A photoacoustic sensor and its fabrication method

Abstract

This invention relates to the field of optical acoustic wave detection technology, and in particular to a photoacoustic sensor and its fabrication method. The invention employs a Fabry-Perot cavity constructed by sequentially coupling a thermally expanded fiber, a hollow fiber, and a composite metal thin film, along with a high-reflectivity dielectric film. This effectively solves the problems of poor mode field matching, high coupling loss, and the difficulty in simultaneously achieving high sensitivity, wideband response, and miniaturization in existing fiber optic photoacoustic sensors. The thermally expanded fiber significantly improves the mode field matching with the hollow fiber, reducing coupling loss. Using the air core of the hollow fiber as the resonant cavity, combined with the high-reflectivity film and the composite metal thin film, a high-Q resonant cavity structure is formed, improving detection sensitivity under miniaturized packaging conditions. Acoustic waves drive the thin film vibration to directly modulate the cavity length, achieving wideband and stable photoacoustic signal conversion, enabling the sensor to simultaneously possess the advantages of miniaturization, high sensitivity, and wideband response.

Description

Technical Field

[0001] This invention relates to the field of optical acoustic wave detection technology, and in particular to a photoacoustic sensor and its fabrication method. Background Technology

[0002] Acoustic wave detection is a core technology for target perception, acoustic recognition, and status monitoring. Fiber optic photoacoustic sensors have become an important direction for replacing traditional electrical sensors due to their advantages such as resistance to electromagnetic interference, passive operation, and long-distance transmission.

[0003] Existing fiber-optic photoacoustic sensors generally use hollow optical fibers and reflective films to form a resonant cavity. In the detection of weak acoustic signals, high sensitivity, wide bandwidth response and miniaturization are mutually restrictive. Size reduction will lead to a sharp drop in sensitivity, and there are problems such as poor mode field matching and high coupling loss.

[0004] Therefore, overcoming the shortcomings of the existing technology is an urgent problem to be solved in this technical field. Summary of the Invention

[0005] The technical problem to be solved by this invention is that existing fiber optic photoacoustic sensors face the challenge of simultaneously achieving high sensitivity, wideband response, and miniaturization, and suffer from poor mode field matching and high coupling loss.

[0006] The present invention adopts the following technical solution: In a first aspect, a photoacoustic sensor is provided, comprising a thermally expanded fiber 1, a hollow fiber 2, and a composite metal thin film 3 coupled in sequence; a high-reflectivity dielectric film 4 is disposed between the thermally expanded fiber 1 and the hollow fiber 2; the air core of the hollow fiber 2 serves as a Fabry-Perot cavity, the high-reflectivity dielectric film 4 serves as a first reflecting surface of the Fabry-Perot cavity, and the composite metal thin film 3 serves as a second reflecting surface of the Fabry-Perot cavity; In this process, the sound wave causes the composite metal film 3 to vibrate, and photoacoustic sensing is achieved by changing the cavity length and modulating the reflected light interference signal.

[0007] Preferably, a micro-air gap 5 is provided between the hollow optical fiber 2 and the composite metal film 3, and the micro-air gap 5 is coaxially arranged with the core of the hollow optical fiber 2.

[0008] Preferably, it also includes a first sleeve 60 and a second sleeve 61, wherein a portion of the thermally expanded fiber 1 and a portion of the hollow fiber 2 are respectively disposed in relative positions within the first sleeve 60, and another portion of the hollow fiber 2 is disposed within the second sleeve 61. The composite metal film 3 is disposed at one end of the second sleeve 61. The first sleeve 60 and the second sleeve 61 are used to coaxially align the core of the thermally expanded fiber 1 with the core of the hollow fiber 2 and encapsulate them into an integral structure.

[0009] Preferably, the composite metal film 3 includes at least a first metal layer 30, a second metal layer 31, and a third metal layer 32 grown sequentially.

[0010] Preferably, the high reflectivity dielectric film 4 is formed by alternating stacking of TiO2 and SiO2 on the end faces of the thermally expanded fiber 1 coupled with the hollow fiber 2.

[0011] Preferably, the acoustic pressure sensitivity and frequency response range of the photoacoustic sensor are adjusted by changing the effective radius, thickness, and cavity reflectivity of the composite metal film 3. The composite metal film 3 has an effective radius ranging from 0.5 mm to 5 mm, a thickness ranging from 100 nm to 1000 nm, and a reflectivity of 80%-99.99% for the first reflective surface and 80%-99.999% for the second reflective surface in the cavity. The photoacoustic sensor has a sound pressure sensitivity ranging from -150 dB re 1 rad / µPa to -100 dB re 1 rad / µPa and a frequency response range of 10 Hz to 20 kHz.

[0012] In a second aspect, a method for fabricating a photoacoustic sensor is provided, for fabricating the photoacoustic sensor as described in the first aspect, the method comprising: A thermally expanded fiber ferrule 7 is prepared, and a high-reflectivity dielectric film 4 is formed on the end face of the thermally expanded fiber 1. Prepare hollow fiber ferrule 8, and align and couple thermally expanded fiber ferrule 7 with hollow fiber ferrule 8; The aligned and coupled thermally expanded fiber ferrule 7 and hollow fiber ferrule 8 are fixed in their relative positions inside the first sleeve 60. A composite metal thin film 3 is prepared and transferred to the end face of the second sleeve 61 to obtain a composite metal thin film sleeve 9. The composite metal thin film sleeve 9 is coupled with the hollow fiber ferrule 8 so that the high reflectivity dielectric film 4, the air core of the hollow fiber 2, and the composite metal thin film 3 together form a Fabry-Perot cavity, thereby obtaining the photoacoustic sensor.

[0013] Preferably, the preparation of the composite metal thin film 3 specifically includes: The silicon wafer surface is subjected to a standard semiconductor cleaning process to remove surface particles, organic contaminants and natural oxide layer, ensuring the surface cleanliness and nanoscale flatness of the silicon wafer. A high-precision spin coater is used to spin coat positive photoresist onto a silicon wafer as a sacrificial layer. The uniformity of the photoresist layer thickness is ensured by adjusting the spin coat speed and time. After spin coating, the silicon wafer is placed on a heating stage for pre-curing treatment to allow the photoresist to form a stable sacrificial layer structure, thus obtaining the silicon wafer substrate. The pre-treated silicon wafer substrate and at least three high-purity metal targets are loaded into the sample stage and corresponding target station of the electron beam evaporation coating system. The coating cavity is subjected to multi-stage vacuum treatment to meet the high vacuum environment requirements of atomic-level evaporation deposition. At least three metal films are sequentially deposited on the surface of a silicon substrate by bombarding the corresponding metal target with a high-energy focused electron beam in stages, and the film thickness is precisely controlled. After the deposition of at least three thin films is completed, the composite metal film 3 is subjected to in-situ low-temperature annealing in the coating chamber to eliminate the internal stress generated during the film deposition process and control the overall residual stress of the composite film within the range of low tensile stress, thereby completing the preparation of the composite metal film 3.

[0014] Preferably, the step of transferring the composite metal film 3 to the end face of the second sleeve 61 to obtain the composite metal film sleeve 9 specifically includes: The second sleeve 61, after being precisely cut and flattened at the end face, is coaxially aligned with the metal film on the silicon wafer substrate, and epoxy resin is used to bond the end face of the second sleeve 61 to the metal film. The bonded assembly is placed on a heating table at a preset temperature and cured at a constant temperature for a first preset time, so that the second sleeve 61 and the metal film form a stable rigid connection. The solidified whole is completely immersed in the organic solution and soaked at a constant temperature for a second preset time. During the soaking process, the organic solution gradually penetrates into the photoresist sacrificial layer, completely dissolves the photoresist, separates the metal film area from the silicon wafer, and the metal film is attached to cover the end face of the second sleeve 61. The second sleeve 61, after the film transfer is completed, is taken out of the organic solution, and the excess metal film outside the end face of the second sleeve 61 is removed by a precision peeling process to obtain a composite metal film sleeve 9 with a smooth surface and no damage.

