Distributed polarization-maintaining fiber bragg grating hydrogen sensing system

Abstract

The invention discloses a distributed polarization-maintaining fiber bragg grating hydrogen sensing system, which relates to the technical field of fiber hydrogen sensing and comprises a fiber bragg grating demodulator and a plurality of sensing units. A plurality of polarization-maintaining fiber bragg grating hydrogen sensing heads with different grating periods in the sensing unit are connected in series through a first connecting optical fiber; the fiber bragg grating demodulator comprises a plurality of channels, and the channels are connected with the sensing unit through first connecting optical fibers; the polarization-maintaining fiber bragg grating hydrogen sensing head comprises a polarization-maintaining fiber bragg grating of which the side surface is plated with a hydrogen sensitive film, an optical fiber flange plate, a second connecting optical fiber, an optical fiber fixing structure and a hollow packaging structure; the polarization-maintaining fiber bragg grating with the hydrogen-sensitive film plated on the side face sequentially comprises a polarization-maintaining fiber bragg grating and the hydrogen-sensitive film from inside to outside. The hydrogen sensitive film deforms after absorbing and releasing hydrogen, and the wavelength interval of the output reflection peak of the polarization maintaining fiber grating is changed; the distributed polarization-maintaining fiber bragg grating hydrogen sensing system provided by the invention is high in sensitivity, easy to network, simple in structure and capable of realizing distributed hydrogen sensing.

Description

Technical Field

[0001] This invention relates to the field of fiber optic hydrogen sensing technology, and in particular to a distributed polarization-maintaining fiber Bragg grating hydrogen sensing system. Background Technology

[0002] Hydrogen, as a representative of clean energy, has demonstrated significant advantages in energy, defense, and industry, and its applications are becoming increasingly widespread. However, hydrogen is highly susceptible to leakage and explosion during production, use, and storage, and because it is colorless and odorless, it is difficult to detect. Therefore, monitoring hydrogen concentration is crucial throughout these processes.

[0003] Sensors used to detect hydrogen concentration include electrochemical catalytic combustion type, electrochemical semiconductor type, optical interferometry type, and fiber optic type. Currently, hydrogen sensors are mainly based on electrochemical methods, and the technology is relatively mature. However, most of these sensors are based on electrical signals, which can easily generate electrical sparks during use. Some even need to operate under high temperature and oxygen-containing conditions, posing significant safety hazards.

[0004] Fiber optic hydrogen sensors are gaining increasing attention in industry due to their advantages such as corrosion resistance, electromagnetic interference resistance, and intrinsic safety. Their good stability, high sensitivity, and relatively simple structure, coupled with the fact that they do not generate electric sparks compared to electrochemical hydrogen sensors, make them a key research area in hydrogen sensors. However, most existing fiber optic hydrogen sensors are point-type structures, making it difficult to achieve distributed network detection, limiting the range of possible solutions and resulting in complex structures. Summary of the Invention

[0005] The purpose of this invention is to provide a distributed polarization-maintaining fiber Bragg grating hydrogen sensing system, which solves the problems of existing fiber optic hydrogen sensors being unable to achieve distributed measurement and having low sensitivity due to relying solely on the longitudinal strain of the fiber Bragg grating.

[0006] To achieve the above objectives, the present invention provides a distributed polarization-maintaining fiber Bragg grating hydrogen sensing system, comprising: a fiber Bragg grating demodulator and multiple sensing units; each sensing unit includes multiple polarization-maintaining fiber Bragg grating hydrogen sensing heads with different grating periods and a first connecting fiber; the multiple polarization-maintaining fiber Bragg grating hydrogen sensing heads are connected in series through the first connecting fiber; the fiber Bragg grating demodulator includes multiple channels, which are connected to the sensing units through the first connecting fiber; thereby realizing distributed hydrogen concentration detection in environments such as hydrogen storage, transportation, and use over a wide range;

[0007] The polarization-maintaining fiber Bragg grating hydrogen sensor head includes a polarization-maintaining fiber Bragg grating with a hydrogen-sensitive film coated on its sides, a fiber optic flange, a second connecting fiber, a fiber fixing structure, and a hollow encapsulation structure. The polarization-maintaining fiber Bragg grating with a hydrogen-sensitive film coated on its sides, the second connecting fiber, and the fiber fixing structure are disposed inside the hollow encapsulation structure. Both sides of the polarization-maintaining fiber Bragg grating with a hydrogen-sensitive film coated on its sides are connected to the fiber optic flange through the second connecting fiber. The fiber optic flange is fixed at both ends of the hollow encapsulation structure. Fiber fixing structures are provided at both ends of the polarization-maintaining fiber Bragg grating with a hydrogen-sensitive film coated on its sides for fixing the second connecting fiber.

