Multi-waveguide non-volatile optical switch

Abstract

An optical switch device comprises a waveguide structure. The waveguide structure comprises: a plurality of optical inputs 120-1, 120-2 and a plurality of optical outputs 130-1, 130-2; a plurality of

Description

Technical Field

[0001] The present disclosure relates to optical switch devices and systems comprising reconfigurable optical switch networks utilising such optical switch devices. Background

[0002] Optical switches find uses in many circumstances. For example, optical switch networks are used widely to implement photonic tensor cores (fast matrix multiplication), neural networks, multiplexing or demultiplexing operations, reconfigurable filters, and optical true-time delay lines for free-space beam steering, among other applications.

[0003] Optical switch networks are often implemented with thermo-optic, electro-optic, or material phase change mechanisms. Thermo- optic switches rely on resistively heating an electrode near a waveguide to thermo-optically change its refractive index and so are typically incapable of maintaining or holding their switching state once power to the electrode is removed. Switches based on phase-change materials might use, for example Ge-Se-Te (GST), which is not compatible with typical CMOS manufacturing processes and accordingly such switches are difficult to manufacture at scale.

[0004] Electro-optic mechanisms may be used in volatile or non-volatile optical switches. Many electro-optic switch designs use lithium niobate (or similar) bonded to silicon-on-insulator (SOI), which may require processes that are not readily compatible with normal CMOS manufacturing processes (e.g. flip chip bonding), or positive-intrinsic-negative (PIN) junction modulators. Accordingly, such electro-optic switches may be difficult to manufacture at scale.

[0005] For those electro-optic switch designs that are CMOS-compatible, the resulting optical switches often have a limited tuning efficiency or are lossy. For example, a conventional non-volatile electro-optic switch may utilise a Mach-Zehnder interferometer with some electro-optic control over the refractive index of an internal arm of that interferometer. Tuning the refractive index may change the distribution of optical power, and therefore the switching state, at the output ports of the interferometer. Often these designs require the use of isolation couplers to optically connect but electrically isolate a portion of the internal arm of the interferometer. However, the resulting switches may be lossy due to the combined losses introduced by the isolation couplers and other elements. Summary

[0006] According to an aspect of the present disclosure an optical switch device is provided. The optical switch device comprises a waveguide structure and a plurality of electrodes. The waveguide structure comprises a plurality of optical inputs and a plurality of optical outputs. The waveguide structure further comprises a plurality of primary optical waveguides, each primary optical waveguide connecting a respective optical input and a respective optical output. The waveguide structure further comprises a set of one or more chargeable optical waveguides electrically isolated from the plurality of primary optical waveguides. The plurality of electrodes is configured to control a charge state of the set of one or more chargeable optical waveguides. When the set of one or more chargeable optical waveguides is in a first charge state, the waveguide structure is configured to optically couple an optical signal received at a first optical input to a first optical output. When the set of one or more chargeable optical waveguides is in a second charge state, the waveguide structure is configured to optically couple an optical signal received at the first optical input to a second optical output.

[0007] In the examples described herein, a chargeable optical waveguide can be charged or discharged by electrodes. In particular, a chargeable optical waveguide may be charged or discharged via quantum tunnelling (e.g. Fowler-Nordheim tunnelling and / or hot carrier injection) through a thin insulator or dielectric layer separating the chargeable optical waveguide from the electrode. By electrically isolating a chargeable optical waveguide from the electrodes and the primary optical waveguides, the chargeable optical waveguide may retain its charged or uncharged state. As may be known to a person skilled in the art, presence or absence of charge in an optical waveguide can influence an index of refraction of that optical waveguide. Accordingly, as a result of this opto-electric effect, the refractive index of the chargeable optical waveguide may have a first value when the waveguide is charged and may have a second value when the waveguide is not charged. Furthermore, different amounts of charge may lead to different refractive indices, and so the charge in the chargeable optical waveguides may be tuned or calibrated to provide a particular refractive index. The charge in the chargeable optical waveguide may be tuneable over a continuous range. As the charged state or uncharged state of the chargeable optical waveguide is non-volatile, the index of refraction of the chargeable optical waveguide is also modifiable / mutable in a non-volatile manner. In this way, the chargeable optical waveguide can function as part of a non-volatile optical switch device.

[0008] In the examples described herein, the chargeable optical waveguides do not act as direct carriers of a received optical signal (in the sense that the chargeable optical waveguide is not directly connected to an optical input or optical output of the optical switch device). Instead, the one or more chargeable optical waveguides are physically separated and electrically isolated from the plurality of primary optical waveguides. An optical signal received at one or more optical inputs may at least partially couple into or out of a chargeable optical waveguide within the waveguide structure via evanescent coupling. The behaviour of the waveguide structure of the optical switch may be analysed by considering supermodes of the device. In particular, the refractive index of a chargeable optical waveguide can influence the evanescent coupling between the different waveguides of the waveguide structure and accordingly influence the behaviour of supermodes therein. By changing the charge state of the set of one or more chargeable optical waveguides, the effect of the waveguide structure as a whole on a received optical signal can be changed. Accordingly, by changing the charge state the switching state of the optical switch device can be changed, for example to select an optical output to which a received optical signal is to be coupled. The charge carriers injected into an optical waveguide may be electrons or holes.

[0009] The optical switch devices described herein provide several advantages. For example, the optical switch devices described herein are examples of non-volatile optical switches that can hold their switching state for extended periods even after electrical power is removed, as opposed to traditional thermo-optic or electro-optic volatile optical switches. Furthermore, the optical switch devices described herein may use less power than conventional optical switches as power may not be required to hold or maintain the switching state of the optical switch device. Further advantageously, as the chargeable optical waveguides are not direct carriers of a received optical signal, the optical switch devices described herein may be less lossy than other optical switch devices in which charge carriers may be injected into a waveguide that directly carries a received optical signal. For example, the non-volatile optical switch devices described herein do not require lossy longitudinal electrical isolation couplers.

