An optical fiber grating hydrogen sensor and its preparation method

Abstract

The present invention discloses a fiber Bragg grating hydrogen sensor, which includes an optical fiber and a housing; a first fiber Bragg grating and a second fiber Bragg grating are connected in series on the optical fiber; the first fiber Bragg grating is close to the free end of the optical fiber and is used for temperature compensation; the surface of the fiber glass cladding of the second fiber Bragg grating has grooves; a hydrogen-sensitive material layer is arranged outside the fiber glass cladding of the second fiber Bragg grating; the optical fiber is encapsulated in the housing, so that the first fiber Bragg grating and the second fiber Bragg grating are in a natural relaxation state. The present invention uses femtosecond laser to perform microfabrication etching on the surface of the fiber Bragg grating, thereby breaking the stress accumulation between the glass cladding with an optically flat surface during operation and the hydrogen-sensitive material layer, improving the adhesion of the hydrogen-sensitive material layer, reducing the probability of fatigue delamination or rupture of the hydrogen-sensitive material thin film. At the same time, the hydrogen-sensitive material adheres to the inner side of the groove, and the optical fiber etched with the groove also becomes softer, enhancing the change in the grating wavelength caused by the expansion strain after hydrogen absorption, and significantly improving the sensitivity of the sensor.

Description

Technical Field

[0001] The present invention belongs to the field of fiber Bragg grating hydrogen sensing, and more specifically, relates to a fiber Bragg grating hydrogen sensor and a preparation method thereof. Background Art

[0002] As a clean energy source, hydrogen plays a crucial role in the new energy field, especially in fuel cells and electric vehicles. However, hydrogen has active chemical properties and is dangerous of explosion when its concentration in the air ranges from 4 to 75 vol%, which greatly limits the production, storage, transportation and use of hydrogen.

[0003] Fiber Bragg grating sensing technology has the advantages of intrinsically safe, strong anti-electromagnetic interference ability, high measurement accuracy, and can realize multi-parameter, long-distance, quasi-distributed measurement, etc., and has been widely applied to multiple industrial production fields.

[0004] The sensing mechanism of the fiber Bragg grating hydrogen sensor is based on the change of the grating wavelength caused by hydrogen being adsorbed onto the hydrogen-sensitive material coating. The performance of the hydrogen-sensitive material directly determines the performance of the hydrogen sensor. Metallic palladium (Pd) is widely used as the hydrogen-sensitive material in hydrogen sensors due to its reversible absorption and special selectivity for hydrogen. However, the phase change caused by high-concentration hydrogen entering palladium will cause phenomena such as bubbles and cracks in the pure palladium film. The adhesion of the palladium film to the substrate is an important factor affecting the stability and lifespan of the gas-sensitive film. If the bonding force between the palladium film and the substrate surface is small, the palladium film will fall off after experiencing the process of hydrogen absorption and desorption, seriously affecting the service life of the sensor.

[0005] The lattice phase of the pure palladium film will change from the α phase to the β phase after contacting hydrogen, and at the same time, the lattice expands. Existing research shows that this phenomenon easily leads to film delamination or rupture due to fatigue, resulting in zero drift and reduced stability of the sensor.

[0006] During the process of measuring hydrogen concentration, the ambient temperature of the sensor may change. Fiber Bragg gratings are sensitive to temperature, which in turn affects the measurement accuracy and sensitivity of hydrogen. In addition, after water molecules and dust particles adhere to the surface of the palladium film, the local diffusion rate of hydrogen atoms will be restricted, which in turn affects the measurement accuracy and sensitivity of hydrogen.

[0007] Therefore, in order to ensure the long-term stable operation of the fiber Bragg grating hydrogen sensor, it is necessary to research a hydrogen sensor with high sensitivity, high stability and long lifespan and its preparation method. Summary of the Invention

[0008] In view of the above defects or improvement needs of the prior art, the present invention provides a fiber Bragg grating hydrogen sensor and a preparation method thereof, the purpose of which is to improve the adhesion and strain amplitude between the fiber Bragg grating and the hydrogen sensitive material layer by machining grooves on the fiber Bragg grating cladding, thereby increasing the sensitivity of the fiber Bragg grating hydrogen sensor and extending its service life, thereby solving the technical problems of the existing fiber Bragg grating hydrogen sensor having a short service life due to the shedding of the hydrogen sensitive material layer and the need to improve the sensitivity of the fiber Bragg grating sensor.

