Carbon dioxide detection sensor and method based on coated fiber bragg grating

Abstract

The application discloses a kind of carbon dioxide detection sensor and method based on coating fiber bragg grating, the sensor includes single-mode optical fiber, single-mode optical fiber includes core, cladding and hollow capillary sleeve sleeved outside cladding;The middle section of the single-mode optical fiber is provided with a core exposed section, a bragg grating FBG is arranged on the core of the exposed section, and a carbon dioxide response expansion polymer coating is provided, and a sleeve is coated outside the exposed section, the sleeve is sealingly connected between the hollow capillary sleeve at both ends, the sleeve is provided with air inlet and outlet hole, and a temperature sensor is arranged between the sleeve and the polymer coating.The CO2 detection sensor of the application can achieve high precision ≤5 ppm, fast response ≤5 s effect, can meet the monitoring of rapidly changing scene, and is easy to maintain.

Description

Technical Field

[0001] This invention belongs to the field of CO2 detection technology and relates to a carbon dioxide detection sensor and method based on coated fiber Bragg gratings. Background Technology

[0002] Optical detection technology, due to its advantages such as high selectivity, rapid response, and long-term stability, has been widely used in the field of gas analysis. Among them, infrared spectroscopy utilizes the absorption characteristics of CO2 molecules to specific wavelengths of infrared light, enabling non-contact measurement of its concentration. In conventional industrial CO2 monitoring, non-dispersive infrared (NDIR) analyzers are commonly used equipment: light emitted from an infrared light source passes through the measuring chamber, is absorbed by characteristic components in the gas, and the residual light is received by an infrared detector to calculate the concentration. However, complex industrial environments (such as high temperature, high humidity, and flue gas with high dust concentrations) pose severe challenges to traditional NDIR sensors: pollutants such as smoke and water vapor can adhere to the surface of optical components or directly interfere with the optical path, leading to a decrease in measurement accuracy. To mitigate these effects, existing NDIR sensors typically add a reference optical path (dual-channel structure) to compensate in wavelengths outside of CO2 absorption, correcting errors caused by dust accumulation or light source intensity drift. Even so, frequent cleaning of optical windows and periodic calibration are still unavoidable, increasing maintenance costs.

[0003] Numerous reports exist on CO2 detection devices based on fiber Bragg gratings, but several shortcomings remain in terms of detection performance. For instance, using dual-parameter optical fibers for CO2 concentration detection presents significant cross-sensitivity issues, increasing the difficulty of signal decoupling and data processing. Furthermore, the response speeds of the two sensitive fibers may differ, leading to asynchronous data acquisition and affecting detection results in rapidly changing environments. This not only results in higher costs and more complex calibration but also greater maintenance difficulties. In addition, existing fiber-based concentration detection schemes often suffer from humidity and air pressure interference, limited sensitivity / detection lower limit, slow response speed, complex calibration prone to drift, multiplexing crosstalk, and mechanical strain interference. Especially when applied to embedded systems for distributed detection, the use of dual-parameter optical fibers in series often presents the following problems:

[0004] 1. Commonly used distributed detection methods require that the interval between FBGs (Fast Step Gears) cannot be too short. This makes it difficult to place the FBGs for temperature compensation and those for CO2 detection in the same location, resulting in temperature differences and errors during the final compensation. 2. The demodulation data volume of FBGs is quite large. For data processing, dual FBG compensation means doubling the data computation, which significantly impacts the difficulty of embedded deployment and the real-time performance of the system. 3. Temperature compensation of FBGs requires temperature calibration of each FBG in a laboratory environment, making it difficult to improve manufacturing efficiency. 4. Dual FBGs also halve the upper limit of the number of distributed detections.

[0005] This invention focuses on the key bottlenecks in the aforementioned detection methods and proposes a novel, targeted solution to significantly improve the accuracy, stability, and response performance of CO2 measurement. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing a carbon dioxide detection sensor and method based on coated fiber Bragg gratings.

[0007] The technical solution adopted in this invention is as follows:

[0008] A carbon dioxide detection sensor based on a coated fiber Bragg grating includes a single-mode fiber, which comprises a core, a cladding, and a hollow capillary sleeve fitted over the cladding. The single-mode fiber has a core-exposed section in its middle segment. The core of this exposed section is coated with a Bragg grating (FBG) and a polymer coating that expands in response to carbon dioxide. A sleeve is then wrapped around this exposed section, and the sleeve is sealed to the hollow capillary sleeves at both ends. The sleeve has inlet and outlet ports. A temperature sensor is positioned between the sleeve and the polymer coating.

