OPTICAL ACOUSTIC SENSOR

An acoustic sensor is disclosed, the sensor including a laser and a membrane configured to vibrate in the presence of an acoustic wave, and to reflect radiation emitted by the laser back toward the laser to produce a self-mixing interference effect corresponding to the acoustic wave. The sensor also includes a cavity separating the membrane from the laser and extending rearward of a radiation-emitting surface of the laser, a majority volume of the cavity being disposed rearward of the radiation-emitting surface of the laser. Also disclosed is an apparatus including the acoustic sensor, and a method of manufacturing the acoustic sensor.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure is in the field of acoustic sensors, and particularly relates to micro-electromechanical system (MEMS) based acoustic sensors.

BACKGROUND

Acoustic sensors may be implemented as microphones in a range of electronic devices such as portable computing devices, tablet devices, smart phones, and the like. Such acoustic sensors may be suitable for detecting acoustic waves, e.g. dynamic pressure changes in a surrounding environment. Typically, an acoustic sensor may be configured to sense acoustic waves in a surrounding environment over a particular acoustic frequency band.

Some acoustic sensors may be manufactured as micro-electromechanical systems (MEMS). For example, capacitive-type MEMs acoustic sensors are well known in the art. Such capacitive-type sensors may exhibit a relatively limited sensitivity, and hence a resultant signal-to-noise ratio may be unsuitable for some audio applications.

In recent years, acoustic sensors using optical devices for readout have been developed. Such optical device-based acoustic sensors may provide some advantages over conventional acoustic sensors in terms of increased sensitivity, increased frequency range, and reduced electronic and acoustic noise. However, such optical device-based acoustic sensors may also be inherently expensive and complex to manufacture, and may not be adequately compact for their target applications.

Acoustic sensors are generally becoming highly integrated components within electronic devices, wherein the acoustic sensors are provided with increasingly sophisticated package designs. Furthermore, stringent size constraints may be imposed upon such sensors particularly when used in mobile devices. As such, components required to manufacture acoustic sensors are required to be relatively small, such that a packaged acoustic sensor is sufficiently compact.

It is therefore desirable to provide a highly sensitive, low-cost, low-complexity and high reliability acoustic sensor, suitable for integration within electronic devices such as portable computing devices, tablet devices, smart phones, and the like.

It is therefore an aim of at least one embodiment of at least one aspect of the present disclosure to obviate or at least mitigate at least one of the above identified shortcomings of the prior art.

SUMMARY

The present disclosure is in the field of acoustic sensors, and particularly relates to micro-electromechanical system (MEMS) based acoustic sensors for use in electronic devices such as portable computing devices, tablet devices, smart phones, and the like.

According to a first aspect of the disclosure, there is provided an acoustic sensor comprising a laser and a membrane configured to vibrate in the presence of an acoustic wave, and to reflect radiation emitted by the laser back toward the laser to produce a self-mixing interference (SMI) effect corresponding to the acoustic wave.

The acoustic sensor also comprises a cavity separating the membrane from the laser and extending rearward of a radiation-emitting surface of the laser, a majority volume of the cavity being disposed rearward of the radiation-emitting surface of the laser.

Advantageously, provision of a majority volume of the cavity being disposed rearward of the radiation-emitting surface of the laser enables implementation of a cavity providing a sufficient acoustic capacitance, but without requiring location of the membrane a substantial distance from the radiation-emitting surface of the laser to achieve the sufficiently large cavity. A sufficiently large acoustic capacitance is a requirement of such acoustic sensors to provide adequate sensitivity, and thus meet signal-to-noise ratio requirements. Advantageously, a larger acoustic capacitance of the air behind the membrane may lead to a reduction in an acoustic damping or acoustic resistance which is induced by the limited compressibility of the air within the cavity.

Advantageously, because the provision of a majority volume of the cavity being disposed rearward of the radiation-emitting surface of the laser enables location of the membrane to be relatively close to the radiation-emitting surface of the laser, relatively high junction voltage variations due to the self-mixing interference effect may be achieved. The higher junction voltages may improve a signal level, and thus a signal-to-noise ratio, of the acoustic sensor. For example, in some embodiments the acoustic sensor may be configured to provide a signal in the range of 10 mV peak for a 1 Pa sound pressure level.

If a relatively large distance was to be implemented between the radiation-emitting surface of the laser and the membrane, then due to a non-ideal collimation of radiation emitted by the laser, the radiation may be insufficiently focused upon a reflective portion of the membrane. Therefore, not all of the emitted radiation would be reflected back into the laser to produce the necessary self-mixing interference effect. That is, in order to have a sufficient self-mixing interference effect, reflectivity of the membrane should be in the region of 90% or higher.

Advantageously, by keeping a distance between the laser and membrane relatively small, as enabled by the provision of the majority of the cavity extending rearward of the radiation-emitting surface of the laser, a greater proportion of radiation emitted by the laser may be reflected back into the laser to provide the self-mixing interference effect.

