Optical displacement sensor

Abstract

This invention provides an optical displacement sensor for use in optical microphones. [Solution] The optical displacement sensor 2 comprises a reflective surface 4 and one or more diffraction gratings 6, each defining an interference configuration together with the reflective surface. Light from the light source 8 propagates through the interference configuration, generating interference patterns in each set of photodetectors 10. Each interference pattern is determined according to the separation between the reflective surface 4 and each grating 6. The collimated optical configuration 14 at least partially collimates the light between the light source 8 and the diffraction gratings 6. The optical displacement sensor may comprise two or more diffraction gratings and may be configured to provide each separated light beam to each grating using a beam separation configuration or multiple light source elements.

Description

Technical Field

[0001] The present invention relates generally to optical displacement sensors, and more particularly, but not exclusively, to optical displacement sensors for use in optical microphones.

[0002] Microphones are typically used to convert sound waves into electrical signals by measuring the displacement of a movable member (e.g., a membrane) that vibrates in response to ambient acoustic vibrations. To measure the displacement of such a movable member, there are a number of ways, including capacitive readout (commonly referred to as a condenser microphone), and electrostatic or electromagnetic readout mechanisms (e.g., dynamic microphones).

[0003] As an alternative way to read the position of the membrane of a microphone, an optical displacement sensor using optical interferometry readout is used. As a typical example of such a system, a diffraction grating is provided on a substrate adjacent to the membrane, and light is directed onto this diffraction grating. A first portion of this light is reflected back from the grating. A second portion passes through the grating, where it diffracts the radiation. The diffracted radiation impinges on the membrane, where it reflects the radiation back towards the grating. The radiation passes through the grating, and these two portions of light interfere with each other to create an interference pattern that can be detected by a detector. The interference pattern has a shape (i.e., a spatial distribution) that corresponds to the diffraction order of the grating, but the intensity of the light directed in these diffraction orders is determined according to the relative phase of these two portions of light, and thus is determined according to the distance between the grating and the membrane. The position (and thus the movement) of the membrane can therefore be determined from the change in the intensity of the light by the detector.

[0004] Such optical displacement sensors are advantageous for use in optical microphones and other applications because they have a high signal-to-noise ratio (SNR) and high sensitivity. However, further improvement in the performance of such optical displacement sensors is desired.

[0005] Viewed from a first aspect, the present invention is an optical displacement sensor, comprising Reflective surface and, One or more diffraction gratings spaced apart from a reflective surface, wherein the diffraction grating or each diffraction grating, together with the reflective surface, defines each interference configuration, and i) the reflective surface or ii) the diffraction grating or each diffraction grating is movable relative to one another, A light source positioned to provide light to the interference configuration, such that for each interference configuration, a first portion of the light propagates through the interference configuration along a first optical path, and a second portion of the light propagates through the interference configuration along a second different optical path, thereby creating an optical path difference between the first and second optical paths, which is determined by the separation between the reflective surface and the diffraction grating of the interference configuration. For each interference configuration, one or more photodetectors in each set are arranged to detect at least a portion of the interference pattern generated by the first and second portions of the light, depending on the optical path difference. A collimating optical arrangement is provided, which is positioned to at least partially collimate the light between the light source and the diffraction grating. For any interference configuration or each interference configuration, when the reflecting surface or diffraction grating is in the zero-displacement position, the diffraction grating is spaced apart from the reflecting surface by a distance such that each first portion of the light travels along the optical path length L between the diffraction grating and the reflecting surface. For diffraction gratings or each diffraction grating, the following relationship exists between the grating period p and the optical path length L for each interference configuration:

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[0006] The applicant has recognized that in optical displacement sensors utilizing diffraction gratings, a significant amount of light is lost due to diffraction, which can impair the performance of the optical displacement sensor (e.g., reduce sensitivity). Diffraction gratings allow for the separation of light into separate channels corresponding to different diffraction orders, generating a signal (e.g., a difference signal) corresponding to the separation between the reaction surface and the diffraction grating. Typically, some light is diffracted in a direction that prevents it from being ultimately collected by the photodetector, and can be lost, for example, due to multiple reflections and absorptions in the gap between the diffraction grating and the reflective surface. The applicant further recognizes that this loss can be mitigated by at least partially collimating the light prior to impacting the diffraction grating, and by configuring the optical displacement sensor such that the grating period p and optical path length L satisfy the relationship in Equation 1 for each grating.

[0007] These losses are improved by the properties of the diffraction pattern that occurs when a plane wave is diffracted by a periodic grating, as shown in Equation 1. When a plane wave is diffracted by a periodic grating, a repeating pattern is generated in which the grating image is repeated at a certain distance from the grating plane. This certain distance is the Talbot length T. z Therefore, at half the Talbot length and odd multiples of half the Talbot length, the diffracted light forms a lattice image, but the image is shifted laterally by half the lattice period.

[0008] According to the present invention, the optical path length L is such that the first portion of light collides, is diffracted by the grating, reflected by the diffraction plane, and then propagates a total distance of 2L before colliding with the grating again.

[0009] According to the relationship in Equation 1, 2L corresponds to an integer multiple of the Talbot length, which means that the repeated self-image of the grating in the first diffracted portion of light coincides with the position of the actual diffraction grating. This allows more light to be coupled through the diffraction grating (i.e., transmitted or reflected by the grating towards the photodetector) and propagated to the photodetector, where it contributes to the detected signal.

[0010] The applicant also recognizes that similar advantages can be obtained when 2L corresponds to an odd multiple of half the Talbot length. Thus, in view of a second aspect, the present invention relates to optical displacement It is a nsa, Reflective surface and, One or more diffraction gratings spaced apart from a reflective surface, wherein the diffraction grating or each diffraction grating, together with the reflective surface, defines each interference configuration, and i) the reflective surface or ii) the diffraction grating or each diffraction grating is movable relative to one another, A light source positioned to provide light to the interference configuration, such that for each interference configuration, a first portion of the light propagates through the interference configuration along a first optical path, and a second portion of the light propagates through the interference configuration along a second different optical path, thereby creating an optical path difference between the first and second optical paths, which is determined by the separation between the reflective surface and the diffraction grating of the interference configuration. For each interference configuration, one or more photodetectors in each set are arranged to detect at least a portion of the interference pattern generated by the first and second portions of the light, depending on the optical path difference. A collimating optical arrangement is provided, which is positioned to at least partially collimate the light between the light source and the diffraction grating. For any interference configuration or each interference configuration, when the reflecting surface or diffraction grating is in the zero-displacement position, the diffraction grating is spaced apart from the reflecting surface by a distance such that each first portion of the light travels along the optical path length L between the diffraction grating and the reflecting surface. For diffraction gratings or each diffraction grating, the following relationship exists between the grating period p and the optical path length L for each interference configuration:

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[0011] According to the relationship of Equation 3, 2L corresponds to an odd multiple of half of the Talbot length, which means that the repeated self-image of the grating in the first diffracted light portion is the same as the actual diffraction grating, but is shifted horizontally by half of the grating period. As a result, most of the light is reflected back from the diffraction grating toward the reflecting surface, and then the reflecting surface reflects the light back toward the diffraction grating again, that is, the light makes another round trip at a distance of 2L. When this light collides with the diffraction grating again, the light has traveled a total distance of 4L, that is, 2 round trips, between the diffraction grating and the reflecting surface. In this case, according to the relationship of Equation 3, 4L corresponds to an integer multiple of the Talbot length. Therefore, the repeated self-image of the grating in the first diffracted light portion after 2 round trips coincides with the position of the actual diffraction grating. This allows more light to be coupled through the diffraction grating (i.e., transmitted or reflected by the grating toward the photodetector) and propagated to the photodetector, where it contributes to the detection signal. This configuration provides an additional effect that, since the light makes 2 round trips in the gap between the reflecting surface and the diffraction grating, the sensitivity of the optical displacement sensor to the movement of the reflecting surface or the diffraction grating can be doubled. repeated self-image coincides with the position of the actual diffraction grating. This allows more light to be coupled through the diffraction grating (i.e., transmitted or reflected by the grating toward the photodetector) and propagated to the photodetector, where it contributes to the detection signal. This configuration provides an additional effect that, since the light makes 2 round trips in the gap between the reflecting surface and the diffraction grating, the sensitivity of the optical displacement sensor to the movement of the reflecting surface or the diffraction grating can be doubled.

