A CO2 detection device and method based on dual TFBG

Abstract

This invention discloses a dual-TFBG-based CO2 detection device and method. The device comprises an optical fiber with a fiber cladding disposed outside the fiber core. Two spaced-apart tilted fiber gratings (TFBGs) (TFBG1 and TFBG2) are formed on the fiber core. A composite sensitive layer is disposed between TFBG1 and the fiber cladding, and an inert protective layer is disposed between TFBG2 and the fiber cladding. The composite sensitive layer comprises, from the inside out, a porous silica layer, a hydrophobic MOF intermediate layer, and a graphene quantum dot (GQD) reinforcement layer. The fiber cladding is provided with an air inlet for admitting the CO2 gas to be detected. By utilizing the composite sensitive layer structure, a dual-channel self-compensating optical path, and a low-cost demodulation system, the invention achieves high-precision, low-drift CO2 detection, providing a reliable solution for environmental monitoring.

Description

Technical Field

[0001] The present invention belongs to the technical field of CO2 detection and relates to a CO2 detection device and method based on dual TFBGs. Background Art

[0002] With the acceleration of global industrialization and urbanization, carbon dioxide (CO2), a major greenhouse gas, has seen a sharp increase in emissions, leading to increasingly severe global warming. Accurate and timely detection of CO2 has become a crucial tool for addressing global climate change and protecting the Earth's environment. It is also crucial for the formulation and implementation of environmental protection policies. Researchers are using TFBGs (tilted fiber gratings) for gas detection due to their unique optical structure and sensitivity to the ambient refractive index.

[0003] However, existing TFBG-based CO2 detection methods often have the following shortcomings:

[0004] 1) Environmental cross-sensitivity: Temperature and humidity fluctuations (especially humidity) can significantly interfere with the refractive index or volume response of CO2-sensitive materials, leading to measurement errors (e.g., humidity fluctuations can cause errors >10%). Traditional solutions lack effective multi-parameter decoupling mechanisms, and detection methods that rely solely on a single wavelength shift are unable to eliminate environmental coupling noise.

[0005] 2) Insufficient long-term stability of sensitive materials: Existing technologies often use PMMA or ordinary MOF materials as sensitive materials, which are susceptible to environmental aging (such as CO2 plasticization, thermal expansion mismatch) or pollution (dust, oil stains), resulting in coating peeling or performance degradation, making it difficult to meet the long-term use requirements of industrial scenarios.

[0006] 3) Conflict between response speed and detection sensitivity: The detection limit of existing TFBG sensors mostly stays at the ppm level, and the detection of low-concentration gases (ppb level) is still limited, especially when the background noise is high, the signal-to-noise ratio is insufficient, which makes it difficult to meet the needs of trace gas detection. At the same time, in order to improve the sensitivity, it is often necessary to increase the thickness of the sensitive layer, but the thick coating will delay the gas diffusion dynamics, which will lead to a long response time (the response time of existing TFBG sensors is often in the minutes level), which makes it difficult to meet the needs of dynamic gas monitoring.

[0007] In addition, the use of a demodulation system, especially a high-precision solution with a multi-channel array, requires reliance on a high-resolution spectrometer, which has high equipment costs, and the algorithm relies on a large amount of calibration data, making it difficult to promote commercially.

[0008] Based on this, the present invention provides a newly designed CO2 detection device and method based on dual TFBG to provide accurate CO2 concentration detection data, thereby providing data support for formulating scientific emission reduction targets and environmental protection policies. Summary of the Invention

[0009] The purpose of the present invention is to provide a CO2 detection device and method based on dual TFBGs to address the deficiencies of the existing technology.

