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Abstract
In this study, a novel method that can detect carbon dioxide (CO2) concentration and realize temperature immunity based on only one fiber Bragg grating (FBG) is proposed. The outstanding contribution lies in solving the temperature crosstalk issue of FBG and ensuring the accuracy of detection results under the condition of anti-temperature interference. To achieve immunity to temperature interference without changing the initial structure of FBG, the optical fiber cladding of FBG and adjacent optical fiber cladding at both ends of FBG are modified by a polymer coating. Moreover, a universal immune temperature demodulation algorithm is derived. The experimental results demonstrate that the temperature response sensitivity of the improved FBG is controlled within the range of 0.00407 nm/°C. Compared with the initial FBG (the temperature sensitivity of the initial FBG is 0.04 nm/°C), it decreases by nearly 10 times. Besides, the gas response sensitivity of FBG reaches 1.6 pm/ppm and has overwhelmingly ideal linearity. The detection error results manifest that the gas concentration error in 20 groups of data does not exceed 3.16 ppm. The final reproducibility research shows that the difference in detection sensitivity between the two sensors is 0.08 pm/ppm, and the relative error of linearity is 1.07%. In a word, the proposed method can accurately detect the concentration of CO2 gas and is efficiently immune to temperature interference. The sensor we proposed has the advantages of a simple production process, low cost, and satisfactory reproducibility. It also has the prospect of mass production.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Rapid economic development has highlighted the problem of global climate change, and carbon dioxide (CO2) emissions have brought challenges to global climate change [1,2]. In the face of the urgent need to address climate change, the “Copenhagen Accord” and the “Paris Agreement” urge governments to set domestic targets to reduce greenhouse gas (GHG) emissions. In February 2021, the United States announced that it rejoined the Paris Agreement and will attempt to reach the goal of carbon neutrality by 2050. The Chinese government has set the national goal of reaching the peak of carbon dioxide emissions by 2030 and announced the goal of carbon neutrality by 2060 [3,4]. Against this background, the detection of CO2 concentration has been regarded as a crucial research direction in the field of intelligent sensing in an array of countries.
Today's shared CO2 sensors mainly include infrared absorption type [5], electrochemical type [6], and thermal conductivity type [7]. Although these sensors possess the advantages of fast detection speed and high sensitivity, they possess certain limitations for gas detection in hazardous environments, such as poor stability and weak anti-interference ability. With the gradual development of optical fiber gas sensors in the 1970s, these problems have been solved. As optical fiber itself is composed of a dielectric, it is suitable for use in flammable and explosive oil, gas and chemical production [8–10]. In addition, generally, the frequency of electromagnetic radiation is lower than that of light waves, accordingly, the optical signal transmitted in optical fiber is not affected by electromagnetic interference [11,12]. Meanwhile, optical fiber also has the advantages of small size, lightweight, and plastic geometry [13,14]. In a multitude of optical fiber gas sensors, the most effective way to distinguish them is the difference of the optical fiber structure. Optical fiber structures with better linearity are more popular in the field of gas detection.
Due to the intrinsic particularity of structure, the fiber Bragg grating (FBG) sensor as the wavelength-modulated fiber sensor has the advantage of positive inherent linearity [15–18]. This feature is widely used in the detection of CO2 gas concentration [19–23]. In 2013, a new type of gas sensor based on cladding etched FBG was proposed by Shivananju B N et al. [19]. Its core surface was coated with polyallylamine amino carbon nanotubes for detecting wide CO2 gas concentration at room temperature. When the CO2 gas molecule interacts with the FBG coated by the polyallylamine amino carbon nanotube, the effective refractive index of the fiber core changes, resulting in the change of the Bragg wavelength. Later, to further expand the scope of application, in 2016, a new optical fiber array type real-time gas detection sensor was developed by Hung S S et al. to measure oxygen, carbon dioxide, and ammonia simultaneously [20]. The optical fiber is etched and polished before coating to improve sensitivity. Fiber Bragg grating is used to detect light intensity. The trisodium 1-hydroxy-3,6,8-pyrene trisulfonate was coated for carbon dioxide sensing. In 2021, a novel polymer-coated fiber grating CO2 gas sensor was proposed by Zhou Z et al [21]. The volume expansion of polymer in CO2 is used to transfer the stress to the grating, resulting in the change of grating period and refractive index. In 2022, A polymer-coated fiber Bragg grating carbon dioxide detection method based on volume expansion mechanism and molecular dynamics simulation was proposed by Xu Y et al [22]. In addition, their research results also obtained a theoretical formula for the relationship between the response strain of the sensor and the gas concentration, which can theoretically be used to detect various gases. In the same year, based on the advantages of convenient sensor network construction and FBG reusability, Xu Y et al. proposed to combine polyimide-coated FBG with etched FBG sensors with different sensitivities to simultaneously detect CO2 concentration and humidity [23]. By changing the coating thickness and etching the grating area, two groups of sensors were made to verify that the sensor has an outstanding linear response to CO2 and humidity. Although this series of results demonstrate that FBG has congenital advantages in detecting CO2, FBG is exceedingly vulnerable to temperature [24–26], which is a paramount variable that interferes with the accuracy of detection results.