[0012] This means that the angle of light propagation does not vary with position across the grating surface (or varies less than if the light were not collimated at least partially), and therefore light collimation contributes to this effect. As a result, for a given perpendicular separation between the diffraction grating and the reflective surface, the optical path length L also does not vary (or varies little) across the surface of the diffraction grating, meaning that the relationship in Equation 1 or Equation 3 is satisfied at all, or substantially all, positions across the surface of the diffraction grating, which can contribute to a reduction in optical loss.

[0013] It should be understood that the relationship in Equation 1 does not need to be satisfied precisely so that the benefit of reduced optical loss is provided to a remarkably useful degree. The applicant states that when the optical path length L is given by Equation 1

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[0014] It should be understood that the relationship in Equation 3 does not need to be satisfied precisely so that the benefit of reduced optical loss is provided to a remarkably useful degree. The applicant states that if the optical path length L is equal to Equation 3

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[0015] The optical displacement sensor preferably includes a membrane, and the reflective surface includes the surface of the membrane or is provided on the surface of the membrane.

[0016] In embodiments where the reflective surface is movable relative to the diffraction grating, the reflective surface may be provided within or on a movable member, or may include a part of the movable member. The diffraction grating may be fixed, for example, to a light source and a photodetector, and the optical displacement sensor may include a substrate, and the diffraction grating may be provided within or on the substrate, or may include a part of the substrate.

[0017] In embodiments in which a diffraction grating or each diffraction grating is movable relative to a reflective surface, the diffraction grating may be provided in or on a movable member, or may include a part of the movable member. The reflective surface may be fixed, for example, to a light source and a photodetector, and the optical displacement sensor may include a substrate, and the reflective surface may be provided in or on the substrate, or may include a part of the substrate.

[0018] As a non-limiting example, the movable member may be a membrane or a proof mass (for example, in an accelerometer or vibrator).

[0019] In the context of "reflective surface," reflection means that a surface is at least partially reflective, but it should be understood that, depending on the specific configuration of an optical displacement sensor, for example, it may be substantially or completely reflective.

[0020] When a diffraction grating or each diffraction grating is said to contain a periodic grating, this should be understood to mean that the diffraction grating or each diffraction grating contains a set of equally spaced parallel grating lines. When multiple diffraction gratings are provided, it is preferable that each diffraction grating be positioned in a plane, and all the planes of the grating are parallel to each other. More generally, it is preferable that the diffraction grating or each diffraction grating be positioned in a plane parallel to the plane of the reflecting surface, so that the relative movement of the reflecting surface and the diffraction grating is perpendicular to the said plane. It should be understood that this allows any deviation from the plane resulting from slight displacements of the reflecting surface or diffraction grating during movement to be ignored.

[0021] It is preferable that the diffraction grating, or each diffraction grating, is manufactured such that the grating lines have high reflectivity, for example, in the region between the grating lines, for light striking the side of the diffraction grating facing the light source. This can help provide high diffraction efficiency.

[0022] In embodiments to which the relationship given by Equation 1 applies, the grid lines preferably have a low reflectivity for light striking the side of the diffraction grating opposite the reflective surface, for example, a reflectivity lower than that of the diffraction grating for light striking the side of the diffraction grating opposite the light source. This helps to reduce multiple internal reflections of light in the gap between the diffraction grating and the reflective surface, which can help to improve the performance of the optical displacement sensor by increasing the advantages provided in the first embodiment of the invention to which Equation 1 applies, as described above.

[0023] In embodiments to which the relationship related to Equation 3 applies, it is preferable that the grid lines have high reflectivity, for example, in the region between the grid lines, for light that strikes the side of the diffraction grating opposite the reflective surface. This helps to reflect the light back toward the reflective surface after the first round trip, and this light can make a second round trip between the diffraction grating and the reflective surface. This can help to improve the performance of the optical displacement sensor by increasing the advantages provided in the second embodiment of the invention to which Equation 3 applies, as discussed above.

[0024] The above-mentioned high reflectivity and low reflectivity characteristics are achieved by applying a suitable coating (e.g., multilayer coating, anti-reflective coating, etc.) or, for example, to the grid lines and / or the regions between the grid lines. It may also be provided by other surface treatments (e.g., anti-reflective treatments).

[0025] The wavelength λ of light may be the wavelength that characterizes the finite spectrum of wavelengths produced by the light source. For example, this wavelength may be the peak wavelength or the center wavelength of the wavelength spectrum of light.

[0026] In this context, "zero displacement position" refers to the equilibrium position that the reflective surface or diffraction grating would take if it were not subjected to the resulting external force that is the subject of measurement in the design.

[0027] The optical arrangement for collimation may consist of a single optical element or multiple elements.

[0028] As used herein, the term "photodetector" is not limited to a specific photodetector device (e.g., a photodiode), but may be a device that separately detects light incident on one or more of its regions, such as a CCD, in which different pixels or regions of pixels each provide a separate signal for the incident incoming light.

[0029] In a series of embodiments, the optical displacement sensor comprises at least two diffraction gratings. The optical path length L may differ for each diffraction grating, and this is preferably achieved by configuring the optical displacement sensor such that there are different perpendicular optical path lengths between each diffraction grating and the reflective surface, as will be discussed below. Such an arrangement may favorably extend the dynamic range of the optical displacement sensor, as described in WO2014 / 202753, for example. Different optical path lengths provide different phase offsets, resulting in multiple optical signals with relative phase offsets. The operating range of the optical displacement sensor can be extended by combining the signals to provide optical measurements.

[0030] In a series of embodiments, a different perpendicular optical path length exists between each diffraction grating and the reflective surface. The difference in perpendicular optical path length may be provided by positioning the diffraction grating at different height offsets relative to the reflective surface, for example, by providing the diffraction grating on a substrate having a stepped shape. The different height offsets may be provided by recesses etched to different depths within the substrate, in which case, for example, the diffraction grating is provided within the recesses. The difference in perpendicular optical path length may also be provided by regions of the reflective surface at different heights (e.g., stepped shape or etched recesses), in which case, for example, the reflective surface is fixed and the diffraction grating moves relative to the reflective surface. Therefore, the reflective surface does not have to be a single planar surface, but may have, for example, multiple regions or facets, each having, for example, a different height offset relative to the movable member having the grating. However, it is not essential that the difference in optical path length be provided by a difference in physical distance between the diffraction grating and the reflective surface. The difference in optical path length may be provided in other ways, for example, by using an optical delay film, without necessarily requiring a substrate having a non-planar outer diameter.

[0031] The reflective surface does not have to be a single planar surface, but may comprise, for example, multiple regions or facets, and it should be understood that this does not imply that the surface may include separate, independently movable surfaces. For example, the reflective surface may be located on or form part of a common movable member (e.g., a membrane), or the diffraction grating may be located on a common movable member (e.g., a membrane).

[0032] In a series of embodiments, at the zero displacement position, the reflective surface has a vertical distance of at least 15 μm. Separate, or separate from the diffraction grating or each diffraction grating.

[0033] In embodiments having more than one diffraction grating, the grating period p is preferably the same for each diffraction grating. Since the height offset for which a phase offset is required is usually very small (e.g., on the wavelength scale), all gratings can satisfy the above relationship in Equation 1 or Equation 3 within the aforementioned margin, even if the phase offset is given by the height offset. For example, the typical zero-displacement gap between the reflecting surface and the grating may be in the range of 30 μm to 40 μm, or may exceed 30 μm, for example, exceed 40 μm. This may be advantageous in reducing attenuation. The corresponding grating period (e.g., corresponding to the range of 30 μm to 40 μm) may be in the range of 5 μm to 6 μm, which can provide a suitable diffraction angle, for example, at a wavelength of 850 nm.

[0034] When a collimating configuration is said to collimate light at least partially, this means that the light leaving the collimating optical configuration has a smaller beam angle than the light that collides with the collimating optical configuration. The beam may converge or diverge, i.e., the beam angle may refer to the angle at which the beam converges or diverges. The collimating optical configuration may completely or substantially completely collimate light such that, for example, the beam angle of the light after collimation is less than 10°, e.g., less than 5°, e.g., less than 2°, e.g., less than 1°, e.g., less than 0.5%, e.g., less than 0.1%. In a series of embodiments, the collimating optical configuration is or comprises a lens.

[0035] As described above, the diffraction grating or reflective surface may be provided in or on a substrate (e.g., the surface of the substrate) (e.g., attached or etched). In some embodiments, the diffraction grating or reflective surface is provided on a first side of the substrate so that the reflective surface and the diffraction grating face each other, and the collimating optical arrangement is provided on a second side of the substrate and faces the light source. In some embodiments, the diffraction grating or reflective surface may be provided in or on a first substrate, and the collimating optical arrangement may be provided on a second substrate between the light source and the first substrate. The collimating optical arrangement may be provided on the surface of the light source, for example, the light source may be a back-emitting VCSEL (i.e., the light emitter is positioned behind the VCSEL and emits light so that it exits the VCSEL through the front), in which case the collimating optical arrangement may be provided on its emitting (i.e., front) surface (e.g., etched).

