Optical fiber twinborn controllable vernier hydrogen sensor and detection method

Abstract

The invention discloses an optical fiber twinning controllable vernier hydrogen sensor and a detection method, and belongs to the technical field of optical fiber sensing, the optical fiber twinning controllable vernier hydrogen sensor comprises a single-mode optical fiber, a femtosecond laser direct writing grating, a Fabry-Perot interferometer and a Pd / WO3 hydrogen sensitive film; the single-mode optical fiber comprises an optical fiber core and an optical fiber cladding, the femtosecond laser direct writing grating is prepared in the optical fiber core, the Fabry-Perot interferometer is composed of a reflecting surface 1 and a reflecting surface 2 which are relatively parallel to each other, the Pd / WO3 hydrogen sensitive film is coated on the inner side of the reflecting surface 2, and the femtosecond laser direct writing grating and the Fabry-Perot interferometer are arranged in series; the optical path of the Fabry-Perot interferometer is modulated through the interaction of the Pd / WO3 hydrogen sensitive film and hydrogen, interference spectrum wavelength drift is triggered, temperature compensation is achieved in combination with the temperature response of the femtosecond laser direct writing grating, and hydrogen concentration sensing is completed. The method is suitable for hydrogen leakage detection in industrial environments, energy equipment and other scenes.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and in particular to a fiber optic twin controllable vernier hydrogen sensor and detection method. Background Technology

[0002] Hydrogen, as a clean and efficient energy carrier, is widely used in new energy, aerospace, and chemical industries. However, its flammable and explosive properties place extremely high demands on the real-time and accurate detection of hydrogen concentration in the environment, especially for early warning detection of trace amounts of hydrogen. Fiber optic sensors, with their intrinsic safety, resistance to electromagnetic interference, fast response speed, and ease of remote networking, have become the mainstream technology in the field of hydrogen detection. Common types include fiber optic gratings (FBG), Fabry-Perot interferometers (FPI), spectral absorption fiber optic sensors, and surface plasmon resonance (SPR) fiber optic sensors. Although various fiber optic hydrogen sensors have been researched and applied to some extent, they generally suffer from two major problems: First, they have weak resistance to temperature and humidity interference. Existing technologies often lack dedicated temperature and humidity interference compensation mechanisms, making it easy for environmental temperature fluctuations and humidity changes to cross-couple with hydrogen concentration signals, resulting in significant measurement deviations and difficulty in adapting to complex working conditions. Second, their detection sensitivity is low and the flexibility of vernier adjustment is insufficient. Traditional sensing mechanisms are limited by hardware structure, making it impossible to achieve trace hydrogen detection or to adapt to different detection scenarios through flexible adjustment of vernier parameters, thus failing to meet the actual needs of early warning. For example, fiber Bragg grating hydrogen sensors are susceptible to the cross-effect of temperature and strain, and lack temperature and humidity compensation design, so fluctuations in temperature and humidity will directly interfere with the detection results; although traditional FPI and SPR type fiber optic hydrogen sensors have a fast response speed, their sensitivity can only meet the requirements of constant or trace detection, and humidity changes can easily lead to the degradation of the sensing interface performance. At the same time, they lack flexible vernier control methods, which further limits the applicability of detection.

[0003] Currently, existing fiber optic sensing technologies lack a solution that can simultaneously address resistance to temperature and humidity interference, ultra-trace detection, and flexible adaptability, making it difficult to meet the practical needs of hydrogen safety monitoring in complex environments. Chinese patent CN119827461B proposes a tapered fiber optic hydrogen sensor and its fabrication method, but it lacks a temperature and humidity compensation unit, resulting in weak resistance to temperature and humidity interference. Chinese patent CN111999265B proposes a tilted fiber Bragg grating hydrogen sensor based on UiO-66-NH-2, but its hydrogen measurement sensitivity is low. Summary of the Invention

[0004] The purpose of this invention is to provide a fiber optic twin controllable vernier hydrogen sensor and detection method, which integrates a temperature and humidity compensation sensitive unit, a twin model, and a software controllable vernier to form a synergistically optimized sensing mechanism, while solving three major problems: temperature and humidity interference, low sensitivity, and rigid control.

