Ultrasonic sensor, preparation method of ultrasonic sensor, underwater ultrasonic detection system and application method of underwater ultrasonic detection system

Abstract

The invention provides an ultrasonic sensor and a preparation method thereof, an underwater ultrasonic detection system and an application method, a single-mode fiber-multimode fiber-grapefruit type photonic crystal fiber cascade structure is adopted, a PDMS elastomer is locally filled in a photonic crystal fiber, a built-in solid-gas Fabry-Perot interference cavity is formed, an external vibrating membrane is not needed, and the ultrasonic sensor is simple in structure and convenient to operate. Therefore, the structure is simplified, and the manufacturing cost is reduced. By optimizing welding parameters and a PDMS filling and curing process, stable and efficient preparation of the sensor is realized. The design enables the sensor to have high sensitivity and broadband ultrasonic response capability, and PDMS built-in packaging improves environmental durability and measurement stability. The all-fiber passive sensing mode has the advantages of electromagnetic interference resistance, miniaturization and narrow space measurement adaptation, the system is easy and convenient to build, and the system is suitable for ultrasonic detection under complex environments such as underwater and high-temperature environments.

Description

Ultrasonic sensors and their fabrication methods, underwater ultrasonic detection systems and their applications Technical Field

[0001] This invention relates to the field of sensor technology, and in particular to an ultrasonic sensor and its preparation method, an underwater ultrasonic detection system and its application method. Background Technology

[0002] Characterizing underwater geological structures and assessing the internal integrity of engineered materials presents fundamental challenges in fields such as geophysics, civil engineering, and petroleum engineering. While direct sampling (e.g., core drilling) provides precise local data, it is inherently destructive, costly, and has limited spatial coverage. Therefore, non-destructive testing (NDT) and evaluation methods are crucial for obtaining comprehensive internal information without altering the properties of the medium. Among these techniques, ultrasonic imaging has emerged as a powerful and versatile tool for probing the internal structure of underwater geological models and man-made materials. Ultrasonic imaging is based on the principles of pulse-echo or transmission acoustics. A transducer generates high-frequency mechanical acoustic waves (typically in the kHz to MHz range) that are coupled into the target medium. As these waves propagate, they encounter interfaces between materials with different acoustic impedances (e.g., rock layers, fractures, pores, or inclusions). At each boundary, some wave energy is reflected, refracted, or scattered. In pulse-echo mode, the same transducer detects the reflected waves (echoes), and by precisely measuring the flight time and amplitude of these echo signals, the location, size, orientation, and properties of internal features can be determined. This data is then processed to reconstruct a two-dimensional or three-dimensional visual representation of the internal structure.

[0003] Existing ultrasonic transducers are primarily based on the piezoelectric effect, such as those made of materials like lead zirconium titanate (PZT). Their operation relies on the direct and inverse piezoelectric effects of the material. The inverse piezoelectric effect causes the transducer to vibrate mechanically under an alternating electric field, generating ultrasonic waves. The direct piezoelectric effect causes the transducer to generate a corresponding charge signal upon receiving ultrasonic pressure waves, thus enhancing ultrasound reception. After decades of development, these transducers possess significant advantages, including mature technology, low cost, high sound pressure output, and good receiving sensitivity. This has made them dominant in fields such as medical ultrasound imaging, industrial ultrasonic cleaning, welding, and high-intensity focused ultrasound therapy. To further improve performance, piezoelectric composite transducers have been developed. By combining piezoelectric ceramics with a polymer matrix, they effectively improve the sensor's bandwidth and acoustic impedance matching characteristics, becoming a core technology for modern high-end medical ultrasound imaging probes. However, piezoelectric sensors have inherent technical limitations: susceptibility to electromagnetic interference, size and shape limitations, the need for acoustic impedance matching, and inherent safety hazards. Therefore, fiber optic ultrasonic sensors have emerged. These sensors detect ultrasonic signals by measuring changes in optical parameters (such as intensity, phase, and wavelength) caused by the interaction between ultrasound and fiber optic sensors. Their main technical methods include Fabry-Perot interferometers, fiber Bragg gratings, and Michelson interferometers. Compared to traditional ultrasonic transducers, fiber optic ultrasonic sensors exhibit a series of revolutionary advantages, directly addressing the shortcomings of traditional technologies: superior immunity to electromagnetic interference, miniaturization and flexibility, inherently good acoustic impedance matching and broadband response, long-distance and distributed sensing capabilities, and high sensitivity. In particular, interferometric fiber optic sensors are extremely sensitive to minute strains induced by ultrasound, with detection thresholds comparable to, or even exceeding, those of traditional piezoelectric hydrophones, especially excelling in the high-frequency range.