A gap between the membrane and the radiation-emitting surface of the laser may be 50 micrometers or less.

In some embodiments the gap between the membrane and the radiation-emitting surface of the laser may be in the range of 50 to 10 micrometers. In some embodiments, the gap between the membrane and the radiation-emitting surface of the laser may be approximately 12 micrometers.

Advantageously, a reduced distance between the membrane and the radiation-emitting surface of the laser may improve acoustic damping characteristics of the gap between the laser and the membrane. That is, air within the gap may exhibit an acoustic impedance, e.g. an effective resistance to being compressed, which may have the effect of improving a frequency response of the acoustic sensors. For example, a higher acoustic impedance in the gap due to a close proximity of the membrane to the laser may help prevent unwanted oscillations in the membrane at particular frequencies.

Furthermore, as described above, a gap in the region of 50 micrometers or less may advantageously improve an overall signal-to-noise ratio of the sensor because of an increased junction voltage incurred due to a greater proportion of radiation emitted by the laser being reflected back into the laser to provide the self-mixing interference effect. Furthermore, due to the selected dimensions of the gap, an acoustic resistance, e.g. damping effect of the air in the gap between the membrane and the radiation-emitting surface of the laser, will not be a dominant noise source in a system comprising the acoustic sensor, yet the particular construction enables sufficient choices in the size of the gap between membrane and the radiation-emitting surface of the laser.

The laser may be configured such that a junction voltage of the laser corresponds to the acoustic wave due to the self-mixing interference effect.

As such, the laser may be implemented a laser diode. The junction voltage of the laser may be measureable at nodes or contacts provided on, or electrically coupled to, the laser.

Advantageously, use of the self-mixing interference effect may enable efficient determination of characteristics of the acoustic wave, such as frequency and amplitude. Furthermore, use of the self-mixing interference effect to provide a measureable junction voltage indicative of characteristics of the acoustic wave obviates a necessity to implement separate sensors, such as separate photodiodes, for detection of radiation reflected by, or propagated through, the membrane.

In some instances a photonics power of radiation emitted by the laser, e.g. the VCSEL, may be readout using a photodiode disposed next to, adjacent, or below the laser. Advantageously, by having the membrane relatively close to the laser, a power of reflected radiation detected by the photodiode may be adequately high to provide a sufficient SNR.

The acoustic sensor may comprise circuitry coupled to the laser and configured to sense the junction voltage.

The circuitry may comprise an analogue-to-digital converted. The circuitry may comprise an amplifier. The circuitry may comprise, or be implemented on, an Application-Specific Integrated Circuit (ASIC). The circuitry may comprise a biasing circuit, e.g. a VCSEL biasing circuit. The circuitry may comprise processing circuitry, such as circuitry configured to enable readout of the SMI. That is, circuitry may be configured to provide data or a signal corresponding to the SMI effect.

Advantageously, due to a relatively small footprint of the acoustic sensor due to the provision of the majority volume of the cavity being disposed rearward of the radiation-emitting surface of the laser, the acoustic sensor may be provided as a packaged module comprising the circuitry. In some embodiments, a PCB that functions as a substrate for coupling to the laser or to the membrane may also comprise the circuitry configured to sense the junction voltage.

In some embodiments, the circuitry coupled to the laser and configured to sense the junction voltage may be provided as part of, or integrated into, a driver circuit for driving the laser.

The acoustic sensor may comprise a first substrate. The laser may be electrically coupled to the first substrate.

In some embodiments, the laser may be electrically coupled to the first substrate using bond wires. In some embodiments, the laser may be electrically coupled, e.g. soldered, to bond pads or vias implemented on the substrate.

Advantageously, the substrate may provide a means to electrically couple the laser to driver circuitry for driving the laser and/or circuitry for sensing the junction voltage, and also a means to support the laser and/or the membrane relative to one another, e.g. to provide the gap between the membrane and the laser.

The laser may be formed on the first substrate.

The laser may be a semiconductor laser that is formed, such as lithographically formed or epitaxially grown, directly onto the first substrate. Thus, the first substrate may advantageously provide a base substrate for the laser in addition to forming at least a portion of the cavity. As such, the laser may be highly integrated into the acoustic sensor, providing a reduced overall sensors size and/or footprint. Furthermore, in such embodiments, manufacturing efficiencies may be realized through an overall reduction in device assembly steps.

The laser may be mounted on the first substrate.

In some embodiments the laser may be manufactured using a particular semiconductor process, e.g. GaAs, and mounted on a separate first substrate that is not for use in the same process, e.g. a silicon substrate or an FR-4 PCB substrate. As such, an overall cost-effectiveness of the acoustic sensor may be optimized.

The membrane may be disposed between an aperture, known in the art as a ‘sound port’, in the first substrate and the radiation-emitting surface of the laser.

