A highly sensitive dual-core D-Shape photonic crystal fiber based on surface plasmon resonance for methane sensing

A highly sensitive dual-core D-Shape photonic crystal fiber based on surface plasmon resonance for methane sensing | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article A highly sensitive dual-core D-Shape photonic crystal fiber based on surface plasmon resonance for methane sensing hongzhi xu, yongkang feng, xiaoyong gan, shubo jiang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4008439/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 06 Jun, 2024 Read the published version in Plasmonics → Version 1 posted 14 You are reading this latest preprint version Abstract This paper presents a gas sensor that uses surface plasmon resonance (SPR) technology and a novel D-type photonic crystal fiber (PCF) structure to detect methane. The sensor's double-sided, side-polished gas holes are the key components for achieving large-area contact with external methane gas. The coating material chosen to stimulate the SPR effect was a gold nanolayer. To increase the sensitivity of methane gas detection, the researchers used polysiloxane-doped cryptane E as a coating material. The study analyzed the sensor characteristics using finite element analysis (FEA) and numerical analysis to examine the effect of optical structure parameters on the sensor performance. The numerical results demonstrate that the sensor has a sensitivity of 11.52 nm/% and a FOM value of 0.409 when measuring methane gas in the concentration range of 0–3.5%. The curve fitted shows excellent linearity. The sensor is a promising technology for the future development of gas leakage detection due to its low cost, simplicity, and real-time detection capability. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1 Introduction Accurately detecting methane gas leaks and determining their concentration is critical to both the mining industry and environmental monitoring. However, due to the flammable and explosive nature of methane, traditional electrical methods are not always feasible [ 1 ]. Fiber optic sensors have been developed for methane detection due to their unique properties, including fast response time, small size, resistance to electromagnetic interference, flexible structural design and the ability to be monitored remotely in real time [ 2 , 3 ]. In addition, a variety of fiber optic sensors based on different optical principles, such as long period fiber grating sensors and interferometric fiber optic sensors, have been proposed for methane gas detection [ 4 , 5 ]. Among them, long period fiber grating sensors have attracted much attention due to their flexible long period fiber grating structure and excellent sensing performance. For example, Yang et al. designed and applied a novel long-period fiber grating (LPFG) thin-film methane sensor with a sensitivity of 0.30 nm/% and a detection limit as low as 0.2% [ 6 ]. There is also a class of sensors based on the principle of light interference, which detects environmental changes by measuring the phase change of light waves. Wang et al. investigated a modified D-type PCF methane sensor based on Sagnac interference, which had an average gas sensitivity as high as 36.64 nm/% over a detection range of 0-3.5% [ 7 ]. However, the sensitivity of these two types of fiber optic sensors can be affected by environmental factors such as temperature variations, light intensity variations leading to measurement instability and errors. Surface plasmon resonance (SPR) fiber optic sensors make use of the surface plasmon resonance phenomenon on metal surfaces to detect the resonance angle changes caused by the substance to be measured in the sensitive region in real time by monitoring the interaction between the incident light and the metal surface, thus realizing highly sensitive real-time monitoring of biomolecular interactions, an effect that has been widely used to monitor refractive indices, gas concentrations and other sensing parameters. A novel D-type telluric crystal photonic crystal fiber optic sensor based on the four-wave mixing (FWM) effect of surface plasmon resonance (SPR) for methane gas concentration measurement was investigated by Hai Liu et al. [ 8 ]. The maximum sensitivities for methane and hydrogen were 4.03 nm/%, respectively, and the linearity was 99.9% for both. The resolution was 1.25 × 10 − 2 per cent for methane and 7.14 × 10 − 3 per cent for hydrogen [ 9 ]. Yui et al. developed a PCF-SPR sensor for SiO 2 gas, which consists of a large air hole in the optical fiber as the sensing channel. The sensitivity of this sensor for SiO 2 was 1.99 nm/% [ 10 ].Wei et al. designed a D-type PCF-SPR sensor using gold nanorods to excite the SPR in methane detection, which was able to detect an average wavelength sensitivity of 4.26 nm/% at a higher wavelength range of 1.8 to 2.3 µm wavelength [ 11 ].Ranju Sardar et al. found out a hollow core photonic crystal fiber (HC-PCF) based gas sensor for monitoring carbon dioxide (CO 2 ).This optimal design provides the relative sensitivity of 93.5% with a confinement loss of 0.004 dB/m at λ = 2.65 µm the absorption wavelength of CO 2 [ 12 ]. Bikash Kumar Paul et al. presented a novel shape photonic crystal fiber based on gas sensor for the first time. Numerical simulations evidences that the sensor shows the sensitivity responses of 64.69% and confinement loss of 4.38 × 10 − 6 dB/cm at the transmission wavelength k = 1.55 mm [ 13 ]. Overall, the proposed methane sensor demonstrates higher gas sensitivity and lower cross-sensitivity effect, and provides a new design idea for the methane gas monitoring. A well-designed PCF structure can enhance the coupling between the core mode and the surface plasmon polariton (SPP) mode, thus improving the sensing performance of PCF-SPR sensors. It is important to note that the plasmonic material plays a crucial role in determining the properties of these sensors [ 14 ]. Furthermore, various PCF-SPR structures, including dual-core and triple-core PCFs, dual-slot PCFs, and V-shaped PCFs, have been studied. Xianli Li et al. have designed a dual-resonant, peaked SMF SPR gas sensor with a V-shaped slot for the highly sensitive detection of methane. The SPR sensor can simultaneously operate on two transmission bands with maximum wavelength sensitivities of 12 nm/% and 8 nm/%. The corresponding resolutions are 0.0083% and 0.013%, respectively [ 15 ]. The detectable refractive index range and sensitivity of the sensor are determined by the plasmonic material. Compound films such as AuTiO 2 , AuMgF 2 and AuTaO 5 have recently been used as plasmonic materials. Ai Hosoki et al. have developed a novel hydrogen sensor based on fiber optic surface plasmon resonance (SPR) technology. The cylindrical cladding surface is uniformly coated with AuTaO 5 and palladium (Pd). The SPR sensor showed a change in transmission loss of 0.23% and a response time of 15 seconds for 4% hydrogen when using 25 nm Au, 60 nm AuTaO 5 , and 3 nm Pd multilayer films [ 16 ]. However, the fiber optic gas sensors described above generally suffer from low wavelength sensitivity or relatively narrow detection limits. Therefore, it is difficult to detect gas concentration in complex environments. In this study, a new dual-core D-shaped photonic crystal fiber (PCF) for methane sensing was designed. It is a special PCF structure with rotational symmetry, which is coated with a gold film and polysiloxane-doped cryptane E on its flattened D-shaped surface. As a result, the SPR effect is enhanced and the sensing performance is improved. The optimal structure and materials resulted in an average wavelength sensitivity of 11.52 nm/%, almost double the previously reported value. 