[0009] To achieve the above object, according to one aspect of the present invention, a fiber Bragg grating hydrogen sensor is provided, comprising an optical fiber and a housing;

[0010] The optical fiber has a first optical fiber grating and a second optical fiber grating connected in series;

[0011] The first fiber grating is close to the free end of the optical fiber and is used for temperature compensation;

[0012] The surface of the optical fiber glass cladding of the second optical fiber Bragg grating has grooves; the outer side of the optical fiber glass cladding of the second optical fiber Bragg grating has a hydrogen sensitive material layer;

[0013] The optical fiber is packaged in the housing so that the first fiber grating and the second fiber grating are in a naturally relaxed state.

[0014] Preferably, in the fiber Bragg grating hydrogen sensor, the groove depth is between 2 and 6 um, the groove width is between 2 and 3 um, and the groove area on the surface of the fiber glass cladding of the second fiber Bragg grating accounts for less than or equal to 15%.

[0015] Preferably, the groove shape of the fiber grating hydrogen sensor is preferably a circular hole, a ring, or a scribed line.

[0016] Preferably, in the fiber grating hydrogen sensor, the hydrogen-sensitive material layer is a Pd-Y alloy film with a thickness of 180 to 200 nm, and the yttrium content in the Pd-Y alloy is 7.5% to 8.0%;

[0017] Preferably, in the fiber Bragg grating hydrogen sensor, a transition film is provided between the hydrogen sensitive material layer and the optical fiber glass cladding, and the transition film is a Ta2O5 transition film, and the thickness of the transition film is between 80 and 100 nm.

[0018] Preferably, in the fiber grating hydrogen sensor, the center wavelengths of the first fiber grating and the second fiber grating differ by more than 5 nm, the spacing is 20 mm, the wavelength range of the first fiber grating and the second fiber grating is 1525-1565 nm, and the grating length is 5 mm.

[0019] Preferably, for the fiber Bragg grating hydrogen sensor, its housing is a capillary steel tube. In a preferred embodiment, its inner diameter is 1 mm and its outer diameter is 5 mm. There are 2 fiber Bragg grating fixing grooves inside the steel tube.

[0020] According to another aspect of the present invention, there is provided a method for manufacturing the fiber Bragg grating hydrogen sensor, including the following steps:

[0021] (1) Grating writing: Use femtosecond laser to write the first fiber Bragg grating and the second fiber Bragg grating on the fiber core; the first fiber Bragg grating and the second fiber Bragg grating are in series on the fiber.

[0022] (2) Grooving: Expose the glass cladding of the second fiber Bragg grating, and use femtosecond laser to etch grooves on the glass cladding of the second fiber Bragg grating.

[0023] (3) Coating: Use magnetron sputtering to deposit a Pd-Y alloy film on the outer side of the glass cladding of the second fiber Bragg grating with grooves.

[0024] (4) Encapsulation: Glue and fix the coated fiber Bragg grating with the housing, so that the fiber Bragg grating is in a natural relaxation state.

[0025] Preferably, for the method for manufacturing the fiber Bragg grating hydrogen sensor, the wavelength of the femtosecond laser in step (2) is 800 nm. When grooving, rotate the fiber Bragg grating circumferentially to form a circumferentially uniform etching on the surface of the glass cladding of the second fiber Bragg grating, and the spot size is 2 - 3 μm; specifically, after the fiber Bragg grating rotates one week, move the spot axially step by step for the grooving process of the glass cladding.

[0026] Preferably, for the method for manufacturing the fiber Bragg grating hydrogen sensor, before depositing the Pd-Y alloy film, use magnetron sputtering to deposit a Ta2O5 film on the outer side of the glass cladding of the second fiber Bragg grating with grooves.

[0027] Generally speaking, compared with the prior art by the above technical solutions conceived by the present invention, the following beneficial effects can be achieved:

[0028] For the fiber Bragg grating sensor provided by the present invention, first use femtosecond laser to perform microfabrication and etching of grooves on the surface of the fiber Bragg grating, thereby breaking the stress accumulation between the optically flat glass cladding and the hydrogen-sensitive material layer during operation, improving the adhesion of the hydrogen-sensitive material layer, reducing the probability of fatigue delamination or rupture of the hydrogen-sensitive material film. At the same time, the hydrogen-sensitive material adheres to the inner side of the groove, and the fiber with the etched groove also becomes softer, enhancing the change in the grating wavelength caused by the expansion strain after hydrogen absorption, and significantly improving the sensitivity of the sensor. The fiber Bragg grating hydrogen sensor and its manufacturing method provided by the present invention can solve the problems of low sensitivity, poor stability, and short sensor life of the existing hydrogen sensors.

