Temperature-insensitive probe type optical fiber hydrogen sensor based on vernier effect

Abstract

The invention discloses a vernier effect-based temperature-insensitive probe-type optical fiber hydrogen sensor, which comprises a broadband light source and a spectrum analyzer, the output end of the broadband light source is connected with the input end of the optical fiber circulator, and the output end of the optical fiber circulator is connected with the spectrum analyzer; the optical fiber circulator, the long-distance transmission single-mode optical fiber, the sensing unit, the single-mode optical fiber and the reference unit are connected in sequence; the sensing unit is used for sensing external hydrogen; the reference unit is used for providing a reference spectrum for the sensing unit, and an optical vernier effect is formed by introducing the reference spectrum, so that the sensitivity of hydrogen sensing is improved, and the influence of external environment temperature on a measurement system is reduced. The hydrogen sensor has the characteristics of compact structure, simple preparation process, high response sensitivity and insensitivity to environment temperature, and has important application requirements and practical value.

Description

Technical Field

[0001] The invention relates to the technical field of hydrogen sensors, and in particular to a temperature-insensitive probe-type optical fiber hydrogen sensor based on a vernier effect. Background Art

[0002] Compared with traditional electrical hydrogen sensors, fiber optic hydrogen sensors have been widely studied because of their strong anti-electromagnetic interference characteristics and no electric sparks during use. Currently, common fiber optic hydrogen sensors are mainly divided into interference type, surface plasmon resonance type, fiber grating type and evanescent field type.

[0003] Interference-type fiber optic hydrogen sensors are more commonly based on the Fabry-Perot interference principle. The sensing performance mainly depends on the hydrogen sensitive film. It has the advantages of simple preparation process, fast response speed, high sensitivity, etc., but it is easily affected by the external environment temperature and has relatively low stability. Surface plasmon resonance fiber optic hydrogen sensors have the advantages of compact structure and high sensitivity, but their manufacturing process is relatively complex and difficult to prepare in large quantities. They are also easily affected by the ambient temperature, which is not conducive to practical applications.

[0004] Fiber Bragg grating hydrogen sensors are used in distributed measurement because of their wavelength multiplexing capability, but their low sensitivity and how to suppress the influence of temperature are the problems that plague their practical application. Surface plasmon resonance fiber hydrogen sensors have the advantages of small size and high sensitivity, but they are also easily affected by ambient temperature and the preparation process is cumbersome.

[0005] The optical fiber hydrogen sensor based on the evanescent field principle has technical advantages such as flexible and compact structure, high sensitivity and easy batch preparation, but it is also easily affected by the external ambient temperature.

[0006] In summary, the technical defects of existing optical fiber hydrogen sensors are low sensitivity and susceptibility to external ambient temperature. Summary of the invention

[0007] In order to overcome the defects of the above existing technologies, the present invention proposes a temperature-insensitive probe-type optical fiber hydrogen sensor based on the vernier effect, which uses the vernier effect to improve the hydrogen measurement sensitivity of the sensor while eliminating the influence of ambient temperature on the measurement results. The hydrogen sensor has the characteristics of compact structure, simple preparation process, high response sensitivity, and insensitivity to ambient temperature, and has important application needs and practical value.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is:

[0009] A temperature-insensitive probe-type optical fiber hydrogen sensor based on vernier effect, comprising a broadband light source 1 and a spectrum analyzer 2;

[0010] The output end of the broadband light source 1 is connected to the port a of the optical fiber circulator 3, and the input end of the spectrum analyzer 2 is connected to the port b of the optical fiber circulator 3;

[0011] The c port on the optical fiber circulator 3 is connected to the input end of the long-distance single-mode optical fiber 4;

[0012] The long-distance transmission single-mode optical fiber 4, the sensing unit 5, the single-mode optical fiber 6, and the reference unit 7 are connected in sequence;

[0013] The sensor unit 5 is used to sense external hydrogen;

[0014] The reference unit 7 is used to provide a reference spectrum for the sensor unit 5. By introducing the reference spectrum, an optical vernier effect is formed to improve the sensitivity of hydrogen sensing and reduce the influence of the external environment temperature on the measurement system.