[0015] Preferably, the preparation of the thermally expanded fiber ferrule 7 and the formation of a high-reflectivity dielectric film 4 on the end face of the thermally expanded fiber 1 specifically includes: The standard single-mode fiber is modified by a standardized heat treatment process of pre-discharge, cyclic expansion and annealing cooling to prepare the thermally expanded fiber 1. The thermally expanded fiber 1 is fixed in the ferrule and the end face of the ferrule is precisely ground and polished to obtain the thermally expanded fiber ferrule 7. An optical coating process is used to deposit a high-reflectivity dielectric film 4 of alternating stacked TiO2 and SiO2 on the end face of the thermally expanded fiber 1.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention employs a Fabry-Perot cavity formed by sequentially coupling a thermally expanded fiber 1, a hollow fiber 2, and a composite metal thin film 3, along with a high-reflectivity dielectric film 4. This effectively solves the problems of poor mode field matching, high coupling loss, and the difficulty in simultaneously achieving high sensitivity, wideband response, and miniaturization in existing fiber optic photoacoustic sensors. The thermally expanded fiber 1 significantly improves the mode field matching with the hollow fiber 2, reducing coupling loss. Using the air core of the hollow fiber 2 as a resonant cavity, a high-Q resonant cavity structure is formed with the high-reflectivity film and the composite metal thin film 3, improving detection sensitivity under the premise of miniaturized packaging. Acoustic waves drive the thin film vibration to directly modulate the cavity length, achieving wideband and stable photoacoustic signal conversion, enabling the sensor to simultaneously possess the advantages of miniaturization, high sensitivity, and wideband response. Attached Figure Description

[0017] 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.

[0018] Figure 1 This is a schematic diagram of the structure of a photoacoustic sensor provided in an embodiment of the present invention; Figure 2a This is a schematic diagram of a photoacoustic sensor including a dual cavity provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the specific structure of a photoacoustic sensor provided in an embodiment of the present invention; Figure 3a This is a schematic diagram of a pressure relief microchannel provided in an embodiment of the present invention; Figure 3b This is a schematic diagram of another pressure relief microchannel provided in an embodiment of the present invention; Figure 3 This is a schematic flowchart of a method for fabricating a photoacoustic sensor according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the structure of a silicon wafer provided in an embodiment of the present invention; Figure 5 This is a schematic diagram of a silicon wafer substrate provided in an embodiment of the present invention; Figure 6 This is a schematic diagram of a structure for depositing a metal layer on the surface of a silicon wafer substrate according to an embodiment of the present invention; Figure 7 This is a schematic diagram of the structure of a composite metal film combined with a second sleeve according to an embodiment of the present invention; Figure 8 This is a schematic diagram of an embodiment of the present invention, which shows a structure combining a composite metal film and a second sleeve immersed in an organic solution. Figure 9 This is a schematic diagram of the structure of a composite metal thin film sleeve provided in an embodiment of the present invention; Figure 10 This is a schematic diagram of the sound pressure sensitivity measured by a sensor in the frequency range of 1k to 14.5k Hz, provided by an embodiment of the present invention. Figure 11 This is a power spectrum of a sensor provided in an embodiment of the present invention in a 2 kHz sound field with a sound pressure amplitude of 6.57 Pa; Figure 12 This is a power spectrum diagram of a sensor in a sound field with a sound pressure amplitude of 6.57 Pa (110.33 dB) provided in an embodiment of the present invention; Figure 13 This is a schematic diagram of the time-domain signal waveform of a 2kHz acoustic signal provided in an embodiment of the present invention; Figure 14 This is a sensor response spectrum diagram of a 2kHz acoustic signal provided in an embodiment of the present invention; Figure 15 This is a schematic diagram of phase noise power spectral density provided in an embodiment of the present invention; Figure 16 This is a schematic diagram of MDP values ​​in the frequency range of 1 kHz to 14.5 kHz provided by an embodiment of the present invention; Figure 17 This is a schematic diagram of a directional test provided in an embodiment of the present invention; Figure 18 This is a directivity test pattern at a frequency of 2 kHz provided in an embodiment of the present invention; Figure 19 This is a schematic diagram comparing the main performance of a photoacoustic sensor research result provided in an embodiment of the present invention.

[0019] In all the accompanying drawings, the same reference numerals denote the same structure, wherein: 1. Thermally expanded fiber; 2. Hollow-core fiber; 3. Composite metal film; 4. High-reflectivity dielectric film; 5. Micro-air gap; 60. First sleeve; 61. Second sleeve; 7. Thermally expanded fiber ferrule; 8. Hollow-core fiber ferrule; 9. Composite metal film sleeve. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0021] Unless the context otherwise requires, throughout the specification and claims, the term "comprising" is interpreted as openly inclusive, meaning "including, but not limited to." In the description of the specification, terms such as "one embodiment," "some embodiments," "exemplary embodiment," "example," "specific example," or "some examples" are intended to indicate that a particular feature, structure, material, or characteristic associated with that embodiment or example is included in at least one embodiment or example of this disclosure. The illustrative representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics mentioned may be included in any suitable manner in any one or more embodiments or examples; that is, although they may be incorporated into embodiments or examples using the above terms for reasons such as order and position, it does not limit them to be incorporated in combination by a single embodiment or example.

[0022] In the description of this invention, 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 indicated technical features. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of embodiments of this disclosure, unless otherwise stated, "a plurality of" means two or more. Furthermore, for example, the description may use the prefix "A" or "B" to describe the same type of nouns as two independent entities. In this case, the corresponding features defined with "A" and "B" are used only to distinguish between similar entities and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features.

[0023] In describing some embodiments, the terms "coupled," "coupled," and "connected," and their derivative expressions, may be used. For example, the term "connected" may be used in describing some embodiments to indicate that two or more components have direct physical or electrical contact with each other. Similarly, the term "coupled" may be used in describing some embodiments to indicate that two or more components have direct physical or electrical contact. However, the terms "connected" or "coupled" may also refer to two or more components that do not have direct contact with each other but still cooperate or interact with each other, such as "optical coupling," "wireless connection," etc. The embodiments disclosed herein are not necessarily limited to the scope of this invention.

[0024] Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0025] Example 1: To address the technical bottleneck of existing acoustic sensors in simultaneously achieving high sensitivity, wideband response, and miniaturization, this embodiment proposes a photoacoustic sensor. In one implementation, such as... Figure 1 As shown, the photoacoustic sensor includes a thermally expanded fiber 1, a hollow fiber 2, and a composite metal film 3 coupled in sequence; a high-reflectivity dielectric film 4 is disposed between the thermally expanded fiber 1 and the hollow fiber 2; the air core of the hollow fiber 2 serves as a Fabry-Perot cavity, the high-reflectivity dielectric film 4 serves as the first reflecting surface of the Fabry-Perot cavity, and the composite metal film 3 serves as the second reflecting surface of the Fabry-Perot cavity.

[0026] In this case, the air core of the hollow fiber 2 directly serves as a Fabry-Perot resonant cavity, with air as the light guiding medium. Compared to air itself serving as a parallel planar cavity, the hollow fiber effectively suppresses the diffraction and divergence of light during propagation by confining the light field laterally, achieving low transmission loss and thus enhancing photoacoustic interaction while balancing broadband response and stability.

[0027] The thermally expanded fiber 1 is used to achieve mode field matching and low-loss coupling, the hollow fiber 2 is used as the main body for optical transmission and resonance, and the composite metal thin film 3 serves as the acoustic sensitive unit and optical reflection end. The three are connected along the same optical axis, and the overall structure is compact, meeting the requirements of miniaturization and integration.

[0028] The high-reflectivity dielectric film 4 is used to improve the quality factor and interference signal intensity of the resonant cavity, providing an optical basis for high-sensitivity detection. In one embodiment, the high-reflectivity dielectric film 4 is disposed between the thermally expanded fiber 1 and the hollow fiber 2. It can be a separately prepared film sandwiched between the two, or it can be directly deposited on the end face of the thermally expanded fiber 1 to form a high-reflectivity film layer integrated with the end face of the thermally expanded fiber 1. In this embodiment, a coating process is used to directly deposit the film layer on the end face of the thermally expanded fiber 1, so that the film layer and the fiber end face are tightly bonded, without gaps or misalignments, effectively improving structural stability and reflection reliability, while simplifying assembly, reducing coupling loss, and better adapting to the requirements of sensor miniaturization and high sensitivity.