[0009] In the above technical solution, the Bragg grating has a center wavelength of 1548 nm to 1568 nm, an interval of 0.8 nm, and a total of 80 channels.

[0010] Furthermore, the polymer coating is a polymer containing amine groups, and the coating thickness is less than 1 μm.

[0011] Furthermore, the polymer coating is a composite formed by mixing an amine-functionalized elastomer (amine / carbonate functional group polymer elastomer) with a high specific surface area nanocarrier, wherein the high specific surface area nanocarrier is porous silica or graphene aerogel; or the coating may also contain an ultraviolet stabilizer.

[0012] Furthermore, the sleeve is a rigid sleeve, and a hydrophobic elastic damping layer is filled between the rigid sleeve and the polymer coating.

[0013] Furthermore, the temperature sensor is an NTC thermistor, with its leads running from the air inlet and outlet ports.

[0014] A carbon dioxide detection method based on coated fiber Bragg gratings (FBGs) is implemented using a sensor as described above. The CO2 to be measured is injected into the cavity through an inlet / outlet port. CO2 of different concentrations is adsorbed by the polymer coating, causing the coating to expand to varying degrees. The FBG is subjected to stress and environmental temperature changes, resulting in a change in the center wavelength of its reflected light, thus creating a difference in the emitted light signal. A temperature sensor in the sensor collects the corresponding signal, and temperature compensation is applied to the measured FBG wavelength shift, thereby obtaining the wavelength shift affected only by stress, from which the CO2 concentration is deduced.

[0015] Furthermore, the temperature sensor employs an NTC thermistor, and its resistance value at a reference temperature and its β parameter are used to determine the relationship between the resistance and temperature. Therefore, after temperature compensation, the FBG wavelength shift caused solely by CO2 is:

[0016]

[0017] in, K is the total center wavelength drift measured by the working grating. T The temperature sensitivity coefficient of the FBG in this working sample; the CO2 sensitivity coefficient. The CO2 concentration C is obtained by reverse calculation:

[0018] .

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] The detection sensor provided by this invention is based on a polymer coating that expands in response to CO2. Through structural design, a temperature sensor is directly integrated into the device, allowing for real-time synchronous monitoring of ambient temperature changes and temperature compensation. This effectively avoids cross-sensitivity issues and facilitates more accurate acquisition of the impact of CO2 concentration on the sensor. Furthermore, the optimized polymer coating improves the response in low-concentration ranges, enhancing detection accuracy. Maintenance is simple and cost-effective. In addition, since the polymer coating may exhibit adsorption or compression effects on humidity, additional expansion / compression spurious strain can be superimposed on the CO2 signal, creating concentration artifacts. This invention further employs a hydrophobic elastic damping layer to fill and cover the polymer coating, not only preventing moisture adsorption but also forming a double strain isolation with a rigid sleeve to isolate external stress. The CO2 detection sensor of this invention can be well integrated into embedded systems for distributed detection, achieving high accuracy (≤5 ppm) and a fast response (≤5 s), meeting the needs of monitoring rapidly changing scenarios and is easy to maintain. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of a detection sensor according to an embodiment of the present invention;

[0022] Figure 2 This is a schematic diagram of the detection system in one embodiment of the present invention;

[0023] In the diagram: 1. Hollow capillary tube, 2. Fiber core, 3. Cladding, 4. Air inlet, 5. Tube, 6. Air outlet, 7. Bragg grating (FBG), 8. NTC temperature sensor, 9. Polymer coating, 10. Antireflection coating, 11. Signal detection and processing module, 12. Signal modulation module, 13. Embedded control module, 14. Connector, 15. Incident broadband light source, 16. Circulator, 17. Fiber optic FBG CO2 sensor, 18. Power supply regulator module, 19. Temperature detection and compensation module, 20. Photoelectric converter. Detailed Implementation

[0024] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. Technical features in the various embodiments of the present invention can be combined accordingly without mutual conflict.

