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EP-4260004-B1 - OPTICAL DISPLACEMENT SENSOR

EP4260004B1EP 4260004 B1EP4260004 B1EP 4260004B1EP-4260004-B1

Inventors

  • SAGBERG, Håkon
  • LACOLLE, MATTHIEU

Dates

Publication Date
20260506
Application Date
20211214

Claims (15)

  1. An optical displacement sensor (2; 34; 70; 112; 134; 152; 172) comprising: a reflective surface (4; 36; 72; 114; 136; 154; 174); one or more diffraction gratings (6; 14, 16, 18; 44, 46; 80; 116; 138; 156; 176) spaced from the reflective surface, wherein the diffraction grating or each of the diffraction gratings together with the reflective surface defines a respective interferometric arrangement, and wherein either: i) the reflective surface is moveable relative to the diffraction grating or to each of the diffraction gratings; or ii) the diffraction grating or each of the diffraction gratings is moveable relative to the reflective surface; a light source (8; 52; 86; 122; 142; 160; 180) arranged to provide light (12; 58; 90; 132; 148; 166; 186) to said interferometric arrangement(s) such that, for each interferometric arrangement, a first portion of said light propagates along a first optical path via the interferometric arrangement and a second portion of said light propagates along a second, different optical path via said interferometric arrangement, thereby giving rise to an optical path difference between the first and second optical paths which depends on a separation between the reflective surface (4; 36; 72; 114; 136; 154; 174) and the diffraction grating (6; 14, 16, 18; 44, 46; 80; 116; 138; 156; 176) of the interferometric arrangement; and for each interferometric arrangement, a respective set of one or more photo detector(s) (10; 20a-b, 22a-b, 24a-b; 54a-c, 56a-c; 88a-c; 104, 106; 124; 144; 162; 182) arranged to detect at least part of an interference pattern generated by said first and second portions of light dependent on said optical path difference; a collimating optical arrangement (15; 82; 130; 150; 168; 188) arranged to at least partially collimate the light between the light source (8; 52; 86; 122; 142; 160; 180) and the diffraction grating(s) (6; 14, 16, 18; 44, 46; 80; 116; 138; 156; 176); wherein, for the interferometric arrangement or each of the interferometric arrangements, when the reflective surface (4; 36; 72; 114; 136; 154; 174) or the diffraction grating (6; 14, 16, 18; 44, 46; 80; 116; 138; 156; 176) is in a zero-displacement position, the diffraction grating is spaced from the reflective surface by a distance such that the respective first portion of light travels along an optical path length L between the diffraction grating and the reflective surface; wherein the diffraction grating or each of the diffraction gratings (6; 14, 16, 18; 44, 46; 80; 116; 138; 156; 176) comprises a periodic diffraction grating with a grating period p such that for the interferometric arrangement or each of the interferometric arrangements, the grating period p and the optical path length L satisfy: A) the relationship: L = T z n 2 , to within 20% of T z 2 , where n is an integer; or B) the relationship: L = T z m 4 , to within 20% of T z 4 , where m is an odd integer; where T z is the Talbot length, defined by: T z = λ 1 − 1 − λ 2 p 2 , where λ is the wavelength of the light (12; 58; 90; 132; 148; 166; 186).
  2. The optical displacement sensor (2; 34; 70; 112; 134; 152; 172) of claim 1, comprising at least two diffraction gratings (14, 16, 18; 44, 46; 80; 116; 138; 156; 176).
  3. The optical displacement sensor (2; 34; 70; 112; 134; 152; 172) of claim 2, wherein the optical path length L is different for each diffraction grating (14, 16, 18; 44, 46; 80; 116; 138; 156; 176), optionally wherein between each diffraction grating and the reflective surface (4; 36; 72; 114; 136; 154; 174) there is a perpendicular optical path length which is different for each diffraction grating.
  4. The optical displacement sensor (2; 34; 70; 112; 134; 152; 172) of any preceding claim, wherein in the zero-displacement position the reflective surface (4; 36; 72; 114; 136; 154; 174) is separated from the diffraction grating or each of the diffraction gratings (6; 14, 16, 18; 44, 46; 80; 116; 138; 156; 176) by a perpendicular distance of at least 15µm.
  5. The optical displacement sensor (2; 34; 70; 152) of any preceding claim, wherein the optical displacement sensor comprises two or more diffraction gratings (14, 16, 18; 44, 46; 80; 156); and a) each diffraction grating (14, 16, 18) comprises a set of parallel grating lines extending in a respective grating line direction, and wherein the grating line direction of each diffraction grating in a set of said diffraction gratings is different from the grating line direction of each other diffraction grating in said set; and/or b) the optical displacement sensor comprises a beam-separating optical arrangement (48; 82; 168) arranged to separate the light (58; 90; 166) into two or more beams (62, 64; 94; 170), wherein each of the two or more beams is directed onto a respective one of the diffraction gratings (44, 46; 80; 156).