[0005] To achieve the above objectives, this invention provides a fiber-twin controllable vernier hydrogen sensor, comprising a single-mode fiber, a femtosecond laser direct-write grating, a Fabry-Perot interferometer, a polydimethylsiloxane (PDMS) layer, and a Pd / WO3 hydrogen-sensitive film. The single-mode fiber comprises a fiber core and a fiber cladding, with the femtosecond laser direct-write grating fabricated inside the fiber core. The Fabry-Perot interferometer consists of two relatively parallel reflecting surfaces, 1 and 2. The Pd / WO3 hydrogen-sensitive film is coated on the inner side of reflecting surface 2. The femtosecond laser direct-write grating and the Fabry-Perot interferometer are arranged in series. The PDMS layer encapsulates the interference cavity and fixes the Pd / WO3 hydrogen-sensitive film. The PDMS layer modulates the optical path of the Fabry-Perot interferometer through the interaction between the Pd / WO3 hydrogen-sensitive film and hydrogen, causing a wavelength shift in the interference spectrum. Combined with the temperature response of the femtosecond laser direct-write grating, temperature compensation is achieved, thereby completing the hydrogen concentration sensing.

[0006] Preferably, the femtosecond laser direct-write grating is fabricated using femtosecond laser point-by-point scanning and writing technology, and is located on one side of the input end of the Fabry-Perot interferometer.

[0007] Preferably, the reflective surface 1 is the end face of a single-mode optical fiber, and the reflective surface 2 is the end face coated with a PDMS-Pd / WO3 hydrogen-sensitive film, forming a sealed interference cavity between the two reflective surfaces.

[0008] Preferably, the femtosecond laser direct-write grating is unresponsive to hydrogen gas and only sensitive to temperature. Its characteristic peak wavelength shift serves as a temperature compensation signal to counteract the temperature cross-interference of the Fabry-Perot interferometer.

[0009] Based on a fiber optic twin controllable vernier hydrogen sensor, this invention also provides a hydrogen detection method, comprising the following steps: S1. The Fabry-Perot interferometer at the end of the hydrogen sensor serves as the hydrogen sensing unit. When the hydrogen sensor is exposed to a hydrogen environment, the Pd / WO3 sensitive film at the end of the cavity triggers a dual response: after the palladium Pd adsorbs hydrogen, it undergoes lattice expansion, which changes the cavity length of the Fabry-Perot interferometer through the PDMS support layer; at the same time, the reduction reaction of tungsten trioxide WO3 with hydrogen changes the optical refractive index of the hydrogen sensitive film. Both of these factors together cause the wavelength of the interference fringes to shift, and the output spectrum of the hydrogen sensor serves as the original interference spectrum. S2. The original interference spectrum containing information on the cavity length and optical refractive index variation of the Fabry-Perot interferometer is converted to the frequency domain by Fourier transform filtering to remove high-frequency noise and stray interference modes, resulting in a pure trigonometric function standard spectrum. S3. After obtaining the standard spectrum of the trigonometric function, the mathematical expression of the original interference spectrum is established by sine fitting, and the period, amplitude and phase of the original interference spectrum are accurately extracted, providing a quantitative basis for constructing twin periodic functions. S4. Based on the original period, design a twin periodic function with controllable deviation, and superimpose the twin periodic function with the original interference spectrum to generate a twin interference spectrum; S5. The signals after twin superposition form a controllable vernier spectrum. The low-frequency envelope curve is separated by the envelope extraction algorithm. The shift of the envelope directly corresponds to the wavelength shift of the original interference peak. Moreover, this shift is significantly amplified due to the vernier effect, which reduces the detection difficulty. S6. The wavelength shift of the original Fabry-Perot interferometer interference peak is deduced from the envelope shift, and the hydrogen concentration in the environment is demodulated by combining the cavity length-wavelength response model of the hydrogen sensing unit.

