Method and apparatus for sensing displacement with photonic integrated circuits

Abstract

According to an example of the present disclosure, there is provided an apparatus comprising a first waveguide; at least one second waveguide including a coupler waveguide arranged to couple light to and from the first waveguide; a micro-electro-mechanical (MEMS) structure from which extends either the first waveguide and the coupler waveguide, or a first interaction portion of the apparatus and a second interaction portion of the apparatus; and an anchoring portion; wherein the MEMS structure is connected to the anchoring portion via a resiliently deformable element configured such that MEMS structure is movable, upon application of a force or perturbation, in a first direction to reduce or increase a first distance between the first waveguide and the first interaction portion; and wherein, upon movement of the MEMS structure: an optical characteristic of the first waveguide and / or an amount of light in the first waveguide is changed based on a first interaction between the first waveguide and the first interaction portion according to the first distance, and an amount of light in an interaction waveguide, among the at least one second waveguide, is changed according to a second interaction between the interaction waveguide and the second interaction portion.

Description

Methods and Apparatus for Sensing Displacement with Photonic Integrated Circuits

[0001] The present disclosure relates generally to apparatus and methods for sensing a displacement of a MEMS structure using photonics. In particular, various examples allowfor both rough sensing and fine sensing of displacement to be performed, with the fine sensing being available for relatively small movements of the MEMS structure from an initial position and the rough sensing being available up to larger movements. Various examples of the present disclosure provide an apparatus comprising a combination of waveguides and interaction portions, whereby movement of a MEMS structure in the apparatus causes relative movement between a waveguide and a corresponding interaction portion. This movement has an effect on the amount of light in or output by a waveguide, where measuring this effect can be used to sense or otherwise determine the magnitude of this movement.BACKGROUND

[0002] A micro-electro-mechanical system, MEMS, is a microscopic device which includes both electronic and moving parts. A MEMS structure may have a thickness in the order of a few to tens of micrometres, e.g., greater than 1 m. A photonic integrated circuit, PIC, (or an integrated optical circuit) is also a microscopic device, and may include a number of photonic components which form a functioning circuit. A PIC structure may have a thickness in the order of hundreds of nanometres, e.g., 70-1000nm.

[0003] Sensing a displacement of an object may be achieved through using an evanescent field of a PIC waveguide; that is, the evanescent field around sub-wavelength geometry waveguides can be used as a non-contact methods for sensing displacement. For example, a displacement of the object within the evanescent field has an effect on the properties or propagation of light in the PIC waveguide or on an optical characteristic of the PIC waveguide itself, such as by changing the effective refractive index of the PIC waveguide, or affecting scattering and / or other loss mechanisms. If the effect is measured, the result can be used to calculate the displacement of the object. It will be understood that two or more waveguides located around an object may be used to detect a displacement of the object in two or more dimensions.

[0004] An evanescent field of a waveguide can be used for sensitive sensing of a displacement of an object in a direction which is perpendicularto the propagation direction of light in the waveguide (or perpendicular to the guided mode, to put it another way) - thismay be regarded as ‘transverse sensing’. For example, if light is input to a straight waveguide and an object is moved towards the waveguide in a direction which is perpendicular to the propagation direction of the light, it is possible to sense the magnitude of this movement with high accuracy or more precision (that is, greater sensitivity, or fine-sensing).

[0005] This sensitive sensing regime is limited to the evanescent field length, which is on the order of approximately the wavelength of the input light (e.g. < 1 micron (pm)) . In other words, displacement, in a direction perpendicular to the propagation direction of light, of an object occurring outside of 1 pm from the edge of the waveguide may not be sensed (or sensed well) because the effect this movement has on the evanescent field (or on the properties of the propagating light or the optical characteristics of the waveguide) may be small, to the point of being unable to distinguish it from background noise.

[0006] To create a sensing device, such as a photonically enhanced MEMS device (an example of which is an inertial sensor), displacements over 200nm are often required for extending the sensing dynamic range (for example, for accelerometers) and / or displacements over 5um are often required for creating drive mode (for example, for gyroscopes).

[0007] Accordingly, there is a need to provide a sensing device which, in addition to allowing for sensing of smaller displacement (e.g. fine sensing, within a small range), can also sense larger displacements (e.g. rough sensing, within a larger range).BRIEF SUMMARY OF THE DISCLOSURE

[0008] Various aspects, embodiments and examples of the present disclosure seek to solve, mitigate or address the problem(s) indicated above. Various examples, aspects and / or embodiments of the present disclosure are provided by the appended claims. The subjectmatter of the claims is to be considered included in this section also.

[0009] According to an aspect of the present disclosure, there is provided an apparatus comprising: a first waveguide; at least one second waveguide including a coupler waveguide arranged to couple light to and from the first waveguide; a micro-electro-mechanical (MEMS) structure from which extends either the first waveguide and the coupler waveguide, or a first interaction portion of the apparatus and a second interaction portion of the apparatus; and an anchoring portion; wherein the MEMS structure is connected to the anchoring portion via a resiliently deformable element configured such that MEMS structure is movable, upon application of a force or perturbation, in a first direction to reduce or increase a first distance between the first waveguide and the first interaction portion; and wherein, upon movement of the MEMS structure: an optical characteristic of the first waveguide and / or an amount of lightin the first waveguide is changed based on a first interaction between the first waveguide and the first interaction portion according to the first distance, and an amount of light in an interaction waveguide, among the at least one second waveguide, is changed according to a second interaction between the interaction waveguide and the second interaction portion.

[0010] According to various examples, the coupler waveguide and the interaction waveguide form a single waveguide, or the coupler waveguide and the interaction waveguide are separate waveguides.

[0011] According to various examples, the first interaction portion and to the second interaction portion extend from the MEMS structure.

[0012] According to various examples, the second interaction portion is arranged such that, upon movement of the MEMS structure, the second interaction portion moves adjacent to the interaction waveguide such that an overlap between the second interaction portion and the interaction waveguide is changed.

[0013] According to various examples, a part of the interaction waveguide is disposed in parallel to the first direction such that the second interaction portion moves adjacent to the part of the interaction waveguide; and / or wherein, as the overlap increases, the first distance decreases.

[0014] According to various examples, the first interaction portion is moved towards the first waveguide upon movement of the MEMS structure in the first direction.

[0015] According to various examples, the first interaction portion comprises a surface angled towards the first waveguide such that a distance between the surface and the first waveguide changes along the length of the surface in the first direction.

[0016] According to various examples, when the first interaction portion is moved adjacent to the first waveguide in the first direction, a change in the first distance corresponds to the angle of the surface of the first interaction portion.

[0017] According to various examples, a surface of the first waveguide is angled relative to the first interaction portion such that a distance between the surface and the first interaction portion changes along the length of the surface in the first direction.

[0018] According to various examples, when the first waveguide is moved adjacent to the first waveguide in the first direction, a change in the first distance corresponds to the angle of the surface of the first waveguide.

[0019] According to various examples, the first interaction portion comprises: a first surface angled relative to the first waveguide; and a second surface extending from the first surface; wherein the second surface is angled relative to the first waveguide differently to the first surface and / or angled relative to the first surface; wherein, when the first interaction portion is moved adjacent to the first waveguide in the first direction: a second distance between the first surface and the first waveguide is increased and a third distance between the second surface and the first waveguide is decreased, or a second distance between the first surface and the first waveguide is decreased and a third distance between the second surface and the first waveguide is increased; and wherein the first distance is one of the second distance and the third distance.

[0020] According to various examples, the first interaction portion comprises: a first surface angled relative to the first waveguide; and a second surface extending from one end of the first surface; wherein the second surface is angled relative to the first waveguide differently to the first surface and / or angled relative to the first surface; and wherein, while the first interaction portion is moved adjacent to the first waveguide in the first direction: the first surface is moved alongside the first waveguide such that the first distance decreases, until the one end of the first surface is moved passed the first waveguide whereupon, as the movement in the first direction is continued, the second surface is moved alongside the first waveguide such that the first distance increases

[0021] According to various examples, the first interaction portion is moved alongside the first waveguide upon movement of the MEMS structure in the first direction.

[0022] According to various examples, the first waveguide is the second interaction portion, and the first interaction portion is the at least one second waveguide.

[0023] According to various examples, the first interaction portion includes a first surface angled relative to the first waveguide; and wherein, upon movement of the MEMS structure in the first direction, the first waveguide moves adjacent to the first interaction portion such that: the first distance is changed and, based on the change in the first distance: an amount of light coupled into the first waveguide from the first interaction portion is changed, an amount of light scattered or lost to the surrounding environment from the first interaction portion and the first waveguide is changed, and / or the optical characteristic is changed.

[0024] According to various examples, the first interaction portion includes a second surface extending from the first surface, angled relative to the first waveguide, and angled relative to the first surface; wherein, when the first waveguide is moved adjacent to the firstinteraction portion in the first direction: a second distance between the first surface and the first waveguide is increased and a third distance between the second surface and the first waveguide is decreased, or a second distance between the first surface and the first waveguide is decreased and a third distance between the second surface and the first waveguide is increased; and wherein the first distance is one of the second distance and the third distance.

[0025] According to various examples, the first waveguide is the second interaction portion; and wherein at least part of the coupler waveguide is disposed on the MEMS structure.

[0026] According to various examples, the first interaction portion has a form which alternates direction along at least a part of a length of the first interaction portion.

[0027] According to various examples, the first interaction portion is formed to have a shape of a zigzag, sawtooth or sinusoid, and / or wherein the part of the length of the first interaction portion extends in the first direction.

[0028] According to various examples, the anchoring portion is a part of a fixed substrate of the apparatus; and / or wherein the first waveguide is a ring resonator.

[0029] According to various examples, one of: the apparatus further comprises a third interaction portion arranged to extend from the MEMS structure such that: a second distance between the first waveguide and the third interaction portion increases as the first distance decreases, and the second distance decreases as the first distance increases; or the first interaction portion comprises a first part disposed the first distance from the first waveguide, and a second part disposed such that: a second distance between the first waveguide and the second part increases as the first distance decreases, and the second distance decreases as the first distance increases.

[0030] According to various examples, wherein according to the first interaction, one or more of the following is changed for the first waveguide: an effective refractive index, a resonance, an amount of light lost to a surrounding environment, an amount of light coupled into the first interaction portion, or scattering; and / or wherein according to the first interaction, the following is changed for the coupler waveguide (or the at least one second waveguide): an amount of light coupled into the coupler waveguide (or the at least one second waveguide) from the first waveguide; and / orwherein accordingto the second interaction, one or more of the following is changed for the interaction waveguide (or the at least one second waveguide): an amount of light lost to a surrounding environment, an amount of light coupled into the second interaction portion, or scattering; and / orwhereinaccording to the second interaction, the following is changed for the first waveguide: an amount of light coupled into the first waveguide from the coupler waveguide (or the at least one second waveguide).

[0031] According to various examples, the coupler waveguide and the interaction waveguide form a single waveguide; and wherein: the first interaction results in a smaller amount of light being present in the interaction waveguide (or the at least one second waveguide) for the second interaction, or the second interaction results in a smaller amount of light being present in the first waveguide for the first interaction.

[0032] According to various examples, the apparatus further comprises one or more driving elements configured to apply a driving force to the MEMS structure to: move the MEMS structure in the first direction, in an opposite direction to the first direction, or in a perpendicular direction to the first direction; or to maintain an initial position of the MEMS structure when an external force is applied to the MEMS structure.