[0029] In one embodiment, the high-reflectivity dielectric film 4 is formed by alternately stacking TiO2 and SiO2 at the end faces coupled to the hollow fiber 2 on the thermally expanded fiber 1. By precisely controlling the thickness and number of each film layer, the film layer achieves a precisely adjustable reflectivity between 80% and 99.99% in the operating wavelength range. It also exhibits excellent mechanical stability, low absorption and scattering losses, and environmental tolerance, making it suitable for mass production and low-cost mass production. Combined with the composite metal thin film 3, it can effectively reduce intracavity optical energy loss, improve the quality factor and interference signal contrast of the Fabry-Perot cavity, and provide a stable and high-intensity optical resonance basis for high-sensitivity photoacoustic detection.

[0030] The photoacoustic sensor is encapsulated within a custom-designed ferrule and fixed in the acoustic field environment to be measured. External sound waves directly act on the surface of the composite metal film 3, driving it to generate mechanical vibrations proportional to the sound pressure. In one embodiment, the working principle of the photoacoustic sensor is as follows: the sound waves cause the composite metal film 3 to vibrate, and photoacoustic sensing is achieved by changing the cavity length and modulating the reflected light interference signal. Specifically, when external sound waves are incident on and act on the surface of the composite metal film 3, the sound pressure drives the film to generate micro-amplitude mechanical vibrations proportional to the sound wave signal. This micro-amplitude mechanical vibration directly changes the optical cavity length of the Fabry-Perot cavity, causing a shift in the wavelength that satisfies the resonance condition within the cavity. This minute change in cavity length continuously modulates the phase and intensity distribution of the multi-beam interference within the cavity, allowing the interference spectrum of the reflected light to carry the amplitude and frequency characteristics of the sound waves. By performing high-precision phase demodulation and spectral analysis on the interference signal of reflected light, key information such as the amplitude and frequency of the sound wave can be extracted, thereby converting the acoustic signal into a detectable optical signal and realizing highly sensitive photoacoustic sensing.

[0031] This embodiment utilizes the inherent limitations of the phase sensitivity of a single FP cavity. Its performance is directly constrained by three core parameters: cavity length, reflectivity of the reflective film, and cavity mass factor. These three parameters are mutually coupled and influence each other, making it difficult to achieve optimal performance simultaneously.

[0032] Among them, improving the reflectivity of the high-reflectivity dielectric film 4 is an important approach to optimizing phase sensitivity. However, simply pursuing high reflectivity will bring two major problems: First, the process difficulty increases significantly. The coating process of the high-reflectivity dielectric film 4 has extremely high requirements for film thickness uniformity, interface flatness, and film adhesion. It is easy to have process defects such as film peeling, cracking, and reflectivity fluctuation, resulting in poor sensor consistency and reduced yield. Second, excessively high reflectivity will lead to a narrow peak value and increased spectral overlap in the FP cavity reflection spectrum, which in turn reduces the dynamic range in the signal demodulation process. It is easy to have problems such as demodulation saturation and signal distortion, which will restrict the actual detection performance of the sensor and fail to meet the requirements of high-precision and high-dynamic-range acoustic signal detection.

[0033] To overcome the inherent bottleneck of phase sensitivity in a single FP cavity, and to avoid the technological and demodulation drawbacks of simply increasing reflectivity, in one embodiment, such as Figure 2a As shown, the Fabry-Perot cavity includes a reference cavity (FP1) and a sensing cavity (FP2), which have different free spectral ranges to amplify the spectral changes caused by the sound waves through the Vernier effect.

[0034] In one embodiment, a partially reflective film (DBR1, reflectivity R1≈50%–70%) is first deposited on the end face of the thermally expanded fiber 1 as the first distributed Bragg reflector. At a distance L1 from this partially reflective film (L1 ranges from tens to hundreds of μm, which can be flexibly adjusted according to the overall size of the sensor, the target frequency response, and sensitivity requirements), a semi-reflective film (DBR2, reflectivity R2≈80%–95%) is then deposited as the second distributed Bragg reflector. These two distributed Bragg reflectors cooperate with the original composite metal film 3 (reflectivity R3≈99.999%) of the sensor to form two cascaded FP cavity structures, namely the reference cavity FP1 and the sensing cavity FP2, thereby amplifying the phase sensitivity and optimizing the performance.

[0035] In one implementation, the structures of the first and second distributed Bragg reflectors can be the same or different. Figure 2a Taking the two distributed Bragg reflector structures shown as identical as an example, they can be obtained by alternately setting TiO2 and SiO2 from left to right. The reference cavity FP1 consists of a first partially reflective membrane (DBR1) and a second partially reflective membrane (DBR2), with a cavity length equal to the fixed distance L1 between the two reflective membranes. This cavity length remains constant after the sensor is fabricated and is unaffected by external sound pressure signals, primarily serving to provide a stable reference phase and spectral standard. The sensing cavity FP2 consists of the second partially reflective membrane (DBR2) and a composite metal film 3. Its cavity length L2 is formed by the end-face spacing of the hollow fiber 2 and the subsequent micro-air gap 5. The core is modulated by external sound pressure signals. When external sound pressure acts on the composite metal film 3, the composite metal film 3 undergoes elastic vibration, thereby changing the thickness of the micro-air gap 5, resulting in a slight change in the cavity length L2 of the sensing cavity FP2, thus realizing the conversion of the sound pressure signal into an optical phase signal.

[0036] The two-stage distributed Bragg reflector and the composite metal thin film 3 form two tandem FP cavities (FP1 and FP2). Their core advantage lies in the ability to amplify phase sensitivity based on the Vernier effect, theoretically breaking through the performance limitations of a single FP cavity. The specific mechanism is described below: First, the two cascaded FP cavities each have an independent Free Spectral Range (FSR), denoted as FSR1 for the reference cavity FP1 and FSR2 for the sensing cavity FP2. In the structural design, the reference cavity length L1 and the initial sensing cavity length L2 can be precisely adjusted to ensure that the FSR1 and FSR2 of the two cavities are "close but not equal," i.e., FSR1≈FSR2, but with a small difference (ΔFSR=|FSR1-FSR2|). The magnitude of this difference directly determines the subsequent phase amplification factor; the smaller the difference, the higher the amplification factor.

[0037] The core physical meaning of the free spectral range (FSR) is the frequency interval between two adjacent reflection peaks of an FP cavity, which is inversely proportional to the cavity length L. The formula can be expressed as FSR≈c / (2nL) (where c is the speed of light in vacuum, n is the refractive index of the medium inside the cavity, and both the reference cavity and the sensing cavity are air media, so n≈1). Since the cavity length L1 of the reference cavity FP1 is fixed, its FSR1 remains constant; while the cavity length L2 of the sensing cavity FP2 undergoes a slight change (ΔL2) due to sound pressure modulation, causing its FSR2 to change synchronously with the sound pressure signal. When the reflection spectra of two tandem FP cavities are superimposed, a periodic envelope structure is formed. The spacing of this envelope (i.e., the frequency interval between adjacent peaks) is defined as the Vernier free spectral range (FSR_vernier), and its calculation formula is: FSR_vernier=FSR1·FSR2 / |FSR1-FSR2|; Since FSR1 and FSR2 are close, |FSR1-FSR2| is much smaller than FSR1 and FSR2. Therefore, FSR_vernier will be much larger than FSR1 and FSR2 of a single FP cavity, thus amplifying the envelope spacing.

[0038] When external sound pressure is applied to the composite metal film 3, it drives a slight change ΔL2 in the cavity length of the sensing cavity FP2. This change in cavity length causes a shift in the peak value of the sensing cavity's reflection spectrum, which in turn causes a shift (ΔL_envelope) in the envelope peak of the superimposed reflection spectrum of the two cavities. According to the amplification principle of the Vinnier effect, the ratio of the envelope peak shift to the change in the sensing cavity length satisfies: ΔL_envelope / ΔL2≈FSR1 / |FSR1-FSR2|. Since the ratio of FSR1 to |FSR1-FSR2| can reach 10–100 times (denoted as the amplification factor M), the shift of the envelope peak will be amplified by a factor of M, thereby indirectly amplifying the phase signal.

[0039] This phase amplification effect can be directly converted into an improvement in the phase sensitivity of the sensor. Without this series structure, the phase sensitivity of a single FP cavity is usually on the order of -120 dB re 1 rad / μPa. With the series FP cavity structure of two-stage distributed Bragg reflectors, the phase sensitivity of the sensor can be improved to the order of -100 dB re 1 rad / μPa by means of a 10-fold phase amplification, which significantly breaks through the inherent bottleneck of a single FP cavity being limited by cavity length, reflectivity and quality factor.