At this stage, there have been a host of solutions to the issue of temperature interference. However, most of these methods focus on increasing the number of FBGs to achieve temperature compensation or realizing simultaneous detection of temperature and other target physical parameters from the demodulation formula. Among them, the most commonly used is to use two FBGs in series [27–30], one to detect the physical quantity of the target and the other to eliminate temperature interference. However, it increases the cost of the sensor and the demodulation difficulty of the spectrometer. Meanwhile, other interferences will be introduced. For example, the spectral cross issue is easy to occur in calibration experiments, which makes demodulation more difficult. There are few reports regarding the influence of immune temperature directly on a single FBG. The existing reports basically focus on the reconstruction of the internal structure of the grating [31,32]. Although these methods have achieved temperature immunity, it diminishes the rate of success and repetition rate of the experiment. Moreover, the modified structure will also affect the detection of other physical quantities. Therefore, the key objective of this research is to observe a low-cost, easy-to-make, high reproduction performance, demodulation speed block, and accurate detection results anti-temperature interference method based on a single FBG.
In this paper, an anti-temperature interference CO2 concentration detection method based on a single modified FBG is proposed. By introducing Polydimethylsiloxane (PDMS) temperature sensitive material and Polyether sulfones (PES) gas sensitive material, the ends and cladding of FBG were coated and modified. In a word, this paper has three contributions to the development of the FBG gas sensing field at this stage:
The first and foremost, the anti-temperature interference performance can be achieved by only one FBG, which fundamentally reduces the production cost of the sensor and elevates the demodulation rate.
There is one more point, the cancellation method is applied to the demodulation formula derivation of immune temperature, which not only improves the detection accuracy but also makes the demodulation formula universal.
The last but not least, the modification method we proposed without changing the initial internal structure of FBG makes the CO2 concentration detection results extremely accurate, stable, and reproducible.
2. Sensing structure and principle
Figure 1 shows the specific method of gas sensor immune temperature interference in this study. Among them, PDMS [33,39,40] has been widely used as a high-sensitivity optical fiber coated temperature sensitive material due to its higher negative thermal optical coefficient (−4.66 × 10−4/°C) and thermal expansion coefficient (3 × 10−4/°C). But the difference this time is that PDMS is smeared on the fiber on both sides of the fiber grating, not the grating. In this way, a counteracting force will be generated at the junction of the fiber and the grating. The numerical values of the thermal optical coefficient and thermal expansion coefficient of the fiber optic grating cladding are smaller than those of PDMS (The thermal optical coefficient and thermal expansion coefficient of the fiber optic grating cladding are 8.3 × 10−6/°C and 5 × 10−7/°C). As the temperature changes, the grating will possess axial strain. When the temperature gradually increases, the grating will stretch axially, and when the temperature gradually decreases, the grating will shrink axially. PDMS also has the same effect. When the temperature increases, F1 and F4 in Fig. 1 are the axial stress generated by the grating, and F2 and F3 are the axial stress caused by PDMS. Thus, there must be a moment when the forces of these two parts are the same to realize offset.
