An optical fiber end face integrated helmholtz resonance enhanced ultrasonic sensor

Abstract

The application provides an optical fiber end face integrated Helmholtz resonance enhanced ultrasonic sensor, and belongs to the technical field of optical fiber ultrasonic sensing. The ultrasonic sensor comprises an optical fiber and a sensing structure integrated on the end face of the optical fiber. The sensing structure comprises a hollow spherical cavity and a pipeline in communication with the cavity. The outer wall of the hollow spherical cavity constitutes an optical whispering gallery mode resonant cavity. The hollow spherical cavity and the pipeline jointly constitute an acoustic Helmholtz resonant cavity. The ultrasonic sensor realizes double enhancement detection of ultrasonic signals through optical resonance and acoustic resonance. The application adopts the above-mentioned optical fiber end face integrated Helmholtz resonance enhanced ultrasonic sensor, and solves the problems of the existing ultrasonic sensor, such as being susceptible to electromagnetic interference, insufficient sensitivity, great preparation difficulty, low miniaturization degree, and the problem that the ultrasonic sensor cannot simultaneously meet the requirements of anti-interference, miniaturization and high-sensitivity weak ultrasonic detection.

Description

Technical Field

[0001] This invention relates to the field of fiber optic ultrasonic sensing technology, and in particular to an ultrasonic sensor with Helmholtz resonance enhancement integrated at the fiber end face. Background Technology

[0002] With the rapid development of fields such as biomedical imaging, industrial non-destructive testing, and environmental monitoring, ultrasonic testing technology has become an important technical means to obtain information about the internal structure of objects due to its advantages such as strong penetration, no ionizing radiation, and real-time dynamic detection. In the biomedical field, ultrasonic imaging is widely used in scenarios such as tumor screening and cardiovascular disease diagnosis, and its non-invasiveness and real-time nature significantly improve the efficiency of clinical diagnosis. In the industrial field, ultrasonic testing can accurately identify internal defects and structural damage in materials, meeting the needs of precision manufacturing and safety monitoring.

[0003] Currently, mainstream ultrasonic sensors are mainly divided into two categories: piezoelectric and fiber optic. Traditional piezoelectric ultrasonic sensors are technologically mature and widely used, but they have significant limitations in complex scenarios: on the one hand, it is difficult to balance sensor sensitivity and size, making it difficult to achieve a balance between miniaturization and high sensitivity; on the other hand, the piezoelectric sensing principle is susceptible to external electromagnetic interference, resulting in poor signal stability in strong electromagnetic environments, making it unsuitable for special detection scenarios such as magnetic resonance imaging.

[0004] To address the inherent limitations of piezoelectric sensors, fiber optic ultrasonic sensors based on optical principles have gradually become a research hotspot. Fiber optic sensors offer advantages such as resistance to electromagnetic interference, small size, high flexibility, and long transmission distance. Existing fiber optic ultrasonic sensing structures are mainly represented by fiber Bragg gratings (FBGs) and Fabry-Perot (FP) cavities. Although these optical sensors can overcome electromagnetic interference problems, significant technical bottlenecks still exist in practical applications.

[0005] For traditional fiber Bragg grating (FBG) ultrasonic sensors, the acoustic pressure response depends on the mechanical deformation of the grating period. However, optical fibers themselves have a high elastic modulus and small cross-sectional area, resulting in low deformation conversion efficiency for weak ultrasonic signals. This makes it difficult to effectively capture extremely weak ultrasonic signals such as those scattered by biological tissue, leading to insufficient overall sensitivity. For Fabry-Perot (FP) cavity sensors, their sensing performance is highly dependent on the parallelism and reflectivity matching of the two reflecting surfaces. This places stringent requirements on the precision of the fabrication process, making fabrication difficult, resulting in low yield rates, and making it difficult to consistently improve the quality factor and sensitivity, thus limiting their application in the field of weak signal detection.

[0006] Meanwhile, existing fiber optic ultrasound sensors mostly employ a single optical resonance mechanism, lacking the ability to pre-amplify ultrasound signals. When faced with low-energy, weak-amplitude ultrasound signals, their response amplitude is limited and their signal-to-noise ratio is low, failing to meet the high sensitivity requirements of fields such as biomedical imaging and precision non-destructive testing. Furthermore, traditional sensor structures are difficult to integrate at the micrometer level, resulting in poor applicability in scenarios with stringent size requirements, such as vascular imaging and detection in confined spaces, significantly limiting their application scope.