[0010] Advantageously, as the chargeable optical waveguides may not function as direct carriers of a received optical signal, there is greater design freedom in the positioning of the plurality of electrodes. For example, some electrodes may be positioned closer together than in alternative optical switches in which charges are injected into a waveguide that directly carries an optical signal, without unduly affecting the loss of the device. By positioning a source electrode and a control electrode closer together, a greater amount of charge may be injected into the chargeable optical waveguide, leading to a greater change in refractive index. This in turn can enable optical transitions within the waveguide structure to be more pronounced, and so the length of the switch device may be reduced.

[0011] Further advantageously, the optical switch devices described herein may be made of traditional materials used in CMOS-processes, in contrast to known non-volatile optical switches that typically use phase-change materials, for example chalcogenide-based alloys such as Ge-Se-Te. The optical switch devices may accordingly be made at scale more easily than known non-volatile optical switches that utilise phase-change materials.

[0012] For example, the primary optical waveguides may comprise silicon or silicon nitride. Furthermore, the primary optical waveguides may be composed of different materials, for example a first primary optical waveguide may be formed from silicon nitride while a second primary optical waveguide may be formed from silicon. Similarly, the set of one or more chargeable optical waveguides may comprise silicon or silicon nitride, and in examples in which the set of chargeable optical waveguides comprises a plurality of chargeable optical waveguides, a first chargeable optical waveguide may be formed of a different material to a second chargeable optical waveguide. In a particular example, the plurality of primary optical waveguides may comprise silicon nitride (SiN) and the set of one or more chargeable optical waveguides may comprise silicon (Si); silicon nitride typically has a lower waveguide loss (e.g. reduced loss from absorption or sidewall scattering) and higher power handling capacity than silicon, particularly at free space wavelengths in the near infra-red (e.g. 1550nm). The choice of waveguide materials may depend on factors such as for example the function of the optical circuit in which the optical switch device is to be used, or the wavelength range on which the switch is designed to operate, or any other factor.

[0013] The different waveguides may be provided in different layers in the optical switch device. For example, the primary optical waveguides may be provided in a first layer of the waveguide structure, and the chargeable optical waveguides may be provided in a second device layer of the waveguide structure above or below the first device layer.

[0014] In some examples, the plurality of primary optical waveguides may consist of two primary optical waveguides, and the set of chargeable optical waveguides may consist of one chargeable optical waveguide. Such an optical switch device may sometimes be referred to herein as a 3-arm switch. The one chargeable optical waveguide may be situated between the two primary optical waveguides. For example within a coupling region of the optical switch device a first primary optical waveguide may be positioned adjacent to and aligned parallel to the one chargeable optical waveguide, and the one chargeable optical waveguide may be positioned adjacent to and aligned parallel to both the first primary optical waveguide and the second primary optical waveguide.

[0015] In some examples, the plurality of primary optical waveguides may consist of two primary optical waveguides, and the set of chargeable optical waveguides may comprise more than one chargeable optical waveguide, for example two chargeable optical waveguides. The two primary optical waveguides may be positioned inside the set of chargeable optical waveguides. For example within a coupling region of the optical switch device a first chargeable optical waveguide may be positioned adjacent to a first primary optical waveguide, the first primary optical waveguide may be positioned adjacent to the first chargeable optical waveguide and a second primary optical waveguide, and the second primary optical waveguide may be positioned adjacent to the first primary optical waveguide and the second chargeable optical waveguide. Advantageously, such an arrangement of the waveguides enables the primary optical waveguides to be positioned closely together and accordingly the optical coupling between the primary optical waveguides may be stronger than if the primary optical waveguides are separated by a third (chargeable) optical waveguide. This in turn enables one to achieve the same device performance as for a 3-arm switch but with a smaller coupling length. Accordingly, the physical dimensions of the optical switch device may be reduced.

[0016] The shapes of the waveguides may be chosen based on the requirements of any system in which the switch device is to be implemented. For example, the shapes of the waveguides may be designed based on which polarization modes are to be processed by the switch device. For instance, the shapes of the waveguides may have a circular, or L-shaped, or mound-shaped, or trapezoidal cross-section depending on different fabrication methods. One or more waveguides (e.g. chargeable optical waveguides) may be tapered. In some examples, the optical switch device may support TE modes. In some examples, the optical switch device may support TM modes. In some examples, the optical switch device may support both TE and TM modes, which may be useful for e.g. polarization multiplexing.

[0017] In some examples, the set of chargeable optical waveguides may have more than two charge states. A chargeable optical waveguide may itself have different charge states, for example an uncharged state in which the waveguide has a first refractive index, a first charged state in which the waveguide has a second refractive index, and a second charged state in which the waveguide has a third refractive index. Additionally or alternatively, the set of chargeable optical waveguides may have further charge states based on different configurations of waveguides being charged. For example, with two chargeable optical waveguides, the two waveguides being uncharged may represent a first charge state, the two waveguides being charged may represent a second charge state, and the one waveguide being charged and the other uncharged may represent a third charge state.

[0018] The plurality of electrodes may be configured to place the set of one or more chargeable electrodes in a third charge state, different to the first charge state or the second charge state. In some examples, when the set of one or more chargeable optical waveguides is in a third charge state, the waveguide structure may be configured to optically couple an optical signal received at the first optical input to a third optical output. In some examples, when the set of one or more chargeable optical waveguides is in a third charge state, the waveguide structure may be configured to optically couple an optical signal received at the first optical input to both the first optical output and the second optical output.

[0019] The skilled person will appreciate that the number of inputs and the number of outputs may be greater than or equal to two, and the optical switch device may have more than two switching states. Switches with more than two inputs and two outputs can enable more complex switching functionality. Furthermore, the number of optical inputs does not have to equal the number of total outputs.

