Optical microphone

Abstract

An optical microphone (10), comprises a laser device (105) configured to emit laser radiation (16), a membrane (15) con- figured to reflect the laser radiation (16), and a transparent substrate (100) attached to the laser device (105). A first diffraction grating (110) is integrated with the transparent substrate (100) and is configured to diffract the laser radiation (17) that has been reflected by the membrane (15). The optical microphone (10) further comprises a detector (119) comprising a set of photodiodes (120). The detector (119) is configured to detect the laser radiation that has been diffracted by the first diffraction grating (110).

Description

[0001] 2024PF01034 1

[0002] OPTICAL MICROPHONE

[0003] Microphones or electroacoustic converters are increasingly employed in a variety of applications . Therefore, attempts are being made to improve microphones .

[0004] It is an obj ect of the present invention to provide an improved optical microphone .

[0005] SUMMARY

[0006] According to embodiments, the above obj ect is achieved by the claimed matter according to the independent claims . Further developments are defined in the dependent claims .

[0007] According to embodiments, an optical microphone comprises a laser device configured to emit laser radiation, a membrane configured to reflect the laser radiation, and a transparent substrate attached to the laser device . A first diffraction grating is integrated with the transparent substrate and is configured to diffract the laser radiation that has been reflected by the membrane . The optical microphone further comprises a detector comprising a set of photodiodes, the detector being configured to detect the laser radiation that has been diffracted by the first diffraction grating.

[0008] The optical microphone may further comprise a collimating element arranged over an aperture of the laser device . For example, the collimating element may comprise a metalens . According to further embodiments, the collimating element may comprise a Fresnel lens .2024PF01034 2

[0009] According to embodiments, the collimating element may be configured to tilt the laser radiation with respect to a vertical direction .

[0010] For example, the collimating element may comprise a lens directly adj acent to the laser device .

[0011] According to embodiments, a center axis of the lens may be shifted in a horizontal direction with respect to the aperture of the laser device .

[0012] According to further embodiments, the first diffraction grating may be configured to diffract the emitted laser radiation and the reflected laser radiation.

[0013] The optical microphone may further comprise a second diffraction grating configured to diffract a beam of the laser radiation. The second diffraction grating may be arranged on a side of the substrate remote from the first diffraction grating.

[0014] For example, the optical microphone may further comprise an intermediate carrier that is arranged between the laser device and the substrate .

[0015] For example, the second diffraction grating may be implemented as a blazed grating.

[0016] An electronic device may comprise the optical microphone as described above .

[0017] For example, the electronic device may be selected from a consumer electronic device and an automotive application.

[0018] BRIEF DESCRIPTION OF THE DRAWINGS2024PF01034 3

[0019] The accompanying drawings are included to provide a further understanding of embodiments of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles . Other embodiments of the invention and many of the intended advantages will be readily appreciated, as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other . Like reference numbers designate corresponding similar parts .

[0020] Fig. 1A is a schematic view of an optical microphone according to embodiments .

[0021] Fig. IB is a schematic view of an optical microphone according to further embodiments .

[0022] Fig. 1C is a schematic view of an optical microphone according to further embodiments .

[0023] Fig. ID is a top view of an optical microphone according to embodiments .

[0024] Figs . 2A and 2B illustrate examples of a collimating element .

[0025] Fig. 3A is a schematic view of a first diffraction grating.

[0026] Fig. 3B is a schematic view of a second diffraction grating.

[0027] Fig. 4 shows an example of signals that may be generated by the optical microphone illustrated in Figs . 1A, IB and 1C .2024PF01034 4

[0028] Fig. 5 is a schematic view of an electronic device according to embodiments .

[0029] DETAILED DESCRIPTION

[0030] In the following detailed description reference is made to the accompanying drawings, which form a part hereof and in which are illustrated by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology such as "top" , "bottom" , "front" , "back" , "over" , "on" , "above" , "leading" , "trailing" etc . is used with reference to the orientation of the Figures being described. Since components of embodiments of the invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope defined by the claims .