[0036] The interference configuration may be set up so that the diffraction grating is positioned between the light source and the reflective surface. The photodetector may be provided on the same side of the interference configuration as the light source, for example, adjacent to the light source. The light source and the photodetector may be provided on a common optoelectronic substrate. The interference configuration may be set up so that the reflective surface is positioned between the light source and the diffraction grating. The photodetector may be provided on the opposite side of the interference configuration from the light source, for example, facing the diffraction grating.

[0037] The substrate may be made of any suitable material, such as silicon or glass.

[0038] The light source may include a laser, for example, a vertical-cavity surface-emitting laser (VCSEL), for example, a back-emitting VCSEL. The VCSEL may have one or more optical elements on its surface (e.g., a collimating optical arrangement and / or a beam-separating optical arrangement).

[0039] In a series of embodiments, the optical displacement sensor comprises two or more diffraction gratings, a) Each diffraction grating includes a set of parallel grid lines extending in each grid line direction, and the grid line direction of each diffraction grating in the set of diffraction gratings is different from the grid line direction of each other diffraction grating in the set. Different and / or b) The optical displacement sensor includes a beam-separating optical arrangement configured to separate light into two or more beams, each of which is directed to one of the diffraction gratings.

[0040] Since this in itself is novel and progressive, from a third aspect of the present invention, an optical displacement sensor is provided, Reflective surface and, Two or more diffraction gratings spaced apart from a reflective surface, each diffraction grating together with the reflective surface defines a specific interference configuration, and either i) the reflective surface or ii) the diffraction gratings are movable relative to each other, A light source is positioned to provide light to the interference configuration, such that for each interference configuration, a first portion of the light propagates through the interference configuration along a first optical path, and a second portion of the light propagates through the interference configuration along a second different optical path, thereby creating an optical path difference between the first and second optical paths, which is determined by the separation between the reflective surface and the diffraction grating of the interference configuration. For each interference configuration, one or more photodetectors in each set are arranged to detect at least a portion of the interference pattern generated by the first and second portions of the light, depending on the optical path difference. A collimating optical arrangement is provided, which is positioned to at least partially collimate the light between the light source and the diffraction grating. a) Each diffraction grating includes a set of parallel grid lines extending in each grid line direction, and the grid line direction of each diffraction grating in the set is different from the grid line direction of each other diffraction grating in the set, and / or b) The optical displacement sensor includes a beam-separating optical arrangement configured to separate light into two or more beams, each of which is directed to one of the diffraction gratings.

[0041] In a series of embodiments according to the first and / or second aspects of the present invention, the optical displacement sensor comprises two or more diffraction gratings, and the light source comprises a plurality of light source elements such that light is provided as a plurality of light beams, each light source element providing one of the beams and directing each light beam to each of the diffraction gratings. Such embodiments may also be provided with feature a) as presented above.

[0042] Since this in itself is novel and progressive, from the perspective of a fourth aspect of the present invention, an optical displacement sensor is provided, Reflective surface and, Two or more diffraction gratings spaced apart from a reflective surface, each diffraction grating together with the reflective surface defines a specific interference configuration, and either i) the reflective surface or ii) the diffraction gratings are movable relative to each other, A light source is positioned to provide light to the interference configuration, such that for each interference configuration, a first portion of the light propagates through the interference configuration along a first optical path, and a second portion of the light propagates through the interference configuration along a second different optical path, thereby creating an optical path difference between the first and second optical paths, which is determined by the separation between the reflective surface and the diffraction grating of the interference configuration. For each interference configuration, one or more photodetectors in each set are arranged to detect at least a portion of the interference pattern generated by the first and second portions of the light, depending on the optical path difference. A collimating optical arrangement is provided, which is positioned to at least partially collimate the light between the light source and the diffraction grating. The light source comprises multiple light source elements such that light is provided as multiple light beams, each light source element providing one of the beams, and directing each beam of light to one of the diffraction gratings, thereby providing an optical displacement sensor.

[0043] Multiple light source elements may be arranged in an array. The light source may include a vertical-cavity surface-emitting laser (VCSEL) with multiple emitters (i.e., each emitter is one of the light source elements). The light source may include a forward-emitting VCSEL or a back-emitting VCSEL. The VCSEL may have one or more optical elements on its surface (e.g., a collimated optical arrangement). The light source elements may be operable independently (e.g., individually or in subgroups). This may allow, for example, operation of an optical displacement sensor in a power-reduced mode.

[0044] The collimating optical arrangement may be configured to collimate at least partially each of the multiple light beams. For example, the collimating optical arrangement may comprise multiple collimating optical elements, each of which is configured to collimate at least partially one of the light beams. The embodiment according to the fourth aspect may also be provided with feature a), as presented above in relation to the third aspect.

[0045] Any feature of the first and second embodiments may also be a feature of the third and fourth embodiments, where applicable.

[0046] The beam separation optical configuration may comprise a single optical element or multiple elements. The beam separation optical configuration may be a beam splitter, for example, a diffraction beam splitter, or may comprise one. The beam separation optical configuration may be refractive or diffractive; for example, the beam separation optical configuration may be a diffraction grating, or may comprise one.

[0047] The beam separation optical configuration may direct the beam onto or relative to each of these diffraction gratings. The beam separation optical configuration may collimate the beam at least partially.

[0048] The beam separation optical arrangement may include a collimating optical arrangement. The collimating optical arrangement and the beam separation optical arrangement may be formed as a single component that performs both the function of collimating light at least partially and the function of separating light into two or more beams. Non-limiting examples of elements that may be used for beam collimation and splitting include a composite phase hologram element and a refractive lens with multiple facets.

[0049] As described above, the diffraction grating or reflective surface may be provided in or on a substrate (for example, having the surface of the substrate) (for example, by being attached or etched). In some embodiments, the diffraction grating or reflective surface is provided on a first side of the substrate so that the reflective surface and the diffraction grating face each other, and the beam separation optical arrangement is provided on a second side of the substrate and faces the light source. In some embodiments, the diffraction grating or reflective surface is provided in or on a first substrate, and the beam separation optical arrangement is provided on a second substrate between the light source and the first substrate.

[0050] The beam separation optical arrangement may be provided on the surface of the light source. For example, the light source may be a back-emitting vertical-cavity surface-emitting laser (VCSEL), in which case the beam separation optical arrangement may be provided on its front surface (for example, integrally formed within / on its front surface, mounted on its front surface, or etched within its front surface). For example, the beam separation optical arrangement may include a lens (e.g., a faceted lens) formed on the surface of the light source. Alternatively, the beam separation optical arrangement may include a diffraction grating etched onto the surface of the light source.

[0051] Providing a beam-separating optical arrangement on the surface of the light source may offer various benefits. For example, it may facilitate the arrangement of optical components of an optical displacement sensor on a wafer scale. The optical displacement sensor may be made more compact. The reduced number of components may make it easier to integrate the optical displacement sensor into a package (e.g., a microphone housing).

[0052] Providing either or both of features a) and b), or providing multiple light source elements that each provide a beam directed to one of the gratings, provides a technical effort to spatially separate the light passing through each interference configuration, making it easier to collect by different photodetectors and generating distinct signals at each diffraction grating.

[0053] In feature a), the beam is spatially separated as a result of at least differently oriented grating lines. When a beam collides with a periodic diffraction grating, the resulting diffraction orders are spatially separated along directions perpendicular to the grating lines. Thus, when a beam collides with multiple diffraction gratings with different grating line orientations, this generates multiple sets of diffraction orders (one for each grating), in which case each set of diffraction orders is spatially separated in a different plane (and therefore the diffraction orders of these sets are spatially separated from each other). Light collides perpendicularly to the gratings, for example, as a light beam incident on a region overlapping multiple gratings. In this case, only the grating orientation acts to separate the light into separate channels containing different sets of diffraction orders.

[0054] In feature b), the light is spatially separated by a beam-separating optical arrangement that separates the light into at least multiple beams (e.g., one for each diffraction grating). In the feature where multiple light source elements provide multiple beams, the rays are spatially separated by being provided by spatially separated light source elements.

[0055] A beam can be described as "off-axis" if, for example, each beam propagates at a constant angle with respect to the beam axis of light colliding with a beam separation optical arrangement. When beams colliding with a diffraction grating are spatially separated, the diffraction orders of these sets generated by the grating are also spatially separated.