Next, PES [34] as a shared gas sensing material for CO2 detection, was applied in this experiment. As shown in Fig. 2, PES is uniformly coated to the upper surface of FBG. When the CO2 concentration increases, the PES will possess axial expansion stress, and FBG will be driven by this stress to generate axial strain, thus causing the central wavelength of FBG to drift. This phenomenon has also been proved by Zhou Z et al. [21]. To better understand these principles, we have also supplemented the image of the fabricated sensor (Fig. 3).
Fig. 2. Structure diagram of CO2 concentration detection method based on FBG immune temperature effect.
When a broadband light beam passes through a fiber grating, the wavelength satisfying the Bragg condition of the FBG will be reflected. This wavelength is referred to as the Bragg wavelength and is given by [35]:
where λB is the Bragg wavelength, neff is the refractive index of the core, and Λ is the periodic spacing of the grating.The research indicates that the FBG will also occur axial deformation under axial stress, which increases the periodicity of the grating, and reduces the radius of the core and the shell of the fiber. The wavelength change of fiber grating under axial strain can be expressed as [36]:
where ɛ is the axial strain of the fiber, and Pe ≈0.22 is the photoelastic coefficient. t1 is the thermo-elastic coefficient, t2 indicates the thermo-optical coefficient. Since the optical fibers at both ends of the FBG are processed by PDMS, the temperature strain generated by the FBG is offset, so that the value of (t1+ t2) T is zero. In this way, the temperature response of the gas sensor is removed. Thus, the wavelength change of fiber grating under axial strain can be expressed as Eq. (3). Meanwhile, it can be seen that the FBG sensor has an excellent linear output with the axial strain changes.3. Experiments and results discussion
3.1 Pre-experiment
Theoretically, the proposed method can achieve the removal of temperature strain, but the specific application of PDMS temperature sensitive materials requires accurate research, such as determining the coating length of PDMS and coating method. For this reason, we carried out a preliminary experiment, and the schematic diagram of modifying the FBG fiber cladding is shown in Fig. 4. Two miniature syringes were used to drip PDMS. And add the same amount on both sides simultaneously. In order to reduce the operation error, the amount of each addition is controlled at one drop. After measurement, the amount of a drop is concerning 2 mm long. Due to the inherent fluidity of PDMS, it is straightforward to cause experimental errors if it is smaller than 2 mm. Thus, when choosing the coating length, we used a 2 mm interval.
By recording six sets of data, Fig. 5 is plotted as shown below. This figure shows the linear graph of the change of the central wavelength of FBG with the external temperature when PDMS is added with different doses. It is not formidable to observe in the figure that when the PDMS applied lengths are 2 mm, 4 mm and 6 mm, the central wavelength of the FBG gradually drifts to the right, but the drift degree gradually decreases. This shows that PDMS plays a role in restraining the temperature strain of FBG. When PDMS continues to increase to 8 mm, the central wavelength curve of FBG is almost a horizontal curve, which indicates that the temperature strain of FBG is well suppressed. However, as PDMS continues to increase to 10 mm and 12 mm, we observe that the central wavelength of FBG starts to move in the direction of decreasing. According to the analysis of Eq. (2) and Eq. (3), the reason for this phenomenon is that the PDMS gradually exceeds the axial tension of FBG due to the influence of temperature, forming a force to extrude FBG, which makes the axial strain of FBG shrink inward, leading to the gradual decrease of the central wavelength.
Fig. 5. Linear graph of the change of the central wavelength of FBG with external temperature when PDMS is added with different doses.
3.2 Temperature detection experiment
After making the gas sensor, according to Fig. 2, the temperature strain test is carried out first. The experimental conditions are to conduct temperature tests at room temperature using a constant temperature box without external interference. As shown in Fig. 6, the experimental system is built. The FBG sensor is placed in the incubator with a temperature range of 0 °C ∼ 10 °C. A spectrometer is used to collect the change in the central wavelength of FBG. Among them, the broadband light source (amplified diffuse emission, ASE) selected in this experiment was the kang-guan optoelectronics kg-ase series broadband light source. This series of broadband light sources possess strong output capacity, high output power, low spectral linewidth, and high screen resolution and clarity. They possess positive high-power stability and average wavelength stability. These characteristics can meet the performance requirements of this experiment. The optical spectrum analyzer (OSA) adopts the spectrum analyzer of Yokogawa company, which supports a wavelength range of 500 nm ∼ 1800nm, a wavelength accuracy of ± 0.01 nm, a wavelength resolution of 0.02 nm, and a wide power range of +30 dB ∼ -80 dB. The initial central wavelength of the FBG used in this experiment is 1550 nm.