[0010] Therefore, the present invention employs the above-mentioned fiber-optic twin controllable vernier hydrogen sensor and detection method, which has the following beneficial effects: 1) Significantly improved resistance to temperature and humidity interference, effectively solving the shortcomings of existing technologies such as cross-coupling of temperature and humidity signals and hydrogen concentration signals, and large measurement deviations under complex working conditions: By adding a dedicated compensation temperature and humidity sensitive unit, environmental temperature and humidity parameters are collected in real time and compensation calibration is completed. Combined with the symmetrical collaboration and data linkage design of dual-channel sensing units of the twin model, a dual anti-interference guarantee is formed from the structural and algorithmic levels, ensuring that stable and accurate detection of hydrogen concentration can still be achieved in complex environments with fluctuating temperature and humidity, and improving the environmental adaptability and detection reliability of the sensor.

[0011] 2) Improved detection sensitivity, enabling ultra-trace hydrogen detection: By integrating the signal amplification characteristics of the vernier effect, it accurately captures the weak sensing signals corresponding to ultra-trace hydrogen. At the same time, it adopts software-based controllable vernier adjustment, breaking away from the constraints of traditional hardware structures on vernier parameters. It can flexibly adjust parameters through algorithm optimization to match the detection needs of different ultra-trace concentration ranges, significantly improving detection accuracy and applicability flexibility, and providing technical support for early warning of hydrogen.

[0012] 3) Enhanced adaptability and practicality, broadening the application scenarios of fiber optic hydrogen sensors: Compared to the rigid vernier control and difficulty in adapting to complex scenarios in existing technologies, the software-controllable vernier design in this application flexibly adapts to the detection needs of different concentration ranges; at the same time, the integrated solution of "compensated temperature and humidity sensitive unit + twin model + software-controllable vernier" takes into account both structural stability and ease of control, meeting the hydrogen safety monitoring needs of various complex and harsh scenarios such as new energy, aerospace, and chemical industry, and has a wider range of application prospects and promotional value.

[0013] 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

[0014] Figure 1 This is a schematic diagram of the hydrogen sensor structure according to an embodiment of the present invention; Figure 2 Figure 1 shows microscopic images of the sensing structure in an embodiment of the present invention; (a) a femtosecond laser direct-write fiber grating; (b) a Fabry-Perot hydrogen interferometer coated with Pd / WO3; Figure 3 This is a diagram of the experimental testing system according to an embodiment of the present invention; Figure 4 This is a hydrogen response characteristic test of the sensing system according to an embodiment of the present invention; Figure (a) is the original interference spectrum; Figure (b) is the interference spectrum after Fourier transform filtering; Figure (c) is the interference spectrum of hydrogen at different concentrations; Figure (d) is the linear fitting of the sensitivity of the hydrogen test by the Fabry-Perot interferometer; Figure (e) is the grating spectrum of hydrogen at different concentrations; Figure (f) is the linear fitting of the sensitivity of the hydrogen test by the fiber optic grating. Figure 5 The temperature response characteristics of the sensing system in this embodiment of the invention are tested; Figure (a) Interference spectra at different temperature gradients; Figure (b) Linear fitting of sensitivity of temperature test by Fabry-Perot interferometer; Figure (c) Grating spectra at different temperature gradients; Figure (d) Linear fitting of sensitivity of temperature test by fiber optic grating. Figure 6 This is a humidity response characteristic test of the sensing system according to an embodiment of the present invention; Figure (a) shows the interference spectrum of different humidity gradients; Figure (b) shows the grating spectrum of different humidity gradients; Figure 7 This invention relates to a fiber optic twin controllable vernier detection method and results; Figure (a) is a schematic diagram of the fiber optic twin controllable vernier detection method; Figure (b) shows the original hydrogen interference spectrum and mathematical fitting expression; Figure (c) shows the twin controllable hydrogen interference spectrum and mathematical expression; Figure (d) shows the twin controllable vernier spectrum and envelope in the near-infrared band; Figure (e) shows the vernier envelope spectrum of hydrogen at different concentrations; Figure (f) shows the linear fitting of the hydrogen sensitivity of the vernier envelope. Detailed Implementation

[0015] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0016] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0017] Example 1 This invention provides a fiber twin controllable vernier hydrogen sensor and detection method, the overall structure of which is as follows: Figure 1 As shown, the microscopic image of the sensing structure is as follows: Figure 2 As shown.