2 Design of sensor 2.1 Geometric construction The cross-section of the proposed double approved D-shaped PCF-SPR sensor is shown in Fig. 1 . The PCF-SPR sensor consists of two layers of equally spaced stomata, with the center being a dual-core structure split by three small stomata to form two symmetric cores. The air holes on the left and right sides are filled with a gold film with a thickness of t 1 and t 2 represents the thickness of the methane gas sensitive film. The methane gas-sensitive films were selected from polysiloxane-doped cryptane E. The radius of the silica cladding is denoted by r, and the polishing depth is denoted by h. The thickness is denoted by t 2 . The spacing between pores is denoted by p, and the thickness of the perfect match layer is 1 µm. The parameters initially chosen were t 1 = 30 nm, t 2 = 1µm, d 1 = 1.2 µm, d 2 = 1.5 µm, h = 7.5 µm. The PCF has a six-fold quasicrystalline structure consisting of square triangles and squares. The PCF preforms can be fabricated by laser punching or 3D printing, and flat d-shaped planes can be fabricated by side polishing methods. The distance between the polished surface and the center of the PQF is h. Gold (Au) is a plasma medium that can be easily plated on the polished surface by magnetron sputtering or chemical vapour deposition (CVD). Finally, polysiloxane-doped cryptane E methane-sensitive film is coated on the outer surface of the Au film using a capillary dip coating technique. The thicknesses of the Au film and the methane-sensitive film are t 1 and t 2 , and the outermost layer is a perfect matching layer (PML). 2.2 Parameters The PCF-SPR can be fabricated by the stacked stretching method and the double stretching method. A gold film is deposited on the inner wall of the anti-resonator tube by high-pressure chemical deposition. Polysiloxane-doped cryptane E can be fabricated by the capillary dip coating technique. In addition, coating can be achieved by sol-gel schemes. The characteristics of the PCF-SPR sensor were analyzed using the finite element method. The dispersion of silicon dioxide can be calculated using the Sellmeier equation [ 17 ]: $${n^2}(\lambda )=1+\frac{{{A_1}{\lambda ^2}}}{{{\lambda ^2} - {B_1}^{2}}}+\frac{{{A_2}{\lambda ^2}}}{{{\lambda ^2} - {B_2}^{2}}}+\frac{{{A_3}{\lambda ^2}}}{{{\lambda ^2} - {B_3}^{2}}}$$ 1 Where A 1 = 0.6961663, A 2 = 0.4079426, A 3 = 0.8974794, B 1 = 0.0684043 µm 2 , B 2 = 0.1162414 µm 2 , B 3 = 9.896161 µm 2 and λ is the wavelength of the incident light. The Drude dispersion model was used to describe the dielectric constant of Au [ 18 ]: $${\varepsilon _{Au}}(\varepsilon )={\varepsilon _\infty } - \frac{{{\omega _p}^{2}}}{{\omega (\omega +i{\omega _\tau })}}$$ 2 where ε ∞ = 9.75, ω p = 1.36×10 16 rad/s, and ω τ = 1.45×10 14 rad/s. The refractive index of polysiloxane-doped cryptane E at room temperature versus methane concentration C_CH 4 can be determined by the following equation [ 19 ]: $$n=1.448 - 0.0046C - {C_ - }C{H_4}$$ 3 The response time of the sensor is determined by the methane-sensitive material, which has an adsorption time of 90 s and an analysis time of 300 s. The performance of the sensor is measured by the confinement loss, as shown in Eq. [ 20 ]: $${\alpha _{loss}}=8.686 \times \frac{{2\pi }}{\lambda }\operatorname{Im} ({n_{eff}}) \times {10^4}dB/cm$$ 4 where Im(n eff ) is the imaginary part of the effective refractive index of the core mode. 2.3 Simulated experimental setup Based on the description of the above principles and equations, we designed the experimental setup shown in Fig. 2 . The complete optical system consists of a light source device, a spectral detection device and a transmission optical path. The broadband light source (BBS) is selected for the light source device. The optical signal is processed by the Optical Spectrum Analyzer (OSA), which quantifies and displays a graph of the optical light source in a given wavelength range. The fiber optic coupler, also known as the splitter, splits the optical signal from one optical fiber into a number of optical fiber components. As a link, the transmission light path passes through a single mode fiber (SMF), a solid core anti-resonance fiber (PCF-SPR) and another SMF, and the final optical signal is output. The Variable Optical Attenuator (VOA) is an important passive optical component in fiber optic communications, allowing real-time control of the signal by attenuating the transmitted optical power. The SMF and PCF with bare fiber adapters are mounted on two six-axis optical stages with a pitch of 40 µm. Experiments have shown that the process can be carried out on the PCF-SPR. The sensors are released into a gas chamber and the concentration of a mixture of nitrogen and methane entering the chamber is Mass Flow Control (MFC). 3 Modeling and analysis 3.1 Principle of the sensor The dispersion relation and loss spectrum of the PCF-SPR sensor with parameters r = 8 µm, d 1 = 1.2 µm, d 2 = 1.5 µm, d 3 = 1.8 µm, h = 7.5 µm, t 1 = 30 nm, and t 2 = 1 µm for a methane concentration of 3% are shown in Fig. 3 . The red solid line and the black solid line indicate the real part of the effective refractive index for the x-polarized core mode and the SPP mode, respectively. At the intersection point, the phase matching condition of SPR is satisfied and the energy of the core mode is coupled to the SPP mode. The physical process is reflected in the electric field distribution in Fig. 3 x-polarized mode loss reaches its maximum value [ 21 ]. Figure 3 shows the electric field distribution of the core mode at 2.34 µm; at the detuning point, the core conduction mode and the SPP mode clearly show a weak coupling. As a result, the loss spectrum shows a single peak [ 22 ]. The effective refractive index of the SPP mode varies with the external refractive index, and the displacement of the crossing point produces a corresponding shift in the loss spectrum. 3.2 Sensor performance It is crucial for us to judge the performance of a sensor. Figure 4 (a) shows the confinement loss spectra of the sensor at different methane concentrations. According to Eq. ( 3 ), the refractive index of the methane-sensitive film decreases with increasing methane concentration; therefore, the effective refractive index of the SPP mode decreases, while the change in the nuclear mode is small. As a result, the phase matching point is shifted to shorter wavelengths, limiting the spectral blue shift of the loss. At the same time, as the methane concentration increases, the effective refractive index difference between the core and SPP modes decreases, increasing the evanescent field and leading to stronger coupling. The performance of the sensor can be evaluated in terms of wavelength sensitivity as follows [ 21 ]: $${S_\lambda }(nm/\% )=\frac{{\Delta \lambda }}{{\Delta C - C{H_4}}}$$ 5 where C_CH 4 denotes the change in methane concentration and λ is the corresponding wavelength shift. The relationship between resonance wavelength and methane concentration was monotonically decreasing as shown in Fig. 4 (b), and the average sensitivity of the linear fit for resonance wavelength was 11.52 nm/%. 3.3 Analysis and optimization 3.3.1 Gold film thickness Fiber cladding diameter, metal film layer thickness, metal type and sensing region length all have an impact on the sensor performance. When the diameter of the fiber core is fixed as D core = 10 µm, the diameter of the cladding D cladding = 20 nm, and the length of the sensing area L = 0.1 mm, the sensing model is simulated and analyzed for the gold film thickness t 1 = 30 nm, 35 nm, 40 nm, 45 nm, and 50 nm in turn. It can be seen from the change of the loss spectrum in Fig. 5 (a)that the position of the resonance peak of the loss spectrum blue shifts with the increase of t 1 from 30 to 50 nm, and the loss intensity decreases continuously, and the resonance effect of the sensor is weakened. From Fig. 5 (b), it can be seen that the sensitivity of the sensor is optimal among the five parameters at a gold film thickness of t 1 of 40 nm. 3.3.2 Cladding aperture d 1 The effect of small stomata d 1 in the cladding is also critical to the performance of the sensor. Fixed methane gas concentrations of 2.0% and 2.5% were used to classify stomatal diameters d 1 as 1.0, 1.2 and 1.4 µm. As shown in Fig. 6 (a), we find that the loss curves shift towards shorter wavelengths as the gas concentration increases, and then redshift to the right as the stomatal diameter increases. This is due to the fact that the size of the small gas pores affects the loss of nuclear energy in the electric field. Figure 6 (b) shows the curve of the resonance wavelength at different concentrations from 0-3.5%. The sensitivity can be calculated separately from Eq. ( 5 ) and the maximum sensitivity is 10.21 nm/% at a stomatal d 1 of 1.0 µm. From this it can be deduced that the optimum size for three different stomatal diameters is d 1 = 1.0 µm. 3.3.3 Cladding aperture d 2 Figure 7 (a) shows the loss profiles of stomatal d 2 for different claddings at methane concentrations of 2.0% and 2.5%. The loss curves are a red-shifted phenomenon with increasing stomatal d 2 at the same concentration and the resonance wavelengths of the d 2 stomata at 2.0% concentration are 2203 nm, 2336 nm, and 2435 nm, respectively, and the loss also increases with increasing stomata. The curves are shifted to the left at both concentrations. Figure 7 (b) shows the fitted curves of sensitivity at different stomatal diameters d 2 . The lines of the three fitted curves are y 1 = − 10.42x + 2531.64, y 2 = -8.52x + 2336.89 and y 3 = − 9.14x + 2152.8. The slope of the fitted curves for sensitivity at d 2 of 1.5 µm is -8.52 nm/%, which tells us that at d 2 of 1.6 µm is when the sensitivity is maximum, so that d 2 = 1.6 µm is the optimum size [ 23 ]. 