[0004] In the field of fiber optic Fabry-Perot (FPI) ultrasonic sensing technology, sensors rely on membrane structures sensitive to external vibrations (such as metal thin films, silicon-based films, polymer thin films, photonic crystal films, etc.) to improve sensitivity. However, these membrane structures are limited by their size and manufacturing processes; sensitivity is often positively correlated with membrane size. High-precision manufacturing requirements not only increase process complexity but also affect sensor stability and repeatability. Existing sensing strategies based on vibrating membranes are largely limited by the membrane's inherent properties. Detection sensitivity is typically proportional to membrane size, and manufacturing such membranes requires high-precision processing control, which in practice increases process complexity and affects sensor stability and repeatability. Furthermore, since the membrane is exposed, most FPI-based fiber optic ultrasonic sensors rely on mechanical encapsulation structures to protect the sensor head, further increasing manufacturing difficulty and cost. Therefore, a new solution specifically for underwater ultrasonic sensing is urgently needed. Summary of the Invention

[0005] In view of this, embodiments of the present invention provide an ultrasonic sensor and its preparation method, an underwater ultrasonic detection system and its application method, in order to eliminate or improve one or more defects existing in the prior art, and solve the problem that the existing fiber optic Fabry-Perot interferometer ultrasonic sensor relies on membrane structure to sense external vibration, resulting in complex manufacturing process and poor stability.

[0006] One aspect of the present invention provides a fiber optic Fabry-Perot interferometric ultrasonic sensor, the ultrasonic sensor comprising: a multimode optical fiber; a single-mode optical fiber fused to a first end of the multimode optical fiber; a grapefruit-shaped photonic crystal fiber fused to a second end of the multimode optical fiber, the connection forming a first reflective surface; the grapefruit-shaped photonic crystal fiber is filled with a first dielectric cavity of a first length at one end connected to the multimode optical fiber, and a second dielectric cavity of a second length at the opposite end; a second reflective surface is formed at the connection between the first dielectric cavity and the second dielectric cavity, and a third reflective surface is formed between the second dielectric cavity and an external contact surface; wherein, a laser beam is introduced into the single-mode optical fiber, and by detecting the intensity of the interference light reflected from the first reflective surface, the second reflective surface, and the third reflective surface, the cavity length variable generated by the second dielectric cavity sensing the ultrasonic wave to be measured is calculated based on the change in the intensity of the interference light, and the ultrasonic wave signal to be measured is solved.

[0007] In some embodiments, the first medium cavity is an air cavity, and the second medium cavity is a polydimethylsiloxane cavity.

[0008] In some embodiments, the access loss between the single-mode fiber and the multimode fiber is less than 0.5 dB.

[0009] In some embodiments, the process of determining the ultrasonic signal to be measured is as follows: Constructing an expression for the intensity of the interference light, and solving for the change in the second length of the polydimethylsiloxane cavity under the action of the ultrasonic signal to be measured. The expression for the intensity of the interference light is as follows: ; ;in, The intensity of the light signal reflected by the first reflecting surface, The light intensity of the light signal reflected by the second reflecting surface and The intensity of the light signal reflected by the third reflecting surface; The optical phase generated by the air cavity. The optical phase generated by the polydimethylsiloxane cavity; For the first length, The second length, The refractive index of the air cavity is... The refractive index of the polydimethylsiloxane cavity; The wavelength of the laser beam is given; ultrasonic pressure acts on the polydimethylsiloxane cavity, causing a change in the second length. According to the principle of elasticity, for a one-dimensional strain model, stress and strain follow Hooke's law: ;in, This indicates the compressive stress. The strain represents E, and the Young's modulus of polydimethylsiloxane is E. Under uniform ultrasonic pressure, compressive stress is formed, and the relationship between the ultrasonic pressure and the compressive stress is as follows: Where P represents the ultrasonic pressure; the strain is defined as the rate of change of the length of the polydimethylsiloxane cavity, expressed as: ;in, Let the second length variable be the variable; then there exists: The ultrasonic signal to be measured corresponds to the ultrasonic pressure denoted as . ,in, For pressure amplitude, Given the ultrasonic angular frequency; then according to Solve for the ultrasonic signal to be measured.

[0010] On the other hand, the present invention also provides a method for fabricating the above-mentioned fiber Fabry-Perot interferometric ultrasonic sensor, the method comprising the following steps: aligning and fusing the first end faces of a single-mode fiber and a multimode fiber in a fiber fusion machine, controlling the discharge intensity to 60-70 bits, the discharge time to 265-275 ms, and controlling the access loss to be less than 0.5 dB; aligning and fusing the second end face of the multimode fiber with a grapefruit-shaped photonic crystal fiber, controlling the discharge intensity to 90-100 bits, and the discharge time to 295-305 ms; The splice points between the single-mode fiber, the multimode fiber, and the grapefruit-shaped photonic crystal fiber are wrapped with heat-shrink tubing; polydimethylsiloxane and a curing agent are weighed according to a set mass ratio and mixed for a first time to obtain a homogeneous solution; the homogeneous solution is placed in a vacuum dryer for a second time to remove air bubbles; the end of the grapefruit-shaped photonic crystal fiber is immersed in the homogeneous solution, making the length of the air cavity at the front end a first length and the length of the polydimethylsiloxane cavity a second length; the polydimethylsiloxane cavity is cured by heating at a preset temperature for a third time.