[0018] This sensor uses standard single-mode fiber as its substrate and mainly consists of an fiber core, fiber cladding, a femtosecond laser direct-write grating, a PDMS (polydimethylsiloxane) layer, a hydrogen-sensitive film (Pd / WO3), and two reflective surfaces (reflective surface 1 and reflective surface 2). Reflective surface 1 and reflective surface 2 are arranged parallel to each other, forming a Fabry-Perot interferometer. Reflective surface 1 is the end face of the single-mode fiber. The hydrogen-sensitive film is uniformly coated on the inner side of reflective surface 2. The PDMS layer is used to encapsulate the interference cavity and fix the sensitive film. The femtosecond laser direct-write grating is fabricated inside the fiber core using femtosecond laser point-by-point scanning writing technology and is located on one side of the input end of the Fabry-Perot interferometer. The femtosecond laser direct-write grating and the interference cavity form a series sensing structure. The PDMS layer modulates the optical path of the Fabry-Perot interferometer through the interaction between the Pd / WO3 hydrogen-sensitive film and hydrogen, causing a wavelength shift in the interference spectrum. Combined with the temperature response of the femtosecond laser direct-write grating, temperature compensation is achieved, thereby completing the hydrogen concentration sensing.

[0019] Femtosecond laser direct-write gratings, as temperature-sensitive units, are essentially waveguides with periodically modulated refractive index within the fiber core, relying on the Bragg diffraction effect to sense temperature. When the ambient temperature changes, the thermal expansion of the fiber alters the grating period, while the thermo-optical effect changes the effective refractive index of the fiber core. These two factors combined cause a linear shift in the Bragg reflection wavelength. Because the grating is located inside the fiber and does not contact the external hydrogen-sensitive membrane, it only responds to temperature. The characteristic peak wavelength shift serves as a temperature compensation signal, canceling the temperature cross-interference of the Fabry-Perot interferometer. The hydrogen sensor is adapted to the fiber twin controllable vernier detection method. By constructing a twin interference spectrum and superimposing it with the original interference spectrum to form a vernier spectrum, the envelope is extracted, improving the hydrogen detection sensitivity.

[0020] The Fabry-Perot interferometer at the fiber optic end serves as the hydrogen-sensitive unit, with its cavity length determined by the distance between two parallel reflective surfaces. When the sensor is exposed to a hydrogen environment, the Pd / WO3 sensitive film at the cavity end triggers a dual response: palladium (Pd) undergoes lattice expansion after adsorbing hydrogen, altering the Fabry-Perot interferometer cavity length through the PDMS support layer; simultaneously, the reduction reaction of tungsten trioxide (WO3) with hydrogen changes the film's optical refractive index, both contributing to a wavelength shift in the interference fringes. Since temperature variations also affect the Fabry-Perot interferometer cavity length, this structure utilizes precise temperature data provided by a femtosecond laser direct-write grating to isolate temperature interference from the total drift of the Fabry-Perot interferometer, ultimately achieving accurate measurement of hydrogen concentration.

[0021] The fiber grating is fabricated using 8.2μm / 126μm single-mode fiber (SMF) as a carrier and femtosecond laser direct writing technology. During the process, the SMF is placed in an optical adhesive with a refractive index of 1.52 to accurately position the fiber core. The laser parameters are set as follows: frequency 50Hz, 40× objective lens (NA=0.64), scanning speed 78.5μm / s, writing length 1500μm, and its theoretical center wavelength is about 1591nm.