3.3.4 Gas-sensitive film thickness Figure 8 (a) shows the loss spectra of different cladding hole radii at methane concentrations of 2.0% and 2.5%. When the methane concentration is constant and the cladding hole diameter increases, the loss spectra are blue-shifted and the peaks become larger and larger. The reason for the above phenomenon is that the cladding holes have a large binding effect on the energy of the core mode, preventing the coupling between the core mode and the metal film [ 24 ]. Figure 8 (b) shows the numerical fitting results of the resonance wavelength versus methane gas concentration. The fitted functional relationships are y 1 = − 8.82x + 2318.45, y 2 = -9.07x + 2314.18, and y 3 = -11.54x + 2286.68, respectively. as the radius of the cladding pores increases, the sensitivity of the designed sensor increases. Considering d 1 = 1.0 µm, the loss peak is not strong and the full width at half maximum (FWHM) is wider. A wider FWHM reduces the resolution of the sensing detection. Therefore, the optimal radius is only 1.0 µm [ 25 ]. The design is not suitable for gas sensing when t 2 = 0.8 µm. Too thin a gas-sensitive film would cause severe energy leakage and increase losses. In conclusion, 1 µm is the optimum gas-sensitive film thickness. 3.3.5 Analysis of results The sensing performance of the proposed SPR fiber-optic methane sensor was compared with some reported fiber-optic methane sensors, as shown in Table 1 . The first two sensors are based on SPR [ 26 ]. Although they can measure two gases, they are not very sensitive. Table 1 Comparison of fiber optic methane gas sensors Refs. Sensing Approach Wavelength(nm) Concentration Range (%) Wave Sens.(nm/%) FOM (% -1 ) [4] External (double- polished PCF) 1300-1400 0-3 5.31 N/A [6] Internal (PCF) 1550-1870 0-3.5 2.052 N/A [8] [11] [15] This work Internal (PCF) Internal (PCF) External (double- polished PCF) External (double- polished PCF) 1435-1680 1120-1450 1520-1700 2000-2800 0-3.5 0-3.5 0-3.5 0-3.5 6.39 1.99 8 11.52 N/A N/A 0.137 0.409 In the LPFG structure, gas sensing using gratings has a fast response time, but its sensitivity is not high. In the Sagnac interferometer, the interferometric fiber-optic methane sensor has the advantage of accurate measurement and high sensitivity. Its disadvantage is that it is strongly affected by temperature, humidity and air pressure and cannot meet the requirements of distributed sensing [ 27 ]. 4 Conclusion In this paper, a dual-core D-type PCF methane gas sensor based on SPR is designed. A gold film is deposited on the polishing groove of the optical fiber to stimulate the SPR effect. A methane sensitive film is deposited on the fiber polishing groove. The effects of different optical fiber parameters on the sensor performance were analyzed. The sensitivities of the sensors were up to 11.52 nm/% at methane gas concentrations ranging from 0–3.5%. The linear fits were all up to 99.8%. The best sensor performance was achieved when t 1 = 30 nm, t 2 = 1 µm, d 1 = 1.2 µm, d 2 = 1.5 µm and h = 7.5 µm. Compared with existing reports, our proposed D-type PCF methane gas sensor is more sensitive and easier to contact with methane gas. Therefore, our designed sensor can be used in methane gas leak detection, coal mining and oil extraction. The sensor is a good example of a D-type PCF gas sensor based on SPR. Declarations Ethical Approval. Indeed, there are no ethical aspects involved in this project at this time. No ethical approval of the program is required from the structural committee. Competing interests. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding. The authors would like to thank the support provided to this work by the National Key Research and Development Program of China, project 2019YFB1705803. Author Contribution Author Contribution Description:Yongkang Feng: Participated in most of the daily work of the experiment, including literature research, execution of the experiment, and data collection. Xu, under the guidance of his supervisor, performed an in-depth analysis of the experimental simulation data and wrote a draft of the results section of the experiment. In addition, he was responsible for the preparation of the literature review.Xiaoyong Gan: was responsible for the creation and maintenance of the experimental model. He supervised the preparation process of the photonic crystal fiber used in the experiment and optimized the experimental data acquisition system. Also participated in the collection and preliminary processing of the experimental data.Hongzhi Xu: Provided theoretical support for the project. He was responsible for the simulation modeling and execution of numerical simulations, helping the team to explain the experimentally observed phenomena. Provided critical feedback during the writing process and assisted in revising and refining the final paper.Shubo Jiang: is the main sponsor and project leader of this research project. She was responsible for the design of the entire project, the development of the experimental protocol, and the writing of the final report. She was also responsible for the preliminary analysis and interpretation of the experimental data. In addition, she undertook the task of funding application and management. Availability of data and materials. 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Cite Share Download PDF Status: Published Journal Publication published 06 Jun, 2024 Read the published version in Plasmonics → Version 1 posted Editorial decision: Revision requested 30 Apr, 2024 Reviews received at journal 29 Apr, 2024 Reviews received at journal 27 Apr, 2024 Reviewers agreed at journal 22 Apr, 2024 Reviewers agreed at journal 21 Apr, 2024 Reviewers agreed at journal 20 Apr, 2024 Reviewers agreed at journal 18 Apr, 2024 Reviews received at journal 04 Apr, 2024 Reviewers agreed at journal 20 Mar, 2024 Reviewers agreed at journal 20 Mar, 2024 Reviewers invited by journal 20 Mar, 2024 Editor assigned by journal 12 Mar, 2024 Submission checks completed at journal 12 Mar, 2024 First submitted to journal 03 Mar, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4008439","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":278933207,"identity":"44027c40-110d-4e85-a0e0-1b1fee516f21","order_by":0,"name":"hongzhi xu","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"hongzhi","middleName":"","lastName":"xu","suffix":""},{"id":278933208,"identity":"025a72da-057a-4dc5-be28-34dbd3b1639c","order_by":1,"name":"yongkang feng","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"yongkang","middleName":"","lastName":"feng","suffix":""},{"id":278933209,"identity":"eac65030-adc5-4e73-aca8-f3c418757f81","order_by":2,"name":"xiaoyong gan","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"xiaoyong","middleName":"","lastName":"gan","suffix":""},{"id":278933210,"identity":"297bfcd7-9e07-400c-9c6b-2e353a35715e","order_by":3,"name":"shubo jiang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABCElEQVRIiWNgGAWjYBACxmYwdYDBgIH5AIMEkGkAxBJEamFLYJBIIEILFIC08AAVE6OFuZ352cMvf+4kbmfv+fbA8oeNvDkD88HbPAx2ebgdxmZuLNv2LHFnz9ntBhIJaYY7G9iSrXkYkovx+MVMWrLhcOKGG7nbJCQSDicYHOAxk+ZhOJDYgFML+zdpiT8gLTnPgFr+A7XwfyOghcdM8gMbWAsbUMsBkC1shLSUSTO2HTbe2XPMTEIiLdlww2E2Y8s5Bsk4tRj2H98m+ePPYdnt7M3PpCVs7OQNjjc/vPGmwg63FqAEMw+UwwyOD2YQYYBDPRDIgxz3A+bKD7gVjoJRMApGwQgGACTaV/9tVcu6AAAAAElFTkSuQmCC","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":true,"prefix":"","firstName":"shubo","middleName":"","lastName":"jiang","suffix":""}],"badges":[],"createdAt":"2024-03-03 11:37:33","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4008439/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4008439/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11468-024-02364-8","type":"published","date":"2024-06-06T08:25:53+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":52763040,"identity":"e54acd1c-2832-4f99-8e6e-f5b26fb6cab4","added_by":"auto","created_at":"2024-03-15 12:49:04","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":75059,"visible":true,"origin":"","legend":"\u003cp\u003eModel diagram of the dual-core PCF-SPR sensor.