A fiber optic sensor based on surface plasmon resonance and its preparation method

Abstract

The present invention provides a fiber optic sensor based on surface plasmon resonance and a method for preparing the same, relating to the field of sensor technology. The fiber optic sensor comprises: a fiber Bragg grating (FBG), comprising a core having a fiber Bragg grating (FBG) and a fiber cladding disposed over the core; the fiber Bragg grating (FBG) is curved into an arc shape according to a preset radius of curvature; the fiber cladding has an opening, exposing a portion of the FBG to the outside of the fiber cladding, forming an exposed core region; a metal film coated on the exposed core region, configured to utilize surface plasmon resonance technology to enhance the optical response to ultrasonic signals incident on the metal film; an insulating protective layer coated on the metal film; and an electromagnetic shielding layer covering the insulating protective layer. The present invention improves the sensitivity of detecting ultrasonic signals generated by partial discharges in power facilities.

Description

Technical Field

[0001] The present invention relates to the field of sensor technology, and more particularly to an optical fiber sensor based on surface plasmon resonance and a preparation method thereof. Background Art

[0002] Arc discharges in power systems pose a threat to their safe and stable operation. In addition to generating significant amounts of light and heat, arc discharges also induce mechanical vibrations in the surrounding medium due to rapidly changing currents, generating ultrasonic signals. Currently, ultrasonic monitoring of power facilities primarily uses piezoelectric ceramic ultrasonic sensors (PZT, Lead Zirconate Titanate). These sensors convert ultrasonic signals into electrical signals through the piezoelectric effect. However, these electrical signals are relatively low in power and suffer from limitations such as susceptibility to electromagnetic interference, insufficient sensitivity, and poor reusability, hindering large-scale measurement. These sensors are therefore unable to meet monitoring requirements in strong electromagnetic environments. Summary of the Invention

[0003] In view of this, the present invention provides an optical fiber sensor based on surface plasmon resonance and a preparation method thereof.

[0004] One aspect of the present invention provides a surface plasmon resonance-based optical fiber sensor, comprising: a fiber Bragg grating (FBG), comprising a core having a fiber Bragg grating (FBG) and a fiber cladding disposed over the core; the fiber Bragg grating being curved into an arc shape according to a preset curvature radius; the fiber cladding having an opening such that a portion of the fiber Bragg grating (FBG) is exposed outside the fiber cladding, forming a core-exposed region; a metal film coated on the core-exposed region for enhancing an optical response to an ultrasonic signal incident on the metal film using surface plasmon resonance technology; an insulating protective layer coated on the metal film; and an electromagnetic shielding layer covering the insulating protective layer. An ultrasonic signal to be measured, after passing through the electromagnetic shielding layer, the insulating protective layer, and the metal film, is incident on the fiber Bragg grating via the core-exposed region. The strain field of the ultrasonic signal to be measured periodically changes the period of the fiber Bragg grating, causing the phase of the optical signal in the core to change. The ultrasonic signal to be measured is detected using the optical signal after the phase change.

[0005] According to an embodiment of the present invention, the flatness of the exposed area of ​​the core is less than or equal to 5 nanometers.

[0006] According to an embodiment of the present invention, a magnetron sputtering process or a chemical vapor deposition process is applied to coat the metal film on the exposed area of ​​the fiber core.

[0007] According to an embodiment of the present invention, a magnetron sputtering process or a chemical vapor deposition process is applied to plate an insulating protective layer on the metal film.

[0008] According to an embodiment of the present invention, the thickness of the metal film is in a range of 10 nanometers to 30 nanometers.

[0009] According to an embodiment of the present invention, the insulating protective layer is made of silicon dioxide or magnesium fluoride.

[0010] According to an embodiment of the present invention, the electromagnetic shielding layer is sleeved on the optical fiber Bragg grating coated with the insulating protective layer.

[0011] Another aspect of the present invention provides a method for preparing an optical fiber sensor based on surface plasmon resonance, the method comprising: bending an optical fiber Bragg grating (FBG) into an arc shape according to a preset curvature radius; the optical fiber Bragg grating (FBG) comprising a core having a fiber Bragg grating (FBG) and an optical fiber cladding sleeved on the core; the optical fiber cladding having an opening such that a portion of the fiber Bragg grating (FBG) is exposed to the outside of the optical fiber cladding, forming an exposed core region; coating the exposed core region with a metal film; coating the metal film with an insulating protective layer; and covering the insulating protective layer with an electromagnetic shielding layer; wherein, after passing through the electromagnetic shielding layer, the insulating protective layer, and the metal film, an ultrasonic signal to be measured is incident on the fiber Bragg grating via the exposed core region, the strain field of the ultrasonic signal to be measured periodically changes the period of the fiber Bragg grating, causing the phase of the optical signal in the core to change, and the ultrasonic signal to be measured is detected by the optical signal after the phase change.

