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

Abstract

The invention discloses a carbon dioxide detection sensor and method based on a coated fiber bragg grating, the sensor comprises a single-mode fiber, and the single-mode fiber comprises a fiber core, a cladding and a hollow capillary sleeve sleeving the cladding; the middle section of the single-mode optical fiber is provided with a fiber core exposed section, the fiber core of the exposed section is provided with a Bragg grating FBG and a polymer coating expanded in response to carbon dioxide, the exposed section is coated with a sleeve, the sleeve is hermetically connected with the hollow capillary sleeves at the two ends, the sleeve is provided with an air inlet and an air outlet, and the air inlet and the air outlet are communicated with the hollow capillary sleeves. And a temperature sensor is arranged between the sleeve and the polymer coating. The CO2 detection sensor provided by the invention can achieve the effects of high precision of less than or equal to 5 ppm and quick response of less than or equal to 5 s, can meet the requirement of quick change scene monitoring, and is easy to maintain.

Description

Technical Field

[0001] The present invention belongs to the technical field of CO2 detection, and relates to a carbon dioxide detection sensor based on a coated optical fiber Bragg grating and a method thereof. Background Art

[0002] Optical detection technology has been widely used in gas analysis due to its advantages such as high selectivity, fast response, and long-term stability. Infrared spectroscopy, among other techniques, exploits the absorption properties of CO2 molecules at specific wavelengths of infrared light to measure its concentration contactlessly. Non-dispersive infrared (NDIR) analyzers are commonly used in conventional industrial CO2 monitoring. Light emitted by an infrared light source passes through a measurement chamber, where it is absorbed by characteristic components in the gas. The residual light is then received by an infrared detector, which then estimates the concentration. However, complex industrial environments (such as high temperature, high humidity, and flue gas with high dust concentrations) pose significant challenges to traditional NDIR sensors. Contaminants such as dust and water vapor can adhere to optical component surfaces or directly interfere with the optical path, reducing measurement accuracy. To mitigate these effects, existing NDIR sensors typically incorporate a reference optical path (dual-channel architecture) to compensate for errors caused by dust accumulation or light source intensity drift in the wavelength bands outside of CO2 absorption. Even so, frequent optical window cleaning and regular calibration are still necessary, increasing operational and maintenance costs.

[0003] There are also many reports on CO2 detection devices based on fiber Bragg gratings, but there are still many deficiencies in detection performance. For example, the use of dual-parameter optical fibers to detect CO2 concentration has prominent cross-sensitivity issues, which increases the difficulty of signal decoupling and data processing. In addition, the response speeds of the two sensitive optical fibers may be different, which will cause data acquisition to be asynchronous in a rapidly changing environment, affecting the detection effect. It is not only costly but also complex to calibrate and difficult to maintain. In addition, existing fiber-based concentration detection solutions often have humidity and air pressure interference, limited sensitivity / detection limit, slow response speed, complex calibration and easy drift, multiplexing crosstalk and mechanical strain interference. Especially when used in embedded systems to implement distributed detection, the use of dual-parameter optical fibers in series often has the following problems: 1. Common distributed detection methods require that the spacing between FBGs cannot be too short, which makes it difficult to place the FBGs for temperature compensation and the FBGs for CO2 detection together. The temperature difference between them causes errors in the final compensation. 2. The amount of demodulated data from FBGs is relatively large. In terms of data processing volume, dual FBG compensation means double the data calculation, which has a significant impact on the difficulty of embedded deployment and the real-time performance of the system. 3. FBG temperature compensation requires temperature calibration of each FBG in a laboratory environment, which makes it difficult to improve manufacturing efficiency. 4. Dual FBGs also halve the upper limit of the number of distributed detections.

[0004] This paper focuses on the key bottlenecks in the above-mentioned detection direction and proposes a newly designed targeted solution to significantly improve the accuracy, stability and response performance of CO2 measurement. Summary of the Invention

[0005] The purpose of the present invention is to address the deficiencies of the prior art and provide a carbon dioxide detection sensor and method based on a coated fiber Bragg grating.