[0010] The technical solution adopted in the present invention is:

[0011] A dual-TFBG-based CO2 detection device comprises an optical fiber with an optical fiber cladding disposed outside the optical fiber core. Two spaced-apart tilted fiber gratings (TFBGs) (TFBG1 and TFBG2) are formed on the optical fiber core, with a composite sensitive layer disposed between TFBG1 and the optical fiber cladding, and an inert protective layer disposed between TFBG2 and the optical fiber cladding. The composite sensitive layer comprises, from the inside to the outside, a porous silica layer, a hydrophobic MOF intermediate layer, and a graphene quantum dot (GQD) reinforcement layer. An air inlet is provided on the optical fiber cladding for introducing the CO2 gas to be detected.

[0012] In the above technical solution, further, the tilt angles of the tilted fiber gratings TFBG1 and TFBG2 are 7-13°, and the grating period is 530 nm.

[0013] Furthermore, the porous silica layer is formed on the TFBG1 by a sol-gel method using a precursor solution, and the pore size thereof is 1-10 nm.

[0014] Furthermore, the hydrophobic MOF intermediate layer is made of UiO-66-(CF3)2 material, which is prepared by coating a DMF solution of UiO-66-(CF3)2 on a porous silica layer and drying the resulting solution.

[0015] Furthermore, the GQD enhancement layer is prepared by applying GQD dispersed droplets on the surface of the hydrophobic MOF intermediate layer and drying.

[0016] Furthermore, the inert protective layer is a PTFE layer.

[0017] A CO2 detection method based on dual TFBGs, implemented based on the device described in any of the above, based on a dual-channel self-compensation optical path formed by TFBG1 and TFBG2, using a dual-parameter decoupling algorithm to combine the wavelength shift caused by CO2 adsorption and power changes The matrix equation is constructed by joint calculation to make CO2 concentration detection independent of environmental interference.

[0018] Furthermore, the constructed matrix equation is:

[0019]

[0020] in, The wavelength shift of TFBG1 caused by CO2 concentration, temperature or humidity, is the wavelength shift of TFBG2 caused by temperature and humidity, is the power change of TFBG1, is the power change of TFBG2, is the wavelength response coefficient of TFBG1 to CO2 concentration, is the wavelength coupling coefficient of TFBG1 to temperature, is the wavelength coupling coefficient of TFBG1 to humidity, is the wavelength coupling coefficient of TFBG2 to temperature, is the wavelength coupling coefficient of TFBG2 to humidity, is the power response coefficient of TFBG1 to CO2 concentration, is the power coupling coefficient of TFBG1 to temperature, is the power coupling coefficient of TFBG1 to humidity, is the power coupling coefficient of TFBG2 to temperature, is the power coupling coefficient of TFBG2 to humidity, is the temperature change, is the humidity change;

[0021] The CO2 concentration C is solved by the least square method to achieve complete compensation of temperature and humidity interference.

[0022] Furthermore, a broadband light source SLD and a tunable filter are used for edge filtering and demodulation, and the wavelength shift is inferred from the change in transmitted power.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The detection device and method provided by the present invention have at least the following advantages: First, the present invention integrates a sensing grating (TFBG1) and a reference grating (TFBG2) in the same optical fiber, and coats them with a composite sensitive layer and an inert PTFE layer respectively, so as to synchronously monitor the changes in ambient temperature and humidity in real time, and combines a dual-parameter decoupling algorithm to make the CO2 concentration detection independent of environmental interference; Second, the unique composite sensitive layer structure can not only effectively improve the structural stability of the sensitive material while allowing the free diffusion of CO2 molecules, but also utilize the design of a porous silica bottom layer combined with a hydrophobic MOF middle layer to effectively improve the stability of the sensitive material structure and delay degradation and cracking. It can also significantly reduce humidity sensitivity from a physical perspective. Third, the present invention utilizes different structural layers, particularly GQDs for enhanced adsorption from the outside to the inside, and MOF and silica layers to form a gradient aperture. This can guide directional gas flow and provide high-density adsorption sites, effectively improving detection sensitivity and shortening response time to less than 30 seconds. Fourth, the present invention can also use a broadband light source + tunable filter (such as an FP filter) to replace the spectrometer, inferring wavelength shift through changes in transmitted power, and combining the dual-grating power ratio to eliminate the influence of light source fluctuations, which can significantly reduce costs and improve resolution. Through a composite sensitive layer structure, a dual-channel self-compensating optical path, and a low-cost demodulation system, the present invention achieves high-precision, low-drift CO2 detection, providing a reliable solution for the field of environmental monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic structural diagram of a detection system according to an embodiment of the present invention;