[0015] The sensing unit 5 comprises a first multi-core optical fiber 8 and a palladium film 9. The first multi-core optical fiber 8 is formed into a cone region and a waist region after being melt-tapered, wherein the cone region is located on both sides of the waist region, and the palladium film 9 is covered on the waist region by magnetron sputtering;

[0016] When the waist diameter of the first multi-core optical fiber 8 reaches tens of microns, the energy of each fiber core is strongly coupled, thereby changing the coupling state between the fiber cores, forming supermode interference, and then realizing spectral modulation of the input light;

[0017] The palladium film 9 is covered on the waist region of the first multi-core optical fiber 8 by magnetron sputtering; magnetron sputtering can ensure that a dense, uniform and highly adhesive palladium film is obtained, so that the palladium film 9 has better uniformity and fits tightly to the optical fiber surface.

[0018] The reference unit 7 includes a second multi-core optical fiber 10 and a gold film 11. The second multi-core optical fiber 10 is formed into a cone region and a waist region after being melt-tapered, wherein the cone region is located on both sides of the waist region, and the end face of the second multi-core optical fiber 10 is covered with a gold film 11 by magnetron sputtering; the gold film 11 is used to reflect light;

[0019] The second multi-core optical fiber 10 is in the same condition as the first multi-core optical fiber 8 after being melt-tapered, and the diameter of each fiber core and the core spacing are reduced in proportion to the tapered length;

[0020] When the waist diameter of the second multi-core optical fiber 10 reaches tens of microns, the energy of each fiber core is strongly coupled, thereby changing the coupling state between the fiber cores, forming supermode interference, and then realizing spectral modulation of the input light;

[0021] The gold film 11 is covered on the end face of the second multi-core optical fiber 10 by magnetron sputtering. Magnetron sputtering can ensure that a dense, uniform and highly adhesive gold film is obtained, so that the gold film 11 has good uniformity and fits closely to the optical fiber end face, which is more conducive to light reflection.

[0022] The output end of the first multi-core optical fiber 8 is connected to the input end of the single-mode optical fiber 6; the output end of the single-mode optical fiber 6 is connected to the input end of the second multi-core optical fiber 10; the output end of the long-distance transmission single-mode optical fiber 4 is connected to the input end of the first multi-core optical fiber 8.

[0023] The first multi-core optical fiber 8 is a seven-core optical fiber or a four-core optical fiber, with one core located in the center and the other side cores arranged in a regular hexagon around the center core, and the distance between the cores is 30-40 μm. The evanescent fields of the adjacent cores are severely overlapped by fusion taper to stimulate supermode interference;

[0024] The second multi-core optical fiber 10 is a seven-core optical fiber or a four-core optical fiber, with one core located in the center and the other side cores distributed in a regular hexagon around the center core, with a core spacing of 30 to 40 μm, and the same fused taper treatment is used to excite supermode interference to form a vernier spectrum.

[0025] The length of the first multi-core optical fiber 8 is about 10-12 mm, the thickness of the palladium film 9 is 5-10 nm, and the length required to cover the waist area is about 1 cm; the length of the second multi-core optical fiber 10 used in the reference unit 7 is about 10-12 mm, and the thickness of the gold film 11 is 100-150 nm.

[0026] A method for using a temperature-insensitive probe-type optical fiber hydrogen sensor based on a vernier effect comprises the following steps;

[0027] Step 1: Light is emitted from a broadband light source 1 (wavelength range 1540nm-1660nm) and enters port a of the optical fiber circulator 3. Then the light passes through port c of the optical fiber circulator 3 and the long-distance transmission single-mode optical fiber 4 and enters the sensing unit 5 and generates supermode interference through the first multi-core optical fiber 8 after fusion taper.

