Gradient strain coupled FBG heat flux density sensor preparation method
By fabricating a gradient strain-coupled metal layer on the surface of FBG, the problems of anti-interference and real-time measurement of electrical heat flux density sensors in complex environments are solved, realizing high-performance heat flux density sensing, which is applicable to fields such as mechanical engineering, bridge and tunnel engineering, oil and gas, aerospace and environmental monitoring.
Patent Information
- Application Number
- CN202610201502.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-02-11
- Publication Date
- 2026-05-26
AI Technical Summary
Existing electrical heat flux density sensors are difficult to operate for long periods under complex boundary conditions, are susceptible to electromagnetic interference, have poor environmental adaptability, are difficult to distribute, and traditional FBG heat flux density measurement methods cannot achieve real-time monitoring.
Microgrooves FBG were prepared by hydrofluoric acid etching, and a high-bonding-strength composite metal layer with gradient strain coupling was prepared on its surface. The heat flux density was reflected by spectral broadening, and a non-uniform distribution of the equivalent thermal expansion coefficient was constructed to achieve high-performance sensing of heat flux density.
It improves the sensor's resistance to electromagnetic interference and measurement stability, reduces dependence on absolute temperature drift, enhances measurement accuracy and reliability in complex thermal environments, and expands the application range.
Smart Images
Figure CN122084155A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to, but is not limited to, the technical field of fiber optic heat flux density sensing, and particularly relates to a method for fabricating a gradient strain-coupled FBG heat flux density sensor. Background Technology
[0002] Temperature is a crucial parameter for measuring the state of a thermal system and has always held a primary position in thermodynamics and heat transfer. However, with the development of thermal management system theory and applications, using temperature alone as the sole parameter for measuring a thermal system is no longer sufficient to meet the needs of thermal analysis. Monitoring heat flux density helps to more accurately describe the heat transfer state of various thermal systems.
[0003] Currently, relatively mature heat flux density sensors based on electrical characteristics are widely used in various industries. However, when using electrical heat flux density sensors to measure heat flux density (v) under complex boundary conditions with multi-field coupling, the complex system structure, harsh working environment, and inherent characteristics of electrical sensor elements such as susceptibility to electromagnetic interference, weak environmental adaptability, difficulty in distributed deployment, complex wiring, and heavy shielding wires make it difficult to operate for extended periods in actual monitoring. Measurement methods based on fiber Bragg grating (FBG) sensing technology have advantages such as inherent resistance to electromagnetic interference, ease of installation, small added weight, and reusable networking. Although FBG packaging methods exist for measuring various parameters such as temperature, strain, vibration, pressure, and flow, there is still a gap in FBG heat flux density measurement packaging methods. Detecting heat flux density through the temperature-time curve of a single FBG temperature measuring point is not suitable for real-time heat flux density measurement. Real-time monitoring of the temperature field through multiple FBG temperature measuring points places high demands on the number of measuring points, the layout scheme, and the complexity of the temperature field of the test object.
[0004] Therefore, there is an urgent need for a real-time, single-point FBG heat flux density sensor.
[0005] To address the aforementioned issues, this patent proposes a method for fabricating a gradient strain-coupled FBG heat flux density sensor. This method involves fabricating a microgroove FBG using hydrofluoric acid etching and then preparing a high-bonding-strength composite metal layer with gradient strain coupling on the FBG surface. This improves the bonding strength between the heterogeneous materials and the spectral broadening of the FBG under the influence of heat flux density. The full width at half maximum (FWHM) of the FBG's reflection spectrum reflects the heat flux density of the gate region, enabling high-performance sensing of heat flux density for various structural parameters of the FBG. The structural characteristics of the composite metal layer can be flexibly controlled to adapt to different working environments. This FBG heat flux density sensor and its fabrication method have significant practical implications for improving the electromagnetic interference resistance, reliability, and expanding the application range of heat flux density sensors. Summary of the Invention
[0006] The purpose of this invention is to address the above-mentioned shortcomings in the prior art by providing a method for fabricating a gradient strain-coupled FBG heat flux density sensor, so as to solve or mitigate at least one of the problems in the prior art.
