Optical modulation

Abstract

An optical modulator 10 for a photonic circuit includes one or more optical waveguides 13. The waveguide(s) are for guiding multiple optical signals Lin1,Lin2 through a modulation region 14 of the modulator. In response to receiving a control input Si, Oi, Xi, the optical modulator simultaneously and non-independently modulates an optical property of the multiple optical signals propagating through the modulating region. The optical waveguide(s) may include where each optical waveguide guides one or more of the optical signals. One or more of the optical waveguides may be a single-mode / multi-mode optical waveguide guiding one / two or more (in two or more respective modes) of the multiple optical signals. An optical system for a photonic circuit may include the optical modulator where the one or more waveguides include first and second waveguides and first and second optical devices that are at least partially integrated by their sharing of the optical modulator. An optical routing network or an optical sensor may include the optical modulator or the optical system. A photonic integrated circuit or a quantum computer may include the optical modulator, the optical system or the optical routing network and the quantum computer may include the photonic integrated circuit.

Description

Technical Field The invention relates generally to an optical modulator for a photonic integrated circuit, a photonic integrated circuit and sensor including the optical modulator, and a method for modulating multiple optical signals in a photonic integrated circuit. Background to the invention Photonic integrated circuits (PICs) have several advantages over their electronic integrated circuit counterparts, including but not limited to high speed and low power consumption, that offer promising potential for current and future large-scale compute and information transfer applications. In PICs, light can be controllably routed through a network of optical waveguides by various active and passive optical / photonic components including optical modulators, couplers, switches and power splitting devices. Active optical components, such as optical modulators, are driven by electrical control signals provided by control electronics, allowing PICs to be programmable and reconfigurable under software / computer control. In current state-of-the art programmable PICs, active optical components are individually addressable and electrical control signals are provided to each active optical component via external electrical connections made to on-chip bond pads and traces that electrically connect to the active optical component. However, in practice, there is a limit to the number of optical components that can fit on a given sized PIC chip due to space requirements, and there is also a limit to the number of bond pads that can fit on a given sized PIC chip due to space and chip packaging requirements (e.g. bond pads should typically be distributed at the periphery of the PIC chip and not staggered) as well as avoiding crosstalk. This places a limitation on the number of active optical components that can be provided on a given PIC chip and represents a bottleneck to developing large-scale reconfigurable PICs. Existing approaches to address this problem include simply avoiding scaling up the number of optical components which limits the functionality of the PIC its use to relatively small scale applications, or to provide additional bond pads on the back side of the PIC chip that connect to the active optical components using vias which increases the manufacturing cost, time and complexity, and in practice also limits the number of foundries and back-end-of-line (BEOL) companies that can produce the PIC chip. There is therefore a need for improved and simplified approaches for increasing the density of optical components and managing electrical connections for large-scale compute and information transfer applications. Aspects and embodiments of the present invention been devised with the foregoing in mind. Summary of the invention According to a first aspect of the invention, there is provided an optical modulator or modulation arrangement for a photonic circuit. The optical modulator comprises at least one optical waveguide for guiding multiple optical signals through a modulation region of the modulator. The optical modulator is configured to simultaneously and non-independently modulate an optical property of the multiple optical signals propagating through the modulating region in response to receiving a control input. By modulating multiple optical signals simultaneously and non-independently using a single optical modulator and a single (common) control input, the number of control inputs (and in various cases the associated electrical connections) provided to the PIC can be reduced. In addition, multiple active optical components that require modulation can be integrated by sharing the same optical modulator. The invention can therefore, at the expense of independent control of the multiple optical signals and optical components that require modulation, reduce the PIC chip footprint, provide for higher efficiency of modulation, reduce the number of electrical connections, and lower chip manufacture and packaging costs, as explained in more detail below. The or each of at least one optical waveguide may be referred to as a modulation waveguide. The modulated optical property of the multiple optical signals may be one or more of: a phase, an amplitude, a wavelength, a polarisation, and a quantum state. The optical modulator is preferably configured to modulate, simultaneously and non-independently, the effective refractive index n©ff of at least a portion of the at least one optical waveguide in the modulation region in response to the control input. The control input can be an electrical signal or an optical signal provided to the modulation region, or a change in external environment at the modulation region, either of which can cause a change of the effective refractive index neff of the at least one optical waveguide in the modulation region. The at least one optical waveguide may be substantially straight and / or curved in at least a portion of the modulation region. The at least one optical waveguide may include a plurality of optical waveguides, where each of the plurality of optical waveguides is configured to guide at least one of the multiple optical signals. At least one of the plurality of optical waveguides may be a single-mode optical waveguide configured to guide one of the multiple optical signals. Additionally or alternatively, at least one of the plurality of optical waveguides may be a multi-mode optical waveguide configured to guide at least two of the multiple optical signals in at least two respective optical modes of the multi-mode waveguide. The plurality of optical waveguides are preferably spaced apart. At least some of the plurality of optical waveguides can be spaced apart by a lateral gap within the modulation region such that their respective optical modes are substantially uncoupled in the modulation region. Alternatively or additionally, at least some of the plurality of optical waveguides can be spaced apart by a lateral gap, over at least a portion of the modulation region, such that their respective optical modes are evanescently coupled in the at least a portion of the modulation region, and optionally such that their optical modes are superposed and support a supermode (a mode which extends between the neighbouring optical waveguides). Preferably, the plurality of optical waveguides are arranged substantially parallel over at least a portion of the modulation region. The plurality of parallel optical waveguides may be substantially straight or curved in at least a portion of the modulation region. Each of the plurality of optical waveguides may be arranged, in the at least a portion of the modulation region, in a spiral configuration or a racetrack configuration. In the case of a spiral, each of the plurality of optical waveguides may have an input portion and an output portion, whereby the input and output portions are arranged, in the at least a portion of the modulation region, as interleaved spirals that are connected in the spiral centre. Each of the plurality of optical waveguides is preferably arranged to be substantially parallel throughout the spiral. The input portion may be configured to guide an optical signal from a first end in an inwards spiral through the modulation region towards to a second end at the centre of the spiral (or modulation region), and the output portion may be configured to guide the optical signal from a first end (connected to the second end of the input portion) in an outwards spiral through the modulation region towards to a second end. The at least one optical waveguide may include a single multi-mode optical waveguide configured to guide at least two of the multiple optical signals in at least two respective optical modes of the multi-mode waveguide. The multi-mode optical waveguide may be arranged, in the at least a portion of the modulation region, in a spiral configuration or a racetrack configuration. In the case of a spiral, the multi-mode optical waveguide may have an input portion and an output portion as described above with reference to the plurality of waveguides. The optical modulator preferably comprises one or more optical inputs for receiving the multiple optical signals and one or more optical outputs for outputting the modulated optical signals. The one at least one optical waveguide is arranged in an optical path between the optical inputs and outputs. The optical modulator may comprise multiple optical inputs for receiving the multiple optical signals, and multiple optical outputs for outputting the multiple optical signals having been guided through the modulation region. The optical inputs and outputs are preferably optical waveguide portions that connect to the at least one optical waveguide and can themselves be single mode or multi-mode. Preferably, single mode inputs are connected to single mode optical waveguides, and multi-mode inputs are connected to multi-mode optical waveguides. Alternatively, multiple single mode inputs and outputs can be used to couple light into and out of a given multimode optical waveguide. The optical modulator may comprise one or more input couplers connected to the optical inputs and / or one or output couplers connected to the optical outputs for coupling light into and / or out of the at least one optical waveguide. Where the optical modulator includes a single multi-mode optical waveguide, the optical modulator can include a single multi-mode optical input or multiple single mode optical inputs for coupling light into the multi-mode optical waveguide, and / or multiple single mode optical outputs for coupling modulated light out of the multi-mode optical waveguide. Wherein the control input is an electrical signal, the optical modulator may be or comprise one of a thermo-optic modulator, an electro-optic modulator, and a magneto-optic modulator. The optical modulator may comprise a modulating element with an electrical input for receiving the electrical control input signal. The modulating element may be configured to modulate, simultaneously and non-independently, the effective refractive index of at least a portion of the at least one optical waveguide in the modulation region in response to the electrical control input signal applied to the electrical input. Where the optical modulating element is a thermo-optic modulator, the modulating element may include a resistive heating element connected to the electrical input and thermally coupled to at least a portion of the at least one optical waveguide in the modulation region. Where the optical modulating element is an electro-optic modulator, the modulating element may include one or more electrodes connected to the electrical input and electrostatically coupled to at least a portion of the at least one optical waveguide in the modulation region. Where the optical modulating element is a magneto-optical modulator, the modulating element may include an electromagnet connected to the electrical input and magnetically coupled to at least a portion of the at least one optical waveguide in the modulation region. Where the control input is an optical signal, the optical modulator may comprise a modulating element configured to provide, guide and / or couple the optical control input signal to the modulation region so as to modulate, simultaneously and non-independently, the effective refractive index of at least a portion of the at least one optical waveguide in the modulation region. Optionally or preferably, the modulating element may be or comprise one or more of: a waveguide, a coupler (such as a grating or directional coupler), and an emitter. Additionally, the optical modulator or the modulating element may further include a layer of non-linear optical material provided over the at least a portion of the at least one optical waveguide in the modulation region. The layer of non-linear optical material exhibits a change in refractive index (in addition to any change exhibited by the waveguide materials) in response to the optical control input signal to thereby modulate or enhance the modulation of effective refractive index neff of the at least one optical waveguide 13. Suitable non-linear optical materials include but are not limited to lithium niobate (e.g. in thin-film form), barium titanate, aluminium gallium arsenide, tantalum pentoxide, and