CO2 detection device and method based on double TFBG

Abstract

The invention discloses a CO2 detection device and method based on double TFBGs. The device comprises an optical fiber, and an optical fiber cladding is arranged on the outer side of a fiber core of the optical fiber; a tilted fiber grating TFBG1 and a tilted fiber grating TFBG2 which are spaced from each other are formed on the fiber core, a composite sensitive layer is arranged between the TFBG1 and the fiber cladding, and an inert protective layer is arranged between the TFBG2 and the fiber cladding; the composite sensitive layer sequentially comprises a porous silicon dioxide layer, a hydrophobic MOF intermediate layer and a graphene quantum dot (GQD) enhancement layer from inside to outside; a gas inlet hole is formed in the optical fiber cladding and is used for introducing gas of CO2 to be detected. Through a composite sensitive layer structure, a dual-channel self-compensation light path and a low-cost demodulation system, high-precision and low-drift CO2 detection is realized, and a reliable solution is provided for the field of 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 TFBG. Background Art

[0002] With the acceleration of the global industrialization and urbanization processes, carbon dioxide (CO2), as one of the main greenhouse gases, has seen a sharp increase in its emissions, leading to an increasingly severe global warming problem. Accurate and timely detection of CO2 has become an important means to address global climate change and protect the Earth's environment, and is also related to the formulation and implementation effect evaluation of environmental protection policies. Due to the unique optical structure of TFBG (tilted fiber Bragg grating) and its sensitivity to environmental refractive index, researchers have used it in gas detection.

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

[0004] 1) Environmental cross-sensitivity problem: Changes in temperature and humidity (especially humidity) will significantly interfere with the refractive index or volume response of CO2-sensitive materials, resulting in measurement errors (e.g., humidity fluctuations can cause errors > 10%). Traditional solutions lack an effective multi-parameter decoupling mechanism, and the detection method relying only on a single wavelength shift cannot eliminate environmental coupling noise.

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

[0006] 3) Contradiction between response speed and detection sensitivity: The detection lower limit of existing TFBG sensors mostly remains 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, making it difficult to meet the requirements of trace gas detection. At the same time, to improve sensitivity, it is often necessary to increase the thickness of the sensitive layer, but the thick coating will delay the gas diffusion kinetics and lead to too long response time (the response time of existing TFBG sensors is often at the minute level), making it difficult to meet the requirements of dynamic gas monitoring.

[0007] In addition, when using a demodulation system, especially if a high-precision multi-channel array solution is adopted, it relies on a high-resolution spectrometer, the equipment cost is high, and the algorithm depends on a large amount of calibration data, making it difficult to be commercially promoted.

[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 and provide data support for formulating scientific emission reduction targets and environmental protection policies. Summary of the Invention

[0009] The object of the present invention is to provide a CO2 detection device and method based on a dual-TFBG in view of the deficiencies of the prior art.

[0010] The technical solution adopted by the present invention is as follows:

[0011] A CO2 detection device based on a dual-TFBG includes an optical fiber, and an optical fiber cladding is provided outside the core of the optical fiber; two inclined fiber Bragg gratings TFBG1 and TFBG2 spaced apart from each other are formed on the core, and 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 is successively a porous silica layer, a hydrophobic MOF intermediate layer, and a graphene quantum dot (GQD) enhancement layer from the inside to the outside; an air inlet hole is provided on the optical fiber cladding for introducing the gas to be detected for CO2.

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

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

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

[0015] Further, the GQD enhancement layer is prepared by drop-coating a GQD dispersion liquid on the surface of the hydrophobic MOF intermediate layer and drying.

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

[0017] A CO2 detection method based on a dual-TFBG is realized based on the device described in any one of the above, and based on the dual-channel self-compensation optical path formed by TFBG1 and TFBG2, a dual-parameter decoupling algorithm is used to combine the wavelength shift and power change resulting from CO2 adsorption for joint solution, and a matrix equation is constructed to make the CO2 concentration detection independent of environmental interference.

[0018] Further, the constructed matrix equation is:

[0019]

[0020] Wherein, is 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 squares method to achieve complete compensation for temperature and humidity interference.