[0040] Meanwhile, this structural design effectively avoids the drawbacks of simply increasing reflectivity. In the two-stage distributed Bragg reflector, the reflectivity R1≈50%–70% of the first-stage partial reflective film and the reflectivity R2≈80%–95% of the second-stage semi-reflective film do not require extremely high reflectivity design, reducing the difficulty of the coating process and improving the consistency and yield of film preparation. Furthermore, a reasonable reflectivity ratio avoids the problem of excessively narrow reflection spectrum peaks, ensuring the dynamic range during demodulation, effectively reducing demodulation saturation and signal distortion, achieving a synergistic balance between high sensitivity and high dynamic range, and further improving the actual detection performance and reliability of the sensor. For ease of description of other implementation methods, the following embodiments all use a single FP cavity as an example.

[0041] In order to increase the effective acoustic sensing area and improve sound pressure sensitivity without increasing transmission loss or device size, in one embodiment, reference is made to... Figure 1 A micro-air gap 5 is provided between the hollow optical fiber 2 and the composite metal thin film 3, and the micro-air gap 5 is coaxially arranged with the core of the hollow optical fiber 2. The coaxially arranged micro-air gap 5 extends the effective acoustic sensing area from the air core region of the anti-resonant hollow optical fiber 2 to the entire composite metal transducer film region. The purpose of providing this micro-air gap 5 is to increase the acoustic wave sensitive area without increasing transmission loss, thereby improving detection sensitivity. In one embodiment, the width of the micro-air gap 5 ranges from 2μm to 5μm.

[0042] The coaxially arranged micro-air gap 5 also prevents direct contact between the composite metal film 3 and the end face of the hollow fiber 2, ensuring that the film can vibrate freely under sound pressure, while not disrupting the internal optical field transmission state of the hollow fiber 2 and introducing additional transmission loss. The effective acoustic sensing area is significantly expanded from a small area limited to the air core of the hollow fiber 2 to the entire effective sensitive area of ​​the composite metal film 3. Without increasing the overall size of the sensor or sacrificing the broadband response, the acoustic signal acquisition capability and detection sensitivity are significantly improved, structurally solving the technical problem of balancing miniaturization and high sensitivity.

[0043] To achieve high-precision coaxial positioning of each fiber optic component, ensure efficient mode field coupling, and improve the overall structural rigidity and packaging consistency of the photoacoustic sensor, thus meeting the requirements of miniaturization and high stability in engineering applications, in one embodiment, such as Figure 2 As shown, the photoacoustic sensor further includes a first sleeve 60 and a second sleeve 61. A portion of the thermally expanded fiber 1 and a portion of the hollow fiber 2 are respectively disposed in relative positions within the first sleeve 60, and another portion of the hollow fiber 2 is disposed within the second sleeve 61. The composite metal film 3 is disposed at one end of the second sleeve 61. The first sleeve 60 and the second sleeve 61 are used to coaxially align the core of the thermally expanded fiber 1 with the core of the hollow fiber 2 and encapsulate them into an integral structure.

[0044] Among them, reference Figure 2 The ferrule 6 is a split design, with a first sleeve 60 corresponding to the thermally expanded fiber ferrule 7 and a partial hollow fiber ferrule 8, and a second sleeve 61 fixed to the composite metal film 3. The manufacturing methods of each ferrule, the preparation method of the composite metal film 3, and the method of transferring it to the ferrule 6 will be described below.

[0045] Based on the above description, a micro-gap 5 is formed between the end face of the hollow optical fiber 2 and the composite metal film. The structural design of this micro-gap can effectively widen the effective acoustic sensing area of ​​the sensor and improve the acoustic signal coupling and reception efficiency and detection sensitivity. However, under actual working conditions, when the composite metal film is excited by the external acoustic signal to be measured and undergoes elastic vibration, the film will repeatedly squeeze the sealed air inside the micro-gap 5, causing the air inside the micro-gap to undergo periodic compression and expansion, thus forming a significant compression film damping effect. This compression film damping caused by the sealed micro-gap will have two major negative effects: on the one hand, it will significantly impede the high-frequency vibration of the composite metal film 3, greatly attenuating the amplitude of the film vibration corresponding to the high-frequency acoustic signal, resulting in the degradation of the sensor's high-frequency response characteristics and the limitation of the high-frequency detection bandwidth; on the other hand, the hysteresis effect of air damping will introduce additional phase shift, destroying the phase consistency in the transmission process of acoustic signals of different frequencies, resulting in acoustic signal demodulation distortion and decreased detection accuracy, which seriously restricts the application of Fabry-Perot cavity photoacoustic sensors in high-precision, wide-frequency domain acoustic detection scenarios.

[0046] To fundamentally eliminate the drawbacks of compression membrane damping caused by the sealed micro-air gap and optimize the sensor's frequency response and phase characteristics, this solution optimizes the ventilation structure of the micro-air gap 5. In one embodiment, such as... Figure 3aThe structure is presented as a single cavity. The micro-air gap 5 is connected to the external atmospheric environment of the sensor through at least one pressure relief microchannel 50. The pressure relief microchannel 50 can be arranged radially through the second sleeve 61 or extended axially along the second sleeve 61. Both arrangements can realize the free flow of gas between the internal cavity of the micro-air gap 5 and the external environment. Figure 3a The following explanation uses the pressure relief microchannel 50, which is radially arranged along the second sleeve 61, as an example. Figure 3b by Figure 2a The dual-cavity structure shown is illustrated, and the pressure relief microchannel 50 is also arranged radially through the second sleeve 61.

[0047] When the composite metal film vibrates and compresses the air inside the micro-gap 5, the compressed air inside the cavity can be promptly discharged to the external environment through the pressure relief microchannel 50. When the film rebounds and expands, external air can be replenished into the micro-gap through the pressure relief microchannel 50, preventing the micro-gap 5 from forming a closed air cavity and generating strong compression film damping. By reasonably setting the number, aperture size, and arrangement of the pressure relief microchannels 50, the gas flow rate inside the micro-gap 5 can be precisely controlled, and the equivalent gas damping of the micro-gap 5 can be adjusted as needed. This retains the advantage of the large effective acoustic sensitive area brought by the micro-gap 5 structure, while significantly reducing the suppression effect of air damping on high-frequency response, ensuring the phase consistency of acoustic signal detection across the entire frequency range, and effectively improving the broadband detection performance and measurement stability of the photoacoustic sensor.

[0048] To enable the composite metal thin film 3 to simultaneously possess high reflectivity, low residual stress, good adhesion, and chemical stability, and to address the problem that a single metal film cannot simultaneously achieve both optical performance and mechanical stability, thereby improving sensor sensitivity and long-term reliability, in one embodiment, referring to... Figure 2The composite metal film 3 comprises at least a first metal layer 30, a second metal layer 31, and a third metal layer 32 grown sequentially. In one embodiment, for example, the first metal layer 30 is a Cr layer, possessing excellent corrosion resistance and adhesion, used to improve the overall adhesion and corrosion resistance of the film; the second metal layer 31 is an Ag layer, serving as an intermediate host layer, used to provide ultra-high reflectivity and regulate residual thermal stress in the film, ensuring uniform and stable film vibration; the third metal layer 32 is a gold (Au) layer, serving as a surface protective layer, used to isolate external environmental corrosion, further improving interface reflectivity and chemical stability. The synergistic effect of the three-layer structure enables the composite metal film 3 to maintain excellent acousto-optic transducer performance and optical reflection performance while achieving miniaturization and high frequency response. This structural design of the composite metal film 3 solves the problem of balancing high reflectivity and mechanical stability in a single metal film, enabling the composite metal film 3 to achieve a reflectivity exceeding 99.99%. On the other hand, as the second reflective surface of the Fabry-Perot cavity, the composite metal film 3 can reduce energy loss from light signal reflection, thereby improving the quality factor of the resonant cavity and the interference contrast of the probe light.