[0006] The technical solution adopted in the present invention is:

[0007] A carbon dioxide detection sensor based on a coated fiber Bragg grating comprises a single-mode optical fiber, wherein the single-mode optical fiber comprises a fiber core, a cladding, and a hollow capillary tube sheathed outside the cladding. An exposed core section is provided in the middle section of the single-mode optical fiber, a Bragg grating (FBG) is provided on the core of the exposed section, and a polymer coating that expands in response to carbon dioxide is provided. A sleeve is coated outside the exposed section, and the sleeve is sealedly connected to the hollow capillary tubes at both ends. The sleeve is provided with air inlet and outlet holes, and a temperature sensor is provided between the sleeve and the polymer coating.

[0008] In the above technical solution, further, the central wavelength of the Bragg grating is 1548 nm to 1568 nm, the interval is 0.8 nm, and there are 80 channels in total.

[0009] Furthermore, the polymer coating is a polymer containing an amine group, and the coating thickness is less than 1 μm.

[0010] Furthermore, the polymer coating is a composite formed by mixing an amine-functionalized elastomer (amine / carbonate-functional 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 further contains a UV stabilizer.

[0011] Furthermore, the sleeve is a rigid sleeve, and a hydrophobic elastic damping layer is filled between the rigid sleeve and the polymer coating.

[0012] Furthermore, the temperature sensor adopts an NTC thermistor, and its lead wires are routed through the air inlet and outlet holes.

[0013] A method for detecting carbon dioxide based on a coated fiber Bragg grating is implemented based on a sensor as described in any of the above items. The CO2 to be measured is injected into the cavity through the inlet and outlet holes. Different concentrations of CO2 will be adsorbed by the polymer coating, causing the polymer coating to expand to varying degrees. The FBG is subjected to stress compression and changes in ambient temperature, resulting in changes in the central wavelength of its reflected light, thereby causing the emitted optical signal to have differences. A temperature sensor in the sensor collects the corresponding signal, and the measured FBG wavelength offset is temperature compensated to obtain the wavelength offset affected only by stress, thereby inferring the CO2 concentration.

[0014] Furthermore, the temperature sensor uses an NTC thermistor, and the relationship between its resistance and temperature is determined based on its resistance at a reference temperature and its β parameter. Therefore, the FBG wavelength drift caused only by CO2 after temperature compensation is:

[0015]

[0016] in, is the total center wavelength drift measured by the working grating, K T is the sensitivity coefficient of the FBG to temperature; CO2 sensitivity coefficient The CO2 concentration C can be obtained by reverse calculation:

[0017] .

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The detection sensor provided by the present 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 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. At the same time, the optimized polymer coating improves the response effect in the low-concentration section, improves detection accuracy, and is simple to maintain and low in cost. In addition, since the polymer coating may also have an adsorption or compression effect on humidity, additional expansion / compression pseudo-strain will be superimposed on the CO2 signal, creating a false concentration. The present invention further uses a hydrophobic elastic damping layer to fill and coat the polymer coating, which not only blocks moisture adsorption but also forms a dual strain isolation in combination with a rigid casing to isolate external stress. The CO2 detection sensor of the present invention can be well integrated into embedded systems for distributed detection, achieving high accuracy of ≤5 ppm and fast response of ≤5 s, meeting the needs of monitoring rapidly changing scenarios and being easy to maintain. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1This is a schematic structural diagram of a detection sensor according to an embodiment of the present invention;

[0021] Figure 2 A schematic structural diagram of a detection system in one embodiment of the present invention;

[0022] Figure: 1. Hollow capillary tube, 2. Fiber core, 3. Cladding, 4. Air inlet, 5. Tube, 6. Air outlet, 7. Fiber 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 FBG CO2 sensor, 18. Power supply regulator, 19. Temperature detection and compensation module, 20. Photoelectric converter. DETAILED DESCRIPTION

[0023] 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.