[0026] Figure 2 Schematic diagram of the structure of a CO2 detection device in one embodiment of the present invention. DETAILED DESCRIPTION

[0027] In order to make the above-mentioned objects, features and advantages of the present invention more clearly understood, the specific embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. The technical features in the various embodiments of the present invention can be combined accordingly without conflicting with each other.

[0028] The present invention provides a CO2 detection device based on dual TFBG, such as Figure 1As shown, in a specific embodiment of the present invention, a system for detecting CO2 includes a broadband light source BBS 1, a single-mode optical fiber SMF 2, an optical fiber sensor 3, and a spectrometer OAS5, wherein the optical fiber sensor 3 integrates a sensing grating (TFBG1) 11 and a reference grating (TFBG2) 12 in the core 10 of the same single-mode optical fiber, which are respectively coated with a composite sensitive layer and an inert polytetrafluoroethylene (PTFE) layer 14, wherein the composite sensitive layer is a porous silica layer 8, a hydrophobic MOF intermediate layer 7, and a graphene quantum dot (GQD) enhancement layer 6; a fiber cladding 9 is provided in the outermost layer, and an air inlet 13 is provided on the fiber cladding 9 for introducing the CO2 gas to be detected.

[0029] This device integrates a sensing grating (TFBG1) and a reference grating (TFBG2) within the same optical fiber, each coated with a composite sensitive layer and an inert PTFE layer. This enables real-time, simultaneous monitoring of ambient temperature and humidity changes, forming a dual-channel, self-compensating structure. Light enters the SMF from the BBS, travels through the fiber sensor, and ultimately transmits its spectrum to the OAS. CO2 gas enters the fiber sensor through the pores and diffuses through the three-layer composite sensitive layer. The CO2 molecules diffuse within the composite sensitive layer. The diffusion rate and degree of adsorption alter TFBG1's light deflection ability, thereby altering the spectrum. TFBG2 compensates for environmental interference, achieving the dual-TFBG CO2 detection effect.

[0030] According to a specific embodiment of the present invention, the double grating is made by writing on the core of a single-mode optical fiber using a femtosecond laser. The parameters of the femtosecond laser are a wavelength of 800 nm, a pulse energy of 0.5 mJ, a tilt angle of 8° for the double gratings TFBG1 and TFBG2, and a grating period of Λ=530 nm.

[0031] After that, a coating process is carried out to form a composite sensitive layer on TFBG1, which directly responds to changes in CO2 concentration, and an inert PTFE protective layer is formed on TFBG2, which is only affected by temperature and humidity and is used for environmental parameter decoupling.

[0032] In the composite sensitive layer:

[0033] 1) Porous silica bottom layer:

[0034] A nanoscale mechanical support layer is formed on the surface of the optical fiber core through the sol-gel method to buffer thermal stress and isolate the direct contact between the hydrophobic MOF intermediate layer and the optical fiber core, prevent interface cracking, and isolate the influence of environmental humidity.

[0035] According to a specific embodiment of the present invention, the preparation method may be:

[0036] Prepare the precursor solution: mix tetraethyl orthosilicate (TEOS), ethanol, and deionized water in a volume ratio of 1:5:2, and add 0.1 M hydrochloric acid as a catalyst.

[0037] Sol-gel reaction: Stir at 60°C for 6 hours to form a transparent sol.

