Hydrogen sensor based on 45-degree inclined fiber bragg grating

Abstract

The invention relates to a hydrogen sensor based on a 45-degree inclined fiber bragg grating, which is applied to the field of chemical or physical analysis and comprises a tunable laser, an optical circulator connected with the tunable laser, a polarizer connected with the optical circulator, a D-shaped optical fiber body connected with the polarizer and an optical detector connected with the optical circulator, through the cooperation of the D-shaped optical fiber body, the hydrogen sensitive dielectric layer, the 45-degree inclined grating and the Fabry-Perot resonant cavity, as the refractive index change of the hydrogen sensitive dielectric layer directly affects the optical path length of the Fabry-Perot resonant cavity, tiny hydrogen concentration change can also be accurately captured; moreover, the design of the D-shaped optical fiber body can effectively promote the formation of a high-quality factor Fabry-Perot resonant cavity, also can increase the coupling area of the optical fiber core and the hydrogen-sensitive dielectric layer, enhances the interaction between light and the external environment, and greatly improves the monitoring sensitivity and precision of the hydrogen concentration change.

Description

Technical Field

[0001] The present invention relates to a hydrogen sensor, in particular to a hydrogen sensor based on a 45° tilted optical fiber grating applied in the field of chemical or physical analysis. Background Art

[0002] As a representative of clean energy, hydrogen is increasingly being used in various energy and industrial fields due to its advantages such as water as a combustion product and high energy density. However, hydrogen is an extremely flammable and explosive gas, and it is colorless and odorless, and it is extremely easy to diffuse in the air, making its leakage difficult to detect. Therefore, rapid and accurate detection of hydrogen leakage is crucial to prevent safety accidents and ensure equipment operation.

[0003] At present, common hydrogen detection technologies include electrochemical sensors, semiconductor sensors, and metal oxide sensors. Although these traditional hydrogen sensors can meet basic detection needs, they also have some limitations: such as limited sensitivity, long response time, susceptibility to temperature and humidity interference, high cost, and difficulty in scalability in large-area or multi-point distributed detection. These problems limit the application of existing hydrogen detection technologies in complex environments or scenarios with high precision requirements.

[0004] Fiber optic sensing technology has attracted extensive attention in the field of gas sensing in recent years due to its inherent anti-electromagnetic interference, chemical inertness, small size, light weight, and stable operation in harsh environments such as high temperature and high pressure. In particular, sensors based on fiber gratings have become an important development direction for gas detection due to their simple structure, high sensitivity, fast response, and easy distributed measurement.

[0005] 45° tilted fiber Bragg grating (45° TFG) has unique optical properties. It can reflect or refract incident light to the side at a specific angle, greatly enhancing the interaction between light and the surrounding medium. Therefore, sensors based on 45° TFG can sensitively respond to changes in the external environment and are particularly suitable for gas detection. In addition, the introduction of Fabry-Perot cavity (FP cavity) can significantly improve the sensitivity of the sensor. Its optical path length changes with the refractive index of the sensitive material, which can achieve high-sensitivity detection of changes in hydrogen concentration.

[0006] Although fiber optic sensors and sensors based on fiber gratings have great potential, the related devices in the existing technology still have problems such as high cost, difficulty in multi-point synchronous demodulation, and limited measurement range. Therefore, how to combine 45° tilted fiber gratings and Fabry-Perot cavity technology to achieve distributed, multi-point high-precision monitoring of hydrogen concentration while maintaining low cost is a technical problem that needs to be solved urgently. Summary of the invention

[0007] In view of the above-mentioned prior art, the technical problem to be solved by the present invention is how to combine 45° tilted fiber Bragg grating and Fabry-Perot cavity technology to achieve distributed, multi-point high-precision monitoring of hydrogen concentration while maintaining low cost.

[0008] In order to solve the above problems, the present invention provides a hydrogen sensor based on a 45° tilted fiber Bragg grating, comprising a tunable laser, an optical circulator connected to the tunable laser, a polarizer connected to the optical circulator, a D-shaped optical fiber body connected to the polarizer, and a photodetector connected to the optical circulator, wherein the optical circulator is connected to the polarizer in a bidirectional signal manner, and the polarizer is connected to the D-shaped optical fiber body in a bidirectional signal manner;

[0009] The D-shaped optical fiber body is composed of a D-shaped large plane body located on the upper side, D-shaped side plane bodies located on the front and rear sides of the D-shaped large plane body, a D-shaped arc body located on the lower side between the two D-shaped side plane bodies, and an optical fiber core located inside the D-shaped arc body, and forms a "D"-shaped cross section;

[0010] The ends of the front and rear D-shaped side plane bodies that are away from each other are coated with a high-reflection film, the ends of the front and rear high-reflection films that are away from each other are deposited with a hydrogen-sensitive dielectric layer, and the ends of the front and rear hydrogen-sensitive dielectric layers that are away from each other are also deposited with a layer of high-reflection dielectric film. The high-reflection film, the hydrogen-sensitive dielectric layer and the high-reflection dielectric film cooperate to form a Fabry-Perot resonant cavity, and a plurality of 45° inclined gratings that cooperate with the Fabry-Perot resonant cavity are engraved on the optical fiber core.