The aperture may allow acoustic waves to be incident upon the membrane. As such, the first substrate may form a portion of the cavity that encloses the laser, yet also provide means for acoustic waves to be incident upon the membrane.

In some embodiments, a diameter of the aperture may correspond to an effective diameter of the membrane.

The acoustic sensor may comprise an enclosure. The enclosure may be acoustically sealed to the first substrate. The enclosure may enclose the laser. The enclosure may define the cavity.

The enclosure may be implemented as a can package, such as a metal can package. The enclosure may be a canister or housing.

An acoustic seal may be formed from a sealing ring or gasket disposed between the enclosure and the first substrate. The acoustic seal may be formed from an adhesive. In some embodiments, the enclosure may be soldered to the first substrate to form the acoustic seal.

The substrate may comprise a recess surrounding the laser and defining the cavity.

The recess may be etched into the substrate. The recess may be formed by means of a lithographic process. The recess may be cut or ground into the substrate.

The substrate may comprise a mesa supporting the laser and at least in part defining the cavity. The mesa may be a raised section of the substrate. The mesa may form a pedestal.

The mesa, or pedestal, may be formed by etching a region surrounding the mesa by means of a lithographic process. The mesa, or pedestal, may be cut or ground into the substrate.

The first substrate may be coupled to a second substrate. A first portion of the cavity may be between the membrane and the first substrate. A second portion of the cavity may be defined by a recess in the second substrate. The first portion of the cavity may be communicably coupled to the second portion of the cavity by at least one opening in the first substrate.

Advantageously, the at least one opening may provide one or more conduits for airflow through the first substrate. As such, the opening may enable the first and second portions of the cavity to operate collectively as a single cavity for providing adequate acoustic capacitance for the acoustic sensor.

The laser may be suspended or supported between the membrane and a portion of the cavity that is rearward of the laser, by an apertured substrate.

The apertured substrate may provide one or more conduits for airflow. As such, the apertured substrate may enable the portion of the cavity that is rearward of the laser to be coupled to a portion of the cavity that is between the laser and the membrane, thus providing adequate acoustic capacitance for the acoustic sensor

The laser may be a vertical cavity surface-emitting laser (VCSEL).

Advantageously, a VCSEL-based self-mixing interference effect using the laser junction voltage as the source of the self-mixing signal may result in cost-savings and reductions in component costs and complexity, when compared to acoustic sensors employing photodiodes, or other discrete sensors for detecting reflections and/or transmission through the membrane.

The membrane may comprise a stretched film provided under tension.

Advantageously, the membrane does not need to be formed as a raised microstructure. The membrane may have a diameter of less than 300 micrometers. The membrane may have a diameter of approximately 270 micrometers.

The membrane may have a thickness of less than 100 nanometers. In some embodiments, a thickness of the membrane may be between 50 nm and 100 nm.

The membrane may comprise a reflector. A diameter of the reflector may be less than 100 micrometers. The reflector may be for reflecting radiation emitted by the laser.

In some embodiments, a diameter of the reflector may be in the range of 30 to 60 micrometers.

The reflector may be a mirror. By providing a majority volume of the cavity being disposed rearward of the radiation-emitting surface of the laser, the membrane may be disposed relatively close to the laser and thus even when accounting for a non-ideal collimation of radiation emitted by the laser, the reflector may be made relatively small, e.g. less than 100 micrometers in diameter.

Furthermore, the provision of a relatively small reflector, e.g. with a diameter of than 100 micrometers, may minimize a mass of the reflector. Thus, an overall mass of the combination of the membrane and the reflector may be minimized, which may advantageously reduce the effects of acoustic noise and increase membrane elasticity.

In some embodiments the reflector may be disposed on a surface of the membrane that is opposing the radiation-emitting surface of the laser.

In some embodiments the reflector may be disposed on an outer surface of the membrane, e.g. an opposite surface of the membrane to the surface of the membrane that is opposing the radiation-emitting surface of the laser. In such embodiments, the membrane may be substantially transparent to radiation emitted by the laser.

In some embodiments, the reflector may be embedded within the membrane. For example, in some embodiments the reflector may be formed as an integral component of the membrane. In some embodiments, the reflector may be disposed between layers of the membrane.

In some embodiments the reflector may comprise gold. In some embodiments the reflector may comprise aluminum.

In some embodiments the reflector may have a thickness in the range of 40 to 60 nanometers.

According to a second aspect of the disclosure, there is provided an apparatus comprising the acoustic sensor according to the first aspect, wherein the apparatus is one of: a smart speaker; a smart phone; a smart-watch; a laptop, a tablet device; or headphones.