[0007] This invention is implemented as follows: a method for fabricating a gradient strain-coupled FBG heat flux density sensor, the method comprising:
[0008] 1) Hydrofluoric acid etching micromachining: After removing the coating layer of the optical fiber and cleaning it with an ultrasonic cleaner, the surface of the optical fiber is wiped clean with a small amount of alcohol on absorbent paper. A grating area (FBG) is placed at the center of the optical fiber. A 40% hydrofluoric acid solution is dripped into the etching area using a dropper and left to stand for a period of time. The cladding is etched to form a stepped microgroove structure in the optical fiber. As the etching time increases, the cladding gradually becomes thinner. After 20 minutes, it is taken out and placed in water for ultrasonic cleaning. The surface of the optical fiber is wiped clean with a small amount of alcohol on absorbent paper. The stepped micromachining FBG is successfully prepared. At this time, the cladding diameter is about 95 μm and the microgroove depth is 15 μm.
[0009] 2) Magnetron sputtering of oxygen-loving metal layer: The stepped micro-machined FBG was placed on a magnetron sputtering platform. The stepped micro-machined FBG was limited and fixed using a magnetron sputtering fixture. After vacuuming, an oxygen-loving metal layer with a thickness of 0.5-2 μm was sputtered on the surface of the stepped micro-machined FBG through a magnetron sputtering nozzle to enhance the adhesion between the metal layer and the optical fiber. The stepped metallized FBG was successfully prepared.
[0010] 3) Variable diameter electroplating gradient strain coupled metal layer: The stepped metallized FBG is placed in deionized water and ultrasonically cleaned for 3 min to remove surface impurities. After cleaning, it is placed in a vacuum dish to dry. Then, it is placed in an electroplating tank to electroplat a temperature-sensitive metal layer. During the electroplating process, the electroplating liquid level is gradually lowered by a peristaltic pump to form a gradient strain coupled temperature-sensitive metal layer on the surface of the oxygen-loving metal layer. After electroplating, the metallized FBG is taken out, ultrasonically cleaned, vacuum dried, and then the encapsulation process is completed to obtain a gradient strain coupled metallized FBG. The diameter of the metal layer changes nonlinearly along the light axis, so that the equivalent thermal expansion coefficient of the gradient strain coupled metallized FBG changes linearly along the light axis.
[0011] Furthermore, in step 1), a microgroove structure with a depth of 15 µm is formed on the surface of the FBG by hydrofluoric acid etching, so that the oxygen-loving metal layer and the fiber step-groove structure form a heterogeneous material mechanical connection structure. The resulting step-microfabricated FBG can improve the bonding strength of heterogeneous materials, and the high bonding strength between the oxygen-loving metal and the fiber can prevent the metal layer from falling off. The temperature-sensitive metal layer coupled by gradient strain can improve the spectral broadening of the FBG affected by heat flux density, optimize the heat flux density sensing performance of the gradient strain coupled metallized FBG, and at the same time improve the mechanical properties of the FBG to protect the fiber from breakage.
[0012] Furthermore, in step 1), the concentration of the hydrofluoric acid solution is 40%, the standing time is 20 min, the corrosion depth is 15 µm, and the corrosion width is 100-500 µm.
[0013] Furthermore, in step 2), the magnetron sputtering fixture can fix and cover both ends of the optical fiber, exposing only the grating region, which can effectively avoid the problem of difficulty in determining the position of the grating region after magnetron sputtering. The magnetron sputtering fixture can clamp 0-50 FBGs at a time. Since the shear strength of the optical fiber is low and multiple optical fibers may slip and cause inaccurate positioning of the grating region when fixed together, they are pasted in a fish-scale pattern from top to bottom in the magnetron sputtering fixture to facilitate the removal of the metallized FBGs after magnetron sputtering.