[0031] The description of the embodiments is not limiting. In particular, elements of the embodiments described hereinafter may be combined with elements of different embodiments .

[0032] The terms "lateral" and "horizontal" as used in this specification intends to describe an orientation parallel to a first surface of a substrate or semiconductor body. This can be for instance the surface of a wafer or a die .

[0033] The term "vertical" as used in this specification intends to describe an orientation which is arranged perpendicular to the first surface of a substrate or semiconductor body.

[0034] Fig. 1A is a schematic view of an optical microphone 10 according to embodiments . The optical microphone 10 comprises a laser2024PF01034 5

[0035] device 105 which is configured to emit laser radiation 16. The optical microphone 10 further comprises a membrane 15 which is configured to reflect the laser radiation 16. The optical microphone 10 further comprises a transparent substrate 100 which is attached to the laser device 105. As is illustrated in Fig.

[0036] 1A, the transparent substrate 100 may be attached to the laser device 105 by means of an intermediate carrier 102 which is transparent for the electromagnetic radiation emitted by the laser device 105. A first diffraction grating 110 is integrated with the transparent substrate 100 and is configured to diffract the laser radiation 17 that has been reflected by the membrane 15. The optical microphone 10 further comprises a detector 119 comprising a set of photodiodes 120. The detector 119 is configured to detect the laser radiation that has been diffracted by the first diffraction grating 110.

[0037] The membrane 15 is attached to a suitable housing 35 which houses the elements of the optical microphone 10. When an acoustic signal (pressure gradient) 20 is incident on the membrane 15, the membrane 15 oscillates e . g. by a distance d which changes the distance between the membrane 15 / ref lector and the first diffraction grating 110. The diffraction grating 110 diffracts the reflected laser radiation 17 to generate diffracted beams 252, 252, and 253corresponding to different diffraction orders . The difference between the detected intensity of the diffracted beam 252and the sum of the detected intensity of the diffracted beams 252and 253can be converted into an electrical signal .

[0038] The laser device 105 may for example be a VCSEL ("vertical cavity surface-emitting laser") or another surface-emitting laser e . g. a PCSEL ("photonic crystal surface-emitting laser") or a HCSEL ("horizontal cavity surface-emitting laser") . According to further embodiments, the laser device 105 may be implemented in2024PF01034 6

[0039] any other manner . According to further embodiments, the laser device 105 may also be implemented as an edge-emitting laser .

[0040] The generated laser radiation is emitted via an aperture 107. The substrate 100 is transparent to laser radiation 16 emitted by the laser device 105. For example, the transparent substrate 100 may comprise SiO2or any other suitable material . For example, a thickness of the transparent substrate 100 may be more than 1 pm, for example some 100 pm. For example, a thickness of the substrate 100 may be less than 1 mm so as to achieve a compact microphone .

[0041] The term "the first diffraction grating being integrated with the transparent substrate" is intended to mean that the first diffraction grating 110 is embedded in the transparent substrate 100 or formed on a surface of the transparent substrate 100 or attached to a surface of the transparent substrate 100. The first diffraction grating 110 is arranged on a side remote from the laser device 105.

[0042] The first diffraction grating 110 is arranged over the detector 119. Accordingly, a horizontal position of the first diffraction grating 110 corresponds to a horizontal position of the detector 119. Further, the first diffraction grating 110 is shifted in a horizontal direction with respect to the position of the laser device 105. Elements of the first diffraction grating 110 will be explained in more detail below with reference to Fig. 3A. The transparent substrate 100 may be attached or connected to the laser device 105 via a transparent adhesive 108. Further, the detector 119 is attached to the transparent substrate 100 via the transparent adhesive 108.

[0043] The optical microphone 10 may further comprise a collimating element 115 which is arranged over the aperture 107 of the laser2024PF01034 7

[0044] device 105. The collimating element 115 may comprise a lens . For example, the collimating element 115 may be implemented as a metalens or a Fresnel lens .

