Optical mems microphone based on apertured strain enhancement and heterodyne readout

Abstract

The application provides an optical MEMS microphone based on open hole strain enhancement and heterodyne reading, relates to the technical field of micro-opto-electro-mechanical system sensing, and comprises a light source module, a beam splitting module, a reference light path, a sensing light path, a beam combining module, a photoelectric detection module and a signal processing module; the input end of the beam splitting module is connected with the light source module; the reference light path and the sensing light path are connected with two output ends of the beam splitting module respectively; the sensing light path comprises a MEMS diaphragm and a sensing waveguide module; and the sensing waveguide module is arranged on a strain sensitive area of the MEMS diaphragm. The application can solve the technical problems of insufficient sensitivity, low strain utilization efficiency and weak anti-interference ability of the reading mode of the existing optical MEMS microphone, effectively improves the robustness of the device to light source fluctuation and environmental interference on the basis of realizing high-sensitivity sound pressure detection, widens the linear detection range and dynamic range, and the overall architecture is adapted to standard semiconductor integrated process, so that the device is convenient for large-scale preparation.

Description

Technical Field

[0001] This invention relates to the field of micro-opto-electro-mechanical systems sensing technology, and in particular to an optical MEMS microphone based on aperture strain enhancement and heterodyne readout. Background Technology

[0002] With the technological advancements in consumer electronics, environmental acoustic monitoring, and industrial non-destructive testing, the application scenarios of MEMS microphones continue to expand, placing higher demands on their electromagnetic interference resistance, weak signal detection capabilities, and dynamic detection range. Traditional capacitive and piezoresistive MEMS microphones, limited by their inherent working principles, face performance bottlenecks that are difficult to overcome in environments with strong electromagnetic interference and ultra-low sound pressure levels. Optical MEMS microphones, based on optical sensing mechanisms to convert sound pressure signals, naturally possess advantages such as strong electromagnetic interference resistance and high sensitivity potential, making them an important research and development direction in the field of MEMS acoustic sensing.

[0003] Most mainstream optical MEMS microphones currently employ Fabry-Perot resonator structures or grating diffraction structures, with some using waveguide-type interference architectures to achieve acousto-optical-electric conversion. However, existing technologies still have significant drawbacks: First, the sensing waveguides often use short straight-line paths, resulting in very limited effective sensing optical paths. The strain generated by diaphragm vibration is difficult to convert into sufficient phase changes, leading to low overall device sensitivity. Second, the diaphragm structure does not actively regulate strain distribution, resulting in insufficient strain concentration in the sensing waveguide area and low strain energy utilization efficiency. Furthermore, the large air film damping of solid diaphragms restricts the diaphragm's vibration amplitude and response characteristics. Third, most solutions use direct light intensity detection for readout, making the detection results susceptible to fluctuations in light source power and optical path loss drift. Low-frequency noise is significant, and the linear detection range and dynamic range are limited, failing to simultaneously meet the requirements for high sensitivity and high stability. Summary of the Invention

[0004] In view of this, the present invention proposes an optical MEMS microphone based on aperture strain enhancement and heterodyne readout, which can solve the technical problems of insufficient sensitivity, low strain utilization efficiency and weak anti-interference capability of existing optical MEMS microphones. On the basis of achieving high-sensitivity sound pressure detection, it effectively improves the robustness of the device to light source fluctuations and environmental interference, broadens the linear detection range and dynamic range, and the overall architecture is compatible with standard semiconductor integration process, which is convenient for large-scale fabrication.

[0005] The technical solution of this invention is implemented as follows: In a first aspect, the present invention provides an optical MEMS microphone based on aperture strain enhancement and heterodyne readout, comprising a light source module, a beam splitter module, a reference optical path, a sensing optical path, a beam combiner module, a photoelectric detection module, and a signal processing module, wherein... The input of the beam splitter module is connected to the light source module; The reference optical path and the sensing optical path are respectively connected to the two outputs of the beam splitter module. The sensing optical path includes a MEMS diaphragm and a sensing waveguide module. The sensing waveguide module is arranged in the strain-sensitive area of ​​the MEMS diaphragm and extends in a serpentine folding path. The MEMS diaphragm is provided with an acoustic aperture array. The reference optical path is provided with a frequency shift module, which is used to apply a fixed frequency shift to the reference light. The two input terminals of the beam combining module are connected to the output terminals of the reference optical path and the sensing optical path, respectively. The beam combining module is used to make the frequency-shifted reference light interfere with the sensing light whose phase is modulated by the strain of the MEMS diaphragm to form a heterodyne beat frequency signal. The photoelectric detection module is connected to the output of the beam combining module, and the photoelectric detection module is used to convert the heterodyne beat frequency signal into an electrical signal. The signal processing module is connected to the output of the photoelectric detection module, and the signal processing module is used to demodulate the electrical signal into external sound pressure information.

[0006] In one embodiment, the sensing waveguide module is a silicon nitride waveguide, which includes multiple parallel straight sensing segments and curved segments connecting adjacent straight sensing segments. The straight sensing segments are arranged along the region of the same tangential strain of the MEMS diaphragm, and the curved segments are located in the low strain region of the MEMS diaphragm.

[0007] In one embodiment, the acoustic aperture array includes multiple segments of rounded corner slits, which are arranged along at least one side of a linear sensing segment of the sensing waveguide module. The major axis of each segment of rounded corner slit is parallel to the extension direction of the corresponding linear sensing segment, and a continuous composite material layer supporting the sensing waveguide module is disposed between the ends of adjacent segments of rounded corner slits.

[0008] In one embodiment, an input waveguide is provided between the light source module and the beam splitter module. The beam splitter module, the reference optical path, the sensing optical path, and the beam combiner module together constitute a Mach-Zehnder interference structure. The frequency shifting module is one of an acousto-optic modulator, an electro-optic modulator, or an integrated optical frequency shifter.

[0009] In one embodiment, the signal processing module includes an orthogonal demodulation unit, a phase extraction unit, and a sound pressure calibration unit connected in sequence.

[0010] Secondly, this invention provides a sound pressure detection method based on aperture strain enhancement and heterodyne readout, comprising the following steps: The incident laser beam is split into two paths: a reference beam and a sensing beam. The reference light is frequency-shifted to create a fixed frequency difference between the reference light and the sensing light. The sensing light is propagated along a serpentine sensing waveguide on the surface of a MEMS diaphragm with an array of acoustic openings, and the propagation phase of the sensing light is modulated by the strain of the MEMS diaphragm. The frequency-shifted reference light and the phase-modulated sensing light are combined and interfered to form a heterodyne beat frequency signal; Phase demodulation is performed on the heterodyne beat frequency signal, the phase change is extracted and converted into an external sound pressure signal for output.