[0007] In summary, existing ultrasonic sensors cannot simultaneously meet the practical application requirements of anti-interference, miniaturization, and high sensitivity. Therefore, to address the aforementioned technical shortcomings, there is an urgent need to develop a fiber optic ultrasonic sensor with a compact structure, controllable fabrication, high integration, and significantly improved sensitivity, in order to overcome existing technological bottlenecks and meet the high-precision sensing needs of fields such as biomedicine and industrial inspection. Summary of the Invention

[0008] The purpose of this invention is to provide an ultrasonic sensor with Helmholtz resonance enhancement integrated at the fiber end face, which solves the problems of existing ultrasonic sensors being susceptible to electromagnetic interference, having insufficient sensitivity, being difficult to manufacture, having low miniaturization, and being unable to simultaneously meet the requirements of anti-interference, miniaturization, and high sensitivity for weak ultrasonic detection.

[0009] To achieve the above objectives, the present invention provides an ultrasonic sensor with Helmholtz resonance enhancement integrated at the end face of an optical fiber. The ultrasonic sensor includes an optical fiber and a sensing structure integrated at the end face of the optical fiber. The sensing structure includes a hollow spherical cavity and a pipe communicating with the cavity. The outer wall of the hollow spherical cavity constitutes an optical whispering-gallery mode resonant cavity, and the hollow spherical cavity and the pipe together constitute an acoustic Helmholtz resonant cavity. The ultrasonic sensor achieves dual enhancement detection of ultrasonic signals through the synergistic effect of optical resonance and acoustic resonance.

[0010] Preferably, the optical fiber is a seven-core optical fiber, which includes a central core and six peripheral cores evenly distributed around the central core.

[0011] Preferably, only two of the seven-core optical fibers are used to achieve optical signal input and output.

[0012] Preferably, the ultrasonic sensor further includes an optical transmission waveguide disposed between the fiber end face and the sensing structure, for coupling the light output from the fiber core to the outer wall of the hollow spherical cavity to excite the whispering-gallery mode resonance.

[0013] Preferably, the ultrasonic sensor also includes a support structure, which connects the fiber end face to the sensing structure, improves the bonding strength, and supports the sensing structure.

[0014] Preferably, the sensing structure, optical transmission waveguide, and support structure are all fabricated using femtosecond laser 3D nanolithography technology, and the materials used are all photosensitive materials.

[0015] Preferably, the acoustic Helmholtz resonant cavity can amplify the external sound pressure under the action of ultrasound, and the resonant frequency can be adjusted by changing the length of the pipe.

[0016] Preferably, the ultrasonic sensor is capable of stable detection of 1MHz sinusoidal ultrasonic signals and pulsed ultrasonic signals, and the peak frequency response is concentrated at 1MHz.

[0017] Therefore, the present invention employs the aforementioned ultrasonic sensor with Helmholtz resonance enhancement integrated at the fiber end face, and the technical effects are as follows: 1. Dual resonance synergistic enhancement significantly improves the sensitivity of weak ultrasound signal detection: By combining optical whispering galvanic mode (WGM) resonance with acoustic Helmholtz resonance, the sound pressure is pre-amplified through the Helmholtz resonant cavity. Combined with the high sensitivity modulation of wavelength changes by optical resonance, the response amplitude of weak ultrasound signals is greatly improved, which can accurately detect weak scattered ultrasound signals in biomedical and precision detection scenarios.

[0018] 2. All-fiber optical structure, fundamentally eliminating electromagnetic interference: The sensor adopts a pure optical transmission and detection mechanism, without conductive components and piezoelectric drive structure, and still works stably in strong electromagnetic environment, perfectly solving the problem that traditional piezoelectric ultrasonic sensors are susceptible to electromagnetic interference and cannot be used in electromagnetically compatible scenarios.