  6. The optical displacement sensor (112; 134; 172) of any of claims 1 to 4, wherein the optical displacement sensor comprises two or more diffraction gratings (116; 138; 176); and the light source (122; 142; 180) comprises a plurality of light source elements (126; 146; 184) such that the light is provided as a plurality of beams of light (132; 148; 186), wherein each light source element provides a respective one of said beams, and wherein each beam of light is directed onto a respective one of the diffraction gratings.
  7. The optical displacement sensor (34; 70; 112; 134; 152; 172) of claim 5 or 6, wherein the beams (62, 64; 94; 132; 148; 170; 186) impinge on the diffraction gratings (44, 46; 80; 116; 138; 156; 176) at an angle to a normal to a plane in which the respective diffraction grating lies, wherein the direction of propagation of each beam is in a plane that is i) parallel to the grating line direction of the diffraction grating on which said beam impinges and ii) perpendicular to a plane in which said diffraction grating lies.
  8. The optical displacement sensor (34; 70; 112; 134; 152; 172) of any preceding claim, wherein the optical displacement sensor comprises two or more diffraction gratings (44, 46; 80; 116; 138; 156; 176), and wherein a or each beam (62, 64; 94; 132; 148; 170; 186) impinges on its respective diffraction grating at a respective incidence angle to a normal to a plane in which the respective diffraction grating lies, wherein the incidence angle for each diffraction grating in a set of said diffraction gratings is different from the incidence angle of each other diffraction grating in said set.
  9. The optical displacement sensor (2) of any preceding claim, wherein a beam direction of the light (12) or of a beam of light impinging on the diffraction gratings (14, 16, 18) is perpendicular to a surface of the diffraction gratings.
  10. The optical displacement sensor (2; 34; 70; 112; 134; 152; 172) of any preceding claim, wherein the optical displacement sensor comprises two or more diffraction gratings (14, 16, 18; 44, 46; 80; 116; 138; 156; 176), and wherein each diffraction grating is oriented along a line of a set of lines extending radially from a centre point between the gratings.
  11. The optical displacement sensor (2; 34; 70; 112; 134; 152; 172) of any preceding claim, wherein the optical displacement sensor comprises N gratings (14, 16, 18; 44, 46; 80; 116; 138; 156; 176), wherein the diffraction gratings are oriented at an angle of (360°)/N or a multiple thereof with respect to each other.
  12. The optical displacement sensor (2) of any preceding claim, wherein the interferometric arrangement or each of the interferometric arrangements comprises a pair of diffraction gratings (14a-b, 16a-b, 18a-b) having the same grating period and the same grating line direction, and being separated from the reflective surface (4) by the same optical distance, such that the pair of diffraction gratings function together to direct light onto the same set of one or more photo detectors (20a-b, 22a-b, 24a-b) corresponding to said interferometric arrangement.
  13. The optical displacement sensor (34; 70) of any preceding claim, further comprising a beam-steering optical arrangement (50; 84) arranged to direct the first and second light portions for each interferometric arrangement onto the respective photo detector(s) (54a-c, 56a-c; 88a-c; 104, 106) provided for said interferometric arrangement.
  14. The optical displacement sensor (2; 34; 70; 112; 134; 152; 172) of any preceding claim, wherein each set of one or more photo detectors comprises two photodetectors (10), and wherein said photo detectors are arranged such that a +1 st diffraction order impinges on a first one of said photo detectors and a -1 st diffraction order impinges on a second one of said photo detectors; or wherein each set of one or more photo detectors comprises three photodetectors (54a-c, 56a-c; 88a-c; 104, 106; 124; 144; 162; 182), and wherein said photo detectors are arranged such that a +1 st diffraction order (66c, 68c) impinges on a first one of said photo detectors (54c, 56c), a 0 th diffraction order (66b, 68b) impinges on a second one of said photo detectors (54b, 56b) and a -1 st diffraction order (66a, 68a) impinges on a third one of said photo detectors (54a, 56a).
  15. An optical microphone comprising the optical displacement sensor (2; 34; 70; 112; 134; 152; 172) of any preceding claim.