According to a third aspect of the disclosure, there is provided a method of manufacturing an acoustic sensor, the method comprising: providing a laser and a membrane in a package such that the membrane is configured to vibrate in the presence of an acoustic wave and to reflect radiation emitted by the laser back toward the laser to produce a self-mixing interference effect corresponding to the acoustic wave; and providing the package with a cavity separating the membrane from the laser and extending rearward of a radiation-emitting surface of the laser, a majority volume of the cavity being disposed rearward of the radiation-emitting surface of the laser.

The above summary is intended to be merely exemplary and non-limiting. The disclosure includes one or more corresponding aspects, embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. It should be understood that features defined above in accordance with any aspect of the present disclosure or below relating to any specific embodiment of the disclosure may be utilized, either alone or in combination with any other defined feature, in any other aspect or embodiment or to form a further aspect or embodiment of the disclosure.

DETAILED DESCRIPTION

FIG.1depicts a cross-sectional view of an acoustic sensor100according to a first embodiment of the disclosure. The acoustic sensor100comprises a laser105. In the example embodiment ofFIG.1, the laser105is a vertical-cavity surface emitting laser (VCSEL). It will be appreciated that in other embodiments, other laser diodes may be implemented.

The laser105is configured to emit radiation from a radiation-emitting surface110at a front of the laser105, relative to a rear surface of the laser105comprising contacts115for providing electrical connectivity to the laser105.

The acoustic sensor100comprises a first substrate120. The first substrate120comprises a mesa125, e.g. a pedestal, configured to support the laser105. In some embodiments, the laser105may be formed on the mesa125. In other embodiments, the laser105is provided as a discrete device which is adhered to the mesa125during an assembly process. The mesa125may, for example, be formed by etching the first substrate120. Electrical contacts (not shown), formed from conductive traces and/or vias may be provided in/on the first substrate120to supply electrical current to the laser105and/or to provide means to sense a junction voltage of the laser105, as described below in more detail.

The first substrate120may comprise glass, silicon, or the like.

The acoustic sensor100also comprises a second substrate130. The second substrate130is formed with an aperture135, such that the acoustic sensor100may be assembled with the mesa125of the first substrate120disposed within the aperture135.

The second substrate130may comprise glass, silicon, or the like.

The acoustic sensor100also comprises a membrane140. The membrane140is provided under tension. That is, the membrane140is provided as a stretched film provided under tension. The membrane140is secured to the second substrate130at at least a portion of a perimeter of the membrane140. In some embodiments, the membrane140may comprise silicon nitride.

In some embodiments, the second substrate130may be a silicon substrate. In some embodiments, the second substrate130may comprise a layer150of silicon dioxide, and the membrane140may be secured, e.g. adhered or clamped, to the layer150of silicon dioxide.

The membrane140and the second substrate130may be provided as an assembly that is coupled, e.g. adhered, to the first substrate120during a process of assembly of the acoustic sensor100.

The membrane140comprises a plurality of holes155. The holes155extend between upper and lower surfaces of the membrane140, thus providing through-passages in the membrane140. In use, the holes155may act as pressure equalization holes. That is, static air pressure levels may typically fluctuate by several tens of hectoPascals at sea level. As sound pressure levels are in the order of 1 Pascal and can be as small as 20 microPascal, which is considered the threshold for human hearing, relatively equal pressure levels in the environment inside and outside the acoustic sensor100are necessary for the detection of vibrations of the membrane140incurred by small pressure fluctuations due to an acoustic wave.

The membrane140comprises a reflector160.

The reflector160is disposed on a surface of the membrane140that is opposing the radiation-emitting surface110of the laser105.

It will be appreciated that, in other embodiments falling within the scope of the disclosure, the reflector160may be disposed on an outer surface of the membrane140, e.g. an opposite surface of the membrane140to the surface of the membrane140that is opposing the radiation-emitting surface110of the laser105. In such embodiments, the membrane140may be substantially transparent to radiation emitted by the laser105, such that radiation emitted by the laser105propagates through the membrane140and is reflected by the reflector back through the membrane towards the laser105.

The reflector160is positioned on the membrane140relative to the laser105such that the reflector160reflects radiation emitted by the laser105back toward the laser105to produce a self-mixing interference effect, as described below in more detail.

In the example embodiment ofFIG.1, the reflector160has a diameter in the region of 100 micrometers. In some embodiments, a diameter of the reflector160may be less than 100 micrometers, e.g. in the range of 30 to 60 micrometers. The provision of a relatively small reflector160, e.g. with a diameter of in the region of 100 micrometers or less, may minimize a mass of the reflector160. Thus, an overall mass of the combination of the membrane140and the reflector160may be minimized, which may advantageously reduce the effects of acoustic noise and increase elasticity of the membrane140.

The reflector160may be a mirror. The reflector160is configured to reflect radiation having a wavelength corresponding to wavelength of radiation emitted by the laser105. In some embodiments, the reflector160may comprise gold. In some embodiments, the reflector160may comprise aluminum. The reflector160may be provided as a discrete element that is adhered to the membrane140during an assembly process. Alternatively, the reflector160may be formed on the membrane140, e.g. by a process of deposition or the like.