[0049] In this embodiment, the composite metal film 3 is preferably prepared by sequentially stacking three materials: Cr, Ag, and Au, to balance the film's adhesion performance, reflectivity, and environmental stability. It is understood that the above-mentioned metal materials are only preferred examples. In other embodiments, other metal or alloy materials with high reflectivity, low stress, good adhesion, and chemical stability can be selected to replace them, depending on the actual sensing requirements, process conditions, and application scenarios, as long as they can achieve the acousto-optic transduction and high reflectivity functions. This embodiment does not impose any particular limitations on this.

[0050] If the sensor is constructed using a uniform, one-piece composite metal film 3 with uniform thickness, material, and number of layers, although the structure is simple and easy to assemble, it has significant performance shortcomings and inherent defects in practical applications: the uniform integrated composite metal film itself has a single modal characteristic, which easily leads to excessively sharp resonance peaks, with sensitivity spikes only appearing at specific resonance frequencies. After deviating from the resonance point, the response decays rapidly, resulting in a narrow usable operating frequency band for the sensor; at the same time, the residual stress distribution during the preparation, coating, bonding, and curing of the uniform film is extremely difficult to control evenly, easily leading to phenomena such as film warping, micro-deformation, and uneven tension, which can easily cause performance drift and deterioration in stability after long-term use; more importantly, high sensitivity and broadband response are mutually restrictive and difficult to achieve simultaneously under the uniform composite metal film structure: if high sensitivity is pursued, the film needs to be made thin and flexible, which will further aggravate resonance peaks and compress the operating bandwidth; if the response bandwidth is broadened, the film stiffness needs to be increased, which will directly sacrifice the acoustic vibration amplitude and detection sensitivity, making it impossible to simultaneously meet the requirements of high precision, high sensitivity, and wide frequency range acoustic signal detection.

[0051] To overcome the technical drawbacks of homogeneous, uniform composite metal thin films 3, such as poor resonance characteristics, uncontrollable residual stress, and difficulty in balancing sensitivity and bandwidth, one embodiment employs a zoned, differentiated structural optimization design for the composite metal thin film 3. The composite metal thin film 3 is divided into two main functional regions: a central vibration region and an outer annular support region. Performance decoupling optimization is achieved through differentiated configuration of structural and physical properties in the two regions. The central vibration region and the annular support region can be differentiated by using different film thicknesses, different numbers of layers, different combinations of metal and dielectric materials, or by pre-setting different residual stress gradients. This allows the central vibration region to emphasize flexible and sensitive vibration, while the annular support region emphasizes rigid fixation and stress bearing, ensuring each region performs its specific function and has clearly defined functional zones.

[0052] In addition, the composite metal film 3 can be constructed into a radial gradient structure with a gradually changing thickness from the center to the edge, and a thinner center and thicker edge. The film layer in the central region is thinner and has lower stiffness, making it easier to generate large-amplitude elastic vibrations when excited by weak acoustic signals, thus ensuring the basic detection sensitivity of the sensor. The film layer in the annular region at the edge is thicker and has higher structural stiffness, which can effectively bear the assembly and fixing stress, constrain boundary deformation, balance the overall residual stress of the film, and suppress invalid resonance peaks.

[0053] Through the aforementioned partitioned differentiated or radial gradient structure design, on the one hand, the resonance peak can be smoothed out and the strong resonance effect of a single frequency can be suppressed, effectively broadening the flat response bandwidth of the sensor; on the other hand, the overall residual stress of the thin film can be precisely guided and balanced, avoiding stress concentration, warping deformation and long-term performance drift; at the same time, high sensitivity and broadband response are achieved in synergy, with the central area ensuring acoustic and vibration sensitivity, and the outer ring support area and gradient structure optimizing frequency response characteristics and structural stability. From the structural level, the inherent technical contradictions of traditional homogeneous composite metal thin films are completely solved, significantly improving the frequency response flatness, detection sensitivity and long-term working reliability of photoacoustic sensors.

[0054] In one embodiment, the acoustic pressure sensitivity and frequency response range of the photoacoustic sensor are adjusted by changing the effective radius, thickness, and cavity reflectivity of the composite metal film 3. Specifically, the effective radius of the composite metal film 3 ranges from 0.5 mm to 5 mm, the thickness ranges from 100 nm to 1000 nm, the cavity reflectivity of the first reflecting surface ranges from 80% to 99.99%, and the reflectivity of the second reflecting surface ranges from 80% to 99.999%. The acoustic pressure sensitivity of the photoacoustic sensor ranges from -150 dB re 1 rad / µPa to -100 dB re 1 rad / µPa, and the frequency response range of the photoacoustic sensor is 10 Hz to 20 kHz.

[0055] Among them, the composite metal thin film 3 serves as the core diaphragm for light absorption, thermal expansion, and acoustic excitation in the photoacoustic sensor. Its effective radius directly determines the mechanical stiffness, inherent resonant frequency, and effective area for light reception. Increasing the effective radius increases the effective light-receiving area of ​​the film under laser irradiation, resulting in more heat generated by photothermal conversion, increased thermal expansion deformation, higher amplitude of the excited sound wave, and a significant improvement in sound pressure sensitivity. Simultaneously, the overall stiffness of the diaphragm decreases, the inherent resonant frequency shifts to lower frequencies, and the low-frequency response range widens. Conversely, decreasing the effective radius increases the film stiffness and resonant frequency, enhancing the sensor's high-frequency response characteristics, but reduces the light-receiving area and weakens thermal expansion deformation, leading to a decrease in sound pressure sensitivity.

[0056] Thin film thickness directly controls the diaphragm's thermal conductivity, mechanical damping, bending stiffness, and thermal relaxation time. Increasing the thickness enhances the overall mechanical stiffness and bending resistance of the film, raising the structural resonant frequency, making it suitable for high-frequency applications. However, it also lengthens the heat conduction path, accelerates photothermal energy dissipation, suppresses local temperature rise and thermal expansion deformation, and reduces sound pressure sensitivity. Decreasing the thickness results in faster thermal relaxation, more sensitive instantaneous photothermal deformation response, improved acoustic excitation efficiency, and increased sound pressure sensitivity. Simultaneously, it reduces structural stiffness and lowers the resonant frequency, allowing coverage of a lower operating frequency range.

[0057] The reflectivity of the microcavity of the sensor determines the number of laser reflections within the cavity, the standing wave distribution of the optical field, and the light energy utilization rate. Increased cavity reflectivity results in multiple reflections and superpositions of the incident laser within the cavity, prolonging the interaction time between the light and the composite metal thin film 3, improving the film's light absorption efficiency and photothermal conversion efficiency, enhancing the intensity of thermoacoustic excitation, and significantly increasing acoustic pressure sensitivity. Simultaneously, the cavity resonant mode changes, allowing for the reconstruction of the sensor's resonant peak position and flexible adjustment of the frequency response's passband range and bandwidth flatness. Conversely, decreased cavity reflectivity leads to greater single-pass laser transmission loss, reduced light energy utilization within the cavity, lower photothermal conversion efficiency, and decreased acoustic pressure sensitivity. Furthermore, the cavity's confinement effect weakens, resulting in a narrower frequency response bandwidth and poorer resonant characteristics.

[0058] In one implementation, the practical application does not involve adjusting a single parameter, but rather optimizing through matching the effective radius, thin film thickness, and cavity reflectivity. For example: For high sensitivity and low-frequency wideband response, the effective radius of the thin film can be appropriately increased, the film thickness reduced, and the cavity reflectivity improved. For high-frequency detection and stable frequency response, the effective radius of the thin film can be reduced, the film thickness moderately increased, and the optimal cavity reflectivity matched. This ensures both bandwidth coverage and sound pressure sensitivity, enabling performance customization of the photoacoustic sensor for different scenarios.

[0059] In summary, this embodiment employs a Fabry-Perot cavity constructed by sequentially coupling a thermally expanded fiber 1, a hollow fiber 2, and a composite metal film 3, along with a high-reflectivity dielectric film 4. This effectively addresses the problems of poor mode field matching, high coupling loss, and the difficulty in simultaneously achieving high sensitivity, wideband response, and miniaturization in existing fiber optic photoacoustic sensors. The thermally expanded fiber 1 significantly improves the mode field matching with the hollow fiber 2, reducing coupling loss. Using the air core of the hollow fiber 2 as the resonant cavity, a high-Q resonant cavity structure is formed with the high-reflectivity film and the composite metal film 3, improving detection sensitivity under miniaturized packaging conditions. Acoustic waves drive the film vibration to directly modulate the cavity length, achieving wideband and stable photoacoustic signal conversion, enabling the sensor to simultaneously possess the advantages of miniaturization, high sensitivity, and wideband response.

