of the device has an optical power the same as, or even higher than, the power of the original incident beam. As an example, the reflector 22 can be configured as a back-to-back connected multi- mode interference (MMI) couplers. An integrated amplifier such as a semiconductor optical amplifier (SOA) can be included within the structure of the MMI, that is, lying along the propagation path through the reflector 22. MMI couplers alone, without any amplification stage, can also be used to provide the basic reflection function discussed with regard to Figures 4 and 5. Also, the reflection function and any amplification function can be provided using any other suitable optical and photonic components that will be apparent to the skilled person. Furthermore, one or more amplification or optical gain stages may be incorporated elsewhere within the device if preferred. The location shown in Figure 6, where the gain occurs during reflection of the light portion, provides a single pass through the amplifier and may be straightforward to control. In particular, the gain is provided separately in association with each phase shifter, so where wavelength selection via spectral separation is provided, as in Figure 4, the gain can be controlled independently for each spectral component. However, locating the amplifier elsewhere within the device element may, for example, allow a double pass and hence a greater total level of gain, although it may become susceptible to lasing effects.

Figure 7 shows a highly schematic representation of an end view of an example steerable reflector device. As noted above, a device comprises a plurality of device elements, which are not optically interconnected. Each device element includes an antenna element, and within the device, the antenna elements are commonly oriented and arranged adjacently into a matrix to provide an optical receiving and emitting area onto which an incident light beam is directed and from which it is subsequently re emitted, or reflected, after passage through the device. The arrangement of the plurality of antenna elements within the matrix forms an antenna. In the depicted example, the device comprises 64 device elements, and the receiving/emitting faces of the 64 antenna elements 14 of these device elements are arranged in a two-dimensional 8x8 matrix 15 to provide the antenna 17 of the device. The matrix 15 is square in this example, but, as noted with regard to Figure 4, the antenna elements 14 may be arranged regularly or irregularly in other shapes if desired, according to the intended use of the device, for example.

Similarly, the number of device elements in the device, and hence the number of antenna elements, may be chosen as required. The number of device elements included in the device will define its overall operational characteristics as an antenna. In the context of a FSO switch, characteristics of significant interest are the achievable beam steering angle, the beam divergence, and side lobes suppression; these factors determine the size of the ports which the switch can address and handle, and how much loss is induced. In the Figure 7 example, a 64-element device arranged in a two- dimensional 8x8 matrix to provide beam steering capabilities in two dimensions can provide a large steering angle of about 16° in each dimension, with a low beam divergence of less than 1°. As example, these values indicate that it is possible to individually address any port in a FSO switch input array of 16x16 devices, making possible a 256-port switch. Since each port would employ one 64-element device, a total of 16384 device elements is required for the switch input array. Other numbers of device elements per device, and devices per array are not excluded however. Simple switching and routing applications with a low quantity of inputs and outputs may be implementable with fewer devices and smaller beam steering angles, for example. Hence, a device might comprise 4 device elements in a 2x2 matrix, 9 device elements in a 3x3 matrix, 16 device elements in a 4x4 matrix, 25 device elements in a 5x5 matrix, 36 device elements in a 6x6 matrix, 49 device elements in a 7x7 matrix, or device element numbers higher than an 8x8 matrix, for example up to 100 device elements or more. The matrix need not be square, so that non-squared amounts of device elements can also be used, arranged in a two-dimensional grid or other format, or as a one-dimensional linear array. These values are purely examples, also, and the invention is not limited in anyway as regards the number of device elements in any device. A minimum requirement is at least two device elements, in order to achieve constructive interference between light waves with different phases. At the other end of the scale, while a larger number of elements can provide more precision in beam steering, there are limitations imposed by fabrication and device architecture that may limit the number of device elements that can be implemented, although this integration density will depend on the chosen structure and type of the various components of the device elements.