[0020] The skilled person will also appreciate that the optical switch devices described herein may be used to controllably interfere multiple optical signals. For example, a first optical signal may be received at a first optical input and a second optical signal may be received at a second optical input, and the effect of the waveguide structure on the two received optical signals (which may have the same or different wavelengths) may depend on the charge state of the set of one or more chargeable optical waveguides. Accordingly, such examples may be used for multiplexing / demultiplexing operations, for example involving optical signals having different wavelengths.

[0021] According to an aspect of the present disclosure, a system is provided. The system comprises a number of optical input ports and a number of optical output ports, an array of individually addressable cells, and control circuitry. The array of individually addressable cells is arranged between the optical input ports and optical output ports. Each cell is optically coupled to at least one other cell. Each cell comprises an optical switch device, wherein each optical switch device comprises a waveguide structure and a plurality of electrodes. The waveguide structure comprises a plurality of optical inputs and a plurality of optical outputs. The waveguide structure further comprises a plurality of primary optical waveguides, each primary optical waveguide connecting a respective optical input of the waveguide structure and a respective optical output of the waveguide structure. The waveguide structure further comprises a set of one or more chargeable optical waveguides electrically isolated from the plurality of primary optical waveguides. The plurality of electrodes is configured to control a charge state of the set of one or more chargeable optical waveguides. The control circuitry is configured to control a charge state of the set of one or more chargeable optical waveguides of each optical switch device.

[0022] Advantageously, by providing a system having an array of optically coupled cells that comprise optical switch devices as described herein, highly configurable and scalable optical networks may be utilised. Such optical switch networks may be suitable for e.g. large scale (optical) matrix operations or optical neural networks. The switch networks can have large and varied topologies (e.g. configurations of optical connections between cells), which may be designed based on the desired application of the system.

[0023] In some examples, there is no primary optical waveguide connecting an optical input port of the system with an optical output port of the system. Put another way, each cell of the system may or may not be directly optically coupled to an adjacent cell. In some examples, cells may be optically coupled via evanescent coupling or intermediate waveguides.

[0024] The skilled person will appreciate that the systems described herein may be suitable for many different applications. In some examples, the system may be used for implementing an optical neural network (ONN) in cases where low power and non-volatile operation are required. Among other uses, the system may be used to perform pre-configured matrix operations, such as Fourier transforms or convolution, on an N-dimensional input vector for signal processing. As part of a neural network, the multi-waveguide non-volatile switches may be trained to set the coefficients (e.g. continuous splitting ratio) for each switch to enable deep learning protocols. This has applications in image processing, signal detection, and machine vision.

[0025] In some examples, the number of optical input ports may be different to the number of optical output ports. For example, the system may have a number N of input ports and a number M of optical output ports.

[0026] The system may further comprise a number of controllable polarization elements for controlling the polarization of optical signals provided to the number of optical input ports of the array.

[0027] Many modifications and other embodiments set out herein will come to mind to a person skilled in the art in light of the teachings presented herein. Therefore, it will be understood that the disclosure herein is not to be limited to the specific embodiments disclosed herein. Moreover, although the description provided herein provides example embodiments in the context of certain example combinations of elements, steps and / or functions, it will be appreciated that different combinations of elements, steps and / or functions may be provided by alternative embodiments without departing from the spirit or scope of the disclosure. Brief Description of the Figures

[0028] Illustrative embodiments of the present disclosure will now be described by way of example only, with reference to the accompanying figures.

[0029] Fig. 1 shows a top-down illustration of a 3-arm optical switch device according to an example.

[0030] Fig. 2 illustrates an isometric view of the part of a waveguide structure of a 3-arm optical switch device according to an example.

[0031] Fig. 3a illustrates the electric field profile for an optical switch device comprising the waveguide structure of Fig- 2, more particularly the magnitude of the electric field when an optical signal is provided to a first optical input and when the optical switch device is in a first charge state.

[0032] Fig. 3b illustrates the electric field profile for an optical switch device comprising the waveguide structure of Fig- 2, more particularly the magnitude of the electric field when an optical signal is provided to a first optical input and when the optical switch device is in a second charge state.

[0033] Fig. 3c illustrates the real part of the three quasi transverse electric (TE) modal cross sections that contribute to optical power transfer in the optical switch device, when the optical switch device is in the first charge state.

[0034] Fig. 3d illustrates the real part of the three quasi transverse electric (TE) modal cross sections that contribute to optical power transfer in the optical switch device, when the optical switch device is in the second charge state.

[0035] Fig. 4 shows a top-down illustration of a 4-arm optical switch device according to an example.

[0036] Fig. 5 illustrates an isometric view of the part of a waveguide structure of a 4-arm optical switch device according to an example.

[0037] Fig. 6a illustrates the electric field profile for an optical switch device comprising the waveguide structure of Fig. 5, more particularly the magnitude of the electric field when an optical signal is provided to a first optical input and when the optical switch device is in a first charge state.

[0038] Fig. 6b illustrates the electric field profile for an optical switch device comprising the waveguide structure of Fig. 5, more particularly the magnitude of the electric field when an optical signal is provided to a first optical input and when the optical switch device is in a second charge state.

[0039] Fig. 6c illustrates the real part of the three quasi transverse electric (TE) modal cross sections that contribute to optical power transfer in the optical switch device, when the optical switch device is in the first charge state.

[0040] Fig. 6d illustrates the real part of the three quasi transverse electric (TE) modal cross sections that contribute to optical power transfer in the optical switch device, when the optical switch device is in the second charge state.

[0041] Fig. 7 illustrates a system according to an example.