[0014] Furthermore, the magnetron sputtering equipment in step 2) is a PD-200C, which magnetron sputters an ultrathin oxygen-loving metal layer of about 500 nm on the stepped micro-machined FBG surface as a reference layer to improve the bonding strength with germanium-doped silica fiber. Then, a 1000 nm temperature-sensitive metal layer is sputtered as an electroplating transition layer to improve the conductivity during electroplating.
[0015] Furthermore, the oxygen-loving metal in step 2) is Ti, and the main component of optical fiber is silicon dioxide. General metal materials have poor bonding with silicon dioxide. Ti has good mechanical properties and good bonding with optical fiber, which is beneficial to the adhesion of the variable diameter electroplating gradient strain coupling metal layer in subsequent steps.
[0016] Furthermore, the temperature-sensitive metal in step 3) includes, but is not limited to, materials with high coefficients of thermal expansion such as Ni, Cu, and Zn, which also have good bonding strength with Ti.
[0017] Furthermore, the temperature-sensitive metal in step 3) includes, but is not limited to, materials with high coefficients of thermal expansion such as Ni, Cu, and Zn, which also have good bonding strength with Ti.
[0018] Furthermore, in step 3), the electroplating temperature is 50°C, the current of a single metal optical fiber is 0.2 mA, and the Ni electroplating solution parameters are: NiSO4-6H2O, 250 g / L; NiCl2-6H2O, 50 g / L; H3BO3, 30 g / L; C 12 H 25 SO4Na, 0.075 g / L; pH 3-5.
[0019] Furthermore, in step 3), the electroplating time is 4 to 10 hours, and the peristaltic pump flow rate changes with time to control the equivalent thermal expansion coefficient of the gradient strain coupled metallized FBG to increase linearly along the axial direction.
[0020] Furthermore, the thickness of the electroplated metal layer is related to the area S of the electroplating tank, the molar mass M of the electroplated metal, the current density J, and the current efficiency. The relationship between the coating metal density ρ, the correction factor k, and the Faraday constant F satisfies: .
[0021] Based on the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solution to be protected by this invention are as follows:
[0022] From the perspective of sensing mechanism, this invention breaks through the single mode of traditional fiber Bragg grating thermal sensing, which mainly relies on center wavelength drift for measurement. By constructing a non-uniform distribution of the equivalent thermal expansion coefficient along the fiber Bragg grating axis, the external heat flux density is transformed into a stable gradient strain field inside the fiber. This gradient strain is continuously distributed along the fiber axis, and the grating sub-units at different locations produce differentiated strain responses under the same thermal load. The superposition of these responses directly manifests as changes in the reflection spectral width. This achieves a heat flux density sensing method that uses spectral broadening as the main characterizing quantity, effectively reducing dependence on absolute temperature drift and improving measurement stability and anti-interference capability under complex thermal environments.
[0023] Regarding the synergy between structure and materials, this invention introduces microstructures with axial geometric variations on the outer surface of the optical fiber, and constructs a composite structure of an oxygen-loving metal reference layer and a temperature-sensitive metal layer on its surface. This allows the thermal expansion behavior of the metal layer to be efficiently and controllably transferred to the optical fiber body. The non-uniform distribution of the metal layer thickness or material composition along the axial direction creates a preset gradient in the equivalent thermal expansion coefficient of the fiber Bragg grating, avoiding the problems of strain concentration or single response in traditional uniform metal cladding structures. This structurally improves the strain conversion efficiency and sensing sensitivity under heat flux density.
[0024] From an engineering reliability perspective, this invention employs an oxygen-loving metal reference layer to form a stable mechanical interlock and material bonding interface with the optical fiber. This significantly improves the adhesion reliability of the metal layer under long-term thermal cycling and thermal shock conditions, reducing the risk of metal layer peeling or performance degradation. Simultaneously, the gradient strain distribution effectively disperses local stress, minimizing the adverse effects of high strain at a single point on the mechanical strength of the optical fiber, and enhancing the structural safety and lifespan of the device in high heat flux density environments.
[0025] At the preparation and application level, the gradient construction method adopted in this invention has good process compatibility, can be achieved through continuous control of the electrochemical deposition process, and has low dependence on specific material systems and deposition parameters, making it easy to design and expand flexibly in different application scenarios. This technical solution can achieve a significant improvement in heat flux density sensing performance without significantly increasing system complexity, and has good engineering promotion value.