[0011] In the above-mentioned hydrogen sensor based on 45° tilted fiber Bragg grating, since the refractive index change of the hydrogen-sensitive medium layer directly affects the optical path length of the Fabry-Perot cavity, even tiny changes in hydrogen concentration can be accurately captured, greatly improving the monitoring sensitivity and accuracy of hydrogen concentration changes.

[0012] As a supplement to the present application, the hydrogen sensitive dielectric layer is made of a gel doped with palladium Pd, and the molecular gaps of the high reflective dielectric film can allow hydrogen molecules to pass through.

[0013] As a supplement to the present application, a plurality of 45° tilted gratings are arranged in series in a manner of being tilted 45° relative to the angle of the incident light beam to form a cascade sensing structure.

[0014] As a further improvement of the present application, in the process of connecting and cooperating the tunable laser, the optical circulator, the polarizer, the D-shaped optical fiber body and the optical detector, the optical fiber is connected by α-angle fusion.

[0015] As a further improvement of the present application, the setting data quantity of the D-shaped optical fiber body is 1 or more. When the setting number of D-shaped optical fiber bodies is multiple, the optical fiber core in the D-shaped optical fiber body is first connected to the delay line, and then fused with the optical fiber core in the next D-shaped optical fiber body through the delay line at an α angle, and multiple D-shaped optical fiber bodies form a cascade sensing structure.

[0016] As a further improvement of the present application, when the number of D-shaped optical fiber bodies is multiple, let the frequency sweep period be T, and the extension line length be L, then the extension line length L should satisfy Where c is the speed of light in vacuum, n1 is the refractive index of the optical fiber core;

[0017] When monitoring, the time from the reflected light decreasing to the deviation is t, then let Wherein, q is the serial number of multiple D-shaped optical fiber bodies, and q is a non-negative integer. q-1 ≤t≤t q When , the reflected light wavelength position deviates from the chain and the qth D-shaped optical fiber body, which indicates that there is a hydrogen leakage at the position of the qth D-shaped optical fiber body.

[0018] As another improvement of the present application, when the Fabry-Perot cavity satisfies 2nhsinθ=mλ, its transmitted light is at a maximum value and its reflected light is at a minimum value, wherein n is the refractive index of the hydrogen-sensitive medium layer, h is the thickness of the hydrogen-sensitive medium layer, θ is the incident angle, when the light is incident normally, θ=90°, m is the resonance order, and m is a non-negative integer, and λ is the wavelength of the resonant light.

[0019] As another improvement of the present application, during the D-shaped optical fiber body application process, the hydrogen concentration can be calculated by the formula It is calculated, where λ0 is the resonant center wavelength in the absence of hydrogen, λ1 is the resonant center wavelength during actual monitoring, and k is the conversion coefficient of the refractive index to the hydrogen concentration.

[0020] As a further improvement of the present application, the manufacturing steps of a hydrogen sensor based on a 45° tilted fiber grating composed of a tunable laser, an optical circulator, a polarizer, a D-shaped optical fiber body and a light detector are as follows:

[0021] S1. The upper end surface and the front and rear end sides of the D-shaped optical fiber body are ground and polished so that the upper end surface of the D-shaped optical fiber body forms a large D-shaped plane body, and the front and rear ends of the D-shaped optical fiber body form a D-shaped side plane body, so that the D-shaped optical fiber body forms a "D" shaped cross section;

[0022] S2. coating a high reflective film on the D-shaped side plane;

[0023] S3. A plurality of 45° tilted gratings are engraved in the fiber core of the D-shaped fiber body by using a mask scanning method, and the period of the 45° tilted gratings is calculated according to the following formula:

[0024]

[0025] Where n1 is the refractive index of the fiber core, and λ is the average wavelength of the tunable laser;

[0026] S4. depositing a hydrogen sensitive dielectric layer on the high reflective film, wherein the hydrogen sensitive dielectric layer has a refractive index of n and a thickness of h;

[0027] S5. When the number of D-shaped optical fiber bodies is one, a hydrogen measurement sensing optical path is constructed, such that the tunable laser is connected to the optical circulator, the optical circulator is connected to the polarizer, the polarizer is connected to the D-shaped optical fiber body, and then the optical circulator is connected to the light detector to obtain a hydrogen sensor based on a 45° tilted fiber grating;

[0028] S6. When there are multiple D-shaped optical fiber bodies, a hydrogen measurement sensing optical path is constructed, such that a tunable laser is connected to an optical circulator, the optical circulator is connected to a polarizer, the polarizer is connected to the first D-shaped optical fiber body, and subsequent D-shaped optical fiber bodies are sequentially connected through extension lines to form a cascade, and then the optical circulator is connected to a light detector, to obtain a hydrogen sensor based on a 45° tilted fiber grating;