[0028] Step 2: The light generating supermode interference enters the reference unit 7 through the single-mode optical fiber 6 and passes through the second multi-core optical fiber 10 after fusion taper to superimpose a second supermode interference spectrum on the original supermode interference spectrum, thereby generating a vernier spectrum;

[0029] Step 3: After the cursor spectrum is generated, the light is reflected by the gold film 11 and passes through the second multi-core optical fiber 10, the single-mode optical fiber 6, the first multi-core optical fiber 8 and the long-distance transmission single-mode optical fiber 4 again, and then returns to the optical fiber circulator 3 through the c port, and finally enters the spectrum analyzer 2 through the b port of the optical fiber circulator 3.

[0030] In step 1, the effective refractive index n of the first multi-core optical fiber 8 is eff1 And the refractive index of the palladium film 9 is expressed as:

[0031]

[0032] Where k0 is the beam in vacuum, η is the transmission ratio of light in the first multi-core optical fiber 8, β is the propagation constant, n1, n Pd are the refractive indices of the first multi-core optical fiber 8 and the palladium film 9, respectively;

[0033] When hydrogen exists in the external environment, the relationship between the refractive index of the palladium film 9 and the hydrogen concentration is expressed as:

[0034]

[0035] Where n0 is the refractive index of palladium in the absence of hydrogen, A and B are constants, is the external hydrogen concentration.

[0036] In step 2, when the free spectrum range (FSR) of the interference spectrum generated by the first multi-core optical fiber 8 and the second multi-core optical fiber 10 is within a few nanometers, a vernier effect will occur, and the corresponding interference spectra FSR1 and FSR2 are:

[0037]

[0038] where n eff1 is the effective refractive index of the first multi-core optical fiber 8, r1 is the distance between the cores, and L1 is the length of the first multi-core optical fiber 8; n eff2 is the effective refractive index of the second multi-core optical fiber 10 , r2 is the distance between the cores, and L2 is the length of the second multi-core optical fiber 10 .

[0039] Among them, r1=r2, and the FSR of the two is similar by adjusting the lengths L1 and L2;

[0040] The free spectral range after the supermode interference spectrum generated by the first multi-core optical fiber 8 and the supermode interference spectrum generated by the second multi-core optical fiber 10 are superimposed is expressed as:

[0041]

[0042] The FSR generated by the vernier effect is several times that of FSR1. The magnification is:

[0043]

[0044] When the hydrogen concentration in the environment changes, the sensor output spectrum drifts regularly with the hydrogen concentration, and the drift Δλ is expressed as:

[0045]

[0046] Further, according to the thermo-optical effect, the refractive indices n1 and n2 of the first multi-core optical fiber 8 and the second multi-core optical fiber 10 change with the temperature, which can be expressed as:

[0047] n1(T0+ΔT)=n1(T0)+ε1ΔT

[0048] n2(T0+ΔT)=n2(T0)+ε2ΔT (7)

[0049] Where T0 is the initial temperature of the environment, ΔT is the change in temperature, ε1 and ε2 are the corresponding thermo-optic coefficients, where n1(T0) = n2(T0), ε1 = ε2;

[0050] When the ambient temperature increases, n1 and n2 increase simultaneously, causing the supermode interference spectra generated by the first multi-core optical fiber 8 and the second multi-core optical fiber 10 to drift to the right simultaneously. The corresponding drift amount is expressed as:

[0051]

[0052] Where λ0 is the wavelength of the incident light, Δn1 and Δn2 are the respective refractive index changes, Δλ1≈Δλ2, that is, the sensitivity S of the first multi-core optical fiber 8 and the second multi-core optical fiber 10 to temperature T1 , S T2 similar;

[0053] The sensitivity of the vernier spectrum to temperature is S T It is expressed as:

[0054]

[0055] Where FSR1≈FSR2, S T1≈ S T2 .