[0011] In some embodiments, the polydimethylsiloxane and the curing agent in the homogeneous solution are prepared at a mass ratio of 10:1, the first duration is 5 to 10 minutes, the curing agent is DC-184 type, and the second duration is at least 30 minutes.

[0012] In some embodiments, the preset temperature is 95~105°C, and the third duration is at least 1 hour.

[0013] On the other hand, the present invention also provides an underwater ultrasonic signal detection system, the system comprising: a tunable laser for generating a laser beam; a fiber optic Fabry-Perot interferometer ultrasonic sensor as described above for detecting ultrasonic echoes; a photodetector; a circulator connecting the tunable laser, the ultrasonic sensor, and the photodetector, for guiding the laser beam into the ultrasonic sensor and guiding the returned interference light into the photodetector; a data acquisition subsystem for acquiring the interference light intensity signal obtained by the photodetector from the interference light; and a processor terminal for calculating the cavity length variable generated by the ultrasonic echo sensed by the second dielectric cavity based on the change in the interference light intensity signal, and solving for the ultrasonic echo signal.

[0014] In some embodiments, the system further includes: an ultrasonic wave generation subsystem for generating emitted waves; the ultrasonic wave generation subsystem includes a linear power supply, a signal driver, and a sound source device connected in sequence.

[0015] On the other hand, the present invention also provides an underwater ultrasonic signal detection method, which uses the above-mentioned fiber optic Fabry-Perot interferometer ultrasonic sensor to detect underwater ultrasonic signals.

[0016] The ultrasonic sensor, its fabrication method, underwater ultrasonic detection system, and application method described in this invention utilize a cascaded fusion structure of single-mode fiber, multi-mode fiber, and grapefruit-shaped photonic crystal fiber. The photonic crystal fiber is partially filled with elastic polymer PDMS to form a built-in solid-gas composite Fabry-Perot interferometer cavity, thus eliminating the need for traditional precision vibrating films. This design not only simplifies the manufacturing process and reduces costs but also effectively improves the sensor's structural stability and environmental durability through the built-in encapsulation of PDMS material. Based on a multi-reflection interface interference mechanism, combined with the high elasticity of PDMS and the microstructural characteristics of photonic crystal fiber, it achieves high sensitivity and wideband response to ultrasonic waves. Simultaneously, the all-fiber passive sensing form offers advantages such as miniaturization, resistance to electromagnetic interference, and adaptability to measurements in complex and confined spaces, providing a stable, sensitive, and easily integrated solution for ultrasonic detection. The fabrication scheme is specifically designed for the ultrasonic sensor structure. By configuring the discharge intensity and discharge time of the fiber fusion splicer, a cascaded fusion structure is precisely formed. A simple immersion operation is used to construct the solid-gas composite structure, achieving highly efficient fabrication of the ultrasonic sensor. The underwater ultrasonic testing system has a simple testing structure and can perform ultrasonic measurements under high-temperature underwater conditions.

[0017] Additional advantages, objects, and features of the invention will be set forth in part in the description which follows, and will also become apparent in part to those skilled in the art upon studying the text, or may be learned by practice of the invention. The objects and other advantages of the invention can be realized and obtained by means of the structures specifically pointed out in the description and drawings.

[0018] Those skilled in the art will understand that the objectives and advantages achievable with the present invention are not limited to those specifically described above, and that the above and other objectives achievable with the present invention will become clearer from the following detailed description. Attached Figure Description

[0019] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, are not intended to limit the scope of the invention. In the drawings: Figure 1 is a schematic diagram of the structure of a fiber optic Fabry-Perot interferometer ultrasonic sensor according to an embodiment of the present invention.

[0020] Figure 2 is a schematic cross-sectional view of the grapefruit-shaped photonic crystal fiber in Figure 1.

[0021] Figure 3 is a schematic diagram of the underwater ultrasonic signal detection system according to an embodiment of the present invention. Detailed Implementation

[0022] 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 embodiments and accompanying drawings. Here, the illustrative embodiments and descriptions of this invention are used to explain the invention, but are not intended to limit the invention.

[0023] It should also be noted that, in order to avoid obscuring the invention with unnecessary details, only the structures and / or processing steps closely related to the solution according to the invention are shown in the accompanying drawings, while other details that are not closely related to the invention are omitted.

[0024] It should be emphasized that the term "including / comprises" as used herein refers to the presence of a feature, element, step, or component, but does not exclude the presence or addition of one or more other features, elements, steps, or components.

[0025] It should also be noted that, unless otherwise specified, the term "connection" in this article can refer not only to a direct connection, but also to an indirect connection involving an intermediary.

[0026] In the following description, embodiments of the invention will be illustrated with reference to the accompanying drawings. In the drawings, the same reference numerals represent the same or similar parts, or the same or similar steps.