[0012] According to an embodiment of the present invention, a magnetron sputtering process or a chemical vapor deposition process is applied to coat the metal film on the exposed area of ​​the fiber core.

[0013] According to an embodiment of the present invention, a magnetron sputtering process or a chemical vapor deposition process is applied to plate an insulating protective layer on the metal film.

[0014] According to an embodiment of the present invention, the fiber Bragg grating is processed into a curved structure, so that the fiber Bragg grating is exposed, forming an exposed core area, thereby overcoming the obstacles to ultrasonic signal transmission; by coating the exposed core area with a metal film, the coupling efficiency between the optical signal in the fiber Bragg grating and the ultrasonic signal to be measured is improved, making the evanescent field of the optical signal more easily excited, providing a basis for the surface plasmon resonance (SPR) effect; and the insulation protective layer and the electromagnetic shielding layer are used to shield the interference of the external environment, thereby at least partially overcoming the technical problem of insufficient sensitivity in detecting ultrasonic signals in the prior art, thereby achieving the technical effect of improving the sensitivity of the optical fiber sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The above and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:

[0016] Figure 1 A half-section schematic diagram of an optical fiber sensor based on surface plasmon resonance according to an embodiment of the present invention is shown.

[0017] Figure 2A A schematic structural diagram of a surface plasmon resonance-based optical fiber sensor according to an embodiment of the present invention is shown.

[0018] Figure 2B A schematic diagram showing the working principle of an optical fiber sensor based on surface plasmon resonance according to an embodiment of the present invention is shown.

[0019] Figure 2C A schematic diagram of the electromagnetic shielding effect of an optical fiber sensor based on surface plasmon resonance according to an embodiment of the present invention is shown.

[0020] Figure 3 A flow chart of a method for preparing an optical fiber sensor based on surface plasmon resonance according to another embodiment of the present invention is shown.

[0021] Figure 4 A schematic diagram of the preparation process of an optical fiber sensor based on surface plasmon resonance according to another embodiment of the present invention is shown.

[0022] Figure 5 A schematic diagram of the electric field distribution inside an optical fiber sensor based on surface plasmon resonance according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0023] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the present invention. In the following detailed description, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of embodiments of the present invention. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of known structures and technologies are omitted to avoid unnecessary confusion of the concept of the present invention.

[0024] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. The terms "comprise", "include", etc. used herein indicate the presence of the features, steps, operations and / or components, but do not exclude the presence or addition of one or more other features, steps, operations or components.

[0025] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification and should not be interpreted in an idealized or overly rigid manner.

[0026] When expressions such as "at least one of A, B, and C, etc." are used, they should generally be interpreted in accordance with the meaning commonly understood by those skilled in the art (for example, "a system having at least one of A, B, and C" should include but is not limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc.).

[0027] In the embodiments of the present invention, the collection, updating, analysis, processing, use, transmission, provision, disclosure, and storage of all data involved (including, but not limited to, user personal information) comply with relevant laws and regulations, are used for legitimate purposes, and do not violate public order and good morals. In particular, necessary measures are taken to prevent unauthorized access to user personal information data and maintain the security of user personal information and network security.

[0028] In the embodiment of the present invention, the user's authorization or consent is obtained before obtaining or collecting the user's personal information.

[0029] Electricity is the cornerstone of modern society. In pursuit of efficiency, convenience, and intelligence, a vast array of electronic devices are being used across all areas of production and life. As electronic devices age, electrical, thermal, and mechanical stresses can damage materials within power facilities, significantly reducing their insulation performance. This can cause arcing, leading to failures and posing a threat to the safe and stable operation of power systems.