[0042] Throughout the description and the drawings, like reference numerals refer to like parts. Additionally, it should be understood that the proportions and dimensions (either relative or absolute) of the various features and elements (and collections and groupings thereof) and the boundaries, separations, and positional relationships presented therebetween, are provided in the accompanying figures merely to facilitate an understanding of the various examples described herein and, accordingly, may not necessarily be presented or illustrated to scale, and are not intended to indicate any preference or requirement for an illustrated example to the exclusion of examples described with reference thereto. Detailed Description

[0043] Embodiments of the disclosure are described with reference to the accompanying drawings. However, it should be appreciated that the disclosure is not limited to the embodiments, and all changes and / or equivalents or replacements thereto also belong to the scope of the disclosure. The same or similar reference denotations may be used to refer to the same or similar elements throughout the specification and the drawings.

[0044] As used herein, the terms “have”, “may have”, “include”, or “may include” a feature (e.g. a number, function, operation, or a component such as a part) indicate the existence of the feature and do not exclude the existence of other features. Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0045] As used herein, the terms “A or B”, “at least one of A and / or B”, or “one or more of A and / or B” may include all possible combinations of A and B. For example, “A or B”, “at least one of A or B”, “at least one of A and B” may indicate all of (1) including at least one A, (2) including at least one B, or (3) including at least one A and at least one B.

[0046] As used herein, the terms “first” and “second” may modify various components regardless of importance and do not limit the components. These terms are only used to distinguish one component from another. For example, reference to a first component and a second component may indicate different components from each other regardless of the order or importance of the components.

[0047] It will be understood that when an element (e.g. a first element) is referred to as being (physically, operatively or communicatively) “coupled with / to”, or “connected with / to” another element (e.g. a second element), it can be coupled with / to the other element directly or via a third element. In contrast, it will be understood that when an element (e.g. a first element) is referred to as being “directly coupled with / to” or “directly connected with / to” another element (e.g. a second element), no element (e.g. a third element) intervenes between the element and the other element.

[0048] The terms as used herein are provided merely to describe some embodiments thereof, but not to limit the scope of other embodiments of the disclosure. It is to be understood that the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. All terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments of the disclosure belong. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealised or overly formal sense unless expressly so defined herein.

[0049] Embodiments described herein relate to photonic circuits and structures having optical properties that are stable, and non-volatile, in two or more states. Such structures can be leveraged by a photonic circuit for nonvolatile field configuration or non-volatile storage, such as in one or more optical memory cells, one or more arrays of memory cells, one or more configurable logic blocks of a field-programmable gate array, one or more bistable switch elements, and so on. More generally, the skilled person may readily appreciate that an optical switch element that exhibits an electrically-programmable, non-volatile, optical property can be used in countless ways in both photonic and electronic circuits.

[0050] Furthermore, because such properties are non-volatile, photonic circuits that incorporate elements as described herein can be operated at significantly reduced power compared to electrically-switched photonic circuits which, as known to a person skilled in the art, already exhibit significantly reduced power as compared to semiconductor circuits. More specifically, a photonic circuit utilising optical switch devices as described herein can be operated, and may perform one or more functions, while consuming only a fraction of the electrical power required to generate light that passes through the photonic circuit. In other words, examples described herein can be leveraged to create passive optical circuits that are electrically programmable and that, once programmed, do not require electrical power to maintain state or function.

[0051] Fig. 1 shows a diagram of an optical switch device according to an example. The optical switch device may be manufactured using any suitable process, for example using a CMOS process and CMOS-compatible materials.

[0052] The optical switch device has a waveguide structure comprising three optical waveguides, and accordingly is sometimes referred to herein as a “3-arm switch”. In particular, the waveguide structure comprises a first primary optical waveguide 110-1, a second primary optical waveguide 110-2, and a chargeable optical waveguide 140. The first primary optical waveguide 110-1 directly connects a first optical input 120-1 to a first optical output 130-1. The second primary optical waveguide 110-2 directly connects a second optical input 120-2 to a second optical output 130-2. Light may be provided to one or more optical inputs and propagate through the device to one or more optical outputs.

[0053] The terms “optical input” and “optical output” as used are to be interpreted broadly as an optical input to or optical output from a coupling region of a waveguide structure of an optical switch device. For example, an optical input may in some examples be an abrupt interface between two media (for example, a fibre-to-chip interface connecting an optical fibre at an edge facet to a waveguide of an integrated photonic circuit) or may be for example a location on a primary optical waveguide through or past which an optical signal propagates prior to entering the coupling region.

[0054] Moreover, while the primary waveguides 110-1, 110-2 directly connect optical inputs 120-1, 120-2 with optical outputs 130-1, 130-2, in other examples the optical connection may be indirect. For example, additional components (e.g. a polarization rotator) may be positioned in between an optical input and optical output.

[0055] Within a coupling region 180 of the optical switch device, the first primary optical waveguide 110-1 and second primary optical waveguide 110-2 are adiabatically curved inwards towards the chargeable optical waveguide 140, extend in the same direction as and substantially parallel to the chargeable optical waveguide 140, and adiabatically curve away from the chargeable optical waveguide 140. Within the coupling region 180, the waveguides 110-1, 110-2, 140 are close enough together that an optical signal may evanescently couple from a primary optical waveguide to a chargeable optical waveguide (or vice versa).

[0056] The chargeable optical waveguide 140 is situated between the first primary optical waveguide 110-1 and the second primary optical waveguide 110-2 and is electrically isolated from the first and second primary optical waveguides. A plurality of metallic electrodes 150, 160, 170 (extending vertically out of the diagram) are situated close to the chargeable optical waveguide 140, separated from the chargeable optical waveguide 140 by some small thickness of dielectric material (not shown in the figure). For example, the thickness of dielectric material between an electrode and a waveguide may be less than 20nm, or less than 15nm, or less than lOnm.

[0057] The plurality of electrodes comprises a first electrode 150, a second electrode 160, and a third electrode 170. The first, second, and third electrodes may herein be referred to as the source electrode or source gate 150, control electrode or control gate 160 and bias electrode or bias gate 170, although such labels are used only to distinguish between the electrodes. The skilled person will appreciate that the optical switch device may comprise further electrodes. While a 3-arm switch may be made with two electrodes, three or more electrodes provides greater control over the charge state of the waveguide 140.