[0027] Current ultrasonic sensing technologies mainly rely on piezoelectric transducers and fiber optic sensors based on Fabry-Perot interferometry. While piezoelectric sensors are technologically mature and low-cost, they are susceptible to electromagnetic interference, have acoustic impedance matching problems, and are limited in size and shape. On the other hand, while existing fiber optic Fabry-Perot interferometry sensors have overcome electromagnetic compatibility issues, they generally require precision-machined external vibrating membrane structures to improve sensitivity, resulting in complex manufacturing processes, increased costs, and poor stability and repeatability of the membrane structures. They often require additional mechanical encapsulation for protection, further increasing the complexity of the overall structure and manufacturing costs.

[0028] In view of this, the present invention provides a fiber optic Fabry-Perot interferometer ultrasonic sensor, as shown in Figure 1. The ultrasonic sensor includes a cascaded fusion structure composed of single-mode fiber (SMF), multimode fiber (MMF), and grapefruit-shaped photonic crystal fiber (PCF). The cross-section of the grapefruit-shaped photonic crystal fiber is shown in Figure 2.

[0029] A single-mode fiber is fused to the first end of a multimode fiber; a grapefruit-shaped photonic crystal fiber is fused to the second end of the multimode fiber, and the connection point forms a first reflective surface; the grapefruit-shaped photonic crystal fiber is filled with a first dielectric cavity of a first length at one end connected to the multimode fiber, and a second dielectric cavity of a second length at the opposite end; a second reflective surface is formed at the connection point of the first dielectric cavity and the second dielectric cavity, and a third reflective surface is formed between the second dielectric cavity and the external contact surface.

[0030] In this process, a single-mode fiber is used to guide a laser beam. The laser beam is reflected from the first, second, and third reflecting surfaces and then interferes. By detecting the intensity of the interference light reflected from the first, second, and third reflecting surfaces, the cavity length variable generated by the second dielectric cavity sensing the ultrasonic wave under test is calculated based on the change in the intensity of the interference light, and the ultrasonic wave signal under test is solved.

[0031] In some embodiments, the first dielectric cavity is an air cavity and the second dielectric cavity is a polydimethylsiloxane cavity (PDMS).

[0032] In some embodiments, the access loss between single-mode fiber and multimode fiber is less than 0.5 dB.

[0033] This sensor performs ultrasonic detection based on the Fabry-Perot interference principle: laser light is introduced through a single-mode fiber, expanded through a multimode fiber, and then enters a grapefruit-shaped photonic crystal fiber partially filled with PDMS. Reflections occur at three interfaces: SMF-air (M1), air-PDMS (M2), and PDMS-air (M3). The three reflected beams return and interfere, with the combined intensity determined by the phase difference between each beam. When ultrasound acts on the sensor end face, the sound pressure causes deformation of the PDMS elastic body, thereby changing the interface position between PDMS and the air cavity (i.e., modulating the air cavity length). This change causes alterations in the optical path difference and corresponding phase difference, ultimately leading to wavelength shift or intensity modulation in the interference spectrum. By monitoring the changes in reflected light intensity in real time using a high-sensitivity photodetector and demodulating the spectral shift using a signal processing system, the amplitude and frequency of the ultrasound can be retrieved, achieving highly sensitive, all-optical ultrasonic signal sensing.

[0034] In some embodiments, the process of solving for the ultrasonic signal to be measured is as follows: Step S101~S103: Step S101: Construct an expression for the intensity of the interference light and solve for the change in the second length of the polydimethylsiloxane cavity under the action of the ultrasonic wave to be measured. The expression for the intensity of the interference light is as follows: ; ;in, The intensity of the light signal reflected by the first reflecting surface The intensity of the light signal reflected by the second reflecting surface and The intensity of the light signal reflected by the third reflecting surface; The optical phase generated by the air cavity. The optical phase generated by the polydimethylsiloxane cavity; The first length, For the second length, Let be the refractive index of the air cavity. is the refractive index of the polydimethylsiloxane cavity; λ is the wavelength of the laser beam.

[0035] Step S102: Ultrasonic pressure is applied to the polydimethylsiloxane cavity, causing a change in the second length. According to the principle of elasticity, for a one-dimensional strain model, stress and strain follow Hooke's law: ;in, Indicates compressive stress. The value represents strain, and E represents the Young's modulus of polydimethylsiloxane.

[0036] Under uniform ultrasonic pressure, compressive stress is generated. The relationship between ultrasonic pressure and compressive stress is as follows: Where P represents ultrasonic pressure.

[0037] Strain is defined as the rate of change of the length of the polydimethylsiloxane cavity, expressed as: ;in, This is the second length variable.

[0038] Then it exists: .

[0039] Step S103: Record the ultrasonic pressure corresponding to the ultrasonic signal to be measured as... ,in, For pressure amplitude, This is the angular frequency of the ultrasonic wave. Then, according to... Solve for the ultrasonic signal to be measured.

[0040] On the other hand, the present invention also provides a method for fabricating the above-mentioned fiber optic Fabry-Perot interferometric ultrasonic sensor, the method comprising the following steps S201~S206: Step S201: Aligning and fusing the first end faces of a single-mode fiber and a multimode fiber in a fiber fusion machine, controlling the discharge intensity to 60~70 bits, the discharge time to 265~275 ms, and controlling the access loss to be less than 0.5 dB. Preferably, the discharge intensity is 65 bits and the discharge time is 270 ms.