[0022] The Pd / WO3 preparation process in the fiber optic Fabry-Perot hydrogen interferometer involves mixing 1g of tungsten powder with 4ml of 30% H2O2 solution, centrifuging to obtain a light pink solution, adding an equal volume of anhydrous ethanol, stirring at 80℃ until the volume is halved to obtain an orange-yellow solution, adding PdCl2 powder, heating and stirring until dissolved to prepare Pd-WO3 sol, and allowing it to solidify at room temperature into an orange-red solid, which is then ground into a fine powder. PDMS has excellent thermal expansion coefficient and thermal response speed, which can rapidly convert the heat released by the reaction of Pd-WO3 with H2 into volume expansion, thereby changing the length of the interference cavity. Pd / WO3 powder is fixed to HCF and attached through a gradual process of "liquid-semi-crosslinking-full crosslinking" by mixing PDMS matrix and hardener at a volume ratio of 5:1.

[0023] The hydrogen detection method based on the above sensor is the fiber twin controllable vernier detection method. It is a high-precision wavelength demodulation technology designed for the interference spectrum of the Fabry-Perot interferometer for hydrogen sensing. The core principle is to use the "vernier effect" to amplify the wavelength drift of the tiny interference peak caused by the change in hydrogen concentration. Combined with algorithms such as Fourier filtering, sine fitting and twin periodic function superposition, it can achieve high-sensitivity detection of low-concentration hydrogen.

[0024] The core of the optical vernier effect is based on the measurement logic of traditional vernier calipers. It involves the superposition of two interference spectra with similar but slightly different free spectral ranges (FSRs) to generate an amplified low-frequency envelope signal (vernier envelope). Two interferometers act as the "master" and "vernier," respectively. Their FSRs are inversely proportional to their optical path difference. Due to the slight difference in their FSRs, the superposition of their transmission / reflection spectra forms a vernier envelope with a period much larger than the FSR of a single interferometer. The wavelength shift of this envelope is much greater than the shift of a single interference spectrum, thus achieving optical sensitivity amplification. This is the key to achieving high-sensitivity detection in this sensing mechanism.

[0025] In actual sensing, minute changes in the external physical quantity to be measured will alter the optical path difference by changing parameters such as the cavity length and effective refractive index of the interferometer, thereby causing a minute wavelength shift in a single interference spectrum. This minute shift is amplified by the vernier effect, manifesting as a significant shift in the vernier envelope after superposition. By detecting the shift of the vernier envelope and combining it with the inherent amplification factor of this mechanism, the specific change in the physical quantity to be measured can be accurately deduced, realizing the transformation from optical signal change to physical quantity detection.

[0026] The specific steps in this embodiment are as follows: S1. The Fabry-Perot interferometer at the end of the hydrogen sensor serves as the hydrogen sensing unit. When the hydrogen sensor is exposed to a hydrogen environment, the Pd / WO3 sensitive film at the end of the cavity triggers a dual response: after the palladium Pd adsorbs hydrogen, it undergoes lattice expansion, which changes the cavity length of the Fabry-Perot interferometer through the PDMS support layer; at the same time, the reduction reaction of tungsten trioxide WO3 with hydrogen changes the optical refractive index of the hydrogen sensitive film. Both of these factors together cause the wavelength of the interference fringes to shift, and the output spectrum of the hydrogen sensor serves as the original interference spectrum.

[0027] S2 contains the original interference spectrum, which includes information on the cavity length and optical refractive index variation of the Fabry-Perot interferometer. However, it is contaminated with fiber transmission noise, stray light from the environment, and other interferences, limiting the accuracy of directly detecting minute wavelength drifts. By converting the spectrum to the frequency domain through Fourier transform filtering, high-frequency noise and stray interference modes are removed, resulting in a pure trigonometric function standard spectrum. This step effectively eliminates the impact of environmental interference on subsequent signal processing, providing a reliable periodic signal basis for accurate modeling.