vanadium dioxide. Where the control input is a change in an external environment at the modulation region, the optical modulator may comprise an encapsulation layer provided over the at least one optical waveguide, wherein the encapsulation layer includes an opening within the modulation region to expose at least a portion of the at least one optical waveguide in the modulation region to the external environment. The change in external environment may include one or more of: a change in refractive index of a medium in contact with the exposed portion of the at least one optical waveguide in the modulation region; and a change in temperature of the medium in contact with the exposed portion of the at least one optical waveguide in the modulation region. Optionally or preferably, the optical modulator may comprise a flow cell or flow chamber / channel arranged over the modulation region for providing a flow of medium over the exposed portion of the at least one optical waveguide in the modulation region. According to a second aspect of the invention, there is provided an optical system for a photonic circuit. The optical system comprises an optical modulator as defined in the first aspect. The at least one optical waveguide of the optical modulator preferably include a first optical waveguide and a second optical waveguide. The system comprises a first optical device comprising: an optical input for receiving a first input optical signal to be modulated, an optical output for outputting a modulated output optical signal, and the first optical waveguide of the optical modulator in a modulation optical path between the optical input and the optical output for guiding at least a portion of the first input optical signal through the modulation region of the optical modulator. The system further comprises a second optical device comprising: an optical input for receiving a second input optical signal to be modulated, an optical output for outputting a modulated output optical signal, and the second optical waveguide of the optical modulator in a modulation optical path between the optical input and the optical output for guiding at least a portion of the second input optical signal through the modulation region of the optical modulator. The first and second optical devices are preferably optical modulating devices. The optical modulator is configured to simultaneously and non-independently modulate an optical property of the at least a portion of the first and second input optical signals to thereby control the optical outputs of the first and second optical devices in dependence on the control input. Preferably, the first and second optical devices are at least partially integrated by virtue of sharing the optical modulator. Preferably, the first and second optical devices each include a further (passive) optical waveguide in a reference optical path between the respective optical input and output for guiding a reference portion of the respective first and second input optical signals. In this case, preferably the modulated output optical signals of the first and second optical devices are a superposition of the reference and modulated portions of the respective first and second input optical signals. At least one of the first and second devices may include a first optical output and a second optical output, and is configured to selectively split or switch the modulated output signal between the first and second optical outputs in response to the control input. The at least one of the first and second devices may therefore be an optical power splitting device or switching device. At least one of the first and second optical devices may further comprise an additional optical modulator, preferably an electro-optic modulator, at the or each optical output to modulate an optical property of the modulated output optical signals. In this way, bulk adjustments to the optical path can be made using the optical modulator, while fine adjustments to individual paths (i.e. trim) can be provided by the additional optical modulators, e.g. to compensate for fabrication imperfections in different optical paths. Preferably, the first and second (modulation) optical waveguides of the optical modulator are arranged substantially parallel and in a dual spiral configuration, in at least a portion of the modulation region of the optical modulator. The further (passive) optical waveguides of the first and second optical devices may be arranged, in the at least a portion of the respective reference optical path, in a spiral configuration. In this case, the or each of the passive optical waveguides may have an input portion and an output portion, whereby the input and output portions are arranged, in the at least a portion of the modulation region, as interleaved spirals that are connected in the spiral centre. The input portion may be configured to guide an optical signal along the reference optical path from a first end in an inwards spiral towards to a second end at the centre of the spiral, and the output portion may be configured to guide the optical signal from a first end (connected to the second end of the input portion) in an outwards spiral towards to a second end. The optical system may further include a further optical modulator as defined in the first aspect and third and fourth optical devices. The at least one optical waveguide of the further optical modulator preferably includes a first optical waveguide and a second optical waveguide. The third optical device comprises: an optical input for receiving a third input optical signal to be modulated, an optical output for outputting a modulated output optical signal, and the first optical waveguide of the further optical modulator in a modulation optical path between the optical input and the optical output for guiding at least a portion of the third input optical signal through the modulation region of the further optical modulator. The fourth optical device comprises: an optical input for receiving a fourth input optical signal to be modulated, an optical output for outputting a modulated output optical signal, and the second optical waveguide of the further optical modulator in a modulation optical path between the optical input and the optical output for guiding at least a portion of the fourth input optical signal through the modulation region of the further optical modulator. The first and second optical devices are preferably optical modulating devices. The further optical modulator is configured to simultaneously and non-independently modulate an optical property of the at least a portion of the third and fourth input optical signals to thereby control the optical outputs of the third and fourth optical devices in dependence on a further control input. Preferably, the third and fourth optical devices are at least partially integrated by virtue of sharing the further optical modulator. Preferably, the third and fourth optical devices each include a further (passive) optical waveguide in a reference optical path between the respective optical input and output for guiding a reference portion of the respective third and fourth input optical signals. In this case, preferably the modulated output optical signals of the third and fourth optical devices are a superposition of the reference and modulated portions of the respective third and fourth input optical signals. Preferably, the first and second (modulation) optical waveguides of the optical modulator are arranged substantially parallel and in a dual spiral configuration, in at least a portion of the modulation region of the optical modulator. Preferably, the further (passive) optical waveguides of the third and fourth optical devices are arranged, in the at least a portion of the respective reference optical path, in a spiral configuration. Preferably, the further (passive) optical waveguides of the second and third optical devices are arranged, in the at least a portion of the respective reference optical path, substantially parallel and in a dual spiral configuration in the at least a portion of the respective reference optical path. In this way, the optical devices in the optical system can be further integrated to reduce the footprint of the system on a PIC. According to a third aspect of the invention, there is provided an optical routing network or system, comprising the optical modulator as defined in the first aspect, or one or more optical systems as defined in the second aspect. According to a fourth aspect of the invention, there is provided a photonic circuit (preferably a photonic integrated circuit, PIC), comprising the optical modulator as defined in the first aspect, or one or more optical systems as defined in the second aspect, or the optical routing network of the third aspect. Preferably, the control input to the or each optical modulator is an electrical signal, and wherein the or each optical modulator comprises a modulating element with an electrical input for receiving the electrical control input signal. The modulating element is preferably configured to modulate, simultaneously and non-independently, the effective refractive index of at least a portion of the at least one optical waveguide in the modulation region in response to the electrical control input signal applied to the electrical input. In this case, the photonic circuit can comprise an electrical contact pad for receiving the electrical control input signal, and a conductive trace for transmitting the electrical control input signal to the optical modulator. According to a fifth aspect of the invention, there is provided an optical sensor comprising an optical modulator as defined in the first aspect, or an optical system as defined in second aspect. Preferably, the control input is a change in external environment at the modulation region, and at least a sensing portion of the at least one (modulation) optical waveguide of the optical modulator is exposed to the external environment. Further preferably, the at least one (modulation) optical waveguide of the optical modulator of the sensor comprises a multi-mode optical waveguide for guiding the multiple optical signals in different modes of the multi-mode waveguide. In this case, where the exposed sensing portion contains more than one mode, and each mode responds differently to the change in the external environment at the exposed sensing portion, a differential sensor signal can be acquired directly from comparing the modulated optical signals from each mode, without the need for a reference arm (which is not exposed to the change in external environment). This may enable calibration free sensing of a surround medium, and / or reduced footprint of the sensor device. According to a sixth aspect of the invention, there is provided a method for modulating multiple optical signals in a photonic circuit. The method may utilise the optical modulator of the first aspect or the optical system of the second aspect, or any other aspect. The method comprises guiding, in at least one optical waveguide, multiple optical signals through a modulation region of an optical modulator; and simultaneously and non-independently modulating the multiple optical signals in response to a control input provided to the optical modulator. Modulating the multiple optical signals preferably comprises simultaneously and non-independently modulating the effective refractive index of at least a portion of the at least one optical waveguide in the modulation region in response to the control input. The method may further comprise providing a control input to the optical modulator, and simultaneously and non-independently modulating the multiple optical signals in response to the control input provided to the optical modulator. The method may further include any steps associated with the features described in any previous aspect. Optionally or preferably, the control input is an electrical signal or an optical signal. According to a seventh aspect of the invention, there is provided a photonic circuit (preferably a PIC). The photonic circuit comprises a first set of optical modulating devices, each comprising a modulation optical waveguide for guiding an input optical signal to be modulated through a modulation region of the respective optical modulating device and an electrical input for receiving a first control input signal. The photonic circuit further comprises a first electrical control line connected to the electrical inputs of each of the first set of optical modulating devices for applying, simultaneously and non-independently, the first control input signal to the first set of optical modulating devices. Each optical modulating device of the first set is preferably configured to modulate an effective refractive index of at least a portion of the respective modulation optical waveguide in the modulation region in response to the first control input signal. The photonic circuit may further comprise a second set of optical modulating devices, each comprising a modulation optical waveguide for guiding an input optical signal to be modulated through a modulation region of the respective optical modulating device and an electrical input for receiving a second control input signal, and a second electrical control line connected to the electrical inputs of each of the second set of optical modulating devices for applying, simultaneously and non-independently, the second control input signal to the second set of optical modulating devices. Each optical modulating device of the second set is preferably configured to modulate an