[0030] In recent years, fiber-optic ultrasonic sensors have attracted increasing attention due to their advantages, including strong resistance to electromagnetic interference, small size, light weight, excellent electrical insulation, corrosion resistance, high sensitivity, good reusability, and suitability for long-distance transmission. Furthermore, they can achieve single-fiber multi-point detection and multi-parameter sensing. All power equipment, from high-voltage transmission lines to wireless communication base stations, from household appliances to smart wearable devices, generates electromagnetic radiation during operation. Although fiber-optic sensors can resist the effects of electromagnetic interference to a certain extent, as power equipment expands in frequency, power, and sensitivity, high-power transmission and high-sensitivity reception enable rapid information transmission and efficient energy utilization, but they also inevitably create a complex and strong electromagnetic environment. For example, near high-voltage insulators in transmission systems, near high-voltage circuit breakers, switchgear, contact points, and within high-voltage power cables, electric field strengths can reach 1–3 MV / m. Electric fields at certain sharp points can exceed 10 MV / m, and the transient electric field strength of lightning discharges can reach 100 MV / m. This electric field strength, equivalent to a 10W laser transmitted through a single-mode optical fiber, can easily lead to problems such as electron cloud distortion and nonlinear polarization in the fiber, limiting the ability of fiber-optic ultrasonic sensors to detect weak signals and early-stage faults in power facilities. Therefore, further in-depth research is urgently needed to study the impact of strong electromagnetic fields on the ultrasonic strain response mechanism of fiber-optic sensors. This will provide effective monitoring tools for early fault detection and prevention, ensuring the stability and reliability of power systems, promoting the development of clean energy, energy storage, and smart grid technologies, and providing technical support for optimizing the energy structure and promoting green and low-carbon development.

[0031] Currently, fiber optic sensors based on surface plasmon resonance are considered an important technical solution to improve sensing sensitivity. However, existing fiber optic sensors based on surface plasmon resonance have the following drawbacks:

[0032] 1. The fiber cladding hinders vibration signal transmission. In existing technologies, the presence of the fiber cladding is a significant factor preventing vibration signals from being effectively transmitted to the fiber core. As the outer protective layer of the optical fiber, the cladding's physical properties (such as elastic modulus and thickness) significantly weaken the energy transfer of acoustic vibrations. This "blocking effect" directly reduces sensor sensitivity, making conventional fiber optic sensors, particularly when detecting weak vibration signals, unable to meet the requirements of high-precision monitoring.

[0033] 2. Interference of strong electromagnetic fields on signals. SPR is a resonance phenomenon between light and free electrons on the metal surface that occurs at the metal-dielectric interface. Its core is to achieve signal amplification through the coupling of the surface charge density wave of the metal film and the optical field, which can effectively improve the sensing sensitivity. Although existing optical fiber sensors use light as a transmission medium, which can avoid the electromagnetic interference problem of traditional piezoelectric sensors to a certain extent, in a strong electromagnetic field environment, the metal coating on the surface of the optical fiber or around it will still be interfered by the external electromagnetic field. This interference manifests itself in the following aspects: (1) Abnormal surface electron oscillation: Strong electromagnetic fields will change the oscillation state of free electrons on the surface of the metal film, thereby causing signal distortion of the surface plasmon resonance (SPR). (2) Nonlinear polarization effect: Strong electromagnetic fields may induce nonlinear polarization effects inside the optical fiber, further interfering with the stability of the optical signal. (3) Reduced signal-to-noise ratio: The superposition of external interference noise causes the signal-to-noise ratio (SNR) to drop significantly, especially in an environment with high electromagnetic field intensity.

[0034] 3. Limitations of existing coating processes. Current metal thin film coating technologies (such as magnetron sputtering or chemical vapor deposition) primarily focus on thicknesses between 50 and 200 nanometers to ensure the activation of the SPR effect. However, this film thickness is insufficient to shield against interference from strong electromagnetic fields, resulting in a significant drop in sensor performance in complex environments. Furthermore, uneven film thickness and insufficient adhesion can also lead to performance degradation over long-term sensor use.

[0035] 4. Traditional sensors lack anti-interference capabilities. While existing SPR fiber optic sensors can partially mitigate the effects of interference through signal conditioning algorithms, this process is complex and relies on algorithm stability, making it impossible to completely address electromagnetic interference at a physical level. Furthermore, traditional sensor designs fail to comprehensively optimize for multiple interferences in the external environment (such as the transmission barrier of vibration signals and the combined effects of electromagnetic interference), limiting their stability and sensitivity in practical applications.

[0036] Figure 1 A half-section schematic diagram of an optical fiber sensor based on surface plasmon resonance according to an embodiment of the present invention is shown.