[0026] This invention introduces the gradient strain mechanism into the field of fiber Bragg grating heat flux density sensing, achieving positive results in terms of sensing principle, structural design, and engineering applicability, and significantly expanding the application potential of fiber Bragg gratings in high heat flux density measurement and complex thermal environment monitoring.
[0027] The expected benefits and commercial value of the technical solution of this invention after transformation are as follows: The FBG heat flux density sensor of this invention has high mechanical strength, small size, light weight and high spatial resolution. Combined with FBG multiplexing technology, it can be networked on a large scale and can be applied to multiple fields such as mechanical engineering, bridge and tunnel, oil and gas, aerospace and environmental monitoring to build fiber optic heat flux density sensing networks.
[0028] The technical solution of this invention solves a long-standing technical problem that has remained unsolved: Currently, FBG heat flux density measurement methods mainly rely on the temperature-time curve of a single FBG temperature sensor to reflect changes in heat flux density, or on multiple FBG temperature sensors to monitor the temperature field under test in real time to reflect changes in heat flux density. These heat flux density measurement methods place high demands on the number of measurement points, their arrangement, and the complexity of the temperature field of the test object. The technical solution of this invention enables single-grating, single-point heat flux density measurement. Attached Figure Description
[0029] Figure 1 This is a schematic diagram of the fabrication process of the gradient strain coupled FBG heat flux density sensor of the present invention;
[0030] Figure 2 This is a schematic diagram of the finished product of the gradient strain-coupled FBG heat flux density sensor of the present invention.
[0031] Figure 3This is a schematic diagram of the calibration of the gradient strain-coupled FBG heat flux density sensor of the present invention;
[0032] Figure 4 This is a schematic diagram of the heat flux density response performance of the gradient strain-coupled FBG heat flux density sensor of the present invention;
[0033] Figure 5 This is an application scenario for measuring the heat flux density at the pump inlet in this invention;
[0034] Figure 6 The results of the pump inlet heat flux density measurement are for this invention.
[0035] In the figure: 1. FBG; 2. Hydrofluoric acid etched area; 3. Stepped micromachining FBG; 4. Magnetron sputtering oxygen-loving metal layer; 5. Magnetron sputtering nozzle; 6. Magnetron sputtering tooling fixture; 7. Magnetron sputtering platform; 8. Stepped metallized FBG; 9. Variable diameter electroplating gradient strain coupled metal layer; 10. Peristaltic pump; 11. Electroplating tank; 12. Gradient strain coupled metallization. Detailed Implementation
[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.
[0037] like Figure 1 As shown, this embodiment of the invention provides a method for fabricating a gradient strain-coupled FBG heat flux density sensor, the method comprising:
[0038] 1) Hydrofluoric acid etching micromachining: After removing the coating layer of the optical fiber and cleaning it with an ultrasonic cleaner, the surface of the optical fiber is wiped clean with a small amount of alcohol on absorbent paper. A grating area (FBG) is placed at the center of the optical fiber. A 40% hydrofluoric acid solution is dripped into the etching area using a dropper and left to stand for a period of time. The cladding is etched to form a stepped microgroove structure in the optical fiber. As the etching time increases, the cladding gradually becomes thinner. After 20 minutes, it is taken out and placed in water for ultrasonic cleaning. The surface of the optical fiber is wiped clean with a small amount of alcohol on absorbent paper. The stepped micromachining FBG is successfully prepared. At this time, the cladding diameter is about 95 μm and the microgroove depth is 15 μm.
[0039] 2) Magnetron sputtering of oxygen-loving metal layer: The stepped micro-machined FBG was placed on a magnetron sputtering platform. The stepped micro-machined FBG was limited and fixed using a magnetron sputtering fixture. After vacuuming, an oxygen-loving metal layer with a thickness of 0.5-2 μm was sputtered on the surface of the stepped micro-machined FBG through a magnetron sputtering nozzle to enhance the adhesion between the metal layer and the optical fiber. The stepped metallized FBG was successfully prepared.