[0029] S7. After the hydrogen sensor based on 45° tilted fiber Bragg grating is manufactured, it is calibrated in a hydrogen environment. At least three hydrogen environments with different concentration gradients are used to calibrate the Fabry-Perot cavity in a step-by-step manner. The resonance order m of the Fabry-Perot cavity and the conversion coefficient k of the refractive index with hydrogen concentration are calibrated. The calibration formula is:

[0030]

[0031] Among them, ρ x is different hydrogen concentrations, λ0 is the resonant wavelength when there is no hydrogen, λ x The resonant wavelengths under different hydrogen concentrations are obtained by establishing a set of equations based on the calibration data, and solving the resonance order m and the conversion coefficient k of the refractive index with the hydrogen concentration.

[0032] In summary, through the coordination of the D-shaped optical fiber body, the hydrogen-sensitive dielectric layer, the 45° tilted grating and the Fabry-Perot cavity, the refractive index change of the hydrogen-sensitive dielectric layer directly affects the optical path length of the Fabry-Perot cavity, so that tiny changes in hydrogen concentration can also be accurately captured. In addition, the design of the D-shaped optical fiber body can effectively promote the formation of a high-quality factor Fabry-Perot cavity, while also increasing the coupling area between the optical fiber core and the hydrogen-sensitive dielectric layer, enhancing the interaction between light and the external environment, and improving the response sensitivity of the hydrogen sensor based on the 45° tilted fiber Bragg grating, thereby greatly improving the monitoring sensitivity and accuracy of hydrogen concentration changes. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Axonometric perspective diagram of the high-reflection film, hydrogen-sensitive dielectric layer and 45° tilted grating in the first to fourth embodiments of the present application;

[0034] Figure 2 This is a flowchart of manufacturing a hydrogen sensor based on a 45° tilted fiber Bragg grating according to the second embodiment of the present application;

[0035] Figure 3 This is an optical path diagram of a hydrogen sensor based on a 45° tilted fiber Bragg grating when the number of the D-shaped optical fiber body in the third embodiment of the present application is one;

[0036] Figure 4 This is an optical path diagram of a hydrogen sensor based on a 45° tilted fiber Bragg grating when the number of D-shaped optical fiber bodies in the fourth embodiment of the present application is multiple;

[0037] Figure 5 Schematic diagram of wavelength scanning of the first to fourth embodiments of the present application;

[0038] Figure 6 This is a diagram showing the determination of the resonant wavelength of the first to fourth embodiments of the present application;

[0039] Figure 7 This is a schematic diagram of hydrogen gas sensing positioning for the first to fourth embodiments of the present application;

[0040] Figure 8 This is an axonometric view of a D-shaped optical fiber body according to the first to fourth embodiments of the present application;

[0041] Fig. 9 This is a front cross-sectional view of a D-shaped optical fiber body according to the first to fourth embodiments of the present application;

[0042] Fig.10 This is a schematic diagram of α-angle welding in the first to fourth embodiments of the present application.

[0043] Description of the numbers in the figure:

[0044] 1D-shaped optical fiber body, 11D-shaped large plane body, 12D-shaped side plane body, 13D-shaped arc body, 14 optical fiber cores, 2 high-reflection films, 3 hydrogen-sensitive medium layers, and 445° tilted grating. DETAILED DESCRIPTION

[0045] The following is a detailed description of four implementation methods of the present application in conjunction with the accompanying drawings.

[0046] The first implementation method:

[0047] Figure 1 and Figure 5 - Fig.10 A hydrogen sensor based on a 45° tilted fiber Bragg grating is shown, comprising a tunable laser, an optical circulator connected to the tunable laser, a polarizer connected to the optical circulator, a D-shaped optical fiber body 1 connected to the polarizer, and an optical detector connected to the optical circulator, wherein the optical circulator is connected to the polarizer in a bidirectional signal manner, and the polarizer is connected to the D-shaped optical fiber body 1 in a bidirectional signal manner;

[0048] The D-shaped optical fiber body 1 is composed of a D-shaped large plane body 11 located on the upper side, a D-shaped side plane body 12 located on the front and rear sides of the D-shaped large plane body 11, a D-shaped arc body 13 located on the lower side between the two D-shaped side plane bodies 12, and an optical fiber core 14 located inside the D-shaped arc body 13, and forms a "D"-shaped cross section;