[0056] The beams can collide with the diffraction gratings at a constant angle with respect to the normal to the plane on which each diffraction grating is mounted. Each beam may be collimated, or substantially collimated, for example, by a separate collimating optical arrangement or by a beam separation optical arrangement. The direction of propagation of each beam is preferably i) parallel to the grating lines of the diffraction grating into which the beams collide, and ii) in a plane perpendicular to the plane on which the diffraction grating is mounted. Such orientations may be advantageous in providing the advantages of the features of the first and second embodiments of the present invention, relating to the reduction of optical loss due to diffraction.

[0057] The optical displacement sensor may have exactly two diffraction gratings. In the context of embodiments having exactly two gratings, any feature described herein may, where applicable, be any feature of embodiments having three or more diffraction gratings.

[0058] In a series of embodiments, each beam collides with each diffraction grating at an angle of incidence relative to the normal to the plane on which each diffraction grating is mounted, in which case the angle of incidence of each diffraction grating is different from the angles of incidence of the other diffraction gratings.

[0059] In a series of embodiments, each beam is normal to the plane on which each diffraction grating is mounted. At each angle of incidence, the light collides with each diffraction grating, and in this case, the angle of incidence for each diffraction grating in a set of diffraction gratings is different from the angle of incidence for each other diffraction grating in the set.

[0060] It should be understood that two angles of incidence may be considered different from each other if they have the same polar angle with respect to the normal but different azimuth angles, or the same azimuth angle with respect to the normal but different polar angles, or different polar angles and different azimuth angles with respect to the normal.

[0061] For example, in an embodiment having exactly two grids, the grids may have the same orientation or may be spaced apart from each other in a direction parallel to the grid line direction. Each beam may collide with each grid at each incident angle, where the azimuth angles differ by 180° for beams having the same polar angle.

[0062] In a series of embodiments, the beam direction of light or light beams striking the diffraction grating is perpendicular to the plane of the diffraction grating.

[0063] In some embodiments, each diffraction grating is oriented along one of a pair of lines radiating from the center point between the gratings. "Oriented along one line" means that the grating coincides with the line and has a grating line direction parallel to the line.

[0064] In a series of embodiments, the optical displacement sensor comprises N grids, in which case the diffraction grids are oriented at an angle of (360°) / N or multiples thereof relative to each other.

[0065] The orientation of a diffraction grating must be understood as referring to the direction of its grating lines.

[0066] In some embodiments, the interference configuration or each interference configuration includes a pair of diffraction gratings having the same grating period and the same grating line direction, and separated from the reflective surface at the same optical distance, and the pair of diffraction gratings together function to direct light to one or more photodetectors of the same set corresponding to the interference configuration.

[0067] In a series of embodiments, the diffraction grating is placed within a rotationally symmetric composite diffraction grating. For example, the diffraction grating may include sectors, pairs of sectors opposite each other in the diametrical direction of a circle, or pairs of triangles opposite each other in the diametrical direction of a triangle or hexagon, octagon, or other polygon. It should be understood that rotational symmetry refers to the position of the diffraction grating around the axis of the composite diffraction grating and does not necessarily refer to all properties of the grating (for example, the grating may have different height offsets in a rotationally symmetric way).

[0068] The spatial separation of these generated diffraction order sets, as discussed above, allows these orders to be advantageously and easily directed by a photodetector, for example, using a beam separation optical configuration.

[0069] In a series of embodiments, the optical displacement sensor includes a beam manipulation optical arrangement that directs the first and second optical portions in each interference arrangement to each photodetector provided for the interference arrangement.

[0070] The beam manipulation optical arrangement may comprise one or more beam manipulation optical elements, for example, one or more refractive and / or diffractive optical elements.

[0071] The beam manipulation optical arrangement may include a plurality of prisms or gratings, which may, for example, be etched into the substrate surface or patterned on the substrate surface using a polymer.

[0072] In a series of embodiments, the beam separation optical arrangement and the beam manipulation optical arrangement are provided on a common substrate. For example, the beam manipulation optical arrangement may be positioned near the beam separation optical arrangement (e.g., around it), such as a plurality of beam manipulation optical elements positioned near the beam separation optical elements.

[0073] Beam manipulation optical arrangements can advantageously provide greater freedom in positioning photodetectors and / or other components. For example, a beam manipulation optical arrangement may comprise a plurality of beam manipulation optical elements, each configured to direct each beam towards one of the photodetectors. This can be particularly useful in embodiments having three, four, or more gratings. It is preferable that the photodetectors and other components associated with each interference arrangement are spaced sufficiently apart to avoid overlapping with the optical paths associated with other interference arrangements. A greater degree of freedom in placement is beneficial in such cases, as a larger number of gratings limits the space available for mounting photodetectors and other components.

[0074] The beam manipulation arrangement is preferably provided in combination with feature b) above, but may also be provided in both features a) and b), or in a combination of feature a) and / or features of multiple light source elements. Separating the light into multiple beams prior to the light colliding with the grating (e.g., here each beam is at a constant angle with respect to the surface normal of the diffraction grating) can advantageously provide further degrees of freedom, which can be useful when manipulating the beam toward the photodetector. This can make the arrangement of optical displacement sensors, in particular photodetectors, easier, for example, by allowing larger tolerances in the positioning of the photodetector and other components. Furthermore, as a special effect, it may be possible to enable the detection of the zeroth order of diffraction (which, otherwise, can be directed, for example, straight back toward the source, and it would be impossible to position the photodetector to receive the zeroth order of diffraction).

[0075] When light strikes a periodic diffraction grating, it will be understood that the light separates in directions perpendicular to the grating lines and is diffracted into multiple diffraction orders, including the central diffraction order (usually referred to as the 0th or zeroth order) and the diffraction orders on each side of the central order. These two orders may be referred to as the first diffraction order or the +1st and -1st order diffraction orders.

[0076] In a series of embodiments, each set of one or more photodetectors comprises two photodetectors, the photodetectors being arranged such that the +1st order diffraction coincides with the first photodetector and the -1st order diffraction coincides with the second photodetector.

[0077] Such a photodetector arrangement may be provided in an embodiment in which the light is separated prior to collision with the diffraction grating, in which case the light is incident perpendicular to the diffraction grating. In such an embodiment, the zeroth order diffraction is directed back to the light source, and the +1st and -1st order diffraction is directed at a constant angle with respect to the grating surface normal so that the +1st and -1st order diffraction is spatially separated from the zeroth order in a direction perpendicular to the grating line direction. This spatial separation means that the +1st and -1st order diffraction does not propagate back in the direction of the source, so that the photodetector can be suitably positioned to receive the +1st and -1st order diffraction.

[0078] In a series of embodiments, each set of one or more photodetectors comprises three photodetectors, in which case the photodetectors are arranged such that the +1st order diffraction coincides with the first photodetector, the 0th order diffraction coincides with the second photodetector, and the -1st order diffraction coincides with the third photodetector.

[0079] Such a photodetector arrangement is an embodiment in which light is separated prior to colliding with the diffraction grating. The diffraction grating may be provided in such a configuration that the light strikes the diffraction grating at a constant angle with respect to the normal to the plane on which the grating is mounted. In such a configuration, the -1st, 0th, and +1st order diffractions typically do not propagate back toward the light source but are spatially separated from each other, allowing the photodetectors to be positioned to receive the -1st, 0th, and +1st order diffractions. This can favorably generate a difference signal due to the characteristics of the diffraction order. Due to the symmetry in the configuration, the detection intensities of the -1st and +1st order diffractions are typically identical, although the 0th order provides a difference signal that can be used together with the -1st and / or +1st order diffractions to generate a difference signal. In these and other embodiments, the signals from the -1st and +1st order diffractions may be combined into a single signal, for example, by integrating the signals from photodetectors positioned to receive the -1st and +1st order diffractions.

[0080] As described above, the present invention extends to optical microphones (and other types of pressure sensors) equipped with optical displacement sensors. However, other applications are possible within the scope of the present invention. The present invention extends to accelerometers or vibrators that include optical displacement sensors as described above. More generally, optical displacement sensors can provide measurement of external stimuli such as pressure differences between two fluids of different capacities, acoustic waves, or vibrations.