[0033] According to various examples, the apparatus further comprises one or more detector or sensor configured to detect light output from the at least one second waveguide (e.g. one or more of the second waveguide(s)). Based on the detected light (or information on the detected light), one or more processor (either included in the apparatus or provided separately and configured to receive information from the one or more detector) is configured to detect a change in the intensity of the light detected by the one or more detectors, and, based on the detected change, identify a displacement of the MEMS structure. For example, the one or more processor may be configured to correlate the change in intensity with displacement to identify a displacement of the MEMS structure. According to various examples, light is coupled into and out of the first waveguide by the coupler waveguide (or the at least one second waveguide); wherein the apparatus further comprises: one or more laser configured to input light to the coupler waveguide and to the interaction waveguide (or the at least one second waveguide); one or more detectors configured to detect light output from the coupler waveguide and from the interaction waveguide (or from the at least one second waveguide); and one or more processor configured to: detect a change in the intensity of the light detected by the one or more detectors, and correlate the change in intensity with displacement to identify a displacement of the MEMS structure.

[0034] According to various examples, one of: detecting the change in the intensity comprises identifying a change in the spectrum of the light detected by the one or more detectors as the wavelength of the light input to the coupler waveguide and / or the interaction waveguide (or the at least one second waveguide) is swept over a range of wavelengths; thelaser is a broadband laser; or the first waveguide is a resonator and the optical characteristic is a resonance of the resonator, and the laser has a fixed wavelength set to the resonance.

[0035] According to various examples, light is coupled into and out of the first waveguide by the coupler waveguide (or the at least one second waveguide); wherein a shape formed by a section of the first interaction portion made up of the first surface and the second surface is sequentially repeated two or more times along the first interaction portion to provide a plurality of the sections; wherein the apparatus further comprises: one or more laser configured to input light to the coupler waveguide and to the interaction waveguide (or the at least one second waveguide); a first detector arranged to receive an output from the coupler waveguide and a second detector arranged to receive an output from the interaction waveguide, wherein the second interaction does not affect the output of the coupler waveguide as detected by the first detector; and one or more processor configured to: detect a first change in the intensity of the light detected by the first detector, and detect a second change in the intensity of the light detected by the second detector: identify, based on the first change, a fourth distance corresponding to a displacement of the first waveguide relative to one section of the first interaction portion made up of the first surface and the second surface; identify, based on the second change, the one section among the plurality of sections along the first interaction portion; and based on the fourth distance and the identification of the one section of the first interaction portion, determine a displacement of the MEMS structure.

[0036] According to another aspect of the present disclosure, there is provided a method of sensing a displacement using an apparatus according to any of the aspect or the examples disclosed above, the method comprising: detecting a change in the intensity of light detected by one or more detectors arranged to receive light output by the coupler waveguide and the interaction waveguide (or the at least one second waveguide), and correlate the change in intensity with displacement to identify a displacement of the MEMS structure.

[0037] According to an aspect of the present disclosure, there is provided an apparatus comprising: a first waveguide; a second waveguide arranged to couple light to and from the first waveguide; a micro-electro-mechanical (MEMS) structure from which extends either the first waveguide and the second waveguide, or a first interaction portion of the apparatus and a second interaction portion of the apparatus; and an anchoring portion; wherein the MEMS structure is connected to the anchoring portion via a resiliently deformable element configured such that MEMS structure is movable, upon application of a force or perturbation, in a first direction to reduce or increase a first distance between the first waveguide and thefirst interaction portion; and wherein, upon movement of the MEMS structure: an optical characteristic of the first waveguide and / or an amount of light in the first waveguide is changed based on a first interaction between the first waveguide and the first interaction portion according to the first distance, and an amount of light in a third waveguide is changed according to a second interaction between the third waveguide and the second interaction portion. Optionally, the second waveguide and the third waveguide are connected (directly or indirectly) to form a single waveguide. Optionally, the second waveguide functions as the coupler waveguide in any of the examples described above. Optionally, the third waveguide functions as the interaction waveguide in any of the examples described above.

[0038] Various examples of the present disclosure provide a method of manufacturing an apparatus according to any of the examples or aspects described above.

[0039] The present disclosure includes other examples, as will be appreciated from the detailed description below.BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Embodiments of the disclosure are further described hereinafter with reference to the accompanying drawings, in which:Figure 1 is a schematic illustrating an apparatus according to various examples of the present disclosure.Figure 2 is a schematic illustrating an apparatus according to various other examples of the present disclosure.Figure 3 is a schematic illustrating an apparatus according to various other examples of the present disclosure.Figure 4A is a schematic illustrating an apparatus according to various other examples of the present disclosure, and Figure 4B shows a representation of a plot of an output of Figure 4A.Figure 5A is a schematic illustrating an apparatus according to various other examples of the present disclosure, and Figure 5B shows a representation of a plot of an output of Figure 5A.Figure 6A is a schematic illustrating an apparatus according to various other examples of the present disclosure, and Figure 6B shows a representation of a plot of the output from each detector in Figure 6A.Figure 7 is a schematic illustrating an apparatus according to various other examples of the present disclosure.Figures 8A, 8B, 8C and 8D schematically illustrate waveguide interaction according to various examples.DETAILED DESCRIPTION

[0041] The evanescent field around sub-wavelength geometry waveguides can be used as a non-contact method for sensing. For displacement sensing, objects can be placed within the evanescent field, typically away from the edge of the waveguide by a distance on the order of the wavelength of light (e.g. 1 um), and the presence of the object will alter the properties of the light in the waveguide due to: scattering or other loss mechanisms, changing of the effective refractive index, and / or taking some of the light and guiding it away (‘directional coupler’).

[0042] For all the above effects, the amplitude of change is greatest when interacting with the evanescent field in a direction perpendicular to the guided mode. For example, referring to Figure 8A, as waveguide 10A gets closer to waveguide 20A, i.e. as separation 30A decreases, more light leaves waveguide 10A and enters into waveguide 20A. Arrow 12A indicates light being input and arrow 14A indicates light being output. As mentioned earlier, this may be termed ‘transverse sensing’.

[0043] Although some sensing is possible with a fixed perpendicular distance between waveguides but translational motion between waveguides in parallel to the guided mode, the signal to noise is significantly less than in the case of perpendicular interaction described above. For example, referring to Figure 8B, if waveguide 20B moves in the +x direction there is greater overlap with waveguide 10B which leads to more light leaving waveguide 10B and going to waveguide 20B; where again, arrow 12B indicates light being input and arrow 14B indicates light being output in the figure. This is because the gradient of the evanescent field is exponential in the perpendicular direction, whereas increasing the interaction length of the parallel waveguides (i.e. increasing the overlap between the waveguides) results in more of a sinusoidal or linear change in the amount of light coupled into waveguide 20B. This may be termed ‘longitudinal sensing’.

[0044] Similar effects can be seen when the ‘evanescent sensing’ region is around the edges of a ring or racetrack resonator, which is a waveguide that contains a resonant opticalfield. Resonators have an even larger signal to noise than directional coupler type waveguides due to the resonance feature.

[0045] Figure 8C, for example, illustrates a waveguide 10C coupling light into a ring waveguide 40C which couples light into waveguide 50C. As ring waveguide 40C gets closer to waveguide 50C, i.e. as separation 60C decreases, there is more of an interaction between waveguide 50C and ring waveguide 40C. For example, more light leaves ring waveguide 40C and enters into waveguide 50C. As another example, more scattering occurs in ring waveguide 40C. As yet another example, waveguide 50C has a greater effect on the refractive index of ring waveguide 40C. Arrow 12C indicates light being input and arrow 14C indicates light being output.

[0046] In contrast, referring to Figure 8D for example, as waveguide 50D moves laterally or alongside to racetrack waveguide 40D (but at a fixed distance / separation to a point on the circumference of racetrack waveguide 40D), in the +x direction shown in the figure, although more light leaves racetrack waveguide 40D to enter waveguide 50D, the effect of this movement is less than the effect of moving the racetrack waveguide 40C closer to the waveguide 50C as shown in Figure 8C. That is, the amount of light leaving racetrack waveguide 40C, 40D to enter waveguide 50C, 50D is more sensitive to movement in the direction illustrated in Figure 8C than to movement in the direction illustrated in Figure 8D.

[0047] For various examples of MEMS devices, such as inertial sensors, measurement of displacement over distances of 200nm to 10um is required or at least desirable for two reasons:• To have a large dynamic sensing range, such as to detect very small motions but also large motions that correspond to increased inertial forces. For example, detecting a robot arm performing detailed surgery but also able to capture sharp forces if the robot arm is making forceful motions. For example, for a sensitivity of 10nm per ‘g’ of acceleration, which is common, this would require a + / - 160nm of linear sensing range for + / - 16g dynamic range.• For resonant MEMS devices like gyroscopes, forced driven motion pumps energy into a mechanical structure that is converted into larger sense mode responses - the sensitivity of the sense mode therefore depends on having a large drive mode - often needing at least 5um of drive.

[0048] Sensing displacement using movement in the perpendicular direction (i.e. transverse sensing) as described above (e.g. one waveguide moving closer to or away fromanother waveguide in a perpendicular direction as illustrated in Figures 8A and 8C) is limited to a relatively small range of displacements - this is inherent to the laws of physics of evanescent fields (decay length is proportional to the wavelength of light). An example of this range is 100nm to 300nm; and, in another example, for convenience a linear readout is often preferred and this can limit sensing to < 50nm of the evanescent field due to its exponential decay length. As such, sensing by this method alone may not be enough to provide dynamic range requirements.

[0049] Sensing displacement using movement in the translational direction (i.e. longitudinal sensing) as described above (e.g. one waveguide moving alongside the other waveguide as illustrated in Figures 8B and 8D, in parallel to the evanescent field) allows for sensing in a larger range, such as up to approximately 15pm of displacement. However, sensing in this manner is not as accurate, small movements cannot be detected in the same way that they can be for movement in the perpendicular direction.

[0050] Various examples of the present disclosure combine sensing using movement in the perpendicular direction and sensing using movement in the translation (or translational direction), i.e. combine transverse sensing and longitudinal sensing as defined herein. In various examples, these sensing schemes are combined with a mechanical test-mass (or other structure) to interact with the evanescent field of PIC structures in both transverse and longitudinal directions. Specific examples are provided below. The examples disclosed herein may be used for sensing displacement, extending the sensing dynamic range and / or creating drive modes (e.g. for gyroscopes).

[0051] The present disclosure provides various apparatus (which may alternatively be termed configurations, arrangements, structures, sensors, detectors or units) in which, through movement of a MEMS structure due to application of a force or perturbation, changes occur to how light propagates through a first waveguide, into which light is being coupled from a second waveguide. These changes arise from an interaction between the second waveguide and a second interaction portion in the apparatus, and / or from an interaction between the first waveguide and a first interaction portion in the apparatus. The interaction between the second waveguide and the second interaction portion in the apparatus (which may be termed the ‘second interaction’) may be regarded as the ‘rough sensing’ (corresponding to longitudinal sensing as described above), while the interaction between the first waveguide and the first interaction portion in the apparatus (which may be termed the first interaction’) may be regarded as the ‘fine sensing’ (corresponding to transverse sensing as described above).