[0029] Preferably, a Ta2O5 transition thin film is sputter-deposited on the surface of the groove, which greatly improves the adhesion of the hydrogen-sensitive material coating to the surface of the optical fiber and significantly increases the service life of the sensor. Description of the Drawings

[0030] Figure 1 is a schematic cross-sectional structure diagram of the fiber Bragg grating sensor provided by the present invention;

[0031] Figure 2 is a schematic diagram of the optical fiber structure adopted by the fiber Bragg grating sensor provided by the present invention;

[0032] Figure 3 is a schematic diagram of the groove on the surface of the fiber Bragg grating glass cladding provided in Embodiment 1 of the present invention;

[0033] Figure 4 is a schematic diagram of the groove on the surface of the fiber Bragg grating glass cladding provided in Embodiment 2 of the present invention;

[0034] Figure 5 is a schematic diagram of the groove on the surface of the fiber Bragg grating glass cladding provided in Embodiment 3 of the present invention.

[0035] In all the drawings, the same reference numerals are used to represent the same elements or structures, where:

[0036] 1 is the optical fiber cladding, 2 is the groove, 3 is the fiber core, 4 is the coating, 5 is the first fiber Bragg grating, 6 is the second fiber Bragg grating, 7 is the Ta2O5 thin film, 8 is the Pd-Y alloy thin film, 9 is the fiber Bragg grating fixing groove, and 10 is the housing. Detailed Embodiments

[0037] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0038] The fiber Bragg grating hydrogen sensor provided by the present invention, as Figure 1 shown, includes an optical fiber and a housing;

[0039] As Figure 2 shown, the optical fiber has a first fiber Bragg grating and a second fiber Bragg grating connected in series; the central wavelengths of the first fiber Bragg grating and the second fiber Bragg grating differ by more than 5 nm, the distance between them is 20 mm, the wavelength ranges of the first fiber Bragg grating and the second fiber Bragg grating are between 1525 and 1565 nm, and the grating region length is 5 mm.

[0040] The optical fiber is encapsulated in the housing such that the first fiber Bragg grating and the second fiber Bragg grating are in a natural relaxation state.

[0041] The first fiber Bragg grating is close to the free end of the optical fiber and is used for temperature compensation. The outer side of the glass part of the second fiber Bragg grating has an optical fiber coating;

[0042] The surface of the fiber glass cladding of the second fiber Bragg grating has grooves; the depth of the grooves is between 2 and 6 μm, the width of the grooves is between 2 and 3 μm, and the area ratio of the grooves on the surface of the fiber glass cladding of the second fiber Bragg grating is less than or equal to 15%. The shape of the grooves is preferably round hole type, annular, or scribed line; the outer side of the fiber glass cladding of the second fiber Bragg grating has a hydrogen-sensitive material layer; the hydrogen-sensitive material layer is a Pd-Y alloy thin film with a thickness of 180 to 200 nm, and the yttrium content in the Pd-Y alloy is 7.5% to 8.0%; there is a transition thin film between the hydrogen-sensitive material layer and the fiber glass cladding, and the transition thin film is a Ta2O5 transition thin film with a thickness between 80 and 100 nm.

[0043] The surface of the fiber glass cladding of the fiber Bragg grating is an optically flat surface, otherwise the grating cannot be accurately written. Based on its sensing mechanism, there is inevitably a stress difference between the glass part of the fiber Bragg grating and the hydrogen-sensitive material layer during use. However, for the hydrogen-sensitive material attached to the surface of the fiber Bragg grating with an optically flat glass cladding, due to stress accumulation, the film delaminates or ruptures due to fatigue, causing zero drift and reduced stability of the sensor. The grooves on the fiber glass cladding of the fiber Bragg grating destroy the optical flatness of the surface of the fiber glass cladding and break the stress accumulation of the hydrogen-sensitive material layer during operation. When the hydrogen-sensitive material layer expands, the stress directions are no longer unified, and there is a component in the radial direction of the optical fiber at the grooves, making it difficult for the hydrogen-sensitive material layer to accumulate stress along the axial direction of the optical fiber, thereby reducing the probability of delamination of the hydrogen-sensitive material. At the same time, the hydrogen-sensitive material attached to the inner side of the grooves has a more obvious strain during operation, and the change in the grating wavelength caused by the expansion strain after hydrogen absorption increases, significantly improving the sensitivity of the sensor. Therefore, the sensor with circular grooves has the same sensitivity as that with long grooves and a longer lifespan.