effective refractive index of at least a portion of the respective modulation optical waveguide in the modulation region in response to the second control input signal. The first and / or second electrical control line may be or comprise a conductive trace for applying / transmitting the respective first and second control input signals to the first and second sets of optical modulating devices. The photonic circuit may further comprise first and / or second electrical contact pad connected to the respective first and / or second electrical control line for receiving the respective first and second control input signals. Each of the first and second sets of optical modulating device may include an optical input and two optical outputs. The modulation optical waveguide of each optical modulating device may be in a modulation optical path between the optical input and outputs of the respective optical modulating device. Preferably, each of the first set of optical modulating devices is configured to selectively split or switch light provided to the respective optical input between the two optical outputs in response to the first input control signal. Preferably, each of the second set of optical modulating devices is configured to selectively split or switch light provided to the respective optical input between the two optical outputs in response to the second input control signal. Optionally or preferably, the optical outputs of the first set of optical modulating devices are connected to the inputs of the second set of optical modulating devices to form at least part of an optical routing network. Each optical output of the first set of optical modulating devices may be connected, directly or indirectly, to a respective optical input of the second set of optical modulating devices. The photonic circuit may comprise an optical routing network, wherein the first and second sets of optical modulating devices form part of the optical routing network. Preferably, the first and second sets of optical modulating devices are configured to selectively route light through the optical routing network in dependence on the first and second control input signals. The first set of optical modulating devices may comprise an optical modulator as defined in the first aspect, wherein the optical modulator comprises an electrical input connected to the first electrical control line for receiving the first control input signal, and wherein the at least one optical waveguide of the optical modulator includes the modulation optical waveguides of at least two of the first set of optical modulating devices for guiding the respective input optical signals through the modulation region of the optical modulator. The optical modulator is preferably configured to modulate, simultaneously and non-independently, the effective refractive index of the at least a portion of the respective modulation optical waveguides of the at least two of the first set of optical modulating devices in response to the first control input signal. Alternatively or additionally, the second set of optical modulating devices may comprise an optical modulator as defined in the first aspect, wherein the optical modulator comprises an electrical input connected to the second electrical control line for receiving the second control input signal, and wherein the at least one optical waveguide of the optical modulator includes the modulation optical waveguides of at least two of the second set of optical modulating devices for guiding the respective input optical signals through the modulation region of the optical modulator. The optical modulator is preferably configured to modulate, simultaneously and non-independently, the effective refractive index of the at least a portion of the respective modulation optical waveguides of the at least two of the second set of optical modulating devices in response to the second control input signal. The optical modulating devices in the first set may be substantially integrated as defined in the optical system of the second aspect. The optical modulating devices in the second set may be substantially integrated as defined in the optical system of the second aspect. The photonic circuit may comprise a first optical system as defined in the second aspect, wherein the first optical system includes the first set of optical modulating devices. The photonic circuit may comprise a second optical system as defined in the second aspect, wherein the second optical system includes the second set of optical modulating devices. According to an eighth aspect of the invention, there is provided a method of operating a photonic circuit including multiple optical modulating devices. Each optical modulating device preferably comprises a modulation optical waveguide for guiding an input optical signal to be modulated through a modulation region of the respective optical modulating device. Each optical modulating device is preferably configured to modulate an effective refractive index of at least a portion of the respective modulation optical waveguide in the modulation region in response to a control input. The photonic circuit is preferably a PIC. The method comprises applying a common control input to the multiple optical modulating devices to simultaneously and non-independently modulate an effective refractive index of at least a portion of a modulation optical waveguide extending through a modulation region of each respective optical modulating device. Each optical modulating device may be or comprise the optical modulator of the first aspect, or have any features associated with the optical system of the second aspect or the photonic circuit of the seventh aspect. The method may utilise the photonic circuit of the seventh aspect, the optical system of the second aspect, and / or the optical modulator of the first aspect. The method may further include any steps associated with the features described in any previous aspect. According to an eighth aspect of the invention, there is provided a quantum computer, comprising the optical modulator as defined in the first aspect, or the optical system as defined in the second aspect, or the optical routing network as defined in the third aspect, or the photonic circuit as defined in the seventh aspect. Any feature or combination of features of the optical modulator, optical system, sensor, or photonic circuit as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure. Any, some and / or all features in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination or sub-combination. In particular, system / apparatus aspects may be applied to method aspects, and vice versa. Similarly, where features are, for brevity, described in the context of a single preferred implementation, these may also be provided separately or in any suitable sub-combination. It should also be appreciated that particular combinations of the various features described and defined in any aspect of the invention can be implemented and / or supplied and / or used independently. The invention extends to methods, photonic integrated circuits, systems and devices substantially as herein described and / or as illustrated with reference to the accompanying figures. The invention also extends to any novel aspects or features described and / or illustrated herein. Whilst the invention has been described in the context of certain applications, it can also be implemented in any photonic integrated circuit application where a large number of optical modulators is required for reconfigurable compute, optical and / or information transfer, and where conservation of electrical connections and chip footprint is desired. The disclosure will now be described, by way of example, with reference to the accompanying drawings. Brief Description of Drawings In order that the invention can be well understood, embodiments will now be discussed by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a prior art optical modulator; Figure 2 is a schematic diagram of an optical modulator according to an embodiment of the present invention; Figure 3 is a schematic diagram of an optical modulator according to an embodiment of the present invention including a multi-mode optical waveguide; Figure 4 is a schematic diagram of an optical modulator according to an embodiment of the present invention including multiple optical waveguides; Figure 5 is a schematic diagram of an optical modulator according to an embodiment of the present invention; Figure 6 is a schematic diagram of an optical modulator according to another embodiment of the present invention; Figures 7(a) to 7(c) show schematic diagrams of different types of modulating elements, according to embodiments of the present invention; Figure 8 is a schematic diagram of an optical modulator in a spiral configuration according to an embodiment of the present invention; Figure 9 is a schematic diagram of an optical modulator in a dual spiral configuration according to an embodiment of the present invention; Figure 10 is a schematic diagram of an optical system including two integrated optical modulating devices according to an embodiment of the invention; Figure 11 shows a specific example of the optical system of figure 9; Figure 12 shows an optical routing network according to an embodiment of the invention; Figure 13 shows a specific example of the optical routing network of figure 11; Figure 14 shows a method according to an embodiment of the invention; and Figure 15 shows a method according to another embodiment of the invention. It should be noted that the figures are diagrammatic and may not be drawn to scale. Relative dimensions and proportions of parts of these figures may have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar features in modified and / or different embodiments. Detailed Description Figure 1 shows a schematic diagram of a conventional optical modulator 1 for modulating the phase of light in a photonic integrated circuit (PIC). The optical modulator 1 comprised an optical input 2i for receiving an input optical signal Uni, an optical output 2o for outputting a modulated output optical signal Louti, and a waveguide 3 for guiding the optical signal Lini through a modulation region 4. Light input at the input end 2i of the optical waveguide 3 with an input phase (j)ini is guided along the waveguide 3 to the output end 2o in the fundamental optical waveguide mode which propagates with a phase velocity of c / neff, where c is the speed of light in vacuum and neff is the effective refractive index of the mode. The modulation region 4 comprises a modulating element (not shown) that modulates the effective refractive index neff of at least a portion of the optical waveguide 3 in the modulation region 4 in response to a control input signal Sh provided via an external electrical connection made to an on-chip bond pad B1, as shown. By way of example, the modulating element can be a resistive heater if the optical modulator 1 is a thermo-optic modulator or one or more electrodes if the optical modulator 1 is an electro-optic modulator. Modulation of the effective refractive index neff in turn modulates the phase louti of the output optical signal Louti, as is known in the art. This modulation arrangement can be combined with various other optical components in a device to provide amplitude modulation and / or other functionality, for example a parallel reference optical waveguide can be included between the input 2i and output 2o in a Mach Zehnder Interferometer (MZI) configuration to split the input optical signal Lini between two paths (one including the modulation region) and recombine it at the output 2o, whereby the modulated output optical signal Louti is a superposition of the light from the two paths. In practice, a PIC will include a large number of optical modulators 1 to provide the required functionality, each individually addressable and independently controllable by a separate control input signal Sn, Si2 in order to modulate multiple different optical signals Lini, Lin2 propagating in the PIC, as indicated by the additional independently driven optical modulator T in figure 1. In this PIC arrangement, each control input signal Su, Si2 for each optical modulator 1,T requires separate bond pads B1, B2 (ignoring ground connections) which places a limitation on the number of active optical components that can be provided on current PIC chips (of a given size and with a given number of bond pads) and represents a bottleneck to developing large-scale reconfigurable PICs. The present invention provides a solution to these problems in existing PICs by reducing the number of bond pads and input control signals required to drive optical components on a PIC, and also by reducing the footprint of the optical components on the PIC chip. Specifically, in various embodiments of the invention, multiple optical signals can be modulated simultaneously using a single optical modulator and a single (common) control input, which in turn allows multiple optical components to be integrated by sharing the same optical modulator. The invention can therefore, at the expense of independent control of the optical modulators, reduce the PIC chip footprint, provide higher efficiency of modulation, reduce the number of electrical connections, and lower chip manufacture and packaging costs, as explained in more detail below. Figure 2 shows an optical modulator 10 for a PIC according to an embodiment of the present invention. The optical modulator 10 is configured to receive multiple optical signals Lni, Lin2 from upstream optical