[0025] This invention provides a carbon dioxide detection sensor based on a coated fiber Bragg grating, such as... Figure 1 As shown, in a specific embodiment of the present invention, the sensor includes a single-mode optical fiber, which includes a core 2, a cladding 3, and a hollow capillary tube 1 sleeved outside the cladding. The single-mode optical fiber has a bare core section in its middle segment. A Bragg grating (FBG) 7 is provided on the core of this bare section, and a polymer coating 9 that expands in response to carbon dioxide is applied. A sleeve 5, which is a rigid sleeve, is wrapped around this bare section. The sleeve is sealed to the hollow capillary tubes at both ends. The sleeve has an inlet 4 and an outlet 6. A temperature sensor 8, using an NTC thermistor, is located between the sleeve and the polymer coating, with its lead passing through the outlet 6. An antireflection coating 10 is provided at the end of the single-mode optical fiber. This sensor is used in a detection system, such as... Figure 2The specific workflow of this invention is as follows: The embedded control module 13 sends a control signal and, after voltage stabilization, supplies power to the incident broadband light source 15. The incident light enters the fiber optic FBG CO2 sensor through the circulator 16, and the CO2 to be measured is injected into the cavity through the air inlet. When the broadband light passes through the FBG, it generates reflected light. CO2 of different concentrations reacts with the amine-containing polymer coating, causing the polymer to expand to different degrees. The FBG is subjected to stress and compression, which causes a change in the center wavelength of its reflected light, resulting in corresponding differences in the spectrum of the reflected and transmitted light signals. The circulator transmits the reflected light signal to the photoelectric converter 20, converts it into an electrical signal, and feeds it back to the embedded control module for further processing and real-time display of the corresponding results. Since the FBG is simultaneously affected by stress and temperature, the resistance change of the NTC temperature sensor is also collected to determine the current temperature and make corresponding temperature compensation.

[0026] The specific signal transmission process of the whole system is as follows: the external power supply provides power to the embedded system, and the voltage regulator circuit in the main control circuit provides power to the broadband light source generator and the signal processing circuit after voltage regulation and division. The broadband light source generates a wide wavelength range of light signal and transmits it to the fiber optic FBG CO2 sensor. The feedback light signal of the fiber optic FBG CO2 sensor is then transmitted to the photoelectric converter to be converted into an electrical signal and amplified. At the same time, the current FBG temperature is collected and transmitted to the signal processing circuit for acquisition, calculation and analysis, and the results are fed back to the main control. The main control displays the corresponding results on the display screen, and the user can freely decide to set the sampling rate through the feedback information. The whole process is automatically controlled and fed back in real time by the embedded system, which organically combines fiber optic sensing with the embedded system.

[0027] The principles and formulas involved in this invention include:

[0028] FBG wavelength-strain-temperature relationship

[0029]

[0030] Where n eff Λ is the effective refractive index of the optical fiber, and Λ is the grating period.

[0031] When the FBG is simultaneously subjected to mechanical strain ε and temperature change ΔT, its wavelength drift is:

[0032]

[0033] Where p e ζ is the photoelastic coefficient of the optical fiber. α is the thermal expansion coefficient of the optical fiber. ζ is the thermo-optical refractive index coefficient of the optical fiber.

[0034] The relationship between CO2 concentration C and adsorption capacity q derived from the Langmuir isotherm model:

[0035]

[0036] Where, q max denoted as saturated adsorption capacity of a single molecular layer, and K is the Langmuir adsorption equilibrium constant.

[0037] Expansion strain It can be approximated as linear:

[0038]

[0039] Where α is the volume expansion coefficient of the polymer (typically 10). -3 ~10 -2 ).

[0040] For an NTC thermistor with nominal parameters R0 (resistance at reference temperature T0) and β parameter B, its resistance R T The relationship with temperature T (absolute temperature, unit K) is approximately as follows:

[0041]

[0042] Where T0 = 298.15K, R0 is the nominal resistance of the NTC at 25℃, and B is given in the device datasheet.

[0043] The temperature-compensated wavelength shift (caused solely by CO2) is as follows:

[0044]

[0045] in, The total center wavelength drift measured by the working grating (including that caused by temperature + CO2, i.e., the one mentioned in the previous analysis) ), K T The temperature sensitivity coefficient (pm / K) of the FBG in this working process is given by ΔT = T(R). T )-T0.

[0046] After temperature compensation is completed, the CO2 sensitivity coefficient can be used. (pm / ppm) Back-estimated gas phase concentration C:

[0047] .