[0058] The electrodes are configured to control a charge state of the chargeable optical waveguide 140. In particular, the electrodes are configured to controllably inject charge carriers (e.g. electrons or holes) into the chargeable optical waveguide 140. For example, by introducing a positive bias between the source electrode 150 (which may, for example, be grounded) and the control electrode 160, electrons may tunnel through the dielectric material separating the source electrode 150 from the chargeable optical waveguide 140 into the chargeable optical waveguide. A bias may be applied using the bias electrode 170 to pull electrons into the waveguide 140 within the coupling region 180. With suitable use of the electrodes, the electrons may be distributed in the chargeable optical waveguide 140. For example, the biases applied to the control gate and bias gate may be relaxed concurrently to enable the electrons injected into the waveguide 140 to distribute more uniformly. Changing the charge state of the chargeable optical waveguide 140 affects the optical properties of the waveguide 140.

[0059] Operation of the optical switch device can be understood in terms of the modes of the waveguide structure, and in particular the “supermodes” that span over multiple waveguides. The charge state of the chargeable optical waveguide 140 characterises the behaviours of the supermodes in the device. For example, when the chargeable optical waveguide 140 is uncharged, an optical signal received at first optical input 120-1 may be coupled to second optical output 130-2; when the chargeable optical waveguide 140 is charged, an optical signal received at first optical input 120-1 may be coupled to first optical output 130-1. This behaviour will be described in further detail below in relation to Fig-2 and Fig- 3

[0060] Fig. 2 and Fig. 3 describe the behaviour of a specific example of an optical switch device such as that of Fig. 1 Fig. 2 shows an isometric view of a portion of the coupling region of a waveguide structure of an optical switch device. In particular, Fig. 2, shows a first primary optical waveguide 210-1 connecting a first optical input (labelled “1” in the figure) to a first optical output (labelled “3” in the figure), a second primary optical waveguide 210-2 connecting a second optical input (labelled “2” in the figure) to a second optical output (labelled “4” in the figure), and a chargeable optical waveguide 220 situated therebetween. The three waveguides are provided over a substrate. Electrodes and cladding material are not shown in Fig. 2.

[0061] In this example, each waveguide has approximately a rectangular cross-section and is 300nm wide and 220nm high and suitable for carrying an optical signal having a free space wavelength of 1550nm. Each waveguide is formed from silicon and embedded in cladding material (silicon dioxide, not shown in the figure). The outer waveguides 210-1, 210-2 are spaced symmetrically on either side of the chargeable optical waveguide 220, separated from the chargeable optical waveguide 220 by a distance of 1 lOOnm. The length of the depicted portion of the waveguide structure is approximately 2.5mm long. The electrodes are separated from the middle waveguide 220 by a thin layer of cladding material having a thickness of approximately lOnm. The skilled person will appreciate that the dimensions used in the example of Fig. 2 are for illustrative purposes only, and that the dimensions may be varied.

[0062] Figs. 3a-3d show field profiles for the specific example of an optical switch device as described in relation to Fig. 2. Fig. 3a illustrates a horizontal cross-section (i.e. in the plane of the silicon waveguides) of the magnitude of the y-component (up) of the electric field when the middle waveguide 220 is uncharged and when the optical signal is provided to optical input “1”. As can be seen in the figure, the optical signal is coupled to optical output “4”. Fig. 3b similarly illustrates a cross-section in the plane of the waveguides of the magnitude of the y-component of the electric field, but with the middle waveguide 220 charged uniformly up to 1019 cm-3. When the middle waveguide 220 is in a charged state, the optical signal received at optical input “1” is coupled to optical output “3”.

[0063] Referring to Fig. 3c, when the optical signal is applied to optical input “1” and the middle waveguide 220 is uncharged, the real part of the three quasi transverse electric (TE) modal cross sections determines the distribution of power between these three supermodes and their interference (as they propagate over the device length) causes the optical power to couple to optical output “4”. Referring to Fig. 3d, when the optical signal is applied to optical input “1” and the middle waveguide 220 is charged, most of the optical power from optical input “1” passes to optical output “3”. When the middle waveguide 220 is charged, mode TE31 separates spatially from modes TE 11 and TE21, practically eliminating coupling into that mode and causing the coupling from optical input “1” to optical output “4” to be a function of the total waveguide spacing between the outer waveguides which is approximately twice the distance of the spacing between adjacent waveguides. This greatly increases the device length that would be required to transfer power between optical input “ 1 ” and optical output “4” using only modes TEI 1 and TE21 and so most of the power remains in the top waveguide and is output on optical output “3”.

[0064] There is one supermode for each waveguide: three in total in the present example, and these are depicted in Figs. 3c and 3d. Each of these supermodes has a different propagation speed and spatial pattern in the waveguide cross-section. Two of the supermodes are symmetric (TEI 1 and TE31) and one is asymmetric (TE21) in the y-axis direction. The input power on “1” will in general excite all 3 supermodes (each with a different amplitude and phase and conserving total power over all the modes) in the coupling region of the waveguide structure. The length of the region in which coupling occurs (coupling length) is set to ensure that the interference pattern of the supermodes, which determines where the total optical power is located in the structure, is switched to “4” (from “1”) when the switch is uncharged. This is accomplished in the present example by setting the waveguide widths to be equal to each other (for the case shown) and the spacing between the waveguides 210 to be symmetrical on either side of the middle, chargeable waveguide 220. This condition maximizes the output power at output “4” for the uncharged case and ensures that the propagation constant of TE21 may be the average of that of TE 11 and TE31. The coupling length Lc may for example be equal to the following: 2Re[ne / / (TEll) - neff(TE21)]

[0065] Here, Ao is the free space wavelength, “Re[x]” denotes the real part of x, and ne^ denotes the effective propagation index of the respective supermode. For more general cases (such as where the middle waveguide has a different width than the outer waveguides or the middle waveguide is made from a different material), a simple formula may not exist and optimization in simulations may be performed to improve the device performance (for example by increasing output power at optical outputs “3” and “4” for uncharged and charged states, respectively).