[0044] However, the depth of the groove is an important factor affecting the service life of the fiber Bragg grating hydrogen sensor. A groove that is too shallow cannot effectively break the stress accumulation in the hydrogen-sensitive material layer during operation, while a groove that is too deep will cause cracks in the fiber Bragg grating glass cladding, increasing the probability of fracture and failure. Both will shorten the service life of the sensor. In addition, the thickness of the hydrogen-sensitive material also affects the performance of the sensor, including service life and sensitivity. When the hydrogen-sensitive material layer is too thick, stress conduction cannot be effectively broken, resulting in a shorter service life of the sensor. When the hydrogen-sensitive material layer is too thin, it cannot effectively sense hydrogen to generate a large enough change amount, resulting in a decrease in the sensitivity of the sensor. The present invention forms a groove on the surface of the fiber glass cladding by laser etching, controls the depth and width of the groove, extends the service life of the fiber Bragg grating hydrogen sensor, and improves stability. At the same time, by controlling the thickness and material of the hydrogen-sensitive material, it better helps to extend the service life and improve the sensitivity.

[0045] Furthermore, adding a transition film between the hydrogen-sensitive material layer and the fiber glass cladding greatly improves the adhesion of the hydrogen-sensitive material coating on the fiber surface and significantly extends the service life of the sensor.

[0046] The housing is a capillary steel pipe. In a preferred embodiment, its inner diameter is 1 mm and its outer diameter is 5 mm. There are 2 fiber Bragg grating fixing grooves inside the steel pipe.

[0047] The preparation method of the fiber Bragg grating hydrogen sensor provided by the present invention includes the following steps:

[0048] (1) Writing gratings: Using femtosecond laser to write the first fiber Bragg grating and the second fiber Bragg grating on the fiber core; the fiber has a series of the first fiber Bragg grating and the second fiber Bragg grating; the central wavelengths of the first fiber Bragg grating and the second fiber Bragg grating differ by more than 5 nm, and the wavelength ranges of the first fiber Bragg grating and the second fiber Bragg grating are 1525 - 1565 nm. The wavelength of the femtosecond laser is 800 nm, and the spot size is 2 - 3 μm;

[0049] (2) Grooving: Exposing the glass cladding of the second fiber Bragg grating, and using femtosecond laser to etch a groove on the glass cladding of the second fiber Bragg grating; the depth of the groove is 2 - 6 μm, and the width of the groove is between 2 - 3 μm. The wavelength of the femtosecond laser is 800 nm. When grooving, the fiber Bragg grating rotates circumferentially to form a circumferentially uniform etching on the surface of the glass cladding of the second fiber Bragg grating, and the spot size is 2 - 3 μm. Specifically, after the fiber Bragg grating rotates one week, the spot moves axially step by step for the grooving process of the glass cladding.

[0050] Both the writing grating and grooving processes are completed using femtosecond laser. However, after grooving, the surface of the glass cladding is uneven, affecting its light transmission performance. Therefore, it is necessary to first perform the writing grating process on the fiber core and then perform the grooving process on the cladding.

[0051] (3) Coating: A Pd-Y alloy film is deposited on the outer side of the glass cladding of the second fiber grating with grooves by magnetron sputtering. The deposition time is 120 seconds. The Pd-Y alloy target has a size of φ50×3 mm and a purity of 4N. In a preferred embodiment, before depositing the Pd-Y alloy film, a Ta2O5 film is deposited on the outer side of the glass cladding of the second fiber grating with grooves by magnetron sputtering. The deposition time is 50 seconds. The Ta2O5 target has a size of φ50×6 mm and a purity of 4N.

[0052] When depositing the Pd-Y alloy film or Ta2O5 film by magnetron sputtering, the fiber grating is rotated circumferentially to form a film with a uniform thickness on the surface of the glass cladding of the second fiber grating.

[0053] (4) Encapsulation: The coated fiber grating is fixedly glued to the outer shell so that the fiber grating is in a natural relaxed state. In a preferred embodiment, the outer shell is a capillary steel tube with 2 fiber grating fixing grooves inside, and the first and second fiber gratings are respectively fixed in the fiber grating fixing grooves.

[0054] The following are the embodiments:

[0055] The fiber grating hydrogen sensor provided by the present invention includes an optical fiber and an outer shell. The optical fiber is encapsulated in the outer shell so that the first fiber grating and the second fiber grating are in a natural relaxed state.

