Optical fiber sensor based on surface plasma resonance and preparation method thereof

Abstract

The invention provides an optical fiber sensor based on surface plasma resonance and a preparation method thereof, and relates to the technical field of sensors. The optical fiber sensor comprises an optical fiber grating, wherein the optical fiber grating comprises a fiber core with a fiber bragg grating and an optical fiber cladding sleeving the fiber core; the fiber bragg grating is bent into an arc shape according to a preset curvature radius; the optical fiber cladding is provided with an opening, so that one part of the optical fiber Bragg grating is exposed to the outer side of the optical fiber cladding to form a fiber core exposed area; the metal film is plated on the exposed area of the fiber core and is used for enhancing the optical response to an ultrasonic signal incident to the metal film by using a surface plasma resonance technology; the insulating protection layer is coated on the metal film; and the electromagnetic shielding layer covers the insulating protection layer. According to the invention, the sensitivity of detecting the ultrasonic signal generated by partial discharge of the electric power facility is improved.

Description

Technical Field

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

[0002] The arc discharge phenomenon generated by the power system poses a threat to the safe and stable operation of the power system. In addition to generating a large amount of light and heat during arc discharge, the rapidly changing current changes will also excite mechanical vibrations in the surrounding medium, forming ultrasonic signals. At present, ultrasonic monitoring of power facilities mainly uses piezoelectric ceramic ultrasonic sensors (PZT, Lead Zirconate Titanate), which convert ultrasonic signals into electrical signals through the piezoelectric effect. Due to the low power of the electrical signal, it is difficult to meet the monitoring needs in strong electromagnetic environments due to its sensitivity to electromagnetic interference, insufficient sensitivity, poor reusability, and difficulty in large-scale measurement. 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 an optical fiber sensor based on surface plasmon resonance, the optical fiber sensor comprising: an optical fiber grating, the optical fiber grating comprising a core having an optical fiber Bragg grating and an optical fiber cladding sleeved on the optical fiber core; the optical fiber grating is bent into an arc shape according to a preset curvature radius; the optical fiber cladding has an opening, so that a portion of the optical fiber Bragg grating is exposed to the outside of the optical fiber cladding to form a core exposure area; a metal film is coated on the core exposure area, and is used to enhance the optical response of an ultrasonic signal incident on the metal film by using surface plasmon resonance technology; an insulating protective layer is coated on the metal film; and an electromagnetic shielding layer is covered on 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 optical fiber Bragg grating via the core exposure area, and the strain field of the ultrasonic signal to be measured periodically changes the period of the optical fiber Bragg grating, so that the phase of the optical signal in the core changes, and the ultrasonic signal to be measured is detected by the optical signal after the phase change.

[0005] According to an embodiment of the present invention, the flatness of the above-mentioned core exposed area 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 used 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 range of the above metal film is from 10 nanometers to 30 nanometers.

[0009] According to an embodiment of the present invention, the material of the above insulation protection layer is silicon dioxide or magnesium fluoride.

[0010] According to an embodiment of the present invention, the above electromagnetic shielding layer is sleeved on the fiber grating coated with the above insulation protection layer.

[0011] Another aspect of the present invention provides a preparation method of an optical fiber sensor based on surface plasmon resonance. The above preparation method includes: bending the fiber grating into an arc shape according to a preset curvature radius; the above fiber grating includes a core having a fiber Bragg grating and a fiber cladding sleeved on the above core; the above fiber cladding has an opening, so that a part of the above fiber Bragg grating is exposed to the outside of the above fiber cladding, forming a core exposure area; plating a metal film on the above core exposure area; coating an insulation protection layer on the above metal film; covering an electromagnetic shielding layer on the above insulation protection layer; wherein, the ultrasonic signal to be measured passes through the above electromagnetic shielding layer, the above insulation protection layer and the above metal film, and then is incident on the above fiber Bragg grating through the above core exposure area. The strain field of the above ultrasonic signal to be measured periodically changes the period of the above fiber Bragg grating, so that the phase of the optical signal in the above core changes, 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 used to plate a metal film on the above core exposure area.