[0038] Coating and drying: The sol was spin-coated (3000 rpm) on the TFBG1 surface, dried at room temperature for 24 h, and then calcined at 400°C for 2 h to form a porous silica layer with a pore size of d≈2 nm.

[0039] The BET specific surface area of ​​the obtained silica layer is: S BET >1000㎡ / g, the pore size distribution peak is 2.1nm; in the relative humidity (RH) range of 20%~80%, the water absorption rate of the silica layer is <0.5%, which has a humidity isolation effect.

[0040] Diffusion kinetics equation:

[0041]

[0042] Where J is the diffusion flux, D is the diffusion coefficient of CO2 in the silica layer, and C is the gas concentration gradient. Nanopores significantly increase the gas transmission rate by increasing the effective diffusion area.

[0043] 2) Hydrophobic MOF middle layer:

[0044] The hydrophobic UiO-66-(CF3)2 material is used, and by introducing a hydrophobic group (-CF3), water molecule adsorption is inhibited, significantly reducing humidity sensitivity. According to one embodiment of the present invention, its preparation method can be as follows:

[0045] Preparation of precursors: zirconium salt (ZrCl4) and 2-(trifluoromethyl) terephthalic acid (H2BDC-CF3) were dissolved in DMF at a molar ratio of 1:1.

[0046] Hydrothermal reaction: react at 120°C for 24 hours, centrifuge and wash, and then vacuum dry to obtain UiO-66-(CF3)2 crystals.

[0047] Coating: Apply the DMF dispersion droplets of UiO-66-(CF3)2 crystals on the porous silica layer and dry.

[0048] The adsorption capacity of MOF for CO2 can be described by the Langmuir adsorption model:

[0049]

[0050] Where θ is the surface coverage, K is the adsorption equilibrium constant, and P is the CO2 partial pressure. H2O < <K CO2 The MOF coating's hydrophobicity was tested, with a contact angle θ > 120° and a CO2 adsorption capacity of 2.8 mmol / g at 25°C and 1 atm, significantly higher than that of conventional MOFs (ZIF-8: 1.2 mmol / g).

[0051] 3) Graphene quantum dot (GQD) enhancement layer:

[0052] According to a specific embodiment of the present invention, graphene oxide was subjected to nitric acid reflux (100°C, 6 hours) followed by ultrasonic exfoliation to obtain GQDs with a diameter of approximately 5 nm. A GQD dispersion (concentration 1 mg / mL) was then drop-coated on the surface of a hydrophobic MOF interlayer and dried. The GQDs stabilize the MOF structure through π-π stacking, and their high specific surface area provides additional adsorption sites, accelerating gas response. The adsorption kinetics can be simplified as:

[0053]

[0054] Among them, K ads and K des are the adsorption / desorption rate constants, respectively. The introduction of GQDs reduces the activation energy E a , significantly improving K ads , improving the adsorption kinetics performance.

[0055] The setting of the composite sensitive layer can greatly suppress humidity interference, which reduces the humidity sensitivity to <1% FS, so that the wavelength shift satisfies the following formula:

[0056]

[0057] Among them, α and β are the temperature-humidity coupling coefficients, respectively. The composite sensitive layer suppresses the influence of temperature and humidity through the synergistic effect of material property design and structure. The composite sensitive layer, especially the silica layer and hydrophobic MOF setting, can make β approach 0. The thermal expansion coefficient of the silica bottom layer is close to that of the optical fiber core, which can buffer the thermal stress caused by temperature changes and reduce the wavelength drift caused by grating structure deformation. The thermal decomposition temperature of the MOF intermediate layer material is often greater than 300°C (MOF materials generally have high thermal stability). In our detection environment, the structure is stable and the adsorption sites will not be degraded due to temperature. The high thermal conductivity of the GQD enhancement layer can evenly distribute the temperature field, reduce the grating wavelength drift caused by local thermal stress, and cooperate with the silica bottom layer to further buffer temperature interference, that is, α also approaches 0.