[0066] In the charged state, the optical switch device causes one of the supermodes (TE31) to be preferentially spatially mismatched from a mode on the input waveguides (i.e. inputs “1” and “2”) such that it is only weakly excited. Additionally, this weakly-coupled mode is also highly attenuated due to the increased absorption in the charged waveguide. Accordingly, this mode may not participate in the mode interference pattern and power is transmitted from optical input “1” to optical output “3”. The output interference pattern is thus only a function of the superposition of modes TEI 1 and TE21. This switch state may be referred to as the “THROUGH” or “BAR” state.

[0067] By varying the amount of charge in the middle waveguide 220 the power splitting ratio on outputs “3” and “4” may be varied continuously.

[0068] Once charged, the charge state of the waveguide 140, 220 may be maintained. In particular, as the waveguide 140, 220 is electrically isolated from the electrodes and from the primary optical waveguide 110, 210, the leakage rate of charge out of the waveguide (for example, via tunnelling through to an electrode) is low compared to the operational time of the switch device (in other words, the time for the switch device to remain in the THROUGH switching state as part of its normal operation) and accordingly, the switch may hold its state statically without requiring further electrical power.

[0069] Fig. 4 shows a diagram of an optical switch device according to another example. The optical switch device may be manufactured using any suitable process, for example using a CMOS process and CMOS-compatible materials.

[0070] The optical switch device has a waveguide structure comprising four optical waveguides and accordingly is sometimes referred to herein as a “4-arm switch”. In particular, the waveguide structure comprises a first primary optical waveguide 410-1, a second primary optical waveguide 410-2, a first chargeable optical waveguide 440-1 and a second chargeable optical waveguide 440-2. The first primary optical waveguide 410-1 directly connects a first optical input 420-1 to a first optical output 430-1. The second primary optical waveguide 410-2 directly connects a second optical input 420-2 to a second optical output 430-2. Light may be provided to one or more optical inputs and propagate through the device to one or more optical outputs. The charge state of the two chargeable optical waveguides 440-1, 440-2 is controllable using a set of electrodes 450-1, 450-2, 460-1, 460-2, 470-1,470-2.

[0071] The 4-arm switch operates in a similar manner to the 3-arm switch (100) described elsewhere herein. The charge state of the set of chargeable waveguides 440-1, 440-2 affects the supermode structure of the 4-waveguide system. However, in contrast to the optical switch device 100 described elsewhere herein, here the chargeable optical waveguides 440-1,440-2 are situated on either side of the coupled, primary optical waveguides 410-1, 410-2. By providing the chargeable optical waveguides outside of the primary optical waveguides, the primary optical waveguides can be positioned closer together than in the 3-arm switch, which leads to a stronger coupling between the primary optical waveguides. This in turn means that the coupling length (that is, the length over which the waveguides in the coupling region 480 are close enough for evanescent coupling of optical modes) may be smaller than for the 3-arm switch. For example, the length of the 4-arm switch may be approximately one third of the length of the 3-arm switch.

[0072] Fig. 5 shows an isometric view of a portion of the coupling region 480 of a waveguide structure of an optical switch device such as that in Fig. 4. In particular, Fig. 5, shows a first primary optical waveguide 510-1 connecting a first optical input (labelled “1”) to a first optical output (labelled “3”), a second primary optical waveguide 510-2 connecting a second optical input (labelled “2”) to a second optical output (labelled “4”), and first and second chargeable optical waveguides 520-1, 520-2 situated outside of the primary optical waveguides. The four waveguides are provided over a substrate. Electrodes and cladding material are not shown in Fig. 5.

[0073] As with the device of Fig- 2, in this example, each waveguide is 300nm wide and 220nm tall and suitable for carrying an optical signal having a free space wavelength of 1550nm. Each waveguide is formed from silicon and embedded in cladding material (silicon dioxide, not shown in the figure). The waveguides are again equally spaced at a distance of approximately HOOnm from one another. The electrodes are separated from the middle waveguide 220 by a thin layer of cladding material having a thickness of approximately lOnm.

[0074] Figs. 6a-6d shows field profiles for the specific example of an optical switch device described in relation to Fig. 5. Fig. 6a illustrates a horizontal cross-section (in the plane of the silicon waveguides) of the magnitude of the y-component (up) of the electric field when the chargeable optical waveguides 520-1, 520-2 are uncharged and when the optical signal is provided to optical input “1”. As can be seen in the figure, the optical signal is coupled to optical output “4”. Fig. 6b similarly illustrates a cross-section in the plane of the waveguides of the magnitude of the y-component of the electric field, but with the chargeable optical waveguide 520-1, 520-2 charged uniformly up to 1019 cm-3. When the chargeable optical waveguides are in a charged state, the optical signal received at optical input “1” is coupled to optical output “3”.

[0075] Figs. 6c and 6d respectively show the cross-sectional field profiles for the uncharged and charged states of the switch when the optical signal is provided to optical input “1”. In particular, Fig. 6c shows the real part of the four supermodes in the uncharged state with two horizontally symmetric (TE11 and TE31) and two horizontally asymmetric (TE21 and TE41). For the uncharged state input on “1” will excite all four supermodes because each will span all four waveguides to some degree, analogous to the 3 -arm waveguide in the uncharged state. In the case where all four waveguides have substantially identical thicknesses and heights, are fabricated on the same device layer, and all three spacings between adjacent waveguides are the same over the coupling length, adjacent modes (in the sense of the modes being ordered by the magnitude of their propagation constant) accrue a phase difference of it radians over the coupling length. This is analogous to the 3-arm switch. For the 4-arm switch of this example, however, not only is the propagation constant of TE21 the average of the propagation constant of TEI 1 and TE31, but TE31’s propagation constant is the average of TE21 and TE41. Thus, in the uncharged state power is transferred from optical input “1” to optical output “4” over a device length equal to Lc.When the chargeable optical waveguides are uncharged, and optical signal received at “1” is coupled to “4”.