[0013] According to an embodiment of the present invention, a magnetron sputtering process or a chemical vapor deposition process is used to plate an insulation protection layer on the above metal film.

[0014] According to an embodiment of the present invention, by processing the fiber grating into a bent structure, the fiber Bragg grating is exposed to form a core exposure area, overcoming the obstacle of ultrasonic signal transmission; by plating a metal film on the core exposure area, the coupling efficiency between the optical signal in the fiber grating and the ultrasonic signal to be measured is improved, making the evanescent field of the optical signal easier to be excited, providing a basis for the surface plasmon resonance (SPR) effect; and by shielding the interference of the external environment through the insulation protection layer and the electromagnetic shielding layer, at least partially overcoming the technical problem of insufficient sensitivity in detecting ultrasonic signals in the prior art, and thus achieving the technical effect of improving the sensitivity of the optical fiber sensor. Description of the Drawings

[0015] Through the following description of the embodiments of the present invention with reference to the accompanying drawings, the above and other objects, features, and advantages of the present invention will become clearer. In the drawings:

[0016] Figure 1 A semi-sectional schematic diagram of a fiber optic sensor based on surface plasmon resonance according to an embodiment of the present invention is shown.

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

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

[0020] Figure 3 A flowchart of a preparation method of a fiber optic 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 a fiber optic sensor based on surface plasmon resonance according to still another embodiment of the present invention is shown.

[0022] Figure 5 A schematic diagram of the internal electric field distribution of a fiber optic sensor based on surface plasmon resonance according to an embodiment of the present invention is shown. Detailed Embodiments

[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 merely exemplary and are not intended to limit the scope of the present invention. In the following detailed description, for the sake of explanation, many specific details are set forth to provide a comprehensive understanding of the embodiments of the present invention. However, obviously, one or more embodiments can also be implemented without these specific details. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessarily confusing the concepts of the present invention.

[0024] The terms used herein are merely for describing specific embodiments and are not intended to limit the present invention. The terms "including", "comprising", etc. used herein indicate the presence of the described 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 of ordinary skill in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted to have a meaning consistent with the context of this specification, and should not be interpreted in an idealized or overly rigid manner.

[0026] In cases where expressions similar to "at least one of A, B, and C, etc." are used, generally, it should be interpreted according to the meaning commonly understood by those of ordinary skill in the art (for example, "a system having at least one of A, B, and C" should include, but not be limited to, a system having only A, only B, only C, having A and B, having A and C, having B and C, and / or having A, B, and C, etc.).

[0027] In the embodiments of the present invention, in aspects such as the collection, update, analysis, processing, use, transmission, provision, disclosure, storage, etc. of the data involved (for example, including but not limited to user personal information), they all comply with the provisions of relevant laws and regulations, are used for legal purposes, and do not violate public order and good customs. In particular, necessary measures are taken for user personal information to prevent illegal access to user personal information data and to maintain the security of user personal information and network security.

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

[0029] Electricity is the cornerstone of modern society. In the pursuit of high efficiency, convenience, and intelligence, a large number of electronic devices are widely used in various fields of production and life. As the usage time of electronic devices increases, electrical, thermal, and mechanical stresses, etc., may damage the materials inside power facilities, significantly reduce their insulation performance, may cause arc discharge phenomena, trigger faults, and pose a threat to the safe and stable operation of the power system.