[0049] The ends of the front and rear D-shaped side plane bodies 12 away from each other are coated with a high-reflection film 2, the ends of the front and rear high-reflection films 2 away from each other are deposited with a hydrogen-sensitive dielectric layer 3, and the ends of the front and rear hydrogen-sensitive dielectric layers 3 away from each other are also deposited with a layer of high-reflection dielectric film, the high-reflection dielectric film is used to improve the reflection efficiency of light and optimize the stability of the Fabry-Perot (FP) resonant cavity, the high-reflection film 2, the hydrogen-sensitive dielectric layer 3 and the high-reflection dielectric film cooperate to form a Fabry-Perot resonant cavity, and a plurality of 45° tilted gratings 4 that cooperate with the Fabry-Perot (FP) resonant cavity are engraved on the optical fiber core 14, and the 45° tilted gratings 4 can reflect the transverse electric (TE) component of the input light into the Fabry-Perot (FP) resonant cavity and allow the transverse magnetic (TM) component to pass losslessly. The reflected light of the Fabry-Perot (FP) resonant cavity can also be reflected back to the input optical fiber through the 45° tilted grating 4. Through the cooperation of the D-shaped optical fiber body 1, the hydrogen-sensitive medium layer 3, the 45° tilted grating 4 and the Fabry-Perot (FP) resonant cavity, since the refractive index change of the hydrogen-sensitive medium layer 3 directly affects the optical path length of the Fabry-Perot (FP) resonant cavity, small changes in hydrogen concentration can also be accurately captured. In addition, the design of the D-shaped optical fiber body 1 can effectively promote the formation of a high-quality factor Fabry-Perot (FP) resonant cavity, while also increasing the coupling area between the optical fiber core 14 and the hydrogen-sensitive medium layer 3, enhancing the interaction between light and the external environment, and improving the response sensitivity of the hydrogen sensor based on the 45° tilted fiber grating, thereby greatly improving the monitoring sensitivity and accuracy of hydrogen concentration changes.

[0050] Figure 1 and Figure 5 - Fig.10 It is shown that the hydrogen sensitive medium layer 3 is made of a gel doped with palladium Pd. The molecular gap of the high reflective medium film can allow hydrogen molecules (H2) to pass through. The molecular gap of the high reflective medium film is relatively loose. When hydrogen exists, the hydrogen sensitive medium layer 3 supported by the gel doped with palladium Pd can cause a change in the refractive index by absorbing hydrogen.

[0051] Figure 1 , Figure 8 and Fig. 9 It is shown that a plurality of 45° tilted gratings 4 are arranged in series in a manner of being tilted 45° relative to the angle of the incident light beam, and form a cascade sensing structure.

[0052] Figure 1 and Figure 5-Figure 10 It is shown that in the Fabry-Perot cavity, 2nhsinθ=mλ ① When , the transmitted light is at the maximum value and the reflected light is at the minimum value, that is, the transmitted light is the strongest and the reflected light is the weakest, wherein n is the refractive index of the hydrogen sensitive medium layer 3, h is the thickness of the hydrogen sensitive medium layer 3, θ is the incident angle, when the light is incident normally, θ=90°, m is the resonance order, and m is a non-negative integer, and λ is the wavelength of the resonant light.

[0053] Figure 1 and Figure 5-Figure 10 It is shown that during the reaction of the D-shaped optical fiber body 1, the hydrogen concentration can be calculated by the formula It is calculated, where λ0 is the resonant center wavelength in the absence of hydrogen, λ1 is the resonant center wavelength during actual monitoring, and k is the conversion coefficient of the refractive index to the hydrogen concentration.

[0054] Figure 1 and Figure 5 - Fig.10 It shows that when hydrogen leaks in the air, the hydrogen contacts the hydrogen sensitive medium layer 3, which is doped with palladium Pd. Palladium Pd has excellent hydrogen adsorption and permeation capabilities. When the hydrogen molecule (H2) contacts the surface of palladium Pd, the hydrogen is first physically adsorbed on the surface of palladium Pd, and then the hydrogen molecule (H2) dissociates into two hydrogen atoms (H). The dissociated hydrogen atoms (H) diffuse into the crystal lattice of palladium Pd to form a palladium-hydrogen solid solution (PdHx). In this process, hydrogen atoms (H) occupy interstitial positions in the lattice of palladium Pd, causing the lattice structure of palladium Pd to expand, resulting in significant changes in the optical properties of the hydrogen-sensitive medium layer 3. The formation of palladium-hydrogen solid solution (PdHx) produces lattice expansion, which increases the free electron density of the material, changes its dielectric constant, and further reduces the refractive index of the hydrogen-sensitive medium layer 3, thereby changing the optical path of the Fabry-Perot (FP) resonant cavity. As can be seen from formula ①, the resonant wavelength is changed. Therefore, the change in the refractive index of the sensitive medium can be obtained by monitoring the resonant wavelength, thereby realizing the monitoring of hydrogen.

[0055] Second implementation method:

[0056] Figure 1 , Figure 2 and Figure 5 - Fig.10 The hydrogen sensor based on 45° tilted fiber Bragg grating is shown, and the setting data amount of the D-shaped fiber body 1 is 1. In the process of connecting and cooperating the tunable laser, the optical circulator, the polarizer, the D-shaped fiber body 1 and the optical detector, the optical fiber is connected by α-angle fusion, wherein the α angle is 5° to 15°, and the optical fiber connecting the tunable laser, the optical circulator, the polarizer, the D-shaped fiber body 1 and the optical detector is a polarization-maintaining fiber.