[0024] The present invention provides a carbon dioxide detection sensor based on 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; a core-exposed section is provided in the middle section of the single-mode optical fiber, a Bragg grating FBG 7 is provided on the core of the exposed section, and a polymer coating 9 that expands in response to carbon dioxide is provided, and a sleeve 5 is wrapped around the exposed section, the sleeve being a rigid sleeve, and the sleeve is sealed to the hollow capillary sleeves at both ends, an air inlet 4 and an air outlet 6 are provided on the sleeve, and a temperature sensor 8 is provided between the sleeve and the polymer coating, which is an NTC thermistor, and its lead wire passes through the air outlet 6, and an anti-reflection film 10 is provided at the end of the single-mode optical fiber. The sensor is used in a detection system, such as Figure 2The specific workflow of the present invention is as follows: the embedded control module 13 sends a control signal and, after voltage stabilization, powers the incident wide-spectrum light source 15. The incident light passes through the circulator 16 and enters the fiber FBG CO2 sensor, injecting the CO2 to be measured into the cavity through the air inlet. When the wide-spectrum light passes through the FBG, reflected light is generated. Different concentrations of CO2 react with the amino-containing polymer coating, causing the polymer to expand to varying degrees. The FBG is subjected to stress and compression, causing the central wavelength of its reflected light to change, resulting in corresponding differences in the spectrum of the reflected and transmitted optical signal. 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. Because the FBG is affected by both stress and temperature, the resistance change of the NTC temperature sensor is simultaneously collected to determine the current temperature and make corresponding temperature compensation.

[0025] The specific transmission process of the entire system signal is as follows: an external power supply supplies energy to the embedded system, the voltage stabilization circuit in the main control circuit performs voltage stabilization and voltage division and then provides power to the wide-spectrum light source generator and signal processing circuit respectively. The wide-spectrum light source generates an optical signal with a wide wavelength range and transmits it to the fiber optic FBG CO2 sensor. The feedback optical signal of the fiber optic FBG CO2 sensor is then transmitted to the photoelectric converter, 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 collection, 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 allows the user to freely decide to set the sampling rate through feedback information. The entire process is automatically controlled and provided with real-time feedback by the embedded system, organically combining fiber optic sensing with the embedded system.

[0026] Some of the principle formulas involved in the present invention are:

[0027] FBG wavelength-strain-temperature relationship

[0028]

[0029] where n eff is the effective refractive index of the optical fiber, and Λ is the grating period.

[0030] When the FBG is subjected to mechanical strain ε and temperature change ΔT at the same time, its wavelength drift is:

[0031]

[0032] where p e is the optical fiber photoelastic coefficient. α is the optical fiber thermal expansion coefficient. ζ is the optical fiber thermo-optical refractive index.

[0033] The relationship between CO2 concentration C and adsorption amount q derived based on the Langmuir isotherm model is:

[0034]

[0035] Among them, q max is the saturated adsorption capacity of a single molecular layer, K is the Langmuir adsorption equilibrium constant; Expansion strain It can be approximately regarded as linear:

[0036]

[0037] Where α is the volume expansion coefficient of the polymer (typically 10 -3 ~10 -2 ).

[0038] For an NTC thermistor with a nominal parameter of R0 (resistance at reference temperature T0) and a β parameter of B, its resistance R T The relationship with temperature T (absolute temperature, unit K) is approximately:

[0039]

[0040] Where T0 = 298.15K, R0 is the nominal resistance of the NTC at 25°C, and B is given in the device manual.

[0041] The wavelength drift after temperature compensation (caused only by CO2) is:

[0042]

[0043] in, is the total center wavelength drift measured by the working grating (including the temperature + CO2 caused, i.e. ), K T is the sensitivity coefficient of the working FBG to temperature (pm / K), ΔT=T(R T )-T0.

[0044] After temperature compensation, the CO2 sensitivity coefficient can be used (pm / ppm) reverse gas phase concentration C:

[0045] .