The spectrogram of FBG is stored and presented in Fig. 7 after fabrication. It can be found that the shift distance of the FBG spectrum to the right is intensely small, and there is almost no trend. This also reflects that this research has played a pleasurable role in suppressing the interference of temperature effects. In order to further determine the immune temperature effect, the linear fitting curve of the change of the central wavelength of the modified FBG with temperature was made and inserted into Fig. 7. It can be found that the line is almost horizontal, and the slope is basically close to 0, which indicates that the temperature sensitivity of the modified FBG is extremely low. In addition, we still use the original FBG for comparison, and the results are shown in Fig. 8. From the figure, we found that the increasing trend of the green bar graph was significantly higher than that of the orange, which means that the modified FBG was insensitive to temperature. In a word, the experimental results of 8 mm indicate that the temperature response sensitivity of the improved FBG is controlled within the range of 0.00407 nm/°C. Besides, The FBG selected this time has a uniform grating length of 10 mm and an initial central wavelength of 1550 nm, the photoelastic coefficient is 0.22, the thermal optical coefficient and thermal expansion coefficient of the fiber optic grating cladding used in this experiment are 8.3 × 10−6/°C and 5 × 10−7/°C. In addition, the thickness of the fiber cladding for this time is 45 µm. The temperature sensitivity of bare FBG is 0.04 nm/°C. Compared with the initial FBG, it decreases by nearly ten times. This experiment content has confirmed the accuracy and practicability of the method proposed by us to eliminate temperature interference and laid an excellent foundation for the subsequent experiments to accurately detect the concentration of CO2.
Fig. 7. Spectral diagram of the change of FBG central wavelength with the change of external temperature and the corresponding quantitative relationship diagram.
3.3 CO2 Gas concentration detection experiment
The following content mainly carries out the carbon dioxide concentration detection experiment. The FBG gas sensor is fabricated as shown in Fig. 2, and the detection device is built as shown in Fig. 9. The manufacturer, source, and model of the spectrometer and light source in the figure are consistent with the previous temperature measurement experiment. The carbon dioxide concentration of the mixed gas is set by adjusting the gas valve. The concentration interval is set as 0 ppm ∼ 1200 ppm, and the concentration interval is set as 150 ppm. The initial central wavelength of the FBG in this experiment is still 1550 nm, consistent with the previous one. In addition, to maintain the accuracy of the experimental data, the temperature of this experiment remains constant, and the test under temperature interference will be carried out later.
Figure 10 is the spectrogram collected in this experiment. When the CO2 concentration gradually increases, the central wavelength of the FBG gradually shifts to the right. Simultaneously, it can be seen that the drift distance is relatively uniform. In order to further verify the linearity of the central wavelength change of FBG, the linear fitting of relevant data is applied to explore the mathematical relationship. The specific mathematical expression, the corresponding fitting result, and the small figure are inserted in Fig. 10. It can be seen that the linearity reaches 0.99141, which indicates that the detection method used in this experiment is ideal. The CO2 concentration sensitivity of the FBG gas sensor is 1.6 pm/ppm. After the linearity calculation, the detection error of the next experiment will be studied. By comparing the linear fitting results obtained this time with the actual data, Fig. 11 is made. A total of 20 groups of data are calculated and compared here. The broken line change in the figure represents the error change of each comparison. It can be concluded that in these 20 data centers, the maximum gas concentration detection error is 3.16 ppm, and the minimum gas concentration detection error is 0.1 ppm. This shows that the detection scheme proposed in this experiment can accurately detect the CO2 concentration.
Fig. 10. Spectral diagram and linear fitting diagram of the central wavelength of FBG changing with CO2 concentration.