[0048] According to a specific embodiment of the present invention, Corning SMF-28e+ single-mode optical fiber is used. The cladding and hollow capillary tube coating are cut and removed using wire strippers to expose the fiber core. Then, FBGs (fiber optic cables) with a length of 5 mm, a center wavelength of 1548 nm to 1568 nm, and a spacing of 0.8 nm, are etched, resulting in a total of 80 channels. Subsequently, a thin polymer coating layer (0.5 μm) is uniformly coated onto the FBG surface. The polymer coating is an amine polymer-porous silica composite. In some embodiments of the present invention, the amine polymer is an amine-functionalized polycarbonate elastomer; in other embodiments, other amine-modified polyethersulfone (PES), polyimide (PI), polyethylene glycol (PEG), etc., may also be used. A temperature compensation module is set up by placing a temperature sensor on the polymer coating. In this example, a Vishay NTCLE100E3 (10 kΩ@25℃, B=3950 K) is used, with a sampling accuracy of ±0.1℃. Then, a hydrophobic elastic stabilizing layer is coated, which can be a polytetrafluoroethylene (PTFE) film in this example. Finally, a rigid sleeve is placed on the outermost layer, and the leads of the temperature sensor are led out through the air inlet and outlet holes on it. The rigid sleeve is then sealed with the hollow capillary sleeves on both sides to obtain the carbon dioxide detection sensor.

[0049] Because FBG is extremely sensitive to ambient temperature while undergoing strain due to polymer expansion in response to CO2, even small temperature fluctuations can cause significant shifts in the center wavelength. Simultaneously, the polymer coating exhibits adsorption or compression effects to humidity and atmospheric pressure, resulting in additional expansion / compression spurious strain superimposed on the CO2 signal, creating a concentration artifact. In this invention, a temperature sensor is used to compensate for temperature drift, effectively eliminating temperature drift terms and achieving zero-temperature cross-compensation. This ensures that CO2 measurement is unaffected by temperature and employs dual strain isolation: an outer rigid sleeve securely wraps the optical fiber, allowing only chemical expansion; a hydrophobic elastic damping layer is filled between the sleeve and the optical fiber, isolating external stress and preventing moisture adsorption. Furthermore, the polymer coating and CO2 adsorption capacity are limited by the Langmuir isotherm, exhibiting weak responses in low-concentration ranges, making detection within 10 ppm difficult. This invention optimizes the polymer coating by combining the polymer with porous silica or graphene aerogel, which has a relatively higher specific surface area. High-specific-surface-area nanocarriers support the amine polymer, significantly improving adsorption capacity and linear response range; the coating thickness is controlled to be <1 μm. To prevent polymer performance from drifting due to aging or contamination over time, anti-aging additives (UV stabilizers) can be added to the coating to extend its service life. Additionally, a known low concentration of CO2 (such as 400ppm standard gas) can be periodically released into the system to automatically compare the grating output with the theoretical value, correct the polymer response coefficient, and avoid frequent manual calibration, which would affect continuous monitoring.

[0050] This invention combines an embedded system and an FBG-based carbon dioxide detection sensor to achieve intelligent and automated detection. According to a specific embodiment of the invention, in addition to the aforementioned sensor, several functional modules are also employed:

[0051] Light source driver module

[0052] Broadband SLD light source: Exalos SEL-3400 (20 nm bandwidth) was used for system self-testing and comparison;

[0053] Driver chip: Maxim MAX191 constant current source, modulation rate up to 200 kHz.

[0054] Signal Demultiplexing and Spectral Analysis Module

[0055] Spectral demultiplexing: With the help of Finisar WaveShaper 1000A, programmable filtering accuracy reaches 0.01 nm, accurately selecting the reflected light of each FBG.

[0056] Photodetector: Hamamatsu G12180-003K InGaAs APD was used, with noise equivalent power <10 pW and bandwidth of 50 MHz.

[0057] Front-end amplification: Analog Devices AD8015 transimpedance amplifier, 50 MHz bandwidth, adjustable gain.

[0058] Analog-to-digital conversion: Analog Devices AD7176-2, 24-bit ΔΣ ADC, maximum sampling rate of 4 kSPS, dynamic range of 110 dB.

[0059] Embedded signal processing module

[0060] Main control MCU: ST Micro STM32L476RG (80 MHz, ARM Cortex-M4DSP), low power standby current <5 μA.