[0056] Beneficial effects of the present invention:

[0057] 1. The optical signal emitted by the broadband light source is injected into the sensing unit and the reference unit through the optical fiber ring and the first single-mode optical fiber. Due to the energy coupling between the cores of the multi-core optical fiber, a periodic optical vernier interference spectrum is generated, which is finally reflected and detected by the spectrum analyzer. When hydrogen exists in the environment, the refractive index of palladium changes after absorbing hydrogen, causing the effective refractive index of the multi-core optical fiber to change, and the interference spectrum detected by the spectrometer drifts. The detection of the hydrogen concentration in the environment is achieved by measuring the spectrum drift.

[0058] 2. Since the supermode interference spectra generated by the sensing unit and the reference unit have similar free spectral ranges, connecting the two in series will produce an optical vernier effect, which can be used to improve the sensitivity of hydrogen measurement and reduce the impact of ambient temperature on the sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 It is a schematic diagram of the structure of the present invention.

[0060] Figure 2 This is a schematic diagram of the cross-sectional structure of a multi-core optical fiber used in the present invention.

[0061] Figure 3 This is a supermode interference spectrum diagram of the multi-core optical fiber after fusion taper of the present invention.

[0062] Figure 4 Output the spectrum for the present invention.

[0063] Figure 5 Schematic diagram of the vernier spectrum envelope of the sensor under different hydrogen concentrations.

[0064] Reference numerals:

[0065] 1. Broadband light source 2. Spectrum analyzer 3. Optical fiber circulator 4. Long-distance transmission single-mode optical fiber 5. Sensing unit 6. Single-mode optical fiber 7. Reference unit 8. First multi-core optical fiber 9. Palladium film 10. Second multi-core optical fiber 11. Gold film. DETAILED DESCRIPTION

[0066] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0067] Reference Figure 1 A temperature-insensitive probe-type optical fiber hydrogen sensor based on the vernier effect includes a broadband light source 1, a spectrum analyzer 2, an optical fiber circulator 3, a long-distance transmission single-mode optical fiber 4, a sensing unit 5, a single-mode optical fiber 6, and a reference unit 7. The sensing unit 5 is composed of a first multi-core optical fiber 8 and a palladium film 9 after fusion taper; the reference unit 7 is composed of a second multi-core optical fiber 10 and a gold film 11 after fusion taper. The output end of the broadband light source 1 and the input end of the spectrum analyzer 2 are respectively connected to the a port and the b port of the optical fiber circulator 3, and the long-distance single-mode optical fiber 4, the sensing unit 5, the single-mode optical fiber 6 and the reference unit 7 are sequentially connected and finally connected to the c port of the optical fiber circulator 3.

[0068] The optical vernier effect is a high-precision measurement method based on the principle of optical interference. It can significantly improve the measurement resolution and sensitivity of optical fiber sensors, interferometers and other equipment. This effect originates from the superposition of two different periodic structures, which is similar to the working principle of a mechanical vernier. By superimposing interference signals of two different periods, a higher sensitivity can be obtained than that of any periodic structure.

[0069] The first multi-core optical fiber 8 and the second multi-core optical fiber 10 are microstructure optical fibers that generate supermode interference spectra through mode coupling between the fiber cores. The cross-sectional schematic diagram thereof is shown in FIG. Figure 2 shown.

[0070] From the attached Figure 2 It can be seen that there are 7 independent cores in the central area of ​​the optical fiber; among them, there is a single core in the center, and the other cores are arranged in a hexagonal shape around it; this arrangement makes the distance between the cores more uniform, which is beneficial to the stability of optical signal transmission; especially in the process of tapering, as the core spacing decreases, the core coupling phenomenon becomes more significant. This symmetrical arrangement has an important influence on the proportional reduction of the core diameter and the core spacing, which plays a vital role in the transmission performance, core coupling and practical application of optical fiber.

