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Abstract
A high-sensitivity photonic crystal fiber long-period grating (PCF-LPG) methane sensor with cryptophane-A-6Me absorbed on a poly(acrylic acid)-carbon nanotubes/ polypropylene amine hydrochloride (PAA-CNTs/PAH) nanofilm was investigated. The sensing film was coated onto the internal surface of a photonic crystal fiber cladding air holes by an electrostatic self-assembly technique. Based on a finite element method and the coupled local-mode theory, the effects of the sensing film’s refractive index (RI) and thickness on the resonant wavelength were theoretically and numerically analyzed. When the sensing film RI decreases from 1.55 to 1.53, and the thickness increases from 100 nm to 200 nm, the resonant wavelength has a blue shift. A higher RI sensitivity with 1.075 × 103 nm RIU–1 is observed for the film thickness of 200 nm. The PCF-LPG methane sensor was fabricated by a pressurized injection method. The sensing experimental result shows that the resonant wavelength of the transmission spectra has a blue shift when the methane concentration increases from 0.0% to 3.5% by volume. The sensor has a good sensitivity of 1.078 nm%–1 and a low detection limit of 0.18% for a film thickness of 210 nm.
© 2017 Optical Society of America
1. Introduction
Methane is the main component of mine gas, with explosive limits about ~4.3% to 16.2% [1], which can easily lead to a gas explosion accident. Thus, monitoring the concentration of methane gas is of great significance. Because of a lot of advantages such as anti-electromagnetic interference, passive components, long-distance monitoring, and so on, optical fiber sensors are especially suitable for detecting the concentration of methane in mines. At present, several optical fiber sensors have been used in the detection of methane gas, such as the evanescent wave sensor [2], the luminescence reflection sensor [3], the mode-filtered light sensor [4], the long-period fiber grating sensor [5, 6], and the photonic crystal fiber (PCF) sensor [7, 8].
Photonic crystal fiber long-period grating (PCF-LPG) is a fiber-optic device that has long-period gratings inscribed in the photonic crystal fiber which can serve as a wavelength modulation device [9]. Because of the mode coupling occurring between the core mode and the cladding mode in PCF-LPG, there is a high sensitivity to the refractive index (RI) change of the medium in the air holes around the core [10, 11], which is especially suitable for RI sensing [12]. By means of numerical simulation for mode-coupling efficiency, and the resonant wavelength shift characteristic of the PCF-LPG RI sensor in the RI range of 1.33 to 1.40, the sensitivity is at 4.3 × 10−8 RIU [13]. Fabricated by point-by-point residual stress relaxation utilizing focused CO2–laser irradiation, the sensitivity for the PCF-LPG sensor reaches 2.27 × 10−6 RIU in the 1.42–1.45 range of RI [14]. By filling air holes with different concentrations of NaCl solution, the sensitivity of the PCF-LPG RI sensor in the RI range of 1.33–1.35 is ~10−7 RIU [15]. In addition, the PCF-LPG sensor is also used for detecting the RI of gas medium [16, 17], ammonia concentration [18], and relative humidity (RH) [19]. By the electrostatic self-assembly deposition process, the sensitive nanofilms were coated onto the surface of air channels in the grating region; the sensitivity of the PCF-LPG humidity sensor reached 0.00022%/10−3 dB m from a relative humidity of 38% to 39%, and the proposed sensor showed excellent thermal stability as well [19].
In this paper, we present a new PCF-LPG methane sensor made by electrostatic self-assembly depositing poly(acrylic acid) (PAA) with carbon nanotubes (CNTs), polypropylene amine hydrochloride (PAH) nanofilms, and absorbing cryptophane-A-6Me onto the internal surface of PCF-LPG cladding air holes. Based on a finite element method and the coupled local-mode theory, the sensing principle is analyzed, and the effect of the sensing film’s RI and thickness on resonant wavelength shift is discussed. In addition, a PCF-LPG methane sensor was fabricated by a pressurized injection method and an electrostatic self-assembly technique, and its sensing performance is also evaluated.