[0019] 3. Micron-level ultra-miniature integrated structure, suitable for minimally invasive detection in confined spaces: Based on femtosecond laser two-photon polymerization technology, it is fabricated in situ on the end face of a seven-core optical fiber. The overall size is on the micron level. The structure is compact and the volume is small. It can enter scenarios where traditional sensors cannot be placed, such as blood vessels, narrow tubes, and precision cavities, to meet the needs of minimally invasive, in situ, and in vivo ultrasound detection. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of an ultrasonic sensor structure with Helmholtz resonance enhancement integrated at the fiber end face according to the present invention. Figure 2 This is a schematic diagram of a seven-core fiber structure for an ultrasonic sensor with Helmholtz resonance enhancement integrated at the fiber end face, according to the present invention. Figure 3 This is a schematic diagram of the ultrasonic sensing structure of an ultrasonic sensor with Helmholtz resonance enhancement integrated at the fiber end face according to the present invention. Figure 4 This is a simulation diagram of the sound pressure intensity inside the Helmholtz cavity in an embodiment of the present invention; Figure 5 This is a schematic diagram of the experimental apparatus and experimental principle in an embodiment of the present invention; Figure 6 This is a time-domain sinusoidal signal response characteristic diagram of the ultrasonic sensor in an embodiment of the present invention; Figure 7 As described in the embodiments of the present invention Figure 6 The frequency domain signal response characteristics of the time-domain sinusoidal signal conversion; Figure 8 This is a characteristic graph of a 1MHz time-domain pulse signal received by a sensor, displayed on an oscilloscope in an embodiment of the present invention. Figure 9 As described in the embodiments of the present invention Figure 8 The frequency domain signal characteristic diagram of the time-domain pulse signal conversion.

[0021] Figure Labels 1. Seven-core optical fiber; 2. Ultrasonic sensing structure; 3. Optical transmission waveguide; 4. Support structure. Detailed Implementation

[0022] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0023] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0024] Example 1 This invention provides an ultrasonic sensor with Helmholtz resonance enhancement integrated at the fiber end face. By integrating an integrated optical resonant structure and an acoustic resonant structure at the fiber end face, it achieves dual enhanced detection of weak ultrasonic signals, solving the technical problems of insufficient sensitivity, large structural size, and high manufacturing difficulty of traditional fiber ultrasonic sensors.

[0025] like Figure 1 As shown, the ultrasonic sensor in this embodiment is integrated on the end face of a single optical fiber and mainly consists of four parts: a seven-core optical fiber 1, an ultrasonic sensing structure 2, an optical transmission waveguide 3, and a support structure 4. The ultrasonic sensing structure 2, the optical transmission waveguide 3, and the support structure 4 are all fabricated in situ on the end face of the optical fiber using femtosecond laser 3D nanolithography technology. The overall structure is compact, with dimensions on the micrometer scale, and features miniaturization, high precision, and high stability.

[0026] The seven-core optical fiber 1 serves as the optical transmission carrier and mounting substrate for the entire sensor, enabling the input, transmission, and output of optical signals. The ultrasonic sensing structure 2 is the core functional component of the sensor, integrating both optical whispering-gallery (WGM) mode resonance and acoustic Helmholtz resonance functions to achieve dual enhancement and conversion of the ultrasonic signal. The optical transmission waveguide 3 is used to achieve efficient optical coupling between the fiber core and the ultrasonic sensing structure 2, ensuring stable excitation of the optical resonant mode. The support structure 4 enhances the connection strength and mechanical stability between the ultrasonic sensing structure 2 and the fiber end face, preventing microstructure displacement, detachment, or shaking during use, thus ensuring detection accuracy and long-term reliability.

[0027] The ultrasonic sensor has no moving connections between its components and is an integrated solid-state microstructure, avoiding problems such as assembly errors, poor contact, and poor stability that exist in traditional assembled sensors. At the same time, it maintains the characteristics of a fully optical structure and does not contain any conductive, piezoelectric, or electromagnetic sensitive elements, fundamentally eliminating the influence of electromagnetic interference on the detection results. It can work stably in strong electromagnetic environments, medical magnetic resonance environments, and high-voltage electrical environments.

[0028] like Figure 2 As shown, this embodiment uses a seven-core optical fiber 1 as the optical transmission carrier. The seven-core optical fiber 1 consists of seven cores and an outer cladding. The core diameter is 9 μm, the core spacing is 42 μm, and the cladding diameter is 150 μm. One core is located at the center of the fiber cross-section, and the remaining six cores are distributed around the central core at a uniform 60° angle, forming a symmetrical seven-core structure. This structure offers advantages such as multi-path optical transmission, compact spatial layout, and high coupling efficiency.