[0040] 3) Variable diameter electroplating gradient strain coupled metal layer: The stepped metallized FBG is placed in deionized water and ultrasonically cleaned for 3 min to remove surface impurities. After cleaning, it is placed in a vacuum dish to dry. Then, it is placed in an electroplating tank to electroplat a temperature-sensitive metal layer. During the electroplating process, the electroplating liquid level is gradually lowered by a peristaltic pump to form a gradient strain coupled temperature-sensitive metal layer on the surface of the oxygen-loving metal layer. After electroplating, the metallized FBG is taken out, ultrasonically cleaned, vacuum dried, and then the encapsulation process is completed to obtain a gradient strain coupled metallized FBG. The diameter of the metal layer changes nonlinearly along the light axis, so that the equivalent thermal expansion coefficient of the gradient strain coupled metallized FBG changes linearly along the light axis.
[0041] In step 1), a microgroove structure with a depth of 15 µm is formed on the surface of the FBG by hydrofluoric acid etching, so that the oxygen-loving metal layer and the fiber optic stepped microgroove structure form a heterogeneous material mechanical connection structure. The resulting stepped micro-fabricated FBG can improve the bonding strength of heterogeneous materials, and the high bonding strength between the oxygen-loving metal and the fiber optic prevents the metal layer from falling off. The temperature-sensitive metal layer coupled by gradient strain can improve the spectral broadening of the FBG affected by heat flux density, optimize the heat flux density sensing performance of the gradient strain coupled metallized FBG, and at the same time improve the mechanical properties of the FBG to protect the fiber from breakage.
[0042] In step 1), the concentration of the hydrofluoric acid solution is 40%, the standing time is 20 min, the corrosion depth is 15 µm, and the corrosion width is 100-500 µm.
[0043] In step 2), the magnetron sputtering fixture can fix and cover both ends of the optical fiber, exposing only the grating region, which can effectively avoid the problem of difficulty in determining the position of the grating region after magnetron sputtering. The magnetron sputtering fixture can clamp 0-50 FBGs at a time. Since the shear strength of the optical fiber is low and multiple optical fibers may slip and cause inaccurate positioning of the grating region when fixed together, they are pasted in a fish-scale pattern from top to bottom in the magnetron sputtering fixture to facilitate the removal of the metallized FBGs after magnetron sputtering.
[0044] The magnetron sputtering equipment in step 2) is a PD-200C. An ultrathin oxygen-loving metal layer of about 500 nm is magnetron sputtered on the surface of the stepped micro-machined FBG as a reference layer to improve the bonding strength with germanium-doped silica fiber. Then, a 1000 nm temperature-sensitive metal layer is sputtered as an electroplating transition layer to improve the conductivity during electroplating.
[0045] The oxygen-loving metal in step 2) is Ti, and the main component of optical fiber is silicon dioxide. General metal materials have poor bonding with silicon dioxide. Ti has good mechanical properties and good bonding with optical fiber, which is beneficial to the adhesion of the variable diameter electroplating gradient strain coupling metal layer in subsequent steps.
[0046] The temperature-sensitive metals in step 3) include, but are not limited to, materials with high coefficients of thermal expansion such as Ni, Cu, and Zn, which also have good bonding strength with Ti.
[0047] The temperature-sensitive metals in step 3) include, but are not limited to, materials with high coefficients of thermal expansion such as Ni, Cu, and Zn, which also have good bonding strength with Ti.
[0048] In step 3), the electroplating temperature is 50℃, the current of a single metal optical fiber is 0.2 mA, and the Ni electroplating solution parameters are: NiSO4-6H2O, 250 g / L; NiCl2-6H2O, 50 g / L; H3BO3, 30 g / L; C 12 H 25 SO4Na, 0.075 g / L; pH 3-5.
[0049] The electroplating time in step 3) is 4 to 10 hours, and the peristaltic pump flow rate changes with time to control the equivalent thermal expansion coefficient of the gradient strain coupled metallized FBG to increase linearly along the axial direction.