[0081] Specific preferred embodiments will be described below for illustrative purposes only, with reference to the attached drawings. [Brief explanation of the drawing]

[0082] [Figure 1] A schematic representation of an optical displacement sensor according to the first embodiment of the present invention is shown. [Figure 2] The diagram shows a composite diffraction grating including three diffraction gratings according to the first embodiment, and includes a representation of the detector locations corresponding to these three gratings. [Figure 3] This graph shows the operating range of an optical displacement sensor with a single grid. [Figure 4a] A schematic front view of a second embodiment of the optical displacement sensor according to the present invention is shown. [Figure 4b] Figure 4a shows a schematic side view of the optical displacement sensor of the second embodiment. [Figure 4c] Figures 4a and 4b show schematic plan views of the optical displacement sensor of the second embodiment. [Figure 5a] A schematic front view of a third embodiment of the optical displacement sensor according to the present invention is shown. [Figure 5b] Figure 5a shows a schematic plan view of the optical displacement sensor of the third embodiment, and the optical paths are shown in a cross-sectional view along with the optical surfaces that these optical paths collide with. [Figure 5c] Figures 5a and 5b show schematic plan views of the photodetector layout as an example of an optical displacement sensor in the embodiment shown. [Figure 6] A schematic front view of an optical displacement sensor according to the fourth embodiment is shown. [Figure 7] A schematic front view of an optical displacement sensor according to the fifth embodiment is shown. [Figure 8] A schematic front view of the optical displacement sensor according to the sixth embodiment is shown. [Figure 9] A schematic front view of the optical displacement sensor according to the seventh embodiment is shown.

[0083] Figure 1 shows an optical displacement sensor 2 according to a first embodiment of the present invention. In this example, the optical displacement sensor 2 forms part of an optical microphone. The optical displacement sensor 2 includes a membrane 4 having a reflective surface that vibrates in response to ambient sound waves. The optical displacement sensor 2 further includes a composite diffraction grating 6, which will be described in detail below with reference to Figure 2. The optical displacement sensor 2 further includes a light source 8 and six detectors 10, two of which can be seen in the diagram shown in Figure 1. The light source 8 is positioned to direct light 12 onto the diffraction grating 6 via a lens 14. The light 12 has a beam angle θ v The light is emitted from the light source. Lens 14 collimates the light 12, which then collides with the composite diffraction grating 6 as a plane wave.

[0084] In this example and other specific embodiments described below, the reflective surface of the membrane moves relative to the diffraction grating, whereas in modifications and other examples, the grating is a fixed reflective surface (for example) It may move relative to the surface of the substrate. For example, the composite diffraction grating in this embodiment can be fabricated on a membrane. In some modifications and other examples, for example, when multiple gratings are attached to a membrane, an optical phase offset may be provided using an optical delay film (for example, instead of the height offset discussed below), or a fixed reflective surface may provide the height offset, for example, by providing a recess in the reflective surface.

[0085] In the presence of sound waves, the membrane vibrates, changing the separation between the membrane and the composite diffraction grating. In Figure 1, this separation is shown as the gap g. When the membrane is not exposed to sound pressure waves, it is separated from the composite diffraction grating at equilibrium separation at the "zero displacement" position, which in this example is a half-integer multiple of the Talbot length of the composite diffraction grating, that is, it satisfies the following equation:

number

number

number

[0086] In the above, L is the optical path length through which light travels between the composite grating and the membrane. In this example, since light is incident on the composite grating normally, the optical path length L through which light travels between the composite grating and the membrane is the same as the equilibrium separation between the composite grating and the membrane.

[0087] As discussed above, this condition regarding the optical path length L (and consequently the distance between the membrane and the composite grating) reduces the loss of light due to diffraction (for example, due to multiple reflections and absorptions in the gap between the membrane and the composite grating). From the above disclosure, it will be understood that, for example, in this embodiment, the optical path length L may instead satisfy Equation 3, and this will also reduce the loss due to diffraction.

[0088] Figure 2 shows a composite diffraction grating 6. In this example, the composite diffraction grating 6 has a circular shape and is divided into six sectors, defining three diffraction gratings, each comprising a pair of diffraction grating regions. The first diffraction grating 14 comprises the first diffraction grating regions 14a and 14b, the second diffraction grating 16 comprises the second pair of diffraction grating regions 16a and 16b, and the third diffraction grating 18 comprises the third pair of diffraction grating regions 18a and 18b. The diffraction grating regions 14a, 14b, 16a, 16b, 18a, and 18b all have the same grating period. The nuclear diffraction gratings 14, 16, and 18 have corresponding pairs of detectors. The first diffraction grating 14 corresponds to the first pair of detectors 20a and 20b. The second diffraction grating 16 corresponds to the second pair of detectors 22a and 22b. The diffraction grating 18 corresponds to the third pair of detectors 24a and 24b.

[0089] The diffraction grating regions 14a and 14b of the first diffraction grating 14 are opposite to each other in the diametrical direction (with respect to the first diameter of the composite diffraction grating 6), and each diffraction region contains a linear diffraction grating such that the grating lines are oriented quasi-radially. In this context, quasi-radial is used to mean that the grating lines are parallel to each other and parallel to the first diameter. The first pair of detectors 20a and 20b are also opposite to each other in the diametrical direction (with respect to the second diameter of the composite diffraction grating which is perpendicular to the first diameter).

[0090] When light 12 is incident on the composite diffraction grating, a portion of the light illuminates the first diffraction grating 14. A first portion of this light passes through the first diffraction grating 14, is reflected by the membrane 4, and then enters the first diffraction grating 14 again. The first diffraction grating 14 transmits this first portion of light and diffracts it to multiple diffraction orders, including -1st, 0th, and +1st order diffraction orders. A second portion of the light illuminating the first diffraction grating is reflected by the first diffraction grating and diffracted to multiple diffraction orders, including -1st, 0th, and +1st order diffraction orders.

[0091] In each case, the zeroth order diffraction is returned to the source and directed. The -1st and +1st order diffractions appear from the first diffraction grating at a constant angle with respect to the zeroth order, and are therefore spatially separated from the zeroth order diffraction in a direction perpendicular to the grating lines of the first diffraction grating. The photodetectors 20a and 20b are positioned so that the -1st order diffraction is incident on one photodetector 20a and the +1st order diffraction is incident on the other photodetector 20b. In each case, the incident light contains light from the first and second parts. The first part of the light travels through the gap between the membrane and the first diffraction grating, while the second part does not, so there is an optical path difference between the paths of the first and second parts of the light, which is determined by the displacement of the membrane. Thus, the first and second parts interfere in such a way that the intensity of the light diffracted to each order is determined by the displacement of the membrane. Therefore, the displacement of the membrane can be determined using the intensities of the -1st and +1st order diffractions detected by the photodetectors 20a and 20b.

[0092] The second and third diffraction gratings have a height offset relative to the first diffraction grating, which will be discussed in detail later. In other respects, the structure and orientation of the second diffraction grating 16 and the second pair of detectors 22a and 22b are identical to those of the first diffraction grating 14 and the first pair of detectors 20a and 20b, but rotated by 60°. Similarly, the third diffraction grating 18 and the third pair of detectors 24a and 24b are rotated by 120° relative to the first diffraction grating 14 and the first pair of detectors 20a and 20b. In each case, the diffraction orders produced by the diffraction gratings are separated in the direction perpendicular to the grating lines, so that diffraction gratings of different orientations can spatially separate the diffraction orders produced by each grating and direct them to different detectors.

[0093] As described above, the three diffraction gratings 14, 16, and 18 have a relative height offset. This means that for the diffraction gratings to have spacing with respect to the membrane, the membrane displacement required for the intensity of the light diffracted to be approximately linear for each diffraction order must be within a relatively small range. The results are shown in Figure 3.

[0094] Figure 3 shows the relative diffraction efficiency of the transmitting and reflecting portions of interfering light. As described above, for each grating 14, 16, and 18, each detector 20a, 20b, 22a, 22b, 24a, and 24b is positioned to receive diffraction peaks of the -1st and 1st orders. However, other orders may be detected additionally or alternatively; for example, in another embodiment, the 0th order may be detected in addition to the -1st and +1st orders. The first line 26 corresponds to the 0th order peak. The second line 28 corresponds to the +1st order peak.

[0095] As shown in Figure 3, the relative diffraction efficiency of the 0th and +1st order peaks fluctuates sinusoidally with respect to the distance between the membrane and the grating, and the 0th and 1st order peaks are out of phase. The sensitivity of the microphone is determined by the change in the output signal in response to a given change in the displacement of the membrane. Therefore, from Figure 3, it can be seen that the maximum sensitivity occurs within the operating range 30, where lines 26 and 28 have the greatest slope and are approximately linear.