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4008439/v1/8ef62180bc8f92243366bba0.png"},{"id":52763066,"identity":"5c1ac20f-c212-41b9-a22a-5fca2eed0673","added_by":"auto","created_at":"2024-03-15 12:49:07","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":193821,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of the sensor unit\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4008439/v1/298ed0e1609d16ebdfe2fe4c.png"},{"id":52763189,"identity":"ad72d5c7-1863-4b1d-aabc-bdf70f189427","added_by":"auto","created_at":"2024-03-15 12:49:13","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":80213,"visible":true,"origin":"","legend":"\u003cp\u003eDispersion relation and loss spectrum of SPP mode and core mode of double polished sensor\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4008439/v1/b4fa9bce43b3b8851fc061bb.png"},{"id":52763042,"identity":"b96a0700-a5e0-4bc2-89ef-4fbd076d6ee1","added_by":"auto","created_at":"2024-03-15 12:49:05","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":70162,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Confinement loss spectrum (b) Resonance wavelength and concentration fit curves at different methane concentrations.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-4008439/v1/9a40b335695ddcc36586c80f.png"},{"id":52762908,"identity":"b3a1202d-4b0b-49bc-b0ab-664eb870debf","added_by":"auto","created_at":"2024-03-15 12:48:56","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":62984,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Confinement loss spectrum (b) Sensitivity for different t\u003csub\u003e1\u003c/sub\u003e (d\u003csub\u003e1\u003c/sub\u003e = 1 μm, d\u003csub\u003e2\u003c/sub\u003e = 1.4 μm, d\u003csub\u003e3\u003c/sub\u003e = 1.8 μm, h = 6 μm, and t\u003csub\u003e2\u003c/sub\u003e = 500 nm).\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-4008439/v1/9bf4fc1b19a6273eb72819f1.png"},{"id":52763170,"identity":"caf1c9ef-8e24-4016-97d9-ea7deea4b747","added_by":"auto","created_at":"2024-03-15 12:49:11","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":56056,"visible":true,"origin":"","legend":"\u003cp\u003e(a) X-pol core modes loss spectrum d\u003csub\u003e1\u003c/sub\u003e from 1.0 to 1.4 μm (b) Linear fitting curves when d\u003csub\u003e1\u003c/sub\u003e from 1.0 to 1.4 μm.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-4008439/v1/debdf97f0ee1b5478838d0f8.png"},{"id":52763194,"identity":"7141673b-e599-45e9-b9c3-e1a6c5930de5","added_by":"auto","created_at":"2024-03-15 12:49:14","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":53531,"visible":true,"origin":"","legend":"\u003cp\u003e(a) X-pol core modes loss spectrum d\u003csub\u003e2\u003c/sub\u003e from 1.4 to 1.6 μm and (b) Linear fitting curves when d\u003csub\u003e2\u003c/sub\u003e from 1.4 to 1.6 μm.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-4008439/v1/82569ace01b4ff7eaff62044.png"},{"id":52763185,"identity":"992ad896-63d3-463c-9dfb-4dff0e54db69","added_by":"auto","created_at":"2024-03-15 12:49:11","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":50920,"visible":true,"origin":"","legend":"\u003cp\u003e(a) X-pol core modes loss spectrum t\u003csub\u003e2\u003c/sub\u003e from 0.8 to 1.2 μm and (b) Linear fitting curves when t\u003csub\u003e2\u003c/sub\u003e from 0.8 to 1.2 μm.\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-4008439/v1/d0338e3ee963ba29c9d94b17.png"},{"id":59088585,"identity":"5def9264-40cf-45a5-80c5-2bdeafa65be5","added_by":"auto","created_at":"2024-06-26 08:25:59","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1097744,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4008439/v1/61346e93-2e5c-4177-9a5b-b144cb680f56.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"A highly sensitive dual-core D-Shape photonic crystal fiber based on surface plasmon resonance for methane sensing","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eAccurately detecting methane gas leaks and determining their concentration is critical to both the mining industry and environmental monitoring. However, due to the flammable and explosive nature of methane, traditional electrical methods are not always feasible [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Fiber optic sensors have been developed for methane detection due to their unique properties, including fast response time, small size, resistance to electromagnetic interference, flexible structural design and the ability to be monitored remotely in real time [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. In addition, a variety of fiber optic sensors based on different optical principles, such as long period fiber grating sensors and interferometric fiber optic sensors, have been proposed for methane gas detection [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Among them, long period fiber grating sensors have attracted much attention due to their flexible long period fiber grating structure and excellent sensing performance. For example, Yang et al. designed and applied a novel long-period fiber grating (LPFG) thin-film methane sensor with a sensitivity of 0.30 nm/% and a detection limit as low as 0.2% [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. There is also a class of sensors based on the principle of light interference, which detects environmental changes by measuring the phase change of light waves. Wang et al. investigated a modified D-type PCF methane sensor based on Sagnac interference, which had an average gas sensitivity as high as 36.64 nm/% over a detection range of 0-3.5% [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. However, the sensitivity of these two types of fiber optic sensors can be affected by environmental factors such as temperature variations, light intensity variations leading to measurement instability and errors.\u003c/p\u003e \u003cp\u003eSurface plasmon resonance (SPR) fiber optic sensors make use of the surface plasmon resonance phenomenon on metal surfaces to detect the resonance angle changes caused by the substance to be measured in the sensitive region in real time by monitoring the interaction between the incident light and the metal surface, thus realizing highly sensitive real-time monitoring of biomolecular interactions, an effect that has been widely used to monitor refractive indices, gas concentrations and other sensing parameters. A novel D-type telluric crystal photonic crystal fiber optic sensor based on the four-wave mixing (FWM) effect of surface plasmon resonance (SPR) for methane gas concentration measurement was investigated by Hai Liu et al. [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The maximum sensitivities for methane and hydrogen were 4.03 nm/%, respectively, and the linearity was 99.9% for both. The resolution was 1.25 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e per cent for methane and 7.14 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e per cent for hydrogen [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Yui et al. developed a PCF-SPR sensor for SiO\u003csub\u003e2\u003c/sub\u003e gas, which consists of a large air hole in the optical fiber as the sensing channel. The sensitivity of this sensor for SiO\u003csub\u003e2\u003c/sub\u003e was 1.99 nm/% [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].Wei et al. designed a D-type PCF-SPR sensor using gold nanorods to excite the SPR in methane detection, which was able to detect an average wavelength sensitivity of 4.26 nm/% at a higher wavelength range of 1.8 to 2.3 \u0026micro;m wavelength [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].Ranju Sardar et al. found out a hollow core photonic crystal fiber (HC-PCF) based gas sensor for monitoring carbon dioxide (CO\u003csub\u003e2\u003c/sub\u003e).This optimal design provides the relative sensitivity of 93.5% with a confinement loss of 0.004 dB/m at λ\u0026thinsp;=\u0026thinsp;2.65 \u0026micro;m the absorption wavelength of CO\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Bikash Kumar Paul et al. presented a novel shape photonic crystal fiber based on gas sensor for the first time. Numerical simulations evidences that the sensor shows the sensitivity responses of 64.69% and confinement loss of 4.38 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e dB/cm at the transmission wavelength k\u0026thinsp;=\u0026thinsp;1.55 mm [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Overall, the proposed methane sensor demonstrates higher gas sensitivity and lower cross-sensitivity effect, and provides a new design idea for the methane gas monitoring.