Figure 8 shows a simplified schematic perspective view of an optical switch according to an example of the present disclosure. Like the generic example of Figure 1, the optical switch comprises a plurality of input ports, a plurality of output ports, and a switching apparatus configured to route optical signals from any of the input ports to any of the output ports. Additionally, the optical switch is able to provide wavelength demultiplexing so that optical channels defined by different spectral components can be directed separately from a single input port to different output ports.

The optical switch 50 comprises a plurality of input ports 52, where each input port 52 comprises an optical fibre 51 to deliver an input optical signal. Collimators, and possibly other beam shaping and/or directing elements (not shown) are included to collimate light as it exits the fibres 51 into the free space optical propagation environment of the switch 50. The fibres 51 comprise 16 fibres in this example, arranged so that the fibre outputs are in a 4x4 grid, but any number M of input ports 52 may be used. The switching apparatus comprises an input steering array 53 comprising an array of steerable reflector devices SRD 10, which in this example are wavelength selective SRD such as that in the example of Figure 4. A total of 16 SRDs 10 are included in the input steering array 53, corresponding to the 16 optical fibres 51 of the input ports 52. The SRDs 10 are arranged on a substrate in a 4x4 grid or array to match the physical arrangement of the input port fibres 52 to give a straightforward mapping of the input port fibres 52 to the SRDs 10 so that each SRD 10 is addressed by one input port fibre 52. Suitable beam directing can allow different physical arrangements while enabling the mapping, however.

The switching apparatus further comprises an output steering array 54 which comprises another array of SRDs 10, in this example also 16 devices located on a substrate as a 4x4 grid. The switch also comprises a plurality N of output ports 56, each comprising an optical fibre 55, where again there are 16 output port fibres 55, one addressed by each of the SRDs 10 in the output steering array 54. Hence, in this example, there are the same number of input port fibres 51, SRDs 10 in the input steering array 53, SRDs 10 in the output steering array 54, and output port fibres 55. In other words, the number of M input ports matches the number of N output ports. More generally, however, there will be the same number of input port fibres 51 as SRDs 10 in the input steering array 53, to achieve a one-to-one mapping at the switch input, and the same number of output port fibres 55 as SRDs 10 in the output steering array 54, to achieve a one-to-one mapping at the switch output, but these two numbers may be different, so that M and N are unequal.

The input steering array 53 and the output steering array 54 are located with respect to one another such that the steering capability of the SRDs 10 in the input steering array 53 allows each SRD 10 in the input steering array to direct light to any of the SRDs 10 in the output steering array 54. In this way, the switch 10 can be fully operational, in that light arriving at any input port can be routed to any output port. In other examples, this capability may be reduced if it sufficient for some or all of the input ports to have access to only some of the output ports; this may achieved with a lesser amount of steering in the steering arrays.

In use, a fibre 51 in the input ports 52 delivers a light beam 24 to the switching apparatus, and the light beam is incident on a SRD 10e which is addressed by the particular fibre 51. The light beam 24 contains light at multiple wavelengths, such as optical signals carried in four optical channels each defined by a different spectral value or range. The SRD 10e has a MUX/DEMUX configured to separate the light beam into different spectral components, in this case, the four optical channels, and operates as described with respect to Figure 4. The input steering array 53 has suitable electrical connections (not shown) to a controller configured to generate and provide electrical control signals to the phase shifters in each of the SRDs 10. By application of appropriate electrical control signals, the SRD 10e emits four spectral components at four different emission angles, determined by the phase differences applied in the different device elements and different phase shifters in the SRD 10e and resulting constructive interference on emission. In this way, each of the four spectral components becomes a separate output light beam 26a, 26b, 26c, 26d from the input steering array 53 which are each directed to a different SRD 10a, 10b, 10c, 10d on the output steering array, according to the output port 56 required for each optical channel. The phase shifters of these SRDs 10a, 10b, 10c, 10d in the output steering array 54 are similarly operated by application of electrical control signals via electrical connections (not shown) to the controller in order to steer the received light to the corresponding output port optical fibre 55. Note that each SRD 10 of the output steering array 54 is not performing any multiplexing or demultiplexing in this example; each receives light of a single wavelength or wavelength range (optical channel) along a single path and emits light at that same wavelength only along a single path. However each SRD 10 of the output steering array may receive light in any of the optical channels, and may moreover simultaneously receive light in more than one optical channel from different SRDs 10 in the input steering array which is required to be multiplexed together for routing to the same output port fibre 55. Accordingly, the SRDs 10 of the output steering array have the same wavelength capability and structure as the SRDs of the input steering array.