[0029] In this sensor, it is not necessary to use all seven fiber cores; only two are selected as the optical input and output channels, respectively. One fiber core is used to couple an external laser into the sensor, and the other fiber core is used to couple the optical signal carrying ultrasonic sensing information to an external demodulation system. The remaining fiber cores are idle and do not participate in the optical transmission and sensing process, which simplifies the structure, ensures a clean optical path, and reduces optical loss and signal crosstalk.

[0030] The end face of the seven-core optical fiber 1 undergoes precision grinding and cleaning, resulting in a smooth, scratch-free, and impurity-free surface. This provides a good processing plane for subsequent femtosecond laser 3D nanolithography, ensuring the precision and adhesion of the microstructure fabrication. The end face of the optical fiber is tightly bonded to the ultrasonic sensing structure 2, the optical transmission waveguide 3, and the support structure 4, with no gaps or misalignments, ensuring a continuous and stable optical transmission path.

[0031] like Figure 3As shown, the ultrasonic sensing structure 2 is an integrated microstructure, comprising a hollow spherical cavity and a pipe connected to the hollow spherical cavity. The hollow spherical cavity and the pipe are coaxially arranged, with one end of the pipe connected to the interior of the hollow spherical cavity and the other end facing the external space to receive external ultrasonic signals.

[0032] In this embodiment, the hollow spherical cavity has an outer diameter of 32 μm and an inner diameter of 27 μm. The cavity wall is a uniform thin-walled structure, possessing good elasticity and light transmission characteristics. It can satisfy the optical whispering-gallery mode resonance condition and generate controllable periodic deformation under acoustic resonance. The pipe is 18.5 μm long, with an outer diameter of 15 μm and an inner diameter of 10 μm. The inner wall of the pipe is smooth and the cross-section is uniform, ensuring that the internal air column can stably reciprocate under ultrasonic action.

[0033] The ultrasonic sensing structure 2 is made of a photosensitive material with a refractive index of 1.54. This material has good optical transmittance, mechanical strength and laser processing compatibility. It can achieve micron-level high-precision molding through femtosecond laser two-photon polymerization. At the same time, it is not easy to age, deform or fade during long-term use, ensuring the stability of the sensor's optical and acoustic performance.

[0034] The core innovation of the ultrasonic sensing structure 2 lies in its dual function: the outer wall layer of the hollow spherical cavity constitutes an optical whispering-gallery mode (WGM) resonant cavity, used for light confinement, cyclic transmission, and resonant modulation; the interior of the hollow spherical cavity, together with the conduit, forms an acoustic Helmholtz resonant cavity, used for sound pressure amplification of the incident ultrasonic signal and cavity deformation driving. The two resonant structures share the same spherical cavity carrier, achieving high spatial integration and synergistic enhancement, which is key to achieving high-sensitivity ultrasonic detection.

[0035] The optical transmission waveguide 3 is a slender strip-shaped microstructure positioned between the core of the seven-core optical fiber 1 and the ultrasonic sensing structure 2. One end is connected to the end face of the core of the seven-core optical fiber 1, and the other end extends to the outer wall of the hollow spherical cavity, achieving efficient optical coupling between the optical fiber and the spherical resonant cavity. The optical transmission waveguide 3 is made of the same photosensitive material as the ultrasonic sensing structure 2, with a consistent refractive index and low optical transmission loss, avoiding light reflection, scattering, and energy loss caused by abrupt changes in refractive index. The waveguide cross-sectional dimensions match the fiber core, ensuring high coupling efficiency and a small divergence angle when light enters the waveguide from the core, allowing most of the optical energy to be stably transmitted along the waveguide to the ultrasonic sensing structure 2.

[0036] The optical transmission waveguide 3 has two functions: First, it couples the laser output from the input core of the seven-core fiber 1 into the outer wall of the hollow spherical cavity, and excites a stable whispering-gallery mode resonance when the optical resonance condition is met; Second, it couples the light that does not participate in the resonance, as well as the optical signal after resonance modulation, back to the output core of the seven-core fiber 1 to complete the closed-loop transmission of the optical signal and provide the optical path basis for subsequent signal demodulation.