[0041] Step S202: Align and fusion splice the second end face of the multimode fiber with the grapefruit-shaped photonic crystal fiber, controlling the discharge intensity to 90~100 bits and the discharge time to 295~305 ms. Preferably, the discharge intensity is 95 bits and the discharge time is 300 ms.

[0042] Step S203: Use heat shrink tubing to wrap the splice points between single-mode fiber, multimode fiber, and grapefruit-shaped photonic crystal fiber.

[0043] Step S204: Weigh the polydimethylsiloxane and curing agent according to the set mass ratio, mix them for a first time to obtain a homogeneous solution; place the homogeneous solution in a vacuum dryer for a second time to remove air bubbles. Preferably, the polydimethylsiloxane and curing agent in the homogeneous solution are prepared at a mass ratio of 10:1, the first time is 5-10 minutes; the curing agent is DC-184 type; the second time is at least 30 minutes.

[0044] Step S205: Immerse the end of the grapefruit-shaped photonic crystal fiber in a uniform solution, so that the length of the air cavity at the front end is the first length and the length of the polydimethylsiloxane cavity is the second length.

[0045] Step S206: Heating at a preset temperature for a third time to cure the polydimethylsiloxane cavity. Preferably, the preset temperature is 95~105°C, and the third time is at least 1 hour.

[0046] The fabrication method described in this invention achieves stable and efficient manufacturing of SMF-MMF-PCF cascade structures by optimizing fusion parameters, standardizing material ratios, and implementing curing processes. By precisely controlling the discharge intensity and time during the fusion process, low fusion loss is ensured while preventing air pocket collapse in the photonic crystal fiber. Subsequently, heat-shrink tubing is used to encapsulate the fusion joints to enhance mechanical stability. During PDMS filling, a precise ratio (10:1) and vacuum degassing ensure the uniformity and bubble-free characteristics of the filling material. Finally, high-temperature curing forms a stable solid-gas composite interference cavity. This fabrication method enables integrated sensor fabrication without an external vibrating membrane, significantly simplifying the process and reducing manufacturing costs. Simultaneously, the built-in PDMS filling and fusion joint protection design effectively improves the sensor's structural stability, environmental durability, and measurement repeatability, providing a reliable process path for the mass production of high-performance, high-reliability all-fiber ultrasonic sensors.

[0047] On the other hand, the present invention also provides an underwater ultrasonic signal detection system, as shown in Figure 3. The system includes: a tunable laser, a fiber optic Fabry-Perot interferometer ultrasonic sensor as described above, a photodetector, a circulator, a data acquisition subsystem, and a processor terminal.

[0048] A tunable laser is used to generate a laser beam; a fiber optic Fabry-Perot interferometer ultrasonic sensor is used to detect ultrasonic echoes; a photodetector is used to sense the intensity of light emitted from the fiber optic Fabry-Perot interferometer ultrasonic sensor; a circulator connects the tunable laser, the ultrasonic sensor, and the photodetector, and is used to guide the laser beam into the ultrasonic sensor and guide the returning interference light into the photodetector.

[0049] The data acquisition subsystem is used to acquire the intensity signal of the interference light obtained by the photodetector detecting the interference light.

[0050] The processor terminal is used to calculate the cavity length variable generated by the ultrasonic echo sensed by the second dielectric cavity based on the change in the intensity signal of the interference light, and to solve for the ultrasonic echo signal.

[0051] In some embodiments, the system further includes an ultrasonic wave generation subsystem for generating emitted waves. The ultrasonic wave generation subsystem includes a linear power supply, a signal driver, and a sound source device connected in sequence.

[0052] On the other hand, the present invention also provides an underwater ultrasonic signal detection method, which uses the above-mentioned fiber optic Fabry-Perot interferometer ultrasonic sensor to detect underwater ultrasonic signals.

[0053] The invention is described below with reference to a specific embodiment: This embodiment provides a fiber optic ultrasonic sensor based on a Fabry-Perot interferometer (FPI). The sensor is constructed by fusing single-mode fiber (SMF) and multimode fiber (MMF), and then fusing them together with a section of grapefruit-shaped photonic crystal fiber (PCF) partially filled with polydimethylsiloxane (PDMS). Filling the PCF with PDMS protects the acoustically sensitive material from external corrosion and enhances the sensor's stability. The proposed sensor has advantages such as simple manufacturing, compact structure, and low cost, and may replace piezoelectric sensors (PZT) for ultrasonic measurements under high-temperature conditions.