[0046] According to one specific embodiment of the present invention, a Corning SMF-28e+ single-mode optical fiber is used. The central cladding and hollow capillary coating are cut and removed with wire strippers to expose the core. A 5 mm long fiber optic bridge (FBG) with a center wavelength of 1548 nm to 1568 nm and 0.8 nm spacing is then inscribed, with a total of 80 channels. A thin layer (0.5 μm) of polymer coating is then uniformly applied to the FBG surface. The polymer coating is an amino-polymer-porous silica composite. In some embodiments of the present invention, the amino-polymer is an amino-functionalized polycarbonate elastomer. In other embodiments, other amino-modified polymers such as polyethersulfone (PES), polyimide (PI), and polyethylene glycol (PEG) can also be used. A temperature compensation module is provided, that is, a temperature sensor is provided on the polymer coating. In this example, Vishay NTCLE100E3 (10 kΩ@25°C, B=3950 K) is used, with a sampling accuracy of ±0.1°C. A hydrophobic elastic stabilization layer is then coated. In this example, a polytetrafluoroethylene (PTFE) film can be used. Finally, a rigid sleeve is placed on the outermost layer, and the leads of the temperature sensor are led out from the inlet and outlet holes opened on the sleeve. The rigid sleeve is sealed with the hollow capillary sleeves on both sides to obtain the carbon dioxide detection sensor.

[0047] Because FBGs are extremely sensitive to ambient temperature, even as CO2-induced strain is induced by polymer expansion, small temperature fluctuations can produce significant shifts in the central wavelength. Furthermore, the polymer coating also experiences adsorption or compression effects in response to humidity and atmospheric pressure. This additional expansion / compression spurious strain can be superimposed on the CO2 signal, creating an illusion of concentration. In this solution, a temperature sensor is installed to compensate for temperature, effectively eliminating temperature drift and achieving zero-temperature crossover compensation, ensuring that CO2 measurement is unaffected by temperature. Dual strain isolation is also employed: an outer rigid sleeve securely encases the optical fiber, allowing only chemical expansion. A hydrophobic elastic damping layer is placed between the sleeve and the optical fiber, isolating it from external stresses while also preventing moisture adsorption. Furthermore, the CO2 adsorption capacity of the polymer coating is limited by the Langmuir isotherm, resulting in a weak response in the low-concentration range, making detection within 10 ppm difficult. This invention optimizes the polymer coating by combining it with porous silica or graphene aerogel, which has a relatively high specific surface area. Using high-specific-surface-area nanocarriers to support the amino polymer significantly increases the adsorption capacity and linear response range. The coating thickness is controlled to less than 1 μm. To prevent polymer performance from drifting over time due to aging or contamination, anti-aging additives (UV stabilizers) can be added to the coating to extend its service life. 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 and correct the polymer response coefficient to avoid frequent manual calibration that affects continuous monitoring.

[0048] The present invention combines an embedded system with an FBG-based carbon dioxide detection sensor to achieve intelligent and automated detection. According to a specific embodiment of the present invention, in addition to the sensor, several major functional modules are also used:

[0049] Light source driver module

[0050] Broadband SLD light source: Exalos SEL-3400 (bandwidth 20 nm) for system self-testing and comparison;

[0051] Driver chip: Maxim MAX191 constant current source, modulation rate can reach 200 kHz.

[0052] Signal demultiplexing and spectrum analysis module

[0053] Spectral demultiplexing: With the Finisar WaveShaper 1000A, programmable filtering with an accuracy of 0.01 nm allows precise selection of the reflected light from each FBG.

[0054] Photodetection: Hamamatsu G12180-003K InGaAs APD with noise equivalent power <10 pW and bandwidth 50 MHz.

[0055] Front-end amplifier: Analog Devices AD8015 transimpedance amplifier, 50 MHz bandwidth, adjustable gain.

[0056] Analog-to-digital conversion: Analog Devices AD7176-2, 24-bit ΔΣ ADC, up to 4 kSPS sampling rate, 110 dB dynamic range.