[0037] The support structure 4 is located between the end face of the seven-core optical fiber 1 and the ultrasonic sensing structure 2, at the bottom and side of the ultrasonic sensing structure 2, serving to support, reinforce, and stabilize it. The support structure 4 is also made of photosensitive material and is integrally connected with the optical fiber end face, the ultrasonic sensing structure 2, and the optical transmission waveguide 3, without splicing or adhesives, resulting in high overall mechanical strength.

[0038] The supporting structure 4 serves several purposes: it enhances the bonding strength between the ultrasonic sensing structure 2 and the fiber optic end face, preventing the microstructure from detaching under liquid, vibration, or minor external forces; it ensures the spatial stability of the ultrasonic sensing structure 2, preventing displacement, tilting, or deformation of the pipe or spherical cavity, and ensuring that the acoustic and optical resonance performance remains unchanged; it provides uniform support for the ultrasonic sensing structure 2, reducing stress concentration, improving the sensor's overall impact and vibration resistance, and extending its service life. Through the reinforcement provided by the supporting structure 4, this sensor is suitable for complex working conditions such as underwater, fluid environments, and mobile devices, ensuring long-term stable operation without failure, meeting the needs of practical engineering and medical testing.

[0039] The ultrasonic detection principle of this sensor is based on the dual synergistic enhancement effect of optical whispering-gallery mode (WGM) resonance and acoustic Helmholtz resonance. It amplifies weak ultrasonic signals through acoustic resonance and converts minute deformations into detectable optical signals through optical resonance, thereby achieving high-sensitivity and high signal-to-noise ratio ultrasonic sensing.

[0040] The laser light output from the input core of the seven-core optical fiber 1 is coupled and transmitted to the outer wall layer of the hollow spherical cavity via optical transmission waveguide 3. When the light meets a specific resonance condition, it will continuously circulate and propagate along the surface of the spherical outer wall, forming a stable optical whispering-gallery mode resonance. The resonance condition satisfies the following formula: ; in, It is a positive integer, representing the resonant order of the optical whispering gallery mode; The resonant wavelength; The effective refractive index of the spherical cavity wall material; It is the circumference of the outer wall of the hollow spherical cavity.

[0041] In the resonant state, light of a specific wavelength is confined within the outer wall of a sphere and circulates continuously, resulting in concentrated energy and extremely low loss, forming a resonant peak with narrow linewidth and high extinction ratio. The free spectral range (FSR) of the resonant wavelength satisfies the formula: ; in, The radius of the hollow spherical cavity is given.

[0042] When ultrasound waves act on the ultrasound sensing structure 2, they cause the effective refractive index of the spherical cavity wall material to change. The radius of the spherical cavity changes, thus altering the shape of the cavity. This generates periodic, minute deformations, which in turn lead to the resonant wavelength. It shifts synchronously with the free spectral range (FSR). High-precision optical detection of ultrasonic signals can be achieved by detecting changes in the resonant wavelength or output light intensity.

[0043] Optical whispering-gallery mode resonances possess extremely high sensitivity, capable of sensing minute deformations and refractive index changes at the nanometer or even sub-nanometer level, providing an optical basis for the detection of weak ultrasonic signals.

[0044] The hollow spherical cavity and the pipe in the ultrasonic sensing structure 2 constitute a typical Helmholtz resonant cavity structure. When external ultrasonic waves propagate to the opening of the pipe, the air column inside the pipe vibrates back and forth along the pipe axis like a piston under the action of ultrasonic pressure; the air inside the hollow spherical cavity acts as an elastic spring, and is periodically compressed and expanded with the movement of the air column.

[0045] When the frequency of the incident ultrasound matches the natural frequency of the Helmholtz resonant cavity, the structure undergoes acoustic resonance, the vibration amplitude of the air column inside the pipe increases sharply, and the compression and expansion effects of the air inside the cavity are significantly enhanced, resulting in a substantial amplification of the sound pressure inside the cavity.