[0052] A feature of various examples of the present disclosure is that the rough sensing (or the rough sensing mechanism, as it may also be referred to) affects the same light path as the fine sensing (or fine sensing mechanism, as it may also be referred to) interacts with - this means the signals are intrinsically coupled together. For example, the rough sensing mechanism affects an amount of light from the second waveguide that is coupled into the first waveguide, the amount of light coupled into the second interaction portion from the second waveguide, or, to put it more generally, the total available light that can go to the second waveguide, or the amount of light output from the second waveguide; and the fine sensing mechanism affects the amount of light from the first waveguide that is coupled into the second waveguide. This is in consideration of the second waveguide losing / giving light to the second interaction portion or to outside of the second waveguide (i.e. light is not only exchanged with the second interaction portion) through coupling occurring (e.g. in the second interaction), and the first waveguide losing / giving light to the first interaction portion or to outside of the first waveguide (i.e. light is not only exchanged with the first interaction portion) through coupling occurring (e.g. in the first interaction). Additionally, in relation to or as a result of the first interaction, the first waveguide may be losing light or changing properties (e.g. optical properties or optical characteristics); an example of this is the first waveguide undergoing a change in effective refractive index causing the resonance of the first waveguide to shift (in the case of a ring resonator), which only changes the amount of light going into the ring at a given wavelength (so the resonance can stay the same shape, the resonance is just shifted so that the resonant wavelength before the first interaction portion moves closer, i.e. that previously caused the most amount of light to be exchanged to the first waveguide, is no longer the wavelength where that happens). In various examples, the second waveguide and second interaction portion may operate together similar to a directional coupler, while the first waveguide may be a ring resonator or racetrack resonator (i.e. having a shape of an oval, such as an oval comprising straight edges and rounded edges connecting corresponding ends of the straight edges).

[0053] While reference is made to a MEMS structure in the various examples disclosed herein, it will be appreciated that, more generally, a micro-mechanical structure may be used. A single MEMS structure, such as a test-mass or MEMS accelerometer structure, is movable in a given direction (e.g. achieved through appropriate coupling of the structure to an anchor).

[0054] The second interaction between the second waveguide and the second interaction portion may be a change in a relative position between the two, as may occur when thesecond interaction portion is moved alongside (e.g. moved substantially parallel with, moved to be adjacent to, passed etc.) or moved to be alongside the second waveguide to increase or decrease an overlap between the second waveguide and the second interaction portion. When there is a greater overlap between the second waveguide and the second interaction portion in the apparatus, the overall available amount of light in the second waveguide is changed. As a result, the amount of light that leaves the second waveguide and goes into the first waveguide may change correspondingly, and / or the output of the second waveguide may change correspondingly. The change in the overall available amount of light occurs whether the second interaction occurs prior to light leaving the first waveguide (i.e. coupling into the second waveguide from the first waveguide) or after light leaves the first waveguide.

[0055] As will be seen for some examples of the present disclosure, movement of the second interaction portion alongside the second waveguide does not always result in an increased overlap or a decrease to the amount of light coupled into the first waveguide from the second waveguide: in these examples, the second interaction portion or the second waveguide has a shape such that an overlap increases and decreases, e.g. alternatively, as one moves alongside the other. For example, moving the second interaction portion 1 m alongside the second waveguide may increase the overlap as more of the second interaction portion is brought closer to a portion (or protruding portion) of the second waveguide (or vice versa); where the portion may be an intersection for two (or more) surfaces of the second waveguide which are angled away from one another (e.g. in a ‘V’ shape), and may protrude towards the second interaction portion. However, continuing to move the second interaction portion in the same direction results in the overlap decreasing as the second interaction portion is moved away from the portion of the second waveguide (or vice versa). In other words: initially, the second interaction portion and / or the second waveguide are moved alongside each other such that more of the length of the second waveguide is brought close to the second interaction portion; following this, continuing the movement causes more of the length of the second waveguide to be moved away from the second interaction portion. Such examples may therefore include the feature that a longitudinal motion induces a change in the transverse coupling.

[0056] Herein, the term “overlap” is being used to refer to an amount of on object that is alongside another object. For example, if a part of the second interaction portion is overlapping with the second waveguide, this means that the part of the second interaction portion is positioned alongside the second waveguide. In another example, if a part of the second interaction portion is not overlapping the second waveguide, this means that the partof the second interaction portion is not positioned alongside the second waveguide. Briefly referring to Figure 1 , in one example, the part 114a of second interaction portion 114 shown with a crosshatch pattern may be regarded as overlapping second waveguide 130. In various examples, a part of the second interaction portion is only regarded as overlapping the second waveguide if the part is having an effect (e.g. a measurable effect, discernible from background noise; or an effect of a certain magnitude) on light propagation or the properties of light within the second waveguide. In various examples, the amount of light coupled from the second waveguide into the first waveguide and / or the output of the second waveguide also depends on the distance between the second interaction portion and the second waveguide. As such, increasing the overlap may also (e.g. additionally or alternatively) be regarded as reducing the distance between two objects, such as between the second interaction portion and the second waveguide. In some examples, a part of one object is said to overlap another object only if the part of one object is both alongside the other object and within a certain distance of the other object.

[0057] The first interaction between the first waveguide and the first interaction portion may be, involve or be dependent on a change in a distance between the two; in particular, the change may occur while the first interaction portion is within the evanescent field of the first waveguide (e.g. within the evanescent field to an extent where a change in the distance between the first waveguide and the first interaction portion has a measurable effect on the evanescent field, on the optical characteristics of the first waveguide, and / or on the properties of light in the first waveguide) or while the first interaction portion is within a distance of 1 pm (or approximately so) from the edge of the first waveguide. When the first interaction portion moves in such a location, a change(s) occurs in the optical characteristics of the first waveguide, which may in turn affect the propagation or properties of light in the first waveguide. For example, an effective refractive index of the first waveguide changes, scattering or other loss mechanisms of the first waveguide are changed, and / or some of the light in the first waveguide may be coupled to the first interaction portion.

[0058] In various examples, the first waveguide and the second waveguide are separate to the first interaction portion and the second interaction portion, with the first and second interaction portions being connected to a MEMS structure and the first and second waveguides not being attached to the MEMS structure (i.e. the MEMS structure can move independently of the first and second waveguides).

[0059] In various examples, the second waveguide acts as, or is, the first interaction portion, e.g. the second waveguide moves in the evanescent field of the first waveguide suchthat a change in the optical characteristics of the first waveguide occur. Additionally, the first waveguide may act as, or be, the second interaction portion, e.g. the first waveguide moves alongside the second waveguide to change an amount of light coupled into the first waveguide from the second waveguide. In an arrangement where the second waveguide comprises one or more protruding portion protruding in a direction perpendicular to a movement direction of the first waveguide, then movement of the first waveguide alongside the second waveguide will result in a change in the distance between the two as the first waveguide moves passed the protruding portion(s).

[0060] The first waveguide and the second waveguide may be PIC waveguides. The first waveguide and the second waveguide have sub-wavelength dimensions, meaning there exists for each an evanescent field which, as described above, allows interaction to moving objects (such as an interaction portion extending from a MEMS structure) without requiring contact. The motion of an object can affect different types of PIC waveguide in different ways. For example, with an optical resonator (e.g. ring resonator, as used in various examples herein), the signal to noise is higher due to the resonance condition of the resonator - the light circulating within the resonator can be affected by motion of the object. For a bus waveguide that supplies light to the resonator, the motion of the object can also affect the signal to noise but to a lesser extent.

[0061] Figure 1 schematically illustrates an apparatus 100 according to various examples of the present disclosure.

[0062] The apparatus 100 comprises MEMS structure 110, first interaction portion 112, second interaction portion 114, first waveguide 120, second waveguide 130, spring 140, and anchoring portion 150. Arrow 132 indicates a direction light is input to or propagates within the second waveguide 130. Arrow 134 indicates a direction light is output from or propagates within the second waveguide 130.

[0063] MEMS structure 110 is connected to anchoring portion 150 via spring 140. It will be appreciated that a spring is simply one example of a resiliently deformable element, that is, an element which can be used to connect MEMS structure 110 to anchoring portion 150 while allowing for movement of MEMS structure 110 relative to anchoring portion 150. As an alternative to a spring, a connector having a suitable degree of elasticity could be used. Therefore, while reference is made to a spring in relation to the component connecting anchoring portion 150 with MEMS structure 110, this should not be seen as limiting and, more generally, a resiliently deformable element is used to attach anchor 150 to MEMS structure 110.

[0064] Sp ring 140 may allow for movement along one axis, i.e. forwards or backwards in a single direction (i.e. in one dimension). Movement in different directions or dimensions may be prohibited or constrained, for example, by spring 140 being configured to be rigid or inflexible in directions perpendicular to the direction in which spring 140 extends. In various examples, spring 140 allows for movement in other directions / dimensions; and in various cases of this, another resiliently deformable element(s) links MEMS structure 110 to another anchoring portion (or a different part of anchoring portion 150) to allow for movement of MEMS structure 110 in another direction perpendicular to that mentioned above (i.e. this configuration allows for two dimensional movement of MEMS structure 110).

[0065] Although a single resiliently deformable element is shown in the form of spring 140, it will be appreciated that a plurality of resiliently deformable elements may be used to link anchoring portion 150 and MEMS structure 110 with MEMS structure 110 being able to move as described.

[0066] In various examples, when the apparatus 100 is at rest or when an unbalanced force is not acting on the MEMS structure 110 (e.g. at rest, or at an equilibrium position) the spring 140 allows for movement in one direction along the axis. For example, initially, the spring 140 may only allow the MEMS structure 110 to move away from anchoring portion 150 or may only allow the MEMS structure 110 to move towards anchoring portion 150. The former may be achieved by the spring 140 being fully compressed when an unbalanced force is not acting on the MEMS structure 110, such as when the apparatus 100 is at rest. Driving forces may also be used / applied to achieve this.

[0067] First interaction portion 112 is connected to MEMS structure 110, and second interaction portion 114 is connected to MEMS structure 110. In Figure 1 , the first and second interaction portions 112, 114 are shown extending from different sides of MEMS structure 110. This is not to be seen as limiting, a similar disposition of first and second interaction portions 112, 114 could be achieved having both extend from the same side of MEMS structure 110 (or to different sides to those shown in Figure 1 ) with one of the interaction portions 112, 114 having an appropriately shaped connection linking it to MEMS structure 110 such that it is positioned as shown in Fig. 1 .

[0068] Although the first and second interaction portions 112, 114 may at times be described as being connected to, linked to or attached to MEMS structure 110, it will be understood that MEMS structure 110 and the first and second interaction portions 112, 114 may be formed from the same whole, e.g. from a MEMS layer which is processed to provide MEMS structure 110 and the first and second interaction portions 112, 114 as opposed toseparately connecting the interaction portions 112, 114 to the MEMS structure 110. Of course, in various examples, the interaction portions 112, 114 are formed separatelyto MEMS structure 110 and then connected to MEMS structure 110.

[0069] Although the term “interaction portion” is used herein, it will be appreciated that these components, i.e. the first interaction portion and the second interaction portion, could instead be termed interaction regions, interaction elements, interactors or manipulators, or more generally regarded as a first portion and a second portion (e.g. first and second portions of a MEMS layer, first and second portions of the MEMS structure 110, or first and second portions connected to the MEMS structure).

[0070] The first and second interaction portions 112, 114 may be attached to MEMS structure 110 by fixed connections, e.g. connectors which are not intended to change shape (unlike spring 140) or to allow movement relative to the MEMS structure 110. It is intended that motion of the MEMS structure 110 results in corresponding motion of the first and second interaction portions 112, 114. For example, the first and second interaction portions 112, 114 are integral with MEMS structure 110.

[0071] The MEMS structure 110 may be formed of a crystalline or polycrystalline material, such as silicon, polysilicon glass, silicon carbide or silicon nitride, to give some non-limiting examples. In various examples, the first interaction portion 112 is formed of a crystalline or polycrystalline material, such as silicon, polysilicon glass, silicon carbide or silicon nitride, to give some non-limiting examples. In a case where the first interaction portion 112 is formed from a same MEMS layer as the MEMS structure 110, the first interaction portion 112 and the MEMS structure 110 will be formed of the same material. The same description applies for the second interaction portion 114. The first interaction portion 112 and the second interaction portion 114 may be formed differently, and may be formed of different materials. For example, the first interaction portion 112 is formed integrally with MEMS structure 110, but the second interaction portion 114 is formed separately and of a different material, and is then connected to the MEMS structure 110.