[0011] In one embodiment, the step of performing phase demodulation on the heterodyne beat frequency signal, extracting the phase change, and converting it into an external sound pressure signal for output specifically includes the following steps: Generate two quadrature local oscillator signals corresponding to the heterodyne frequency; The two quadrature local oscillator signals are respectively mixed with the heterodyne beat frequency signal; The two mixed signals are low-pass filtered to obtain two baseband signals. Phase information is extracted from two baseband signals using a four-quadrant arctangent algorithm. Phase jumps are eliminated by phase expansion algorithm to obtain continuous phase change; Based on the preset phase-sound pressure correspondence, the continuous phase change is converted into an external sound pressure signal and output.

[0012] Thirdly, the present invention provides a method for fabricating an optical MEMS microphone, used to fabricate the aforementioned optical MEMS microphone based on aperture strain enhancement and heterodyne readout, comprising the following steps: Select an SOI wafer and fabricate a silicon dioxide undercladding layer on the front side of the device layer; A waveguide core layer is deposited on a silicon dioxide cladding layer, and a beam splitter module, a reference waveguide, a sensing waveguide module, and a beam combiner module are formed by photolithography and etching processes. A cladding layer is deposited on silicon dioxide to fabricate an optical input-output coupling structure; An array of acoustic apertures is formed by etching in the region that avoids the waveguide core layer; The release diaphragm area is etched from the back of the wafer to form a suspended MEMS diaphragm and release cavity.

[0013] In one embodiment, the reference waveguide is fabricated in the fixed frame region of the wafer, the sensing waveguide module is fabricated in the diaphragm region to be released, and the sensing waveguide module is fabricated as a serpentine foldback path.

[0014] In one embodiment, the acoustic aperture array adopts a segmented rounded corner slit structure, arranged on both sides of the straight segment of the sensing waveguide module, with the major axis of the segmented rounded corner slit parallel to the extension direction of the straight segment of the sensing waveguide module.

[0015] The optical MEMS microphone based on aperture strain enhancement and heterodyne readout of the present invention has the following advantages over the prior art: 1. By setting an acoustic aperture array on the MEMS diaphragm, the gas film damping during diaphragm vibration can be reduced, and the vibration response performance of the diaphragm can be improved. At the same time, the strain distribution state inside the diaphragm can be actively controlled, so that the bending strain is concentrated in the area where the sensing waveguide is located, thereby improving the strain utilization efficiency. The sensing waveguide module is set as a serpentine return path along the strain-sensitive area of ​​the diaphragm, which greatly extends the effective sensing optical path within the limited diaphragm area. This allows the micro-strain generated by the diaphragm to continuously accumulate along the waveguide propagation path, significantly amplifying the phase modulation amplitude corresponding to the sound pressure. Meanwhile, a frequency shift module is set in the reference optical path to form a stable frequency difference between the reference light and the sensing light. After the two are combined, a heterodyne beat frequency signal is generated, and finally the sound pressure information is restored through phase demodulation. 2. The sensing waveguide module is divided into multiple parallel straight sensing segments and connecting curved segments. The straight sensing segments are arranged along the high amplitude region of the tangential strain of the MEMS diaphragm with the same sign, while the curved segments are set in the low strain region of the diaphragm. This can effectively prevent the tangential strains of different signs from canceling each other in the path integral, and improve the effective proportion of strain accumulation. In conjunction with the segmented rounded slit openings arranged on both sides of the straight sensing segments, a local compliant structure can be formed in the diaphragm, which further guides the bending strain to concentrate in the sensing waveguide path. Under the premise of ensuring the continuity of the mechanical structure below the waveguide, the path strain integral and phase detection sensitivity are greatly improved. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram illustrating the principle of the optical MEMS microphone based on aperture strain enhancement and heterodyne readout of the present invention. Figure 2 This is a partial cross-sectional schematic diagram of the optical MEMS microphone based on perforated strain enhancement and heterodyne readout according to the present invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0019] In the description of the embodiments of the present invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of the present invention based on the specific circumstances.

[0020] In the description of the embodiments of the present invention, it should be noted that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", and "outer" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of the present invention.

[0021] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0022] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0023] The following disclosure provides numerous different embodiments or examples for implementing various structures of the invention. To simplify the disclosure, specific examples of components and arrangements are described below. These are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples. Such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed. Additionally, examples of various specific processes and materials are provided in this invention; however, those skilled in the art will recognize the applicability of other processes and / or the use of other materials.

[0024] This embodiment provides an optical MEMS microphone based on aperture strain enhancement and heterodyne readout. The overall design employs a coherent interferometric optical sensing architecture, using a MEMS diaphragm as the sound pressure sensing unit and waveguide optical path modulation as the signal conversion mechanism. Combined with a heterodyne interferometric readout scheme, it achieves high-precision sound pressure detection. The device can be fabricated using on-chip integration technology, has a compact overall structure, is compatible with standard semiconductor manufacturing processes, and can be applied to various scenarios such as consumer electronics, acoustic monitoring, and industrial non-destructive testing.

[0025] like Figure 1-2 As shown, the optical MEMS microphone includes, in sequence along the optical path transmission direction, a light source module 1, a beam splitter module 2, a reference optical path 3, a sensing optical path 4, a beam combiner module 5, a photoelectric detection module 6, and a signal processing module 7.

[0026] Light source module 1 provides a coherent detection light source for the entire detection system. It can be a laser source that outputs single-mode linearly polarized laser, with an output wavelength adapted to the waveguide transmission band, and has stable output power and center frequency. The optical output end of light source module 1 and the optical input end of beam splitter module 2 are aligned and connected through an optical coupling structure. The output coherent laser is incident on beam splitter module 2 at a fixed power, providing a stable optical signal reference for subsequent interferometric sensing.

[0027] Beam splitter module 2 is a power beam splitter device with one optical input and two optical outputs. After the incident laser enters beam splitter module 2, it is split into two coherent output beams according to a preset power ratio, which are injected into reference optical path 3 and sensing optical path 4, respectively. In this embodiment, beam splitter module 2 adopts an equal-power beam splitting design, with a power ratio of 1:1 between the two output beams to ensure the contrast of the interference signal; in other embodiments, the beam splitting ratio can be adjusted according to actual signal-to-noise ratio requirements. The two outputs of beam splitter module 2 are connected one-to-one with the inputs of reference optical path 3 and sensing optical path 4, respectively, to ensure that the initial phase of the two beams remains synchronized.