[0008] The polarization-maintaining fiber grating with a hydrogen-sensitive film on the side consists of a polarization-maintaining fiber grating and a hydrogen-sensitive film, from the inside out.

[0009] The hydrogen-sensitive membrane deforms after absorbing and releasing hydrogen, changing the wavelength spacing of the reflection peaks output by the polarization-maintaining fiber grating with the hydrogen-sensitive membrane on its side. The hydrogen concentration can be measured by measuring the change in wavelength spacing.

[0010] The expression for the wavelength interval of the reflection peak is:

[0011]

[0012] In the formula, Δλ is the wavelength interval, and K H is the hydrogen sensitivity coefficient, and C is the hydrogen concentration;

[0013] In equation (1), the expression for the hydrogen sensitivity coefficient is:

[0014]

[0015] In the formula, η is the strain-induced birefringence coefficient; Λ is the grating period; and E Pd E represents the elastic modulus of the hydrogen-sensitive membrane. F The elastic modulus of a polarization-maintaining fiber grating; A Pd A is the cross-sectional area of ​​the hydrogen-sensitive membrane. F K is the cross-sectional area of ​​the polarization-maintaining fiber grating. S υ is the Sievert constant; υ is the Poisson's ratio of the polarization-maintaining fiber grating.

[0016] Furthermore, the hydrogen-sensitive membrane consists of two metal layers: an inner transition layer and an outer alloy hydrogen-sensitive membrane.

[0017] Furthermore, a transition layer and an alloy hydrogen-sensitive film are deposited using multi-target magnetron sputtering. This method allows for adjustment of the film thickness and atomic ratio as needed, completing the deposition in one step. This reduces the number of times the film is charged and released and the sample is removed and placed. The resulting film is of high quality and the deposition process is simple.

[0018] Furthermore, the polarization-maintaining fiber grating with a hydrogen-sensitive film deposited on the side is positioned in the center of the hollowed-out encapsulation structure.

[0019] Furthermore, each polarization-maintaining fiber Bragg grating hydrogen sensor head is provided with a polarization-maintaining fiber Bragg grating with a hydrogen-sensitive film coated on the side, two fiber fixing structures, two second connecting fibers, and two fiber flanges; the fiber fixing structures, second connecting fibers, and fiber flanges are symmetrically arranged on both sides of the polarization-maintaining fiber Bragg grating with a hydrogen-sensitive film coated on the side.

[0020] Furthermore, the transition layer is a metal film with good quartz affinity; the alloy hydrogen-sensitive film is a palladium-silver hydrogen-sensitive film.

[0021] Furthermore, when the hydrogen-sensitive membrane absorbs and releases hydrogen, the lattice constant changes, causing changes in the refractive index and volume of the hydrogen-sensitive membrane itself.

[0022] When the volume of the hydrogen-sensitive membrane changes, it generates axial and radial stresses on the polarization-maintaining fiber grating, altering the birefringence of the fiber grating and causing a change in the wavelength spacing of the different reflection peaks output by the polarization-maintaining fiber grating with the hydrogen-sensitive membrane on its side.

[0023] Furthermore, the fiber grating demodulator emits a broadband light source, which passes through the first and second connecting fibers to reach the polarization-maintaining fiber grating with a hydrogen-sensitive film on its side. The broadband light source is reflected at the polarization-maintaining fiber grating with a hydrogen-sensitive film on its side. The reflected light at the polarization-maintaining fiber grating with a hydrogen-sensitive film on its side passes through the first and second connecting fibers and returns to the fiber grating demodulator.