[0050] The thickness of the electroplated metal layer is related to the area S of the electroplating tank, the molar mass M of the electroplated metal, the current density J, and the current efficiency. The relationship between the coating metal density ρ, the correction factor k, and the Faraday constant F satisfies: .
[0051] This embodiment provides a method for fabricating a gradient strain-coupled FBG heat flux density sensor. This embodiment can fabricate a gradient strain-coupled FBG heat flux density sensor that meets the requirements and apply it to the monitoring of heat flux density at the pump inlet of a hydraulic system. Compared with existing technologies, this method enables ordinary FBGs to possess single-grid area heat flux density sensing performance, reduces installation space, and lightens the added weight. (Reference) Figure 1The preparation process mainly includes hydrofluoric acid etching micromachining (1), magnetron sputtering of an oxygen-loving metal layer (4) and variable diameter electroplating of a gradient strain coupled metal layer (9), and using a dropper to drop hydrofluoric acid solution into the etched area (2) and let it stand for a period of time. After etching to form a stepped microgroove structure for the optical fiber, it was taken out and placed in water for ultrasonic cleaning. A small amount of alcohol was used to wipe the surface of the optical fiber clean with absorbent paper, and a stepped micro-machined FBG (3) was successfully prepared. The stepped micro-machined FBG (3) was placed on a magnetron sputtering platform (7), and the stepped micro-machined FBG (3) was limited and fixed using a magnetron sputtering fixture (6). A 0.5 μm titanium layer was sputtered on the surface of the stepped micro-machined FBG (3) through a magnetron sputtering nozzle (6) to enhance the adhesion between the metal layer and the optical fiber. A 0.5 μm nickel layer was sputtered on the surface of the titanium layer through a magnetron sputtering nozzle (6) to enhance the conductivity as a pretreatment for variable diameter electroplating, and a stepped metallized FBG (8) was successfully prepared. The stepped metallized FBG (8) was placed in deionized water for ultrasonic cleaning for 3 min to remove surface impurities. After cleaning, it was placed in a vacuum dish to dry. Then, it is placed in an electroplating tank (11) for hot nickel plating. During the electroplating process, the electroplating liquid level is gradually lowered by a peristaltic pump (10) to form a gradient strain-coupled nickel layer with a thickness gradually increasing from 400 μm to 600 μm on the surface of the oxygen-loving metal layer. After the electroplating is completed, the metallized FBG is taken out, ultrasonically cleaned, vacuum dried and then the encapsulation process is completed to obtain the gradient strain-coupled metallized FBG (12).
[0052] In this example, the magnetron sputtering equipment is PD-200C. The magnetron sputtering fixture (6) can fix and cover both ends of the optical fiber, exposing only the grating area. This can effectively avoid the problem of difficulty in determining the position of the grating area after magnetron sputtering. The high-performance polyester elastomer yellow tube at the metallized tail of the optical fiber protects against the breakage of the pigtail and the optical fiber at the metallized optical fiber connector during the preparation process, thus avoiding the impact on signal transmission.
[0053] In this example, the gradient strain-coupled metallized FBG pigtail is connected to the fiber optic patch cord via a fiber optic welding machine to transmit the optical signal to the spectrometer. After processing by the host computer, the temperature signal and heat flux density signal are obtained.
[0054] In this example, the prepared gradient strain coupled metallized FBG has the advantages of single-grid heat flux density measurement, small installation space, and high mechanical strength compared with the traditional fiber heat flux density measurement method. It is 10 mm long, 3 mm grid area, 1 mm in diameter, and weighs 0.1 g. It can be installed inside electromechanical components in various complex working environments to realize real-time measurement of the heat flux density of the object under test.
[0055] In this example, the finished product is prepared by Figure 2 As shown.
[0056] In this example, the calibration diagram of the prepared finished product is derived from... Figure 3 As shown.
[0057] In this example, the heat flux density response performance of the prepared product is as follows: Figure 4 As shown.
[0058] In this example, the prepared product is applied to the scenario of measuring the heat flux density at the pump inlet of a hydraulic system. Figure 5 As shown.