[0028] Reference optical path 3 forms the reference arm for interferometric detection, providing a stable phase reference. The optical input end of reference optical path 3 is connected to the corresponding output end of beam splitting module 2, and the optical output end is connected to one input end of beam combining module 5. Reference optical path 3 is located in the fixed substrate region of the device and does not pass through the suspended deformation region of the MEMS diaphragm throughout its transmission. Therefore, it is not modulated by external sound pressure during transmission, and both the optical path and phase remain stable.

[0029] A frequency shift module is connected in series in the optical transmission path of the reference optical path 3. The frequency shift module applies a fixed frequency offset to the passing reference light, so that a stable frequency difference is formed between the output reference light and the sensing light in the sensing optical path 4, thereby constructing the carrier frequency required for heterodyne interference. The frequency shift module needs to maintain a stable operating state, and the output frequency offset should not change with the external sound pressure to ensure that the beat frequency carrier frequency is constant. When using discrete field devices such as acousto-optic modulators or electro-optic modulators, the frequency shift module can be set in the discrete coupling section of the reference optical path and connected to the on-chip reference waveguide through spatial coupling or fiber coupling. When using an integrated optical frequency shifter, the frequency shift module and the reference waveguide are integrally fabricated on the substrate and directly connected in series in the on-chip transmission path of the reference optical path.

[0030] The sensing optical path 4 constitutes the sensing arm of the interferometric detection and is the core unit for converting sound pressure signals into optical phase signals. Its optical input end is connected to the corresponding output end of the beam splitter module 2, and its optical output end is connected to the other input end of the beam combiner module 5. The sensing optical path 4 includes two parts: a MEMS diaphragm 41 and a sensing waveguide module 42.

[0031] The MEMS diaphragm 41 is a suspended flexible vibration structure supported by a surrounding fixed frame. Specifically, the MEMS diaphragm 41 is a composite thin film structure, composed of a top-layer silicon device layer of the SOI wafer, a silicon dioxide lower cladding layer, a silicon nitride waveguide core layer, and a silicon dioxide upper cladding layer. The diaphragm can adopt a square or circular configuration with a side length of 300~800μm and a total thickness ranging from 2~10μm. Its periphery is connected to the substrate silicon through the fixed frame. A release cavity is correspondingly provided below the diaphragm to provide deformation space for the diaphragm's bending vibration. The upper surface of the MEMS diaphragm 41 is used to support the sensing waveguide module 42, and the lower surface is opposite to the release cavity. When external sound pressure is applied to the MEMS diaphragm 41, the diaphragm is driven by the sound pressure to generate up-and-down bending vibration, resulting in a corresponding mechanical strain distribution inside. The MEMS diaphragm 41 is provided with an acoustic aperture array, which is formed by multiple apertures that penetrate the thickness direction of the diaphragm and are arranged according to a preset rule. On the one hand, the apertures can break the air compression effect of the closed air cavity under the diaphragm, reduce the air film damping during diaphragm vibration, and improve the vibration response amplitude and high frequency characteristics of the diaphragm. On the other hand, by changing the local stiffness distribution of the diaphragm, the strain distribution state inside the diaphragm can be actively controlled, so that the mechanical strain is concentrated in the area where the sensing waveguide module 42 is located, thereby improving the utilization efficiency of strain energy.

[0032] The sensing waveguide module 42 is attached to the upper surface of the MEMS diaphragm 41 and extends along a serpentine folding path along the strain-sensitive region of the MEMS diaphragm 41. The sensing waveguide module 42 is tightly integrated with the MEMS diaphragm 41 and can undergo synchronous mechanical deformation with the bending vibration of the diaphragm. When the MEMS diaphragm 41 is subjected to sound pressure and undergoes bending strain, the sensing waveguide module 42 simultaneously generates tangential strain along the propagation direction: on the one hand, the strain stretches or compresses the geometric length of the waveguide, changing the geometric path of light propagation; on the other hand, the strain changes the refractive index of the waveguide core material through photoelasticity, thereby changing the effective refractive index of the waveguide. The combined effect of the change in geometric length and the change in effective refractive index causes a change in the total optical path of the sensing light propagating within the waveguide, ultimately manifesting as modulation of the sensing light propagation phase, so that the amount of phase change corresponds to the amplitude of the external sound pressure. The serpentine return path design can significantly extend the effective sensing optical path within a limited diaphragm area, allowing the micro-strain distributed along the path to continuously accumulate, ultimately forming a detectable phase change and improving the sensitivity of sound pressure detection.

[0033] The beam combining module 5 has two optical input terminals and one optical output terminal. The two input terminals are connected to the output terminals of the reference optical path 3 and the sensing optical path 4, respectively. The frequency-shifted reference light and the phase-modulated sensing light enter the beam combining module 5 and are coupled into the same optical transmission path. Since the two beams are coherent from the same source and have a fixed frequency difference, they interfere when superimposed, forming a heterodyne beat frequency optical signal whose intensity changes periodically with the beat frequency period. The phase change information caused by sound pressure is modulated into the phase of this beat frequency optical signal. The output terminal of the beam combining module 5 is connected to the optical input terminal of the photodetector module 6, outputting the heterodyne beat frequency optical signal to the photodetector module 6.

[0034] The photoelectric detection module 6 employs a detection device with photoelectric conversion effect. Its optical receiving end is aligned with the output end of the beam combining module 5 to receive the heterodyne beat frequency optical signal output by the beam combining module, and converts the light intensity-time-varying optical signal into an electrical signal with corresponding amplitude variation. The response bandwidth of the photoelectric detection module 6 covers the frequency range of the heterodyne beat frequency, ensuring distortion-free conversion of the beat frequency signal. Its electrical signal output end is electrically connected to the input end of the signal processing module 7, outputting the converted electrical signal to the subsequent demodulation unit.

[0035] Signal processing module 7 is an electrical signal demodulation and calculation unit, and its signal input terminal is electrically connected to the output terminal of photoelectric detection module 6. Signal processing module 7 receives the electrical signal after photoelectric conversion, extracts the phase change of the sensing light from the heterodyne beat frequency signal through a heterodyne phase demodulation algorithm, and then converts the phase change into the corresponding external sound pressure information according to the pre-calibrated phase-sound pressure correspondence and outputs it, thus completing the detection and reconstruction of the sound pressure signal.