[0072] The first interaction portion 112 is positioned relative to the first waveguide 120 such that movement of the MEMS structure 110 in or opposite to the direction in which the spring 140 extends (referred to as a first direction, e.g. being a direction, or directions, in which movement is supported by the spring 140) causes the first interaction portion 112 to move towards or away from the first waveguide 120. The disposition of the MEMS structure 110, the first interaction portion 112 and / or the first waveguide 120 within the apparatus 100 may be determined based on allowing for movement of the first interaction portion 112 towardsand / or away from the first waveguide 130. The distance 164 between the first interaction portion 112 and the first waveguide 120 changes as the first interaction portion 112 (or the MEMS structure 110 linked thereto) moves.

[0073] The spring 140 may be configured (e.g. manufactured or selected) such thatthe first interaction portion 112 is not capable of touching or contacting the first waveguide 120. For example, distance 164 cannot decrease reach zero. For example, in a case where, at rest or in the absence of an unbalanced force, the spring 140 does not allow for movement of the MEMS structure 110 towards the anchoring portion 150; upon application of a suitable force the spring 140 may allow the MEMS structure 110, and so the first interaction portion 112, to move towards the first waveguide 120 but not so far as to allow the first interaction portion 112 to contact the first waveguide. In various examples, the first interaction portion 112 is prevented from being able to touch or contact the first waveguide 120 through application of a driving force to the MEMS structure 110, e.g. to at least partially counteract a force (e.g. an external force) which moves the MEMs structure 110 towards the first waveguide 120. In other examples, a stopper or hard stop is provided which prevents the first interaction portion 112 from contacting the first waveguide 120, such as by impeding further movement of the MEMS structure 110 or the first interaction portion 112.

[0074] In various examples of the apparatus 100 of Figure 1 , the MEMS structure 110 is initially (e.g. when the apparatus 100 is at rest, or the MEMS structure 110 is not subject to an unbalanced force) located such that the first interaction portion 112 is adjacent to, but noncontiguous with, the first waveguide 120. Here, the first interaction portion 112 is within the evanescent field of the first waveguide 120, e.g. within 1 m, 500nm or even less, of the edge of the first waveguide 120. Movement of the first interaction portion 112 towards the first waveguide 120 is not allowed in this state, or at least is constrained to avoid contact between the first waveguide 120 and the first interaction portion 112. That is, the spring 140 is configured (e.g. through choice of a spring with appropriate tension or spring constant) to constrain further extension of the spring 140, but allow contraction of the spring 140.

[0075] Accordingly, in this configuration, movement of the apparatus 100 or, more specifically, of the MEMS structure 110 away from the first waveguide 120 can be sensed with a high degree of accuracy (this may be referred to as fine sensing herein). This is because even a small movement of the first interaction portion 112 away from the first waveguide 120 (a small increase in distance 164) will cause an interaction with the evanescent field (e.g. exponential change in the strength of the coupling between the first waveguide 120 and the first interaction portion 112), prompting a measurable change in the behaviour of light in thefirst waveguide 120 (or output from the second waveguide 130, following coupling of light from the first waveguide 120 back into the second waveguide 130). By measuring or identifying this change, a determination (or calculation, identification, sensing etc.) of the displacement can be performed. Once the first interaction portion 114 moves further away from the first waveguide 120, i.e. outside of the evanescent field or where the evanescent field tails off, it is no longer possible to perform the fine sensing as any interaction between the first interaction portion 112 and the evanescent field will be indistinguishable from the background noise or loss mechanisms. At which point, the rough sensing mechanism may be used, i.e. the rough sensing mechanism dominates the output signal. Accordingly, in this configuration, the fine sensing mechanism can be used immediately, as the first interaction portion 112 is initially (e.g. when the MEMS structure 110 is at rest or not accelerating) sufficiently close to the first waveguide 120 to interact with it via the evanescent field. This may be beneficial for sense modes: it is useful to have the finer resolution of sensing in relation to small motions about zero (i.e. about a rest or initial position), while sensing of larger motions may not typically require as fine a resolution.

[0076] The rough sensing mechanism relates to the second interaction, as described above. Here, as the MEMS structure 110 moves away from the first waveguide 120 (e.g. in a first direction, being a direction directly away from the first waveguide 120 or the direction in which the spring 140 extends from the MEMS structure 120), the second interaction portion 114 moves alongside the second waveguide 130 to change overlap 162.

[0077] With reference to the figure - e.g. the relative disposition of the MEMS structure 110, the spring 140, the second waveguide 130 and the second interaction structure 114, and also the shape of the second waveguide 130 - it can be seen that the description of the second interaction portion 114 moving alongside the second waveguide 130 may be intended to mean that the second interaction portion 114, while maintaining a fixed distance from an edge of the second waveguide 130, moves along the length of the second waveguide 130 or a portion thereof (e.g. moves in a direction parallel to the longitudinal direction or long axis of the second waveguide 130). Where the second waveguide 130 is not straight over its entire length (e.g. noting there is a curve in the second waveguide 130 in Figure 1 ), it will be understood that the second waveguide 130 has a straight (or substantially straight) portion alongside which the second interaction portion 114 moves. It will be understood that, in various examples, the description of the second interaction portion 114 moving alongside the second waveguide 130 may be intended to mean that the second interaction portion 114 moves from a position below, and laterally offset from, the second waveguide 130 to bebrought alongside the second waveguide 130, i.e. the second interaction portion 114 moves upwards and, in doing so, may enter a plane of the second waveguide 130 such that an overlap may be said to occur between the two (or, to consider another way, is brought closer to the second waveguide 130, where a minimum distance (i.e. at least some distance) exists between the second interaction portion 114 and the second waveguide 130 when the two are in the same plane). In other words, examples of the present disclosure include cases where the second interaction portion 114 moves laterally with respect to the second waveguide 130, to increase or decrease the overlap between the two, and cases where the second interaction portion 114 moves vertically with respect to the second waveguide 130, to increase of decrease the overlap between the two. Of course, the present disclosure also includes examples combining these two cases.

[0078] As the second interaction portion 114 moves (e.g. towards or away from the location of the first waveguide 120), the overlap 162 (as defined earlier) between the second interaction portion 114 and the second waveguide 130 changes. This may be achieved through appropriate positioning of the two features, and / or through appropriate selection of dimensions or shape for the two features. As seen in Figure 1 , the curve in the second waveguide 130 provides an edge (or corner, extent, curve, or, more generally, a point where the distance to the second interaction portion 114 will change) which the second interaction portion 114 can move beyond or passed. Also, the length of the second interaction portion 114 may be set such that, upon movement of the MEMS structure 110 from its initial or resting position, the overlap 162 changes with some of the length of the second interaction portion 114 being moved beyond the edge or corner of the second waveguide 130.

[0079] The amount of light lost from the second waveguide 130 (i.e. the second interaction referred to earlier), such as the amount of light coupled into the second interaction portion 114 from the second waveguide 130 or the amount of light lost to the environment, is based on (i.e. depends on) the amount of overlap 162 between the second interaction portion 114 and the second waveguide 130. This, in turn, may affect the amount of light coupled into the first waveguide 120 from the second waveguide 130. When the entire length of the second interaction portion 114 overlaps with the second waveguide 130, such as may be the case when the MEMS structure 110 is at its initial position, coupling between the two is at its greatest. As the MEMS structure 110 moves away from the first waveguide 120, the second interaction portion 114 moves alongthe length of the second waveguide 130 (i.e. the straight portion shown in Figure 1 ), keeping the same distance, thereby reducing the size of overlap 162. The coupling effect therefore reduces as the second interaction portion 114 moves,meaningthe amount of light coupled into the second interaction portion 114 is reduced. Compared to when the MEMS structure 110 is at its initial position, the amount of light coupled into the first waveguide 120 from the second waveguide 130 increases as the second interaction portion 114 moves to reduce the size of overlap 162. This change in the amount of light coupled from the second waveguide 130 into the first waveguide 120 is measurable, and allows for the determination (or identification, calculation, sensing etc.) of the displacement of the second interaction portion 114, MEMS structure 110 and / or apparatus 100. In view of the difference between rough sensing mechanism and fine sensing mechanism, this determination may not be as precise as the determination of displacement based on the movement of the first interaction portion 112 away from the first waveguide 120.

[0080] In various examples of the apparatus 100 of Figure 1 , the MEMS structure 110 is initially (e.g. when the apparatus 100 is at rest, or when the MEMS structure 110 is not subject to an unbalanced force) located such that the first interaction portion 112 is adjacent to or nearby, but non-contiguous with, the anchoring portion 150, or is otherwise located away from the first waveguide 120. The extent to which the first interaction portion 112 is located away from the first waveguide 120 may be set based on a distance the second interaction portion 114 can move before it entirely overlaps with the second waveguide 130, or on a maximum distance which the first interaction portion 112 is permitted to move (bearing in mind the constraint that the first interaction portion 112 should not be able to contact the first waveguide 120).

[0081] As such, in these examples, the first interaction portion 112 is initially located welloutside of the evanescent field of the first waveguide 120, such that there is effectively no coupling between the two. In this configuration, the MEMS structure 110 is moveable towards the first waveguide 120 from its initial position, and may be constrained or prevented from moving further from the first waveguide 120. In other examples, the MEMS structure 110 may be able to move both towards and away from the first waveguide 120 from its initial (e.g. at rest) position, but fine sensing would only be possible for movements towards the first waveguide 120 once the first interaction portion 112 to suitably close to the first waveguide 120 (e.g. within or interacting with the evanescent field of the first waveguide 130.

[0082] Accordingly, in examples such as this, rough sensing occurs or may be performed first, with fine sensing occurring (e.g. able to be performed) only if the MEMS structure 110 is moved such that the first interaction portion 112 is brought sufficiently close to the first waveguide 120. Moving the MEMS structure 110 to achieve this can result from an external force being applied to the apparatus 100, or by applying a driving force to the MEMS structure110 (e.g. using one or more driving element, which may drive the MEMS structure 110 using a / an electromagnetism, induction, electrostatic, optical, thermal and / or piezoelectric force). Accordingly, the fine sensing mechanism can be accessed by driving the MEMS structure 110 to move the first interaction portion 112 closed to the first waveguide. Figure 7, described below, illustrates an apparatus 700 related to apparatus 100 (in terms of having some similar components), where apparatus 700 includes additional refinements facilitating sensing of a drive mode / signal.

[0083] It will be appreciated that applying a driving force to the MEMS structure 110 may effectively allow for switching between a configuration where the first interaction portion 112 is initially located next to the first waveguide 120 and a configuration where the first interaction portion 112 is initially located away from the first waveguide. Accordingly, various examples of the present disclosure use a driving force to allow for sensing a displacement of the apparatus 100 or MEMS structure 110 in either direction along the axis correspondingthe direction along which the spring 140 extends (i.e. in the first direction or a direction opposite to the first direction), by moving the MEMS structure 110 to a position relative to the first waveguide 120 such that application of an external force then moves the MEMS structure 110 from this position. In other words, it is not the case that displacement in only a single direction can be detected when the rest / initial position of the first interaction portion 112 is next to the first waveguide 120, for example, but rather the MEMS structure 110 can be driven (e.g. through repulsion or attraction) such that the rest / initial position of the first interaction portion 112 is moved away from the first waveguide 120 to then allow for movement towards the first waveguide 120 upon application of an appropriate external force.

[0084] It will be understood that the shape of the various components shown in Figure 1 is not to be seen as limiting. The second waveguide 130 may have a different shape, e.g. more curves or corners, so long as a portion of its length allows for the second interaction portion 114 to move alongside it to change the overlap 162 while a distance between the two remains fixed (or substantially so). The first waveguide 120 may be a ring resonator, and may have a shape of a circle, oval or racetrack.