[0057] Figure 1 , Figure 2 and Figure 5 - Fig.10The application process of the hydrogen sensor based on 45° tilted fiber Bragg grating is shown as follows: a tunable laser is used to emit a continuously tuned swept laser, which enters an optical circulator, and the output light of the optical circulator enters a polarizer to generate transverse electric (TE) linear polarized light; the linear polarized light is converted into elliptically polarized light containing transverse electric (TE) and transverse magnetic (TM) components, and the light intensity of the transverse electric (TE) component is I0cos 2 α, the intensity of the transverse magnetic (TM) component is I0sin 2 α, I0 is the input light intensity before welding. The transverse electric (TE) component is reflected by the 45° tilted grating 4 and enters the Fabry-Perot (FP) resonant cavity. The tunable laser wavelength is scanned. When the wavelength meets the matching condition shown in formula ①, the reflected light is maximum. The reflected light is reflected back to the input optical fiber, passes through the polarizer and the optical circulator, and reaches the detector. When the reflected light minimum appears, the scanning wavelength at this time is recorded.

[0058] During the application process, the D-shaped optical fiber body 1 can be mounted on the area to be detected with a risk of hydrogen leakage. After mounting, when there is no hydrogen leakage, the resonant center wavelength is λ0. When a hydrogen bevel appears, the refractive index n of the hydrogen sensitive medium layer 3 becomes lower, and the thickness h remains unchanged. The resonance order m and the conversion coefficient k of the refractive index to the hydrogen concentration can be calibrated when the sensor leaves the factory, and the hydrogen concentration can be calculated by formula ②.

[0059] The third implementation method:

[0060] Figure 1 , Figure 3 and Figure 5 - Fig.10A hydrogen sensor based on 45° tilted fiber Bragg grating is shown. In actual engineering applications, it may be necessary to monitor hydrogen leakage at multiple points at the same time, but the deployment of multiple systems has the problem of high cost. In view of this contradiction, on the basis of a single-point monitoring sensor, a time-division multiplexing method is used to realize multi-point quasi-distributed measurement of hydrogen. The setting data volume of the D-shaped optical fiber body 1 is multiple. When the setting number of the D-shaped optical fiber body 1 is multiple, the optical fiber core 14 in the D-shaped optical fiber body 1 is first connected to the delay line, and then the optical fiber core 14 in the next D-shaped optical fiber body 1 is welded at an α angle through the delay line, and multiple D-shaped optical fiber bodies 1 form a cascade sensing structure. Two adjacent D-shaped optical fiber bodies 1 are connected in a bidirectional signal manner. A tunable laser with pulse output is used to emit a sweep pulse. A delay line is added between every two D-shaped optical fiber bodies 1. The delay time of the delay line is greater than the sweep period T, and the α-angle welding method can effectively ensure that the input light has a reflectable transverse electric (TE) component. By cascading multiple D-shaped optical fiber bodies 1, quasi-distributed multi-point detection is achieved, and the simultaneous monitoring of hydrogen concentrations at multiple locations is effectively realized, which greatly expands the scope of hydrogen leakage monitoring and is effectively applicable to hydrogen leakage monitoring in complex environments. It promotes the application scope and environmental adaptability of hydrogen sensors based on 45° tilted fiber gratings, and promotes its economic benefits during the application process. In addition, since multiple D-shaped optical fiber bodies 1 can share a unified light source, namely, a tunable laser and a detector, the overall cost of the hydrogen sensor based on a 45° tilted fiber grating is significantly reduced. Compared with the traditional solution of multiple independent sensors, the multi-point cascade design of this embodiment simplifies the structure of the hydrogen leakage monitoring system, reduces its manufacturing and maintenance costs, and is particularly suitable for large-scale industrial applications.

[0061] Figure 1 , Figure 3 and Figure 5 - Fig.10 It is shown that when the number of D-shaped optical fiber bodies 1 is multiple, let the frequency sweep period be T, and the extension line length be L, then the extension line length L should satisfy Where c is the speed of light in vacuum, n1 is the refractive index of the optical fiber core 14;

[0062] When monitoring, the time from the reflected light decreasing to the deviation is t, then let Wherein, q is the serial number of the plurality of D-shaped optical fiber bodies 1, and q is a non-negative integer. q-1 ≤t≤t q When , the reflected light wavelength position deviates from the chain and the qth D-shaped optical fiber body 1, which indicates that there is a hydrogen leakage at the position of the qth D-shaped optical fiber body 1.