[0036] In a complete operation, the light source module 1 outputs a stable single-mode coherent laser, which, after coupling, is incident on the beam splitter module 2. The beam splitter module 2 splits the incident laser into two equal-power paths: one is injected into the reference optical path 3, and the other into the sensing optical path 4. When the laser flowing into the reference optical path 3 passes through the frequency shift module, a fixed frequency offset is applied, forming a reference light with a fixed frequency offset. It then propagates along the reference optical path 3 to the beam combiner module 5. The laser flowing into the sensing optical path 4 is incident on the sensing waveguide module 42 mounted on the MEMS diaphragm 41 and propagates along a serpentine return path. When external sound pressure is applied to the MEMS diaphragm 41, the diaphragm... The bending vibration generates synchronous mechanical strain in the sensing waveguide module 42. The propagation phase of the sensing light is modulated by the change in geometric length and the photoelastic effect, so that the sensing light carries the phase information corresponding to the sound pressure. It is then transmitted to the beam combining module 5. The frequency-shifted reference light and the phase-modulated sensing light are coupled and superimposed in the beam combining module 5, interfering and forming a heterodyne beat frequency light signal. The heterodyne beat frequency light signal is incident on the photoelectric detection module 6, and is converted into an electrical signal carrying beat frequency and phase information by photoelectric conversion. The electrical signal is input to the signal processing module 7, and the phase change corresponding to the sound pressure is extracted by phase demodulation and finally converted into external sound pressure information output.

[0037] Based on the above embodiments, this embodiment further elaborates on the waveguide structure, aperture configuration and optical path architecture of the optical MEMS microphone. The refined structures can be combined with each other to jointly improve the detection performance and integration adaptability of the device.

[0038] The sensing waveguide module 42 is made of silicon nitride waveguide. Silicon nitride material has the characteristics of low optical loss, moderate photoelasticity, high mechanical strength and good temperature stability in the near-infrared communication band. At the same time, it is highly compatible with standard CMOS and MEMS processing technology, and can be monolithically integrated with the MEMS diaphragm 41 structure, taking into account both optical transmission performance and mechanical sensing performance. The silicon nitride waveguide core layer thickness ranges from 200 to 500 nm, and the waveguide width ranges from 400 to 1000 nm, which meets the single-mode transmission conditions of the near-infrared communication band.

[0039] The sensing waveguide module 42 includes multiple parallel straight sensing segments and curved segments connecting adjacent straight sensing segments. Under uniform normal sound pressure, the tangential strain on the surface of the MEMS diaphragm 41 exhibits a fixed spatial distribution pattern: the tangential strain in different regions has different signs, and the strain amplitude changes in a gradient along the radial direction of the diaphragm. Based on this distribution characteristic, the straight sensing segments are arranged along the regions of the MEMS diaphragm 41 where the tangential strain has the same sign and a higher amplitude, so that the tangential strain direction of each straight sensing segment is consistent, and the strain integral along the waveguide propagation path can be accumulated in the same direction, avoiding the mutual cancellation of positive and negative strains. The curved segments are arranged in the relatively low strain regions of the MEMS diaphragm 41, and only serve the connection function of optical path folding. The strain generated by the curved segments is extremely low, which can effectively reduce the cancellation effect of tangential strains of different signs on the total strain integral.

[0040] The smooth transition between adjacent straight sensing segments via arc-shaped bends reduces optical transmission loss at waveguide bends and ensures low-loss propagation of sensing light within the serpentine path.

[0041] The acoustic aperture array includes multiple segments of rounded corner slits. These segmented rounded corner slits are arranged along at least one side of the straight sensing segment of the sensing waveguide module 42, and the major axis of each segmented rounded corner slit is parallel to the extension direction of the corresponding straight sensing segment. The slit structure arranged along both sides of the waveguide can form a stiffness weakening zone locally on the diaphragm. When the MEMS diaphragm 41 undergoes bending deformation under sound pressure, the bending strain is concentrated in the strip-shaped region between the slits. The sensing waveguide module 42 is precisely located at the center of this strip-shaped region, thereby significantly increasing the strain amplitude at the waveguide location and improving the utilization efficiency of strain energy.

[0042] The segmented rounded corner slits are intermittent segmented structures, with a continuous composite material layer supporting the sensing waveguide module 42 maintained between the ends of adjacent segmented rounded corner slits. This continuous composite material layer ensures the mechanical continuity of the diaphragm structure, avoiding the decrease in mechanical strength of the diaphragm caused by a continuous slit, while ensuring the continuous support structure under the sensing waveguide, guaranteeing the uniformity and stability of strain transmission along the waveguide path. Both ends of the segmented rounded corner slits adopt a rounded transition configuration, which can eliminate stress concentration at the slit ends, improve the fatigue resistance and long-term operational reliability of the MEMS diaphragm 41. The width of a single segmented rounded corner slit ranges from 5 to 20 μm, the length ranges from 50 to 200 μm, and the spacing between adjacent slit ends ranges from 10 to 30 μm.

[0043] An input waveguide is provided between the light source module 1 and the beam splitter module 2. The input waveguide adopts an on-chip waveguide structure with the same material system as the sensing waveguide module 42, which is used to realize low-loss on-chip transmission of the light source output optical signal. The light source module 1 can be aligned and coupled to the input end of the input waveguide through end-face coupling, grating coupling, etc., so that the overall optical path architecture can be fully integrated on the chip, reducing the device size.

[0044] The beam splitter module 2, reference optical path 3, sensing optical path 4, and beam combiner module 5 together constitute a Mach-Zehnder interference structure. The reference optical path 3 is the reference arm of the interference structure, and the sensing optical path 4 is the sensing arm of the interference structure. The optical signals of the two arms originate from the same incident light source. After beam splitting, they are transmitted separately and then interfered after beam combining. The phase difference between the two arms is modulated by the change of the optical path of the sensing arm to realize the phase detection of the sound pressure signal. During the device design, the static optical path of the reference optical path and the sensing optical path are kept matched so that the static operating point is in the orthogonal region of the interference curve, ensuring the linearity and sensitivity of the phase detection.

[0045] This interference structure has the advantages of optical path symmetry, high integration, and compatibility with fabrication processes, and can be seamlessly adapted to silicon nitride waveguide technology.

[0046] The frequency shift module can be any one of an acousto-optic modulator, an electro-optic modulator, or an integrated optical frequency shifter. Among them, the acousto-optic modulator achieves optical frequency shift based on the acousto-optic effect, and has the characteristics of stable frequency shift and high diffraction efficiency, making it suitable for solutions that integrate discrete devices. The electro-optic modulator achieves frequency modulation based on the electro-optic effect, and has the characteristics of fast response speed and large modulation bandwidth, making it suitable for high-frequency sound pressure detection scenarios. The integrated optical frequency shifter can be monolithically integrated with the on-chip waveguide system without the need for discrete device coupling, which can further reduce the overall device size and improve the system integration.

[0047] The signal processing module 7 is the core of the optical MEMS microphone for signal demodulation. It is used to convert the heterodyne beat frequency electrical signal output by the photoelectric detection module 6 into external sound pressure information. The signal processing module 7 includes an orthogonal demodulation unit, a phase extraction unit and a sound pressure calibration unit connected in sequence. The three units transmit data step by step along the signal processing link to complete the complete conversion from high-frequency beat frequency signal to sound pressure signal.