[0045] Generally, the term "metalens" or "metaoptic structure" refers to a lens that bends light using an array of nanostructures that are formed on a (ideally flat) surface of a substrate, instead of via refraction. More specifically, a metalens includes a metasurface that includes an array of nanostructures, wherein the nanostructure array is configured to bend light incident thereon by altering its phase . For example, the phase change imparted by the metasurface can create a new wave front in a region down field of the lens . For example, a metalens can include an array of nanostructures that can impart a phase change to incident light having a spherical or hemispherical wave front up field of a lens, such that the light in a region down field of the lens has a planar wave front (i . e . , a plane wave) .

[0046] Stated more generally, metalenses are designed to shape light to a dedicated radiation pattern. This may be accomplished due to phase shifts and interference effects of geometrical structures having sizes below the wavelength of the electromagnetic radiation to be shaped by the metalenses . According to an approximation, the phase shifts may be interpreted as waveguide modes propagating in the nanostructures, generating a phase shift dependent on the height of the nanostructure of a given geometry and size .

[0047] Furthermore, the propagation in materials may also change the polarization direction of light . After passing areas having a structured pattern of polarization rotation material, interferences may be generated, the interferences redirecting the light .2024PF01034 8

[0048] For example, nanostructures that may be a component of metalenses are generally configured to function as resonators or waveguides that impart a phase change to light incident thereon. According to implementations, the metalenses may comprise pillars or ridges and a planarization layer filling the spaces between adj acent pillars or ridges . In this context, the pillars / ridges or the planarization layer in between may constitute the nanostructures mentioned above .

[0049] A Fresnel lens may be a lens in which the continuous surface of a standard lens is divided into a set of surfaces of the same curvature, with stepwise discontinuities between them. Accordingly, the Fresnel lens may reduce the amount of material required compared to a conventional lens by dividing the lens into a set of concentric annular sections .

[0050] Further details of the collimating element 115 implemented as a metalens or a Fresnel lens will be given below while referring to Figs . 2A and 2B, respectively. Due to the presence of the collimating element 115, the laser device 105 implements a plane wave illumination source . As a consequence, the readout performance may be improved. Due to the presence of a transparent substrate and due to the first diffraction grating being integrated with the transparent substrate, the stability of the grating may be improved even when a high sound pressure is incident on the optical microphone . As will be explained below with reference to Fig. 3A, a grating having a small thickness measured in a vertical direction is beneficial to improve the directional readout . Due to the integration of the first diffraction grating with the transparent substrate, the first diffraction grating may be made very thin while at the same time maintaining the mechanical stability of the first diffraction grating. In particular, due to the presence of the transparent substrate 100,2024PF01034 9

[0051] the first diffraction grating 110 is not bent or deformed due to the high sound pressure of the acoustic signal 20.

[0052] The optical microphone 10 further comprises a second diffraction grating 112 that is configured to tilt the laser radiation from a vertical orientation by up to 45 degrees . As a result, selfmixing interference (SMI ) within the laser device 105 may be reduced or even avoided. For example, the second diffraction grating 112 may be implemented as a blazed grating which will be described in more detail below with reference to Fig. 3B . According to embodiments illustrated in Figs . 1A and IB, the second diffraction grating 112 may be integrated with the intermediate carrier 102. For example, the second diffraction grating 112 may be arranged over a surface of the intermediate carrier 102. According to further embodiments, the second diffraction grating 112 may be formed in the intermediate carrier 112. According to still further embodiments, the second diffraction grating 112 may be arranged at a surface of the substrate 100 facing the intermediate carrier 102.