[0024] The birefringence of a polarization-maintaining fiber grating generates two main polarization axes within the fiber: the fast axis, which is the polarization axis with a faster light propagation speed, and the slow axis, which is the polarization axis with a slower light propagation speed, passing through the midpoint of the two circular stress regions. The reflected light along the fast axis forms the first peak in the emission spectrum of the polarization-maintaining fiber grating with a hydrogen-sensitive film on its side; the reflected light along the slow axis forms the second peak in the emission spectrum of the polarization-maintaining fiber grating with a hydrogen-sensitive film on its side.

[0025] The fiber grating demodulator receives the reflection spectrum of a polarization-maintaining fiber grating with a hydrogen-sensitive film coated on its side, demodulates the reflection spectrum to obtain the center wavelengths of the two reflection peaks, and outputs the wavelength interval between the two reflection peaks.

[0026] Therefore, the present invention employs the above-mentioned distributed polarization-maintaining fiber Bragg grating hydrogen sensing system, which has high sensitivity, is easy to network, and has a simple structure, and can realize distributed hydrogen sensing.

[0027] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the structure of a distributed polarization-maintaining fiber grating hydrogen sensing system according to the present invention.

[0029] Figure 2 This is a schematic diagram of a packaging structure for a polarization-maintaining fiber Bragg grating hydrogen sensor head according to an embodiment of the present invention.

[0030] Figure 3 This is a cross-sectional schematic diagram of a polarization-maintaining fiber grating according to an embodiment of the present invention;

[0031] Figure 4 This is a schematic diagram of another packaging structure for the polarization-maintaining fiber Bragg grating hydrogen sensor head according to an embodiment of the present invention.

[0032] Figure 5 The spectrum output by the polarization-maintaining fiber Bragg grating hydrogen sensor;

[0033] Figure 6 The curve shows the transformation between hydrogen concentration and the wavelength interval of the two reflection peaks in the output spectrum of the polarization-maintaining fiber grating hydrogen sensor.

[0034] Figure Labels

[0035] 1. Fiber Bragg grating demodulator; 2. Polarization-maintaining fiber Bragg grating hydrogen sensor head; 21. Polarization-maintaining fiber Bragg grating with hydrogen-sensitive film on the side; 211. Polarization-maintaining fiber Bragg grating; 212. Transition layer; 213. Palladium-silver hydrogen-sensitive film; 22. Fiber optic flange; 23. Second connecting fiber; 24. Fiber optic fixing structure; 25. Hollow-out encapsulation structure; 3. First connecting fiber. Detailed Implementation

[0036] The following detailed description of embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0037] Example 1

[0038] Please see Figure 1 A distributed polarization-maintaining fiber Bragg grating hydrogen sensing system, comprising:

[0039] The fiber grating demodulator 1 includes multiple sensing units; each sensing unit includes multiple polarization-maintaining fiber grating hydrogen sensing heads 2 and a first connecting fiber 3; the multiple polarization-maintaining fiber grating hydrogen sensing heads 2 are connected in series through the first connecting fiber 3; the fiber grating demodulator 1 includes multiple channels, which are connected to the sensing units through the first connecting fiber 3; thus realizing distributed hydrogen concentration detection in environments such as large-scale hydrogen storage, transportation and use.

[0040] Please see Figure 2The polarization-maintaining fiber Bragg grating hydrogen sensor head 2 includes a polarization-maintaining fiber Bragg grating 21 with a hydrogen-sensitive film coated on its sides, an optical fiber flange 22, a second connecting optical fiber 23, an optical fiber fixing structure 24, and a hollow encapsulation structure 25. The polarization-maintaining fiber Bragg grating 21 with a hydrogen-sensitive film coated on its sides, the second connecting optical fiber 23, and the optical fiber fixing structure 24 are disposed inside the hollow encapsulation structure 25. Both sides of the polarization-maintaining fiber Bragg grating 21 with a hydrogen-sensitive film coated on its sides are connected to the optical fiber flange 22 through the second connecting optical fiber 23. The optical fiber flange 22 is fixed at both ends of the hollow encapsulation structure 25. Optical fiber fixing structures 24 are provided at both ends of the polarization-maintaining fiber Bragg grating 21 with a hydrogen-sensitive film coated on its sides for fixing the second connecting optical fiber 23.