[0046] This embodiment uses, as follows Figure 4 Simulation verification showed that when an incident ultrasonic pressure of 10 Pa was applied at the pipe opening, the average sound pressure inside the cavity could reach 300 Pa to 15000 Pa, and the sound pressure amplification factor reached 10. 2 -10 3 This allows for the pre-amplification of weak ultrasonic signals.

[0047] The amplified alternating sound pressure exerts periodic thrust and tension on the inner wall of the hollow spherical cavity, causing the elastic spherical cavity wall to undergo synchronous periodic expansion and contraction vibrations in accordance with the ultrasonic frequency, resulting in an increase in the cavity radius. The deformation undergoes periodic changes. This deformation directly affects the optical resonant cavity, causing a greater shift in the resonant wavelength, thereby significantly improving the overall output signal strength of the sensor.

[0048] Compared to traditional single-optical sensing structures such as FBG and FP, this sensor, under the same ultrasonic signal, outputs a higher signal amplitude, a better signal-to-noise ratio, and a lower detection limit. It can stably capture weak scattered ultrasonic signals from biological tissues and precision devices, breaking through the sensitivity bottleneck of existing technologies. Furthermore, by changing the pipe length L, the natural frequency of the Helmholtz resonator can be flexibly adjusted, enabling the sensor to selectively enhance its response to ultrasonic waves in specific frequency bands, adapting to the detection needs of different application scenarios.

[0049] In this embodiment, the ultrasonic sensing structure 2, the optical transmission waveguide 3, and the support structure 4 are all fabricated in situ on the end face of the seven-core optical fiber 1 using femtosecond laser 3D nanolithography technology. The specific process is as follows: Fiber pretreatment: The seven-core fiber 1 is cut and ground into a flat end face, and then ultrasonically cleaned with acetone, ethanol and deionized water in sequence. After drying, the end face is ensured to be clean and free of impurities. Material coating: A layer of liquid photosensitive polymer with a refractive index of 1.54 is uniformly coated on the end face of the optical fiber, with uniform thickness to meet the requirements of microstructure molding; Laser processing: A femtosecond laser two-photon polymerization system is used to perform high-precision scanning and solidification of photosensitive materials according to the preset structural dimensions and path, and to sequentially form support structure 4, optical transmission waveguide 3, and ultrasonic sensing structure 2. Development and shaping: The processed optical fiber is immersed in the developing solution to remove excess uncured photosensitive material while retaining the integrated microstructure; Cleaning and drying: Clean the residual developer with isopropanol and blow dry with nitrogen to complete the overall sensor fabrication.

[0050] To verify the actual detection performance of the sensor in this embodiment, a system was built as follows: Figure 5 The ultrasonic response testing system shown tests the sensor's sinusoidal signal response, pulse signal response, frequency selectivity, signal fidelity, and other indicators.

[0051] The testing system consists of an ultrasonic generation module and a signal demodulation module. The ultrasonic generation module comprises a signal generator, a power amplifier, and a piezoelectric ultrasonic transducer, used to generate stable single-frequency ultrasonic waves. The signal demodulation module comprises a narrow-linewidth laser, a piezoelectric sensor, a photodetector, an oscilloscope, and a spectrometer, used to achieve optical signal excitation, sensor modulation, signal acquisition, and waveform display. The test is conducted in a degassed water medium, where ultrasonic waves have low propagation loss and stable waveforms, accurately reflecting the sensor's response characteristics. The distance between the piezoelectric transducer and the sensor is 15mm to ensure stable ultrasonic wave incidence.

[0052] The signal generator outputs a 1MHz sinusoidal signal with a driving voltage of 20Vpp, which drives the piezoelectric transducer to generate a 1MHz continuous sinusoidal ultrasonic wave. The ultrasonic wave propagates through the water medium to the sensor, and the ultrasonic sensing structure 2 converts the ultrasonic signal into a change in optical signal under the action of double resonance.

[0053] The laser wavelength is locked at the position of maximum slope on the resonant curve. The resonant wavelength shift is directly converted into a change in output light intensity, which is then converted into an electrical signal by a photodetector. This signal is then acquired and displayed as a time-domain waveform by an oscilloscope. The test results are as follows: Figure 6 As shown, the sensor outputs a stable, regular, and distortion-free 1MHz sinusoidal time-domain signal with a smooth waveform and high signal-to-noise ratio, indicating that the sensor has good linear response and signal restoration capabilities for continuous sinusoidal ultrasound.