[0053] As is illustrated in Fig. 1A, the laser radiation 16 is reflected by the membrane 15 towards the first diffraction grating 110. The first diffraction grating 110 is configured to diffract the reflected laser radiation 17 to generate diffracted beams 25x, 252, and 253which are spatially separated along a horizontal direction. The first diffraction grating 110 is shifted along a horizontal direction, e . g. in the x-y-plane, with respect to the second diffraction grating 112. For example, the first diffraction grating 110 and the second diffraction grating 112 do not overlap or overlap only to less than 20 % in a vertical direction, e . g. in a z-direction or in a direction perpendicular to an extension direction of the first diffraction grating 110. The detector 119 comprises photodiodes 120 which are arranged at different horizontal positions so as to detect the different2024PF01034 10

[0054] diffracted beams 253... 253separately from each other . In more detail, the detector 119 comprises different portions that may be configured to selective detect the diffracted beams 253... 253in accordance with their diffraction order . Accordingly, separate signals for the different diffraction orders may be detected .

[0055] As is further illustrated in Fig. 1A, the membrane 15 and the first diffraction grating 110 may be connected to a voltage source 30. By setting an appropriate voltage between the membrane 15 and the first diffraction grating 110, a distance between the membrane 15 and the first diffraction grating 110 may be tuned. For example, the distance between the membrane 15 and the first diffraction grating 110 may be in a range of 3 to 8 pm, e . g. 4 to 6 pm, e . g. approximately 5 pm. Due to tuning the distance, an operating point of the optical microphone 10 may be set so as to achieve operation within the linear region 128 which is illustrated in Fig. 4 below. The optical microphone 10 may further comprise an interposer 118 so as to adjust the height of the detector 119.

[0056] As is further illustrated in Fig. 1A, a horizontal extension, e . g. measured in the x-y plane, of the substrate 100 may be equal to a horizontal extension of the intermediate carrier 102.

[0057] Fig. IB is a schematic view of an optical microphone 10 according to further embodiments . The optical microphone 10 of Fig. IB comprises similar elements as the optical microphone 10 illustrated in Fig. 1A so that a detailed description thereof will be omitted here . Differing from embodiments illustrated in Fig. 1A, the horizontal extension of the substrate 100 may be less than the horizontal extension of the intermediate carrier 102. For example, a difference between the horizontal extension of the substrate 100 and the horizontal extension of the intermediate2024PF01034 11

[0058] carrier 102 may be 2 * d. For example, d may be approximately more than 50pm. For example, d may be less than 500 pm. Due to the smaller horizontal extension of the substrate 100 in comparison with the horizontal extension of the intermediate carrier 102, the optical microphone 10 may reduce the acoustic noise between the membrane 15 and first diffraction grating 110.

[0059] Fig. 1C is a schematic view of an optical microphone 10 according to further embodiments . The optical microphone 10 of Fig. 1C comprises similar elements as the optical microphone 10 illustrated in Fig. 1A so that a detailed description thereof will be omitted here . Differing from embodiments illustrated in Fig. 1A, the collimating element 115 is implemented by a lens 121, e . g. a microlens, which collimates the generated laser radiation 16. According to embodiments, the lens 121 may be shifted in a horizontal direction with respect to the aperture 107 of the laser device 105. For example, the lens 121 may be arranged so that a center axis 129 of the lens 121 does not coincide with a center of the aperture 107. As a result, generated laser radiation 16 is tilted by the surface of the lens 121 in a horizontal direction. As a consequence, a second diffraction grating 112 may be dispensed with. Further, the intermediate carrier 102 may also be dispensed with. Consequently, according to embodiments, the substrate 100 is directly attached to the laser device 105 without an intermediate carrier being arranged in between.

[0060] In this case, for example, the first diffraction grating may be implemented as a blazed tilting grating. In addition to the tilting, the grating 110 may polarize the laser beam.

[0061] Fig. ID is a top view of the optical microphone 10 according to embodiments . As is shown, the laser device 105 and the collimating element 115 are arranged at the same horizontal position. The horizontal position of the detector 119 as well as of the2024PF01034 12

[0062] first diffraction grating 110 are shifted with respect to the position of the laser device 105. The laser device 105 and the detector 119 are arranged within a housing 35. The optical microphone 10 further comprises an air hole 122 through which the acoustic signal may enter the housing 35. For example, the hole 122 may be arranged in a bottom part of the housing 35.