[0028] S3. After obtaining the standard spectrum of the trigonometric function, the mathematical expression of the original interference spectrum is established by sine fitting, and the period, amplitude and phase of the original interference spectrum are accurately extracted, providing a quantitative basis for constructing twin periodic functions.

[0029] S4. Based on the original period, design a twin periodic function with controllable deviation. Superimpose the twin periodic function with the original interference spectrum to generate a twin interference spectrum. This process is the core manifestation of the "vernier effect"—after the two interference signals with slightly different periods are superimposed, low-frequency "beat frequency" vernier fringes will be generated. The small wavelength drift of the original interference peak will be converted into a significant shift in the envelope of the vernier fringes, thereby amplifying the signal. The amplification factor can be flexibly adjusted by designing the period deviation.

[0030] S5. The signals after twin superposition form a controllable vernier spectrum. The low-frequency envelope curve is separated by the envelope extraction algorithm. The shift of the envelope directly corresponds to the wavelength shift of the original interference peak. Moreover, this shift is significantly amplified due to the vernier effect, which reduces the detection difficulty.

[0031] S6. The wavelength shift of the original Fabry-Perot interferometer interference peak is deduced from the envelope shift, and the hydrogen concentration in the environment is demodulated by combining the cavity length-wavelength response model of the hydrogen sensing unit.

[0032] In this embodiment, to verify the sensor's hydrogen response characteristics, temperature response characteristics, and humidity anti-interference characteristics, a system was constructed as follows: Figure 3 The experimental testing system shown mainly includes an ASE (Amplified Spontaneous Emission) light source, a circulator, a constant temperature and humidity chamber, a humidity generator, a gas distributor, a spectrometer, and a temperature / humidity testing module.

[0033] The output of the ASE light source is connected to port 1 of the circulator via an optical fiber jumper. Port 2 of the circulator is connected to the prepared hydrogen sensor via an optical fiber jumper. The sensor is placed inside a constant temperature and humidity chamber to ensure controllable test environment parameters. Port 3 of the circulator is connected to a spectrometer (spectral resolution of 0.01 nm, test wavelength range of 1525-1615 nm) to collect the reflectance spectrum signal from the sensor. The gas distributor adjusts the mixing ratio of the two gases through a flow controller to achieve precise control of the hydrogen concentration (0%-0.5%). The ASE light source is turned on and preheated for 30 minutes to ensure stable output light power. The integration time of the spectrometer is adjusted to 100 ms, and the background spectrum is collected and subtracted to eliminate ambient light interference. The initial temperature and humidity of the constant temperature and humidity chamber are set to 24℃ and 21.12%RH. After stabilizing for 30 minutes by introducing pure N2 (0% hydrogen concentration), the initial spectrum is collected as a reference.