[0096] Therefore, for each grid, the movement of the membrane can only be determined with high sensitivity within an operating range 30 of approximately ±λ / 16 (corresponding to a membrane displacement of approximately ±50 nm) around the point of application, which corresponds to a distance of (2n+1)λ / 8 between the membrane and the grid where n is an integer. At other distances, there is a region of low sensitivity 32. As a result, the dynamic range detectable by a single grid is limited. Therefore, in the optical displacement sensor 2, the three grids 14, 16, and 18 are positioned at slightly different distances from the membrane, corresponding to different points of application, to cover a wider range of membrane positions and, consequently, to extend the dynamic range of the optical microphone.

[0097] Since the height offset is on the scale of the wavelength of light, it is understood that Equation 5 (or Equation 3) is still satisfied for multiple gratings with relative height offsets within a suitable margin.

[0098] Figures 4a to 4c show a second embodiment of the optical displacement sensor 34 according to the present invention. Figure 4a shows a schematic front view of the optical displacement sensor 34, Figure 4b shows a schematic side view of the optical displacement sensor 34, and Figure 4c shows a schematic top view of the optical displacement sensor 34.

[0099] The optical displacement sensor 34 includes a membrane 36, a first transparent substrate 38, a second transparent substrate 40, and an optoelectronic substrate 42. Two diffraction gratings 44, 46 are fabricated on the side of the first transparent substrate 38 facing the membrane 36. The second transparent substrate 40 includes a beam separation optical arrangement 48 and a beam manipulation optical arrangement 50 surrounding the beam separation arrangement 48. In this example, the beam separation arrangement 48 is in the form of a lens with multiple facets (although other arrangements may be used), and as a result, also provides collimator functionality. In this example, the beam manipulation optical arrangement 50 includes multiple prisms etched to form a substrate surface, although other arrangements are possible. The optoelectronic substrate 42 includes a light source 52, in this example a vertical-cavity surface-emitting laser (VCSEL) and six photodetectors 54a, 54b, 54c, 56a, 56b, 56c (two of which are visible in Figure 4a).

[0100] The VCSEL is positioned to direct the uncollimated light 58 along axis 60 to a beam separation configuration 48. The beam separation configuration 48 separates and collimates the uncollimated light 58 into two separate collimated beams 62, 64, and propagates the uncollimated light 58 at a constant angle with respect to axis 60 so that each beam 62, 64 can be directed to one of the diffraction gratings 44, 46. Each beam 62, 64 interacts with each grating 44, 46 and membrane 36 in the same manner as described above with reference to Figure 1. That is, the first portions 62a, 64a pass through the gratings 44, 46 so as to be reflected by the membrane 36 to the gratings 44, 46, and are then transmitted by the gratings 44, 46 and diffracted to diffraction orders 66a, 66b, 66c, 68a, 86b, 68c, -1, 0, and 1. The second portions 62b and 64b are reflected and diffracted by the grating 36 to -1st, 0th, and 1st order diffractions 66a, 66b, 66c, 68a, 68b, and 68c. As can be seen in Figure 4c, the -1st and +1st order diffractions 66a, 66c, 68a, and 68c are separated from the 0th order diffractions 66b and 86b, so that the -1st, 0th, and +1st order diffractions 66a, 66b, 66c, 68a, 86b, and 68c are spatially separated along directions perpendicular to the grating lines of each diffraction grating 44 and 46.

[0101] The beam separation configuration 48 causes the two beams 62 and 64 to propagate at a constant angle with respect to the axis 60, so that the diffracted first and second portions of the returning light (i.e., -1st, 0th, and +1st order) 66a, 66b, 66c, 68a, 86b, and 68c also propagate at a constant angle with respect to the axis 60. This means that the -1st, 0th, and +1st order 66a, 66b, 66c, 68a, 86b, and 68c pass through the second transparent substrate 40 in a region displaced laterally with respect to the beam separation configuration 48, and that these pass through the beam manipulation optical configuration 50. The beam manipulation optical arrangement 50 refracts the -1st, 0th, and +1st order beams 66a, 66b, 66c, 68a, 86b, and 68c, and reorients them so that each diffraction order of each beam collides with one of the photodetectors 54a, 54b, 54c, 56a, 56b, and 56c.

[0102] The beam separation configuration 48, combined with the diffraction gratings 44 and 46, spatially separates all six diffraction gratings (i.e., the -1st, 0th, and 1st order from each beam), making them operable on each photodetector 54a, 54b, 54c, 56a, 56b, and 56c by the beam manipulation optical configuration 50. Next, the signals detected by the photodetectors 54a, 54b, 54c, 56a, 56b, and 56c can be used to generate difference signals for each beam (i.e., corresponding to each diffraction grating).

[0103] The two diffraction gratings 44 and 46 have a relative height offset to extend the operating range of the optical displacement sensor 34, as described above with reference to Figure 3 (i.e., so that the signal for each beam corresponds to a different point of action on the membrane 36). In this example, the spacing between the membrane 36 and the gratings 44 and 46 is such that the optical path length from the gratings 44 and 46 to the membrane 36 for each light beam is a half-integer multiple of the Talbot length (as described above, so that it is within a preferred range), but this is not essential, and other embodiments and modifications of this embodiment may be provided without this feature.

[0104] Figures 5a and 5b show a schematic front view and a schematic top view, respectively, of a third embodiment of the optical displacement sensor 70 according to the present invention. The embodiment in Figure 5a is the same as the embodiments in Figures 4a to 4c, except that the optical displacement sensor 70 comprises three grids and three corresponding sets of photodetectors. For clarity, only one grid and only one set of photodetectors are shown in Figures 5a and 5b. The placement of the other two grids and sets of photodetectors will be discussed below with reference to Figures 5b and 5c.

[0105] The optical displacement sensor 70 includes a membrane 72, a first transparent substrate 74, a second transparent substrate 76, and an optoelectronic substrate 78. Figures 5a and 5b show a first diffraction grating 80 fabricated on the side of the first transparent substrate 74 facing the membrane 72. The second transparent substrate 76 includes a beam separation configuration 82 and a beam manipulation optical configuration 84 surrounding the beam separation configuration 82. In this example, the beam separation configuration 82 is in the form of a lens with multiple facets to also provide collimator functionality, but other configurations are possible. In this example, the beam manipulation optical configuration 84 includes multiple prisms etched to form the substrate surface, each prism configured to diffract one beam to one of the photodetectors, but other beam manipulation optical configurations are possible. The optoelectronic substrate 78 includes a light source 86, which in this example is a vertical-cavity surface-emitting laser (VCSEL). Figure 5b shows one set of the three photodetectors 88a, 88b, and 88c (one of which can be seen in Figure 5a).

[0106] As shown in Figure 5a, VCSEL86 has a beam angle θ L It emits uncollimated light 90. The uncollimated light 90 is directed along the beam axis 92. The beam separation configuration 82 separates the uncollimated light 90 into three parts. The beams are collimated to separate, substantially collimated beams, the first beam 94 of which is shown in Figure 5a. The first beam 94 is perpendicular to the first grating surface and parallel to the grating lines of the first grating 80, at an angle θ to the beam axis 92. B It then propagates to the first diffraction grating 80.

[0107] A first portion 96 of the first beam 94 passes through the first diffraction grating 80 and is reflected from the membrane 72. The reflected first portion then passes back through the diffraction grating 80, where it is diffracted to the -1st, 0th, and 1st order diffractions. A second portion 98 of the first beam 94 is reflected by the first grating 80, where the second portion 98 is diffracted to the -1st, 0th, and 1st order diffractions. The diffraction orders are spatially separated in a direction perpendicular to the grating lines of the first grating 80. The beam manipulation optical arrangement 84 directs each of the -1st, 0th, and +1st order diffractions to one of the photodetectors 88a, 88b, and 88c. The diffraction orders of the first and second portions 96 and 98 interfere in the photodetectors 88a, 88b, and 88c, so that the intensity of light detected in each photodetector 88a, 88b, and 88c is determined according to the separation between the first grating 80 and the membrane 72. Therefore, when the membrane position changes, the detected light intensity can be used to determine the displacement of the membrane.

[0108] As described above, the beam separation configuration 82 splits the uncollimated light 90 into three beams. In addition to the first beam 94, a second beam and a third beam are generated. Using the second and third diffraction gratings, diffraction orders -1, 0, and +1 are generated from the second and third beams, respectively, and these are directed onto each photodetector in the same manner as described above in relation to the first grating.

[0109] Figure 5b shows the relative positions of the regions occupied by the VCSEL 86, beam separation configuration 82, first grating 80, beam manipulation elements 100 of the beam manipulation optical configuration 84 (configured to direct the 0th order diffraction beam onto the corresponding photodetector), and photodetectors 88a, 88b, and 88c, as viewed in plan view. The dotted circle 102 indicates the region of the beam colliding with each of these elements for the 0th order diffraction beam. It can be seen that the beam width in each case is smaller than the region of the element it collides with, which is to avoid edge diffraction effects.