[0056] The optical fiber has a first fiber grating and a second fiber grating connected in series. The central wavelength of the first fiber grating is 1540 nm, the central wavelength of the second fiber grating is 1550 nm, the grating region length of the grating is 5 mm, and the distance between the two fiber gratings is 20 mm.

[0057] The first fiber grating is close to the free end of the optical fiber and is used for temperature compensation. The outer side of the glass part of the first fiber grating has an optical fiber coating.

[0058] The surface of the fiber glass cladding of the second fiber grating has grooves, and its specification parameters are shown in Table 1. The outer side of the fiber glass cladding of the second fiber grating has a hydrogen-sensitive material layer. The hydrogen-sensitive material layer is a Pd-Y alloy film with a thickness of 180 - 200 nm, and the yttrium content in the Pd-Y alloy is 7.5% - 8.0%. There is a transition film between the hydrogen-sensitive material layer and the fiber glass cladding, and the transition film is a Ta2O5 transition film with a thickness between 80 - 100 nm.

[0059] Table 1 Specification parameters of the second fiber grating

[0060]

[0061] The outer shell is a capillary steel tube. In the preferred embodiment, its inner diameter is 1 mm and its outer diameter is 5 mm. There are two fiber Bragg grating fixing grooves inside the steel tube.

[0062] The preparation method of the fiber Bragg grating hydrogen sensor provided by the present invention includes the following steps:

[0063] (1) Grating writing: Use femtosecond laser to write the first fiber Bragg grating and the second fiber Bragg grating on the fiber core; the wavelength of the femtosecond laser is 800 nm. Write the grating point by femtosecond. The laser beam is focused on the fiber core, and the spot size is 2 - 3 μm. After writing one spot, the fiber is displaced to the next writing point under the displacement device, and the displacement interval is 1.608 μm. Then repeat the above steps.

[0064] (2) Groove etching: Use CO2 laser to strip the coating of the second fiber Bragg grating through an ablation process, and use femtosecond laser to etch grooves on the glass cladding of the second fiber Bragg grating. The wavelength of the femtosecond laser is 800 nm. When etching the grooves, rotate the fiber Bragg grating circumferentially to form a circumferentially uniform etching on the surface of the glass cladding of the second fiber Bragg grating. The laser beam is focused on the fiber cladding, and the spot size is 2 - 3 μm. When etching the grooves, the fiber Bragg grating rotates at a constant speed. After etching one cross-section, the fiber is displaced to the next writing cross-section, and then repeat the above steps. The writing interval is 20 μm.

[0065] Use CO2 laser to strip the coating of the second fiber Bragg grating through an ablation process, so that the glass cladding of the second fiber Bragg grating is exposed. The specific parameters of groove etching are shown in Table 2.

[0066] (3) Film coating: Use magnetron sputtering to deposit a Pd - Y alloy film on the outer side of the glass cladding of the second fiber Bragg grating with grooves. The deposition time is 120 seconds. The size of the Pd - Y alloy target is φ50×3 mm, and the purity is 4N; in the preferred embodiment, before depositing the Pd - Y alloy film, use magnetron sputtering to deposit a Ta2O5 film on the outer side of the glass cladding of the second fiber Bragg grating with grooves. The deposition time is 50 seconds. The size of the Ta2O5 target is φ50×6 mm, and the purity is 4N.

[0067] When depositing the Pd - Y alloy film or Ta2O5 film by magnetron sputtering, rotate the fiber Bragg grating circumferentially to form a film with a uniform thickness on the surface of the glass cladding of the second fiber Bragg grating.

[0068] (4) Encapsulation: Glue and fix the film - coated fiber Bragg grating with the outer shell, so that the fiber Bragg grating is in a natural relaxation state; the outer shell is a capillary steel tube, and there are two fiber Bragg grating fixing grooves inside, so that the first and second fiber Bragg gratings are respectively fixed in the fiber Bragg grating fixing grooves.

[0069] The preparation parameters are shown in Table 2 below:

[0070] Table 2 Preparation parameters

[0071]

[0072]

[0073] The fiber Bragg grating hydrogen sensors prepared in Examples 1 to 3 and Comparative Examples were subjected to sensitivity and life tests as follows:

[0074] Hydrogen with a concentration of 1% was introduced into the gas chamber. After 3 minutes, the change in the central wavelength of the grating was observed through a fiber Bragg grating demodulator to obtain the wavelength sensitivity of the fiber Bragg grating.