[0054] A schematic diagram of the proposed ultrasonic sensor is shown in Figure 1. The sensor consists of a single-mode fiber (SMF) spliced ​​with a section of multimode fiber (MMF), which is then electrically fused to a grapefruit-shaped photonic crystal fiber (PCF) partially filled with polydimethylsiloxane (PDMS). The sensor incorporates three different media: silica (SMF), air, and PDMS. Due to the different refractive indices of each medium, different reflections occur at each interface between two media. There are three reflecting interfaces: M1, M2, and M3, representing the SMF-air, air-PDMS, and PDMS-air interfaces, respectively. When incident light passes through these three reflecting interfaces, three reflected beams are generated. These beams interfere with each other due to phase delay. When external ultrasonic waves reach the PCF endface, the vibration of the PDMS changes the length of the air cavity, thereby modulating the interference spectrum. Therefore, by detecting the change in the reflection spectrum, the external ultrasonic signal can be detected.

[0055] The sensor fabrication process is as follows: First, single-mode fiber (SMF) and multimode fiber (MMF) are aligned and fused together using a fusion splicer. In the fiber fusion splicer, the end faces of the SMF and MMF are precisely aligned and spliced ​​using preset discharge parameters (discharge intensity: 65 bits, discharge time: 270 ms) to form an SMF-MMF structure. In this step, the introduction of MMF effectively expands the optical field modes and improves the sensor's sensitivity. After splicing, the splicing quality is evaluated using the fusion splicer's loss assessment function to ensure that the insertion loss is less than 0.5 dB. Then, the other end of the MMF is spliced ​​onto a pebble-shaped photonic crystal fiber (PCF). Due to the air hole structure of the PCF, the discharge parameters need to be optimized (discharge intensity: 95 bits, discharge time: 300 ms) to prevent the collapse or deformation of the air holes. During the splicing process, the PCF end face is aligned with the MMF by real-time monitoring of microscope images to form a complete SMF-MMF-PCF structure. After splicing, heat-shrink tubing is used to protect the splice point and improve mechanical stability. After fiber optic splicing, PCF is filled with PDMS (Corning, DC-184) to form an FP cavity. PDMS is a transparent elastic polymer with an adjustable refractive index, suitable for optical sensing. The filling process includes the preparation, injection, and curing of the PDMS solution. First, the PDMS substrate and curing agent are weighed at a mass ratio of 10:1, thoroughly mixed, and stirred for 5-10 minutes to form a homogeneous solution. Then, the solution is placed in a vacuum degasser for at least 30 minutes to remove air bubbles and avoid introducing scattering defects during filling. During filling, the PCF end is immersed in the PDMS solution. Utilizing the adsorption effect of the PCF air channels and the control of a precision translation stage, the filling process is observed under a microscope, and the sensor's reflectance spectrum is monitored in real time. After filling, the sensor is placed in a constant temperature oven and heated at 100°C for 1 hour to fully cure the PDMS. After curing, the air pores of the PCF are filled with PDMS, forming a solid-gas composite structure.

[0056] Fiber optic sensors based on Fabry-Perot interferometers (FPI) achieve high-sensitivity sensing through the interference of multiple reflective surfaces. Light enters from a single-mode fiber (SMF) and undergoes partial reflection at three reflective interfaces. The three reflected beams return to the SMF and interfere with each other; the output light intensity is determined by the interference relationship between these three beams. The total output light intensity can be expressed as: The interference light intensity reflected by the sensor can be expressed as: (1) of which , and These represent the light intensity of the three reflective surfaces. and These are the optical phases generated by the air cavity and the PDMS composite material, respectively, and can be defined as: (2) When ultrasound acts on the sensor, it causes the PDMS to vibrate, thereby changing the length of the internal air cavity. This change modulates the phase difference of the reflected light, and the ultrasound signal can be measured by dynamically detecting the shift of the interference spectrum.

[0057] In addition, the PCF is filled with the elastomer polydimethylsiloxane (PDMS). When ultrasonic pressure is applied to the PDMS, it deforms and transfers strain to the PCF air cavity boundary. According to the principle of elasticity, for a one-dimensional strain model, the stress ( ) and strain ( The relationship between the two follows Hooke's Law: (3) Where E is the Young's modulus of PDMS. Under the applied ultrasonic pressure P, assuming the pressure is uniform and constitutes compressive stress, then Strain is defined as the rate of change of the PDMS cavity length: (4) (5) The change in ultrasonic pressure over time is denoted as... ,in For pressure amplitude, Where is the ultrasonic angular frequency. Therefore, the change in the PDMS cavity length is: (6) Figure 3 shows a schematic diagram of the underwater ultrasonic signal detection principle of the sensor. The experimental system mainly consists of two parts: ultrasonic transmission and ultrasonic detection. The transmitting unit generates an electrical signal (continuous sine wave or pulse wave) of a specific frequency and waveform through a signal driver to control the sound source to generate ultrasonic waves; a high-precision linear power supply provides a stable operating voltage. The receiving unit is based on the FPI principle and uses a tunable laser to inject light into the fiber optic sensor. The ultrasonic pressure modulates the length of the interference cavity through the PDMS elastomer, resulting in a change in the output light intensity, which is converted into an electrical signal by a photodetector. The data acquisition system synchronously collects the signal and transmits it to the computer for real-time processing and analysis, thereby completing the entire detection process from ultrasonic excitation to optical sensing.