[0063] Figure 1 , Figure 3 and Figure 5 - Fig.10 It is shown that a continuously tuned swept-frequency laser is emitted by a tunable laser, and the laser enters a polarizer through an optical circulator to generate linearly polarized light. In the optical path, the polarization-maintaining optical fiber is fused at a specific α angle to generate elliptically polarized light containing transverse electric (TE) and transverse magnetic (TM) components; the transverse electric (TE) component of the light enters the 45° inclined grating 4 of each D-shaped optical fiber body 1 in turn through the delay line, and the transverse magnetic (TM) component passes through without loss; the reflected light of the transverse electric (TE) component passes through the Fabry-Perot (FP) resonant cavity, and returns to the optical fiber in turn, continues to pass through the polarizer and the optical circulator, and finally reaches the detector, and the scanning wavelength at this time is recorded by the detector; when hydrogen leakage occurs at one of the monitoring points, the minimum value of the reflected light at this position deviates from the original wavelength position, and the time t of the minimum drift of the monitored reflected light is obtained by solving formula ④, the drift of the reflected light wavelength is calculated, and the position of the hydrogen leakage is determined.

[0064] The fourth implementation method:

[0065] Figure 1 , Figure 4 and Figure 5 - Fig.10 The hydrogen gas sensor based on 45° tilted fiber Bragg grating is shown. The manufacturing steps of the hydrogen gas sensor based on 45° tilted fiber Bragg grating, which is composed of a tunable laser, an optical circulator, a polarizer, a D-shaped fiber body 1 and a light detector, are as follows:

[0066] S1. The upper end surface and the front and rear end sides of the D-shaped optical fiber body 1 are ground and polished so that the upper end surface of the D-shaped optical fiber body 1 forms a D-shaped large plane body 11, and the front and rear ends of the D-shaped optical fiber body 1 form a D-shaped side plane body 12, so that the D-shaped optical fiber body 1 forms a "D"-shaped cross-section;

[0067] S2. A high-reflective film 2 is coated on the D-shaped side plane body 12;

[0068] S3. A plurality of 45° tilted gratings 4 are engraved in the fiber core 14 of the D-shaped fiber body 1 by using a mask scanning method, and the period of the 45° tilted gratings 4 is calculated according to the following formula:

[0069]

[0070] Wherein, n1 is the refractive index of the optical fiber core 14, and λ is the average wavelength of the tunable laser;

[0071] The 45° inclined grating 4 has unique optical properties, which can reflect the transverse electric (TE) component in the optical fiber core 14 into the Fabry-Perot (FP) resonant cavity and allow the transverse magnetic (TM) component to pass through without loss. After the light of the transverse electric (TE) component is reflected into the Fabry-Perot (FP) resonant cavity, when the resonance condition is met, the reflected light is minimum. By detecting the minimum value of the reflected light, the change of the resonant wavelength can be inferred, thereby calculating the change of the hydrogen concentration.

[0072] S4. depositing a hydrogen sensitive dielectric layer 3 on the high reflective film 2, wherein the hydrogen sensitive dielectric layer 3 has a refractive index of n and a thickness of h. The hydrogen sensitive dielectric layer 3 is made of a gel doped with palladium Pd. When the hydrogen sensitive dielectric layer 3 contacts hydrogen, palladium Pd can absorb hydrogen and form a palladium hydrogen solid solution (PdHx), resulting in a change in the refractive index of the hydrogen sensitive dielectric layer 3. The change in the refractive index of the hydrogen sensitive dielectric layer 3 directly affects the optical path length of the Fabry-Perot (FP) resonant cavity, thereby affecting the resonant wavelength.

[0073] S5. When the number of D-shaped optical fiber bodies 1 is one, a hydrogen measurement sensing optical path is constructed, so that the tunable laser is connected to the optical circulator, the optical circulator is connected to the polarizer, the polarizer is connected to the D-shaped optical fiber body 1, and then the optical circulator is connected to the light detector to obtain a hydrogen sensor based on a 45° tilted fiber grating;

[0074] A continuously tuned frequency-sweeping laser is emitted through a tunable laser, and the laser enters a polarizer through an optical circulator to generate linearly polarized light. In the optical path, the polarization-maintaining optical fiber is fused at a specific α angle to generate elliptically polarized light containing transverse electric (TE) and transverse magnetic (TM) components. The transverse electric (TE) component of the light enters the Fabry-Perot (FP) resonant cavity through reflection from the 45° inclined grating 4, while the transverse magnetic (TM) component passes through without loss. The reflected light of the transverse electric (TE) component passes through the Fabry-Perot (FP) resonant cavity and returns to the optical fiber, continues to pass through the polarizer and the optical circulator, and finally reaches the detector, which records the scanning wavelength at this time.