2. Theoretical analysis
2.1 Methane-sensing principle based on coupled local-mode theory
Coupled local-mode theory (CLMT) [20], which comes from improvement of the coupled mode theory (CMT), is suitable for analyzing fiber gratings with larger RI modulation. Because photonic crystal fiber (PCF) is composed of nonphotosensitive material of pure silica and air medium, long-period gratings can be only inscribed in PCF by means of the periodical perturbation of waveguide geometry. The periodical perturbation results in strong refractive index modulation and significant modification of local-mode fields. Thus, the CLMT can be used to analyze the sensing characteristics of the PCF-LPG methane sensor.
The inscription of long period fiber grating in PCF will destroy the orthogonality between the j-order mode and the l-order mode propagating in PCF and leading to energy exchange among these different modes, which means the mode-coupling occurrence. The coupling equations can be expressed as [11, 20]
In Eq. (1), βj(z), bj and Cjl denotes the propagation constant of j-order mode, the intensity of j-order mode and the coupling coefficient between j-order mode and l-order mode, respectively. bj and Cjl can be expressed as In Eq. (2), a ± j(z) denotes the amplitude of j-order mode, the positive and negative sign represents forward or backward propagating direction of modes. , represents normalized transverse electric field and magnetic field of j-order mode distribution, can be defined asIn long period fiber gratings, mode-coupling occurs between core mode and co-propagating cladding modes. Substitute the Eq. (2) into Eq. (1), the coupling equations between the core mode and cladding mode can be obtained,
In Eq. (6), aco and acl denotes the intensity amplitude of core mode and cladding mode. βco(z) and βcl(z) denotes the function of core mode and cladding mode propagation constants to the axis z. The coupling coefficient C(z) can be expanded into the form of Fourier series,fN is the coefficient of each harmonic after expansion, N is the order of each harmonic, and Λ denotes the grating period.The phase term can be obtained by substituting Eq. (7) into (6),
Similar to the conventional coupled mode theory, resonant coupling occurs strongly when the phase-matching condition is satisfied (i.e. the phase detuning is zero). Since the propagation constants are changing periodically, the phase-matching condition can also be calculated in a grating period. After the transformation, the phase-matching condition of the resonant wavelength can be expressed as: , where neff, co(z) and neff,cl (z) denotes the effective refractive index (RI) of the local core and cladding modes, respectively. The effective RI of cladding mode neff, cl (z) is related to the RI and thickness of the sensing film of our sensor. While the PCF-LPG sensor coated with PAA-CNTs/PAH and cryptophane-A-6Me sensing film, which RI is sensitive to methane concentration, the tiny variation of film RI will result in the change of the neff,cl (z), and leading to resonant wavelength shift (∆λres) of the transmission spectra.According to the resonant wavelength λres before and after the PCF-LPG methane sensor interaction with methane gas, the methane concentration can be obtained by means of wavelength shift value (∆λres = λres-λres0).
2.2 Effects of refractive index and thickness of sensing film
The structural diagram of the PCF-LPG methane-sensing film sensor is shown in Fig. 1(a). The sensing film consists of PAA with CNTs, PAH, and cryptophane-A-6Me. An idealized collapsed model of a PCF-LPG notch is shown in Fig. 1(b), and Fig. 1(c) shows a typical scanning electron microscope (SEM; TESCAN, VEGA 2 LUM) image of the PCF. The PCF is an endlessly single-mode photonic crystal fiber (ESM-PCF) (YOFC, China), with the core diameter of 7.1 μm, the cladding diameter of 125 μm, the pitch Λp of 5.5 μm, the normalized holes diameter d/Λp of 0.56 (the holes diameter d is 3.1 μm), and the core RI of 1.457. In addition, the period of PCF-LPG is 480 μm with 35 periods.
Fig. 1 (a) Structural diagram of the PCF-LPG methane sensor. (b) An idealized collapsed model for the longitudinal structural variation around the PCF-LPG notch shown later in Fig. 5(a). (c) SEM image of the PCF used. (d) Cross-sections at different locations from A to E.