[0048] The signal input terminal of the quadrature demodulation unit is connected to the signal output terminal of the photoelectric detection module 6. It is used to perform quadrature mixing and baseband extraction on the input heterodyne beat frequency electrical signal. The quadrature demodulation unit integrates a local oscillator generation subunit, two mixing subunits, and two low-pass filter subunits. The local oscillator generation subunit generates two quadrature local oscillator signals with the same frequency as the heterodyne beat frequency carrier frequency and a 90° phase difference, namely the in-phase local oscillator signal and the quadrature local oscillator signal. The input terminals of the two mixing subunits are respectively connected to the heterodyne beat frequency input signal and the corresponding local oscillator signal, and the two signals are subjected to analog mixing or digital mixing to output a mixed signal containing a sum frequency component and a difference frequency component. The two low-pass filter subunits are connected to the output terminals of the two mixing subunits respectively to filter out the high-frequency sum frequency component and background high-frequency noise in the mixed signal, and only retain the low-frequency difference frequency component carrying sound pressure phase information. Finally, the in-phase baseband signal and the quadrature baseband signal are output to the phase extraction unit.

[0049] Through orthogonal mixing and low-pass filtering, the phase signal carrying sound pressure information is demodulated from the high-frequency heterodyne carrier to the baseband. At the same time, it can effectively suppress common-mode interference caused by light source power fluctuations and optical path loss drift, thereby improving the stability of the detection results.

[0050] The input of the phase extraction unit is connected to the output of the quadrature demodulation unit. It is used to calculate the continuous phase change caused by sound pressure from the two quadrature baseband signals. The phase extraction unit first performs point-by-point calculation on the in-phase baseband signal and the quadrature baseband signal through four-quadrant arctangent operation to obtain the instantaneous wrapped phase at each sampling time. The phase value range is [-π, π]. Then, the wrapped phase is unwrapped through the phase expansion algorithm to eliminate the 2π jump generated when the phase crosses the ±π boundary, and recover the continuous phase change that corresponds to the sound pressure amplitude. The continuous phase change is then output to the sound pressure calibration unit.

[0051] In this embodiment, the phase extraction unit can use a digital signal processor or a field-programmable gate array to implement the above operations, ensuring the real-time performance and calculation accuracy of phase solution, and adapting to the response requirements of different sound pressure detection scenarios.

[0052] The input of the sound pressure calibration unit is connected to the output of the phase extraction unit. It is used to convert the continuous phase change into an external sound pressure signal that can be directly read. The sound pressure calibration unit has a built-in storage medium that pre-stores the phase-sound pressure calibration relationship. This calibration relationship is obtained through pre-calibration: the standard reference microphone and this microphone are placed in the same uniform sound field, and multiple sets of phase changes corresponding to known standard sound pressures are collected. The phase change and sound pressure value are established through data fitting and then stored in the storage medium.

[0053] In actual operation, the sound pressure calibration unit calls the pre-stored calibration mapping relationship based on the input continuous phase change, and converts the phase change into the corresponding external sound pressure value by looking up a table or calculating a function and outputs it, thus completing the complete detection of the sound pressure signal.

[0054] This embodiment provides a sound pressure detection method based on aperture strain enhancement and heterodyne readout, which can be implemented using the aforementioned optical MEMS microphone. This method obtains the waveguide phase change caused by sound pressure through a heterodyne interference phase demodulation mechanism, thereby reconstructing the external sound pressure information. The sound pressure detection method specifically includes steps S100 to S500.

[0055] Step S100: Split the incident laser beam into two paths: a reference beam and a sensing beam.

[0056] The light source module 1 outputs a stable single-mode coherent laser. After coupling, the laser beam enters the beam splitter module 2 and is split into two coherent beams of the same origin according to a preset power ratio. One beam is used as a reference beam input to the reference optical path 3, and the other is used as a sensing beam input to the sensing optical path 4. The two beams maintain a consistent initial phase. In this embodiment, equal-power beam splitting is used, with a power ratio of 1:1 between the two beams to ensure the contrast of the subsequent interference signal. In other embodiments, the beam splitting ratio can be adjusted according to the signal-to-noise ratio requirements.

[0057] Step S200: The reference light is frequency shifted to create a fixed frequency difference between the reference light and the sensing light.

[0058] The reference light in the input reference optical path 3 flows through a frequency shift module connected in series in the optical path. The frequency shift module applies a constant frequency offset to the reference light, creating a stable frequency difference Δf between the output reference light and the sensing light in the sensing optical path 4, thereby constructing the carrier frequency for heterodyne interference. The frequency shift Δf is set according to the detection bandwidth and device parameters, and must be higher than the highest frequency of the sound pressure signal to be measured to ensure that the sound pressure signal can be completely modulated to the carrier frequency band and avoid the low-frequency noise range. The typical range of the frequency shift Δf is 10kHz to 10MHz, and it must be higher than the highest frequency of the sound pressure signal to be measured.

[0059] Step S300: The sensing light is propagated along the serpentine sensing waveguide on the surface of the MEMS diaphragm 41 with an acoustic aperture array, and the propagation phase of the sensing light is modulated by the strain of the MEMS diaphragm 41.

[0060] After the sensing light is input into the sensing light path 4, it propagates into the serpentine sensing waveguide laid in the strain-sensitive area of ​​the MEMS diaphragm 41. When external sound pressure is applied to the MEMS diaphragm 41, the diaphragm bends and vibrates, causing the sensing waveguide to generate mechanical strain synchronously. The strain simultaneously causes changes in the waveguide's geometric length and effective refractive index, which together change the propagation path of the sensing light, causing the propagation phase of the sensing light to be modulated accordingly with the sound pressure amplitude.

[0061] The phase change caused by waveguide geometric strain satisfies the following calculation relationship:

[0062] In the formula, λ is the vacuum wavelength of the sensing light, and ε tt The tangential strain component at a local location in the waveguide along the propagation direction is integrated along the total length L of the sensing waveguide. This phase change includes two contributions: one is the change in the physical length of the waveguide caused by geometric deformation, and the other is the change in the refractive index of the waveguide caused by photoelasticity. The acoustic aperture array on the MEMS diaphragm 41 reduces the gas film damping of the diaphragm vibration and increases the vibration response amplitude; on the other hand, it modulates the strain distribution, causing the strain to concentrate in the sensing waveguide region, increasing the total strain of the path integral; the serpentine return path, by extending the integration length L, can obtain a larger phase change at the same unit strain amplitude, thus amplifying the phase modulation amplitude.

[0063] Step S400: Combine the frequency-shifted reference light with the phase-modulated sensing light to form a heterodyne beat frequency signal.