In a more generic example, however, each wavelength from a wavelength selective SRD (WSSRD) of the input steering array (which may also be considered as a transmit WSSRD) could be directed to an individual, spatially separated, and monochromatic (in that it lacks individual wavelength steering functionality, like the Figure 5 example) SRD in the output steering array (which may also be considered as a receive SRD) that would receive the incoming wavelength and redirect it to a respective output port fibre. In that case, where there is n wavelength operation for the transmit WSSRD (n channels are supported), n times the number of monochromatic SRD are required in the output steering array to preserve full multi-wavelength operation. In the examples of Figures 4 and 8, n=4, but as discussed, a greater or less number of channels or wavelength bands are possible, as desired.

Furthermore, the monochromatic SRDs in the output steering array may be SRDs without MUX/DEMUX as in the Figure 5 example. However, other optical reflectors that are adjustable to provide beam steering by changing the reflection angle, but lacking wavelength capability, may alternatively be used, such as MEMS (Figure 2) or OPA (Figures 3A and 3B).

In another example, wavelength handling may be dispensed with entirely. Both the input steering array and the output steering array can be formed using monochromatic SRDs, in place of the WSSRDs 10 shown in Figure 8. This will enable switching between any of a plurality of input ports and any of a plurality of output ports without reference to the wavelength of the light beams, in a manner that addresses the drawbacks of colourless MEMS-based switches noted in the Background section, and also reduces complexity compared with the back-to-back OPA-based switches of US 2020/0393737.

For WSSRDs, however, the number of wavelengths, wavelength ranges or channels a device can individually support and handle is equal to the number of phase shifters used in each device element in the WSSRD. For example, wavelength capability for eight wavelengths, channels or spectral components requires eight phase shifters per device element. As an example, if a WSSRD employs 32 device elements, giving an antenna formed from 32 antenna elements, an 8 wavelength or 8 channel capability requires 8 phase shifters for each of the 32 device elements, in other words, 256 phase shifters per WSSRD. This number of components can also be a limiting factor for the complexity of a switch with wavelength selection capability. The number of phase- shifters required for a whole WSSRD practically limits the number of wavelengths which can be handled. However, in the data centre-based applications, telecommunication optical links are normally 4 wavelengths wide at present, with a future projection of 8 wavelengths, which is not unfeasible with the presently-proposed designs. However, any number n of spectral components per device, and therefore phase shifters per device element, is possible according to the current disclosure, and the invention is not limited in this regard. As an example, n may be in the range of 1 to 10 or , or appropriate to cover wavelength bands defined by the ITU (International Telecommunications Union) such as O-band and/or C-band channels, but as noted, values of 4 and 8 are particularly useful, relevant and achievable.

In some examples, every SRD in a switch’s input steering array and output steering array may comprise the same quantity of device elements, in other words every antenna is made up of the same number of elements in the same matrix pattern, and has the same performance as the others. Alternatively, however, a switch may be configured such that each SRD has its own emission pattern, being a pattern of possible light reflection angles or directions, which can be provided by an optimised arrangement of the antenna elements to define the antenna. In this case, different antenna element types and arrangements and different antenna element numbers may be used within the same steering array. For example, a SRD at the middle of a steering array preferably has a symmetrical beam steering capability in all directions, while SRDs at the edge of the array require an asymmetric steering pattern, with no steering required in a direction beyond the array edge. SRDs with different numbers of device elements and/or different patterns or arrangements of their antenna elements in the matrix forming the antenna can satisfy these different requirements for emission patterns for differently located SRDs. Note that the same or a similar effect may be achieved with appropriate offsets of the phase biasing or phase differences provided by the phase shifters, however.