[0030] In recent years, fiber optic ultrasonic sensors have attracted increasing attention from researchers due to their strong anti-electromagnetic interference ability, small size, light weight, good electrical insulation performance, corrosion resistance, high sensitivity, good multiplexing performance, and suitability for long-distance transmission. They can also achieve single-fiber multi-point detection and multi-parameter sensing. From high-voltage transmission lines to wireless communication base stations, from household appliances to smart wearable devices, all electrical equipment generates electromagnetic radiation during operation. Although fiber optic sensors can resist the influence of electromagnetic interference to a certain extent, with the broadening of the frequency range, increase in power, and improvement in sensitivity of electrical equipment, high-power transmission and high-sensitivity reception, while enabling the rapid transmission of information and efficient utilization of energy, inevitably form a complex strong electromagnetic environment. For example, near high-voltage insulators in the power transmission system, near high-voltage circuit breakers, switchgear, and contact points, the electric field intensity in high-voltage power cables can reach 1 MV / m - 3 MV / m, the electric field at some tip structures may exceed 10 MV / m, and the instantaneous electric field intensity during lightning discharge can reach 100 MV / m. This electric field intensity is equivalent to the transmission of a 10 W laser in a single-mode fiber, which easily causes problems such as electron cloud distortion and non-linear polarization in the fiber, limiting the monitoring ability of fiber optic ultrasonic sensors for weak signals and early faults in power facilities. Therefore, it is urgently necessary to further study the influence of the strong electromagnetic field environment on the ultrasonic strain response mechanism of fiber optic sensors, provide an effective monitoring tool for early fault detection and prevention, ensure the stability and reliability of the power system, promote the development of technologies such as clean energy, energy storage, and smart grids, and provide 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 defects:

[0032] 1. The obstruction of the fiber cladding to the transmission of vibration signals. In the existing technology, the presence of the fiber cladding is an important factor that prevents vibration signals from being effectively transmitted to the fiber core. As the external protective layer of the fiber, its physical properties (such as elastic modulus and thickness) will significantly weaken the energy transmission of acoustic vibrations. This "blocking effect" directly reduces the sensitivity of the sensor. Especially when detecting weak vibration signals, the performance of traditional fiber optic sensors is difficult to meet the requirements of high-precision monitoring.

[0033] 2. Interference of strong electromagnetic fields on signals. SPR is a resonance phenomenon of light and free electrons on the metal surface occurring at the metal-dielectric interface. Its core is to achieve signal amplification through the coupling of the surface charge density wave of the metal thin film and the photoelectric field, which can effectively improve the sensing sensitivity. Although existing fiber optic sensors use light as the transmission medium and 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 or around the fiber optic still will be interfered by the external electromagnetic field. This interference is manifested in the following aspects: (1) Abnormal surface electron oscillation: The strong electromagnetic field will change the oscillation state of free electrons on the metal thin film surface, resulting in signal distortion of surface plasmon resonance (SPR). (2) Nonlinear polarization effect: The strong electromagnetic field may trigger the nonlinear polarization effect inside the fiber optic, further interfering with the stability of the optical signal. (3) Decrease in signal-to-noise ratio: The superposition of external interference noise significantly reduces the signal-to-noise ratio (SNR), 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) mainly focus on the range of 50 nanometers to 200 nanometers in thickness to ensure the excitation of the SPR effect. However, this film thickness is not sufficient to shield the interference of strong electromagnetic fields, resulting in a significant decline in sensor performance in complex environments. In addition, the non-uniformity of the film thickness and the insufficient adhesion may also cause performance degradation of the sensor during long-term use.

[0035] 4. Insufficient anti-interference ability of traditional sensors. Although existing SPR fiber optic sensors can partially reduce the interference effect through signal adjustment algorithms, the signal adjustment process is complex and depends on the stability of the algorithm, and cannot completely solve the electromagnetic interference problem from a physical level. In addition, the traditional sensor design does not comprehensively optimize multiple interferences in the external environment (such as the transmission blockage of vibration signals and the superimposed effect of electromagnetic interference), resulting in limitations in stability and sensitivity in practical applications.

[0036] Figure 1 Fig. shows a half-sectional schematic view of a fiber optic sensor based on surface plasmon resonance according to an embodiment of the present invention.