[0064] The frequency-shifted reference light and the phase-modulated sensing light are transmitted to the two input terminals of the beam combining module 5, respectively. They are coupled to the same transmission path and coherently superimposed within the beam combining module 5. Since the two beams have a fixed frequency difference and the sensing light carries sound pressure phase information, the superposition forms a heterodyne beat frequency light signal whose light intensity changes periodically with time. The phase change caused by sound pressure is modulated into the phase of the beat frequency signal.

[0065] The expression for the intensity of the heterodyne beat frequency optical signal output by the combined beam is:

[0066] In the formula, A is the amplitude of the interference beat frequency signal, Δf is the fixed frequency difference between the reference light and the sensing light, and Δφ(t) is the time-varying phase change of the sensing light caused by the sound pressure. The sound pressure information is entirely contained in the phase term of the beat frequency signal and is not directly affected by fluctuations in the light intensity amplitude.

[0067] Step S500: Demodulate the phase of the heterodyne beat frequency signal, extract the phase change, and convert it into an external sound pressure signal for output.

[0068] The heterodyne beat frequency optical signal is converted into an electrical signal by the photoelectric detection module 6 and then input into the signal processing link. It is processed step by step through quadrature demodulation, phase extraction and sound pressure calibration, and finally outputs a readable external sound pressure value.

[0069] In this embodiment, step S500 is specifically implemented through sub-steps S501 to S506.

[0070] Step S501: Generate two quadrature local oscillator signals corresponding to the heterodyne frequency.

[0071] The local oscillator generation subunit inside the quadrature demodulation unit generates two quadrature local oscillator signals with the same frequency as the heterodyne beat frequency carrier frequency Δf and a phase difference of 90°. These are the in-phase local oscillator signal and the quadrature local oscillator signal, respectively. The frequency and phase accuracy of the two local oscillator signals meet the demodulation error requirements.

[0072] Step S502: Mix the two quadrature local oscillator signals with the heterodyne beat frequency signal respectively.

[0073] The in-phase local oscillator signal and the quadrature local oscillator signal are multiplied and mixed with the input heterodyne beat frequency electrical signal by two mixing sub-units. Each mixing output contains a high-frequency sum frequency component and a low-frequency difference frequency component, in which the phase change information caused by sound pressure is carried in the low-frequency difference frequency component.

[0074] Step S503: Perform low-pass filtering on the two mixed signals to obtain two baseband signals.

[0075] The two mixed output signals are respectively connected to the corresponding low-pass filter subunits. The high-frequency sum and frequency components and high-frequency noise are filtered out by the low-pass filter with the preset cutoff frequency, and only the low-frequency difference component carrying the sound pressure phase information is retained. Finally, the in-phase baseband signal and the quadrature baseband signal are output.

[0076] Step S504: Extract phase information from the two baseband signals using the four-quadrant arctangent algorithm.

[0077] The phase extraction unit receives the in-phase baseband signal and the quadrature baseband signal, performs a four-quadrant arctangent operation on the two signal values ​​at each sampling time, and calculates the wrap-around phase value at the corresponding time. The wrap-around phase value ranges from [-π, π], and this phase value corresponds to the waveguide phase change caused by the current sound pressure.

[0078] Step S505: Eliminate phase jumps by using a phase expansion algorithm to obtain continuous phase change amounts.

[0079] A phase unwrapping algorithm is used to untangle the discrete wrapped phase sequence: the phase difference between adjacent sampling points is compared sequentially, and when the absolute value of the difference exceeds π, ±2π is added to the phase value of the next sampling point for correction, eliminating the 2π jump that occurs when the phase crosses the ±π boundary, and finally obtaining a sequence of phase changes that vary continuously with sound pressure. Step S506: Based on the preset phase-sound pressure correspondence, convert the continuous phase change into an external sound pressure signal and output it.

[0080] The phase-sound pressure correspondence is pre-established through calibration experiments and solidified in the sound pressure calibration unit. The calibration process employs least-squares linear fitting: first, the microphone and a calibrated standard reference microphone are placed in the same uniform standard sound field, and multiple standard sound pressure points with known amplitudes are applied sequentially. The stable phase change corresponding to each standard sound pressure is simultaneously collected, resulting in multiple sets of discrete sample data. Then, with the phase change as the independent variable and the standard sound pressure value as the dependent variable, a univariate linear fitting is performed using the least-squares method to obtain a linear mapping relationship including slope and intercept, and the fitting parameters are solidified and stored. During actual testing, the sound pressure calibration unit substitutes the input continuous phase change into the linear mapping relationship, calculates the corresponding sound pressure value, and outputs it, thus completing the reconstruction of the sound pressure signal.

[0081] This embodiment provides a method for fabricating an optical MEMS microphone, used to prepare the aforementioned optical MEMS microphone based on aperture strain enhancement and heterodyne readout. This method is implemented using standard SOI wafer fabrication technology, exhibiting strong compatibility with conventional MEMS fabrication processes and silicon nitride photonic integration processes, enabling large-scale mass production. The fabrication method specifically includes steps S1-S5.

[0082] Step S1: Select an SOI wafer and prepare a silicon dioxide undercladding layer on the front side of the device layer.

[0083] An SOI wafer, consisting of a single-crystal silicon device layer, a BOX buried oxide layer, and a silicon substrate, is selected as the fabrication substrate from top to bottom. First, the front side of the wafer undergoes standard cleaning to remove surface particles and organic impurities. Then, a silicon dioxide lower cladding layer is grown on the front side of the device layer using plasma-enhanced chemical vapor deposition or thermal oxidation. The thickness of the lower cladding layer is set according to the waveguide's single-mode transmission conditions, serving to provide optical confinement for the subsequent waveguide core layer and simultaneously achieving electrical and optical isolation between the waveguide and the single-crystal silicon device layer.

[0084] Step S2: Deposit a waveguide core layer on the silicon dioxide underlayer, and form a beam splitter module 2, a reference waveguide, a sensing waveguide module 42 and a beam combiner module 5 through photolithography and etching processes.

[0085] A silicon nitride waveguide core layer is deposited on the surface of the silicon dioxide underlayer using low-pressure chemical vapor deposition (LPCVD). After deposition, high-temperature annealing is performed according to material stress and optical loss requirements to release residual stress and reduce transmission loss. Subsequently, a complete on-chip optical path pattern is defined on the surface of the waveguide core layer using ultraviolet lithography (UV lithography). Then, inductively coupled plasma etching (ICP-CPE) is used to vertically etch the silicon nitride core layer to a depth consistent with the core layer thickness, ultimately forming an integrated on-chip optical path structure including a beam splitter module 2, a reference waveguide, a sensing waveguide module 42, and a beam combiner module 5. The reference waveguide corresponds to the waveguide portion of the reference optical path 3 in the finished product, and the sensing waveguide module 42 corresponds to the waveguide sensing portion of the sensing optical path 4 in the finished product.