[0060] Example 2: This embodiment proposes a method for fabricating a photoacoustic sensor, used to fabricate the photoacoustic sensor as described in Embodiment 1. In one embodiment, as shown... Figure 3 As shown, the preparation method includes: Step 101: Prepare the thermally expanded fiber ferrule 7 and form a high-reflectivity dielectric film 4 on the end face of the thermally expanded fiber 1.

[0061] Among them, reference Figure 1 and Figure 2 First, thermally expanded fiber 1 is prepared by using a standardized heat treatment process of pre-discharge, cyclic expansion and annealing cooling to modify a standard single-mode fiber, thereby obtaining the thermally expanded fiber 1.

[0062] Then, the thermally expanded fiber 1 is fixed inside the ferrule, and the end face of the ferrule is precisely ground and polished to obtain the thermally expanded fiber ferrule 7.

[0063] In one embodiment, the high-reflectivity dielectric film 4 formed on the end face of the thermally expanded fiber 1 can be achieved using an optical coating process, wherein a high-reflectivity dielectric film 4 consisting of alternating stacks of TiO2 and SiO2 is deposited on the end face of the thermally expanded fiber 1. In one embodiment, referring to… Figure 1 The high reflectivity dielectric film 4 can be a separately prepared film sandwiched between the two, or it can be directly deposited on the end face of the thermally expanded fiber 1 to form a high reflectivity film layer integrated with the end face of the thermally expanded fiber 1. In this embodiment, a coating process is used to directly deposit the film on the end face of the thermally expanded fiber 1, so that the film layer is tightly bonded to the end face of the fiber without gaps or misalignment, which effectively improves the structural stability and reflection reliability, while simplifying assembly, reducing coupling loss, and better adapting to the requirements of sensor miniaturization and high sensitivity.

[0064] Step 102: Prepare hollow fiber ferrule 8, and align and couple thermally expanded fiber ferrule 7 with hollow fiber ferrule 8.

[0065] Among them, reference Figure 1 and Figure 2 First, hollow optical fibers are fabricated. A precision fiber cleaver can be used to cut the end face of the negative curvature anti-resonant hollow optical fiber, ensuring that the end face is flat and free of chipping or bevel. Considering the critical length of anti-resonance, demodulation module, and miniaturization design requirements, the length of the negative curvature anti-resonant hollow optical fiber is controlled to obtain hollow optical fiber 2. Then, hollow optical fiber 2 is fixed inside a ceramic ferrule, and the end face is cleaned to remove surface contaminants, thus obtaining hollow optical fiber ferrule 8.

[0066] In one embodiment, a six-dimensional high-precision electric coupling system is constructed. A thermally expanded fiber ferrule 7 is inserted into a coaxial ceramic sleeve and fixed to the stationary end of a displacement stage; a hollow fiber ferrule 8 is positioned at the moving end. The two ferrules are coaxially coarsely calibrated using a CCD microscope, and the parallelism of the two end faces is monitored and optimized in real time using an optical power meter. Multi-point fixation is applied to the mating area inside the ceramic sleeve using ultra-low shrinkage optical adhesive, and the joint is cured in segments using an ultraviolet lamp to form an integrated rigid structure. After curing, the transmission power is tested to ensure no degradation in coupling performance, thereby aligning and coupling the thermally expanded fiber ferrule 7 and the hollow fiber ferrule 8, and fixing them inside the coaxial ceramic sleeve.

[0067] Step 103: Fix the aligned and coupled thermally expanded fiber ferrule 7 and hollow fiber ferrule 8 in their relative positions within the first sleeve 60.

[0068] Step 104: Prepare the composite metal thin film 3 and transfer the composite metal thin film 3 to the end face of the second sleeve 61 to obtain the composite metal thin film sleeve 9. Couple the composite metal thin film sleeve 9 with the hollow fiber ferrule 8 so that the high reflectivity dielectric film 4, the air core of the hollow fiber 2, and the composite metal thin film 3 together form a Fabry-Perot cavity, thereby obtaining the photoacoustic sensor.

[0069] In one embodiment, the preparation of the composite metal thin film 3 specifically includes: Step 1031: Perform standard semiconductor cleaning process on the silicon wafer surface to remove surface particles, organic contaminants and natural oxide layer, ensuring the surface cleanliness and nanoscale flatness of the silicon wafer.

[0070] Among them, such as Figure 4 As shown, a single-layer polished silicon wafer can be selected as the growth substrate for the deposition of composite metal thin films 3.

[0071] Step 1032: A positive photoresist is spin-coated onto the silicon wafer as a sacrificial layer using a high-precision spin coater. The uniformity of the photoresist layer thickness is ensured by adjusting the spin coat speed and time. After spin coating, the silicon wafer is placed on a heating stage for pre-curing treatment to allow the photoresist to form a stable sacrificial layer structure, thus obtaining the silicon wafer substrate.

[0072] Among them, such as Figure 5As shown, photoresist is uniformly spin-coated onto the silicon wafer.

[0073] Step 1033: The pre-treated silicon wafer substrate and at least three high-purity metal targets are loaded into the sample stage and corresponding target station of the electron beam evaporation coating system. The coating cavity is subjected to multi-stage vacuum treatment to meet the high vacuum environment requirements of atomic-level evaporation deposition.

[0074] In one embodiment, the composite metal film 3 includes at least a first metal layer 30, a second metal layer 31, and a third metal layer 32 grown sequentially. In one embodiment, as... Figure 6 As shown, for example, the first metal layer 30 is a Cr layer, which has excellent corrosion resistance and adhesion, and is used to improve the overall adhesion and corrosion resistance of the film; the second metal layer 31 is an Ag layer, which serves as an intermediate host layer, and is used to provide ultra-high reflectivity and regulate the residual thermal stress of the film, ensuring uniform and stable film vibration; the third metal layer 32 is a gold (Au) layer, which serves as a surface protective layer, and is used to isolate the film from external environmental corrosion, further improving the interface reflectivity and chemical stability. The synergistic effect of the three-layer structure enables the composite metal film 3 to still possess excellent acousto-optic transduction performance and optical reflection performance under the premise of miniaturization and high frequency response.

[0075] Step 1034: Use a high-energy focused electron beam to bombard the corresponding metal target in stages, and deposit at least three metal thin films sequentially on the surface of the silicon wafer substrate, and precisely control the film thickness.

[0076] Step 1035: After the deposition of at least three thin films is completed, in-situ low-temperature annealing is performed in the coating chamber to eliminate the internal stress generated during the film deposition process and control the overall residual stress of the composite film within the range of low tensile stress, thereby completing the preparation of the composite metal film 3.

[0077] In one embodiment, transferring the composite metal film 3 to the end face of the second sleeve 61 to obtain the composite metal film sleeve 9 specifically includes: like Figure 7 As shown, the second sleeve 61, after being precisely cut and flattened at the end face, is coaxially aligned with the metal film on the silicon wafer substrate, and epoxy resin is used to bond the end face of the second sleeve 61 to the metal film; the bonded assembly is placed on a heating table at a preset temperature and cured at a constant temperature for a first preset time to form a stable rigid connection between the second sleeve 61 and the metal film; wherein, the preset temperature range is 75°C to 85°C, and the first preset time range is 85 minutes to 95 minutes.

[0078] In one implementation, such as Figure 8As shown, the cured assembly is completely immersed in an organic solution and kept at a constant temperature for a second preset time. During the immersion process, the organic solution gradually penetrates to the photoresist sacrificial layer, completely dissolving the photoresist and separating the metal film area from the silicon wafer. The metal film then adheres to and covers the end face of the second sleeve 61. The second preset time ranges from 23 to 25 hours. Acetone can be used as the organic solution.

[0079] In one implementation, such as Figure 2 and Figure 9 As shown, the second sleeve 61, after the film transfer is completed, is removed from the organic solution. A precision peeling process is used to remove excess metal film from the end face of the second sleeve 61, resulting in a smooth, undamaged composite metal film sleeve 9. In one embodiment, after obtaining the composite metal film sleeve 9, it is fixed to the moving end of a high-precision electric displacement stage, and the anti-resonant hollow fiber ferrule 8 is fixed to the stationary end of the stage. The coaxiality and parallelism of the two end faces are calibrated using a high-precision CCD microscope and an optical microscope, and adjusted to the optimal position for high-precision interference spectral contrast. The displacement stage is then precisely adjusted axially, and the distance between the two end faces is monitored to form a coaxial micro-air gap 5 between the end face of the hollow fiber 2 and the composite metal film 3.