[0037] like Figure 1As shown, an embodiment of the present invention provides an optical fiber sensor based on surface plasmon resonance, which includes: an optical fiber Bragg grating (FBG), which includes a core 1 having a fiber Bragg grating (FBG) 6 and an optical fiber cladding 3 sleeved on the core 1; the optical fiber Bragg grating is bent into an arc shape according to a preset curvature radius; the optical fiber cladding 3 has an opening, so that a portion of the fiber Bragg grating 6 is exposed to the outside of the optical fiber cladding 3, forming a core exposure area; a metal film 5, which is coated on the core exposure area and is used to enhance the optical response to an ultrasonic signal incident on the metal film 5 using surface plasmon resonance technology; an insulating protective layer 4, which is coated on the metal film 5; and an electromagnetic shielding layer 2, which is covered on the insulating protective layer 4; wherein, after the ultrasonic signal to be measured passes through the electromagnetic shielding layer 2, the insulating protective layer 4 and the metal film 5, it is incident on the fiber Bragg grating 6 through the core exposure area, and the strain field of the ultrasonic signal to be measured periodically changes the period of the fiber Bragg grating 6, so that the phase of the optical signal in the core 1 is changed, and the ultrasonic signal to be measured is detected by the optical signal after the phase change.

[0038] According to an embodiment of the present invention, a fiber Bragg grating (FBG) comprises a core 1 having a fiber Bragg grating (FBG) 6 and a fiber cladding 3 disposed over the core 1. The FBG (Fiber Bragg Grating) is the core sensitive element of a fiber sensor. Its periodic refractive index variation structure is capable of reflecting optical signals of specific wavelengths. The Bragg wavelength of the FBG 6 is highly sensitive to strain changes caused by external vibrations and can sense tiny ultrasonic signals generated by vibrations through wavelength shifts. To enhance the transmission efficiency of ultrasonic signals, the fiber cladding 3 (Cladding) is removed from the sensitive area of ​​the FBG, directly exposing the core 1 of the FBG and forming an exposed core area. This design reduces the obstruction of the ultrasonic strain field by the fiber cladding 3, allowing the ultrasonic signal to be measured to act directly on the core 1 of the FBG, greatly improving the sensitivity of the fiber sensor.

[0039] According to an embodiment of the present invention, a metal film is deposited on the exposed core area of ​​the optical fiber after the cladding is removed. The deposition process uses magnetron sputtering or chemical vapor deposition (CVD) to ensure uniform thickness of the metal film and good adhesion to the fiber core.

[0040] According to an embodiment of the present invention, the insulating protective layer is made of silicon dioxide or magnesium fluoride. A 1-micron-thick insulating protective layer of silicon dioxide (SiO2) or magnesium fluoride (MgF2) is applied to the metal film. The choice of the insulating protective layer is based on the following properties: silicon dioxide has high chemical stability and is suitable for use in a variety of complex environments (such as high humidity or acidic or alkaline environments), while also exhibiting good mechanical strength. Magnesium fluoride, as a low-refractive-index material, can further reduce optical loss and also offers good corrosion resistance. An electromagnetic shielding layer is applied over the fiber Bragg grating coated with the insulating protective layer. The insulating protective layer isolates conductive interference from the external environment, ensuring that the metal film is not directly affected by external electric fields. It also protects the exposed fiber core and metal film, preventing mechanical damage to the fiber optic sensor from external forces or environmental factors.

[0041] According to an embodiment of the present invention, the outermost portion of the optical fiber sensor is covered with an electromagnetic shielding layer, typically made of a metal material (such as copper or aluminum) with a thickness of several microns. This electromagnetic shielding layer, located on the outermost portion of the optical fiber sensor, serves to shield against interference from external electromagnetic fields. Through the skin effect, the electromagnetic shielding layer effectively prevents external high-frequency electromagnetic fields from entering the optical fiber sensor, while ensuring that the SPR effect stimulated by the metal film is not subject to external interference. The metal film achieves high sensitivity to external ultrasonic signals by stimulating SPR. The electromagnetic shielding layer effectively shields against interference from strong external electromagnetic fields, protecting the stability of the optical signal within the fiber core. It also exhibits excellent corrosion resistance under most environmental conditions, extending the service life of the optical fiber sensor. Furthermore, the design of the electromagnetic shielding layer balances electrical conductivity and mechanical strength to ensure durability and reliability in complex environments. The surface of the electromagnetic shielding layer can also be treated with an anti-corrosion treatment to adapt to various harsh environments, such as humid, high-temperature, or strongly acidic or alkaline environments. The introduction of the electromagnetic shielding layer further enhances the stability of the optical fiber sensor in environments with strong electromagnetic interference, ensuring accurate detection of the strain field of the ultrasonic signal.