components and comprises at least one optical waveguide 13 for guiding the multiple optical signals through a modulation region 14 of the optical modulator 10. Each input optical signal Lm, Lin2 can propagate in an optical waveguide mode of the same or a different one of the at least one optical waveguide 13. The optical modulator 10 is configured to simultaneously and non-independently modulate an optical property of the multiple optical signals Lini, Linz propagating through the modulating region 14 in response to receiving a single control input Si, Oi, Xi that causes a simultaneous and non-independent modulation of the effective refractive index neff of at least a portion of the at least one optical waveguide 10 in the modulation region 14. As such, like the conventional arrangement in figure 1, a change in effective refractive index neff is used to modulate an input optical signal Uni, Lin2, but unlike the arrangement in figure 1, the optical modulator 10 of the present invention can modulate multiple optical signal Lini, Lin2 using a single common control input Si, Oi, Xi. The modulated optical property is preferably a phase ¢, of the optical signals Um, Un2 as shown, but it will be appreciated that the arrangement can also be used to modulate an amplitude, a wavelength, a polarisation, and a quantum state. For example, amplitude can be modulated using electro-optic absorption, wavelength can be modulated through a nonlinear multi-photon effect (or by using resonant effect in the modulation region 14 to modulate a resonant wavelength), polarisation can be modulated using a non-linear effect or asymmetric (polarisation) coupling between evanescently coupled waveguides (e.g. see D. Daoxin et al. in “On-chip polarisation handling for silicon photonics”, SPIE 2012). A quantum state can be the presence or not of light at the output, a polarisation state or a spatial mode. A range of suitable modulation arrangements are known in the art. In preferred implementations the control input is an input electrical signal Si provided from a bond pad B1 for controlling a modulating element in the modulation region 14 (such as a resistive heating element, not shown), as will be described in more detail below with reference to figure 5. However, the invention is not limited to electrical control signals Si. In alternative implementations, the control input is an optical signal Oi signal provided to the modulation region 14 or a change in external environment Xi at the modulation region 14, either of which can also cause a change of the effective refractive index neff of the at least one optical waveguide 13 in the modulation region 14. With reference to figure 3, where the optical modulator 10 comprises one optical waveguide 13 for guiding the multiple input optical signals Lini, Lin2 through the modulation region 14, this is a multi-mode waveguide 13m whereby the multiple optical signals Um, Lin2 propagate in different optical modes Mini, Min2 of the multi-mode waveguide 13m. In this case, the phase of each mode Mi, M2 is modulated simultaneously in response to the change in effective refractive index neff of the optical waveguide 13, and each mode Mi, M2 will respond differently to a given change in effective refractive index neff. For example, in the case of a multi-mode waveguide 13m comprising a core and cladding materials with different refractive indexes (e.g. a silicon waveguide with silica cladding), each mode Mi, M2 will have a different effective refractive index neff dependent the spatial extent of the mode with respect to the optical waveguide core and cladding materials. When a change occurs, e.g. in temperature, the refractive indexes of the core and cladding materials will change at different rates due to having different thermo-optic coefficients, which in turn means that the change in effective index neff will also be different for different modes in the multi-mode waveguide 13m. The transverse dimensions of the waveguide 13 determine which modes can exist. Preferably, the multi-mode waveguide 13 is a few-mode waveguide, e.g. supporting 2 to 10 guided optical modes. In embodiments where one multi-mode waveguide 13m is used to guide the multiple optical signals Lw, Lm through the modulation region 14, the optical modulator 10 can comprise multiple optical inputs 12i-1, 12i-2 and outputs 12o-1,12o-2 to couple light into and out of the multi-mode waveguide 13m, as described in more detail below with reference to figure 5. In various applications, using multi-mode waveguides 13m as opposed to single mode waveguide 13 may have various advantages, e.g. multimode waveguides 13m allow more light to be passed through the optical modulator 10 (two-photon effects limit the maximum optical power in a single mode), and multi-mode control allows for development of PICs involving mode-multiplexing which could improve bandwidth in telecoms applications. Figure 4 shows an embodiment of the optical modulator 10 including a plurality of spaced apart optical waveguides 13-1, 13-2, each optical waveguide 13 configured to guide at least one of the multiple input optical signals Lini, Lin2 through the modulation region 14. In this case, the optical modulator 10 is configured to simultaneously and non-independently modulate the effective refractive index neff of at least a portion of each of the plurality of optical waveguides 13 in the modulation region 14. Although two waveguides 13 are shown, it will be appreciated that the modulator 10 can in principle include any number of optical waveguides 13. The plurality of optical waveguides 13 can include single mode optical waveguides, multi-mode optical waveguides 13m, or a combination of both. Each single mode optical waveguides 13 (where present) is configured to guide a single input optical signal Lini, Lin2, and each multi-mode optical waveguides 13 (where present) is configured to guide at least two of the multiple optical signals Lini, Lin2 in at least two respective optical modes Mi, M2 of the multi-mode waveguide 13m. The plurality of optical waveguides 13-1,13-2 are preferably arranged substantially parallel over at least a portion of the modulation region 14, separated by a lateral gap g as shown in figure 4. The lateral gap g between neighbouring optical waveguides 13 can be the same or it may vary for different neighbouring pairs of optical waveguides 13, depending on the particular implementation. The degree of optical evanescent coupling between neighbouring optical waveguides 13-1,13-2 is dependent on the lateral gap between them, as is known in the art. Ina preferred implementation, the plurality of optical waveguides 13-1, 13-2 are spaced apart by lateral gap g within the modulation region 14 sufficiently large (e.g. substantially greater than the width of the waveguides) such that their respective optical modes are substantially uncoupled in the modulation region 14, and do not interact. In another implementation, at least some of the neighbouring optical waveguides 13-1,13-2 are spaced apart by lateral gap g, over at least a portion of the modulation region, sufficiently small (e.g. on the order of or substantially less than the width of the waveguides) such that their respective optical modes are evanescently coupled in the at least a portion of the modulation region 14. Optionally or preferably, in such cases the lateral gap g is sufficiently small such that their respective optical modes are superposed and support a supermode extending between the neighbouring optical waveguides 13-1, 13-2 that can be controllably routed to the outputs of the optical modulator 10 in response to the control input. For example, by changing the interference pattern between the modes that superpose to form the supermode, the supermode can be reconfigured (see e.g., P. Xu et al. in “Low-loss and broadband nonvolatile phase-change directional coupler switches” ACS Photonics 2019, 6 553 (2019)). In practice, the gap g required for supporting a supermode will vary with the material system and length of the interaction region (i.e. the length of the parallel waveguides 13-1, 13-2 in the modulation region 14). By way of example, for a silicon waveguide with silica cladding a suitable gap may be in the range of 50-300 nm. Figure 5 shows a schematic diagram of an embodiment of the optical modulator 10 in more detail. The optical modulator 10 comprises multiple optical inputs 12i-1, 12i-2 for receiving the multiple input optical signals L ini, Lin2, multiple optical outputs 12o-1, 12o-2 for outputting the modulated optical signals Louti, Lout2, and at least one optical waveguide 13 between the inputs 12i-1, 12i-2 and outputs 12o-1, 12o-2 for guiding the input optical signals Lini, Ln2 through the modulation region 14. The optical modulator 10 further comprises a modulating element 16 with an electrical input for receiving an electrical control input Si. The modulating element 16 is configured to modulate, simultaneously and non-independently, the effective refractive index neff of at least a portion of the at least one optical waveguide 13 in the modulation region 14 in response to an electrical input control signal applied to the electrical input. In this case, the optical modulator 10 can be configured as a thermo-optic modulator, an electro-optic modulator, or a magneto-optic modulator. As described above, the optical modulator 10 can include a single multi-mode optical waveguide 13m for guiding all the multiple input optical signals Lini, Ln2 through the modulation region 14, a plurality of single mode optical waveguides 13-1, 13-2, a plurality of multi-mode optical waveguides 13m-1, 13m-2, or a combination of single and multimode optical waveguides 13-1, 13-2 connected between the inputs 12i-1, 12i-2 and outputs 12o-1, 12o-2. The optical inputs 12i-1, 12i-2 and outputs 12o-1, 12o-2 are preferably optical waveguide portions that connect to the at least one optical waveguide 13 and can themselves be single mode or multi-mode. Preferably, single mode inputs 12i-1, 12i-2 are connected to single mode optical waveguides 13, and multi-mode inputs 12i-1, 12i-2 are connected to multi-mode optical waveguides 13. However, in some embodiments, multiple single mode inputs 12i-1, 12i-2 and outputs 12o-1, 12o-2 can be used to couple light into and out of a given multi-mode optical waveguide 13m via respective input and output couplers Ci, Co as illustrated in figure 6. Similarly, where the optical modulator 10 includes just a single multi-mode optical waveguide 13m, the optical modulator 10 can include a single multimode input 2i or multiple single mode inputs 12i-1, 12i-2 that couple light into the multimode optical waveguide 13m (and optical modulator 10), and also multiple single mode outputs 12o-1, 12o-2 for coupling modulated light out of the multi-mode optical waveguide 13m (and optical modulator 10). In preferred implementations, the optical modulator 10 is a thermo-optic modulator whereby the modulating element 16 includes a resistive heating element 16t connected to the electrical input and thermally coupled to at least a portion of the at least one optical waveguide 13 in the modulation region 14, as shown in figure 7(a). In an alternative implementation, the optical modulator 10 can be configured as an electrooptic modulator whereby the modulating element 16 includes one or more electrodes 16e connected to the electrical input and electrostatically coupled to at least a portion of the at least one optical waveguide 13 in the modulation region 14. In this case, an additional layer of electro-optic material can be provided over the at least a portion of the at least one optical waveguide 13 in the modulation region 14 to increase the modulation depth. An electro-optic material is one whose refractive index n is sensitive to an applied electric field, including but not limited to silicon, lithium niobate, indium phosphide, and electrooptic polymers. The one or more electrodes 16e can be arranged in various configurations provided the electric-field E produced from the electrodes 16e interacts with the waveguide 13, as is known in the art. In examples, the optical modulator 10 comprises electrodes 16e positioned transverse with respect to the at least one waveguide 13 so as to produce an electric field E across the waveguide 13 (i.e. laterally or above / below), as shown in figure 7(b). In an alternative implementation, the optical modulator 10 can be configured as a magneto-optical modulator whereby the modulating element 16 includes an electromagnet 16mconnected to the electrical input and magnetically coupled to at least a portion of the at least one optical waveguide 13 in the modulation region 14. In this case, an additional layer of magneto-optic material can be provided over the at least a portion of the at least one optical waveguide 13 in the modulation region 14 to increase the modulation depth. A magneto-optic material is one whose refractive index n is sensitive to an applied magnetic field, such as bismuth yttrium iron garnet (Bi-YIG) grown on top of a gadolinium gallium garnet (GGG). The electromagnet 16m can be arranged and provided in various configurations provided the magnetic-field M produced from the electromagnet 16m interacts with the waveguide 13, as is known in the art. In one example, the electromagnet 16m includes one or more conductive coils positioned over the at least one waveguide 13 so as to produce an magnetic field M within the waveguide 13, as shown in figure 7(c). With reference again to figure 5, in another alternative implementation, the optical modulator 10 is configured as