[0086] Step S3: Deposit a cladding layer on silicon dioxide to prepare an optical input-output coupling structure.

[0087] A silicon dioxide cladding layer is deposited on the etched optical path structure surface using plasma-enhanced chemical vapor deposition (PECVD), completely encapsulating the waveguide core layer with the upper and lower cladding layers to form a symmetrical cladding-core-cladding waveguide structure, achieving total internal reflection confinement of the light field. Subsequently, optical input-output coupling structures are fabricated according to the device's coupling scheme: if an end-face coupling scheme is used, the corresponding end face of the wafer is precisely polished to form a flat waveguide end face; if a grating coupling scheme is used, grating couplers are fabricated at the waveguide input and output ends using photolithography and etching processes to achieve efficient coupling between the external light source and the on-chip optical path.

[0088] Step S4: Etch an acoustic aperture array in the area that avoids the waveguide core layer.

[0089] An acoustic aperture array pattern was defined in a pre-defined region 41 of the MEMS diaphragm using photolithography. The aperture pattern strictly avoided the waveguide core layer and its necessary optical cladding to prevent damage to the waveguide's optical transmission structure. Subsequently, a deep inductively coupled plasma etching process was used to etch the upper silicon dioxide cladding, lower silicon dioxide cladding, and single-crystal silicon device layer in the aperture region from top to bottom. The etching depth penetrated the entire diaphragm thickness, forming a pervasive acoustic aperture array. During the etching process, the etching morphology was controlled to ensure the perpendicularity of the aperture sidewalls, while retaining rounded corners at the aperture ends to reduce the risk of stress concentration.

[0090] Step S5: Etch the release diaphragm area from the back of the wafer to form a suspended MEMS diaphragm 41 and a release cavity.

[0091] The wafer is flipped over, and photolithographic alignment is performed on the back side of the wafer to precisely align the back release window with the MEMS diaphragm 41 area on the front side. Deep reactive ion etching is used to etch the substrate silicon from the back side until the BOX buried oxide layer is reached. Then, hydrofluoric acid wet etching is used to remove the BOX buried oxide layer in the release area, so that the top composite film structure in this area is completely detached from the substrate support, and finally a suspended MEMS diaphragm 41 circumferentially supported by a peripheral fixed frame and a release cavity below the diaphragm are formed, completing the fabrication of the core structure of the device.

[0092] In some embodiments, the reference waveguide is fabricated in the fixed frame region of the wafer, and the sensing waveguide module 42 is fabricated in the region of the diaphragm to be released. The sensing waveguide module 42 is fabricated in a serpentine foldback path. The fixed frame region corresponds to the region where the substrate silicon and BOX buried oxide layer are subsequently retained. The waveguide in this region is always supported by a complete substrate, and the optical path remains stable during operation, forming the reference arm of the interference structure. The diaphragm region to be released corresponds to the region that is subsequently etched and suspended. The sensing waveguide module 42 in this region vibrates synchronously with the MEMS diaphragm 41 and undergoes phase change due to strain modulation, forming the sensing arm of the interference structure. The serpentine foldback path design can extend the effective sensing length within a limited diaphragm area and improve the phase modulation sensitivity.

[0093] In some embodiments, the acoustic aperture array adopts a segmented rounded corner slit structure, arranged on both sides of the straight segment of the sensing waveguide module 42, with the major axis of the segmented rounded corner slits parallel to the extension direction of the straight segment of the sensing waveguide module 42. This arrangement can form local stiffness weakening regions on both sides of the waveguide, causing the bending strain of the diaphragm under sound pressure to concentrate in the waveguide bearing area between the two slits, thus improving strain utilization efficiency. The segmented slit design ensures the continuity of the composite layer material under the waveguide throughout, avoiding a decrease in the mechanical strength of the diaphragm due to a continuous slit, thus balancing strain enhancement effect and structural reliability.

[0094] In a verification example, to verify the technical effects of local apertures, serpentine waveguides, and heterodyne I / Q readout, a composite layer structure mechanical model, a geometric optical path phase post-processing model, and signal models for direct intensity detection and heterodyne I / Q detection were established. The structural mechanical model adopted a COMSOL Shell equivalent laminated structure. The diaphragm was a square with a side length of 1.5 mm, fixed on all four sides, and a uniform pressure of 1 Pa along the -z direction was applied to the diaphragm surface. The SOI device layer had a single-crystal silicon thickness of 1.5 μm, a silicon dioxide lower cladding thickness of 2.0 μm, a silicon nitride waveguide core layer thickness of 0.4 μm, and a silicon dioxide upper cladding thickness of 2.0 μm, for a total equivalent thickness of 5.9 μm. The center of the silicon nitride core layer was located approximately 1.217 μm above the equivalent neutral plane. The outer circular aperture reference structure was compared with g=3.5d. h L=4d h W=0.5d h The recommended segmented rounded slit structure is used. Strain is extracted at the center height of the silicon nitride waveguide core. However, this model does not include residual film stress, air load, back cavity fluid, or gas film damping.

[0095] Optical post-processing uses λ=1550nm and neff=1.8, and initially only considers the change in waveguide geometry. The current results are strictly defined as geometric optical path phase sensitivity and do not yet include strain optical effects; the silicon nitride photoelastic coefficient, mode overlap factor, and strain-induced effective refractive index change can be incorporated into subsequent optical mode calculations.

[0096] The results shown in Table 1 were obtained under the above composite layer model and a load of 1 Pa.

[0097] Table 1 Comparison results of geometric phase sensitivity

[0098] Compared to the peripheral circular aperture, the rounded corner slits increased the silicon nitride path strain integral by approximately 41.64%, while reducing the first-order natural frequency by approximately 2.59%. This result demonstrates that, compared to a circular aperture only on the periphery, segmented rounded corner slits arranged along both sides of the sensing waveguide can create a more effective local compliance region in the composite diaphragm and guide more bending strain to the waveguide path. The serpentine waveguide is approximately 7.960 times longer than the straight waveguide, and its path strain integral and geometric phase sensitivity are approximately 7.946 times greater. This result indicates that the serpentine waveguide extends the effective sensing optical path within the same sensitive region.

[0099] The signal readout simulation uses a phase sensitivity of S = 8.6961 × 10⁻⁶. -4 The parameters used were rad / Pa, 1kHz sinusoidal sound pressure level, 100kHz heterodyne frequency, and 2MHz sampling rate. Both methods employed the same optical power, interferometric visibility, white noise, 1 / f noise, and optical power fluctuation. Direct light intensity detection was ideally set at the quadrature operating point, and a fixed quadrature slope was used to recover the sound pressure level; heterodyne detection employed quadrature mixing, low-pass filtering, atan2, and unwrap to recover the phase.