[0042] According to an embodiment of the present invention, in order to simultaneously achieve the high-sensitivity signal detection capability of the optical fiber sensor and the anti-interference capability in a strong electromagnetic environment, a self-shielding structure is designed comprising a double-layer metal film, an inner metal film, and an outer electromagnetic shielding layer. The inner metal film is used to stimulate the surface plasmon resonance effect and enhance the optical response of the ultrasonic strain signal; the outer electromagnetic shielding layer is used to isolate external electromagnetic interference and protect the stability of the optical signal transmission within the optical fiber sensor. The inner metal film and the outer electromagnetic shielding layer are isolated by an insulating protective layer to avoid electrical coupling while ensuring that the ultrasonic signal can penetrate the insulating protective layer and act on the inner metal film, achieving a balance between signal selective response and anti-interference capability. The self-shielding structure of the double-layer metal film can fully utilize the functions of the inner metal film and the outer electromagnetic shielding layer, effectively shielding electromagnetic noise in complex environments while ensuring efficient detection of ultrasonic signals.

[0043] According to an embodiment of the present invention, the flatness of the exposed area of ​​the core is less than or equal to 5 nanometers. The fiber Bragg grating is bent into an arc shape according to a preset curvature radius, which can be set according to actual conditions, for example, the preset curvature radius can be set to 3 centimeters.

[0044] According to an embodiment of the present invention, a metal film is plated on the exposed area of ​​the fiber core by a magnetron sputtering process or a chemical vapor deposition process, and an insulating protective layer is plated on the metal film by a magnetron sputtering process or a chemical vapor deposition process.

[0045] Figure 2A FIG2 shows a schematic structural diagram of an optical fiber sensor based on surface plasmon resonance according to an embodiment of the present invention; Figure 2B A schematic diagram showing the working principle of an optical fiber sensor based on surface plasmon resonance according to an embodiment of the present invention is shown; Figure 2C A schematic diagram of the electromagnetic shielding effect of an optical fiber sensor based on surface plasmon resonance according to an embodiment of the present invention is shown.

[0046] like Figure 2A As shown, a fiber Bragg grating (FBG) system includes a core with a fiber Bragg grating (FBG) and a fiber cladding surrounding the core. An ultrasonic signal 7 is incident from the outside of the fiber sensor corresponding to the exposed core region. After passing through the electromagnetic shielding layer, insulating protective layer, and metal film, it is incident on the FBG through the exposed core region.

[0047] like Figure 2BAs shown, the thickness of the metal film ranges from 10 nanometers to 30 nanometers. For example, after the metal film is coated on the exposed area of ​​the fiber core, the metal film is polished so that the thickness of the polished metal film is on the order of 10 nm. The thickness of the metal film, the insulating protective layer, and the electromagnetic shielding layer can all be selected to be 10 nanometers. The ultrasonic signal passes through the electromagnetic shielding layer, the insulating protective layer, and the metal film in sequence, and excites plasma resonance at the interface between the exposed area of ​​the fiber core and the metal film through the evanescent field. The fiber Bragg grating provides a wave vector to assist the optical signal in the communication band to resonate with the surface plasmon at the interface between the exposed area of ​​the fiber core and the metal film. The strain field of the ultrasonic signal to be measured affects the coupling excitation conditions of the surface plasmon by changing the period of the FBG, thereby completing high-sensitivity detection of the ultrasonic signal to be measured. That is to say, the strain field of the ultrasonic signal to be measured periodically stretches the fiber Bragg grating, changes the period of the fiber Bragg grating, and affects the coupling excitation conditions of the surface plasmon. Fiber Bragg gratings of different periods will excite surface plasmons of different modes. The wavelength of the resonant wave of the surface plasmon-coupled optical signal shifts, causing the phase of the optical signal to change. The optical signal after the phase change is used as the output signal, and the ultrasonic signal to be measured is detected based on the output signal.

[0048] According to an embodiment of the present invention, the core of the existing optical fiber sensor is wrapped by the optical fiber cladding and has no exposed core area. The propagation of the strain field of the ultrasonic signal will be blocked by the optical fiber cladding, which reduces the sensitivity of the optical fiber sensor. Figure 2C As shown in the figure, SP refers to the surface plasmon obtained by plasma resonance (SPR); light refers to the optical signal in the fiber core; the optical fiber sensor provided by the present invention is respectively plated with an insulating protective layer and an electromagnetic shielding layer on the outer layer of the metal film supporting SPR, and the metal film is used instead of the optical fiber cladding to eliminate the strain field blocking effect of the cladding; the strain field of the ultrasonic signal can penetrate the electromagnetic shielding layer, changing the coupling characteristics of the optical signal and SPR, while the electric field lines of the external electromagnetic interference cannot penetrate the electromagnetic shielding layer, thereby achieving "electromagnetic shielding" of the exposed areas of the metal film and the fiber core.