[0022] Furthermore, an edge filtering demodulation is performed using a broadband light source SLD and a tunable filter, and the wavelength shift is inversely deduced from the transmitted power change.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] The detection device and method provided by the present invention have at least the following advantages: First, by integrating a sensing grating (TFBG1) and a reference grating (TFBG2) in the same optical fiber, and respectively coating a composite sensitive layer and an inert PTFE layer, the present invention can monitor the changes in environmental temperature and humidity in real time and synchronously. Combining with a dual-parameter decoupling algorithm, the CO2 concentration detection is independent of environmental interference. Second, by adopting a unique composite sensitive layer structure, not only can the structural stability of the sensitive material be effectively improved while allowing CO2 molecules to freely diffuse, delaying degradation and cracking, but also the humidity sensitivity can be greatly reduced from a physical level by using the design of a porous silica bottom layer combined with a hydrophobic MOF intermediate layer. Third, in the present invention, by adopting different structural layers, especially forming a gradient pore size as a whole from the outside to the inside with GQD for enhanced adsorption, MOF layer and silica layer, the gas can be guided to flow in a specific direction and a high-density adsorption site can be provided, effectively improving the detection sensitivity and shortening the response time to within 30 seconds. Fourth, the present invention can also use a broadband light source + tunable filter (such as an F-P filter) to replace the spectrometer. By inferring the wavelength shift from the change in transmission power and combining the power ratio of the two gratings to eliminate the influence of light source fluctuations, the cost can be greatly reduced and the resolution can be improved. Through the composite sensitive layer structure, dual-channel self-compensating optical path and low-cost demodulation system, the present invention realizes high-precision and low-drift CO2 detection, providing a reliable solution for the environmental monitoring field. Description of the Drawings

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

[0026] Figure 2 It is a schematic structural diagram of a CO2 detection device according to an embodiment of the present invention. Detailed Embodiments

[0027] In order to make the above objects, features, and advantages of the present invention more obvious and understandable, the following detailed description of the specific embodiments of the present invention will be given with reference to the accompanying drawings. Many specific details are set forth in the following description in order to fully understand the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below. The technical features in the various embodiments of the present invention can be combined correspondingly without conflict.

[0028] The present invention provides a CO2 detection device based on dual TFBG, as Figure 1As shown in the figure, in a specific embodiment of the present invention, the system for detecting CO2 includes a broadband light source BBS 1, a single-mode optical fiber SMF 2, an optical fiber sensor 3, and an optical spectrum analyzer OAS 5. 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. They are respectively coated with a composite sensitive layer and an inert polytetrafluoroethylene PTFE layer 14. The composite sensitive layer is a porous silica layer 8, a hydrophobic MOF intermediate layer 7, and a graphene quantum dot (GQD) enhanced layer 6. An optical fiber cladding 9 is provided on the outermost layer, and an air inlet hole 13 is provided on the optical fiber cladding 9 for introducing the gas to be detected for CO2.

[0029] In this device, by integrating a sensing grating (TFBG1) and a reference grating (TFBG2) in the same optical fiber and respectively coating a composite sensitive layer and an inert PTFE layer, real-time synchronous monitoring of environmental temperature and humidity changes can be realized, forming a dual-channel self-compensation structure. Light is incident from the BBS into the SMF, conducted to the optical fiber sensor, and finally the optical spectrum is transmitted to the OAS. The CO2 gas enters the optical fiber sensor through the air hole. After passing through the three-layer composite sensitive layer structure, the CO2 molecules diffuse in the composite sensitive layer. The diffusion speed and adsorption degree will change the light deflection ability of the TFBG1, thereby changing the optical spectrum change. The TFBG2 compensates for environmental interference, so as to achieve the effect of detecting CO2 using dual TFBGs.

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

[0031] After that, a coating process is carried out. A composite sensitive layer is formed on the TFBG1 to directly respond to the change in CO2 concentration. An inert PTFE protective layer is formed on the TFBG2, which is only affected by temperature and humidity and is used for decoupling environmental parameters.