[0110] From Figure 5b, it can also be seen that, apart from the VCSEL 86 and beam separation configuration 82, the remaining elements are positioned within the 120° sector 103 of the circle. These remaining two diffraction gratings, along with the corresponding beam manipulation elements and photodetector sets (not shown in Figure 5b), are similarly positioned within 120° sectors rotated 120° and 240° relative to the sensor shown in Figure 5b, so that these three sectors together form a circle in plan view, where the circle includes all the elements of the optical displacement sensor 70.

[0111] Figure 5c shows a schematic plan view of an example photodetector layout for the optical displacement sensor 70 of the embodiment shown in Figures 5a and 5b. The layout has a 3x rotational symmetry so that three sets of photodetectors 104, 106 are provided, each set contained within one of three 120° sectors 108 within a circle 110 indicated by a dashed line. The photodetector 104, drawn with a solid line, is positioned to receive the 0th order diffraction from each grating. The photodetector 106, drawn with a dotted line, is positioned to receive the -1st and +1st order diffraction.

[0112] In the examples in Figures 5a and 5b, these three grids have a relative height offset, providing multiple points of action on the membrane and extending the dynamic range of the microphone in the same manner as described above with reference to Figure 3.

[0113] In this example, although not essential, the separation between the membrane and the diffraction grating is selected such that the optical path through which the first optical portion propagates for each hypothetical grating is a half-integer multiple of the Talbot length (within the preferred margin as discussed above), thereby providing the effects discussed above in relation to the reduction of optical loss.

[0114] Figure 6 shows a schematic front view of the optical displacement sensor 112 according to the fourth embodiment. The fourth embodiment may be considered a modification of the embodiments shown in Figures 4a to 4c, but in other embodiments, for example, the embodiments shown in Figures 5a to 5c may provide the same or similar modifications.

[0115] The optical displacement sensor 112 comprises a membrane 114 and two diffraction gratings 116. The diffraction gratings 116, together with the membrane, each form an interference arrangement 118.

[0116] The membrane 112 and diffraction grating 114 are supported above the substrate 120. (For clarity, the support structure for the membrane 114 and diffraction grating 116 is not shown in Figure 6.) The VCSEL 122 and six photodetectors 124 (two of which are visible in Figure 6) are mounted on the substrate 120.

[0117] In this embodiment, the VCSEL 122 is a back-emitting VCSEL with two light emitters 126. Two prisms 128 are formed on the front (light-emitting) surface of the VCSEL 122. A collimated optical arrangement including a lens 130 is positioned between the VCSEL 122 and the diffraction grating 116.

[0118] During operation, each light emitter 126 emits a light beam 132. Each beam passes through one of the prisms 128. The prisms 128 direct the beams 132 upwards through one of the lenses 130, where they collide the beams 132. The beams 132 then collide with one of the diffraction gratings 116.

[0119] The beam 132 interacts with each grating 116 and membrane 114 in the same manner as described above with reference to Figures 1, 4a, and 5a, namely, the first portion passes through each grating 116 so as to be reflected by the membrane 114 and returned to the grating 116, then passes through the grating 116 and is diffracted to diffraction orders -1, 0, and 1, respectively, colliding with one of the detectors 124. The second portion is reflected by each grating 116 and diffracted to diffraction orders -1, 0, and 1, each colliding with one of the detectors 124, where it interferes with the corresponding first portion, and the resulting signal is measured by the detector.

[0120] Therefore, this embodiment differs from the embodiments in Figures 4a-4c in that it uses multiple light sources (i.e., light emitters 126) to provide separated beams, rather than using a beam separation optical arrangement to separate a single beam from a single light source. In addition, in this particular example, a prism 124 is provided on the VCSEL surface that provides the operating function and directs the beam 132 to the lens 130, but this is not an essential feature of this embodiment or any other embodiment.

[0121] Figure 7 shows a schematic front view of the optical displacement sensor 134 according to the fifth embodiment. This embodiment is also a modification of the embodiments shown in Figures 4a to 4c. The optical displacement sensor 134 is identical in all respects to the optical displacement sensor 112 of Figure 6 and operates similarly. However, the method of directing the light beam onto the grating is different.

[0122] The optical displacement sensor 134 consists of a membrane 136 and two sensors positioned above the substrate 140. The substrate 140 is equipped with two diffraction gratings 138, and a VCSEL 142 and six photodetectors 144 are mounted on it. The VCSEL 142 is a back-emitting VCSEL with two emitters 146, each providing a light beam 148. However, instead of having a prism on the VCSEL surface with a separate lens mounted to direct and collimate the light beam 148, the VCSEL 142 has a lens 150 on its front (emitting) surface. The lens 150 both collimates and directs the light beam 148 so that it collides with the gratings 138.

[0123] Figure 8 is a schematic front view of the optical displacement sensor 152 according to the sixth embodiment. This embodiment is also a modification of the embodiments shown in Figures 4a to 4c. The optical displacement sensor 152 is identical in all respects to the optical displacement sensor 134 of Figure 7 and operates similarly. However, it differs in how it directs the light beam onto the grating.

[0124] The optical displacement sensor 152 comprises a membrane 154 and two diffraction gratings 156 positioned above the substrate 158, on which a VCSEL 160 and six photodetectors 162 are mounted. The VCSEL 160 is a back-emitting VCSEL. However, in this embodiment, the VCSEL 160 has only one emitter 164 that provides a single light beam 166. Furthermore, instead of a lens, a diffractive optical element 168 is present on the front (emitting) surface of the VCSEL 160.

[0125] The beam 168 from the VCSEL emitter 164 collides with the diffractive optical element 168, separating and collimating the beam 166 to produce two separated, collimated beams 170, which then collide with the diffraction grating 156.

[0126] Figure 9 shows a schematic front view of the optical displacement sensor 172 according to the seventh embodiment. This embodiment is also a modification of the embodiments shown in Figures 4a to 4c. The optical displacement sensor 172 is identical in all respects to the optical displacement sensor 112 of Figure 6 and operates similarly. However, it differs in the way it directs the light beam onto the grating.

[0127] The optical displacement sensor 172 comprises a membrane 174 and two diffraction gratings 176 positioned above a substrate 178, on which a VCSEL 180 and six photodetectors 182 are mounted. Each VCSEL 180 has two emitters 184 that provide each light beam 186, but in this embodiment, the VCSEL 180 is a forward-emitting VCSEL, and no prisms, lenses, or other optical elements are formed inside or on its front surface. Instead, a separate single lens 188 is positioned between the VCSEL 180 and the gratings 176. In this example, the lens 188 is a refractive lens, but this is not mandatory, and in other embodiments, the lens 188 may be a diffracting lens. The two light beams 186 are directed towards the lens 188, and each beam 186 passes through a different portion of the lens 188, where the beams 186 are collimated and directed towards one of the gratings 176.

[0128] Where technically applicable, the optical features and their variations described in the context of a particular embodiment having exactly two diffraction gratings may also be applied to embodiments having three or more diffraction gratings, and vice versa.

[0129] The embodiments described above are merely illustrative, and it will be understood that other embodiments and modifications are also possible within the scope of the present invention as defined in the appended claims.

Claims

1. An optical displacement sensor, Reflective surface and, One or more diffraction gratings spaced apart from the reflective surface, wherein the diffraction grating or each diffraction grating, together with the reflective surface, defines each interference arrangement, and i) the reflective surface or ii) either the diffraction grating or each diffraction grating is movable relative to one or more diffraction gratings, A light source is positioned to provide light to the interference configuration, such that for each interference configuration, a first portion of the light propagates through the interference configuration along a first optical path, and a second portion of the light propagates through the interference configuration along a second different optical path, thereby creating an optical path difference between the first and second optical paths, which is determined according to the separation between the reflective surface and the diffraction grating of the interference configuration. For each interference configuration, one or more photodetectors are provided in each set, arranged to detect at least a portion of the interference pattern generated by the first and second portions of the light, according to the optical path difference. A collimating optical arrangement is provided, which is arranged to at least partially collimate the light between the light source and the diffraction grating. With respect to the interference configuration or each interference configuration, when the reflecting surface or the diffraction grating is in the zero-displacement position, the diffraction grating is spaced apart from the reflecting surface by a distance such that each of the first portions of light travels along the optical path length L between the diffraction grating and the reflecting surface. The diffraction grating or each diffraction grating has the following relationship for the interference configuration or each interference configuration: [Number 14] [Number 15] The formula includes a periodic diffraction grating with the aforementioned grating period p such that it satisfies up to 20% of the following conditions, where n is an integer: T ｚ The Talbot length is defined as follows: [Number 16] An optical displacement sensor in which λ is the wavelength of the light.