[0049] Figure 3 A flow chart of a method for preparing an optical fiber sensor based on surface plasmon resonance according to another embodiment of the present invention is shown.

[0050] like Figure 3 As shown, the preparation method includes operations S310 to S340.

[0051] In operation S310, the fiber Bragg grating is bent into an arc shape according to a preset curvature radius; the fiber Bragg grating includes a core having a fiber Bragg grating and a fiber cladding sleeved on the core; the fiber cladding has an opening so that a portion of the fiber Bragg grating is exposed to the outside of the fiber cladding, forming a core exposed area.

[0052] In operation S320, a metal film is plated on the exposed region of the fiber core.

[0053] In operation S330, an insulating protective layer is coated on the metal film.

[0054] In operation S340, the electromagnetic shielding layer is covered with the insulating protective layer; wherein, after the ultrasonic signal to be measured passes through the electromagnetic shielding layer, the insulating protective layer and the metal film, it is incident on the fiber Bragg grating through the exposed area of ​​the fiber core, and the strain field of the ultrasonic signal to be measured periodically changes the period of the fiber Bragg grating, so that the phase of the optical signal in the fiber core changes, and the ultrasonic signal to be measured is detected by the optical signal after the phase change.

[0055] Figure 4 A schematic diagram of the preparation process of an optical fiber sensor based on surface plasmon resonance according to another embodiment of the present invention is shown.

[0056] like Figure 4 As shown, first the original fiber Bragg grating is macro-bent, and then the original fiber Bragg grating is side-polished at the location of the fiber Bragg grating, that is, the outer area of ​​the original fiber Bragg grating after macro-bending is polished, and a constant pressure polishing method is adopted to remove the fiber cladding, so that the fiber Bragg grating is exposed, forming a core exposure area, so that the light evanescent field can leak to the surface of the object to be measured, and the surface of the object to be measured will emit a measured ultrasonic signal. A layer of metal film is coated on the core exposure area, and the surface plasma resonance effect of the metal film is used to enhance the signal response of the ultrasonic signal. Subsequently, on the basis of the metal film, a layer of insulating protective layer is coated to isolate the influence of the external environment and protect the stability and durability of the sensor structure. The material of the insulating protective layer can be selected from silicon dioxide or magnesium fluoride. Finally, an electromagnetic shielding layer is added to the outside of the insulating protective layer to effectively isolate the interference of the strong external electromagnetic field through the skin effect, ensuring the stable excitation of the SPR effect, thereby obtaining the optical fiber sensor provided by the present invention.

[0057] According to an embodiment of the present invention, the fiber cladding blocks the transmission of the ultrasonic strain field, reducing the sensor's sensitivity. To overcome this problem, the macrobent fiber Bragg grating is side-polished to remove the cladding, allowing the ultrasonic signal to act directly on the exposed core area, which is also the sensitive area of ​​the fiber core. Side-polishing removes the fiber cladding, reducing its barrier to the transmission of the ultrasonic strain field.

[0058] According to an embodiment of the present invention, before polishing, the original fiber Bragg grating will be processed into a macro-bend structure, which can cause the light field in the fiber core to leak into the fiber cladding area. The exposed area of ​​the core after polishing can significantly improve the coupling efficiency between the light field and the ultrasonic signal to be measured. The macro-bend structure can also make the evanescent field of the optical signal more easily excited, thereby providing a basis for the SPR effect; during the polishing process, a grinding wheel is used to polish the fiber Bragg grating after macro-bending, and the pressure between the fiber Bragg grating after macro-bending and the grinding wheel must be kept constant to avoid uneven polishing depth and surface smoothness, which affects the subsequent optical signal coupling. At the same time, the polishing time and depth need to be precisely controlled, and the polishing depth must match the metal film sputtering thickness and the period parameter of the fiber Bragg grating. For example, when polishing, the distance between the two farthest points in the exposed area of ​​the core obtained after polishing is selected to be 8 mm. When the distance between the two farthest points in the exposed area of ​​the core is 8 mm, it is considered that the corresponding polishing depth is appropriate.

[0059] According to embodiments of the present invention, surface plasmon resonance (SPR) is a phenomenon in which electrons on a metal surface collectively oscillate. When a light signal enters a metal film through the evanescent field of a fiber Bragg grating (FBG), it can stimulate the SPR effect at the metal / fiber interface. Macrobending the FBG to produce a macrobent FBG allows the light signal to leak through the fiber cladding into the external dielectric metal film, insulating protective layer, and electromagnetic shielding layer. This allows for higher coupling efficiency between the light signal in the fiber core and the strain field generated by the ultrasonic signal.