[0058] To improve sensitivity, the thickness of the sensitive layer often needs to be increased. However, thick coatings can slow gas diffusion dynamics, resulting in excessively long response times, making them unsuitable for real-time monitoring. In the present invention, the composite sensitive layer can be controlled to approximately 500 nm. This design optimizes response time by combining a gradient pore size design within the composite sensitive layer with a GQD enhancement layer that provides a high density of adsorption sites. This design can reduce response time to <30 seconds.

[0059] According to the Prague conditions:

[0060]

[0061] Among them, n eff is the effective refractive index, and Λ is the grating period. CO2 adsorption leads to n eff changes, causing the wavelength to shift Δλ B The present invention is based on the above-mentioned dual-channel self-compensating optical path design and adopts a dual-parameter decoupling algorithm to eliminate environmental interference by jointly solving the wavelength offset (Δλ) and power change (ΔP):

[0062]

[0063] in, The wavelength shift of TFBG1 caused by CO2 concentration, temperature and humidity, The wavelength shift of TFBG2 caused by temperature and humidity, is the power change of TFBG1, is the power change of TFBG2, is the wavelength response coefficient of TFBG1 to CO2 concentration, is the wavelength coupling coefficient of TFBG1 to temperature, is the wavelength coupling coefficient of TFBG1 to humidity, is the wavelength coupling coefficient of TFBG2 to temperature, is the wavelength coupling coefficient of TFBG2 to humidity, is the power response coefficient of TFBG1 to CO2 concentration, is the power coupling coefficient of TFBG1 to temperature, is the power coupling coefficient of TFBG1 to humidity, is the power coupling coefficient of TFBG2 to temperature, is the power coupling coefficient of TFBG2 to humidity, is the temperature change, is the humidity change;

[0064] The above parameters such as wavelength response coefficient, wavelength coupling coefficient, power response coefficient, power coupling coefficient, etc. can be obtained by experimental data acquisition and fitting. For example, the response coefficient of CO2 concentration is 、 In a constant temperature and humidity environment at 25°C and 50% RH, CO2 gas with different concentrations ranging from 0 to 2000 ppm was introduced into a TFBG1 fiber, and the wavelength shift of TFBG1 was recorded on a spectrometer. and power changes The value of this type of response coefficient can be obtained through linear fitting. The temperature and humidity response coefficient is obtained by controlling the fixed CO2 concentration and changing the temperature and humidity (experiments are conducted under common indoor temperature and humidity).

[0065] Construct the matrix equation:

[0066]

[0067] The least squares method is used to solve for C, fully compensating for temperature and humidity interference. Without compensation, humidity fluctuations (ΔRH = 60%) can cause a measurement error, ΔC, of ​​up to ±150 ppm. However, after matrix calculation, this error is reduced to ΔC = ±10 ppm.

[0068] In view of the problem that existing high-precision solutions often rely on high-resolution spectrometers, which are expensive (>100,000 yuan) and the algorithms need to rely on a large amount of calibration data, making them difficult to commercialize, according to one embodiment of the present invention, edge filtering can be used to achieve low-cost demodulation, that is, a broadband light source + tunable filter (such as FP filter) is used to replace the spectrometer, and the transmission power P is adjusted according to the transmission power P. out Relationship with wavelength:

[0069]

[0070] Where T(λ) is the filter transfer function. By calibrating the T(λ) curve, the wavelength shift Δλ can be inferred from the change in transmitted power. Combined with the dual-grating power ratio (ΔP1 / ΔP2), the effect of light source fluctuations can be eliminated, achieving a resolution of ±10 ppm. This method can reduce the cost of using a high-resolution spectrometer by approximately 80%. Combined with a calibration model based on transfer learning, only a small amount of data (<100 sets) is required to adapt to different environments. Kalman filtering can also be used to correct for temperature and humidity drift in real time, eliminating the need for continuous manual calibration:

[0071]

[0072] in, is the state vector (CO2 concentration, temperature and humidity, etc.), is the Kalman gain, is the measurement value at time k, and H is the observation matrix.