[0037] As Figure 1As shown in the figure, an embodiment of the present invention provides an optical fiber sensor based on surface plasmon resonance. The optical fiber sensor includes: an optical fiber grating, which includes a core 1 having an optical fiber Bragg grating 6 and an optical fiber cladding 3 sleeved on the core 1; the optical fiber grating is bent into an arc shape according to a preset curvature radius; the optical fiber cladding 3 has an opening, so that a part of the optical fiber Bragg grating 6 is exposed to the outside of the optical fiber cladding 3, forming a core exposure area; a metal film 5, plated on the core exposure area, for enhancing the optical response to the ultrasonic signal incident on the metal film 5 by using the surface plasmon resonance technology; an insulating protective layer 4, coated on the metal film 5; an electromagnetic shielding layer 2, covering the insulating protective layer 4; wherein, the ultrasonic signal to be measured passes through the electromagnetic shielding layer 2, the insulating protective layer 4 and the metal film 5, and then is incident on the optical fiber Bragg grating 6 through the core exposure area. The strain field of the ultrasonic signal to be measured periodically changes the period of the optical fiber Bragg grating 6, so that the phase of the optical signal in the core 1 changes, 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, the optical fiber grating includes a core 1 having an optical fiber Bragg grating 6 and an optical fiber cladding 3 sleeved on the core 1. The FBG (Fiber Bragg Grating) is the core sensitive unit of the optical fiber sensor, and its periodic refractive index change structure can reflect optical signals of a specific wavelength. The Bragg wavelength of the optical fiber Bragg grating 6 is highly sensitive to the strain change caused by external vibration, and can sense tiny ultrasonic signals generated by vibration through wavelength shift. In order to enhance the transmission efficiency of the ultrasonic signal, the optical fiber cladding 3 is removed from the sensitive area of the optical fiber grating, so that the core 1 of the optical fiber grating is directly exposed, forming a core exposure area. This design reduces the barrier of the optical fiber cladding 3 to the ultrasonic strain field, enables the ultrasonic signal to be measured to directly act on the core 1 of the optical fiber grating, and greatly improves the sensitivity of the optical fiber sensor.

[0039] According to an embodiment of the present invention, a metal film is plated on the core exposure area where the optical fiber cladding is removed. The magnetron sputtering or chemical vapor deposition (CVD) process is adopted in the plating process to ensure the thickness uniformity of the metal film and the good adhesion to the core.

[0040] According to an embodiment of the present invention, the material of the insulating protective layer is silicon dioxide or magnesium fluoride. A 1-micron-thick silicon dioxide (SiO 2 ) or magnesium fluoride (MgF 2An insulating protective layer. The material selection of the insulating protective layer is based on the following characteristics: silicon dioxide has high chemical stability and is suitable for various complex environments (such as high humidity or acid-base environments), and at the same time has good mechanical strength. Magnesium fluoride, as a low refractive index material, can further reduce optical loss and has good corrosion resistance. The electromagnetic shielding layer is sleeved on the fiber grating coated with the insulating protective layer. The insulating protective layer can isolate conductive interference in the external environment, ensure that the metal film is not directly affected by the external electric field, and at the same time protect the bare fiber core and the metal film, so that the fiber optic sensor is protected from mechanical damage caused by external force factors or environmental factors.

[0041] According to an embodiment of the present invention, the outermost layer of the fiber optic sensor is covered with an electromagnetic shielding layer, which is usually made of a metal material (such as copper or aluminum) with a thickness of several micrometers. The electromagnetic shielding layer is located on the outermost layer of the fiber optic sensor and is used to shield the interference of the external electromagnetic field. Through the skin effect, the electromagnetic shielding layer can effectively prevent the external high-frequency electromagnetic field from entering the inside of the fiber optic sensor, and at the same time ensure that the SPR effect excited by the metal film is not interfered by the outside world. The metal film realizes high-sensitivity perception of external ultrasonic signals by exciting SPR. The electromagnetic shielding layer can effectively shield the interference of the external strong electromagnetic field, protect the stability of the optical signal in the fiber core, and has good corrosion resistance under most environmental conditions, extending the service life of the fiber optic sensor. In addition, the design of the electromagnetic shielding layer takes into account both conductivity and mechanical strength to ensure its durability and reliability in complex environments. The surface of the electromagnetic shielding layer can also be subjected to anti-corrosion treatment to adapt to various harsh environments, such as humid, high-temperature or strong acid-base environments. The introduction of the electromagnetic shielding layer further improves the working stability of the fiber optic sensor in a strong electromagnetic interference environment and ensures that the strain field of the ultrasonic signal is accurately detected.