A cavity145separates the membrane140from the laser105and extends rearward of the radiation-emitting surface110of the laser105. A majority volume of the cavity145is disposed rearward of the radiation-emitting surface110of the laser105. Advantageously, by providing a majority volume of the cavity145rearward of the radiation-emitting surface110of the laser105, the membrane140may be disposed relatively close to the laser105. Thus, even when accounting for a non-ideal collimation of radiation emitted by the laser105, the reflector160may be made relatively small, e.g. less than 100 micrometers in diameter.

In the example embodiment ofFIG.1, the membrane140has a diameter in the region of 1.0 to 1.2 millimeters. In some embodiments, the reflector160may have a thickness in the range of 40 to 60 nanometers. In some embodiments, the reflector160may be as thick as 100 nm. In the example embodiment ofFIG.1, the cavity extends a height of approximately 500 micrometers from the membrane140to a base of the mesa125. The mesa125has a cross-sectional width of approximately 290 micrometers. The laser has a thickness extending from the mesa125in a direction towards the membrane140of approximately 100 micrometers. A gap between the membrane140and the radiation-emitting surface110of the laser105is 50 micrometers or less. An overall cross-sectional width of the acoustic sensor100may be between 2.4 and 1.4 millimeters.

It will be appreciated that such dimensions are for purposes of example only. Thus, it will be understood that embodiments with dimensions that may generally be comparable to, yet individually or collectively vary from, those of the embodiment ofFIG.1, will also fall within the scope of the disclosure.

In use, an acoustic wave incident upon the membrane140will cause a vibration in the membrane140. Radiation emitted from the laser105is reflected from the reflector160back into the laser105to produce a self-mixing effect, where the self-mixing effect is modulated by the vibrations of the membrane140. Said self-mixing effect causes detectable variations in a junction voltage of the laser105. As such, the junction voltage of the laser105corresponds to the acoustic wave due to the self-mixing interference effect. In some embodiments the acoustic sensor100may comprise, or may be coupled to, circuitry configured to sense the junction voltage of the laser105. Specifically, in some embodiments, the laser105may comprise, or may be coupled to, circuitry configured to sense the junction voltage of the laser105.

FIG.2depicts a cross-sectional view of an acoustic sensor200according to a second embodiment of the disclosure. The acoustic sensor200comprises a laser205. In the example embodiment ofFIG.2, the laser205is a vertical-cavity surface emitting laser (VCSEL). It will be appreciated that in other embodiments, other laser diodes may be implemented.

The laser205is configured to emit radiation from a radiation-emitting surface210at a front of the laser205, relative to a rear surface of the laser205comprising contacts215for providing electrical connectivity to the laser205.

The acoustic sensor200comprises a first substrate220. The first substrate220comprises a recess290. In some embodiments, the recess290may be formed as a trench. The recess290is formed to comprise a mesa225. The mesa225is configured to support the laser205. In some embodiments, the laser205may be formed on the mesa225. In other embodiments, the laser205is provided as a discrete device which is adhered to the mesa225during an assembly proves. The recess290may, for example, be formed by etching the first substrate220. Electrical contacts (not shown), formed from conductive traces and/or vias may be provided in the first substrate220to supply electrical current to the laser205and/or to provide means to sense a junction voltage of the laser205, as described below in more detail.

The first substrate220may comprise glass, silicon, or the like.

The acoustic sensor200also comprises a second substrate230. The second substrate230is formed with an aperture235, such that the acoustic sensor200may be assembled with the aperture235aligned with the recess290.

The acoustic sensor200may be assembled with the mesa225of the first substrate220disposed within the second aperture235.

The second substrate230may comprise glass, silicon, or the like.

The acoustic sensor200also comprises a membrane240. The membrane240, and associated reflector260and pressure equalization holes255, are generally similar to the membrane140, reflector160and pressure equalization holes155respectively ofFIG.1, and are not described in further detail for purposes of brevity.

In some embodiments, the second substrate230may be a silicon substrate. In some embodiments, the second substrate230may comprise a layer250of silicon dioxide, and the membrane240may be secured to the layer250of silicon dioxide.

The membrane240and the second substrate230may be provided as an assembly that is coupled, e.g. adhered, to the first substrate220during a process of assembly of the acoustic sensor200.

Similar to the example embodiment ofFIG.1, the second embodiment ofFIG.2also comprises a cavity245separating the membrane240from the laser205and extending rearward of the radiation-emitting surface210of the laser205. A majority volume of the cavity245is disposed rearward of the radiation-emitting surface210of the laser205.

The example dimensions of the embodiments ofFIG.1andFIG.2are generally similar, and therefore also not described in more detail.