[0060] According to an embodiment of the present invention, the core exposed area of ​​FBG and metal film, insulating protective layer and electromagnetic shielding layer constitute an SPR coupler, by which the optical fiber sensor is made more compact and integrated. The core exposed area of ​​FBG and the synergistic effect of metal film, insulating protective layer and electromagnetic shielding layer can optimize the collection and transmission of signals, and does not require additional complex circuits and equipment, thereby reducing the complexity and cost of the system and improving the convenience of installation and maintenance. It is suitable for small-scale equipment and systems requiring high integration.

[0061] According to an embodiment of the present invention, the sides of a macrobent fiber Bragg grating are polished, and a metal film, an insulating protective layer, and an electromagnetic shielding layer are sequentially applied to the exposed core area. The metal film excites the SPR, and the electromagnetic shielding layer effectively isolates external electromagnetic interference through an "electromagnetic shielding" effect. Specifically, the combined thickness of the metal film, insulating protective layer, and electromagnetic shielding layer is approximately 1 micron. This ensures that ultrasonic signals excite the SPR through the combined action of the skin effect and the SPR effect, while significantly reducing the impact of external electromagnetic interference.

[0062] According to an embodiment of the present invention, SPR can enhance the response of the fiber Bragg grating to tiny strain changes caused by ultrasonic signals, so that the vibration caused by the ultrasonic signal can effectively change the reflection wavelength of the fiber Bragg grating, thereby changing the phase of the optical signal. The sensitivity of the optical fiber sensor is also related to the period of the fiber Bragg grating. The period of the fiber Bragg grating matches the wave vector of the SPR, thereby maximizing the coupling effect of the ultrasonic strain field on the optical signal. When the period of the fiber Bragg grating and the wave vector of the SPR are optimally matched, the energy of the strain field of the ultrasonic signal can be more effectively converted into changes in the optical signal, further improving the sensitivity of the optical fiber sensor to the ultrasonic signal, and also improving the ability to capture ultrasonic signals, making it possible to accurately capture weak ultrasonic signals, and enhancing the stability and measurability of the ultrasonic signal incident on the optical fiber sensor, thereby providing high-precision monitoring capabilities in complex environments.

[0063] According to an embodiment of the present invention, a metal film is plated on the exposed area of ​​the fiber core using a magnetron sputtering process or a chemical vapor deposition process. An insulating protective layer is plated on the metal film using a magnetron sputtering process or a chemical vapor deposition process. The magnetron sputtering process of the metal film can also use other materials for coating or surface treatment of the optical fiber, for example, nanomaterial coatings, including carbon nanotubes, metal nanoparticle coatings or polymer coatings, which can be achieved through process steps such as chemical deposition and solution immersion; the surface contact process can be used to change the distance between the fiber grating and the surface of the metal film, the surface of the insulating protective layer and the surface of the electromagnetic shielding layer, and can also be replaced by indirectly stimulating the SPR effect by adding a layer of insulating material or a tiny air gap.

[0064] According to an embodiment of the present invention, multiple fiber optic sensors can be deployed on the same optical fiber, and each fiber optic sensor can independently sense the ultrasonic signals of different objects to be measured, so that the fiber optic sensor can cover multiple monitoring points in a fiber optic transmission system, greatly improving the flexibility and efficiency of applying multiple fiber optic sensors. In particular, in applications that require simultaneous monitoring of multiple monitoring points, the wiring cost of multiple fiber optic sensors is reduced, providing an efficient solution.

[0065] Figure 5 A schematic diagram of the electric field distribution inside an optical fiber sensor based on surface plasmon resonance according to an embodiment of the present invention is shown.

[0066] like Figure 5As shown in the figure, the horizontal axis is the radius of curvature and the vertical axis is the electric field intensity. As the radius of curvature decreases, that is, along the direction of the horizontal axis, the electric field intensity at different locations inside the fiber optic sensor also varies. The electric field intensity of the external electric field corresponding to the external electromagnetic interference at the location of the object to be measured is relatively high. Because the electromagnetic shielding layer effectively isolates the strong external electromagnetic field interference through the skin effect, the electric field intensity of the electromagnetic shielding layer decreases rapidly, and the electric field intensity within the insulating protective layer is also relatively low. After the surface plasmons at the interface between the metal film and the exposed area of ​​the fiber core are excited, the corresponding electric field intensity increases slightly, while the electric field intensity within the fiber Bragg grating remains relatively low.