[0058] This embodiment is based on a grapefruit-shaped photonic crystal fiber (PCF) and PDMS-partially filled FPI ultrasonic sensor structure. A single-mode fiber (SMF), a multimode fiber (MMF), and a PCF are cascaded, with PDMS partially filled into the PCF to form a composite FP cavity. This structure utilizes the high elasticity of PDMS and the microstructure of PCF to achieve a high-sensitivity response to ultrasound.

[0059] PDMS serves as an embedded protective layer within the air pores of the PCF (Polymer Carbon Fiber) as the acoustic sensing material. The PDMS fills these pores, preventing direct exposure to the external environment and enhancing the sensor's stability and durability. Compared to surface-coated PDMS sensors, this structure offers superior mechanical protection and optical stability.

[0060] This embodiment features a non-vibrating membrane structure design. Traditional FPI ultrasonic sensors rely on vibrating membranes to enhance sensitivity, while this design eliminates the need for an additional vibrating membrane, simplifying the structure, reducing costs, and avoiding the process complexity and instability associated with membrane structures.

[0061] The fiber optic ultrasonic sensor consists of an optical fiber and a flexible sensing probe. Its maximum diameter is 125μm, so the sensor not only has high spatial resolution, but also meets the measurement requirements in confined spaces.

[0062] Fiber optic ultrasonic sensors are passive devices, thus overcoming the limitation of traditional ultrasonic sensors that cannot operate under strong electromagnetic interference. Compared with traditional ultrasonic sensors, they also have the advantages of small size and light weight.

[0063] In summary, the ultrasonic sensor, its fabrication method, underwater ultrasonic detection system, and application method described in this invention utilize a cascaded fusion structure of single-mode fiber, multimode fiber, and grapefruit-shaped photonic crystal fiber. The photonic crystal fiber is partially filled with elastic polymer PDMS to form a built-in solid-gas composite Fabry-Perot interferometer cavity, thus eliminating the need for traditional precision vibrating films. This design not only simplifies the manufacturing process and reduces costs but also effectively improves the sensor's structural stability and environmental durability through the built-in encapsulation of PDMS material. Based on a multi-reflection interface interference mechanism, combined with the high elasticity of PDMS and the microstructural characteristics of photonic crystal fiber, it achieves high sensitivity and wideband response to ultrasonic waves. Simultaneously, the all-fiber passive sensing form offers advantages such as miniaturization, resistance to electromagnetic interference, and adaptability to measurements in complex and confined spaces, providing a stable, sensitive, and easily integrated solution for ultrasonic detection. The fabrication scheme is specifically designed for the structure of the ultrasonic sensor. By configuring the discharge intensity and discharge time of the fiber fusion splicer, a cascaded fusion structure is precisely formed. A simple immersion operation is used to construct the solid-gas composite structure, achieving highly efficient fabrication of the ultrasonic sensor. The underwater ultrasonic testing system has a simple testing structure and can perform ultrasonic measurements under high-temperature underwater conditions.

[0064] Those skilled in the art will understand that the exemplary components, systems, and methods described in conjunction with the embodiments disclosed herein can be implemented in hardware, software, or a combination of both. Whether implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this invention. When implemented in hardware, it can be, for example, electronic circuits, application-specific integrated circuits (ASICs), appropriate firmware, plug-ins, function cards, etc. When implemented in software, the elements of this invention are programs or code segments used to perform the desired tasks. The programs or code segments can be stored in a machine-readable medium or transmitted over a transmission medium or communication link via data signals carried in a carrier wave.

[0065] It should be clarified that the present invention is not limited to the specific configurations and processes described above and shown in the figures. For the sake of brevity, detailed descriptions of known methods are omitted here. In the above embodiments, several specific steps are described and shown as examples. However, the method process of the present invention is not limited to the specific steps described and shown. Those skilled in the art can make various changes, modifications, and additions, or change the order of steps, after understanding the spirit of the present invention.

[0066] In this invention, features described and / or illustrated for one embodiment may be used in the same or similar manner in one or more other embodiments, and / or combined with or in place of features of other embodiments.

[0067] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, various modifications and variations of the embodiments of the present invention are possible. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A fiber optic Fabry-Perot interferometer ultrasonic sensor, characterized in that, The ultrasonic sensor includes: a multimode optical fiber; a single-mode optical fiber fused to a first end of the multimode optical fiber; and a grapefruit-shaped photonic crystal fiber fused to a second end of the multimode optical fiber, with the connection forming a first reflective surface; the grapefruit-shaped photonic crystal fiber is filled with a first dielectric cavity of a first length at one end connected to the multimode optical fiber, and a second dielectric cavity of a second length at the opposite end; a second reflective surface is formed at the connection between the first dielectric cavity and the second dielectric cavity, and a third reflective surface is formed between the second dielectric cavity and an external contact surface; wherein, a laser beam is introduced into the single-mode optical fiber, and the intensity of the interference light reflected from the first reflective surface, the second reflective surface, and the third reflective surface is detected. Based on the change in the intensity of the interference light, the cavity length variable generated by the second dielectric cavity sensing the ultrasonic wave to be measured is calculated, and the ultrasonic wave signal to be measured is solved.