[0085] For all examples disclosed herein, the second waveguide may be regarded as at least one second waveguide (e.g. a single second waveguide, or a plurality of second waveguides). In the case of there being one second waveguide, as in Figure 1 , this second waveguide 130 both couples light into and out of the first waveguide 120 and interacts with the second interaction portion 114 (i.e. is involved in the second interaction). In such a case, the single second waveguide may be regarded as including an interaction waveguide and acoupler waveguide, where the interaction waveguide and the coupler waveguide together form the second waveguide (e.g. the two form a continuous waveguide, being parts / portions of the single second waveguide). The interaction waveguide interacts with the second interaction portion (i.e. the interaction waveguide is the part of the second waveguide that interacts with (or is involved mainly in the interaction with) the second interaction portion), while the coupler waveguide couples to / from the first waveguide (i.e. the coupler waveguide is the part of the second waveguide that couples light into / from (or is involved mainly in the coupling of light into / from) the first waveguide). In the case of there being two (or more) second waveguides, the second waveguides may include a coupler waveguide and an interaction waveguide separately. In otherwords, the coupler waveguide may be distinct from the interaction waveguide, yet togetherthe couplerwaveguide and the interaction waveguide perform the function(s) of the second waveguide described above or elsewhere herein. Therefore, although various embodiments of the present disclosure illustrate a single second waveguide, this should not be seen as limiting. In other examples, two or more second waveguides may be provided separately, to provide a coupler waveguide and an interaction waveguide as described above.

[0086] Figure 2 schematically illustrates an apparatus 200 according to various examples of the present disclosure.

[0087] The apparatus 200 comprises MEMS structure 210, first interaction portion 212, second interaction portion 214, first waveguide 220, second waveguide 230 (or at least one second waveguide 230, in a non-illustrated alternative arrangement where separate coupler waveguide and interaction waveguide are provided), spring 240, and anchoring portion 250. Arrow 232 indicates a direction light is input to or propagates within the second waveguide 230. Arrow 234 indicates a direction light is output from or propagates within the second waveguide 230. Note that the direction of light can be reversed according to various examples, in which case the second interaction does not affect the coupling of light between the first and second waveguides 220, 230 but still affects the output of the second waveguide 230 (which would correspond to arrow 232 in this case) - the same applies for the examples given in the other figures.

[0088] In general, the description given for a feature in relation to Figure 1 may be applied to a corresponding feature in Figure 2, unless indicated otherwise or there is a difference between said features. For example, there is a correspondence between: MEMS structures 110 and 210; first waveguides 120 and 220; second waveguides 130 and 230; second interaction portions 114 and 214; springs 140 and 240; and anchoring portions 150 and 250.For example, one of these components in Figure 2 has a similarfunction to the corresponding component in Figure 1 .

[0089] As can be seen, a difference between Figure 2 and Figure 1 is the shape of the first interaction portion 212 and its position relative to the first waveguide 220.

[0090] The first interaction portion 212 is positioned such that, when the MEMS structure 210 moves in the first direction (e.g. in the direction the spring extends from the anchoring portion 240), the first interaction portion 212 will move closer to the first waveguide 220 but is capable of passing alongside the first waveguide 220. That is, the first interaction portion 212 would not collide with the first waveguide 220, it is offset from the first waveguide 220 in a direction perpendicular to the first direction.

[0091] First interaction portion 212 has an angled surface 212a. First interaction portion 212 may itself be regarded as a protrusion angled relative to a surface of the first waveguide 220. The angled surface 212a can be considered to be angled with respect to: the surface of the first waveguide 220, or a closest point thereon; and / or to one or more other surfaces of the first interaction portion 212.

[0092] The distance between the point of the first interaction portion 212 farthest from the MEMS structure 210, e.g. the apex of the first interaction portion 212, and the first waveguide 220 may be set such that, when the MEMS structure 210 is at its initial or resting position, the first interaction portion 212 is sufficiently far from the first waveguide 220 such that coupling does not occur between the two (i.e. where the evanescent field of the first waveguide 220 is very weak). This may correspond to distance 266 in Figure 2.

[0093] As the first interaction portion 212 moves due to movement of the MEMS structure 210, it moves towards the first waveguide 220 but offset from the first waveguide 220. As such, the apex moves passed the first waveguide 220, followed by the angled surface 212a. As the angled surface 212a angles towards the first waveguide 220 from the apex, the angled surface 212a will get progressively closer to the first waveguide 220 as the first interaction portion 212 continues to move in the first direction. Eventually, the point or edge at the opposite end of the angled surface 212a to the apex is brought alongside the first waveguide 220 or otherwise becomes the closest part of the first interaction portion 212a to the first waveguide 220. That is, distance 262 is less than distance 266, and distance 262 is a smallest distance the first interaction portion 212 can be from the first waveguide 220. Distance 262 may be set by manufacturing limitations, e.g. to avoid damaging components during manufacture or to prevent (or at least reduce the chances of) the first interactionportion 212 from contacting the first waveguide 210. Between at least some of distance 266 and distance 262, the first interaction portion 212 may measurably affect the evanescent field of the first waveguide 220.

[0094] Accordingly, moving through length 268 of the angled surface in the direction of motion allows for exploiting of the fine sensing mechanism. In this manner, fine sensing can be performed for small motions, upon to length 268. For example, if length 268 is 2pm, and if the difference between distance 266 and distance 262 is approximately 1 m (with distance 262 itself being very small, to reflect close proximity to the first waveguide 220), it will be possible to perform fine sensing for 2pm of movement of the MEMS structure 210. For displacements greater than 2pm, the rough sensing mechanism will allow for their sensing through movement of the second interaction portion 214. As discussed in relation to Figure 1 , movement of the second interaction portion 214 in the first direction will increase the overlap 272 with the second waveguide 230, changing an amount of light coupled between the two and therefore changing the amount of light coupled from the second waveguide 230 into the first waveguide 220 (and vice versa). One way to describe the configuration of Figure 2, i.e. using a slanted surface / protrusion on / as the first interaction portion 212, is as a deamplification (e.g. compared to the apparatus of Figure 1 ), that lies between pure ‘fine sensing’ versus ‘rough sensing’. That is, there is a trade-off in that some of the measurement is now de-amplified; to refer to the example above, sensing 1 pm change via the transverse sensing mechanism represents 2pm of the actual motion / displacement of the MEMS structure 210. That is, when there is a first interaction the effects of which correspond to a 1 pm change in how close the first interaction portion 212 is from the first waveguide 220, this can be understood to indicate a 2pm change in the position of the MEMS structure 210.

[0095] An advantage of the use of an angled surface 212a in the first interaction portion 212 is that a length 268 can be chosen in which fine sensing is possible, where angle 0 and distances 262 and 266 and the initial position of the first interaction portion 212 are then selected to support this length 268. Fine sensing is possible for displacements of magnitude less than this length 268, and for larger displacements the rough sensing mechanism takes over. Also, as explained above, a smaller change in how close the first interaction portion 212 is, as sensed by the fine sensing, can be used to determine a larger actual change in the position of the MEMS structure 210. A potential limiting factor for this arrangement is that, the smaller the angle 0 becomes, the less feasible it is to fabricate or etch the first interaction portion 212 - i.e. in manufacturing it can be difficult to resolve a very small slant.

[0096] Figure 3 schematically illustrates an apparatus 300 according to various examples of the present disclosure.

[0097] The apparatus 300 comprises MEMS structure 310, first interaction portion 312, second interaction portion 314, first waveguide 320, second waveguide 330 (or at least one second waveguide 330, in a non-illustrated alternative arrangement where separate coupler waveguide and interaction waveguide are provided), spring 340, and anchoring portion 350. Arrow 332 indicates a direction light is input to or propagates within the second waveguide 330. Arrow 334 indicates a direction light is output from or propagates within the second waveguide 330.

[0098] In general, the description given for a feature in relation to Figure 1 may be applied to a corresponding feature in Figure 3, unless indicated otherwise or there is a difference between said features. For example, there is a correspondence between: MEMS structures 110 and 310; second waveguides 130 and 330; first interaction portions 112 and 312; second interaction portions 114 and 314; springs 140 and 340; and anchoring portions 150 and 350. For example, one of these components in Figure 3 has a similarfunction to the corresponding component in Figure 1 .

[0099] One difference between apparatus 300 and apparatus 100 is that first waveguide 320 is angled relative to the first interaction portion 312. Essentially, this creates a similar configuration to that of Figure 2, except there the first interaction portion 212 includes an angled surface 212a which allows for a distance between the first interaction portion 212 and the first waveguide 220 to change as the first interaction portion 212 is moved alongside the first waveguide 220 through movement of the MEMS structure 210 in the first direction. In contrast, in Figure 3 the first waveguide 330 is angled or arranged at an angle with respect to the first interaction portion 312 such that, as the first interaction portion 312 is moved with motion of the MEMS structure 310 away from the anchoring portion 350, the first interaction portion 312 passes alongside the first waveguide 310 with a distance between the two decreasing. The minimum distance between the two is shown by distance 364. Similar to distance 262 in Figure 2, distance 364 may be set in consideration of a manufacturing tolerance or capability, or to prevent damage between components.

[0100] Similar to Figure 2, when the MEMS structure 310 is at its initial position, the distance between the first interaction portion 312 and the first waveguide 320 may be such that there is effectively no coupling between the two (e.g. approximately 1 pm) - e.g. the first interaction does not occur, or at least occurs only so weakly as to be negligible. This may correspond to when the gap between the first interaction portion 312 and the first waveguide320 is distance 366. Upon displacement of the MEMS structure 310 in the direction the spring 340 extends from the anchoring portion 350 (that is, in a direction in which MEMS structure 310 is permitted to move, or a sensing direction), first interaction portion 312 will move alongside the first waveguide 320 (e.g. in the first direction) such that a distance between the first interaction portion 312 and the first waveguide 320 will reduce due to the angle of the first waveguide 320 relative to the first interaction portion 312. This reduction will continue as the MEMS structure 320 continues to move, until the first interaction portion 312 is distance 364 from the first waveguide 320. Up to this point, the fine sensing mechanism can be used to sense the displacement of the MEMS structure 310. That is, upon movement of the MEMS structure 310 from its initial position, the distance between the first interaction portion 312 and the first waveguide 320 reduces such that the first interaction occurs through the first interaction portion 312 moving further into the evanescent field of the first waveguide 320. As described above, this exponentially increases the coupling effect between the two, allowing for fine measurement of the change in the amount of light coupled into the second waveguide 330 from the first waveguide 320 and so output from the second waveguide 320. From this measurement, fine sensing, or measurement, of the displacement can be performed.

[0101] As in the examples of Figure 2, further movement of the MEMS structure 320 can then be sensed using the rough sensing mechanism, based on the overlap 362 between the second interaction portion 314 and the second waveguide 330. It should be noted that, as indicated in Figure 3, a shape of the second waveguide 330 may be determined to allow for the adjacent movement of the second interaction portion 314 and for coupling light into and out of the first waveguide 320. It will be appreciated that the first waveguide 320, the second waveguide 330, the first interaction portion 312 and the second interaction portion 314 may be arranged in the apparatus 300 such that the second interaction portion 314 can continue to move alongside the second waveguide 330, increasing the overlap 362, for a distance several times the distance needed to move the first interaction portion 312 to be the distance 364 from the first waveguide 320. In one example, the rough sensing mechanism may allow for sensing of displacement of the MEMS structure of up to 5pm, while the fine sensing mechanism may all for sensing of displacement of the MEMS structure of up to 1 pm. Here, both the first and second interaction portions 312, 314 may be capable of moving 5pm, but displacement or motion sensing after the first 1 pm of movement is only possible via the second interaction portion (i.e. rough sensing).