[0067] According to an embodiment of the present invention, external electromagnetic field may change the dielectric properties of metal film by nonlinear effect, thereby affecting the excitation conditions of SPR;Under the action of strong electromagnetic field, the nonlinear effect of metal film and optical fiber can further interfere with the stability of SPR excitation. In addition, the interaction between external electromagnetic field and surface electron resonance may cause the offset, intensity variation and half-height width of SPR resonance peak to increase, and these phenomena all reduce the sensitivity of sensor. Therefore, the present invention enables optical fiber sensor to stably operate in a strong electromagnetic environment by metal film, insulating protective layer and electromagnetic shielding layer.

[0068] The above describes embodiments of the present invention. However, these embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention. Although each embodiment has been described separately above, this does not mean that the measures in each embodiment cannot be advantageously used in combination. Without departing from the scope of the present invention, those skilled in the art may make various substitutions and modifications, which should all fall within the scope of the present invention.

Claims

1. An optical fiber sensor based on surface plasmon resonance, characterized in that: The optical fiber sensor comprises: A fiber Bragg grating (FBG) comprising a core having a fiber Bragg grating (FBG) and a fiber cladding disposed over the core; the fiber Bragg grating (FBG) is curved into an arc shape according to a preset curvature radius; the fiber cladding has an opening such that a portion of the fiber Bragg grating (FBG) is exposed outside the fiber cladding, forming a core exposed region; a metal film coated on the exposed region of the fiber core, for enhancing the optical response to an ultrasonic signal incident on the metal film by using surface plasmon resonance technology; an insulating protective layer coated on the metal film; An electromagnetic shielding layer, covering the insulating protective layer; the electromagnetic shielding layer is made of metal material; Among them, after the ultrasonic signal to be measured passes through the electromagnetic shielding layer, the insulating protective layer and the metal film, it is incident on the fiber Bragg grating through the exposed area of ​​the fiber core. The strain field of the ultrasonic signal to be measured periodically changes the period of the fiber Bragg grating, so that the phase of the optical signal in the fiber core changes, and the ultrasonic signal to be measured is detected by the optical signal after the phase change.

2. The optical fiber sensor based on surface plasmon resonance according to claim 1, characterized in that: The flatness of the exposed area of ​​the fiber core is less than or equal to 5 nanometers.

3. The optical fiber sensor based on surface plasmon resonance according to claim 1, characterized in that: A magnetron sputtering process or a chemical vapor deposition process is used to coat a metal film on the exposed area of ​​the fiber core.

4. The optical fiber sensor based on surface plasmon resonance according to claim 1, characterized in that An insulating protective layer is plated on the metal film using a magnetron sputtering process or a chemical vapor deposition process.

5. The optical fiber sensor based on surface plasmon resonance according to claim 1, characterized in that: The thickness of the metal film ranges from 10 nanometers to 30 nanometers.

6. The optical fiber sensor based on surface plasmon resonance according to claim 1, characterized in that: The material of the insulating protective layer is silicon dioxide or magnesium fluoride.

7. The optical fiber sensor based on surface plasmon resonance according to claim 1, characterized in that: The electromagnetic shielding layer is sleeved on the optical fiber Bragg grating coated with the insulating protective layer.

8. A method for preparing an optical fiber sensor based on surface plasmon resonance, characterized in that: The preparation method comprises: The fiber Bragg grating is bent into an arc shape according to a preset curvature radius; the fiber Bragg grating comprises a core having a fiber Bragg grating and a fiber cladding sleeved on the core; the fiber cladding has an opening so that a portion of the fiber Bragg grating is exposed outside the fiber cladding to form a core exposed area; Coating a metal film on the exposed area of ​​the fiber core; coating an insulating protective layer on the metal film; The electromagnetic shielding layer is covered on the insulating protective layer; the electromagnetic shielding layer is made of metal material; Among them, after the ultrasonic signal to be measured passes through the electromagnetic shielding layer, the insulating protective layer and the metal film, it is incident on the fiber Bragg grating through the exposed area of ​​the fiber core. The strain field of the ultrasonic signal to be measured periodically changes the period of the fiber Bragg grating, so that the phase of the optical signal in the fiber core changes, and the ultrasonic signal to be measured is detected by the optical signal after the phase change.

9. The method for preparing a surface plasmon resonance-based optical fiber sensor according to claim 8, wherein: A magnetron sputtering process or a chemical vapor deposition process is used to coat a metal film on the exposed area of ​​the fiber core.

10. The method for preparing a surface plasmon resonance-based optical fiber sensor according to claim 8, wherein: An insulating protective layer is plated on the metal film using a magnetron sputtering process or a chemical vapor deposition process.

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