[0042] According to an embodiment of the present invention, in order to simultaneously achieve the high-sensitivity signal detection ability of the fiber optic sensor and the anti-interference ability in a strong electromagnetic environment, a self-shielding structure of a double-layer metal thin film including an inner metal film and an outer electromagnetic shielding layer is designed. Among them, the inner metal film is used to excite 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 inside the fiber optic sensor. The inner metal film and the outer electromagnetic shielding layer are isolated by an insulating protective layer to avoid electrical coupling, and at the same time ensure that the ultrasonic signal can penetrate the insulating protective layer and act on the inner metal film to achieve a balance between signal selective response and anti-interference ability. The self-shielding structure of the double-layer metal thin film can make full use of the functions of the inner metal film and the outer electromagnetic shielding layer, effectively shielding electromagnetic noise in the complex environment and ensuring the efficient detection of ultrasonic signals at the same time.

[0043] According to an embodiment of the present invention, the flatness of the core exposed area is less than or equal to 5 nanometers. The fiber grating is bent into an arc shape according to a preset curvature radius, and the preset curvature radius 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 deposited on the core exposed area by applying a magnetron sputtering process or a chemical vapor deposition process. An insulating protective layer is deposited on the metal film by applying a magnetron sputtering process or a chemical vapor deposition process.

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

[0046] As Figure 2A shown, the fiber grating includes a core having a fiber Bragg grating and an optical fiber cladding sleeved on the core. The ultrasonic signal 7 is incident from the outside of the optical fiber sensor corresponding to the core exposed area, and after passing through the electromagnetic shielding layer, the insulating protective layer and the metal film, it is incident on the fiber Bragg grating through the core exposed area.

[0047] As Figure 2B shown, the thickness range of the metal film is 10 nanometers to 30 nanometers. For example, after the metal film is deposited on the core exposed area, the metal film is polished so that the thickness of the polished metal film is on the order of 10 nm. The thicknesses 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 plasma resonance is excited at the interface between the core exposed area 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 core exposed area and the metal film. The strain field of the ultrasonic signal to be measured affects the coupling excitation condition of the surface plasmon by changing the period of the FBG, and high-sensitivity detection of the ultrasonic signal to be measured is completed. 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, affects the coupling excitation condition of the surface plasmon, and fiber Bragg gratings with different periods will excite different modes of surface plasmons. The wavelength of the resonant wave of the surface plasmon-coupled optical signal shifts, causing a change in the phase of the optical signal. The optical signal with the changed phase is used as the output signal, and the ultrasonic signal to be measured is detected according to the output signal.

[0048] According to an embodiment of the present invention, the core of an existing optical fiber sensor is wrapped by an optical fiber cladding and does not have a core exposed area. The propagation of the strain field of the ultrasonic signal will be blocked by the optical fiber cladding, reducing the sensitivity of the optical fiber sensor. As Figure 2C shown, in the figure, SP refers to surface plasmons obtained by surface plasmon resonance SPR; light refers to the optical signal in the core; the optical fiber sensor provided by the present invention is respectively coated with an insulating protective layer and an electromagnetic shielding layer on the outer layer of the metal film supporting SPR. Using the metal film instead of the optical fiber cladding can eliminate the strain field blocking effect of the cladding; the strain field of the ultrasonic signal can penetrate the electromagnetic shielding layer and change the coupling characteristics of the optical signal and SPR, while the electric field lines of external electromagnetic interference cannot penetrate the electromagnetic shielding layer, thereby realizing "electromagnetic shielding" for the metal film and the core exposed area.