[0102] Figure 4A schematically illustrates an apparatus 400 according to various examples of the present disclosure.

[0103] The apparatus 400 comprises MEMS structure 410, first waveguide 420, second waveguide 430 (or at least one second waveguide 430, in a non-illustrated alternative arrangement where separate coupler waveguide and interaction waveguide are provided), spring 440, and anchoring portion 450.

[0104] In general, the description given for a feature in relation to Figure 1 may be applied to a corresponding feature in Figure 4A, unless indicated otherwise or there is a difference between said features. For example, there is a correspondence between: MEMS structures 110 and 410; first waveguides 120 and 420; second waveguides 130 and 430; springs 140 and 440; and anchoring portions 150 and 450. For example, one of these components in Figure 3 has a similar function to the corresponding component in Figure 1 .

[0105] The configuration of Figure 4A otherwise differs from that of Figures 1 to 3 in several ways, as will be described below.

[0106] A main difference is the shape of the second waveguide 430. The second waveguide 430 is shown to have a zigzag shape, in Fig. 4A. More generally, the second waveguide 430 may have a shape formed by two surfaces which intersect with one another along an edge and are angled with respect to each other and the first waveguide 420. Additional angled surfaces may then interact at another end / edge of each surface, for example to provide a zigzag shape as shown in Figure 4A. For example, the shape formed by the two surfaces may be repeated periodically (e.g. patterned).

[0107] The shape is to be selected such that, while the first waveguide 420 moves with the MEMS structure 410, a distance between the first waveguide 420 and the second waveguide 430 changes in a periodic way. For example, when the MEMS structure 410 is at its initial position, a distance between the first waveguide 420 and the second waveguide 430 corresponds to distance 466. As the MEMS structure 410 moves in the first direction, the first waveguide 420 moves alongside the second waveguide such that the distance between the two decreases until it corresponds to distance 464. This corresponds to the first interaction mentioned above in relation to Figures 1 -3, with the second waveguide 430 acting as the first interaction portion. That is, the relative movement of the first waveguide 420 and the second waveguide 430, achieved in Figure 4A by placing the first waveguide 420 on the movable / deflectable MEMS structure 410 (e.g. connecting the two), causes a distance between the first waveguide 420 and the second waveguide 430 to decrease, resulting in the second waveguide 430 being further into the evanescent field of the first waveguide 420 (and vice versa) and so more coupling between the two. This provides the fine sensing mechanism, with an exponential change in coupling as the distance between the first and secondwaveguides 420, 430 changes as the first waveguide 420 moves / is moved. Although direction of light input to / output from the second waveguide 430 is not shown in Figure 4A, it will be appreciated that light may be input to the second waveguide 430. In other examples, a coupler waveguide is provided at least partially on the MEMS structure 410 to couple light into the first waveguide 420. This may be achieved by the coupler waveguide being provided along spring 440, or by arranging a laser on anchoring portion 450 and arranging the coupler waveguide on the MEMS structure 410 to receive light from the laser and couple it into the first waveguide 410.

[0108] Continued movement of the MEMS structure 410 when the first waveguide 420 is distance 464 from the second waveguide 430 causes the distance between the first waveguide 420 and the second waveguide 430 to increase again as the first waveguide 420 continues to move alongside the second waveguide 430, until there is again distance 466 between the two.

[0109] Through knowledge of distances 464 and 466 and the distance the first waveguide 420 needs to travel to move from being distance 464 from the second waveguide 430 to be distance 466 from the second waveguide 430 (or to move from being distance 466 from the second waveguide 430 to be distance 464 from the second waveguide 430), fine sensing can be performed over the length of the second waveguide 430 because the distance between the two waveguides 420, 430 changes between a minimum and a maximum over a given length. This change in the distance corresponds to a change in the strength of the coupling between the two waveguides 420, 430, and so the amount of light coupled into the first waveguide 420 from the second waveguide 430 and into the second waveguide 430 from the first waveguide 420, thereby affecting the amount of light output from the second waveguide 430. The coupling strength may increase and decrease as the distance decreases and increases, with a similar or the same periodicity. For example, if plotted against distance moved by the first waveguide 420, the coupling strength would have a sinusoidal shape, having turning points corresponding to positions where the distance between the first and second waveguides is distance 464 or distance 466. Each turning point corresponds to a maximum or minimum of the distance between the two waveguides 420, 430.

[0110] Therefore, by measuring the output of the second waveguide 430 as the first waveguide 420 is moved alongside it by motion of the MEMS structure 410, the coupling strength can be determined and, in particular, the changes in the coupling strength as it increases and decreases. Knowing that each turning point (i.e. change from increasing to decreasing, or from decreasing to increasing) in the measured / recorded coupling strengthcan be related to a location along the second waveguide 430 where two angled surfaces intersect or meet. The distance which the first waveguide 420 needs to be moved to be next to this location (in the sense of it being alongside the first waveguide 420, or the point along the second waveguide 430 nearest to the first waveguide 420) is known. Therefore, it can be determined how many angled sections the first waveguide 420 has moved passed and thus the displacement of the first waveguide 420.

[0111] An example of this is illustrated in Figure 4B, which shows a representation of a plot 470 of distance on the ‘x’ axis against the output of the second waveguide (or, more generally, a representation of output of second waveguide 430) on the ‘y’ axis. It should be noted that the sinusoid represented on the plot 470 is merely to give an illustration of the changes in output, and not an realistic indication of how the output changes. Additionally, the locations of the peaks and troughs in relation to the points along the second waveguide 430 are merely illustrative examples: the actual position of the peaks and troughs in the output will depend on the resonance of the first waveguide 420, with the resonance shifting as the distance between the first waveguide 420 and the second waveguide 430 changes; this shift may be such that a drop in output occurs at a point along the second waveguide 430 that protrudes towards the first waveguide 420 relative to another point along the second waveguide 340 that protrudes away from the first waveguide 420, but it is also possible that the opposite case occurs or that a peak or trough occurs between these two points (i.e. it depends on the resonance).

[0112] It is assumed that the first waveguide 420 is initially level with point 430a on the second waveguide 430, i.e. point 430a is nearest on the second waveguide 430 to the first waveguide 420. This is where distance between the two waveguides is distance 464, so the output is lowest (i.e. assumed to be the lowest value in view of e.g. a shift in the resonance due to the second waveguide 430 being distance 464 from the first waveguide). This is reflected by trough 472a on plot 470, which is a turning point and corresponds to distance travelled 474a (this can be regarded as zero, if this initial state corresponds to a rest position of the MEMS structure 410).

[0113] The MEMS structure 410 then moves causing movement of the first waveguide 420 alongside the second waveguide. The changes in output (i.e. signal output from the second waveguide 430) are measured and shown on plot 470. As can be seen, as the first waveguide 420 moves from point 430a to point 430b, the output increases from 472a to 472b. Point 430b corresponds to a separation of distance 466 between the first and second waveguides 420, 430, where the effects of coupling is weaker. This is reflected by peak 472b on plot 470, whichis also a turning point because first waveguide 420 then moves towards point 430c which is distance 464 from the first waveguide 420 and so corresponds to trough 472c. Peak 472b corresponds to distance travelled 474b, while trough 472c corresponds to distance travelled 474c.

[0114] This continues as the first waveguide 420 continues to move: point 430d corresponds to peak 472d and distance travelled 474d; point 430e corresponds to trough 472e and distance travelled 474e; and point 430f corresponds to peak472f and distance travelled 474f.

[0115] Accordingly, if the output is measured and recorded such that, at point 480 on plot 470 (corresponding to point 435 along the second waveguide 430), the output becomes constant (indicating that the first waveguide 420 has stopped moving alongside the second waveguide 430, i.e. is holding position), through viewing the recorded data (e.g. the turning points in general, or the troughs or peaks), it can be seen that the first waveguide 420 has moved passed point 430e on the second waveguide so has travelled at least an amount corresponding to distance travelled 474e. The additional distance travelled after 474e can be calculated through the change in coupling strength or output between peak 472e and point 480, bearing in mind that the distance between the first and second waveguides 420, 430 is distance 464 at point 430e. Therefore, distance travelled 485 corresponding to point 480 on plot 470 and point 435 along second waveguide 430 can be calculated. In other words, the pattern of increasing and decreasing output as the first waveguide 420 moves can be used to identify a number of sections of the second waveguide 430 (e.g. a section being the shape that is repeated along the second waveguide, being a ‘V’, “<” or “>” shape in this case) that have been moved past, so knowledge of the length of each section allows for a calculation of the displacement of the first waveguide 420 for it to have moved past that number of sections.

[0116] Figure 5A schematically illustrates an apparatus 500 according to various examples of the present disclosure.

[0117] Examples according to Figure 5A are similar to those according to Figure 4A, except in Figure 5Athe first waveguide 520 is not connected to or positioned on the MEMS structure 510, but rather a first interaction portion 512a (having a shape according to that of the second waveguide 430 of Figure 4A) is linked to the MEMS structure 510 and movement of this relative to the first waveguide 510 allows for sensing of the displacement of the MEMS structure 510. Although only one second waveguide 530 is illustrated, it will be appreciatedthat, in an alternative arrangement, two or more second waveguides including separate coupler waveguide and interaction waveguide are provided.

[0118] The second waveguide 530 couples light into and out of the first waveguide 520, similar to in Figures 1 to 3.

[0119] Optionally, second interaction portion 514 is provided. The second interaction portion 514 moves with the MEMS structure 510 and, similar to the second interaction portion of Figure 1 , overlaps with a part of the second waveguide 530 to differing degrees depending on a position or movement of the MEMS structure 510. The overlap 562, i.e. of the change in the amount of light coupling from the second waveguide 530 into the second interaction portion 514, has an effect on the coupling between the first waveguide 520 and the second waveguide 530 or, more generally, an effect on the output 534 of the second waveguide 530. As the overlap increases so more light is coupled from the second waveguide 530 into the second interaction portion 514, the amount of light coupled from the second waveguide 530 into the first waveguide 520 decreases or the output of the second waveguide 530 drops. As the overlap 562 increases as the MEMS structure 510, and so also the first interaction portion 512, moves further in the first direction, this has the effect of reducing (e.g. linearly) the amount of light coupled from the second waveguide 530 into the first waveguide 520 or reducing (e.g. linearly) the output of the second waveguide 530.

[0120] An example of this is illustrated in Figure 5B, which is a plot 570 representing the change in output 570 of the second waveguide 530 (or, more generally, a measurement relating to the output from the second waveguide, e.g. at 534) based on the distance the MEMS structure 510 travels. The effect of the increasing overlap 562 is to introduce a gradient into the output 570; i.e., the output drops as the MEMS structure 510 continues to move so, while the output continues to oscillate between peaks and troughs, it is decreasing continuously as the amount of light coupled from the second waveguide 530 into the second interaction portion 514 increases.

[0121] Figure 6A schematically illustrates an apparatus 600 according to various examples of the present disclosure. Apparatus 600 comprises MEMS structure 610; first waveguide 620; second waveguide 630 (or at least one second waveguide 630, in a non-illustrated alternative arrangement where separate coupler waveguide and interaction waveguide are provided); spring 640; anchoring portion 650; first interaction portion 612; and second interaction portion 614.

[0122] Apparatus 600 is related to apparatus 500 of Figure 5A (e.g. the same description applies to corresponding components), but where the second waveguide 630 is split to output to detector 636 and detector 638. Instead of splitting the second waveguide 630, balanced detection may alternatively be used to extract the same information. It will be appreciated that the part of second waveguide 630 before the split may be regarded as a coupler waveguide, while the part of second waveguide 630 between the split and detector 638 may be regarded as an interaction waveguide.