[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 by the sol-gel method to buffer the thermal stress, 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, its preparation method can be:

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

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

[0038] Coating and drying: Spin-coat the sol (rotation speed 3000 rpm) on the surface of TFBG1, dry at room temperature for 24 hours, and then calcine at 400 °C for 2 hours to form a porous silica layer with a pore diameter d≈2 nm.

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

[0040] Diffusion kinetic 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. The nano-pore size significantly improves the gas transmission rate by increasing the effective diffusion area.

[0043] 2) Hydrophobic MOF interlayer:

[0044] Use the hydrophobic UiO-66-(CF3)2 material. By introducing hydrophobic groups (-CF3), the adsorption of water molecules is inhibited, and the humidity sensitivity is significantly reduced. According to an embodiment of the present invention, its preparation method can be as follows:

[0045] Prepare the precursor: Zirconium salt (ZrCl4) and 2-(trifluoromethyl) terephthalic acid (H2BDC-CF3) are dissolved in DMF in a molar ratio of 1:1.

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

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

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

[0049]

[0050] Among them, θ is the surface coverage rate, K is the adsorption equilibrium constant, and P is the CO2 partial pressure. The K of the hydrophobic MOF H2O <<K CO2 , and its selectivity is significantly better than that of traditional materials. The hydrophobicity of the MOF coating was tested: the contact angle θ > 120°, and the CO2 adsorption capacity: at 25 °C and 1 atm, the adsorption capacity was 2.8 mmol / g, which was significantly higher than that of ordinary MOF (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 refluxed with nitric acid (100 °C, 6 hours) and then ultrasonically exfoliated to obtain GQDs with a diameter of about 5 nm. Then, the GQD dispersion (concentration 1 mg / mL) was drop-coated on the surface of the hydrophobic MOF intermediate layer and dried. The GQD stabilized the MOF structure through π-π stacking, and its high specific surface area could provide additional adsorption sites to accelerate 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 GQD significantly increases K a by reducing the activation energy E ads , improving the adsorption kinetic performance.

[0055] By setting up this composite sensitive layer, the humidity interference can be greatly suppressed, and its humidity sensitivity is reduced 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 material property design and structural synergistic effects. The setting of the composite sensitive layer, especially the silica layer and the hydrophobic MOF in it, can make β approach 0. The thermal expansion coefficient of the silica bottom layer is close to that of the fiber core, which can buffer the thermal stress caused by temperature changes and reduce the wavelength drift caused by the deformation of the grating structure. The thermal decomposition temperature of the MOF intermediate layer material is often greater than 300 °C (MOF materials generally have high thermal stability), and the structure is stable in our detection environment and will not cause degradation of adsorption sites due to temperature. The high thermal conductivity of the GQD enhancement layer can evenly distribute the temperature field and reduce the grating wavelength drift caused by local thermal stress, and cooperate with the silica bottom layer to further buffer the temperature interference, that is, α also approaches 0.

[0058] To improve sensitivity, it is often necessary to increase the thickness of the sensitive layer. However, a thick coating will slow down the gas diffusion kinetics, resulting in an excessively long response time, which is not conducive to meeting the requirements of real-time monitoring. In the solution of the present invention, the composite sensitive layer can be controlled to be about 500 nm. By forming a gradient pore size design in the composite sensitive layer and combining with a GQD enhancement layer that can provide high-density adsorption sites, the response time is optimized. With this design, the response time can be shortened to <30 seconds.

[0059] According to the Bragg condition:

[0060]

[0061] where n eff is the effective refractive index and Λ is the grating period. The adsorption of CO2 causes a change in n eff , resulting in a wavelength shift Δλ B . Based on the above dual-channel self-compensating optical path design, the present invention adopts a dual-parameter decoupling algorithm. By jointly solving the wavelength shift (Δλ) and power change (ΔP), environmental interference is eliminated:

[0062]

[0063] where is the wavelength shift of TFBG1 caused by CO2 concentration, temperature, and 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;

[0064] Parameters such as the above wavelength response coefficient, wavelength coupling coefficient, power response coefficient, and power coupling coefficient can all obtain data through experiments and be obtained by fitting. The following is an example: For example, the response coefficient to CO2 concentration , , in a constant temperature and humidity environment of 25 °C and 50% RH, for a single TFBG1 optical fiber, different concentrations of CO2 gas with a concentration range of 0 - 2000 ppm are introduced, and the wavelength shift and power change of TFBG1 are recorded on a spectrometer. By linear fitting, the value of this type of response coefficient can be obtained. The temperature and humidity response coefficient is the same principle, that is, controlling a fixed CO2 concentration and changing the temperature and humidity to obtain (experiments are carried out under common indoor temperature and humidity).