an all-optical modulator whereby by the control input is an optical signal Oj. In this case, the modulating element 16 comprises a means for providing, guiding and / or coupling the optical control input signal Oi to the modulation region 14 so as to modulate, simultaneously and non-independently, the effective refractive index netf of at least a portion of the at least one optical waveguide 13 in the modulation region 14. The source (i.e. an emitter) of the optical control input signal Oi can be on-chip or off-chip (i.e. the PIC chip). Where it is on-chip, the PIC or the optical modulator 10 includes an emitter for producing the optical control input signal Oi, typically directed in the plane of the chip. In one example implementation, the modulating element 16 preferably comprises a coupling waveguide to direct the optical control input signal Oi onto a least a portion of the at least one optical waveguide 13 in the modulation region 14. Where the source of the optical control input signal Oi is off-chip (e.g. this could be provided by another PIC chip with an emitter, not shown), the optical control input signal Oi, may be directed out of the plane of the chip. In one example implementation, the modulating element 16 preferably comprises a coupler, such as a grating coupler (not shown), for coupling the optical control input signal Oi into the modulation region 14. In both cases, whether the source of the optical control input signal Oi is on or off chip, the modulating element 16 may further include a layer of non-linear optical material provided over the at least a portion of the at least one optical waveguide 13 in the modulation region 14, whereby the layer of nonlinear optical material exhibits a change in refractive index (in addition to any change exhibited by the waveguide materials) in response to the optical control input signal Oi to thereby modulate (or enhance the modulation) the effective refractive index netf of the at least one optical waveguide 13. Suitable non-linear optical materials include but are not limited to lithium niobate (e.g. in thin-film form), barium titanate, aluminium gallium arsenide, tantalum pentoxide, and vanadium dioxide. The optical waveguides 13 can be substantially straight or curved as they pass through the modulation region 14. In preferred implementations, the optical waveguide(s) 13 are substantially curved and arranged in a spiral to maximise the modulation path length for a given footprint. Figure 8 shows a schematic diagram of an optical waveguide 13 arranged in a spiral configuration in accordance with embodiments of the invention. In this case, the optical waveguide 13 can be conceptually divided into two portions, an input portion 13i and an output portion 13o that are arranged as interleaved spirals and connected in the spiral centre D via an S-shaped curve as shown. The input portion 13i guides light received from an optical input 12i inwards towards the centre D of the modulation region 14 in a spiral, and the output portion 13o guides light (received from the input portion 13i) from the centre D of the modulation region 14 back outwards in a spiral towards the optical output 12o. The modulation region 14 extends over the interleaved spiral as shown. For example, a heating element 16 can be provided over the spiral (see below). The above spiral arrangement can be extended to embodiments of the optical modulator 10 with multiple optical waveguides 13, as shown in figure 9. In the example of figure 9, the optical modulator 10 includes first and second optical waveguides 13-1, 13-2 arranged in an interleaved dual or double spiral configuration. In this case, the first and second optical waveguides 13-1, 13-2 each comprise an input portion 13i-1, 13i-2 and an output portion 13o-1, 13o-2 that are arranged as interleaved spirals and that are connected in the spiral centre D via an S-shaped curve as shown. The input portions 13i-1, 13i-2 and output portions 13o-1,13o-2 of the respective first and second optical waveguides 13-1,13-2 are substantially parallel (i.e. separated by a constant gap g) throughout the modulation region 14. In the example shown, the optical modulator 10 is configured as a thermo-optic modulator and the modulation element 16 is a substantially ring-shaped heating element 16t configured to heat at least a portion of the optical waveguides 13-1, 13-2 in response to a control input Si to thereby cause a modulation of the effective refractive index neff. Although a dual spiral is shown in figure 9, it will be appreciated that any number of optical waveguides 13 can be arranged in the spiral configuration together, allowing for all of optical waveguides 13 to be modulated simultaneously and non-independently via the modulating element 16 (e.g. a triple spiral, a quadruple spiral, etc.). It will be appreciated that, depending on the application and configuration of the PIC (in particular the upstream components feeding optical signals to the optical modulator 10), all, some, or only one of the multiple optical signals Lni, Lnzmay be provided to the optical modulator 10 at any one time. In the examples of the dual spiral modulator 10 shown in figure 9, where only one of the optical signals Lini, Lin2 is provided at a time, the optical modulator 10 of the present invention is as energy efficient in terms of power consumption to drive the modulator 10 as an equivalent single spiral modulator (e.g. equivalent to figure 9 with only one waveguide), but occupies only half the footprint compared to two such standard independently driven spiral modulators. On the other hand, where both of the input optical signals Lini, Lin2 are provided at the same time, the optical modulator 10 of the present invention is twice as energy efficient in addition to the saved footprint. This benefit in footprint and efficiency scales with the number of optical waveguides in the spiral modulator 10 (e.g. a triple spiral would be three times as efficient, a quadruple spiral would be four times as efficient, etc.). With reference again to figure 9, where input optical signals Lini, Lin2 are provided to both the first and second optical waveguides 13-1, 13-2 and these are separated by a lateral gap g sufficiently small to allow evanescent coupling and the formation of a supermode, the modulation of the effective refractive index neff effects the supermode and can allow switching between the two outputs 13o-1, 13o-2. Figure 10 shows a schematic diagram of an optical system 100 for a photonic circuit according to an embodiment of the invention, whereby the optical modulator 10 is used to (at least partially) integrate two or more separate optical modulating devices in order to reduce the footprint of the devices on the PIC chip. This arrangement is based on the optical modulating devices sharing the same optical modulator 10. The system 100 comprises an optical modulator 10 as described above, a first modulating optical device 110 and a second optical modulating device 120. The first optical modulating device 110 comprises an optical input 112i for receiving a first input optical signal Uni to be modulated, an optical output 112o for outputting a modulated output optical signal Louti, and an optical waveguide 13 of the optical modulator 10 in a modulation optical path MP-1 between the optical input 112i and the optical output 112o for guiding at least a portion of the first input optical signal Lini through the modulation region 14 of the optical modulator 10. The second optical modulating device 120 comprises an optical input 122i for receiving a second input optical signal Lins to be modulated, an optical output 122o for outputting a modulated output optical signal Lout2, and an optical waveguide 13 of the optical modulator 10 in a modulation optical path MP-2 between the optical input 122i and the optical output 122o for guiding at least a portion of the second input optical signal Lin2 through the modulation region 14 of the optical modulator 10. In an example implementation, each optical modulating device 110, 120 uses a separate optical waveguide 13 of the optical modulator 10 to guide first and second input optical signals Lini, Lin2 through the modulation region 14. In other implementations, the first and second input optical signals Lini, Lin2 can be guided in different modes M1, M2 of a multi-mode optical waveguide 13m of the optical modulator 10. In this configuration, the optical modulator 10 is configured to simultaneously and non-independently modulate an optical property of the at least a portion of the first and second input optical signals Lini, Lin2 to thereby control the optical outputs 112o, 122o of the first and second optical modulating devices 110, 120 in dependence on the control input (not shown), and the first and second optical modulating devices 110, 120 are at least partially integrated by virtue of sharing the optical modulator 10, as indicated schematically by the overlapping dashed-dotted lines in figure 10. In some embodiments, the first and second optical modulating devices 110, 120 each include a further (passive) optical waveguide in a reference optical path RP-1, RP-2 between the respective optical input 112i, 122i and output 112o, 122o for guiding a reference portion of the respective first and second input optical signals Lini, Lh2. In this case, first and second input optical signals Lhi, Ln2 are split between reference and modulation optical paths, and then recombined to provide the respective modulated output optical signals Louti, Lout2. As such, modulated output optical signals Louti, Lout2 of the first and second optical devices are a superposition of the reference and modulated portions of the respective first and second input optical signals. In this case, the optical system 100 represents two integrated MZI-type devices 110, 120. In further examples, an additional optical modulator 112t, 122t, preferably an electro-optic modulator, can be included at each optical output 112o, 122o to fine tune the modulated output optical signals Louti, Louti, as illustrated in figure 10. In this way, bulk adjustments to the optical path can be made using the optical modulator 10, while fine adjustments to individual paths (i.e. trim) can be provided by the electro-optic modulators, e.g. to compensate for fabrication imperfections in different optical paths. The use of thermo-optic bulk modulation in the optical modulator 10 and electro-optic modulators for trim may be particularly effective in silicon-based material systems whereby thermo-optic modulation primarily affects the phase with little effect on amplitude, but electro-optic modulation affects both phase and amplitude. The use of additional electro-optic modulators 112t, 122t requires additional electrical signals. However, the advantage is in the combination of using a thermo-optical modulator 10 for simultaneous and non-independent modulation of the multiple optical signals in an integrated system 100 with fewer electrodes to not take up as much space on the PIC chip and to modulate without any optical loss, and then using electro-optic modulation for fast / fine trim only (large electro-optic modulation will typically cause large optical loss, while small / fine electro-optic modulation as proposed here will typically provide comparatively little optical loss). Figure 11 shows a specific example of an optical system 100 comprising two integrated modulating optical devices 110,120 that share the same optical modulator 10 as described above. In this example, the at least one optical waveguide 13 of the optical modulator 10 includes a first (active) optical waveguide 13-1 and a second (active) optical waveguide 13-1 arranged in parallel and in an interleaved dual spiral configuration as shown in figure 9. The optical modulator 10 is preferably a thermo-electric modulator and includes a modulating element 16 in the form of a resistive heater with an electrical input for receiving a control input Si (not shown). The first optical modulating device 110 comprises an optical input 112i for receiving a first input optical signal Ljni, an input coupler 111i for splitting the first input optical signal Lini between a modulation optical path MP-1 comprising the first (active) optical waveguide 13-1 of the optical modulator 10 and a reference optical path RP-1 comprising a further (passive) optical waveguide 113r. The first optical modulating device 110 further comprises an output coupler 111i for combining the reference and modulated optical signals at the end of the modulation and reference paths MP-1, RP-1 and coupling them to an optical output 112o to provide a modulated output optical signal LOuti. Similarly, the second optical modulating device 120 comprises an optical input 122i for receiving a second input optical signal Lin2, an input coupler 121 i for splitting the second input optical signal Lin2 between a modulation optical path MP-2 comprising the second (active) optical waveguide 13-2 of the optical modulator 10 and a reference optical path RP-2 comprising a further (passive) optical waveguide 123r. The second optical modulating device 120 further comprise an output coupler 121o for combining the modulated and reference optical signals at the end of the modulation and reference paths MP-1, RP-1 and coupling them to an optical output 112o to provide a modulated output optical signal Lout2. Preferably, the passive optical waveguides 113r, 123r in the reference optical paths RP-1, RP-2 of the respective first and second optical modulating devices 110, 120 are also arranged in respective spirals 115,125 so