[0075] S6. When there are multiple D-shaped optical fiber bodies 1, a hydrogen measurement sensing optical path is constructed, so that the tunable laser is connected to the optical circulator, the optical circulator is connected to the polarizer, the polarizer is connected to the first D-shaped optical fiber body 1, and the subsequent D-shaped optical fiber bodies 1 are sequentially cascaded through extension lines, and then the optical circulator is connected to the light detector to obtain a hydrogen sensor based on a 45° tilted fiber grating;

[0076] By setting the extension line, the reflected light of each monitoring point can have a clear time mark, and the length of the extension line satisfies the condition shown in formula ③;

[0077] Each D-shaped optical fiber body 1 is fused at a small angle, i.e., α-angle fusion, to ensure that each D-shaped optical fiber body 1 can effectively reflect the transverse electric (TE) component;

[0078] A continuously tuned swept-frequency laser is emitted through a tunable laser, and the laser enters a polarizer through an optical circulator to generate linearly polarized light. In the optical path, the polarization-maintaining optical fiber is fused at a specific α angle to generate elliptically polarized light containing transverse electric (TE) and transverse magnetic (TM) components. The transverse electric (TE) component of the light enters the 45° inclined grating 4 of each D-shaped optical fiber body 1 in turn through a delay line, while the transverse magnetic (TM) component passes through without loss. The reflected light of the transverse electric (TE) component passes through a Fabry-Perot (FP) resonant cavity, and returns to the optical fiber in turn, continues to pass through the polarizer and the optical circulator, and finally reaches the detector, and the scanning wavelength at this time is recorded by the detector.

[0079] When hydrogen leakage occurs at one of the monitoring points, the minimum value of the reflected light at that location deviates from the original wavelength position. The time t of the minimum value drift of the monitored reflected light is obtained by solving equation ④, the drift of the reflected light wavelength is calculated, and the location of the hydrogen leakage is determined;

[0080] S7. After the hydrogen sensor based on 45° tilted fiber Bragg grating is manufactured, it is calibrated in a hydrogen environment. At least three hydrogen environments with different concentration gradients are used to calibrate the Fabry-Perot (FP) resonant cavity in a step-by-step manner. The resonance order m of the Fabry-Perot (FP) resonant cavity and the conversion coefficient k of the refractive index with hydrogen concentration are calibrated. The calibration formula is:

[0081]

[0082] Among them, ρ x is the different hydrogen concentrations, λ0 is the resonant wavelength when there is no hydrogen, λ x is the resonant wavelength at different hydrogen concentrations;

[0083] Based on the calibration data, a set of equations is established to solve the resonance order m and the conversion coefficient k of the refractive index to the hydrogen concentration. In the subsequent application process, they are substituted into equation ② to obtain the hydrogen concentration under different wavelength drifts.

[0084] In view of current practical needs, the above-mentioned implementation mode adopted in this application is not limited to the scope of protection. Various changes made within the knowledge scope of technical personnel in this field without departing from the concept of this application still fall within the scope of protection of the present invention.

Claims

1. A hydrogen sensor based on 45° tilted fiber Bragg grating, characterized in that: The invention comprises a tunable laser, an optical circulator connected to the tunable laser, a polarizer connected to the optical circulator, a D-shaped optical fiber body (1) connected to the polarizer, and an optical detector connected to the optical circulator, wherein the optical circulator and the polarizer are connected in a bidirectional signal manner, and the polarizer and the D-shaped optical fiber body (1) are connected in a bidirectional signal manner; The D-shaped optical fiber body (1) is composed of a D-shaped large plane body (11) located on the upper side, a D-shaped side plane body (12) located on the front and rear sides of the D-shaped large plane body (11), a D-shaped arc body (13) located on the lower side between the two D-shaped side plane bodies (12), and an optical fiber core (14) located inside the D-shaped arc body (13), thereby forming a "D"-shaped cross section; The ends of the two front and rear D-shaped side plane bodies (12) that are away from each other are both coated with a high reflection film (2), the ends of the two front and rear high reflection films (2) that are away from each other are both deposited with a hydrogen sensitive medium layer (3), and the ends of the two front and rear hydrogen sensitive medium layers (3) that are away from each other are further deposited with a layer of high reflection medium film, the high reflection film (2), the hydrogen sensitive medium layer (3) and the high reflection medium film cooperate to form a Fabry-Perot resonant cavity, and the optical fiber core (14) is engraved with a plurality of 45° inclined gratings (4) that cooperate with the Fabry-Perot resonant cavity.

2. A hydrogen sensor based on 45° tilted fiber Bragg grating according to claim 1, characterized in that: The hydrogen sensitive medium layer (3) is made of a gel doped with palladium (Pd), and the molecular gaps of the high reflectivity medium film are capable of allowing hydrogen molecules to pass through.

3. A hydrogen sensor based on 45° tilted fiber Bragg grating according to claim 1, characterized in that: A plurality of the 45° inclined gratings (4) are arranged in series in a manner of being inclined at 45° relative to the incident light beam angle, and form a cascade sensing structure.

4. A hydrogen sensor based on 45° tilted fiber Bragg grating according to claim 1, characterized in that: In the process of connecting and matching the tunable laser, the optical circulator, the polarizer, the D-shaped optical fiber body (1) and the optical detector, the optical fiber is connected by α-angle fusion.