[0075] Thermal shock method: The fiber of the example was placed in a material thermal shock test chamber, heated to 250 °C and maintained for 1 hour. After maintaining for the specified time, the sample was sent to the 0 °C cold water area, and the above steps were repeated 3 times. Whether there were phenomena such as peeling, flaking, and blistering of the sample was observed under a microscope;

[0076] The test results are shown in Table 3:

[0077] Table 3 Fiber Bragg Grating Hydrogen Sensor Test

[0078] Example 1 Example 2 Example 3 Comparative Example Sensitivity 38 pm 52 pm 44 pm 29 pm Lifetime The coating has no peeling The coating has no peeling The coating has no peeling The coating peels off

[0079] It is easy for those skilled in the art to understand that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. An optical fiber grating hydrogen sensor, characterized in that, It includes an optical fiber and a housing; A first fiber grating and a second fiber grating are connected in series on the optical fiber; The first fiber grating is close to the free end of the optical fiber and is used for temperature compensation; The surface of the fiber glass cladding of the second fiber grating has a groove; a hydrogen-sensitive material layer is provided outside the fiber glass cladding of the second fiber grating; the depth of the groove is between 2 and 6 μm, the width of the groove is between 2 and 3 μm, and the area ratio of the groove on the surface of the fiber glass cladding of the second fiber grating is less than or equal to 15%; the groove breaks the stress accumulation of the hydrogen-sensitive material layer during operation. When the hydrogen-sensitive material layer expands, the stress directions are no longer unified, and there is a component in the radial direction of the optical fiber at the groove; The optical fiber is encapsulated in the housing so that the first fiber grating and the second fiber grating are in a natural relaxation state.

2. The fiber Bragg grating hydrogen sensor according to claim 1, characterized in that The groove shape is round hole type, annular, or engraved line.

3. The fiber Bragg grating hydrogen sensor according to claim 1, wherein, The hydrogen-sensitive material layer is a Pd-Y alloy thin film with a thickness of 180 - 200 nm, and the yttrium content in the Pd-Y alloy is 7.5% - 8.0%.

4. The fiber Bragg grating hydrogen sensor according to claim 1, characterized in that, A transition thin film is provided between the hydrogen-sensitive material layer and the fiber glass cladding. The transition thin film is a Ta2O5 transition thin film, and the thickness of the transition thin film is between 80 and 100 nm.

5. The fiber Bragg grating hydrogen sensor according to claim 1, characterized in that, The central wavelengths of the first fiber grating and the second fiber grating differ by more than 5 nm, the distance between them is 20 mm, the wavelength range of the first fiber grating and the second fiber grating is 1525 - 1565 nm, and the grating region length is 5 mm.

6. The fiber Bragg grating hydrogen sensor according to claim 1, wherein The housing is a capillary steel tube.

7. The fiber Bragg grating hydrogen sensor according to claim 6, wherein, The inner diameter of the housing is 1 mm, the outer diameter is 5 mm, and two fiber grating fixing grooves are provided inside the steel tube.

8. A method for preparing an optical fiber grating hydrogen sensor according to any one of claims 1 to 7, characterized in that, It includes the following steps: (1) Grating writing: Use femtosecond laser to write the first fiber grating and the second fiber grating on the fiber core; a first fiber grating and a second fiber grating are connected in series on the optical fiber; (2) Grooving: Expose the glass cladding of the second fiber grating, and use femtosecond laser to etch a groove on the glass cladding of the second fiber grating; (3) Coating: Use magnetron sputtering to deposit a Pd-Y alloy thin film on the outside of the glass cladding of the second fiber grating with a groove; (4) Encapsulation: Glue and fix the coated fiber grating and the housing so that the fiber grating is in a natural relaxation state.

9. The preparation method of the fiber grating hydrogen sensor according to claim 8, characterized in that, In step (2), the wavelength of the femtosecond laser is 800 nm. When grooving, the fiber grating rotates circumferentially to form circumferentially uniform etching on the surface of the glass cladding of the second fiber grating, and the spot size is 2 - 3 μm; specifically, after the fiber grating rotates one week, the spot axially steps and moves.

10. The preparation method of the fiber grating hydrogen sensor according to claim 8, characterized in that, Before depositing the Pd-Y alloy thin film, use magnetron sputtering to deposit a Ta2O5 thin film on the outside of the glass cladding of the second fiber grating with a groove.

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