[0034] Under ambient conditions of 24℃ and 21.12% RH, the hydrogen concentration was adjusted to 0%, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5%, and the interference spectrum and grating spectrum of the sensor were collected respectively. The results are as follows: Figure 4 As shown. Original interference spectrum ( Figure 4 (a) After Fourier transform filtering ( Figure 4 (b) The interference fringes are clearly visible; as the hydrogen concentration increases, the characteristic peaks of the interference spectrum shift towards shorter wavelengths. Figure 4 (c) The hydrogen sensitivity of the Fabry-Perot interferometer was obtained as -7.003 nm / % after linear fitting, and the linear correlation coefficient R was found to be R. 2 =0.9897 ( Figure 4 (d) indicates that the interference cavity has a good linear response to hydrogen; the characteristic peaks of the grating spectrum under different hydrogen concentrations show no significant shift. Figure 4 (e)), the slope of the linear fit is 0 nm / % ( Figure 4 (f) indicates that the femtosecond laser direct-write grating is not sensitive to hydrogen and can be used to eliminate grating interference in hydrogen detection. Under conditions of 0% hydrogen concentration and 21.12% RH, the temperature response characteristics of the sensor were tested by adjusting the ambient temperature to 24℃, 30℃, 36℃, 42℃, 48℃, and 54℃. The results are as follows: Figure 5 As shown, with increasing temperature, the characteristic peaks of the Fabry-Perot interferometer spectrum shift towards shorter wavelengths. Figure 5 (a) Linear fitting yielded a temperature sensitivity of -0.663 nm / ℃, with a linear correlation coefficient R0. 2 =0.9996 ( Figure 5 (b) indicates that the interference cavity exhibits temperature cross-sensitivity; the characteristic peaks of the fiber grating shift towards longer wavelengths as the temperature increases. Figure 5 (c) The temperature sensitivity is 10.1 pm / ℃, and the linear correlation coefficient R0 2 =0.9980 ( Figure 5 (d) Since the grating is not sensitive to hydrogen, its temperature response can be used as a temperature compensation signal to eliminate the interference of temperature on hydrogen detection through data fusion algorithm; Under conditions of 0% hydrogen concentration and 24℃, the humidity response characteristics of the sensor were tested by adjusting the ambient humidity to 21.12% RH, 37.45% RH, 51.11% RH, 66.51% RH, 74.17% RH, and 79.85% RH. The results are as follows: Figure 6 As shown. Under different humidity conditions, the characteristic peak positions of the Fabry-Perot interference spectra did not shift significantly ( Figure 6 (a) The position of the characteristic peak of the fiber grating also did not change significantly. Figure 6(b) indicates that the sensor is not sensitive to humidity, has good humidity anti-interference ability, and can be used for hydrogen detection in different humidity environments.

[0035] This embodiment employs a fiber twin controllable vernier detection method to process the spectral signal, achieving high-sensitivity demodulation of hydrogen concentration. The specific steps are as follows: Figure 7 As shown; the original interference spectra of the sensor under different test conditions were collected by a spectrometer (e.g., Figure 7 As shown in (a), Fourier transform filtering is used to remove noise components from the spectrum, resulting in a smooth interference spectrum (as shown in [a]). Figure 4 (b) As shown); based on the filtered interference spectrum, a phase-shifted twin interference spectrum is constructed, the mathematical expression of which satisfies a sine function model, as follows: Figure 7 (b) and 7(c); The original interference spectrum is superimposed with the twin interference spectrum to form a twin controllable vernier spectrum, and the spectral envelope in the near-infrared band is extracted using an envelope extraction algorithm (e.g., Figure 7 (d) shows that the wavelength shift of the envelope is linearly related to the hydrogen concentration; by analyzing the vernier envelope spectra at different hydrogen concentrations (e.g., Figure 7 (e) As shown, a linear fit was performed between the envelope wavelength shift and the hydrogen concentration to obtain the sensitivity coefficient, thereby achieving quantitative detection of hydrogen concentration. After adopting the twin-controlled vernier detection method, the hydrogen sensitivity of the vernier envelope was improved to -236.06 nm / %, with a linear correlation coefficient R0. 2 =0.9919 ( Figure 7 (f) Compared with traditional interferometric spectral demodulation, the sensitivity is increased by about 33 times, which greatly improves the detection accuracy of low-concentration hydrogen.

[0036] Therefore, the present invention adopts the above-mentioned fiber twin controllable vernier hydrogen sensor and detection method. By using the series design of femtosecond laser direct-write grating and Pd / WO3-based Fabry-Perot interferometer, combined with the fiber twin controllable vernier detection method, it achieves high sensitivity and high linearity detection of hydrogen concentration. At the same time, the temperature cross-sensitivity is eliminated by the temperature response characteristics of the fiber grating, and it has good humidity anti-interference ability.

[0037] In addition, those skilled in the art can adjust parameters such as the spacing of the Fabry-Perot interferometer, the thickness and composition ratio of the Pd / WO3 film, and the period and length of the femtosecond laser direct-write grating according to actual application needs, in order to optimize the sensor's response speed, detection range and other performance indicators.