[0049] Figure 3 The flowchart of the preparation method of the optical fiber sensor based on surface plasmon resonance according to another embodiment of the present invention is shown.

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

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

[0052] In operation S320, a metal film is deposited on the core exposed area.

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

[0054] In operation S340, an electromagnetic shielding layer is covered on the insulating protective layer; wherein, the ultrasonic signal to be measured passes through the electromagnetic shielding layer, the insulating protective layer and the metal film, and then enters the fiber Bragg grating through the core exposed area. 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.

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

[0056] As Figure 4As shown, first, the original fiber Bragg grating is subjected to macro-bending. Then, the original fiber Bragg grating is side-polished at the position where the fiber Bragg grating is located, that is, the outer region of the original fiber Bragg grating after macro-bending is polished. The constant-pressure polishing method is adopted to remove the fiber cladding, expose the fiber Bragg grating, and form a core-exposed area, so that the optical evanescent field can leak to the surface of the object to be measured, and the object to be measured surface will emit the ultrasonic signal to be measured. A metal film is deposited on the core-exposed area, and the surface plasmon 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, an insulating protective layer is deposited to isolate the influence of the external environment and protect the stability and durability of the sensor structure. Among them, the material of the insulating protective layer can be selected from silicon dioxide or magnesium fluoride. Finally, an electromagnetic shielding layer is added outside the insulating protective layer, and the skin effect is used to effectively isolate the interference of the external strong electromagnetic field and ensure the stable excitation of the SPR effect, thereby obtaining the fiber optic sensor provided by the present invention.

[0057] According to the embodiments of the present invention, the fiber cladding has a blocking effect on the transmission of the ultrasonic strain field, reducing the sensitivity of the sensor. To overcome this problem, the fiber Bragg grating after macro-bending is side-polished to remove the cladding, so that the ultrasonic signal can directly act on the core-exposed area, which is also the sensitive area of the fiber core. By side-polishing to remove the fiber cladding, the blocking of the ultrasonic strain field transmission by the cladding is reduced.

[0058] According to the embodiments of the present invention, before polishing, the original fiber Bragg grating is processed into a macro-bending structure, which can make the optical field in the core leak to the fiber cladding area. The core-exposed area after polishing can significantly improve the coupling efficiency between the optical field and the ultrasonic signal to be measured. The macro-bending structure can also make the evanescent field of the optical signal easier to be excited, 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 needs to be kept constant to avoid uneven polishing depth and surface smoothness, which will affect the subsequent optical signal coupling. At the same time, the polishing time and depth need to be precisely controlled, and the polishing depth is matched with the sputtering thickness of the metal film and the period parameters of the fiber Bragg grating. For example, when polishing, the distance between the two farthest points in the core-exposed area obtained after polishing is selected to be 8 mm. When the distance between the two farthest points in the core-exposed area is 8 mm, it is considered that the corresponding polishing depth is appropriate.

[0059] According to an embodiment of the present invention, surface plasmon resonance is a phenomenon of collective oscillation of electrons on a metal surface. When an optical signal enters a metal film through the evanescent field of a fiber grating, an SPR effect can be excited at the metal / fiber interface. The fiber grating is macro-bent to obtain a macro-bent fiber grating. The macro-bent fiber grating can leak the optical signal through the fiber cladding to the external dielectric metal film, the insulating protective layer and the electromagnetic shielding layer. The optical signal in the fiber core has a higher coupling efficiency with the strain field generated by the ultrasonic signal.

[0060] According to an embodiment of the present invention, the core exposed area of ​​the fiber grating and the metal film, the insulating protective layer and the electromagnetic shielding layer constitute an SPR coupler, through which the optical fiber sensor is made more compact and integrated. The synergy of the core exposed area of ​​the fiber grating and the metal film, the insulating protective layer and the electromagnetic shielding layer can optimize the acquisition and transmission of signals, does not require additional complex circuits and equipment, reduces the complexity and cost of the system, and improves the convenience of installation and maintenance. It is suitable for small devices and systems requiring high integration.