[0041] A polarization-maintaining fiber grating 21 with a hydrogen-sensitive film coated on its side is positioned at the center of the hollow encapsulation structure 25. Each polarization-maintaining fiber grating hydrogen sensor head 2 contains one polarization-maintaining fiber grating 21 with a hydrogen-sensitive film coated on its side, two fiber fixing structures 24, two second connecting fibers 23, and two fiber flanges 22; the fiber fixing structures 24, the second connecting fibers 23, and the fiber flanges 22 are symmetrically arranged on both sides of the polarization-maintaining fiber grating 21 with a hydrogen-sensitive film coated on its side.

[0042] Please see Figure 3 The polarization-maintaining fiber grating 21 with a side-coated hydrogen-sensitive film consists of a polarization-maintaining fiber grating 211 and a hydrogen-sensitive film, arranged from the inside out. The hydrogen-sensitive film comprises two metal layers: an inner transition layer 212 and an outer alloy hydrogen-sensitive film. The transition layer 212 is a metal film with good affinity for quartz; the alloy hydrogen-sensitive film is a palladium-silver hydrogen-sensitive film 213. When the hydrogen-sensitive film absorbs or releases hydrogen, it deforms, thereby altering the birefringence of the side-coated hydrogen-sensitive film. During the absorption and release of hydrogen, the lattice constant of the hydrogen-sensitive film changes, causing changes in its refractive index and volume. This volume change generates axial and radial stresses on the polarization-maintaining fiber grating 211, altering its birefringence and changing the wavelength spacing of the different reflection peaks output by the side-coated hydrogen-sensitive film.

[0043] The expression for the wavelength interval of the reflection peak is:

[0044]

[0045] In the formula, Δλ is the wavelength interval, and K H is the hydrogen sensitivity coefficient, and C is the hydrogen concentration;

[0046] In equation (1), the expression for the hydrogen sensitivity coefficient is:

[0047]

[0048] In the formula, η is the strain-induced birefringence coefficient; Λ is the grating period; and EPd E represents the elastic modulus of the hydrogen-sensitive membrane. F The elastic modulus of a polarization-maintaining fiber grating; A Pd A is the cross-sectional area of ​​the hydrogen-sensitive membrane. F K is the cross-sectional area of ​​the polarization-maintaining fiber grating. S is the Sievert constant; is the Poisson's ratio of the polarization-maintaining fiber grating.

[0049] The transition layer 212 and the alloy hydrogen-sensitive film were deposited using multi-target magnetron sputtering. This method allows for adjustment of the film thickness and atomic ratio as needed, completing the deposition in one step, reducing the number of gas filling and venting operations and sample handling, resulting in high-quality films and a simple deposition process.

[0050] The fiber optic demodulator 1 emits a broadband light source, which travels through the first connecting fiber 3 and the second connecting fiber 23 to a polarization-maintaining fiber grating 21 with a hydrogen-sensitive film deposited on its side. The broadband light source is reflected at the polarization-maintaining fiber grating 21. The reflected light from the polarization-maintaining fiber grating 21 travels through the first connecting fiber 3 and the second connecting fiber 23 back to the fiber optic demodulator 1. The birefringence of the polarization-maintaining fiber grating generates two principal polarization axes within the fiber; the faster axis is the polarization axis with the higher light propagation speed. The slow axis is the polarization axis that passes through the midpoint of the two circular stress regions and has a slower light propagation speed; the reflected light in the fast axis direction forms the first peak in the emission spectrum of the polarization-maintaining fiber grating 21 with a hydrogen-sensitive film on its side; the reflected light in the slow axis direction forms the second peak in the emission spectrum of the polarization-maintaining fiber grating 21 with a hydrogen-sensitive film on its side; the fiber grating demodulator 1 receives the reflection spectrum of the polarization-maintaining fiber grating 21 with a hydrogen-sensitive film on its side, demodulates the reflection spectrum to obtain the center wavelengths of the two reflection peaks, and outputs the wavelength interval of the two reflection peaks.

[0051] Example 2

[0052] Please see Figure 4 Another packaging method for the polarization-maintaining fiber grating hydrogen sensor head 2 is as follows: The polarization-maintaining fiber grating hydrogen sensor head 2 includes a polarization-maintaining fiber grating 21 with a hydrogen-sensitive film coated on the side, an optical fiber fixing structure 24, and a hollow packaging structure 25; the optical fiber fixing structure 24 is installed at both ends of the hollow packaging structure 25, and the polarization-maintaining fiber grating 21 with a hydrogen-sensitive film coated on the side passes through the optical fiber fixing structure 24 and crosses the central axis of the hollow packaging structure 25.