[0123] Figure 6B illustrates an output time trace from each of detectors 636, 638, as the MEMS structure 610 moves in the first direction (e.g. vertically, or away from the anchoring portion 650). Trace 670 represents the output from detector 636, and trace 680 represents the output of detector 638. The horizontal lines running across each trace 670, 680 represent ‘bit’ steps of an analogue to digital converter (ADC), which may be used to obtain information from the detectors 636, 638 - these lines are just shown to provide further detail and need not be considered further.

[0124] As detector 636 receives output from the second waveguide 630 after the second waveguide 630 has interacted with the second interaction portion 614, the output represented by the trace 670 experiences more loss overtime (i.e. as the MEMS structure 610 continues to move) compared to the output represented by trace 680. Furthermore, in view of detectors 636, 638 being positioned at different positions along the second waveguide 630 (e.g. a different length of waveguide leads to each detector 636, 638), there is a phase difference between the outputs at each detector 636, 638, as represented by the two traces 670, 680. The difference in lengths of waveguide leading to each detector 636, 638 may be chosen to provide a specific phase difference between the outputs at each detector 636, 638.

[0125] Figure 6A shows different parts of the first interaction portion 612 labelled with a number, with each of (1), (2) and (3) correspondingto a peak on the right hand side of the first interaction portion 612. Note that, at a rest or initial position of the MEMS structure 610, the tip of the first interaction portion 612 (i.e. the point furthest from the MEMS structure 610 in the direction away from the anchoring portion 650) may be level with the first waveguide 620 (e.g. a nearest point on the first waveguide 620).

[0126] In Figure 6B, arrows 672, 674, 676 extend from parts of trace 670, and arrows 682, 684, 686 extend from corresponding parts of trace 680. This is to represent that the information from trace 670 (i.e. from the bottom plot) can be used to identify which peak (or part) of the first interaction portion 612 corresponds to the given part of trace 680 (e.g. a particular peak in trace 680). Similar to as mentioned in relation to Figure 4B, the relationshipbetween peaks and troughs along trace 670 and how they align with points (1), (2), (3) is merely to provide an illustrative example - how the peaks and troughs coincide with distance travelled by the first interaction portion 612 alongside the first waveguide 620 will depend on the optical properties / characteristics of the first waveguide 620 (e.g. its resonance).

[0127] The arrangement of apparatus 600 may be regarded as similar to a linear encoder, where each two connected surfaces of the first interaction portion 612 (e.g. for a zigzag shape, this is each ‘V’ section or portion of the zigzag shape) is similar to a rotation or act as an incremental track (i.e. the first waveguide 620 passing by one set of two connected surfaces is regarded as incrementing a counter), and the perturbation of the second waveguide 630 by the second interaction portion 614 (e.g. the change of light output to detector 636) can be used to determine which part of the first interaction portion 612 (e.g. a part of the zigzag shape is one of the points or intersections of two surfaces, i.e. the tips of the protrusions) the first waveguide 620 is level with (i.e. is nearest to), e.g. this acts as a reference track (i.e. as the second interaction portion 614 passes by the second waveguide 630, this has a known, e.g. linear, effect on the light output from the second waveguide 630 which allows for rough sensing of a displacement of the second interaction portion 614).

[0128] This may provide additional benefits for more efficient usage, or maximisation, of bit resolution, e.g. of an ADC. For example, where 16 bits are available, it is difficult to have a large dynamic range and also resolve very small or the smallest change of light. For apparatus 600, each detector 636, 638 may have 16 bits, so all 16 bits of detector 638 may be used for detecting position along one section of the first interaction portion 612 (e.g. for the case of a zigzag shape, this is along one of the ‘V’ sections) to provide fine sensing, and all 16 bits of detector 636 may be used for detecting which section of the first interaction portion 612 is level with the first waveguide 620 due to motion of the MEMS structure 610 (i.e. which section is the ‘one section’ that detector 612 is detecting the position of the first waveguide along).

[0129] For example: the 16 bits of detector 636 can be used to determine whether the first waveguide 620 is between points (1) and (2) or between points (2) and (3) as labelled in Figure 6A; the 16 bits of detector 638 can be used to determine how far between two consecutive points the first waveguide 620 (e.g. a nearest part thereof) has moved; and then this information can be combined, along with knowledge of how far the MEMS structure 610 needs to move such that point (1), point (2) or point (3) is level with the first waveguide 620, to determine a displacement of the MEMS structure 610.

[0130] In another example, if a total number, e.g. 16, of bits is provided by both detectors 636, 638, then the majority of the bits can be dedicated to the fine sensing (e.g. to the outputof detector 636) while the smaller, remaining amount of bits can be dedicated to the rough sensing (e.g. to the output of detector 638). While detectors are specifically described in the context of Figures 6A and 6B, it will be understood that any of the other embodiments and examples provided herein may also comprise one or more detector, to allow for detecting changes to light propagating in the first waveguide and the second waveguide so as to detect a change in displacement.

[0131] Figure 7 schematically illustrates an apparatus 700 accordingto various examples of the present disclosure. Apparatus 700 comprises MEMS structure 710; first waveguide 720; second waveguide 730 (or at least one second waveguide 730, in a non-illustrated alternative arrangement where separate coupler waveguide and interaction waveguide are provided); spring 740; anchoring portion 750; first interaction portion 712; second interaction portion 714; and a third interaction portion 716,

[0132] Apparatus 700 may be considered to relate to apparatus 100 of Figure 1 , in that a similar description may be applied to like components. Apparatus 700 differs from apparatus 100 in that it includes a third interaction portion 716. In various examples, third interaction portion 716 is actually part of, or combined with, the first interaction portion 712. To achieve this, first interaction portion 712 may curve around first waveguide 720 such that a part of it is also present on an opposite side of the first waveguide 730. If a separate third interaction portion 716 is provided, this extends from the MEMS structure 710 to be disposed on the opposite side of the first waveguide 720 to the first interaction portion 712.

[0133] When the MEMS structure 710 moves in either direction shown by arrow 760 (e.g. if the first direction is towards or away from the anchoring portion 750, or towards or away from the first waveguide 720 etc.), one of the first and third interaction portions 712, 716 will move closer to the first waveguide 720 (changing an amount of light coupled into said interaction portion from the first waveguide 720) while the other one of the first and third interaction portions 712, 716 will move away from the first waveguide 720 (changing an amount of light coupled into said other interaction portion from the first waveguide 720). As a result, the amount of light coupled between the first waveguide 720 and the second waveguide 730 will change (e.g. due to a change in the resonance of the first waveguide 720). To put another way, the movement of one of the first and third interaction portions 712, 716 closer to the first waveguide 720 and corresponding movement of the other one of the first and third interaction portions 712, 716 away from the first waveguide 720 will change an optical characteristic of the first waveguide 720, such as the resonance; this will in turn affect an amount of light in the first waveguide 720.

[0134] This arrangement may be particularly beneficial for a case of measuring a drive mode of a gyroscope (e.g. MEMS gyroscope), such as when a gyroscope includes the arrangement . This may be thought of as requiring accurate resolving of a sinusoidal movement, and so may be applied to other cases of this (i.e. extended to uses other than for measuring a drive mode of a gyroscope). Here, an accurate measurement at both the peak and the trough of the sinusoid (i.e. of the motion of the gyroscope) is needed. To give an example: a gyroscope may be regarded as a mechanical test-mass designed to move in two ways - a driven mode and a sense mode; this test-mass design for the gyroscope incorporates the arrangement (i.e. apparatus shown in Figure 7) . Providing an interaction portion on either side of the first waveguide 720, as shown, allows for this accurate measurement to be performed, i.e. using the fine sensing mechanism that is provided by the interaction (i.e. the first interaction) between one of the first and third interaction portions 712, 716 and the first waveguide 720 at either the peak or the trough, and using the fine sensing mechanism that is provided by the interaction (i.e. the first interaction) between the other one of the first and third interaction portions 712, 716 and the first waveguide 720 at the other one of the peak or the trough.

[0135] Accordingly, in examples corresponding to apparatus 700, it will be appreciated that applying a driving force to the MEMS structure 710 may effectively allow for switching between a configuration where the first interaction portion 712 is located next to the first waveguide 720 while the third interaction portion 716 is located further from the first waveguide 720, and a configuration where the first interaction portion 712 is located away from the first waveguide 720 while the third interaction portion is located next to the first waveguide 720. Accordingly, various examples of the present disclosure use a driving force to allow for sensing a displacement of the apparatus 700 or MEMS structure 710 in either direction along the axis corresponding the direction along which the spring 740 extends (i.e. in the first direction or a direction opposite to the first direction), by moving the MEMS structure 710 to a position relative to the first waveguide 730 such that application of an external force then moves the MEMS structure 710 from this position. The driving force may be sensed through identifying the distance between the first interaction portion 712 and the first waveguide 710 and the distance between the third interaction portion 716 and the first waveguide 710. In a case where both the first and third interaction portions 712, 716 are too far from the first waveguide 720 to have an appreciable or measurable effect on the first waveguide 720, the rough sensing allows for identifying details of the driving force.

[0136] A number of apparatus according to different examples of the present disclosure are described above. In the case of each, the output of the second waveguide may be detected and used to sense (e.g. determine, identify, calculate, detect, sense etc.) the displacement of the MEMS structure (or, more generally, the apparatus). It will be understood that this may be achieved in various different ways.

[0137] I n various examples, the sensing is performed by sweeping the laser wavelength and looking at the change of the spectrum produced. Here, the sweep is performed faster than the motion to be detected. For example, the rough sensing mechanism will create an overall change to the total amount of light (this may be thought of as the background level of the whole spectrum), whereas the fine sensing causes a shift and broadening / narrowing of the ring resonator resonance (this appears as a dip in the spectrum). In the case where a directional coupler is used instead of a ring resonator (i.e. as the first waveguide), the change in distance between the parts of the directional coupler (e.g. between the first waveguide and the first interaction portion) will produce an extra but more significant change to the spectrum intensity compared to the change produces by the change in overlap distance between the second waveguide and the second interaction portion.

[0138] In other examples, the sensing is performed using a broadband laser. This provides a reading that is related to the area under the spectrum, which can be used to sense the displacement.

[0139] In other examples, the sensing is performed is using a fixed wavelength laser where the intensity change can be identified. That is, with a ring resonator as the first waveguide, the laser wavelength could be set to the resonance of the ring resonator, and then a drop in intensity occurs as the distance between the first interaction portion and the ring resonator gets smaller. This drop is due to a change in the resonance, which in turn arises due to a change in the effective refractive index of the ring resonator (e.g. due to the first interaction, with the first interaction portion moving perpendicularly in the evanescent field of the ring resonator).

[0140] It will be appreciated that, in general terms, each method involves using intensity changes to determine the displacement. Additional processing may then be performed to correlate the change in intensity with displacement. For example, tests could be performed to detect how the intensity changes (e.g. a magnitude of the change) for a number of different displacements. This information could be stored, and then looked-up when a change in intensity is detected so as to identify a corresponding displacement. The example sensing methods given above should not be seen as limiting.

[0141] Optionally, additional processing may be performed to stabilise the laser and / or the photonics components (e.g. first waveguide, second waveguide, first interaction portion and / or second interaction portion) so as not to drift with temperature.

[0142] Optionally, additional processing may involve using a reference waveguide (e.g. a ring or racetrack waveguide) that isn’t interactingwith the MEMS structure to identify the background (e.g. background spectrum), where this can be used to subtract unwanted drifts from the output (or data processed from the output) of the second waveguide.