[0071] After the hydrogen flame is used to taper the fiber at a uniform speed, the radius of each core of the multi-core optical fiber and the corresponding core spacing are proportionally reduced. When the core diameter and core spacing are reduced to a certain extent, the core's ability to constrain the light field becomes weaker, resulting in the continuous enhancement of the coupling between adjacent cores, the gradual increase of the coupling coefficient, the significant change of the energy coupling state between the cores, and the serious overlap of the evanescent fields between adjacent cores. As a result, multiple supermodes will be excited in the core and supermode interference will be formed, eventually producing a periodic interference spectrum, such as Figure 3 shown.

[0072] The working mode of the present invention is:

[0073] Light is emitted from a broadband light source 1 and enters port a of a fiber circulator 3. Then, the light passes through port c of the fiber circulator 3 and a long-distance transmission single-mode optical fiber 4, enters a sensing unit 5, and generates supermode interference through a first multi-core optical fiber 8 after fusion taper.

[0074] The light generating supermode interference enters the reference unit 7 through the single-mode optical fiber 6 and passes through the second multi-core optical fiber 10 after fusion taper to superimpose the second supermode interference spectrum on the original supermode interference spectrum, thereby generating a vernier spectrum;

[0075] The light is then reflected by the gold film 11 and passes through the second multi-core optical fiber 10, the single-mode optical fiber 6, the first multi-core optical fiber 8 and the long-distance transmission single-mode optical fiber 4 again, and then returns to the optical fiber circulator 3 through the c port, and finally enters the spectrum analyzer 2 through the b port of the optical fiber circulator 3.

[0076] Since the evanescent field generated by the first multi-core optical fiber 8 after tapering is large, the effective refractive index n of the first multi-core optical fiber 8 is eff1 It depends on the refractive index of the multi-core optical fiber and the refractive index of the palladium film 9 and is expressed as:

[0077]

[0078] Where k0 is the beam in vacuum, η is the transmission ratio of light in the first multi-core optical fiber 8, β is the propagation constant, n1, n Pd are the refractive indices of the first multi-core optical fiber 8 and the palladium film 9 respectively.

[0079] When hydrogen exists in the external environment, the palladium film 9 absorbs hydrogen, causing its refractive index n Pd changes, which in turn causes the effective refractive index of the first multi-core optical fiber 8 to change, and ultimately causes the periodic interference spectrum to drift;

[0080] The relationship between the refractive index of the palladium film 9 and the hydrogen concentration is expressed as:

[0081]

[0082] Among them, n0 is the refractive index of palladium in the absence of hydrogen, A and B are constants, which are related to the thickness and particle size of the palladium film. As the concentration of hydrogen increases, the effect of hydrogen on the palladium film 9 becomes more intense, so the effective refractive index of the first multi-core optical fiber 8 changes more significantly.

[0083] The reference unit 7 does not directly participate in the hydrogen measurement, but serves as an auxiliary tool to provide a reference spectrum for the sensing unit 5 and form a vernier effect with it to improve the sensitivity of hydrogen measurement, so that the distance of the vernier spectrum drift observed on the spectrum analyzer 2 will be increased. By establishing a relationship between the fitted vernier spectrum envelope drift and the corresponding hydrogen concentration, accurate measurement of the hydrogen concentration can be achieved.

[0084] When the free spectrum range (FSR) of the interference spectrum generated by the first multi-core optical fiber 8 and the second multi-core optical fiber 10 is within a few nanometers, a vernier effect will occur, and the corresponding interference spectra FSR1 and FSR2 are:

[0085]

[0086] where n eff1 is the effective refractive index of the first multi-core optical fiber 8, r1 is the distance between the cores, and L1 is the length of the first multi-core optical fiber 8; n eff2 is the effective refractive index of the second multi-core optical fiber 10 , r2 is the distance between the cores, and L2 is the length of the second multi-core optical fiber 10 .

[0087] The multi-core optical fiber used in the present invention has the same structural parameters, so r1=r2. The surface of the first multi-core optical fiber 8 is coated with a palladium film 9, resulting in an effective refractive index n eff1 and n eff2There is a slight difference between them, but the FSR of the two can be made similar by adjusting the lengths L1 and L2.