as to substantially match the length of the modulation paths MP-1, MP-2, as shown. In the example shown, the output couplers 111o, 121o are 2x2 multimode interference (MMI) devices, such that the first and second optical modulating devices 110, 120 each include a two optical outputs 112o, 122o. Light from the modulation and reference optical paths interferes within the 2x2 MMI 111 o, 121o to affect its output. The MMI 111o, 121o can be configured such that the interference creates a null at one optical output 112o of the MMI (destructive interference) and a bright spot at the other optical output 112o of the MMI (constructive interference), or another splitting ratio. As such, in this example the first and second optical modulating devices 110, 120 are configured as optical switching or power splitting devices that selectively split or switch the modulated output signal between the two optical outputs in response to the control input. However, in other examples the first and second optical modulating devices 110, 120 can each include a single optical output 112o, 112o (e.g. the output couplers 111o, 121ocan be 2x1 MMI devices, or any other suitable optical component for combining the reference and modulated optical signals at the end of the modulation and reference paths MP-1, RP-1 and coupling them into a single optical output 112o to provide an modulated output optical signal Louti), in which case the first and second optical modulating devices 110, 120 are configured as MZIs whose output signals Louti, Lout2 are modulated according to the relative phase difference between the between the reference and modulated optical signals induced by the optical modulator 10. In the example shown, the input couplers 111i, 121i are 1x2 multimode interference (MMI) devices, but it will be appreciated that any other suitable splitting device can be used, e.g. a y-junction splitter, or directional coupler. In further examples, an additional optical modulator 112t (not shown), preferably an electro-optic modulator, can be included at each optical output 112o to fine tune the modulated output optical signals Louti, Louti, similar to that illustrated in figure 10. The optical system 100 can comprise further pairs of optical modulating devices integrated in the same way as shown in figures 10 and 11, whereby each pair (e.g. the first and second optical modulating devices 110, 120 as shown, and third and fourth optical modulating devices identical to that shown in figures 10 and 11) shares a respective optical modulator 10. With reference to figure 11, where integrated first and second optical modulating devices 110,120 and the integrated third and fourth optical modulating devices are arranged substantially side by side with the second optical modulating device adjacent the third optical modulating device, the reference optical waveguides of the second and third optical modulating devices can be integrated into a dual spiral configuration (similar to that shown in figure 9 but without the modulation region 14) to further reduce the footprint on the PIC. Figure 12 shows a schematic diagram of an optical routing network 200 for a photonic integrated circuit according to an embodiment of the invention. The optical routing network 200 comprises an optical input IN on an input side of the network 200 and a plurality of optical outputs OUT1-OUT16 on an output side of the network 200, with multiple optical power splitting (or switching) devices 20 in the optical paths between the optical input IN and the outputs OUT1-OUT16 to selectively route light from the optical input IN to one or more of the optical outputs OUT1-OUT16. The optical switching devices 20 are grouped into multiple different levels, whereby the optical switching devices 20 in each level are electrically driven together using a respective common control input to reduce the number of bond pads required to operate the network 200, as will be described in more detail below. Each optical switching device 20 comprises an optical input 21 i for receiving an input optical signal to be modulated, two optical outputs 21 o, and an electrical input for receiving a control input signal Si (indicated by the solid dot in figure 12). Each optical switching device 20 is configured to selectively split or switch an input optical signal provided to the respective optical input 21 i between its two optical outputs 21o in response to a control input signal Si applied to its electrical input. With reference to figure 12, the optical switching devices 20 of the network 200 are grouped into multiple different levels connected in a cascading manner: a zeroth level comprises zeroth optical switching device 20 to receive the input optical signal provided to the input IN of the network 200; a first level comprising a first set Set1 (two in this example) of optical switching devices 20 arranged to receive the light output from the zeroth level; a second level comprising a second set S2 (four in this example) of optical switching devices 20 arranged to receive the light output from the first level; and a third level comprising a third set Set3 (eight in this example) of optical switching devices 20 arranged to receive the light output from the second level and provide the optical outputs OUT1-OUT16 of the network 200. Each optical output 21 o of the of the zeroth level is connected to a respective optical input 21 i of the first set of optical switching devices 20, each optical output 21 o of the first set Set1 of optical switching devices 20 is connected to a respective optical input 21 i of the second set Set2 of optical switching devices 20, and each optical output 21o of the second set Set2 of optical switching devices 20 is connected to a respective optical input 21 i of the third set Set3 of optical switching devices 20 in the third level. A zeroth electrical control line ELO connects the electrical input of the zeroth optical switching device 20 to a zeroth bond pad B0 for applying a zeroth control input signal S® thereto. A first electrical control line EL1 connects the electrical inputs of each of the first set Set1 of optical switching devices 20 to a first bond pad B1 for applying, simultaneously and non-independently, a first control input signal thereto. A second electrical control line EL2 connects the electrical inputs of each of the second set Set2 of optical switching devices 20 to a second bond pad B2 for applying, simultaneously and non-independently, a second control input signal thereto. A third electrical control line EL3 connects the electrical inputs of each of the third set Set3 of optical switching devices 20 to a third bond pad B2 for applying, simultaneously and non-independently, a third control input signal thereto. In this way, the optical switching devices 20 in each level of the network 200 are electrically driven together using a respective common control input signal to reduce the number of bond pads required to operate the network 200. By way of example, in prior art configurations whereby each optical switching device 20 is independently driven by a separate control input, a 1 x 16 optical switching network as shown in figure 12 would require 15 electrical connections or bond pads (16 connections including a ground). By contrast, the parallel driving arrangement of figure 12 requires only 4 bond pads to operate the network (5 including ground, not shown). This is particularly effective where the network 200 is used to route a single optical signal from the input IN to one of the outputs OUT1-OUT16, whereby light is only routed through one optical switching device 20 in any given level of the network 200 (depending on the states of the previous levels) - while the other switching devices 20 in a given level are being driven they would be redundant in this instance. The saving in electrical input / bond pads scales with the size of the network. For example, for a 1 x N network, the number of electrical connections needed (including one ground connection) is N, while for the arrangement of figure 12 it is only 1+log2N. Although in figure 12 all the optical switching devices 20 of a given level are driven together, this is not essential. In practice, at least some of the levels can be divided into a number of subsets of optical switching devices 20 (e.g. 2-6 modulators) which are driven together, so that the resistance for the driving electronics does not vary too much from level to level. In one example, each optical switching device 20 is configured to operate in a similar way to the first or second optical modulating device 110, 120 shown in figure 11 (i.e. one half of the optical system 100), whereby the input optical signal is split between a reference optical path and a modulation optical path that includes an optical modulator 10 driven by the control input, and then combined in a 2x2 MM I device at the end of the reference and modulation optical paths to provide selective switching between the two optical outputs 21 o in response to the control input. Further, in preferred implementations, adjacent optical switching devices 20 in the first, second and third sets Set1-Set3 are integrated as shown in figure 11 to reduce the footprint of the network 200. Figure 13 shows a specific example of the optical routing network 200, whereby adjacent optical switching devices 20 in the first, second and third sets Set1-Set3 are configured and integrated in the same way as the optical modulating devices 110, 120 of the optical system 100 shown in figure 11. As with the optical system 100 of figure 11, two optical switching devices 20 share an optical modulator 10 in which the optical waveguides are arranged in parallel and in an interleaved dual spiral configuration as shown. ln figure 13, the zeroth level is omitted, only the features of the first set Set1 of optical devices 20 are labelled for clarity. Each one of the integrated optical switching devices 20 comprises an optical input 21 i, an input coupler 22i for splitting the input optical signal between a modulation optical path MP comprising a (modulation) optical waveguide of the optical modulator 10 (arranged in a spiral) and a reference optical path RP comprising a passive optical waveguide 25 (also arranged in a spiral), and an output coupler 22o for combining the reference and modulated optical signals at the end of the modulation and reference paths MP, RP and coupling them to the optical outputs 21 o. The output coupler 22o is preferably a 2x2 MMI device configured to selective spit or switch light between the two optical outputs 21 o in dependence on the superposition of the reference and modulated optical signals (which in turn is controlled by the control input). As such, the first set Set1 comprises one optical system 100 according to figure 11, the second set Set2 comprises two optical systems 100 according to figure 11, and the third set Set3 comprises four optical systems 100 according to figure 11. Ina further preferred implementation, the optical switching devices 20 of the second and third sets are further integrated by integrating the adjacent passive optical waveguides 25 (as indicated by the regions labelled A in figure 13) into dual spirals (not shown). Figure 14 shows a method 500 for modulating multiple optical signals in a photonic circuit. The method 500 may utilise the optical modulator 10, or the optical system 100 as previously described. In step 502, the method comprises guiding, in at least one optical waveguide 13, multiple optical signals Lw, Lin2 through a modulation region 14 of an optical modulator 10. In step 504, the method comprises simultaneously and non-independently modulating the multiple optical signals Lini, Lin2 in response to a control input provided to the optical modulator 10. Step 504 may be preceded by a step 503 providing a control input to the optical modulator 10. Modulating the multiple optical signals preferably comprises simultaneously and non-independently modulating the effective refractive index of at least a portion of the at least one optical waveguide in the modulation region in response to the control input. Figure 15 shows a method 600 of operating the optical routing network 200 described above. In step 602 the method comprises applying a common control input to the multiple optical modulating devices 20 to simultaneously and non-independently modulate an effective refractive index of at least a portion of a modulation optical waveguide 13 extending through a modulation region 14 of each respective optical modulating device 20. Step 602 may comprise applying a common control input to each level of the network to selectively route light from the input IN to one or more outputs OUT1-OUT16. The various embodiments of the invention can be implemented in any suitable photonic material system, including but not limited to: silicon, InP, lithium niobate (e.g. in thin film form), barium titanate, and silicon nitride. From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art, and which may be used instead of, or in addition to, features already described herein. Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination. Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims. Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. ln this specification the word 'or' can be interpreted in the exclusive or inclusive sense unless stated otherwise. For the sake of completeness it is also stated that the term "comprising" does not exclude other elements or steps, the term "a" or "an" does not 5 exclude a plurality, and any reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