[0080] Finally, the sensor is encapsulated using ultra-low shrinkage UV adhesive. This mainly involves the following steps: First, using a small amount of adhesive applied multiple times, bare fibers are dipped in UV adhesive for initial fixation on the outside. Second, by controlling the UV lamp irradiation time and power, the sensor is pre-cured and cured in layers. Finally, a small amount of adhesive is applied evenly to form an additional fixing structure, improving the stability of the sensor during use.

[0081] For the specific structure of the photoacoustic sensor, please refer to Embodiment 1, which will not be repeated in this embodiment.

[0082] In summary, this embodiment employs a Fabry-Perot cavity constructed by sequentially coupling a thermally expanded fiber 1, a hollow fiber 2, and a composite metal film 3, along with a high-reflectivity dielectric film 4. This effectively addresses the problems of poor mode field matching, high coupling loss, and the difficulty in simultaneously achieving high sensitivity, wideband response, and miniaturization in existing fiber optic photoacoustic sensors. The thermally expanded fiber 1 significantly improves the mode field matching with the hollow fiber 2, reducing coupling loss. Using the air core of the hollow fiber 2 as the resonant cavity, a high-Q resonant cavity structure is formed with the high-reflectivity film and the composite metal film 3, improving detection sensitivity under miniaturized packaging conditions. Acoustic waves drive the film vibration to directly modulate the cavity length, achieving wideband and stable photoacoustic signal conversion, enabling the sensor to simultaneously possess the advantages of miniaturization, high sensitivity, and wideband response.

[0083] Example 3: This embodiment will use actual test results of multiple parameters to illustrate the testing effect of the photoacoustic sensor proposed in this invention.

[0084] (1) Sensitivity and frequency response range.

[0085] like Figure 10 The acoustic pressure sensitivity of the photoacoustic sensor was demonstrated in the frequency range of 1 kHz to 14.5 kHz, reaching -127 dB re 1 rad / μPa at 1 kHz and -123.02 dB re 1 rad / μPa at 12 kHz. Besides high sensitivity, frequency response is another key performance parameter of the photoacoustic sensor. The results show that the designed photoacoustic sensor achieves a flat response in the 1 kHz to 14.5 kHz range, with sensitivity fluctuations of ±2 dB. In the simulation, the thin-film resonant frequency is approximately 24.51 kHz. In the actual measurements, the sensitivity of the photoacoustic sensor shows an increasing trend above 14.5 kHz. The measured results are largely consistent with the simulated results. To ensure improved signal-to-noise ratio, phase demodulation was performed at the sixth-order peak, resulting in an increase in photoacoustic sensor sensitivity of approximately 15 dB, verifying that the cavity enhancement mechanism can effectively improve the sensitivity of the photoacoustic sensor.

[0086] (2) Linearity and dynamic response range.

[0087] An acoustic signal with a frequency of 2 kHz and an intensity range of 0 to 6.57 Pa was applied to the system. The sound pressure level output value and the phase response amplitude of the photoacoustic sensor were linearly fitted. Within the effective linear operating range, the photoacoustic sensor exhibited excellent linear response characteristics, with a linear regression determination coefficient as high as 0.999. Figure 11 As shown.

[0088] Figure 12 The power spectrum of the photoacoustic sensor in a sound field with a sound pressure level of 6.57 Pa (110.33 dB) is shown. The power spectrum reveals the presence of higher harmonics, with a total harmonic distortion (THD) of -31.12 dB; some of the harmonic energy originates from the sound source itself. Within a 1 Hz bandwidth, the minimum detectable sound pressure level at a frequency of 2 kHz is 158.8 μPa (18 dB). Therefore, the dynamic range of the photoacoustic sensor exceeds 92.33 dB, and the THD is below -30 dB. Figure 13 The time-domain waveform of a 2kHz acoustic signal is shown. Figure 14 The photoacoustic sensor response spectrum of a 2kHz acoustic signal is shown, demonstrating that the photoacoustic sensor has excellent frequency response characteristics and signal fidelity.

[0089] (3) Minimum detectable sound pressure and directionality.

[0090] To evaluate the noise spectral density, the photoacoustic sensor was placed in a fully anechoic chamber to minimize environmental noise interference. Without any applied signal, the average background noise level of the photoacoustic sensor's noise spectrum was measured to be -79.06 dB. Figure 15 As shown. To quantify the noise level at the target frequency, the root mean square value of the spectral amplitude is calculated within a bandwidth of ±1 / 6 octave band from the center of the target frequency. Let the total noise sampling time be T, then the frequency interval (or frequency resolution) of the horizontal axis of the spectrum after Fourier transform is... . Figure 16 The MDP values ​​at different frequencies are shown. The MDP values ​​at 1 kHz and 12 kHz are 251.78 μPa / Hz. 1 / 2 and 158.86 μPa / Hz 1 / 2 Therefore, the proposed acoustic-photoacoustic sensor has low noise characteristics and maintains good performance within its frequency response range.

[0091] Directivity refers to the acoustic response characteristics of a photoacoustic sensor, under fixed sound pressure level and fixed frequency sound wave excitation, in relation to the change in the incident direction (spatial angle) of the sound wave. When measuring the directivity characteristics of a photoacoustic sensor, a rotary stage drives a loudspeaker to rotate 360° along the horizontal axis, simultaneously measuring the sensitivity of the photoacoustic sensor in all incident directions. Figure 17 A schematic diagram of the directivity measurement of an acoustic sensing system is shown. For example... Figure 18 As shown, a 2kHz acoustic signal was used as the sound source, and the speaker rotated around the photoacoustic sensor in 30° increments, simultaneously recording the incident angle and corresponding sensitivity data. The sensitivity fluctuation range was controlled within ±1.5 dB, indicating that the acoustic-photoacoustic sensor has omnidirectional characteristics.

[0092] like Figure 19 The main performance characteristics of photoacoustic sensors based on hollow fiber 2 structures and thin-film transducer structures were compared, with a focus on key parameters such as structure, sensitivity, minimum detectable sound pressure level, and frequency response range. The comparison shows that this embodiment achieves the widest flat-range response bandwidth and a high signal-to-noise ratio of 76 dB, which is superior to other hollow fiber 2 structures. Its sensitivity is also superior to PET and gold films, demonstrating significant advantages in overall performance within the field of acoustic detection.

[0093] Based on the above test results, this embodiment establishes a coupling structure between anti-resonant hollow-core fiber and thermally expanded fiber. By analyzing the mode field matching conditions between AR-HCF (Anti-resonant Hollow-core Fiber) and thermally expanded fiber, the coupling loss is reduced to 0.45 dB. Optical field confinement is achieved using AR-HCF, resulting in a wavelength of 1.43 × 10⁻⁶ at 1550 nm. 4A high-Q optical resonator was developed. Through MEMS (Micro-Electro-Mechanical System) fabrication, integration, and transfer processes using a Cr-Ag-Au three-layer composite metal transducer film, the film achieved high reflectivity (>99.9%), low residual stress, and good mechanical strength, laying the technological foundation for a high-Q resonator. A micro-air gap of approximately 3 μm was introduced between the composite sensing film and the AR-HCF ferrule end face. This design expanded the effective acoustic pressure sensing area of ​​the film, further improving detection sensitivity.

[0094] A photoacoustic coupling enhancement effect was achieved through cavity enhancement mechanism, increasing the photoacoustic signal by 6 times compared to single-mode fiber-based sensors. A three-layer composite metal transducer film was designed to solve the problem of balancing high reflectivity and mechanical stability in a single metal film, thus improving the acoustic-optical conversion efficiency. When the Q value reaches 1.43 × 10⁻⁶... 4 The sensor achieves sensitivity parameters of -127 dB re 1 rad / μPa at 1 kHz and -123.02 dB re 1 rad / μPa at 12 kHz, respectively, enabling the detection of weak acoustic signals as low as 251.78 µPa / Hz1 / 2 and 158.86 µPa / Hz1 / 2, thus improving detection resolution. It also exhibits a wide bandwidth response from 1 kHz to 14.5 kHz with a linearity of 0.999. Its compact size of only 3 mm × 3 mm × 10 mm, coupled with a dynamic range of 92.33 dB, makes it a promising candidate for applications in the field of weak acoustic sensing.