[0053] The polarization-maintaining fiber grating hydrogen sensor head 2 is cascaded with the next polarization-maintaining fiber grating hydrogen sensor head 2 through the first connecting fiber 3.

[0054] Experimental verification

[0055] A chromium transition layer 212 with a thickness of 20 nm and a palladium-silver hydrogen-sensitive film 213 with a thickness of 500 nm are sequentially deposited on the surface of the polarization-maintaining fiber grating 211 to obtain a polarization-maintaining fiber grating 21 with a hydrogen-sensitive film deposited on the side. The ratio of palladium to silver atoms in the palladium-silver hydrogen-sensitive film 213 is 75:25.

[0056] The polarization-maintaining fiber grating 21 with the hydrogen-sensitive film coated on its side is assembled with the second connecting fiber 23, the fiber fixing structure 24, the fiber flange 22, and the hollow encapsulation structure 25 to obtain the polarization-maintaining fiber grating hydrogen sensor head 2.

[0057] A polarization-maintaining fiber Bragg grating hydrogen sensor head 2 is placed in a hydrogen experimental chamber, and standard hydrogen gases of different concentrations are introduced into the chamber. A platinum resistance thermometer is built into the hydrogen experimental chamber, which is then placed in a high-low temperature chamber. A fiber Bragg grating demodulator 1 is connected to the polarization-maintaining fiber Bragg grating hydrogen sensor head 2 via a first connecting fiber 3. The output spectra of the polarization-maintaining fiber Bragg grating hydrogen sensor head 2 at different hydrogen concentrations are tested and recorded, such as... Figure 5 As shown; after demodulating the output spectrum, the following is obtained: Figure 6 The curves showing the variation of hydrogen concentration with the wavelength interval of the hydrogen sensor with a side-coated hydrogen-sensitive film are shown. Figure 6 As can be seen, when the hydrogen concentration increases, the wavelength interval between the two reflection peaks increases, exhibiting a monotonic response characteristic.

[0058] Therefore, the present invention employs the above-mentioned distributed polarization-maintaining fiber Bragg grating hydrogen sensing system, which has high sensitivity, is easy to network, and has a simple structure, and can realize distributed hydrogen sensing.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A distributed polarization-maintaining fiber Bragg grating hydrogen sensing system, characterized in that, include: A fiber grating demodulator and multiple sensing units; each sensing unit includes multiple polarization-maintaining fiber grating hydrogen sensing heads with different grating periods and a first connecting fiber; Multiple polarization-maintaining fiber Bragg grating hydrogen gas sensor heads are connected in series via a first connecting fiber; the fiber Bragg grating demodulator includes multiple channels, which are connected to the sensing unit via the first connecting fiber. The polarization-maintaining fiber Bragg grating hydrogen sensor head includes a polarization-maintaining fiber Bragg grating with a hydrogen-sensitive film coated on its sides, a fiber optic flange, a second connecting fiber, a fiber fixing structure, and a hollow encapsulation structure. The polarization-maintaining fiber Bragg grating with a hydrogen-sensitive film coated on its sides, the second connecting fiber, and the fiber fixing structure are disposed inside the hollow encapsulation structure. Both sides of the polarization-maintaining fiber Bragg grating with a hydrogen-sensitive film coated on its sides are connected to the fiber optic flange through the second connecting fiber. The fiber optic flange is fixed at both ends of the hollow encapsulation structure. Fiber fixing structures are provided at both ends of the polarization-maintaining fiber Bragg grating with a hydrogen-sensitive film coated on its sides for fixing the second connecting fiber. The polarization-maintaining fiber grating with a hydrogen-sensitive film on the side consists of a polarization-maintaining fiber grating and a hydrogen-sensitive film, from the inside out. The hydrogen-sensitive membrane deforms after absorbing and releasing hydrogen, which changes the wavelength interval of the reflection peak output of the polarization-maintaining fiber grating with the hydrogen-sensitive membrane on the side. The hydrogen concentration can be measured by measuring the change in the wavelength interval of the reflection peak. The expression for the wavelength interval of the reflection peak is: In the formula, Δλ is the wavelength interval, and K H is the hydrogen sensitivity coefficient, and C is the hydrogen concentration; In equation (1), the expression for the hydrogen sensitivity coefficient is: In the formula, η is the strain-induced birefringence coefficient; Λ is the grating period; and E Pd E represents the elastic modulus of the hydrogen-sensitive membrane. F The elastic modulus of the polarization-maintaining fiber grating; A Pd A is the cross-sectional area of ​​the hydrogen-sensitive membrane. F K is the cross-sectional area of ​​the polarization-maintaining fiber grating. S υ is the Sievert constant; υ is the Poisson's ratio of the polarization-maintaining fiber grating.