2. The optical displacement sensor according to claim 1, comprising at least two diffraction gratings.

3. The optical displacement sensor according to claim 2, wherein the optical path length L differs depending on each diffraction grating.

4. At the zero displacement position, the reflective surface is separated from the diffraction grating or each diffraction grating by a vertical distance of at least 15 μm, as described in any one of the prior claims. Nsa.

5. The optical displacement sensor comprises two or more optical gratings, a) Each diffraction grating includes a set of parallel grid lines extending in each grid line direction, wherein the grid line direction of each diffraction grating in the set is different from the grid line direction of each other diffraction grating in the set, and / or b) The optical displacement sensor according to any one of the prior claims, comprising a beam-separating optical arrangement configured to separate the light into two or more beams, and directing each of the two or more beams to one of the diffraction gratings.

6. The optical displacement sensor according to any one of claims 1 to 4, wherein the optical displacement sensor comprises two or more diffraction gratings, and the light source comprises a plurality of light source elements such that the light is provided as a plurality of light beams, each light source element providing one of the beams and directing each beam of light to one of the diffraction gratings.

7. An optical displacement sensor, Reflective surface and, Two or more diffraction gratings spaced apart from the reflective surface, each diffraction grating together with the reflective surface defines a respective interference configuration, and i) either the reflective surface or ii) the diffraction gratings are movable relative to each other, A light source is positioned to provide light to the interference configuration, such that for each interference configuration, a first portion of the light propagates through the interference configuration along a first optical path, and a second portion of the light propagates through the interference configuration along a second different optical path, thereby creating an optical path difference between the first and second optical paths, which is determined according to the separation between the reflective surface and the diffraction grating of the interference configuration. For each interference configuration, one or more photodetectors are provided in each set, arranged to detect at least a portion of the interference pattern generated by the first and second portions of the light, according to the optical path difference. A collimating optical arrangement is provided, which is arranged to at least partially collimate the light between the light source and the diffraction grating. b) Each diffraction grating includes a set of parallel grid lines extending in each grid line direction, wherein the grid line direction of each diffraction grating in the set of diffraction gratings is different from the diffraction grid line direction of each other diffraction grating in the set, and / or b) The optical displacement sensor according to any one of the prior claims, comprising a beam-separating optical arrangement configured to separate the light into two or more beams, and directing each of the two or more beams to one of the diffraction gratings.

8. The optical displacement sensor according to claim 5 or 7, wherein the collimating optical arrangement and the beam-separating optical arrangement are formed as a single component that performs both the function of collimating the light at least partially and the function of separating the light into two or more beams.

9. An optical displacement sensor, Reflective surface and, Two or more diffraction gratings spaced apart from the reflective surface, each diffraction grating together with the reflective surface defines a specific interference configuration, and either i) the reflective surface or ii) the diffraction grating is movable relative to the other, By being positioned to provide light to the interference arrangement, for each interference arrangement, a first portion of the light propagates through the interference arrangement along a first optical path, and a second portion of the light propagates through the interference arrangement along a second different optical path, the first and the second, which are determined according to the separation between the reflective surface and the diffraction grating of the interference arrangement. A light source that creates an optical path difference between two optical paths, For each interference configuration, one or more photodetectors are provided in each set, arranged to detect at least a portion of the interference pattern generated by the first and second portions of the light, according to the optical path difference. A collimating optical arrangement is provided, which is arranged to at least partially collimate the light between the light source and the diffraction grating. The optical displacement sensor comprises a plurality of light source elements such that the light is provided as a plurality of light beams, each light source element providing one of the beams and directing each beam of light to one of the diffraction gratings.

10. An optical displacement sensor according to any one of the prior claims, wherein a different perpendicular optical path length exists between each diffraction grating and the reflective surface with respect to each diffraction grating.

11. The collimated optical arrangement includes a lens, as described in any one of the prior claims, for the optical displacement sensor.

12. The optical displacement sensor according to any one of the prior claims, wherein the beam collides with the diffraction grating at a constant angle with respect to the normal of the plane on which each diffraction grating is mounted.

13. The optical displacement sensor according to claim 12, wherein the direction of propagation of each beam is i) parallel to the grid lines of the diffraction grating with which the beam collides, and ii) in a plane perpendicular to the surface on which the diffraction grating is mounted.

14. The optical displacement sensor according to any one of the prior claims, comprising two or more diffraction gratings, wherein one beam or each beam collides with each diffraction grating at each angle of incidence with respect to the normal to the plane on which each diffraction grating is mounted, and the angle of incidence for each diffraction grating in a pair of diffraction gratings is different from the angle of incidence for each other diffraction grating in the pair.

15. The optical displacement sensor according to any one of the prior claims, wherein the beam direction of the light, or the beam direction of the light beam colliding with the diffraction grating, is perpendicular to the surface of the diffraction grating.

16. The optical displacement sensor according to any one of the prior claims, comprising two or more diffraction gratings, each diffraction grating oriented along one of a pair of lines extending radially from the center point between the gratings.

17. The optical displacement sensor according to any one of the prior claims, comprising N grids, wherein the diffraction grids are oriented at an angle of (360°) / N or a multiple thereof with respect to each other.

18. The optical displacement sensor according to any one of the prior claims, wherein the interference configuration includes a pair of diffraction gratings having the same grating period and the same grating line direction, and the pair of diffraction gratings together function to direct light onto the same set of one or more photodetectors corresponding to the interference configuration, separated from the reflective surface at the same optical distance.

19. The optical displacement sensor according to any one of the prior claims, wherein the diffraction grating is arranged as a rotationally symmetric composite diffraction grating.

20. The optical displacement sensor according to any one of the prior claims, further comprising a beam manipulation optical arrangement arranged to direct the first and second optical portions for each interference arrangement onto each photodetector provided in the interference arrangement.

21. The optical displacement sensor according to claim 20, wherein the beam separation optical arrangement or the beam separation optical arrangement and the beam manipulation optical arrangement are provided on a common substrate.

22. An optical displacement sensor according to any one of the prior claims, wherein one or more photodetectors in each set comprises two photodetectors, the photodetectors being arranged such that the +1st order diffraction coincides with a first photodetector and the -1st order diffraction coincides with a second photodetector.

23. An optical displacement sensor according to any one of the prior claims, wherein one or more photodetectors in each set comprises three photodetectors, the photodetectors being arranged such that the +1st order diffraction coincides with the first photodetector, the 0th order diffraction coincides with the second photodetector, and the -1st order diffraction coincides with the third photodetector.

24. The optical displacement sensor according to any one of the prior claims, further comprising a membrane, wherein the reflective surface includes the surface of the membrane or is provided on the surface of the membrane.

25. An optical microphone comprising an optical displacement sensor as described in any one of the prior claims.

26. An optical displacement sensor, Reflective surface and, One or more diffraction gratings spaced apart from the reflective surface, wherein the diffraction grating or each diffraction grating, together with the reflective surface, defines each interference arrangement, and i) the reflective surface or ii) either the diffraction grating or each diffraction grating is movable relative to one or more diffraction gratings, A light source is positioned to provide light to the interference configuration, such that for each interference configuration, a first portion of the light propagates through the interference configuration along a first optical path, and a second portion of the light propagates through the interference configuration along a second different optical path, thereby creating an optical path difference between the first and second optical paths, which is determined according to the separation between the reflective surface and the diffraction grating of the interference configuration. For each interference configuration, one or more photodetectors are provided in each set, arranged to detect at least a portion of the interference pattern generated by the first and second portions of the light, according to the optical path difference. A collimating optical arrangement is provided, which is arranged to at least partially collimate the light between the light source and the diffraction grating. With respect to the interference configuration or each interference configuration, when the reflecting surface or the diffraction grating is in the zero-displacement position, the diffraction grating is spaced apart from the reflecting surface by a distance such that each of the first portions of light travels along the optical path length L between the diffraction grating and the reflecting surface. The diffraction grating or each diffraction grating has the following relationship for each interference configuration: the grating period p and the optical path length L [Number 17] [Number 18] The formula includes a periodic diffraction grating with the aforementioned grating period p such that it satisfies up to 20% of the following conditions, where m is an odd integer. T ｚ The Talbot length is defined as follows: [Number 19] In the formula, λ is the wavelength of light; this is an optical displacement sensor.

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