[0057] Embedded signal processing module

[0058] Main control MCU: ST Micro STM32L476RG (80 MHz, ARM Cortex-M4DSP), low power standby current <5 μA.

[0059] Storage and interfaces: Winbond W25Q128JV (128 Mb SPI FLASH) for storing calibration curves and historical data; I²C interface for expanding temperature and humidity sensing; UART interface for debugging.

[0060] Through the modular design and precise chip selection, this embodiment outperforms existing technologies in key performance indicators such as multi-channel distribution, 5ppm resolution, <5s response, and ±0.1°C temperature compensation, achieving large-area grid-based CO2 monitoring with high precision and high stability.

[0061] Through structural design, the system of the present invention directly implements the temperature compensation function by the NTC thermistor integrated in the sleeve, which can effectively solve the problem that existing sensing solutions are not conducive to integration with embedded systems and the implementation of distributed detection. It has the following advantages: 1. The integrated NTC thermistor is located close to the FBG, and its temperature can better reflect the current temperature of the FBG; 2. The temperature-resistance calculation formula of the NTC thermistor is extremely mature, and it can achieve high temperature accuracy even when used directly without temperature correction in the laboratory; 3. The solution using a single optical fiber and built-in NTC has a more compact structure and the calculation formula of the thermistor is not complicated, which can be easily deployed in the embedded system, improving the timely response time of the system; 4. The FBG with integrated NTC thermistor can provide more dense carbon dioxide detection points, improving the distribution density of the detection system.

[0062] 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 carbon dioxide detection sensor based on coated fiber Bragg grating, characterized in that: The invention comprises a single-mode optical fiber, which comprises a core, a cladding and a hollow capillary tube sheathed outside the cladding. The middle section of the single-mode optical fiber is provided with an exposed core section, the core of the exposed section is provided with a Bragg grating (FBG), and a polymer coating that expands in response to carbon dioxide is provided. A sleeve is covered outside the exposed section, and the sleeve is sealed to the hollow capillary tubes at both ends. The sleeve is provided with air inlet and outlet holes, and a temperature sensor is provided between the sleeve and the polymer coating.

2. The carbon dioxide detection sensor based on coated fiber Bragg grating according to claim 1, characterized in that: The central wavelength of the Bragg grating is 1548 nm to 1568 nm, the interval is 0.8 nm, and there are 80 channels in total.

3. The carbon dioxide detection sensor based on coated fiber Bragg grating according to claim 1, characterized in that: The polymer coating is a polymer containing amino groups, and the coating thickness is less than 1 μm.

4. The carbon dioxide detection sensor based on coated fiber Bragg grating according to claim 3, characterized in that: The polymer coating is a composite formed by mixing an amine-functionalized elastomer with a nano-carrier having a high specific surface area, wherein the nano-carrier having a high specific surface area is porous silica or graphene aerogel; or the coating further contains an ultraviolet 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 coated fiber Bragg grating according to claim 1, characterized in that: The temperature sensor adopts an NTC thermistor, and its lead wires are routed through the air inlet and outlet holes.

7. A method for detecting carbon dioxide based on coated fiber Bragg grating, characterized in that: Based on the sensor according to any one of claims 1 to 6, the CO2 to be measured is injected into the cavity through the inlet and outlet holes. 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, resulting in changes in the central wavelength of its reflected light, thereby making the emitted optical signal different. The temperature sensor in the sensor collects the corresponding signal, and the measured FBG wavelength offset is temperature compensated to obtain the wavelength offset affected only by stress, thereby obtaining the CO2 concentration by reverse calculation.

8. The method for detecting carbon dioxide based on coated fiber Bragg grating 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 the reference temperature and its β parameter. Therefore, the FBG wavelength drift caused only by CO2 after temperature compensation is: ， in, is the total center wavelength drift measured by the working grating, K T is the sensitivity coefficient of the working FBG to temperature, T0 is the reference temperature; CO2 sensitivity coefficient The CO2 concentration C can be obtained by reverse calculation: 。

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