According to the change characteristic of air holes, they have different deformation degree due to the PCF geometry change. In each collapsed region, air holes located closer to the outer cladding show greater collapse. The maximum collapsed depth Δs = 20 μm, and the length of the collapsed region l = 80 μm (see Fig. 1(b)). In order to obtain the modes variation in the collapsed region, the first section is divided into five planes from A to E (the latter section is a symmetrical distribution from E' to A') and the distance of each plane is set to 8 μm. The cross-sections of plane A, which has no deformation, and collapsed plane B to E are shown in Fig. 1(d). Figure 2 presents a simulation model of the PCF-LPG sensor (a) and the distribution of cladding mode LP11 in plane A and plane E (b).
Fig. 2 (a) Simulation model of the PCF-LPG sensor with the sensing film on the internal surface of cladding air holes; the blue line and the red line represent PAA-CNTs/PAH and cryptophane-A-6Me film, respectively. (b) Distribution of LP11 mode in the uncollapsed plane A and collapsed plane E.
The effective refractive index of LP01 and LP11 in the collapsed region are calculated with a full-vector finite element method [21]. Values of the two modes electronic and magnetic field are solved and substituted into Eq. (3) to calculate the coupling coefficient between the adjacent planes. Figure 3 presents the effective RI difference Δneff between the LP01 and LP11 modes (a) and the distribution of the coupling coefficient in the collapsed region (b). It shows that the coupling coefficient decreases in the former part and then increases with the change of PCF cladding collapsed depth.
Fig. 3 (a) The effective RI difference (Δneff) between the LP01 and LP11 modes for the incident wavelength λ of 1550 nm. (b) Distribution of coupling coefficient in the collapsed region.
Substitute the coupling coefficient into the coupled local-mode Eq. (6), with the boundary conditions of PCF-LPG aco(0) = 1, acl(0) = 1, and the amplitude of the modes propagating in the waveguide can be calculated. By calculating the transmission T(λ) at different incident wavelength with the transmittance formula T(λ) = |aco(z)|2/|aco(0)|2, the corresponding transmission spectrum of PCF-LPG can be obtained. As shown in Fig. 4, the resonant wavelength of the transmission spectra has a blue shift when the sensing film RI decreases from 1.55 to 1.53 and the thickness of the sensing film increases from 100 nm to 250 nm. The resonant wavelength shift can be up to –21.5 nm with 200 nm sensing film thickness in the RI range, and the RI sensitivity is 1.075 × 103 nm RIU–1. The thickness of 200 nm is used as an optimum value, and the sensitivity decreases beyond this value.
3. Sensor fabrication and experimental setup
To evaluate the sensing performance of the PCF-LPG methane sensors, the CO2–laser irradiation method was used to scan the PCF cladding in equal spacing, which can inscribe the long-period grating in PCF with the periodical collapse of air holes in cladding (Shanghai Kuhn Electronic Co. Ltd.). Figure 5(a) presents the side view of a cladding notch formed by CO2–laser irradiation. The sensors coated with PAA-CNTs/PAH and cryptophane-A-6Me films were fabricated by electrostatic self-assembly technique using a pressurized injection method [19]. Cryptophane-A-6Me was synthesized by direct method using acetovanillone as the starting material (replacing vanillyl alcohol in the traditional method) [3, 5]. The following 1H NMR and 13C NMR data for cryptophane-A-6Me were acquired: 1H NMR (CDCl3, 500 MHz) δ 6.59 (1H, s, ArH), 6.54 (1H, s, ArH), 4.80–4.76 (1H, m, CH), 4.24 (2H, t, J = 10.0 Hz, OCH2), 3.74 (3H, s, OCH3), 1.41 (3H, d, J = 20.0 Hz, CH3). 13C NMR: (125 MHz, CDCl3) δ 153.39, 144.01, 136.42, 126.97, 112.52, 109.51, 61.92, 53.21, 45.30, 20.89. The complexation process of the cryptophane-A-6Me with methane is shown in Fig. 5(b).
Fig. 5 (a) Side view of a cladding notch formed by CO2–laser irradiation, with a depth of 23 μm. (b) Interaction between the cryptophane-A-6Me and the methane molecule.