FIG.3adepicts a cross-sectional view of an acoustic sensor300according to a third embodiment of the disclosure. Similar to the acoustic sensors100,200ofFIGS.1and2, the acoustic sensor300comprises a laser305and a membrane340, wherein the membrane comprises a reflector360.

The acoustic sensor300comprises a first substrate320. The first substrate320is configured to support the laser305. The first substrate320may comprise glass, silicon, or the like. The first substrate320is an apertured substrate.

The acoustic sensor300also comprises a second substrate330. The second substrate330is formed with a recess325. The recess325may, for example, be formed by etching the second substrate330. The second substrate330may comprise glass, silicon, or the like.

The acoustic sensor300comprises a third substrate395. The third substrate395is configured to support the membrane340.

The acoustic sensor300is assembled such that the first substrate320is disposed between the second substrate330and the third substrate395, such that openings, e.g. apertures365in the first substrate320are aligned with the recess325in the second substrate, and the laser305is supported by the first substrate320between the second substrate330and the third substrate395.

The recess325and a gap between the laser305and the membrane340define a cavity. A first portion of the cavity is between the membrane340and the first substrate320and a second portion of the cavity is defined by the recess325in the second substrate330, wherein the first portion is communicably coupled to the second portion by the apertures365in the first substrate320.

That is, the laser305is suspended or supported between the membrane340and a portion of the cavity that is rearward of the laser, by the apertured first substrate320.

Advantageously, the absence of a mesa on the second substrate330, when compared to the example embodiments ofFIGS.1and2, enables a volume of the cavity formed by the recess325to be relatively large, thereby increasing an acoustic capacitance of the cavity when compared to that of the embodiments ofFIGS.1and2.

FIG.3bdepicts a top view of the first substrate320as implemented in the third embodiment depicted inFIG.3a. The first substrate320comprises the plurality of apertures365. For purposes of example, four apertures365are depicted, although it will be appreciated that in other embodiments fewer than or greater than four apertures365may be implemented The apertures365are formed between a central portion for supporting the laser305and an outer portion, wherein the central portion is coupled to the outer portion by spokes350. The apertures may be formed in the substrate by etching, or the like.

FIG.4adepicts a cross-sectional view of an acoustic sensor400according to a fourth embodiment of the disclosure.

The acoustic sensor400comprises a laser405. In the example embodiment ofFIG.4a, the laser405is a VCSEL. It will be appreciated that in other embodiments, other laser diodes may be implemented.

The laser405is configured to emit radiation from a radiation-emitting surface410of the laser405. The laser405also comprises comprising terminals465for providing electrical connectivity to the laser405.

The acoustic sensor400comprises a first substrate420. The first substrate420may be a printed circuit board (PCB) substrate, such as an FR-4 substrate or the like. The first substrate comprises electrical contacts415. In the example embodiment ofFIG.4a, the electrical contacts415are provided as vias, e.g. conductive elements extending through the first substrate420.

The electrical contacts415of the first substrate420are conductively coupled to the terminals465of the laser405. In the example embodiment ofFIG.4a, a conductive adhesive470is used to couple the electrical contacts415to the terminals465. It will be appreciated that in other embodiments, the electrical contacts415may be soldered or otherwise conductively coupled to the terminals465.

The acoustic sensor400comprises a membrane440. The membrane is supported between the first substrate420and the laser405by a first support structure430and a second support structure450. The first support structure430couples the membrane to the laser405. The second support structure450couples the membrane440to the first substrate420. The first support structure430supports the membrane440such that a first cavity portion488is provided between the membrane440and the radiation-emitting surface410of the laser405. The first support structure430is configured to communicably couple the first cavity portion488to a second cavity portion490, as described in more detail below with reference toFIG.4b.

The membrane440also comprises pressure equalization holes455, which serve the same purposes as those described in respect of the embodiment ofFIG.1above. Although not shown inFIG.4a, the membrane440also comprises a reflector, as described above with reference toFIG.1.

The second support structure450supports the membrane440between an aperture460in the first substrate420and the radiation-emitting surface of the laser405. As such, in use an acoustic wave may propagate through the aperture460in the first substrate420to be incident upon the membrane440.

The laser405, the membrane440, the first support structure430and the second support structure450may be provided as an VCSEL assembly, which is assembled with the enclosure480and the first substrate420during an acoustic sensor400assembly process.

The acoustic sensor400comprises an enclosure480. The enclosure480is acoustically sealed to the first substrate420. For example, in some embodiments, the enclosure480is sealed to the first substrate using a sealing ring or gasket disposed between the enclosure480and the first substrate420. In some embodiments the acoustic seal may be formed from an adhesive. In some embodiments, the enclosure480may be soldered to the first substrate420to form the acoustic seal.

The enclosure480is implemented as a can package. For example, in some embodiments the enclosure480is implemented as a metal can package.

The enclosure480encloses the laser405, and as such the enclosure defines the second cavity portion490.