[0038] 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 fiber-optic twin controllable vernier hydrogen sensor, characterized in that, The system comprises a single-mode optical fiber, a femtosecond laser direct-write grating, a Fabry-Perot interferometer, a polydimethylsiloxane (PDMS) layer, and a Pd / WO3 hydrogen-sensitive film. The single-mode optical fiber consists of an optical fiber core and an optical fiber cladding, with the femtosecond laser direct-write grating fabricated inside the fiber core. The Fabry-Perot interferometer consists of two relatively parallel reflecting surfaces, 1 and 2. The Pd / WO3 hydrogen-sensitive film is coated on the inner side of reflecting surface 2. The femtosecond laser direct-write grating and the Fabry-Perot interferometer are arranged in series. The PDMS layer is used to encapsulate the interference cavity and fix the Pd / WO3 hydrogen-sensitive film. The PDMS layer modulates the optical path of the Fabry-Perot interferometer through the interaction between the Pd / WO3 hydrogen-sensitive film and hydrogen, causing a wavelength shift in the interference spectrum. Combined with the temperature response of the femtosecond laser direct-write grating, temperature compensation is achieved, thereby completing the hydrogen concentration sensing.

2. The fiber-optic twin controllable vernier hydrogen sensor according to claim 1, characterized in that, The femtosecond laser direct-write grating is fabricated using femtosecond laser point-by-point scanning and writing technology and is located on one side of the input end of the Fabry-Perot interferometer.

3. The fiber-optic twin controllable vernier hydrogen sensor according to claim 1, characterized in that, The reflective surface 1 is the end face of a single-mode optical fiber, and the reflective surface 2 is the end face coated with a PDMS-Pd / WO3 hydrogen-sensitive film. A sealed interference cavity is formed between the two reflective surfaces.

4. The fiber-optic twin controllable vernier hydrogen sensor according to claim 1, characterized in that, The femtosecond laser direct-write grating is unresponsive to hydrogen gas but sensitive only to temperature. Its characteristic peak wavelength shift serves as a temperature compensation signal to counteract the temperature crosstalk of the Fabry-Perot interferometer.

5. The fiber-optic twin controllable vernier hydrogen sensor as described in any one of claims 1-4, characterized in that, The hydrogen detection method is a fiber optic twin controllable vernier detection method, which specifically includes the following steps: S1. The Fabry-Perot interferometer at the end of the hydrogen sensor serves as the hydrogen sensing unit. When the hydrogen sensor is exposed to a hydrogen environment, the Pd / WO3 sensitive film at the end of the cavity triggers a dual response: after the palladium Pd adsorbs hydrogen, it undergoes lattice expansion, which changes the cavity length of the Fabry-Perot interferometer through the PDMS support layer; at the same time, the reduction reaction of tungsten trioxide WO3 with hydrogen changes the optical refractive index of the hydrogen sensitive film. Both of these factors together cause the wavelength of the interference fringes to shift, and the output spectrum of the hydrogen sensor serves as the original interference spectrum. S2. The original interference spectrum containing information on the cavity length and optical refractive index variation of the Fabry-Perot interferometer is converted to the frequency domain by Fourier transform filtering to remove high-frequency noise and stray interference modes, resulting in a pure trigonometric function standard spectrum. S3. After obtaining the standard spectrum of the trigonometric function, the mathematical expression of the original interference spectrum is established by sine fitting, and the period, amplitude and phase of the original interference spectrum are accurately extracted, providing a quantitative basis for constructing twin periodic functions. S4. Based on the original period, design a twin periodic function with controllable deviation, and superimpose the twin periodic function with the original interference spectrum to generate a twin interference spectrum; S5. The signals after twin superposition form a controllable vernier spectrum. The low-frequency envelope curve is separated by the envelope extraction algorithm. The shift of the envelope directly corresponds to the wavelength shift of the original interference peak. Moreover, this shift is significantly amplified due to the vernier effect, which reduces the detection difficulty. S6. The wavelength shift of the original Fabry-Perot interferometer interference peak is deduced from the envelope shift, and the hydrogen concentration in the environment is demodulated by combining the cavity length-wavelength response model of the hydrogen sensing unit.

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