Description

This invention relates generally to optical displacement sensors and in particular, but not exclusively, to optical displacement sensors for use in optical microphones. Microphones are used to convert sound waves into electrical signals, typically by measuring the displacement of a moveable member (e.g. a membrane) that vibrates in response to ambient acoustic vibrations. There are a number of ways of measuring the displacement of such a moveable member, including capacitive readout (commonly called condenser microphones) and electrostatic or electromagnetic readout mechanisms (e.g. dynamic microphones). An alternative way of reading out the position of a microphone membrane is to use an optical displacement sensor that uses optical interferometric read out. In typical examples of such systems, a diffraction grating is provided on a substrate adjacent to a membrane, and light is directed onto the diffraction grating. A first portion of the light is reflected back from the grating. A second portion is transmitted through the grating, which diffracts the radiation. The diffracted radiation impinges on the membrane, which reflects it onto the grating. The radiation passes through the grating and the two portions of light interfere to create an interference pattern that can be detected by a detector. The interference pattern has a shape (i.e. spatial distribution) matching the diffraction orders of the grating, but the light intensity directed into these diffraction orders depends on the relative phase of the two portions of light, and therefore on the distance between the grating and the membrane. The position (and therefore the movement) of the membrane can thus be determined from changes in the intensity of the light at the detector. WO 2019/220103, US 2004/130728 and Hall et al., "Micromachined optical microphone structures with low thermal-mechanical noise levels", J. Acoust. Soc. Am. 122(4), 2007 (pp. 2031-2037) disclose examples of optical displacement sensors known in the art. Such optical displacement sensors have a high signal to noise ratio (SNR) and high sensitivity, which is beneficial for use in optical microphones and other applications. However, further improvements in the performance of such optical displacement sensors are desirable. The present invention is directed to an optical displacement sensor as defined by independent claim 1. Further aspects of the invention are according to the dependent claims. The invention provides an optical displacement sensor comprising: a reflective surface;one or more diffraction gratings spaced from the reflective surface, wherein the diffraction grating or each of the diffraction gratings together with the reflective surface defines a respective interferometric arrangement, and wherein either: i) the reflective surface is moveable relative to the diffraction grating or to each of the diffraction gratings; orii) the diffraction grating or each of the diffraction gratings is moveable relative to the reflective surface;a light source arranged to provide light to said interferometric arrangement(s) such that, for each interferometric arrangement, a first portion of said light propagates along a first optical path via the interferometric arrangement and a second portion of said light propagates along a second, different optical path via said interferometric arrangement, thereby giving rise to an optical path difference between the first and second optical paths which depends on a separation between the reflective surface and the diffraction grating of the interferometric arrangement; andfor each interferometric arrangement, a respective set of one or more photo detector(s) arranged to detect at least part of an interference pattern generated by said first and second portions of light dependent on said optical path difference;a collimating optical arrangement arranged to at least partially collimate the light between the light source and the diffraction grating(s);wherein, for the interferometric arrangement or each of the interferometric arrangements, when the reflective surface or the diffraction grating is in a zero-displacement position, the diffraction grating is spaced from the reflective surface by a distance such that the respective first portion of light travels along an optical path length L between the diffraction grating and the reflective surface;wherein the diffraction grating or each of the diffraction gratings comprises a periodic diffraction grating with a grating period p such that for the interferometric arrangement or each of the interferometric arrangements, the grating period p and the optical path length L satisfy: A the relationship: L=Tzn2, to within 20% of Tz2, where n is an integer; orB) the relationship: L=Tzm4, to within 20% of Tz4, where m is an odd integer; where Tz is the Talbot length, defined by: Tz=λ1−1−λ2p2,where λ is the wavelength of the light. The Applicant has appreciated that in optical displacement sensors utilising diffr