[0076] Fig. 6d shows the real part of the four supermodes with the outer waveguides 520-1, 520-2 charged uniformly to 1019 cm-3 with electrons with two of the supermodes, TE31 and TE41 spatially separated from the two optical path waveguides (i.e. primary optical waveguides) 510-1, 510-2 in the center. The modes TE31 and TE41 are only weakly excited by input power on “ 1 ” and accordingly only negligibly contribute to the coupling of the optical signal from the input ports to the output ports. The coupling in the charged case is primarily determined by the two supermodes of the central waveguides 510-1, 510-2, TEI 1 and TE21, and accordingly an optical signal received at optical input “1” is coupled to optical output “3” as shown in Fig. 6b.

[0077] The skilled person will appreciate that the optical switch devices described above in relation to Figs. 1 to 6d may be varied in any of a number of ways.

[0078] For example, the switch devices may comprise greater numbers of primary optical waveguides or chargeable optical waveguides. The optical switch devices are not restricted to having only two inputs and two outputs only, instead switches with more than two inputs or two outputs may be made. Generally speaking, the number of supermodes affected by the waveguide structure may be equal to the total number of primary optical waveguides and chargeable optical waveguides in the coupling region of the switch device, and the shapes, sizes and positions of the waveguides may be adapted to obtain the desired interference patterns.

[0079] Furthermore, the waveguides may be formed from any suitable waveguide material. For example, the primary optical waveguides may be formed of silicon or may be formed of silicon nitride. The one or more chargeable optical waveguides may be formed from the same material as the primary optical waveguides or from a different material. For example, the primary optical waveguides may be formed from silicon nitride, and the chargeable optical waveguide(s) may be formed from silicon. In this case, the waveguide and coupler geometries may be adapted to enable continuously tuneable switching behaviour between the optical outputs as a function of the amount of injected charge.

[0080] The waveguides may be formed in the same or in different device layers (e.g. different distances above the substrate). Furthermore, while rectangular waveguides were described in relation to the optical switch devices herein, the skilled person would appreciate that other waveguide shapes may be used depending on the desired function of the switch. For example, a different cross-sectional geometry may enable mode hybridization between TE and TM polarizations, for example an L-shaped waveguide (e.g. a rectangular waveguide partially etched in one quadrant).

[0081] While in the examples described above, three electrodes are used to control the amount of charge in an individual chargeable optical electrode, the skilled person may appreciate that different numbers of electrodes may be used. Furthermore, the placement of the electrodes on the chargeable optical waveguide may not be in the same locations as shown in e.g. Fig- 1 and Fig- 4 For example, the bias electrode 170, 470-1, 470-2 may be positioned anywhere about the charging waveguide 140, 440-1, 440-2 to obtain a desired injected charge distribution for best performance.

[0082] Fig. 7 shows a simplified system diagram of a system 700 having an optical switch array according to an example. The system 700 may be operated, for example, in connection with an optical field programmable gate array or a non-volatile optical memory array. The system 700 comprises a number of optical input ports (710-1 to 710-8) and a number of optical output ports (720-1 to 720-8). Arranged therebetween is an array of individually addressable cells 730. Each cell 730 comprises at least one optical switch device substantially as described herein. In other words, each optical switch device may comprise a waveguide structure (comprising two optical inputs and two optical outputs, two primary optical waveguides, one or more chargeable optical waveguides) and a set of electrodes for controlling a charge state of the set of one or more chargeable optical waveguides. For example, the optical switches may be 3-arm switches or 4-arm switched as described elsewhere herein.

[0083] Each cell 730 is optically coupled to at least one other cell 730. In the example shown in the figure, each cell 730 is configured to controllably operate on two optical inputs. For example, one cell 730 is arranged to receive optical signals from input port 710-1 and / or 710-2, a second cell is arranged to receive optical signals from input port 710-3 and / or 710-4, a third cell is arranged to receive any optical signal output from each of the first and the second cells, and so on.

[0084] The system 700 further comprises control circuitry in the form of controller 740, coupled to the cells. For example, the controller 740 may be electrically coupled to the electrodes of the one or more optical switch devices of each cell 730. The controller is configured to control a charge state of the set of one or more chargeable optical waveguides of each optical switch device. In this way, the controller 740 may control the flow of light between the optical input ports 710-1 to 710-8 and the optical output ports 720-1 to 720-8. The controller 740 may be any kind of dedicated or general processor, such as a central processing unit (CPU), a graphics processing unit (GPU), or an integrated (electronic) circuit). In some examples, the controller 740 may comprise an application-specific integrated circuit (ASIC), an application-specific standard product (ASSP) or another domain-specific architecture (DSA). Alternatively, the controller 740 may be implemented in adaptive computing hardware (that is, hardware comprising configurable hardware blocks / configurable (electronic) logic blocks) that has been configured to perform the required functions, for example in a configured field programmable gate array (FPGA).

[0085] The skilled person will appreciate that the array of Fig. 7 may be varied in many different ways. For example, each cell may comprise an optical switch device that has more than two inputs and / or more than two outputs, and each cell may have more than one optical switch device. Furthermore, in some examples a cell 740 may operate on more than two optical inputs. Moreover, the optical configuration of cells in the array may be adapted to provide different optical functions. For example, the number of input ports may be greater than, less than, or equal to the number of output ports.