2. The fiber optic Fabry-Perot interferometric ultrasonic sensor according to claim 1, characterized in that, The first medium cavity is an air cavity, and the second medium cavity is a polydimethylsiloxane cavity.

3. The fiber optic Fabry-Perot interferometric ultrasonic sensor according to claim 2, characterized in that, The access loss between the single-mode fiber and the multimode fiber is less than 0.5 dB.

4. The fiber optic Fabry-Perot interferometric ultrasonic sensor according to claim 3, characterized in that, The process of solving for the ultrasonic signal to be measured is as follows: Construct an expression for the intensity of the interference light, and solve for the change in the second length of the polydimethylsiloxane cavity under the action of the ultrasonic signal to be measured. The expression for the intensity of the interference light is as follows: ； ;in, The intensity of the light signal reflected by the first reflecting surface, The light intensity of the light signal reflected by the second reflecting surface and The intensity of the light signal reflected by the third reflecting surface; The optical phase generated by the air cavity. The optical phase generated by the polydimethylsiloxane cavity; For the first length, The second length, The refractive index of the air cavity is... The refractive index of the polydimethylsiloxane cavity; The wavelength of the laser beam is given; ultrasonic pressure acts on the polydimethylsiloxane cavity, causing a change in the second length. According to the principle of elasticity, for a one-dimensional strain model, stress and strain follow Hooke's law: ;in, This indicates the compressive stress. The strain represents E, and the Young's modulus of polydimethylsiloxane is E. Under uniform ultrasonic pressure, compressive stress is formed, and the relationship between the ultrasonic pressure and the compressive stress is as follows: Where P represents the ultrasonic pressure; the strain is defined as the rate of change of the length of the polydimethylsiloxane cavity, expressed as: ;in, Let the second length variable be the variable; then there exists: The ultrasonic signal to be measured corresponds to the ultrasonic pressure denoted as . ,in, For pressure amplitude, Given the ultrasonic angular frequency; then according to Solve for the ultrasonic signal to be measured.

5. A method for fabricating a fiber optic Fabry-Perot interferometric ultrasonic sensor according to any one of claims 1 to 4, characterized in that, The method includes the following steps: Aligning and fusing the first end faces of a single-mode fiber and a multimode fiber in a fiber fusion machine, controlling the discharge intensity to 60-70 bits, the discharge time to 265-275 ms, and controlling the access loss to be less than 0.5 dB; aligning and fusing the second end face of the multimode fiber with a grapefruit-shaped photonic crystal fiber, controlling the discharge intensity to 90-100 bits, and the discharge time to 295-305 ms; wrapping the fusion joint between the single-mode fiber, the multimode fiber, and the grapefruit-shaped photonic crystal fiber with heat-shrink tubing; weighing polydimethylsiloxane and a curing agent according to a set mass ratio, mixing and immersing for a first time to obtain a homogeneous solution; placing the homogeneous solution in a vacuum dryer for a second time to remove air bubbles; immersing the end of the grapefruit-shaped photonic crystal fiber in the homogeneous solution, making the length of the air cavity at the front end a first length and the length of the polydimethylsiloxane cavity a second length; heating at a preset temperature for a third time to cure the polydimethylsiloxane cavity.

6. The preparation method according to claim 5, characterized in that, The polydimethylsiloxane and the curing agent in the homogeneous solution are prepared at a mass ratio of 10:1, the first time is 5 to 10 minutes, the curing agent is DC-184 type, and the second time is at least 30 minutes.

7. The preparation method according to claim 5, characterized in that, The preset temperature is 95~105℃, and the third duration is at least 1 hour.

8. An underwater ultrasonic signal detection system, characterized in that, The system includes: a tunable laser for generating a laser beam; a fiber optic Fabry-Perot interferometric ultrasonic sensor as described in any one of claims 1 to 4 for detecting ultrasonic echoes; a photodetector; a circulator connecting the tunable laser, the ultrasonic sensor, and the photodetector, for guiding the laser beam into the ultrasonic sensor and guiding the returned interference light into the photodetector; a data acquisition subsystem for acquiring the interference light intensity signal obtained by the photodetector detecting the interference light; and a processor terminal for calculating the cavity length variable generated by the ultrasonic echo sensed by the second dielectric cavity based on the change in the interference light intensity signal, and solving for the ultrasonic echo signal.

9. The underwater ultrasonic signal detection system according to claim 8, characterized in that, The system further includes an ultrasonic wave generation subsystem for generating emitted waves; the ultrasonic wave generation subsystem includes a linear power supply, a signal driver, and a sound source device connected in sequence.

10. A method for detecting underwater ultrasonic signals, characterized in that, The fiber optic Fabry-Perot interferometric ultrasonic sensor described in any one of claims 1 to 4 is used to detect underwater ultrasonic signals.

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