1. An optical modulator for a photonic circuit, comprising:at least one optical waveguide for guiding multiple optical signals through a modulation region of the modulator; andwherein the optical modulator is configured to simultaneously and non-independently modulate an optical property of the multiple optical signals propagating through the modulating region in response to receiving a control input.

2. The optical modulator of claim 1, wherein the at least one optical waveguide includes a plurality of optical waveguides, each optical waveguide configured to guide at least one of the multiple optical signals.

3. The optical modulator of claim 2, wherein at least one of the plurality of optical waveguides is a single-mode optical waveguide configured to guide one of the multiple optical signals.

4. The optical modulator of claim 2 or 3, wherein at least one of the plurality of optical waveguides is a multi-mode optical waveguide configured to guide at least two of the multiple optical signals in at least two respective optical modes of the multi-mode waveguide.

5. The optical modulator of claim 2 or 3 or 4, wherein the plurality of optical waveguides are spaced apart, and wherein:at least some of the plurality of optical waveguides are spaced apart by a lateral gap within the modulation region such that their respective optical modes are substantially uncoupled in the modulation region; and / orat least some of the plurality of optical waveguides are spaced apart by a lateral gap, over at least a portion of the modulation region, such that their respective optical modes are evanescently coupled in the at least a portion of the modulation region; and optionally or preferably, such that their respective optical modes are superposed and support a supermode.

6. The optical modulator of any of claims 2 to 5, wherein the plurality of optical waveguides are arranged substantially parallel over at least a portion of the modulation region.

7. The optical modulator of claim 6, wherein each of the plurality of optical waveguideshas an input portion and an output portion arranged, in the at least a portion of the modulation region, as interleaved spirals that are connected in the spiral centre.

8. The optical modulator of claim 1, wherein the at least one optical waveguide includes a multi-mode optical waveguide configured to guide at least two of the multiple optical signals in at least two respective optical modes of the multi-mode waveguide.

9. The optical modulator of claim 8, wherein the multi-mode optical waveguide has an input portion and an output portion arranged, in at least a portion of the modulation region, as interleaved spirals that are connected in the spiral centre.

10. The optical modulator of any preceding claim, wherein the optical modulator is configured to modulate, simultaneously and non-independently, the effective refractive index of at least a portion of the at least one optical waveguide in the modulation region in response to the control input.

11. The optical modulator of any preceding claim, wherein the optical modulator comprises multiple optical inputs for receiving the multiple optical signals, and multiple optical outputs for outputting the multiple optical signals having been guided through the modulation region, wherein the multiple optical inputs and outputs are connected directly or indirectly by the at least one optical waveguide.

12. The optical modulator of any preceding claim, wherein the control input is an electrical signal, and the optical modulator is or comprises one of: a thermo-optic modulator, an electro-optic modulator, and a magneto-optic modulator.