Typically, the input steering array and the output steering array will each comprise a two-dimensional regular grid of devices, but this is not essential. An array may be one dimensional, and the individual devices may be regularly or irregularly arranged and/or spaced within the array, for example in a square or rectangular grid, or in some other shape.

While the SRDs, WSSRDs and switches described herein may be fabricated or assembled from the relevant optical and optoelectronic components in any format, a useful arrangement is as a photonic device that may be suitable for use in multiple applications. The photonic device can have the form of a planar waveguide such as a strip waveguide or a rib waveguide written, etched or otherwise fabricated in a suitable substrate by any known waveguide fabrication technique. The waveguide is shaped, defined and otherwise formed to provide the various routing, reflective, MUX/DEMUX and phase shifting components. The dimensions of the waveguide and the depth at which it is formed in the substrate will depend on the material used, as is known. A range of substrate materials are suitable, such as lll/V semiconductors (including gallium arsenide, GaAs; indium phosphide, InP; aluminium gallium arsenide, AIGaAs; gallium nitride, GaN and others), and group IV-based materials in amorphous or crystalline form (germanium and silicon) or glasses (including silicon nitride, SiN; silicon oxynitride, SiON; aluminium oxide, AI ₂ O ₃ and others). Other materials are not excluded, however. For use in telecommunications switching the material may be transparent between about 1250 nm and 2000 nm, being the wavelength range typically used for optical signals in telecommunications. For other applications, for example use in sensing, the material forming the photonic devices and associated system may be selected for different wavelength requirements such as transparency in the visible range or the near infrared (such as 350 nm to 1 pm), or the mid-infrared (about 2 pm and above).

Any approach may be used to implement the phase shifters. Examples include configurations utilising the thermo-optic effect, electro-refraction (plasma dispersion effect, quantum configured Stark effect, Franz-Keldysh effect), in a PIN semiconductor structure, and the electro-optic effect by use of a material forming or surrounding the waveguide which has a c ² optical nonlinearity and shows a Pockels effect (such as electro-optical polymers and ferroelectric materials), enabling a phase shift to occur when an electric field or heat is applied.

In order to provide an array of devices, all devices may be formed in a single substrate or chip. Alternatively, individual devices (such as in Figures 4 or 5) or multiple devices may be formed on separate substrates or chips, which are then subsequently assembled into an array suitable for use as a steering array (such as in Figure 8). This assembly can be achieved through bonding of the chips or dies on a larger substrate for connectivity and alignment purposes. For example, a single chip which contains one or more devices can be considered as a tile, and the tiles may be assembled or tiled together into an array. Relatively large scale integration is required to implement an optical switch with a high number of ports. This may be achieved, for example, by fabrication of a tile in the form of one chip with one device or multiple integrated devices, which is then tiled together with copies of the tile to form a larger array.

Figure 9 shows a simplified plan view of a steering array formed in this way. The array 53 is formed from individual WSSRDs 10 each having four device elements 12. Tiles or chips 60 are fabricated each having 16 WSSRDs 10 arranged in a 4x4 grid, and four identical chips 60 are tiled together in a 2x2 grid by mounting on a substrate 62, in order to create a 8x8 array 53 of devices, in other words an array 53 comprising 64 devices 10.