\u003c/p\u003e \u003cp\u003eA well-designed PCF structure can enhance the coupling between the core mode and the surface plasmon polariton (SPP) mode, thus improving the sensing performance of PCF-SPR sensors. It is important to note that the plasmonic material plays a crucial role in determining the properties of these sensors [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Furthermore, various PCF-SPR structures, including dual-core and triple-core PCFs, dual-slot PCFs, and V-shaped PCFs, have been studied. Xianli Li et al. have designed a dual-resonant, peaked SMF SPR gas sensor with a V-shaped slot for the highly sensitive detection of methane. The SPR sensor can simultaneously operate on two transmission bands with maximum wavelength sensitivities of 12 nm/% and 8 nm/%. The corresponding resolutions are 0.0083% and 0.013%, respectively [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. The detectable refractive index range and sensitivity of the sensor are determined by the plasmonic material. Compound films such as AuTiO\u003csub\u003e2\u003c/sub\u003e, AuMgF\u003csub\u003e2\u003c/sub\u003e and AuTaO\u003csub\u003e5\u003c/sub\u003e have recently been used as plasmonic materials. Ai Hosoki et al. have developed a novel hydrogen sensor based on fiber optic surface plasmon resonance (SPR) technology. The cylindrical cladding surface is uniformly coated with AuTaO\u003csub\u003e5\u003c/sub\u003e and palladium (Pd). The SPR sensor showed a change in transmission loss of 0.23% and a response time of 15 seconds for 4% hydrogen when using 25 nm Au, 60 nm AuTaO\u003csub\u003e5\u003c/sub\u003e, and 3 nm Pd multilayer films [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. However, the fiber optic gas sensors described above generally suffer from low wavelength sensitivity or relatively narrow detection limits. Therefore, it is difficult to detect gas concentration in complex environments.\u003c/p\u003e \u003cp\u003eIn this study, a new dual-core D-shaped photonic crystal fiber (PCF) for methane sensing was designed. It is a special PCF structure with rotational symmetry, which is coated with a gold film and polysiloxane-doped cryptane E on its flattened D-shaped surface. As a result, the SPR effect is enhanced and the sensing performance is improved. The optimal structure and materials resulted in an average wavelength sensitivity of 11.52 nm/%, almost double the previously reported value.\u003c/p\u003e"},{"header":"2 Design of sensor","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Geometric construction\u003c/h2\u003e \u003cp\u003eThe cross-section of the proposed double approved D-shaped PCF-SPR sensor is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The PCF-SPR sensor consists of two layers of equally spaced stomata, with the center being a dual-core structure split by three small stomata to form two symmetric cores. The air holes on the left and right sides are filled with a gold film with a thickness of t\u003csub\u003e1\u003c/sub\u003e and t\u003csub\u003e2\u003c/sub\u003e represents the thickness of the methane gas sensitive film. The methane gas-sensitive films were selected from polysiloxane-doped cryptane E. The radius of the silica cladding is denoted by r, and the polishing depth is denoted by h. The thickness is denoted by t\u003csub\u003e2\u003c/sub\u003e. The spacing between pores is denoted by p, and the thickness of the perfect match layer is 1 \u0026micro;m. The parameters initially chosen were t\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;30 nm, t\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1\u0026micro;m, d\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.2 \u0026micro;m, d\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.5 \u0026micro;m, h\u0026thinsp;=\u0026thinsp;7.5 \u0026micro;m. The PCF has a six-fold quasicrystalline structure consisting of square triangles and squares.\u003c/p\u003e \u003cp\u003eThe PCF preforms can be fabricated by laser punching or 3D printing, and flat d-shaped planes can be fabricated by side polishing methods. The distance between the polished surface and the center of the PQF is h. Gold (Au) is a plasma medium that can be easily plated on the polished surface by magnetron sputtering or chemical vapour deposition (CVD). Finally, polysiloxane-doped cryptane E methane-sensitive film is coated on the outer surface of the Au film using a capillary dip coating technique. The thicknesses of the Au film and the methane-sensitive film are t\u003csub\u003e1\u003c/sub\u003e and t\u003csub\u003e2\u003c/sub\u003e, and the outermost layer is a perfect matching layer (PML).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Parameters\u003c/h2\u003e \u003cp\u003eThe PCF-SPR can be fabricated by the stacked stretching method and the double stretching method. A gold film is deposited on the inner wall of the anti-resonator tube by high-pressure chemical deposition. Polysiloxane-doped cryptane E can be fabricated by the capillary dip coating technique. In addition, coating can be achieved by sol-gel schemes.\u003c/p\u003e \u003cp\u003eThe characteristics of the PCF-SPR sensor were analyzed using the finite element method. The dispersion of silicon dioxide can be calculated using the Sellmeier equation [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$${n^2}(\\lambda )=1+\\frac{{{A_1}{\\lambda ^2}}}{{{\\lambda ^2} - {B_1}^{2}}}+\\frac{{{A_2}{\\lambda ^2}}}{{{\\lambda ^2} - {B_2}^{2}}}+\\frac{{{A_3}{\\lambda ^2}}}{{{\\lambda ^2} - {B_3}^{2}}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere A\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.6961663, A\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.4079426, A\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.8974794, B\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.0684043 \u0026micro;m\u003csup\u003e2\u003c/sup\u003e, B\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.1162414 \u0026micro;m\u003csup\u003e2\u003c/sup\u003e, B\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;9.896161 \u0026micro;m\u003csup\u003e2\u003c/sup\u003e and λ is the wavelength of the incident light. The Drude dispersion model was used to describe the dielectric constant of Au [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$${\\varepsilon _{Au}}(\\varepsilon )={\\varepsilon _\\infty } - \\frac{{{\\omega _p}^{2}}}{{\\omega (\\omega +i{\\omega _\\tau })}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere ε\u003csub\u003e\u0026infin;\u003c/sub\u003e = 9.75, ω\u003csub\u003ep\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.36\u0026times;10\u003csup\u003e16\u003c/sup\u003e rad/s, and ω\u003csub\u003eτ\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.45\u0026times;10\u003csup\u003e14\u003c/sup\u003e rad/s.\u003c/p\u003e \u003cp\u003eThe refractive index of polysiloxane-doped cryptane E at room temperature versus methane concentration C_CH\u003csub\u003e4\u003c/sub\u003e can be determined by the following equation [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]:\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$n=1.448 - 0.0046C - {C_ - }C{H_4}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe response time of the sensor is determined by the methane-sensitive material, which has an adsorption time of 90 s and an analysis time of 300 s. The performance of the sensor is measured by the confinement loss, as shown in Eq.