13. The optical modulator of claim 12, wherein the optical modulator comprises a modulating element with an electrical input for receiving the electrical control input signal, and wherein the modulating element is configured to modulate, simultaneously, the effective refractive index of at least a portion of the at least one optical waveguide in the modulation region in response to the electrical control input signal applied to the electrical input; and, optionally or preferably,wherein the optical modulating element is or comprises:(i) a thermo-optic modulator including a resistive heating element connected to the electrical input and thermally coupled to at least a portion of the at least one optical waveguide in the modulation region;(ii) an electro-optic modulator including an electrode connected to the electrical input and electrostatically coupled to at least a portion of the at least one optical waveguide in the modulation region; or(iii) a magneto-optical modulator including an electromagnet connected to the electrical input and magnetically coupled to at least a portion of the at least one optical waveguide in the modulation region.

14. The optical modulator of claim 10 or 11, wherein the control input is an optical signal, and wherein optical modulator comprises a modulating element configured to provide, guide and / or couple the optical control input signal to the modulation region to modulate, simultaneously, the effective refractive index of at least a portion of the at least one optical waveguide in the modulation region; and optionally or preferably, wherein the modulating element is or comprises one or more of: a waveguide, a coupler, and an emitter.

15. The optical modulator of any of claims 1 to 10, wherein the control input is a change in an external environment at the modulation region, and wherein the optical modulator comprises an encapsulation layer provided over the at least one optical waveguide, wherein the encapsulation layer includes an opening within the modulation region to expose at least a portion of the at least one optical waveguide in the modulation region to the external environment.

16. The optical modulator of claim 15, wherein the change in external environment comprises one or more of: a change in refractive index of a medium in contact with the exposed portion of the at least one optical waveguide in the modulation region, and a change in temperature of the medium in contact with the exposed portion of the at least one optical waveguide in the modulation region; and, optionally or preferably,wherein the optical modulator comprises a flow cell arranged over the modulation region for providing a flow of medium over the exposed portion of the at least one optical waveguide in the modulation region.

17. The optical modulator of any preceding claim, wherein the modulated optical property of the multiple optical signals is one or more of: a phase, an amplitude, a wavelength, a polarisation, and a quantum state.

18. An optical system for a photonic circuit, comprising:an optical modulator as defined in any of claims 1 to 17, wherein the at least one optical waveguide includes a first optical waveguide and a second optical waveguide;a first optical device comprising: an optical input for receiving a first input optical signal to be modulated, an optical output for outputting a modulated output optical signal, and the first optical waveguide of the optical modulator in a modulation optical path between the optical input and the optical output for guiding at least a portion of the first input optical signal through the modulation region of the optical modulator; anda second optical device comprising: an optical input for receiving a second input optical signal to be modulated, an optical output for outputting a modulated output optical signal, and the second optical waveguide of the optical modulator in a modulation optical path between the optical input and the optical output for guiding at least a portion of the second input optical signal through the modulation region of the optical modulator,wherein the optical modulator is configured to simultaneously and non-independently modulate an optical property of the at least a portion of the first and second input optical signals to thereby control the optical outputs of the first and second optical devices in dependence on the control input, and preferablywherein the first and second optical devices are at least partially integrated by virtue of sharing the optical modulator.

19. The optical system of claim 18, wherein the first and second optical devices each include a further optical waveguide in a reference optical path between the respective optical input and output for guiding a reference portion of the respective first and second input optical signals; andwherein the modulated output optical signals of the first and second optical devices are a superposition of the reference and modulated portions of the respective first and second input optical signals.

20. The optical system of claim 19, wherein at least one of the first and second devices includes a first optical output and a second optical output, and is configured to selectively split or switch the modulated output signal between the first and second optical outputs in response to the control input.

21. An optical routing network, comprising the optical modulator as defined in any of claims 1 to 17 or the optical system as defined in any of claims 18 to 20.

22. A photonic integrated circuit, comprising the optical modulator as defined in any of claims 1 to 17, or the optical system as defined in any of claims 18 to 20, or the optical routing network of claim 21.

23. The photonic integrated circuit of claim 22, wherein the control input is an electrical signal, and wherein the optical modulator comprises a modulating element with an electrical input for receiving the electrical control input signal, and wherein the modulating element is configured to modulate, simultaneously, the effective refractive index of at least a portion of the at least one optical waveguide in the modulation region in response to the electrical control input signal applied to the electrical input; andwherein the photonic circuit comprises an electrical contact pad for receiving the electrical control input signal, and a conductive trace for transmitting the electrical control input signal to the optical modulator.

24. An optical sensor comprising the optical modulator as defined in any of claims 1 to17, or the optical system as defined in any of claims 18 to 20, and preferably wherein the control input is a change in external environment at the modulation region.

25. A method for modulating multiple optical signals in a photonic circuit, comprising:guiding, in at least one optical waveguide, multiple optical signals through a modulation region of an optical modulator; andsimultaneously and non-independently modulating the multiple optical signals in response to a control input provided to the optical modulator.

26. The method of claim 25 wherein simultaneously and non-independently modulating the multiple optical signals comprises simultaneously and non-independently modulating the effective refractive index of at least a portion of the at least one optical waveguide in the modulation region in response to the control input.

27. The method of claim 25 or 26, further comprising providing a control input to the optical modulator, and simultaneously and non-independently modulating the multiple optical signals in response to the control input provided to the optical modulator; and optionally or preferably, wherein the control input is an electrical signal or an optical signal.

28. A photonic circuit, comprising:a first set of optical modulating devices, each comprising a modulation optical waveguide for guiding an input optical signal to be modulated through a modulation region of the respective optical modulating device and an electrical input for receiving a first control input signal, wherein each optical modulating device of the first set is configured to modulate an effective refractive index of at least a portion of the respective modulation optical waveguide in the modulation region in response to the first control input signal, anda first electrical control line connected to the electrical inputs of each of the first set of optical modulating devices for applying, simultaneously and non-independently, the first control input signal to the first set of optical modulating devices.

29. The photonic circuit of claim 28, further comprising:a second set of optical modulating devices, each comprising an modulation opticalwaveguide for guiding an input optical signal to be modulated through a modulation region of the respective optical modulating device and an electrical input for receiving a second control input signal, wherein each optical modulating device of the second set is configured to modulate an effective refractive index of at least a portion of the respective modulation optical waveguide in the modulation region in response to the second control input signal, anda second electrical control line connected to the electrical inputs of each of the second set of optical modulating devices for applying, simultaneously and non-independently, the second control input signal to the second set of optical modulating devices.

30. The photonic circuit of claim 29, wherein each of the first and second sets of optical modulating device includes an optical input and two optical outputs, wherein the modulation optical waveguide of each optical modulating device is in a modulation optical path between the optical input and outputs of the respective optical modulating device, andwherein each of the first set of optical modulating devices is configured to selectively split or switch light provided to the respective optical input between the two optical outputs in response to the first input control signal; andwherein each of the second set of optical modulating devices is configured to selectively split or switch light provided to the respective optical input between the two optical outputs in response to the second input control signal; and, optionally or preferably,wherein each optical output of the first set of optical modulating devices is connected, directly or indirectly, to a respective optical input of the second set of optical modulating devices.

31. The photonic circuit of claim 29 or 30, comprising an optical routing network, wherein the first and second sets of optical modulating devices form part of the optical routing network, and wherein the first and second sets of optical modulating devices are configured to selectively route light through the optical routing network in dependence on the first and second control input signals.

32. The photonic circuit of any of claims 28 to 31, wherein the first set of optical modulating devices comprises an optical modulator as defined in any of claims 1 to 17, wherein the optical modulator comprises an electrical input connected to the first electrical control line for receiving the first control input signal, and wherein the at least one optical waveguide of the optical modulator includes the modulation optical waveguides of at least two of the first set of optical modulating devices for guiding the respective input optical signals through the modulation region of the optical modulator, such that the optical modulator is configured to modulate, simultaneously and non-independently, the effective refractive index of the at least a portion of the respective modulation optical waveguides of the at least two of the first set of optical modulating devices in response to the first control input signal; and / orwherein the second set of optical modulating devices comprises an optical modulator as defined in any of claims 1 to 17, wherein the optical modulator comprises an electrical input connected to the second electrical control line for receiving the second control input signal, and wherein the at least one optical waveguide of the optical modulator includes the modulation optical waveguides of at least two of the second set of optical modulating devices for guiding the respective input optical signals through the modulation region of the optical modulator, such that the optical modulator is configured to modulate, simultaneously and non-independently, the effective refractive index of the at least a portion of the respective modulation optical waveguides of the at least two of the second set of optical modulating devices in response to the second control input signal.

33. A method of operating a photonic integrated circuit including multiple optical modulating devices, the method comprising:applying a common control input signal to the multiple optical modulating devices to simultaneously modulate an effective refractive index of at least a portion of an optical waveguide extending through a modulation region of each respective optical modulating device.

34. A quantum computer, comprising the optical modulator as defined in any of claims 1 to 17, or the optical system as defined in any of claims 18 to 20, or the optical routing network of claim 21, or the photonic integrated circuit as defined in any of claims 28 to 32.

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