Figure 10 shows a simplified perspective view of a further example of an optical switch. As before, the switch 50 comprises an input steering array 53 and an output steering array 54, which in this example comprise 16 SRDs 10 formed on a single chip 60 and arranged in a 4x4 regular two-dimensional grid. Hence, each array can be substantially the same, each being a copy of the other, although this is not necessary. Each steering array 53, 54 could comprise a different number of SRDs 10, and the SRDs might be formed on multiple chips assembled or tiled together to create an array, as in the Figure 8 example. The input steering array 53 and the output steering array 54 are arranged in a same plane (in this example immediately adjacent, but they may be spaced apart within the plane), with the antennae of the SRDs 10 facing in the same direction. Alternatively, the two steering arrays 53, 54 might occupy parallel planes, having an offset along the general beam input and output direction. Locating the two arrays 53, 54 in this manner enables the arrays to be mounted or fabricated on a same substrate, or fabricated directly on the same chip or die. Fabrication, manufacturing and assembly of the switch can thereby be simplified.

This arrangement of the input steering array 53 and the output steering array 54 places the antennae of the SRDs 10 on one array out of the line of sight of the antennae of the SRDs 10 on the other array. There is no direct optical path between the two arrays 53, 54. In order to provide optical communication between the two arrays 53, 54, the switch 50 additionally comprises a reflective element 64 (which may be a planar or non- planar reflective surface such as a mirror, or a differently shaped element with reflective capability such as a prism) arranged to face the plane occupied by the arrays 53, 54 and lying in a second plane parallel or substantially parallel to the plane or planes of the arrays 53, 54, but spaced apart therefrom. In use, an incident beam 24 from an input port (not shown) is directed onto a corresponding SRD 10e in the input steering array 53. The SRD 10e operates to reflect the beam to the reflective surface 64 at a suitable angle such that the beam reflects from the reflective surface 64 to a desired SRD 10a in the output steering array 54 that corresponds to an output port (not shown) to which the incident beam 24 is to be routed. The SRD 10a is operated to reflect the beam to the output port, so the output steering array 54 thus produces the required output beam 26. The reflective surface 64 hence enables an optical path for beam propagation between the input steering array 53 and the output steering array 54.

More broadly, one or more static reflective elements (in other words, reflective elements or surfaces without tuneable beam steering capability) may be used to provide an optical path between the two arrays under any geometry in which the input beam array and the output beam array are positioned so that there is no direct line of sight from one array to the other, in other words, where a SRD in the input beam steering array is not able to steer a beam directly onto a SRD in the output beam steering array. Instead, the SRDs in the input beam steering array steer beams to the reflective element, and the SRDs in the output beam steering array receive those beams from the reflective element. The optical path may be broken into additional stages with one or more additional reflective elements if required. One or more reflective elements may also be included in switch configurations where there is a line of sight between the arrays in order to facilitate, supplement or otherwise modify the beam steering, for example by reducing or removing extreme angles from the optical path, which might be at the edge of a SRD’s steering range. Overall, reflective elements can be used in conjunction with input and output steering arrays arranged at any angle with respect to one another.

Thus far the various examples have been presented as free space devices with the suggestion that the optical propagation space between components (input fibres to steering array, steering array to steering array, steering array to output fibres, for example) is air-filled. However, the space or spaces in a switch or other optical apparatus or component comprising the devices described herein may be occupied by any suitable material with adequate transparency (with regard to the level of optical loss which can be tolerated) at the wavelength or wavelengths for which the apparatus is intended, such as a glass material. Filling the free space in this way can improve the mechanical and/or thermal stability of the apparatus. However, the filled space can still be considered as free space in that optical beams are able to pass through the space along any propagation direction, in contrast to being constrained to specific optical paths defined by waveguides, for example.

The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and/or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and/or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in the future. REFERENCES

[1] US 2020/0393737

[2] R. Fatemi, A. Khachaturian and A. Hajimiri, "A Nonuniform Sparse 2-D Large-

FOV Optical Phased Array With a Low-Power PWM Drive," IEEE Journal of

Solid-State Circuits, vol. 54, no. 5, pp. 1200-1215, May 2019, doi: 10.1109

/JSSC.2019.2896767]