Also shown inFIG.4ais a corresponding top view of the acoustic sensor400according to a fourth embodiment of the disclosure. The top view shows the first substrate420comprising an aperture460, through which the membrane440is visible. Also depicted are the electrical contacts415of the first substrate420, which are conductively coupled to the terminals465of the laser405as described above. For purposes of example, four electrical contacts415are depicted, arranged in pairs labelled “N” and “P”. The electrical contacts415labelled “N” are coupled to an “N” terminal of the laser405, e.g. a cathode, and the electrical contacts415labelled “P” are coupled to an “P” terminal of the laser405, e.g. an anode. In the example ofFIG.4a, each pair of terminals provides a terminal for supplying electrical current to the laser405and a corresponding terminal for measuring a junction voltage of the laser405. It will be appreciated that, in other embodiments, there may be as few as one “N” terminal and one “P” terminal.

Also depicted in the top view is a further terminal485. In some embodiments, the further terminal485provides a ground connection from the first substrate420to a base or substrate of the laser405.

FIG.4bdepicts a first cross sectional view425, a second cross sectional view435, a top view445, and a partial perspective view475of the VCSEL assembly for use in the acoustic sensor400according to the fourth embodiment of the disclosure.

The top view445of the VCSEL assembly depicts the laser405with terminals465disposed at an upper surface, wherein the terminals465are for conductively coupling the laser405to the electrical contacts415of the first substrate420.

Also depicted is the first support structure430. The first support structure430is provided as a plurality of support elements. The membrane440is supported between the support elements of the first support structure430and the second support structure450.

The first cross sectional view425depicts a cross section along the line denoted X-X in the top view445. The first cavity portion488is provided between the membrane440and the radiation-emitting surface410of the laser405, wherein the membrane440is supported by the support elements of the first support structure430. In contrast, the second cross sectional view435depicts a cross section along the line denoted Y-Y in the top view445. It can be seen in the second cross sectional view435that gaps between the plurality of support elements of the first support structure430enable airflow498to and from the first cavity portion488.

This is more clearly shown in the partial perspective view475of the VCSEL assembly, wherein airflow498between the plurality of support elements of the first support structure430is depicted.

FIG.4cdepicts a further cross-sectional view of the acoustic sensor400according to the fourth embodiment of the disclosure.FIG.4cis annotated with equivalent impedances, which may be considered when assessing the effects of features of the particular construction of the acoustic sensor400. For example:R_port: a resistance corresponding to a component of an acoustic impedance to compression of air in the aperture460in the first substrate420;M_port: an inductance corresponding to a component of an acoustic impedance to compression of air in the aperture460in the first substrate420;Cfv: a capacitance corresponding to an acoustic capacitance of the front volume, e.g. the first cavity portion488;Rsqueeze: a resistance corresponding to a resistance of air in the first cavity portion488between the laser405and the membrane440to compression;Rsilt: a resistance corresponding to a resistance of air between the laser405and the membrane440to flow through the gaps between the support elements of the first support structure430;Rpe: a resistance corresponding to a resistance of air to flow through the pressure equalization holes in the membrane440, and wherein Rpeis substantially larger in magnitude than a series combination of Rsqueezeand Rsilt; andCbv: a capacitance corresponding to an acoustic capacitance of the back volume, e.g. the second cavity portion490formed by the enclosure480enclosing the laser405.

The particular dimensions of the construction of the acoustic sensors400ofFIG.4censure that Rsqueezeand Rslitprovide adequate damping, thus giving a sufficient acoustical response. Furthermore, the values of Rsqueezeand Rsiltare selected to also provide a relatively low acoustical noise.FIG.5adepicts a cross-sectional view and a corresponding top view of an acoustic sensor500according to the fifth embodiment of the disclosure.

The acoustic sensor500comprises a laser505. In the example embodiment ofFIG.5a, the laser505is a VCSEL.

Features of the acoustic sensor500, such as the enclosure580, the first substrate520, the electrical contacts515of the first substrate520, and the membrane540are generally comparable to that of the embodiment ofFIG.4a, and therefore are not described in further detail.

In contrast to the fourth embodiment of the acoustic sensor400which comprises a “top-emitting” VCSEL laser405, the fifth embodiment of the acoustic sensor500comprises a “bottom-emitting” VCSEL laser505. That is, the VCSEL is configured to emit radiation through the substrate that the laser is formed on, e.g. though an opposite side of the laser505than the side comprising the terminals565for providing electrical connectivity to the laser505.

Furthermore, the terminals565of the laser505are connected to the electrical contacts515of the first substrate520by bondwires570.

The membrane540is supported between the first substrate520and the laser505by a first support structure530and a second support structure550. The first support structure530couples the membrane540to the laser505. The second support structure550couples the membrane540to the first substrate520. The first support structure530supports the membrane540such that a first cavity portion588is provided between the membrane540and a radiation-emitting surface510of the laser505. The first support structure530is configured to communicably couple the first cavity portion588to a second cavity portion590, as described in more detail below with reference toFIG.5b.