[0059] In this example, the measured heat flux density of the prepared pump inlet is obtained from... Figure 6 As shown.
[0060] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention, and within the spirit and principles of the present invention, should be covered within the scope of protection of the present invention.
Claims
1. A method for fabricating a gradient strain-coupled fiber Bragg grating heat flux density sensor, characterized in that, Includes the following steps: Microfabricated structures with stepped variations along the fiber axis are constructed on the surface of a fiber grating region containing a fiber Bragg grating. An oxygen-loving metal reference layer is formed on the surface of the micro-machined structure, so that a composite interface of mechanical interlocking and material bonding is formed between the metal layer and the optical fiber. A temperature-sensitive metal layer is electrodeposited on the surface of the oxygen-loving metal reference layer, and the deposition conditions are continuously changed along the fiber axis during the electrodeposition process, so that the equivalent thermal expansion coefficient of the obtained temperature-sensitive metal layer exhibits a monotonic variation distribution along the fiber axis. By utilizing the axial distribution of the equivalent thermal expansion coefficient, a gradient strain field is introduced into the fiber Bragg grating along its axis under the influence of external heat flux density, causing the reflection spectrum of the fiber Bragg grating to produce a broadened response related to heat flux density.
2. The method according to claim 1, characterized in that, The stepped microfabricated structure is formed by local thinning of the fiber cladding, the depth of the microfabricated structure is 15, and the microfabricated structure forms at least one dimension variation segment in the axial direction.
3. The method according to claim 1, characterized in that, The oxygen-loving metal reference layer is a titanium metal layer, which covers the outer surface of the grating region where the fiber Bragg grating is located.
4. An electroplating control method for constructing gradient strain coupled metallized fiber Bragg gratings, characterized in that, When performing metal electrodeposition on the surface of a conductive reference layer, the electrodeposition time or deposition rate at different positions along the optical fiber axis can be made different by continuously changing the height of the electroplating solution or the equivalent deposition area. The formed metal deposition layer has a continuously varying thickness distribution along the fiber axis, and this thickness distribution, together with the thermal expansion properties of the metal material, determines the distribution of the equivalent thermal expansion coefficient of the fiber Bragg grating along the axis. The thickness of the metal deposition layer satisfies the Faraday electrodeposition mass conservation relationship with respect to the effective area of the electroplating tank, the molar mass of the electroplated metal, the current density, the current efficiency, the density of the coating metal, the Faraday constant, and the correction coefficient.
5. The method according to claim 4, characterized in that, The material of the metal deposition layer is selected from one or more of nickel, copper, and zinc.
6. The method according to claim 4, characterized in that, The electrodeposition process is carried out at an electroplating solution temperature of 50°C, and the electrodeposition current for a single optical fiber is 0.2 ohms.
7. A gradient strain-coupled fiber Bragg grating heat flux density sensing system, characterized in that, include: An optical fiber substrate, wherein an optical fiber grating region is provided on the optical fiber substrate; A stepped microfabricated structure is disposed on the fiber cladding on the outer surface of the gate region; An oxygen-loving metal reference layer covers the stepped micromachining structure and forms a mechanically interlocked interface with the optical fiber; A temperature-sensitive metal layer is covered on the outside of the oxygen-loving metal reference layer, and the temperature-sensitive metal layer has a continuously varying thickness distribution along the optical fiber axis. The temperature-sensitive metal layer and the fiber Bragg grating form a gradient strain coupling under thermal load, causing the reflection spectrum width of the fiber Bragg grating to change with the heat flux density.
8. The system according to claim 7, characterized in that, The axial length of the stepped micromachining structure is 100 to 500.
9. The system according to claim 7, characterized in that, The equivalent thermal expansion coefficient corresponding to the axial thickness change of the temperature-sensitive metal layer changes linearly along the optical fiber axis.
10. The system according to claim 7, characterized in that, The gradient strain coupling structure produces greater fiber Bragg grating spectral broadening compared to a uniform metal-clad structure under the same heat flux density.