[0100] Table 2 Comparison of Direct Light Intensity Detection and Heterodyne Demodulation Performance under Ideal Conditions

[0101] Under ideal noise and filtering conditions in this embodiment, heterodyne I / Q detection improves the SNR by approximately 7.6 dB compared to direct light intensity detection. However, this result is a simulation result under the parameters of this embodiment and does not represent a fixed improvement of 7.6 dB under any hardware and any filtering conditions.

[0102] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An optical MEMS microphone based on aperture strain enhancement and heterodyne readout, characterized in that, It includes a light source module (1), a beam splitter module (2), a reference optical path (3), a sensing optical path (4), a beam combiner module (5), a photoelectric detection module (6), and a signal processing module (7), among which, The input end of the beam splitter module (2) is connected to the light source module (1); The reference optical path (3) and the sensing optical path (4) are respectively connected to the two output ends of the beam splitting module (2). The sensing optical path (4) includes a MEMS diaphragm (41) and a sensing waveguide module (42). The sensing waveguide module (42) is arranged in the strain-sensitive area of ​​the MEMS diaphragm (41) and extends in a serpentine folding path. The MEMS diaphragm (41) is provided with an acoustic aperture array. The reference optical path (3) is provided with a frequency shift module. The frequency shift module is used to apply a fixed frequency shift to the reference light. The two input terminals of the beam combining module (5) are respectively connected to the output terminals of the reference optical path (3) and the sensing optical path (4). The beam combining module (5) is used to make the frequency-shifted reference light interfere with the sensing light whose phase is modulated by the strain of the MEMS diaphragm (41) to form a heterodyne beat frequency signal. The photoelectric detection module (6) is connected to the output end of the beam combining module (5), and the photoelectric detection module (6) is used to convert the heterodyne beat frequency signal into an electrical signal; The signal processing module (7) is connected to the output of the photoelectric detection module (6). The signal processing module (7) is used to demodulate the electrical signal into external sound pressure information.

2. The optical MEMS microphone based on aperture strain enhancement and heterodyne readout as described in claim 1, characterized in that, The sensing waveguide module (42) adopts a silicon nitride waveguide. The sensing waveguide module (42) includes multiple parallel straight sensing segments and curved segments connecting adjacent straight sensing segments. The straight sensing segments are arranged along the tangential strain region of the MEMS diaphragm (41), and the curved segments are located in the low strain region of the MEMS diaphragm (41).

3. The optical MEMS microphone based on aperture strain enhancement and heterodyne readout as described in claim 2, characterized in that, The acoustic aperture array includes multiple segments of rounded corner slits. The segments of rounded corner slits are arranged along at least one side of the straight sensing segment of the sensing waveguide module (42). The major axis of the segments of rounded corner slits is parallel to the extension direction of the corresponding straight sensing segment. A continuous composite material layer supporting the sensing waveguide module (42) is provided between the ends of adjacent segments of rounded corner slits.

4. The optical MEMS microphone based on aperture strain enhancement and heterodyne readout as described in claim 1, characterized in that, An input waveguide is provided between the light source module (1) and the beam splitting module (2). The beam splitting module (2), the reference optical path (3), the sensing optical path (4), and the beam combining module (5) together constitute a Mach-Zehnder interference structure. The frequency shifting module is one of an acousto-optic modulator, an electro-optic modulator, or an integrated optical frequency shifter.

5. The optical MEMS microphone based on aperture strain enhancement and heterodyne readout as described in claim 1, characterized in that, The signal processing module (7) includes an orthogonal demodulation unit, a phase extraction unit, and a sound pressure calibration unit connected in sequence.

6. A sound pressure detection method based on open-hole strain enhancement and heterodyne readout, characterized in that, Includes the following steps: The incident laser beam is split into two paths: a reference beam and a sensing beam. The reference light is frequency-shifted to create a fixed frequency difference between the reference light and the sensing light. The sensing light is propagated along the serpentine sensing waveguide on the surface of the MEMS diaphragm (41) which is provided with an acoustic aperture array, and the propagation phase of the sensing light is modulated by the strain of the MEMS diaphragm (41). The frequency-shifted reference light and the phase-modulated sensing light are combined and interfered to form a heterodyne beat frequency signal; Phase demodulation is performed on the heterodyne beat frequency signal, the phase change is extracted and converted into an external sound pressure signal for output.

7. The sound pressure detection method based on opening strain enhancement and heterodyne readout as described in claim 6, characterized in that, The process of performing phase demodulation on the heterodyne beat frequency signal, extracting the phase change, and converting it into an external sound pressure signal for output includes the following steps: Generate two quadrature local oscillator signals corresponding to the heterodyne frequency; The two quadrature local oscillator signals are respectively mixed with the heterodyne beat frequency signal; The two mixed signals are low-pass filtered to obtain two baseband signals. Phase information is extracted from two baseband signals using a four-quadrant arctangent algorithm. Phase jumps are eliminated by phase expansion algorithm to obtain continuous phase change; Based on the preset phase-sound pressure correspondence, the continuous phase change is converted into an external sound pressure signal and output.

8. A method for fabricating an optical MEMS microphone, characterized in that, The method for fabricating an optical MEMS microphone based on aperture strain enhancement and heterodyne readout as described in any one of claims 1-5 includes the following steps: Select an SOI wafer and fabricate a silicon dioxide undercladding layer on the front side of the device layer; A waveguide core layer is deposited on the silicon dioxide cladding, and a beam splitter module (2), a reference waveguide, a sensing waveguide module (42), and a beam combiner module (5) are formed by photolithography and etching processes. A cladding layer is deposited on silicon dioxide to fabricate an optical input-output coupling structure; An array of acoustic apertures is formed by etching in the region that avoids the waveguide core layer; The release diaphragm area is etched from the back of the wafer to form a suspended MEMS diaphragm (41) and release cavity.

9. The method for fabricating an optical MEMS microphone as described in claim 8, characterized in that, The reference waveguide is fabricated in the fixed frame region of the wafer, and the sensing waveguide module (42) is fabricated in the diaphragm region to be released. The sensing waveguide module (42) is fabricated as a serpentine return path.

10. The method for fabricating an optical MEMS microphone as described in claim 8, characterized in that, The acoustic aperture array adopts a segmented rounded corner slit structure, which is arranged on both sides of the straight segment of the sensing waveguide module (42). The long axis of the segmented rounded corner slit is parallel to the extension direction of the straight segment of the sensing waveguide module (42).

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