[0088] The free spectral range after the supermode interference spectrum generated by the first multi-core optical fiber 8 and the supermode interference spectrum generated by the second multi-core optical fiber 10 are superimposed is expressed as:

[0089]

[0090] Therefore, compared with the FSR1 generated by the first multi-core optical fiber 8, the FSR generated by the vernier effect is several times greater than the FSR1. The magnification is:

[0091]

[0092] When the hydrogen concentration in the environment changes, the sensor output spectrum drifts regularly with the hydrogen concentration, and the drift Δλ can be expressed as:

[0093]

[0094] Through the optical vernier effect, the spectral drift can be greatly amplified, thereby improving the sensitivity of the sensor and realizing the measurement of low-concentration hydrogen in the environment.

[0095] The output spectrum of the temperature-insensitive probe-type optical fiber hydrogen sensor based on the vernier effect described in the present invention is as follows: Figure 4 As shown, the spectrum is the superposition result of the supermode interference spectra output by the sensing unit 5 and the reference unit 7.

[0096] In addition to forming a vernier effect with the sensing unit 5 to improve the measurement sensitivity of the sensor, the reference unit 7 also has another important function, namely, reducing the influence of the ambient temperature on the measurement result of the sensor.

[0097] According to the thermo-optical effect, the refractive indexes n1 and n2 of the first multi-core optical fiber 8 and the second multi-core optical fiber 10 change with the temperature, which can be expressed as:

[0098] n1(T0+ΔT)=n1(T0)+ε1ΔT

[0099] n2(T0+ΔT)=n2(T0)+ε2ΔT (16)

[0100] Where T0 is the initial temperature of the environment, ΔT is the change in temperature, and ε1 and ε2 are the corresponding thermo-optical coefficients. In the present invention, the first multi-core optical fiber 8 and the second multi-core optical fiber 10 used have the same material and structure, and it can be considered that n1(T0)=n2(T0), ε1=ε2. Therefore, the change in the external ambient temperature has the same effect on n1 and n2. When the external ambient temperature rises, n1 and n2 increase at the same time, causing the supermode interference spectra generated by the first multi-core optical fiber 8 and the second multi-core optical fiber 10 to drift to the right at the same time, and the corresponding drift amount is expressed as:

[0101]

[0102] Where λ0 is the wavelength of the incident light, Δn1 and Δn2 are the respective refractive index changes. From the above analysis, it can be seen that Δλ1≈Δλ2, that is, the sensitivity S of the first multi-core optical fiber 8 and the second multi-core optical fiber 10 to temperature T1 , S T2 similar.

[0103] The sensitivity of the vernier spectrum to temperature is S T It is expressed as:

[0104]

[0105] Where FSR1≈FSR2, S T1≈ S T2 Therefore, the sensitivity of the sensor to the ambient temperature is approximately zero, which can greatly reduce the impact of the ambient temperature on the sensor measurement results and achieve accurate measurement of the hydrogen concentration.

[0106] The thickness of the palladium film 9 is 5 to 10 nanometers. The advantage is that the nanometer-thick palladium can greatly shorten the hydrogen adsorption and desorption time, thereby improving the response speed of the sensor and achieving rapid measurement of hydrogen.

[0107] The sensor output vernier spectrum is monitored by a spectrometer to measure the hydrogen concentration. The vernier spectrum envelope of the sensor under different hydrogen concentrations is as follows: Figure 5 As shown. It can be clearly seen that with the increase of ambient hydrogen concentration, the spectral envelope red shifts. In the hydrogen concentration range of 0% to 1%, the sensor has a high response sensitivity of about 6.9nm / %.