[0065] Construct a matrix equation:

[0066]

[0067] Solve for C by the least squares method to achieve complete compensation for temperature and humidity interference. Before compensation, the measurement error ΔC caused by humidity fluctuation (ΔRH = 60%) can reach ±150 ppm, while after matrix calculation, the error is reduced to ΔC = ±10 ppm.

[0068] Regarding the problem that existing high-precision solutions often rely on high-resolution spectrometers, with high equipment costs (> 100,000 yuan), and the algorithm requires a large amount of calibration data and is difficult to commercialize and promote. According to an 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 an F-P filter) is used to replace the spectrometer. According to the relationship between the transmitted power P out and wavelength:

[0069]

[0070] where T(λ) is the filter transfer function. By calibrating the T(λ) curve, the wavelength shift Δλ can be inversely deduced from the change in transmitted power, and combined with the double-grating power ratio (ΔP1 / ΔP2) to eliminate the influence of light source fluctuations, achieving a resolution of ±10 ppm. Using this method, the cost can be reduced by about 80% compared with using a high-resolution spectrometer. It can be further combined with a calibration model based on transfer learning, which only requires a small amount of data (< 100 groups) to adapt to different environments, and can also correct temperature and humidity drift in real time based on Kalman filtering without continuous manual calibration:

[0071]

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

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

[0074] The above-described embodiments have detailed the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the present invention. Any modifications, supplements, and equivalent replacements made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A CO2 detection device based on a dual-TFBG, characterized in that, It includes an optical fiber, and an optical fiber cladding is provided outside the core of the optical fiber; two inclined fiber Bragg gratings TFBG1 and TFBG2 spaced apart from each other are formed on the core, a composite sensitive layer is arranged 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 is successively a porous silica layer, a hydrophobic MOF intermediate layer, and a graphene quantum dot (GQD) enhancement layer from the inside to the outside; air inlet holes are provided on the optical fiber cladding for introducing the gas to be detected of CO2.

2. The CO2 detection device based on a dual-TFBG according to claim 1, wherein The inclination angles of the inclined fiber Bragg gratings TFBG1 and TFBG2 are 7 to 13°, and the grating period is 530 nm.

3. The CO2 detection device based on a dual-TFBG according to claim 1, wherein The porous silica layer is formed on TFBG1 by a sol-gel method using a precursor solution, and its pore diameter is 1 to 10 nm.

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

5. The CO2 detection device based on a dual-TFBG according to claim 1, wherein, The GQD enhancement layer is prepared by drop-coating a GQD dispersion liquid on the surface of the hydrophobic MOF intermediate layer and drying.

6. The CO2 detection device based on a dual-TFBG according to claim 1, wherein, The inert protective layer is a PTFE layer.

7. A CO2 detection method based on dual-TFBG, characterized in that, It is implemented based on the device described in any one of claims 1-6. A dual-channel self-compensating optical path formed by TFBG1 and TFBG2 is adopted, and a dual-parameter decoupling algorithm is used to combine the wavelength shift and power change caused by CO2 adsorption for joint calculation, and a matrix equation is constructed to make the CO2 concentration detection independent of environmental interference. ​ 8. The CO2 detection method based on a dual-TFBG according to claim 7, characterized in that, The constructed matrix equation is: ， Among them, is 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 squares method to achieve complete compensation for temperature and humidity interference.

9. The CO2 detection method based on a dual-TFBG according to claim 7, wherein, An edge filtering demodulation is carried out by using a broadband light source SLD and a tunable filter, and the wavelength shift is inversely deduced through the change of the transmission power.

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