[0143] Various examples of the present disclosure provide an apparatus according to any of those disclosed above (e.g. as in Figure 1 , Figure 2, Figure 3, Figure 4A, Figure 5A, Figure 6A or Figure 7) in which another MEMS structure, including extending interaction portions in a similar configuration to the first and second interaction portions, is provided on another side of the first waveguide or in combination with another first waveguide to be moveable in a direction perpendicular to the first direction. The other MEMS structure is constrained to move in a different direction to the MEMS structure, e.g. in a perpendicular direction, while still in the same plane as the first waveguide (e.g., for a ring resonator first waveguide, the other MEMS structure is facing a point 90 degrees along the circumference of the ring resonator relative to a point faced by the MEMS structure). Accordingly, the apparatus of such examples is arranged to allow for displacement sensing in two dimensions (e.g. along an x- axis and a y-axis, where the first waveguide is viewed from above along a z-axis); that is, inplane sensing in two different direction is possible. In these examples, the MEMS structure and other MEMS structure may be designed to have minimum cross axis sensing.

[0144] Various examples provide an apparatus according to any of the above in which another MEMS structure (e.g. a yet additional MEMS structure, if another MEMS structure is already provided) is provided above the first waveguide (i.e. out of plane) and is capable of moving towards and away from the first waveguide. Movement of this other MEMS structure towards the first waveguide will have a similar interaction as the first interaction mentioned above, thereby allowing for fine sensing in the out of plane direction. Therefore, this may allow for sensing in three dimensions, i.e. along three axis. The second waveguide may also be modified such that movement of this other MEMS structure outside of the evanescent field of the first waveguide may be sensed via a rough sensing mechanism. For example, a grating may be provided on the second waveguide to shine light upwards towards the other MEMS structure, where this light can interact with the MEMS structure when the MEMS structure is not suitably close to the first waveguide to interact with the first waveguide.

[0145] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0146] The phrases or expressions "at least one of,", “one or more of”, and “and / or”, when used with a list of items, means that different combinations of one or more of the listed items may be used, and only one item in the list may be needed. For example, "at least one of: A, B, and C" (and “one or more of A, B and C”, and “A, B and / or C”) includes any of the following combinations: A, B, C, A and B, A and C, B and C, and A and B and C.

[0147] Herein, references to a “first step”, a “second step”, a “third step” (or similar, e.g. “operation” or “sub-process” may be used in place of “step”) are not intended to indicate a chronological order for performing the steps, but rather the terms “first”, “second” and “third” are used to distinguish the various steps.

[0148] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0149] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

CLAIMS1. An apparatus comprising: a first waveguide; at least one second waveguide including a coupler waveguide arranged to couple light to and from the first waveguide; a micro-electro-mechanical (MEMS) structure from which extends either the first waveguide and the coupler waveguide, or a first interaction portion of the apparatus and a second interaction portion of the apparatus; and an anchoring portion; wherein the MEMS structure is connected to the anchoring portion via a resiliently deformable element configured such that MEMS structure is movable, upon application of a force or perturbation, in a first direction to reduce or increase a first distance between the first waveguide and the first interaction portion; and wherein, upon movement of the MEMS structure: an optical characteristic of the first waveguide and / or an amount of light in the first waveguide is changed based on a first interaction between the first waveguide and the first interaction portion, wherein the first interaction includes a movement of the first interaction portion within an evanescent field of light propagating in the first waveguide, and an amount of light in an interaction waveguide, among the at least one second waveguide, is changed according to a second interaction between the interaction waveguide and the second interaction portion, wherein the second interaction is a relative movement of the second interaction portion adjacent to the interaction waveguide, and the change in the amount of light in the interaction waveguide corresponds to a change in an overlap between the second interaction portion and the interaction waveguide caused by the relative movement.

2. The apparatus of claim 1 , wherein the coupler waveguide and the interaction waveguide form a single waveguide, orwherein the coupler waveguide and the interaction waveguide are separate waveguides.

3. The apparatus of claim 1 or claim 2, wherein the first interaction portion and the second interaction portion extend from the MEMS structure.

4. The apparatus of claim 3, wherein the first interaction portion is moved towards the first waveguide upon movement of the MEMS structure in the first direction.

5. The apparatus of claim 4, wherein a part of the interaction waveguide is disposed in parallel to the first direction such that the second interaction portion is moved adjacent to the part of the interaction waveguide; wherein a part of the interaction waveguide is disposed perpendicular to the first direction such that the second interaction portion is moved adjacent to the part of the interaction waveguide; and / or wherein, as the overlap increases, the first distance decreases.

6. The apparatus of any one of claims 3 to 5, wherein the first interaction portion comprises a surface angled towards the first waveguide such that a distance between the surface and the first waveguide changes along the length of the surface in the first direction.

7. The apparatus of claim 6, wherein, when the first interaction portion is moved adjacent to the first waveguide in the first direction, a change in the first distance corresponds to the angle of the surface of the first interaction portion.

8. The apparatus of any one of claims 3 to 5, wherein a surface of the first waveguide is angled relative to the first interaction portion such that a distance between the surface and the first interaction portion changes along the length of the surface in the first direction.

9. The apparatus of claim 8, wherein, when the first interaction portion is moved adjacent to the first waveguide in the first direction, a change in the first distance corresponds to the angle of the surface of the first waveguide.

10. The apparatus of any one of claims 3 to 5, wherein the first interaction portion comprises: a first surface angled relative to the first waveguide; and a second surface extending from the first surface; wherein the second surface is angled relative to the first waveguide differently to the first surface and / or angled relative to the first surface; wherein, when the first interaction portion is moved adjacent to the first waveguide in the first direction: a second distance between the first surface and the first waveguide is increased and a third distance between the second surface and the first waveguide is decreased, or a second distance between the first surface and the first waveguide is decreased and a third distance between the second surface and the first waveguide is increased; and wherein the first distance is one of the second distance and the third distance.11 . The apparatus of any one of claims 3 to 5, wherein the first interaction portion comprises: a first surface angled relative to the first waveguide; and a second surface extending from one end of the first surface; wherein the second surface is angled relative to the first waveguide differently to the first surface and / or angled relative to the first surface; and wherein, while the first interaction portion is moved adjacent to the first waveguide in the first direction: the first surface is moved alongside the first waveguide such that the first distance decreases, until the one end of the first surface passes the first waveguide whereupon, as the movement in the first direction is continued, the second surface is moved alongside the first waveguide such that the first distance increases.

12. The apparatus of claim 1 or claim 2, wherein the first waveguide is the second interaction portion, and the at least one second waveguide includes the first interaction portion.

13. The apparatus of claim 12, wherein the first interaction portion includes a first surface angled relative to the first waveguide; and wherein, upon movement of the MEMS structure in the first direction, the first waveguide moves adjacent to the first interaction portion such that: the first distance is changed and, based on the change in the first distance: an amount of light coupled into the first waveguide from the first interaction portion is changed, an amount of light scattered or lost to the surrounding environment from the first interaction portion and the first waveguide is changed, and / or the optical characteristic is changed.

14. The apparatus of claim 13, wherein the first interaction portion includes a second surface extending from the first surface, angled relative to the first waveguide, and angled relative to the first surface; wherein, when the first waveguide is moved adjacent to the first interaction portion in the first direction: a second distance between the first surface and the first waveguide is increased and a third distance between the second surface and the first waveguide is decreased, or a second distance between the first surface and the first waveguide is decreased and a third distance between the second surface and the first waveguide is increased; and wherein the first distance is one of the second distance and the third distance.

15. The apparatus of claim 1 or claim 2, wherein the first waveguide is the second interaction portion; and wherein at least part of the coupler waveguide is disposed on the MEMS structure.

16. The apparatus of claim 10, claim 13, claim 14 or claim 15, wherein the first interaction portion has a form configured to alternate direction along at least a part of a length of the first interaction portion; and / orwherein the first interaction portion is formed to have a shape of a zigzag, sawtooth or sinusoid, and / or wherein the part of the length of the first interaction portion extends in the first direction.

17. The apparatus of any previous claim, wherein the anchoring portion is a part of a fixed substrate of the apparatus; and / or wherein the first waveguide is a ring resonator.

18. The apparatus of any one of claims 3 to 6, wherein one of: the apparatus further comprises a third interaction portion arranged to extend from the MEMS structure such that: a second distance between the first waveguide and the third interaction portion increases as the first distance decreases, and the second distance decreases as the first distance increases; or the first interaction portion comprises a first part disposed the first distance from the first waveguide, and a second part disposed such that: a second distance between the first waveguide and the second part increases as the first distance decreases, and the second distance decreases as the first distance increases.

19. The apparatus of any previous claim, wherein according to the first interaction, one or more of the following is changed for the first waveguide: an effective refractive index, a resonance, an amount of light lost to a surrounding environment, an amount of light coupled into the first interaction portion, or scattering; and / or wherein according to the first interaction, the following is changed for the coupler waveguide: an amount of light coupled into the coupler waveguide from the first waveguide; and / or wherein according to the second interaction, one or more of the following is changed for the interaction waveguide: an amount of light lost to a surrounding environment, an amount of light coupled into the second interaction portion, or scattering; and / or wherein according to the second interaction, the following is changed for the first waveguide: an amount of light coupled into the first waveguide from the coupler waveguide.

20. The apparatus of any previous claim, wherein the coupler waveguide and the interaction waveguide form a single waveguide; and wherein: the first interaction results in a smaller amount of light being present in the interaction waveguide for the second interaction, or the second interaction results in a smaller amount of light being present in the first waveguide for the first interaction.21 . The apparatus of any previous claim, further comprising one or more driving elements configured to apply a driving force to the MEMS structure to: move the MEMS structure in the first direction, in an opposite direction to the first direction, or in a perpendicular direction to the first direction; or to maintain an initial position of the MEMS structure when an external force is applied to the MEMS structure.

22. The apparatus of any previous claim, wherein light is coupled into and out of the first waveguide by the coupler waveguide; wherein the apparatus further comprises: one or more laser configured to input light to the coupler waveguide and to the interaction waveguide; one or more detectors configured to detect light output from the coupler waveguide and from the interaction waveguide; and one or more processor configured to: detect a change in the intensity of the light detected by the one or more detectors, and correlate the change in intensity with displacement to identify a displacement of the MEMS structure.

23. The apparatus of claim 22, wherein one of:detecting the change in the intensity comprises identifying a change in the spectrum of the light detected by the one or more detectors as the wavelength of the light input to the coupler waveguide and / or interaction waveguide is swept over a range of wavelengths; the laser is a broadband laser; or the first waveguide is a resonator and the optical characteristic is a resonance of the resonator, and the laser has a fixed wavelength set to the resonance.

24. The apparatus of claim 16, wherein light is coupled into and out of the first waveguide by the coupler waveguide; wherein a shape formed by a section of the first interaction portion made up of the first surface and the second surface is sequentially repeated two or more times along the first interaction portion to provide a plurality of the sections; wherein the apparatus further comprises: one or more laser configured to input light to the coupler waveguide and to the interaction waveguide; a first detector arranged to receive an output from the coupler waveguide and a second detector arranged to receive an output from the interaction waveguide, wherein the second interaction does not affect the output of the coupler waveguide as detected by the first detector; and one or more processor configured to: detect a first change in the intensity of the light detected by the first detector, and detect a second change in the intensity of the light detected by the second detector: identify, based on the first change, a fourth distance corresponding to a displacement of the first waveguide relative to one section of the first interaction portion made up of the first surface and the second surface; identify, based on the second change, the one section among the plurality of sections along the first interaction portion; and based on the fourth distance and the identification of the one section of the first interaction portion, determine a displacement of the MEMS structure.

25. A method of sensing a displacement using an apparatus according to any of claims 1 to 21 , the method comprising: detecting a change in the intensity of light detected by one or more detectors arranged to receive light output by the coupler waveguide and the interaction waveguide, and correlate the change in intensity with displacement to identify a displacement of theMEMS structure.

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