[0061] According to an embodiment of the present invention, the side of the optical fiber grating after macro bending is polished, and a metal film, an insulating protective layer and an electromagnetic shielding layer are sequentially arranged in the exposed area of ​​the core, the metal film can excite SPR, and the electromagnetic shielding layer can effectively isolate external electromagnetic interference through the "electromagnetic shielding" effect. Specifically, the total thickness of the metal film, the insulating protective layer and the electromagnetic shielding layer is about 1 micron, and the skin effect and the SPR effect can be used together to ensure the excitation of the SPR by the ultrasonic signal, while the influence of external electromagnetic interference is significantly reduced.

[0062] According to an embodiment of the present invention, SPR can enhance the response of the fiber Bragg grating to the small strain changes caused by the ultrasonic signal, 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, and 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 matches the wave vector of the SPR to the best, the energy of the strain field of the ultrasonic signal can be more effectively converted into a change in the optical signal, further improving the sensitivity of the optical fiber sensor to the ultrasonic signal, and also improving the capture ability of the ultrasonic signal, which can accurately capture weak ultrasonic signals, and enhance the stability and measurability of the ultrasonic signal incident into 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 chemical deposition, solution immersion and other process steps; 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 adding a layer of insulating material or a tiny air gap to indirectly stimulate the SPR effect.

[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, especially in applications that need to monitor multiple monitoring points at the same time, reducing the wiring cost of multiple fiber optic sensors, and providing an efficient solution.

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

[0066] like Figure 5 As shown in the figure, the abscissa is the radius of curvature and the ordinate is the electric field strength. As the radius of curvature decreases, that is, along the direction of the abscissa axis, the electric field strength at different positions inside the optical fiber sensor is also different. The electric field strength of the external electric field corresponding to the external electromagnetic interference at the location of the object to be measured is relatively high. Since the electromagnetic shielding layer effectively isolates the strong external electromagnetic field interference through the skin effect, the electric field strength of the electromagnetic shielding layer decreases rapidly, and the electric field strength in the insulating protective layer is also relatively small. After the surface plasmons at the interface of the metal film and the exposed area of ​​the fiber core are excited, the corresponding electric field strength increases slightly, while the electric field strength in the fiber grating is still relatively small.

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

[0068] The embodiments of the present invention have been described above. However, these embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention. Although the embodiments have been described separately above, this does not mean that the measures in each embodiment cannot be used advantageously in combination. Without departing from the scope of the present invention, those skilled in the art can make various substitutions and modifications, and these substitutions and modifications should 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, wherein the fiber Bragg grating comprises a core having a fiber Bragg grating and a fiber cladding sleeved on the core; the fiber Bragg grating is bent into an arc shape according to a preset curvature radius; the fiber cladding has an opening, so that a portion of the fiber Bragg grating is exposed to the outside of the fiber cladding to form a core exposure area; 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 a surface plasmon resonance technique; An insulating protective layer coated on the metal film; An electromagnetic shielding layer, covering the insulating protective layer; Among them, after the ultrasonic signal to be measured passes through the electromagnetic shielding layer, the insulating protection layer and the metal film, it is incident on the fiber Bragg grating through the exposed area of ​​the 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 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 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 by 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 protection 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 grating coated with the insulating protection 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 to the outside of the fiber cladding to form a core exposure area; Coating a metal film on the exposed area of ​​the fiber core; Coating an insulating protective layer on the metal film; Covering the electromagnetic shielding layer on the insulating protective layer; Among them, after the ultrasonic signal to be measured passes through the electromagnetic shielding layer, the insulating protection layer and the metal film, it is incident on the fiber Bragg grating through the exposed area of ​​the 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 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 optical fiber sensor according to claim 8, 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.

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

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