\u0026nbsp;[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]:\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$${\\alpha _{loss}}=8.686 \\times \\frac{{2\\pi }}{\\lambda }\\operatorname{Im} ({n_{eff}}) \\times {10^4}dB/cm$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere Im(n\u003csub\u003eeff\u003c/sub\u003e) is the imaginary part of the effective refractive index of the core mode.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Simulated experimental setup\u003c/h2\u003e \u003cp\u003eBased on the description of the above principles and equations, we designed the experimental setup shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The complete optical system consists of a light source device, a spectral detection device and a transmission optical path. The broadband light source (BBS) is selected for the light source device. The optical signal is processed by the Optical Spectrum Analyzer (OSA), which quantifies and displays a graph of the optical light source in a given wavelength range. The fiber optic coupler, also known as the splitter, splits the optical signal from one optical fiber into a number of optical fiber components. As a link, the transmission light path passes through a single mode fiber (SMF), a solid core anti-resonance fiber (PCF-SPR) and another SMF, and the final optical signal is output.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe Variable Optical Attenuator (VOA) is an important passive optical component in fiber optic communications, allowing real-time control of the signal by attenuating the transmitted optical power. The SMF and PCF with bare fiber adapters are mounted on two six-axis optical stages with a pitch of 40 \u0026micro;m. Experiments have shown that the process can be carried out on the PCF-SPR. The sensors are released into a gas chamber and the concentration of a mixture of nitrogen and methane entering the chamber is Mass Flow Control (MFC).\u003c/p\u003e \u003c/div\u003e"},{"header":"3 Modeling and analysis","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Principle of the sensor\u003c/h2\u003e \u003cp\u003eThe dispersion relation and loss spectrum of the PCF-SPR sensor with parameters r\u0026thinsp;=\u0026thinsp;8 \u0026micro;m, d\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.2 \u0026micro;m, d\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.5 \u0026micro;m, d\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.8 \u0026micro;m, h\u0026thinsp;=\u0026thinsp;7.5 \u0026micro;m, t\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;30 nm, and t\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1 \u0026micro;m for a methane concentration of 3% are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The red solid line and the black solid line indicate the real part of the effective refractive index for the x-polarized core mode and the SPP mode, respectively. At the intersection point, the phase matching condition of SPR is satisfied and the energy of the core mode is coupled to the SPP mode. The physical process is reflected in the electric field distribution in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ex-polarized mode loss reaches its maximum value [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the electric field distribution of the core mode at 2.34 \u0026micro;m; at the detuning point, the core conduction mode and the SPP mode clearly show a weak coupling. As a result, the loss spectrum shows a single peak [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The effective refractive index of the SPP mode varies with the external refractive index, and the displacement of the crossing point produces a corresponding shift in the loss spectrum.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Sensor performance\u003c/h2\u003e \u003cp\u003eIt is crucial for us to judge the performance of a sensor. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a) shows the confinement loss spectra of the sensor at different methane concentrations. According to Eq.\u0026nbsp;(\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), the refractive index of the methane-sensitive film decreases with increasing methane concentration; therefore, the effective refractive index of the SPP mode decreases, while the change in the nuclear mode is small.\u003c/p\u003e\u003cp\u003eAs a result, the phase matching point is shifted to shorter wavelengths, limiting the spectral blue shift of the loss. At the same time, as the methane concentration increases, the effective refractive index difference between the core and SPP modes decreases, increasing the evanescent field and leading to stronger coupling. The performance of the sensor can be evaluated in terms of wavelength sensitivity as follows [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]:\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$${S_\\lambda }(nm/\\% )=\\frac{{\\Delta \\lambda }}{{\\Delta C - C{H_4}}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere C_CH\u003csub\u003e4\u003c/sub\u003e denotes the change in methane concentration and λ is the corresponding wavelength shift. The relationship between resonance wavelength and methane concentration was monotonically decreasing as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b), and the average sensitivity of the linear fit for resonance wavelength was 11.52 nm/%.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Analysis and optimization\u003c/h2\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e3.3.1 Gold film thickness\u003c/h2\u003e \u003cp\u003eFiber cladding diameter, metal film layer thickness, metal type and sensing region length all have an impact on the sensor performance. When the diameter of the fiber core is fixed as D\u003csub\u003ecore\u003c/sub\u003e = 10 \u0026micro;m, the diameter of the cladding D\u003csub\u003ecladding\u003c/sub\u003e = 20 nm, and the length of the sensing area L\u0026thinsp;=\u0026thinsp;0.1 mm, the sensing model is simulated and analyzed for the gold film thickness t\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;30 nm, 35 nm, 40 nm, 45 nm, and 50 nm in turn.\u003c/p\u003e \u003cp\u003eIt can be seen from the change of the loss spectrum in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a)that the position of the resonance peak of the loss spectrum blue shifts with the increase of t\u003csub\u003e1\u003c/sub\u003e from 30 to 50 nm, and the loss intensity decreases continuously, and the resonance effect of the sensor is weakened. From Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b), it can be seen that the sensitivity of the sensor is optimal among the five parameters at a gold film thickness of t\u003csub\u003e1\u003c/sub\u003e of 40 nm.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e \u003ch2\u003e3.3.2 Cladding aperture d\u003csub\u003e1\u003c/sub\u003e\u003c/h2\u003e \u003cp\u003eThe effect of small stomata d\u003csub\u003e1\u003c/sub\u003e in the cladding is also critical to the performance of the sensor. Fixed methane gas concentrations of 2.0% and 2.5% were used to classify stomatal diameters d\u003csub\u003e1\u003c/sub\u003e as 1.0, 1.2 and 1.4 \u0026micro;m. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a), we find that the loss curves shift towards shorter wavelengths as the gas concentration increases, and then redshift to the right as the stomatal diameter increases.\u003c/p\u003e \u003cp\u003eThis is due to the fact that the size of the small gas pores affects the loss of nuclear energy in the electric field. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b) shows the curve of the resonance wavelength at different concentrations from 0-3.5%. The sensitivity can be calculated separately from Eq.\u0026nbsp;(\u003cspan refid=\"Equ5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) and the maximum sensitivity is 10.21 nm/% at a stomatal d\u003csub\u003e1\u003c/sub\u003e of 1.0 \u0026micro;m. From this it can be deduced that the optimum size for three different stomatal diameters is d\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.0 \u0026micro;m.