The second support structure550supports the membrane540between an aperture560in the first substrate520and the radiation-emitting surface of the laser505. As such, in use an acoustic wave may propagate through the aperture560in the first substrate520to be incident upon the membrane540.

The laser505, the membrane540, the first support structure530and the second support structure550may be provided as a VCSEL assembly, which is assembled with the enclosure580and the first substrate520during an acoustic sensor500assembly process.

The acoustic sensor500also comprises a third support structure555. The third support structure555couples the laser505to the first substrate520, and is also configured to communicably couple the first cavity portion588to a second cavity portion590, as described in more detail below with reference toFIG.5b. In some embodiments, the third support structure555may be provided or formed as part of, or together with, the first support structure530. In some embodiments, the third support structure555may be provided or formed as part of, or together with, the second support structure550. The third support structure555provides structural support to the acoustic sensor500.

FIG.5bdepicts a first cross sectional view525, a second cross sectional view535and a top view545of a VCSEL assembly for use in an acoustic sensor according to a fifth embodiment of the disclosure, and a further representation of a cross-section the acoustic sensor500.

The top view545of the VCSEL assembly depicts the laser505coupled to the first support structure530and the third support structure555.

The first support structure530is provided as a plurality of support elements. The membrane540is supported between the support elements of the first support structure530and the second support structure550.

In some embodiments, the first support structure530is formed from an epoxy, or a photoresist material such as SU-8 or the like. In some embodiments, the first support structure530may be formed using a lithographic process.

The third support structure555is also provided as a plurality of elements, arranged to form a cruciform trench arrangement generally centered around the first support structure530.

In some embodiments, a total height of the third support structure555, e.g. a distance from the radiation-emitting surface510of the laser505to the first substrate520, is in the region of 16 micrometers.

In some embodiments, a total height of the first support structure530, e.g. a distance from the radiation-emitting surface510of the laser505to the membrane540, is in the region of 12 micrometers.

The first cross sectional view525depicts a cross section along the line denoted A in the top view545. The first cavity portion588is provided between the membrane540and the radiation-emitting surface510of the laser505, wherein the membrane540is supported by the plurality of support elements of the first support structure530.

The second cross sectional view535depicts a cross section along the line denoted B in the top view545. It can be seen in the second cross sectional view535that a trench between the plurality of support elements of the third support structure555enable airflow to and from the first cavity portion588. A corresponding representation of a cross-section the acoustic sensor500is also depicted.

FIG.5cdepicts a further cross-sectional view of the acoustic sensor500according to the fifth embodiment of the disclosure. Similar to the embodiment ofFIG.4c, the particular dimensions of the construction of the acoustic sensor500ofFIG.5censures that Rsqueezeand Rslitprovide adequate damping, thus providing a sufficient acoustical response. Furthermore, the values of Rsqueezeand Rslitare selected to also provide a relatively low acoustical noise.

FIG.6depicts an apparatus600comprising an acoustic sensor610according to an embodiment of the disclosure. The acoustic sensor600may be an acoustic sensor100,200,300,400,500as described with reference toFIGS.1to5d. The apparatus600is depicted as a generic apparatus and may correspond to, for example, a smart speaker; a smart phone; a smart-watch; a laptop, a tablet device; or headphones.

The apparatus600comprises a laser driver620. The laser driver620may be configured to provide an electrical current to drive a laser of the acoustic sensor610.

The apparatus600also comprises sensor circuitry630. The sensor circuitry630of configured to sense a junction voltage of a laser of the acoustic sensor610. As such, the sensor circuitry630may be configured to determine characteristics of an acoustic wave incident upon the acoustic sensor620. The sensor circuitry630may, for example, comprise an analogue to digital converter. The sensor circuitry630may be coupled to, or integrated with, processing circuitry (not shown).

It will be appreciated that, in some embodiments, the laser driver620and the sensor circuitry630may be integrated into a single device.

FIG.7depicts a method of manufacturing an acoustic sensor100,200,300,400,500,600according to an embodiment of the invention. The method comprising a step710of providing a laser and a membrane in a package such that the membrane is configured to vibrate in the presence of an acoustic wave and to reflect radiation emitted by the laser back toward the laser to produce a self-mixing interference effect corresponding to the acoustic wave.

The method also comprises a step720of providing the package with a cavity separating the membrane from the laser and extending rearward of a radiation-emitting surface of the laser, a majority volume of the cavity being disposed rearward of the radiation-emitting surface of the laser.

It will be understood that the above description is merely provided by way of example, and that the present disclosure may include any feature or combination of features described herein either implicitly or explicitly of any generalisation thereof, without limitation to the scope of any definitions set out above. It will further be understood that various modifications may be made within the scope of the disclosure.