[0086] A system as described herein may comprise further optical components. For example, the system 700 may comprise one or more lasers, or one or more photodetectors. The system may comprise active optical elements such as photodiodes, amplifiers, piezo-electrically actuated elements, heaters, electro-optic devices that use Pockel’s effect (i.e. the linear electro-optic effect), electro-optic devices that use the DC Kerr effect (i.e. the quadratic electro-optic effect), ferroelectric materials, electro-absorptive materials or structures that manipulate charge distributions near optical modes such as PIN diodes that utilize the plasma dispersion effect to change the propagation characteristics of optical modes, structures that utilized acousto-optic effects such as stimulated Brillouin scattering and stimulated Raman scattering.

[0087] The arrays of cells, or individual optical switch devices, may utilize further passive optical structures such as splitters, combiners, isolators, circulators, frequency comb generators, filters (e.g. ring resonators), photonic crystals, or self-phase modulation structures.

[0088] The systems described herein may comprise a number of controllable polarization elements, for controlling the polarization of optical signals provided to the number of optical input ports of the system. For example, the switch networks may utilise metallic structures to absorb, rotate or otherwise manipulate one or more polarizations of light prior to entering the waveguides. As described elsewhere herein, the polarization states that are coupled between optical inputs and optical outputs of an optical switching device may be polarization dependent, dependent at least in part on the shapes and sizes of the waveguides of the optical switching device. A polarization element may accordingly be used as a filter to filter unwanted signals prior to entering the optical input ports or may otherwise influence the operation of the optical switching device on the optical signals.

[0089] The systems described herein may be applicable to several different applications.

[0090] In some examples, the system may be used as part of a reconfigurable filter, for instance, different filter elements (Bragg gratings, ring resonators, loop filters, etc.) may be positioned between layers of the switch array and the switch settings may select a particular filter function for each path through the network. This may have applications in signal processing and spectrometry. Further, as a reconfigurable signal routing network, a switch array may be utilized to perform multiplex / demultiplex operations (mux-demux) on a vector of input optical signals.

[0091] In some examples, the switch array may be utilized to control the amount of time delay for optical truetime delay switch networks. These can be used in beamforming protocols for phased arrays and can also be used to implement finite impulse response (FIR) microwave optical filters.

[0092] CMOS-compatible non-volatile optical switch networks such as those described herein may be used in a wide array of technologies for low SWaP (Size, Weight, and Power) applications. These include (1) Dynamic signal routing, (2) Reconfigurable filters, (3) Tunable true-time delays (used for e.g. beam steering or microwave FIR filters), (4) Photonic tensor cores (used to e.g. perform matrix operations), (5) Optical neural networks.

[0093] Each feature disclosed in this specification (including any accompanying claims, abstract or drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.

Claims

1. An optical switch device comprising:a waveguide structure comprising:a plurality of optical inputs and a plurality of optical outputs;a plurality of primary optical waveguides, each primary optical waveguide connecting a respective optical input and a respective optical output; anda set of one or more chargeable optical waveguides electrically isolated from the plurality of primary optical waveguides; anda plurality of electrodes configured to control a charge state of the set of one or more chargeable optical waveguides;wherein, when the set of one or more chargeable optical waveguides is in a first charge state, the waveguide structure is configured to optically couple an optical signal received at a first optical input to a first optical output; andwherein, when the set of one or more chargeable optical waveguides is in a second charge state, the waveguide structure is configured to optically couple an optical signal received at the first optical input to a second optical output.

2. An optical switch device according to claim 1, wherein the one or more chargeable optical waveguides are formed of a different material to the primary optical waveguides.

3. An optical switch device according to any preceding claim, wherein the one or more chargeable optical waveguides are in a different physical device layer to the plurality of primary optical waveguides.

4. An optical switch device according to any preceding claim, wherein the plurality of primary optical waveguides comprise silicon nitride.

5. An optical switch device according to any preceding claim, wherein the set of one or more chargeable optical waveguides comprise silicon.

6. An optical switch device according to any preceding claim, wherein the plurality of primary optical waveguides consists of two primary optical waveguides.

7. An optical switch device according to any of claims 1 to 6, wherein the set of chargeable optical waveguides comprises more than one chargeable optical waveguide.

8. An optical switch device according to claim 7, wherein the plurality of primary optical waveguides is situated between the set of chargeable optical waveguides.

9. An optical switch device according to any of claims 1 to 6, wherein the set of chargeable optical waveguides consists of one chargeable optical waveguide.

10. An optical switch device according to claim 9, wherein the one chargeable optical waveguide is situated between the two primary optical waveguides.

11. An optical switch device according to any preceding claim, wherein, when the set of one or more chargeable optical waveguides is in a third charge state, the waveguide structure is configured to optically couple an optical signal received at the first optical input to a third optical output.

12. An optical switch device according to any preceding claim, wherein, when the set of one or more chargeable optical waveguides is in a third charge state, the waveguide structure is configured to optically couple an optical signal received at the first optical input to both the first optical output and the second optical output.

11. A system comprising:a number of optical input ports and a number of optical output ports;an array of individually addressable cells arranged between the optical input ports and optical output ports, each cell optically coupled to at least one other cell, each cell comprising an optical switch device, wherein each optical switch device comprises:a waveguide structure comprising:a plurality of optical inputs and a plurality of optical outputs;a plurality of primary optical waveguides, each primary optical waveguide connecting a respective optical input and a respective optical output; anda set of one or more chargeable optical waveguides electrically isolated from the plurality of primary optical waveguides; anda plurality of electrodes configured to control a charge state of the set of one or more chargeable optical waveguides; andcontrol circuitry configured to control a charge state of the set of one or more chargeable optical waveguides of each optical switch device.

12. A system according to claim 11, wherein the number of optical inputs is different to the number of optical outputs.

13. A system according to claim 11 or claim 12, further comprising a number of controllable polarization elements, for controlling the polarization of optical signals provided to the number of optical input ports of the system.

Images