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.3.3 Cladding aperture d\u003csub\u003e2\u003c/sub\u003e\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a) shows the loss profiles of stomatal d\u003csub\u003e2\u003c/sub\u003e for different claddings at methane concentrations of 2.0% and 2.5%. The loss curves are a red-shifted phenomenon with increasing stomatal d\u003csub\u003e2\u003c/sub\u003e at the same concentration and the resonance wavelengths of the d\u003csub\u003e2\u003c/sub\u003e stomata at 2.0% concentration are 2203 nm, 2336 nm, and 2435 nm, respectively, and the loss also increases with increasing stomata. The curves are shifted to the left at both concentrations.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(b) shows the fitted curves of sensitivity at different stomatal diameters d\u003csub\u003e2\u003c/sub\u003e. The lines of the three fitted curves are y\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;\u0026minus;\u0026thinsp;10.42x\u0026thinsp;+\u0026thinsp;2531.64, y\u003csub\u003e2\u003c/sub\u003e = -8.52x\u0026thinsp;+\u0026thinsp;2336.89 and y\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;\u0026minus;\u0026thinsp;9.14x\u0026thinsp;+\u0026thinsp;2152.8. The slope of the fitted curves for sensitivity at d\u003csub\u003e2\u003c/sub\u003e of 1.5 \u0026micro;m is -8.52 nm/%, which tells us that at d\u003csub\u003e2\u003c/sub\u003e of 1.6 \u0026micro;m is when the sensitivity is maximum, so that d\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.6 \u0026micro;m is the optimum size [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e3.3.4 Gas-sensitive film thickness\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a) shows the loss spectra of different cladding hole radii at methane concentrations of 2.0% and 2.5%. When the methane concentration is constant and the cladding hole diameter increases, the loss spectra are blue-shifted and the peaks become larger and larger. The reason for the above phenomenon is that the cladding holes have a large binding effect on the energy of the core mode, preventing the coupling between the core mode and the metal film [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(b) shows the numerical fitting results of the resonance wavelength versus methane gas concentration. The fitted functional relationships are y\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;\u0026minus;\u0026thinsp;8.82x\u0026thinsp;+\u0026thinsp;2318.45, y\u003csub\u003e2\u003c/sub\u003e = -9.07x\u0026thinsp;+\u0026thinsp;2314.18, and y\u003csub\u003e3\u003c/sub\u003e = -11.54x\u0026thinsp;+\u0026thinsp;2286.68, respectively. as the radius of the cladding pores increases, the sensitivity of the designed sensor increases.\u003c/p\u003e\u003cp\u003eConsidering d\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.0 \u0026micro;m, the loss peak is not strong and the full width at half maximum (FWHM) is wider. A wider FWHM reduces the resolution of the sensing detection. Therefore, the optimal radius is only 1.0 \u0026micro;m [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. The design is not suitable for gas sensing when t\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.8 \u0026micro;m. Too thin a gas-sensitive film would cause severe energy leakage and increase losses. In conclusion, 1 \u0026micro;m is the optimum gas-sensitive film thickness.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section3\"\u003e \u003ch2\u003e3.3.5 Analysis of results\u003c/h2\u003e \u003cp\u003eThe sensing performance of the proposed SPR fiber-optic methane sensor was compared with some reported fiber-optic methane sensors, as shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The first two sensors are based on SPR [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Although they can measure two gases, they are not very sensitive.\u003c/p\u003e \u003cp\u003e\u003cstrong\u003eTable 1\u003c/strong\u003e Comparison of fiber optic methane gas sensors\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.852112676056338%\" valign=\"top\"\u003e\n \u003cp\u003eRefs.\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.654929577464788%\" valign=\"top\"\u003e\n \u003cp\u003eSensing Approach\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"18.309859154929576%\" valign=\"top\"\u003e\n \u003cp\u003eWavelength(nm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.549295774647888%\" valign=\"top\"\u003e\n \u003cp\u003eConcentration Range (%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.725352112676056%\" valign=\"top\"\u003e\n \u003cp\u003eWave Sens.(nm/%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.908450704225352%\" valign=\"top\"\u003e\n \u003cp\u003eFOM (%\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.852112676056338%\" valign=\"top\"\u003e\n \u003cp\u003e[4]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.654929577464788%\" valign=\"top\"\u003e\n \u003cp\u003eExternal (double-\u003c/p\u003e\n \u003cp\u003epolished PCF)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"18.309859154929576%\" valign=\"top\"\u003e\n \u003cp\u003e1300-1400\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.549295774647888%\" valign=\"top\"\u003e\n \u003cp\u003e0-3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.725352112676056%\" valign=\"top\"\u003e\n \u003cp\u003e5.31\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.908450704225352%\" valign=\"top\"\u003e\n \u003cp\u003eN/A\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.852112676056338%\" valign=\"top\"\u003e\n \u003cp\u003e[6]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.654929577464788%\" valign=\"top\"\u003e\n \u003cp\u003eInternal (PCF)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"18.309859154929576%\" valign=\"top\"\u003e\n \u003cp\u003e1550-1870\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.549295774647888%\" valign=\"top\"\u003e\n \u003cp\u003e0-3.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.725352112676056%\" valign=\"top\"\u003e\n \u003cp\u003e2.052\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.908450704225352%\" valign=\"top\"\u003e\n \u003cp\u003eN/A\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.852112676056338%\" valign=\"top\"\u003e\n \u003cp\u003e[8]\u003c/p\u003e\n \u003cp\u003e[11]\u003c/p\u003e\n \u003cp\u003e[15]\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003cp\u003eThis work\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.654929577464788%\" valign=\"top\"\u003e\n \u003cp\u003eInternal (PCF)\u003c/p\u003e\n \u003cp\u003eInternal (PCF)\u003c/p\u003e\n \u003cp\u003eExternal (double-\u003c/p\u003e\n \u003cp\u003epolished PCF)\u003c/p\u003e\n \u003cp\u003eExternal (double-\u003c/p\u003e\n \u003cp\u003epolished PCF)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"18.309859154929576%\" valign=\"top\"\u003e\n \u003cp\u003e1435-1680\u003c/p\u003e\n \u003cp\u003e1120-1450\u003c/p\u003e\n \u003cp\u003e1520-1700\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e2000-2800\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.549295774647888%\" valign=\"top\"\u003e\n \u003cp\u003e0-3.5\u003c/p\u003e\n \u003cp\u003e0-3.5\u003c/p\u003e\n \u003cp\u003e0-3.5\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e0-3.