[0095] For the specific structure of the photoacoustic sensor, please refer to Embodiment 1, which will not be repeated in this embodiment.

[0096] In summary, this embodiment employs a Fabry-Perot cavity constructed by sequentially coupling a thermally expanded fiber 1, a hollow fiber 2, and a composite metal film 3, along with a high-reflectivity dielectric film 4. This effectively addresses the problems of poor mode field matching, high coupling loss, and the difficulty in simultaneously achieving high sensitivity, wideband response, and miniaturization in existing fiber optic photoacoustic sensors. The thermally expanded fiber 1 significantly improves the mode field matching with the hollow fiber 2, reducing coupling loss. Using the air core of the hollow fiber 2 as the resonant cavity, a high-Q resonant cavity structure is formed with the high-reflectivity film and the composite metal film 3, improving detection sensitivity under miniaturized packaging conditions. Acoustic waves drive the film vibration to directly modulate the cavity length, achieving wideband and stable photoacoustic signal conversion, enabling the sensor to simultaneously possess the advantages of miniaturization, high sensitivity, and wideband response.

[0097] 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, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A photoacoustic sensor, characterized in that, It includes a thermally expanded fiber (1), a hollow fiber (2), and a composite metal film (3) coupled in sequence; a high-reflectivity dielectric film (4) is disposed between the thermally expanded fiber (1) and the hollow fiber (2); the air core of the hollow fiber (2) serves as a Fabry-Perot cavity, the high-reflectivity dielectric film (4) serves as the first reflecting surface of the Fabry-Perot cavity, and the composite metal film (3) serves as the second reflecting surface of the Fabry-Perot cavity; The sound wave causes the composite metal film (3) to vibrate, and photoacoustic sensing is achieved by changing the cavity length and modulating the reflected light interference signal.

2. The photoacoustic sensor according to claim 1, characterized in that, A micro-air gap (5) is provided between the hollow optical fiber (2) and the composite metal film (3), and the micro-air gap (5) is coaxially arranged with the core of the hollow optical fiber (2).

3. The photoacoustic sensor according to claim 1, characterized in that, It also includes a first sleeve (60) and a second sleeve (61), a portion of the thermally expanded fiber (1) and a portion of the hollow fiber (2) are respectively disposed in relative positions within the first sleeve (60), and another portion of the hollow fiber (2) is disposed within the second sleeve (61); The composite metal film (3) is disposed at one end of the second sleeve (61). The first sleeve (60) and the second sleeve (61) are used to align the core of the thermally expanded fiber (1) with the core of the hollow fiber (2) coaxially and encapsulate them into an integral structure.

4. The photoacoustic sensor according to claim 1, characterized in that, The composite metal film (3) includes at least a first metal layer (30), a second metal layer (31) and a third metal layer (32) grown sequentially.

5. The photoacoustic sensor according to claim 1, characterized in that, The high reflectivity dielectric film (4) is formed by alternating stacking of TiO2 and SiO2 on the end faces of the thermally expanded fiber (1) coupled with the hollow fiber (2).

6. The photoacoustic sensor according to claim 1, characterized in that, The acoustic pressure sensitivity and frequency response range of the photoacoustic sensor can be adjusted by changing the effective radius, thickness and cavity reflectivity of the composite metal film (3). The effective radius of the composite metal film (3) ranges from 0.5 mm to 5 mm, the thickness of the composite metal film (3) ranges from 100 nm to 1000 nm, the reflectivity of the first reflective surface in the cavity ranges from 80% to 99.99%, and the reflectivity of the second reflective surface ranges from 80% to 99.999%; the acoustic pressure sensitivity of the photoacoustic sensor ranges from -150 dB re 1 rad / µPa to -100 dB re 1 rad / µPa; and the frequency response range of the photoacoustic sensor ranges from 10 Hz to 20 kHz.

7. A method for fabricating a photoacoustic sensor, used to fabricate the photoacoustic sensor as described in any one of claims 1-6, characterized in that, The preparation method includes: Prepare a thermally expanded fiber ferrule (7) and form a high-reflectivity dielectric film (4) on the end face of the thermally expanded fiber (1); Prepare a hollow fiber ferrule (8), and align and couple the thermally expanded fiber ferrule (7) with the hollow fiber ferrule (8); The aligned and coupled thermally expanded fiber ferrule (7) and hollow fiber ferrule (8) are fixed in relative positions inside the first sleeve (60); A composite metal thin film (3) is prepared and transferred to the end face of the second sleeve (61) to obtain a composite metal thin film sleeve (9). The composite metal thin film sleeve (9) is coupled with the hollow fiber ferrule (8) so that the high reflectivity dielectric film (4), the air core of the hollow fiber (2), and the composite metal thin film (3) together constitute a Fabry-Perot cavity, thereby obtaining the photoacoustic sensor.

8. The method for fabricating a photoacoustic sensor according to claim 7, characterized in that, The preparation of the composite metal thin film (3) specifically includes: The silicon wafer surface is subjected to a standard semiconductor cleaning process to remove surface particles, organic contaminants and natural oxide layer, ensuring the surface cleanliness and nanoscale flatness of the silicon wafer. A high-precision spin coater is used to spin coat positive photoresist onto a silicon wafer as a sacrificial layer. The uniformity of the photoresist layer thickness is ensured by adjusting the spin coat speed and time. After spin coating, the silicon wafer is placed on a heating stage for pre-curing treatment to allow the photoresist to form a stable sacrificial layer structure, thus obtaining the silicon wafer substrate. The pre-treated silicon wafer substrate and at least three high-purity metal targets are loaded into the sample stage and corresponding target station of the electron beam evaporation coating system. The coating cavity is subjected to multi-stage vacuum treatment to meet the high vacuum environment requirements of atomic-level evaporation deposition. At least three metal films are sequentially deposited on the surface of a silicon wafer by bombarding the corresponding metal target in stages with a high-energy focused electron beam, and the film thickness is precisely controlled. After the deposition of at least three thin films is completed, the composite metal film (3) is annealed in situ in the coating cavity at low temperature to eliminate the internal stress generated during the film deposition process and control the overall residual stress of the composite film within the range of low tensile stress, thereby completing the preparation of the composite metal film (3).

9. The method for fabricating a photoacoustic sensor according to claim 8, characterized in that, The process of transferring the composite metal film (3) to the end face of the second sleeve (61) to obtain the composite metal film sleeve (9) specifically includes: The second sleeve (61), after being precisely cut and flattened at the end face, is coaxially aligned with the metal film on the silicon wafer substrate, and epoxy resin is used to bond the end face of the second sleeve (61) to the metal film. The bonded assembly is placed on a heating table at a preset temperature and cured at a constant temperature for a first preset time so that the second sleeve (61) and the metal film form a stable rigid connection. The solidified whole is completely immersed in the organic solution and soaked at a constant temperature for a second preset time. During the soaking process, the organic solution gradually penetrates into the photoresist sacrificial layer, completely dissolves the photoresist, separates the metal film area from the silicon wafer, and the metal film is attached to cover the end face of the second sleeve (61). The second sleeve (61) after the film transfer is completed is taken out of the organic solution, and the excess metal film outside the end face of the second sleeve (61) is removed by a precision peeling process to obtain a composite metal film sleeve (9) with a smooth surface and no damage.

10. The method for fabricating a photoacoustic sensor according to claim 7, characterized in that, The preparation of the thermally expanded fiber ferrule (7) and the formation of a high-reflectivity dielectric film (4) on the end face of the thermally expanded fiber (1) specifically includes: The standard single-mode fiber is modified by a standardized heat treatment process of pre-discharge, cyclic expansion and annealing cooling to prepare the thermally expanded fiber (1). The thermally expanded fiber (1) is fixed in the ferrule and the end face of the ferrule is precisely ground and polished to obtain the thermally expanded fiber ferrule (7). An optical coating process was used to deposit a high-reflectivity dielectric film (4) with alternating stacks of TiO2 and SiO2 on the end face of the thermally expanded fiber (1).

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