Claims

1. A temperature-insensitive probe-type optical fiber hydrogen sensor based on the vernier effect, characterized in that: It comprises a broadband light source (1) and a spectrum analyzer (2); The output end of the broadband light source (1) is connected to the port a of the optical fiber circulator (3), and the input end of the optical spectrum analyzer (2) is connected to the port b of the optical fiber circulator (3); The c-port on the optical fiber circulator (3) is connected to the input end of the long-distance single-mode optical fiber (4); The long-distance transmission single-mode optical fiber (4), the sensing unit (5), the single-mode optical fiber (6), and the reference unit (7) are connected in sequence; The sensing unit (5) is used to sense external hydrogen; The reference unit (7) is used to provide a reference spectrum for the sensing unit (5), and an optical cursor effect is formed by introducing the reference spectrum to improve the sensitivity of hydrogen sensing and reduce the influence of the external environment temperature on the measurement system.

2. The temperature-insensitive probe-type optical fiber hydrogen sensor based on the vernier effect according to claim 1 is characterized in that: The sensing unit (5) comprises a first multi-core optical fiber (8) and a palladium film (9), wherein the first multi-core optical fiber (8) is subjected to melt tapering to form a cone region and a waist region, wherein the cone region is located on both sides of the waist region, and the palladium film (9) is covered on the waist region by magnetron sputtering; When the waist diameter of the first multi-core optical fiber (8) reaches tens of micrometers, the energy of each fiber core is strongly coupled, thereby changing the coupling state between the fiber cores, forming supermode interference, and further realizing spectral modulation of the input light; The palladium film (9) is covered on the waist region of the first multi-core optical fiber (8) by magnetron sputtering.

3. The temperature-insensitive probe-type optical fiber hydrogen sensor based on the vernier effect according to claim 2 is characterized in that: The reference unit (7) comprises a second multi-core optical fiber (10) and a gold film (11); the second multi-core optical fiber (10) is formed into a cone region and a waist region after being melt-tapered, wherein the cone region is located on both sides of the waist region; the end face of the second multi-core optical fiber (10) is covered with a gold film (11) by magnetron sputtering; the gold film (11) is used to reflect light; When the waist diameter of the second multi-core optical fiber (10) reaches tens of micrometers, the energy of each fiber core is strongly coupled, thereby changing the coupling state between the fiber cores, forming supermode interference, and further realizing spectral modulation of the input light.

4. The temperature-insensitive probe-type optical fiber hydrogen sensor based on the vernier effect according to claim 2 is characterized in that: The output end of the first multi-core optical fiber (8) is connected to the input end of the single-mode optical fiber (6); the output end of the single-mode optical fiber (6) is connected to the input end of the second multi-core optical fiber (10); and the output end of the long-distance transmission single-mode optical fiber (4) is connected to the input end of the first multi-core optical fiber (8).

5. The temperature-insensitive probe-type optical fiber hydrogen sensor based on the vernier effect according to claim 4 is characterized in that: The first multi-core optical fiber (8) is a seven-core optical fiber or a four-core optical fiber, with one core located in the center and the remaining side cores distributed in a regular hexagon around the center core, with a distance between the cores of 30 to 40 μm, and the evanescent fields of adjacent cores are severely overlapped by fusion taper to stimulate supermode interference; The second multi-core optical fiber (10) is a seven-core optical fiber or a four-core optical fiber, with one core located in the center and the remaining side cores distributed in a regular hexagon around the center core, the distance between the cores is 30 to 40 μm, and the same fused taper treatment is used to excite supermode interference to form a vernier spectrum.

6. The temperature-insensitive probe-type optical fiber hydrogen sensor based on the vernier effect according to claim 5 is characterized in that: The length of the first multi-core optical fiber (8) is 10-12 mm, the thickness of the palladium film (9) is 5-10 nm, and the length required to cover the waist area is 1 cm; the length of the second multi-core optical fiber (10) is 10-12 mm, and the thickness of the gold film (11) is 100-150 nm.