[0054] Performing a Fourier transform on the time-domain signal yields the frequency-domain curve, such as... Figure 7 As shown, the signal energy is concentrated at 1MHz, with a significant peak and no obvious spurious frequency components, proving that the sensor can accurately identify 1MHz ultrasonic signals, with good frequency selectivity and strong anti-interference ability.

[0055] The signal generator outputs a 1MHz pulse signal with a drive voltage of 20Vpp and a pulse period of 8μs, simulating the pulsed ultrasonic signal commonly used in actual imaging and detection. The time-domain response waveform acquired by the oscilloscope is shown below. Figure 8 As shown, the sensor can clearly capture the envelope and timing characteristics of the pulse signal, and output a complete waveform with a rapid response and no obvious distortion or delay.

[0056] Performing a Fourier transform on the pulse time-domain signal yields the frequency-domain curve as shown below. Figure 9 As shown, the signal peak is also concentrated at 1MHz, with good sidelobe suppression and a clean spectrum, indicating that the sensor also has high-fidelity detection capability for pulsed ultrasound, which can meet the practical application requirements of ultrasonic imaging, non-destructive testing and other applications.

[0057] Therefore, this invention employs the aforementioned fiber-optic end-face integrated Helmholtz resonance-enhanced ultrasonic sensor. Using a seven-core optical fiber as a carrier, a hollow spherical cavity and pipe composite structure are integrally fabricated on its end face via femtosecond laser 3D nanolithography. This allows the outer wall of the spherical cavity to form an optical whispering-gallery (WGM) resonant cavity, and the cavity and pipe to constitute an acoustic Helmholtz resonant cavity. The weak ultrasonic sound pressure is amplified by 10⁻¹⁰ using acoustic resonance. 2 -10 3 It can drive the cavity to deform, and then convert the deformation into a detectable light signal through optical resonance, realizing dual resonance synergistic enhancement. It has the advantages of full optical anti-electromagnetic interference, micron-level miniaturization, high sensitivity and good stability. It can accurately detect 1MHz sine and pulse ultrasound signals and is suitable for biomedical minimally invasive imaging, industrial non-destructive testing and in-situ monitoring in confined spaces.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. An ultrasonic sensor with Helmholtz resonance enhancement integrated at the fiber end face, characterized in that, The ultrasonic sensor includes an optical fiber and a sensing structure integrated on the end face of the optical fiber; the sensing structure includes a hollow spherical cavity and a pipe connected to the cavity, the outer wall of the hollow spherical cavity constitutes an optical whispering-gallery mode resonant cavity, and the hollow spherical cavity and the pipe together constitute an acoustic Helmholtz resonant cavity; the ultrasonic sensor achieves dual enhancement detection of ultrasonic signals through the synergy of optical resonance and acoustic resonance. The ultrasonic sensor also includes an optical transmission waveguide, which is disposed between the end face of the optical fiber and the sensing structure. It is used to couple the light output from the fiber core to the outer wall of the hollow spherical cavity to excite the whispering-gallery mode resonance. The ultrasonic sensor also includes a support structure, which connects the fiber end face to the sensing structure, improves the bonding strength, and supports the sensing structure. The sensing structure, optical transmission waveguide, and support structure are all fabricated using femtosecond laser 3D nanolithography technology, and the materials used are all photosensitive materials. An acoustic Helmholtz resonant cavity can amplify external sound pressure under the action of ultrasound, and the resonant frequency can be adjusted by changing the length of the tube.

2. The ultrasonic sensor with fiber endface integrated Helmholtz resonance enhancement according to claim 1, characterized in that, The optical fiber is a seven-core fiber, which consists of a central core and six peripheral cores evenly distributed around the central core.

3. The ultrasonic sensor with fiber endface integrated Helmholtz resonance enhancement according to claim 2, characterized in that, A seven-core optical fiber uses only two of its cores to achieve both optical signal input and output.

4. The ultrasonic sensor with fiber endface integrated Helmholtz resonance enhancement according to claim 1, characterized in that, The ultrasonic sensor can stably detect 1MHz sinusoidal ultrasonic signals and pulsed ultrasonic signals, and the peak frequency response is concentrated at 1MHz.

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