2. The distributed polarization-maintaining fiber Bragg grating hydrogen sensing system according to claim 1, characterized in that: The hydrogen-sensitive membrane consists of two metal layers: an inner transition layer and an outer alloy hydrogen-sensitive membrane.

3. The distributed polarization-maintaining fiber Bragg grating hydrogen sensing system according to claim 2, characterized in that: The transition layer and alloy hydrogen-sensitive film were deposited using multi-target magnetron sputtering.

4. The distributed polarization-maintaining fiber Bragg grating hydrogen sensing system according to claim 1, characterized in that: The polarization-maintaining fiber grating with a hydrogen-sensitive film on the side is positioned in the center of the hollowed-out encapsulation structure.

5. A distributed polarization-maintaining fiber Bragg grating hydrogen sensing system according to claim 1, characterized in that: Each polarization-maintaining fiber Bragg grating hydrogen sensor head is provided with a polarization-maintaining fiber Bragg grating with a hydrogen-sensitive film coated on the side, two fiber fixing structures, two second connecting fibers, and two fiber flanges; the fiber fixing structures, second connecting fibers, and fiber flanges are symmetrically arranged on both sides of the polarization-maintaining fiber Bragg grating with a hydrogen-sensitive film coated on the side.

6. A distributed polarization-maintaining fiber Bragg grating hydrogen sensing system according to claim 2, characterized in that: The transition layer is a metal film with good affinity for quartz; the alloy hydrogen-sensitive film is a palladium-silver hydrogen-sensitive film.

7. A distributed polarization-maintaining fiber Bragg grating hydrogen sensing system according to claim 1, characterized in that: When the hydrogen-sensitive membrane absorbs and releases hydrogen, the lattice constant changes, causing changes in the refractive index and volume of the hydrogen-sensitive membrane itself. When the volume of the hydrogen-sensitive membrane changes, it generates axial and radial stresses on the polarization-maintaining fiber grating, altering the birefringence of the fiber grating and causing a change in the wavelength spacing of the reflection peaks output by the fiber grating with the hydrogen-sensitive membrane on its side.

8. A distributed polarization-maintaining fiber Bragg grating hydrogen sensing system according to claim 1, characterized in that: The fiber grating demodulator emits a broadband light source, which passes through the first and second connecting fibers to reach the polarization-maintaining fiber grating with a hydrogen-sensitive film on its side. The broadband light source is reflected at the polarization-maintaining fiber grating with a hydrogen-sensitive film on its side. The reflected light from the polarization-maintaining fiber grating with the hydrogen-sensitive film on the side passes through the first connecting fiber and the second connecting fiber and returns to the fiber grating demodulator. The birefringence of a polarization-maintaining fiber grating generates two main polarization axes within the fiber: the fast axis, which is the polarization axis with a faster light propagation speed, and the slow axis, which is the polarization axis with a slower light propagation speed, passing through the midpoint of the two circular stress regions. The reflected light along the fast axis forms the first reflection peak in the emission spectrum of the polarization-maintaining fiber grating with a hydrogen-sensitive film on its side; the reflected light along the slow axis forms the second reflection peak in the emission spectrum of the polarization-maintaining fiber grating with a hydrogen-sensitive film on its side. The fiber grating demodulator receives the reflection spectrum of a polarization-maintaining fiber grating with a hydrogen-sensitive film coated on its side, demodulates the reflection spectrum to obtain the center wavelengths of the two reflection peaks, and outputs the wavelength interval between the two reflection peaks.

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