[0063] Fig. 2A is a schematic view of the collimating element 115 that may be implemented as a metalens . For example, the metalens may comprise a plurality of structural elements 123 that may be implemented as pillars . Irrespective of the realization of the optical element as a metalens or a Fresnel lens, a material of the structural elements 123 may be selected that a high index contrast is achieved between the structural elements 123 and the surrounding material, e . g. the transparent substrate 100. For example, when the transparent substrate 100 is made of silicon oxide having a comparatively low refractive index, the structural elements 123 may be made of a high refractive index material, e . g. TiO2, Si, Nb2O5. According to further implementations, the structural elements may be made of silicon and may be surrounded by air holes . Moreover, when the transparent substrate is made of a high index material, the structural elements may be implemented by air holes .

[0064] As is illustrated in Fig. 2B, for implementing a Fresnel lens, the structural elements 123 may form concentric rings .

[0065] Fig. 3A shows an example of a first diffraction grating 110. A material of the grating elements 124 may be selected so as to achieve a high refractive index contrast to the surrounding material, e . g. the material of the transparent substrate . For example, the grating elements 124 may be made of Si, Nb2O5or TiO2. Further, the substrate 100 may be made of a low index material such as SiO2. The periodicity P, i . e . the pitch between adj acent2024PF01034 13

[0066] grating elements 124 and the width between adj acent grating elements 124 are selected so that only the zero-th and the +1- and -1st diffraction orders are generated. The first diffraction grating 110 may be designed so that all other diffraction orders are suppressed. As a consequence, all the diffracted power is detected by the photodetector 119. For example, a width w of the slits between adj acent grating elements 124 may be at least half of the periodicity P .

[0067] A thickness of the grating elements 124 measured in a vertical direction, is selected so that only the fundamental modes are generated. In more detail, a thickness may be approximately 0.2 to X, wherein X denotes the wavelength of radiation emitted by the laser device 105 in the substrate medium. For example, a thickness of the grating elements may be 200 nm or 100 nm to 940 nm for an emission wavelength of 940 nm.

[0068] Fig. 3B shows an example of the second diffraction grating 112 which may implement a beam filter . The second diffraction grating 112 may be implemented as a blazed grating. Generally, the term "blazed grating" or echelette grating as used within the present disclosure may refer to a diffractive element having a two-or three-dimensional subwavelength structure, e . g. formed in a dielectric material . For example, the structure may comprise a sawtooth pattern as illustrated in the lower portion of Fig.

[0069] 3B . According to further implementations, the blazed grating may comprise a blazed-index element, i . e . a diffractive element having a locally varying refractive index at a subwavelength distance . According to still further implementations, the blazed grating may be implemented as a pattern of grating elements 124, e . g. subwavelength pillars or ridges having a periodically varying pitch (blazed-binary grating) as illustrated in the upper portion of Fig. 3B . For example, when the transparent substrate 100 is made of a low index material such as SiO2, the grating2024PF01034 14

[0070] elements 124 may be made of a high index material such as Si, Nb2O5or TiO2.

[0071] Fig. 4 shows an example of signals detected by the different sets of photodiodes . Signal 125 represents the zero-order signal and signal 126 represents the + / - first-order signal . Signal 127 represents a differential signal corresponding to a difference between the zero-order signal 125 and the sum of the first-order signal 126. The intensity of the respective signals is represented in dependence from a displacement d of the membrane 15. The voltage applied by the voltage source 30 is set so that the optical microphone is operated in a linear region 128 so as to result in an optimum sensitivity.

[0072] Fig. 5 is a schematic view of an electronic device 18 according to embodiments . For example, the electronic device 18 comprises the optical microphone 10 which has been discussed above . For example, the electronic device 18 may be selected from a consumer electronic device such as a smart phone, an industrial application and an automotive device .