5. The hydrogen sensor based on 45° tilted fiber Bragg grating according to claim 1, characterized in that: The setting data quantity of the D-shaped optical fiber body (1) is one or more. When the setting quantity of the D-shaped optical fiber body (1) is more than one, the optical fiber core (14) in the D-shaped optical fiber body (1) is first connected to a delay line, and then fused with the optical fiber core (14) in the next D-shaped optical fiber body (1) through the delay line at an α angle, and the multiple D-shaped optical fiber bodies (1) form a cascade sensing structure.

6. A hydrogen sensor based on 45° tilted fiber Bragg grating according to claim 5, characterized in that: When the number of D-shaped optical fiber bodies (1) is plural, let the frequency sweep period be T and the extension line length be L. Then the extension line length L should satisfy Where c is the speed of light in vacuum, n1 is the refractive index of the optical fiber core (14); When monitoring, the time from the reflected light decreasing to the deviation is t, then let Wherein, q is the serial number of the plurality of D-shaped optical fiber bodies (1), and q is a non-negative integer. When t q-1 ≤t≤t q When , the wavelength position of the reflected light deviates from the chain and the qth D-shaped optical fiber body (1), which indicates that there is a hydrogen leak at the position of the qth D-shaped optical fiber body (1).

7. The hydrogen sensor based on 45° tilted fiber Bragg grating according to claim 1, characterized in that: When the Fabry-Perot resonant cavity satisfies 2nhsinθ=mλ, its transmitted light is at a maximum value and its reflected light is at a minimum value, wherein n is the refractive index of the hydrogen sensitive medium layer (3), h is the thickness of the hydrogen sensitive medium layer (3), θ is the incident angle, when the light is incident normally, θ=90°, m is the resonance order, and m is a non-negative integer, and λ is the wavelength of the resonant light.

8. A hydrogen sensor based on 45° tilted fiber Bragg grating according to claim 7, characterized in that: During the reaction of the D-shaped optical fiber body (1), the hydrogen concentration can be calculated by the formula It is calculated, where λ0 is the resonant center wavelength in the absence of hydrogen, λ1 is the resonant center wavelength during actual monitoring, and k is the conversion coefficient of the refractive index to the hydrogen concentration.

9. A hydrogen sensor based on 45° tilted fiber Bragg grating according to claim 1, characterized in that: The steps for making a hydrogen sensor based on a 45° tilted fiber grating, which is composed of a tunable laser, an optical circulator, a polarizer, a D-shaped optical fiber body (1) and a light detector, are as follows: S1. The upper end surface and the front and rear end side surfaces of the D-shaped optical fiber body (1) are ground and polished so that the upper end surface of the D-shaped optical fiber body (1) forms a D-shaped large plane body (11), and the front and rear ends of the D-shaped optical fiber body (1) form a D-shaped side plane body (12), so that the D-shaped optical fiber body (1) forms a "D"-shaped cross section; S2. A high-reflective film (2) is coated on the D-shaped side plane body (12); S3. A plurality of 45° tilted gratings (4) are engraved in the optical fiber core (14) of the D-shaped optical fiber body (1) by using a mask scanning method, and the period of the 45° tilted gratings (4) is calculated according to the following formula: Wherein, n1 is the refractive index of the optical fiber core (14), is the average wavelength of the tunable laser; S4. Depositing a hydrogen-sensitive dielectric layer (3) on the high-reflective film (2), the hydrogen-sensitive dielectric layer (3) having a refractive index of n and a thickness of h; S5. When the number of D-shaped optical fiber bodies (1) is one, a hydrogen measurement sensing optical path is constructed, so that the tunable laser is connected to the optical circulator, the optical circulator is connected to the polarizer, the polarizer is connected to the D-shaped optical fiber body (1), and then the optical circulator is connected to the light detector to obtain a hydrogen sensor based on a 45° tilted fiber grating; S6. When there are multiple D-shaped optical fiber bodies (1), a hydrogen gas measurement sensing optical path is constructed, such that a tunable laser is connected to an optical circulator, the optical circulator is connected to a polarizer, the polarizer is connected to a first D-shaped optical fiber body (1), and subsequent D-shaped optical fiber bodies (1) are sequentially connected in cascade through an extension line, and then the optical circulator is connected to a light detector, so as to obtain a hydrogen gas sensor based on a 45° tilted fiber grating; S7. After the hydrogen sensor based on 45° tilted fiber Bragg grating is manufactured, it is calibrated in a hydrogen environment. At least three hydrogen environments with different concentration gradients are used to calibrate the Fabry-Perot cavity in a step-by-step manner. The resonance order m of the Fabry-Perot cavity and the conversion coefficient k of the refractive index with hydrogen concentration are calibrated. The calibration formula is: Among them, ρ x is the different hydrogen concentrations, λ0 is the resonant wavelength when there is no hydrogen, λ x The resonant wavelengths under different hydrogen concentrations are obtained by establishing a set of equations based on the calibration data, and solving the resonance order m and the conversion coefficient k of the refractive index with hydrogen concentration.

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