Next, the concentration detection experiment under temperature interference will be carried out. We have made a schematic diagram of the quantitative relationship between gas concentration and central wavelength at different temperatures, as shown in Fig. 12. From the figure, it can be seen that the curves have high overlap and similarity at different temperatures, which verifies that the influence of temperature on concentration detection is minimal. To further verify this, we collected the concentration detection sensitivity at different temperatures and plotted it as Fig. 13. In the experiment, the concentration sensitivity of the FBG gas sensor was calculated at seven different temperatures. It is concluded from the figure that the numerical height curve corresponding to the 7th sensitivity is almost the same height (blue image in the figure). The broken line statistical chart of the 7th sensitivity is relatively straight (the red part in the figure). The cross-sensitivity is 0.000824 pm/ppm·°C. It shows that this research has achieved its goal, the influence of temperature has been largely eliminated, and the sensitivity of CO2 concentration detection has been maintained at a stable level.
Fig. 13. Test results of concentration detection sensitivity and stability of FBG gas sensor under temperature interference.
3.4 Reproducibility of the FBG gas sensor
A novel FBG gas sensor is made in the same way as this sensor. To distinguish, the reproduced FBG sensor is named FBG2. Previously, the whole experiment process was named FBG1. All the previous experimental processes were copied on FBG2, and the obtained data were processed and analyzed and linear fit. For clearer analysis reproducibility, the linear fitting of gas concentration detection of FBG1 and FBG2 is plotted in Fig. 14. As shown in the figure, the coincidence of two straight lines is overwhelmingly high. The fitting results manifest that the mathematical expressions of the central wavelength of FBG1 and FBG2 are also exceedingly close to each other. The error of concentration sensitivity of two FBGs is 0.08 pm/ppm. The relative error shall not exceed 5%. The linearity error is 0.00107, and the relative error is not more than 1.07%. The overall results demonstrate that the design method of the FBG sensor proposed in this paper is relatively simple, easy to realize repeated manufacturing, and the detection results are relatively stable and free from temperature interference.
4. Conclusions
In this paper, a method for detecting the CO2 concentration with immune temperature interference based on only one modified FBG is proposed. Gas sensing material PES and temperature sensing material PDMS are respectively attached to FBG and fiber cladding on both sides as optical fiber coatings. The use of PDMS is to introduce a force to restrain the axial strain of grating under temperature change. Meanwhile, the PES is used to introduce a force to enhance the axial strain of the grating under the change of CO2 concentration. Through the analysis of experimental data, it is concluded that the temperature strain of FBG is well restrained within the temperature range of 0 °C∼ 60 °C. The temperature response sensitivity of the improved FBG is controlled within the range of 0.00407 nm/°C. Compared with the initial FBG (the temperature sensitivity of the initial FBG is 0.04 nm/°C), it decreases by nearly 10 times. In the range of CO2 concentration 0 ppm ∼ 1200 ppm, the gas response sensitivity of FBG reaches 1.6 pm/ppm, and has extremely ideal linearity. The detection error results infer that the gas concentration error in 20 groups of data does not exceed 3.16 ppm. The temperature interference experiment shows that the concentration sensitivity of the sensors at seven different temperatures is almost at the same level. The final reproducibility research shows that the difference in detection sensitivity between the two sensors is 0.08 pm/ppm, and the relative error of linearity is 1.07%. These results indicate that the sensor has the advantages of anti-temperature interference, easy fabrication, high repeatability, and strong stability. The previously reported works are compared with our work to conduct a proper performance evaluation, as shown in Table 1. Besides, the sensor needs to continue to find faster methods in its manufacturing process, laying the groundwork for future mass production capacity. In addition, further efforts are needed to improve the applicability of sensors further. One is to continue searching for ways to reduce temperature crosstalk, and the other is to improve the sensor's sensitivity further while ensuring it is not affected by temperature interference.
Table 1. Comparison of our work and previously reported works.
Funding
National Natural Science Foundation of China (61833006, 61973058, 62033002, 62071112); 111 Project (B16009); Fundamental Research Funds for the Central Universities (N2201008, N2304024); Natural Science Foundation of Hebei Province (F2020501040); Natural Science Foundation of Liaoning Province (2020-KF-11-04).
Acknowledgments
This work was supported by the National Natural Science Foundation of China (61973058, 62033002, 61833006, 62071112), the 111 Project (B16009), the Fundamental Research Funds for the Central Universities in China (N2304024, N2201008), the Hebei Natural Science Foundation (F2020501040) and the Liaoning Province Natural Science Foundation (2020-KF-11-04).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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