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.725352112676056%\" valign=\"top\"\u003e\n \u003cp\u003e6.39\u003c/p\u003e\n \u003cp\u003e1.99\u003c/p\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e11.52\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.908450704225352%\" valign=\"top\"\u003e\n \u003cp\u003eN/A\u003c/p\u003e\n \u003cp\u003eN/A\u003c/p\u003e\n \u003cp\u003e0.137\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e0.409\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e \u003cp\u003eIn the LPFG structure, gas sensing using gratings has a fast response time, but its sensitivity is not high. In the Sagnac interferometer, the interferometric fiber-optic methane sensor has the advantage of accurate measurement and high sensitivity. Its disadvantage is that it is strongly affected by temperature, humidity and air pressure and cannot meet the requirements of distributed sensing [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eIn this paper, a dual-core D-type PCF methane gas sensor based on SPR is designed. A gold film is deposited on the polishing groove of the optical fiber to stimulate the SPR effect. A methane sensitive film is deposited on the fiber polishing groove. The effects of different optical fiber parameters on the sensor performance were analyzed. The sensitivities of the sensors were up to 11.52 nm/% at methane gas concentrations ranging from 0\u0026ndash;3.5%. The linear fits were all up to 99.8%. The best sensor performance was achieved when t\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;30 nm, t\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1 \u0026micro;m, d\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.2 \u0026micro;m, d\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.5 \u0026micro;m and h\u0026thinsp;=\u0026thinsp;7.5 \u0026micro;m. Compared with existing reports, our proposed D-type PCF methane gas sensor is more sensitive and easier to contact with methane gas. Therefore, our designed sensor can be used in methane gas leak detection, coal mining and oil extraction. The sensor is a good example of a D-type PCF gas sensor based on SPR.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eEthical Approval.\u003c/h2\u003e \u003cp\u003eIndeed, there are no ethical aspects involved in this project at this time. No ethical approval of the program is required from the structural committee.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eCompeting interests.\u003c/strong\u003e \u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding.\u003c/h2\u003e \u003cp\u003eThe authors would like to thank the support provided to this work by the National Key Research and Development Program of China, project 2019YFB1705803.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAuthor Contribution Description:Yongkang Feng: Participated in most of the daily work of the experiment, including literature research, execution of the experiment, and data collection. Xu, under the guidance of his supervisor, performed an in-depth analysis of the experimental simulation data and wrote a draft of the results section of the experiment. In addition, he was responsible for the preparation of the literature review.Xiaoyong Gan: was responsible for the creation and maintenance of the experimental model. He supervised the preparation process of the photonic crystal fiber used in the experiment and optimized the experimental data acquisition system. Also participated in the collection and preliminary processing of the experimental data.Hongzhi Xu: Provided theoretical support for the project. He was responsible for the simulation modeling and execution of numerical simulations, helping the team to explain the experimentally observed phenomena. Provided critical feedback during the writing process and assisted in revising and refining the final paper.Shubo Jiang: is the main sponsor and project leader of this research project. She was responsible for the design of the entire project, the development of the experimental protocol, and the writing of the final report. She was also responsible for the preliminary analysis and interpretation of the experimental data. In addition, she undertook the task of funding application and management.\u003c/p\u003e\u003ch2\u003eAvailability of data and materials.\u003c/h2\u003e \u003cp\u003eData sharing is not applicable to this article as no datasets were generated or analyzed during the current study.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLiu Q, Zhao J, Sun Y et al (2022) Highly sensitive dual-core photonic quasicrystal fiber methane sensor based on surface plasmon resonance[J]. JOSA A 39(9):1723\u0026ndash;1728\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi Y, Chen H, Li H et al (2023) Highly sensitive methane gas sensor based on Au/UVCFS films coated D-shaped photonic crystal fiber[J]. 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Biosensors 11:104\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"plasmonics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"plas","sideBox":"Learn more about [Plasmonics](https://www.springer.com/journal/11468)","snPcode":"11468","submissionUrl":"https://submission.nature.com/new-submission/11468/3","title":"Plasmonics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4008439/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4008439/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis paper presents a gas sensor that uses surface plasmon resonance (SPR) technology and a novel D-type photonic crystal fiber (PCF) structure to detect methane. The sensor's double-sided, side-polished gas holes are the key components for achieving large-area contact with external methane gas. The coating material chosen to stimulate the SPR effect was a gold nanolayer. To increase the sensitivity of methane gas detection, the researchers used polysiloxane-doped cryptane E as a coating material. The study analyzed the sensor characteristics using finite element analysis (FEA) and numerical analysis to examine the effect of optical structure parameters on the sensor performance. The numerical results demonstrate that the sensor has a sensitivity of 11.52 nm/% and a FOM value of 0.409 when measuring methane gas in the concentration range of 0\u0026ndash;3.5%. The curve fitted shows excellent linearity. The sensor is a promising technology for the future development of gas leakage detection due to its low cost, simplicity, and real-time detection capability.\u003c/p\u003e","manuscriptTitle":"A highly sensitive dual-core D-Shape photonic crystal fiber based on surface plasmon resonance for methane sensing","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-03-15 12:47:36","doi":"10.21203/rs.3.rs-4008439/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-04-30T10:41:12+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-04-30T01:44:46+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-04-27T10:35:55+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"708547bc-7759-4a36-9311-e952fcf776fc","date":"2024-04-22T04:17:03+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"5c697dce-bb94-4628-9980-bfcbedd916b9_SNPRID","date":"2024-04-22T03:17:44+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"42803efc-2294-482a-b286-346d23ce2fa0","date":"2024-04-20T11:49:48+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"498bb670-52d7-4f9e-902f-24a22c2790eb","date":"2024-04-18T12:16:07+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-04-04T08:34:38+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"368aacf8-e314-4ab5-a9cc-5be3f2058afe","date":"2024-03-20T15:10:56+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"0b421388-bea6-49fa-b3b7-e52d14e4022b_SNPRID","date":"2024-03-20T13:22:26+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-03-20T13:19:58+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-03-12T23:36:51+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-03-12T23:36:51+00:00","index":"","fulltext":""},{"type":"submitted","content":"Plasmonics","date":"2024-03-03T11:31:56+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"plasmonics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"plas","sideBox":"Learn more about [Plasmonics](https://www.springer.com/journal/11468)","snPcode":"11468","submissionUrl":"https://submission.nature.com/new-submission/11468/3","title":"Plasmonics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"00b4b225-3e8e-4b2a-9f6c-308b852dd9b9","owner":[],"postedDate":"March 15th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-06-26T08:25:53+00:00","versionOfRecord":{"articleIdentity":"rs-4008439","link":"https://doi.org/10.1007/s11468-024-02364-8","journal":{"identity":"plasmonics","isVorOnly":false,"title":"Plasmonics"},"publishedOn":"2024-06-06 08:25:53","publishedOnDateReadable":"June 6th, 2024"},"versionCreatedAt":"2024-03-15 12:47:36","video":"","vorDoi":"10.1007/s11468-024-02364-8","vorDoiUrl":"https://doi.org/10.1007/s11468-024-02364-8","workflowStages":[]},"version":"v1","identity":"rs-4008439","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4008439","identity":"rs-4008439","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}