7. The operating method of a temperature-insensitive probe-type optical fiber hydrogen sensor based on vernier effect according to any one of claims 1 to 6, characterized in that: The steps include: Step 1: Light is emitted from a broadband light source (1) and enters port a of an optical fiber circulator (3). The light then passes through port c of the optical fiber circulator (3) and a long-distance transmission single-mode optical fiber (4) and enters a sensing unit (5) and generates supermode interference through a first multi-core optical fiber (8) after fusion taper. Step 2: The light generating supermode interference enters the reference unit (7) through the single-mode optical fiber (6) and passes through the second multi-core optical fiber (10) after fusion taper to superimpose a second supermode interference spectrum on the original supermode interference spectrum, thereby generating a vernier spectrum; Step 3: After the vernier spectrum is generated, the light is reflected by the gold film (11) and passes through the second multi-core optical fiber (10), the single-mode optical fiber (6), the first multi-core optical fiber (8) and the long-distance transmission single-mode optical fiber (4) again, and then returns to the optical fiber circulator (3) through the c port, and finally enters the optical spectrum analyzer (2) through the b port of the optical fiber circulator (3).

8. The operating method of the temperature-insensitive probe-type optical fiber hydrogen sensor based on the vernier effect according to claim 7 is characterized in that: In step 1, the effective refractive index n of the first multi-core optical fiber (8) is eff1 and the refractive index of the palladium film (9), expressed as: Where k0 is the wave beam in vacuum, η is the transmission ratio of light in the first multi-core optical fiber (8), β is the propagation constant, n1, n Pd are the refractive indices of the first multi-core optical fiber (8) and the palladium film (9), respectively; When hydrogen exists in the external environment, the relationship between the refractive index of the palladium film 9 and the hydrogen concentration is expressed as: Where n0 is the refractive index of palladium in the absence of hydrogen, A and B are constants, is the external hydrogen concentration.

9. The method for using the temperature-insensitive probe-type optical fiber hydrogen sensor based on the vernier effect according to claim 7, characterized in that: In step 2, when the free spectral range of the interference spectra generated by the first multi-core optical fiber (8) and the second multi-core optical fiber (10) is within a few nanometers, a vernier effect will occur, and the interference spectra FSR1 and FSR2 corresponding to the two are respectively: Where n eff1 is the effective refractive index of the first multi-core optical fiber (8), r1 is the distance between the cores, and L1 is the length of the first multi-core optical fiber (8); n eff2 is the effective refractive index of the second multi-core optical fiber (10), r2 is the distance between the cores, and L2 is the length of the second multi-core optical fiber (10); Among them, r1=r2, and the FSR of the two is similar by adjusting the lengths L1 and L2; The free spectral range after the supermode interference spectrum generated by the first multi-core optical fiber (8) and the supermode interference spectrum generated by the second multi-core optical fiber (10) are superimposed is expressed as: The FSR generated by the vernier effect is several times that of FSR1. The magnification is: When the hydrogen concentration in the environment changes, the sensor output spectrum drifts regularly with the hydrogen concentration, and the drift Δλ is expressed as:

10. The operating method of the temperature-insensitive probe-type optical fiber hydrogen sensor based on the vernier effect according to claim 9, characterized in that: According to the thermo-optical effect, the refractive indices n1 and n2 of the first multi-core optical fiber (8) and the second multi-core optical fiber (10) change with the change of temperature, which can be expressed as: n1(T0+ΔT)=n1(T0)+ε1ΔT n2(T0+ΔT)=n2(T0)+ε2ΔT (7) Where T0 is the initial temperature of the environment, ΔT is the change in temperature, ε1 and ε2 are the corresponding thermo-optic coefficients, where n1(T0) = n2(T0), ε1 = ε2; When the ambient temperature increases, n1 and n2 increase simultaneously, causing the supermode interference spectra generated by the first multi-core optical fiber (8) and the second multi-core optical fiber (10) to drift to the right simultaneously. The corresponding drift amount is expressed as: Where λ0 is the wavelength of the incident light, Δn1 and Δn2 are the respective refractive index changes, Δλ1≈Δλ2, that is, the sensitivity S of the first multi-core optical fiber (8) and the second multi-core optical fiber (10) to temperature is T1 , S T2 similar; The sensitivity of the vernier spectrum to temperature is S T It is expressed as: Where FSR1≈FSR2, S T1≈ S T2 .

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