[0073] While embodiments of the invention have been described above, it is obvious that further embodiments may be implemented. For example, further embodiments may comprise any subcombination of features recited in the claims or any subcombination of elements described in the examples given above . Accordingly, this spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein .2024PF01034 15

[0074] LIST OF REFERENCES

[0075] 10 optical microphone

[0076] 15 membrane

[0077] 16 laser radiation

[0078] 17 reflected laser radiation

[0079] 18 electronic device

[0080] 20 acoustic signal

[0081] 25x, 252, 253diffracted beam

[0082] 30 voltage source

[0083] 35 housing

[0084] 100 substrate

[0085] 102 intermediate carrier

[0086] 105 laser device

[0087] 107 aperture

[0088] 108 transparent adhesive layer

[0089] 110 first diffraction grating

[0090] 112 second diffraction grating

[0091] 115 collimating element

[0092] 118 interposer

[0093] 119 detector

[0094] 120 photodiode

[0095] 121 lens

[0096] 122 hole

[0097] 123 structural element

[0098] 124 grating element

[0099] 125 zero-order signal

[0100] 126 first order signal

[0101] 127 differential signal

[0102] 128 linear region

Claims

2024PF01034 16CLAIMS1 . An optical microphone ( 10 ) , comprising :a laser device ( 105 ) configured to emit laser radiation ( 16 ) ,a membrane ( 15 ) configured to reflect the laser radiation ( 16 ) ,a transparent substrate ( 100 ) attached to the laser device ( 105 ) , a first di f fraction grating ( 110 ) being integrated with the transparent substrate ( 100 ) and being configured to di f fract the laser radiation ( 17 ) that has been reflected by the membrane ( 15 ) ; anda detector ( 119 ) comprising a set of photodiodes ( 120 ) , the detector ( 119 ) being configured to detect the laser radiation that has been di f fracted by the first di f fraction grating ( 110 ) .2 . The optical microphone ( 10 ) according to claim 1 , further comprising a collimating element ( 115 ) arranged over an aperture ( 107 ) of the laser device ( 105 ) .3 . The optical microphone ( 10 ) according to claim 2 , wherein the collimating element ( 115 ) comprises a metalens .4 . The optical microphone ( 10 ) according to claim 2 , wherein the collimating element ( 115 ) comprises a Fresnel lens .5 . The optical microphone ( 10 ) according to claim 2 , wherein the collimating element ( 115 ) is configured to tilt the laser radiation ( 16 ) with respect to a vertical direction .2024PF01034 176 . The optical microphone ( 10 ) according to claim 2 , wherein the collimating element ( 115 ) comprises a lens ( 121 ) directly adj acent to the laser device ( 105 ) .7 . The optical microphone ( 10 ) according to claim 6 , wherein a center axis ( 129 ) of the lens ( 121 ) is shi fted in a hori zontal direction with respect to the aperture ( 107 ) of the laser device ( 105 ) .8 . The optical microphone ( 10 ) according to claim 5 or 7 , wherein the first di f fraction grating ( 110 ) is configured to di f fract the emitted laser radiation ( 16 ) and the reflected laser radiation ( 17 ) .9 . The optical microphone ( 10 ) according to any of the preceding claims , further comprising a second di f fraction grating ( 112 ) configured to di f fract a beam of the laser radiation ( 16 ) , the second di f fraction grating ( 112 ) being arranged on a side of the substrate ( 100 ) remote from the first di f fraction grating ( 110 ) .10 . The optical microphone ( 10 ) according to claim 9 , further comprising an intermediate carrier ( 102 ) arranged between the laser device ( 105 ) and the substrate ( 100 ) , the second di f fraction grating ( 112 ) being arranged on a side of an intermediate carrier ( 102 ) remote from the laser device ( 105 ) .11 . The optical microphone ( 10 ) according to claim 9 or 10 , wherein the second di f fraction grating ( 112 ) is implemented as a blazed grating .12 . An electronic device ( 18 ) comprising the optical microphone ( 10 ) according to any of the preceding claims .2024PF01034 1813 . The electronic device ( 18 ) according to claim 12 , the electronic device ( 18 ) being selected from a consumer electronic device and an automotive application .

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