[0061] Storage and Interfaces: The Winbond W25Q128JV (128 Mb SPI FLASH) stores calibration curves and historical data; the I²C interface expands temperature and humidity sensing; and the UART interface is used for debugging.

[0062] Through the modular design and precise chip selection described above, this embodiment outperforms existing technologies in key performance indicators such as multi-channel distribution, 5ppm resolution, <5s response, and ±0.1℃ temperature compensation, achieving high-precision and high-stability CO2 monitoring with large-area gridding.

[0063] The system of this invention, through structural design, directly implements the temperature compensation function using an NTC thermistor integrated within the sleeve. This effectively solves the problem that existing sensing solutions are not conducive to integration with embedded systems and the realization of distributed detection. It has the following advantages: 1. The integrated NTC thermistor is located close to the FBG (Fiber Optic Generator), allowing its temperature to better reflect the current temperature of the FBG; 2. The temperature-resistance calculation formula for the NTC thermistor is extremely mature, requiring no laboratory temperature calibration for direct use with high temperature accuracy; 3. The single-fiber and built-in NTC design results in a more compact structure, and the thermistor calculation formula is not complex, allowing for easy deployment in embedded systems and improving system response time; 4. The FBG with integrated NTC thermistor can provide a denser distribution of carbon dioxide detection points, increasing the density of the detection system.

[0064] The embodiments described above provide a detailed explanation of the technical solutions and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions, and equivalent substitutions 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 carbon dioxide detection sensor based on a coated fiber Bragg grating, characterized in that, The invention includes a single-mode optical fiber, comprising a core, a cladding, and a hollow capillary tube surrounding the cladding. The single-mode optical fiber has a bare core section in its middle segment. The core of this bare section is provided with a Bragg grating (FBG) and a polymer coating that expands in response to carbon dioxide. A tube surrounds this bare section, and the tube is sealed to the hollow capillary tubes at both ends. The tube has inlet and outlet vents, and a temperature sensor is located between the tube and the polymer coating. The polymer coating is a composite formed by mixing an amino-functionalized elastomer with a high specific surface area nanocarrier, wherein the high specific surface area nanocarrier is porous silica or graphene aerogel.

2. The carbon dioxide detection sensor based on coated fiber Bragg grating according to claim 1, characterized in that, The Bragg grating has a center wavelength of 1548 nm to 1568 nm, with an interval of 0.8 nm, and a total of 80 channels.

3. The carbon dioxide detection sensor based on coated fiber Bragg gratings according to claim 1, characterized in that, The thickness of the polymer coating is less than 1 μm.

4. The carbon dioxide detection sensor based on coated fiber Bragg grating according to claim 1, characterized in that, The polymer coating also contains a UV stabilizer.

5. The carbon dioxide detection sensor based on coated fiber Bragg grating according to claim 1, characterized in that, The sleeve is a rigid sleeve, and a hydrophobic elastic damping layer is filled between the rigid sleeve and the polymer coating.

6. The carbon dioxide detection sensor based on a coated fiber Bragg grating according to claim 1, characterized in that, The temperature sensor is an NTC thermistor, and its leads run from the air inlet and outlet.

7. A method for carbon dioxide detection based on coated fiber Bragg gratings, characterized in that, Based on the sensor described in any one of claims 1-6, the CO2 to be measured is injected into the cavity through the inlet and outlet ports. CO2 of different concentrations will be adsorbed by the polymer coating, causing the polymer coating to expand to different degrees. The FBG is subjected to stress compression and changes in ambient temperature, which causes the center wavelength of its reflected light to change, thus resulting in differences in the emitted light signal. The temperature sensor in the sensor collects the corresponding signal and performs temperature compensation on the measured FBG wavelength shift, thereby obtaining the wavelength shift affected only by stress, and then inversely deducing the CO2 concentration.

8. The carbon dioxide detection method based on coated fiber Bragg gratings according to claim 7, characterized in that, The temperature sensor uses an NTC thermistor. The relationship between its resistance and temperature is determined based on its resistance at a reference temperature and its β parameter. Therefore, after temperature compensation, the FBG wavelength shift caused solely by CO2 is: ， in, K is the total center wavelength drift measured by the working grating. T The temperature sensitivity coefficient of this working FBG is given by T0, where T0 is the reference temperature; the CO2 sensitivity coefficient is used. The CO2 concentration C is obtained by reverse calculation: 。

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