[0073] The solution of the present invention achieves high-precision, low-drift CO2 detection through the design of a composite sensitive layer structure, a dual-channel self-compensating optical path and a low-cost demodulation system. It has industrial potential and provides a new and reliable solution for the field of environmental monitoring.

[0074] The embodiments described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above descriptions are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A CO2 detection device based on dual TFBG, characterized in that: The device comprises an optical fiber, wherein an optical fiber cladding is provided on the outer side of the optical fiber core; two spaced-apart tilted optical fiber gratings (TFBG1 and TFBG2) are formed on the optical fiber core, a composite sensitive layer is provided between TFBG1 and the optical fiber cladding, and an inert protective layer is provided between TFBG2 and the optical fiber cladding; the composite sensitive layer comprises, from the inside to the outside, a porous silica layer, a hydrophobic MOF intermediate layer, and a graphene quantum dot (GQD) enhanced layer; and an air inlet is provided on the optical fiber cladding for introducing the CO2 gas to be detected.

2. The CO2 detection device based on dual TFBG according to claim 1, characterized in that: The tilt angles of the tilted fiber gratings TFBG1 and TFBG2 are 7-13°, and the grating period is 530 nm.

3. The CO2 detection device based on dual TFBG according to claim 1, characterized in that: The porous silicon dioxide layer is formed on TFBG1 by a sol-gel method using a precursor solution, and its pore size is 1-10 nm.

4. The CO2 detection device based on dual TFBG according to claim 1, characterized in that: The hydrophobic MOF intermediate layer adopts UiO-66-(CF3)2 material and is prepared by coating a DMF solution of UiO-66-(CF3)2 on a porous silicon dioxide layer and drying the resulting solution.

5. The CO2 detection device based on dual TFBG according to claim 1, characterized in that: The GQD enhancement layer is prepared by applying GQD dispersed droplets on the surface of the hydrophobic MOF intermediate layer and drying.

6. The CO2 detection device based on dual TFBG according to claim 1, characterized in that: The inert protective layer is a PTFE layer.

7. A CO2 detection method based on dual TFBG, characterized in that: Based on the device according to any one of claims 1 to 6, a dual-channel self-compensation optical path formed by TFBG1 and TFBG2 is used, and a dual-parameter decoupling algorithm is used to combine the wavelength shift caused by CO2 adsorption. and power changes The matrix equation is constructed by joint calculation to make CO2 concentration detection independent of environmental interference.

8. The CO2 detection method based on dual TFBG according to claim 7, characterized in that: The constructed matrix equation is: ， in, The wavelength shift of TFBG1 caused by CO2 concentration, temperature or humidity, is the wavelength shift of TFBG2 caused by temperature and humidity, is the power change of TFBG1, is the power change of TFBG2, is the wavelength response coefficient of TFBG1 to CO2 concentration, is the wavelength coupling coefficient of TFBG1 to temperature, is the wavelength coupling coefficient of TFBG1 to humidity, is the wavelength coupling coefficient of TFBG2 to temperature, is the wavelength coupling coefficient of TFBG2 to humidity, is the power response coefficient of TFBG1 to CO2 concentration, is the power coupling coefficient of TFBG1 to temperature, is the power coupling coefficient of TFBG1 to humidity, is the power coupling coefficient of TFBG2 to temperature, is the power coupling coefficient of TFBG2 to humidity, is the temperature change, is the humidity change; The CO2 concentration C is solved by the least square method to achieve complete compensation of temperature and humidity interference.

9. The CO2 detection method based on dual TFBG according to claim 7, characterized in that: A broadband light source SLD and a tunable filter are used for edge filtering and demodulation, and the wavelength shift is inferred from the change of transmitted power.

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