The internal surface of the PCF cladding air holes was initially positively charged by a surface treatment process in a mixture of concentrated hydrochloric acid (HCl) and methanol (MeOH). Negatively charged a PAA-CNTs film, and positively charged a PAH film was alternately attached to the inner surface of the air holes to form electrostatic self-assembled inner layer films [22]. Meanwhile, the cryptophane-A-6Me molecules were absorbed by the delocalized π bonds of carbon nanotubes in electrostatic self-assembled film to form the methane sensors. The experimental solutions were mainly composed of 0.02 mL/L PAA solution (4 mL, Aldrich Co. Ltd.), including single-walled carbon nanotubes-COOH (8 mg, TNSSRC SWCNTs, Chengdu Organic Chemicals Co. Ltd.), 2 mg/mL PAH solution (4 mL, Aldrich Co. Ltd.), and cryptophane-A-6Me (0.2 g) with dichloromethane (4 mL). Figure 6 depicts the coating process of PAA-CNTs/PAH nanofilms and the cryptophane-A-6Me layer (a), and the SEM images of the PCF-LPG coated sensing film (b).
Fig. 6 (a) Coating process of PAA-CNTs/PAH nanofilms and the cryptophane-A-6Me layer onto the inner surface of the PCF cladding air holes, and red, yellow, purple, and green represent PAA, PAH, CNTs, and cryptophane-A-6Me film, respectively. (b) SEM cross-section of the PCF-LPG coated sensing film, with the thickness of 105 nm, 155 nm, and 210 nm, respectively.
To test the fabricated PCF-LPG methane sensors, the experimental test system consisted of super-luminescent diode (SLD; Dense Light CO. Ltd.), an optical spectrum analyzer (OSA; Agilent 86140B), personal computer, single-mode fiber (SMF-28), and a stainless steel gas chamber. The chamber seal cover was equipped with a vacuum pressure gauge. The inlet end of the chamber was connected with a mass flow controller, and the outlet end was connected with a vacuum pump. The prepared PCF-LPG methane sensor was connected with single-mode fiber by two optic fiber connectors to achieve butt coupling, as shown in Fig. 7.
4. Results and discussion
To further demonstrate the response characteristic of the sensing film to the methane, a thin film RI measurement system was used to measure the film RI in the methane concentration range of 0.0% to 3.5% in situ. The measurement system was composed of a refractometer (F20-UVX, Filmetrics, Inc.), a high-power UV-Vis optical fiber light source (L10290, Hamamatsu, Inc.), an acrylic gas chamber containing single crystal silicon wafer coated with PAA-CNTs /PAH nanofilms and cryptophane-A-6Me layer, a CS-1 sample stage with fiber-optic cable, a mass flow controller, methane and nitrogen. The result shows that the film RI (n) decreases linearly with the increase in methane concentration (c), where n = 1.5370–0.0014c.
Figure 8 shows the resonant wavelengths of the sensors in different methane concentrations. The wavelength shifts to shorter wavelengths when the methane concentration ranges from 0.0% to 3.5% by volume, which is in accordance with the simulation results.
Fig. 8 The resonant wavelengths of the sensor transmission spectra with (a) 105 nm and (b) 210 nm film thickness in the methane concentration range of 0.0% to 3.5% (including respective partial enlarged drawings).
Figure 9(a) shows the relationships between the wavelength shift and methane concentration, with the increasing of the sensing film thicknesses. When the film thickness increases from 105 nm to 210 nm, the resonant wavelength has a blue shift, which is in accordance with the simulation results. For the sensor with a film thickness of 210 nm, the sensitivity S (the ratio of sensor signal transmitted per percent of methane concentration) is 1.078 nm%–1, with a correlation coefficient (R2) of 0.998 and a detection limit of 0.18%, and the blue shift of the resonant wavelength is 3.80 nm (shifting from 1545.80 nm to 1542.00 nm), with increasing methane concentration from 0.0% to 3.5% (v/v). Figure 9(b) further presents the wavelength shifts versus time of the sensor when it was exposed repeatedly to 1.5% of methane. As is shown, when the sensor is exposed to 1.5% methane, the shift declines rapidly from zero to a stable value. The sensor exhibits a good reversibility and repeatability. The response time (t90) of the sensor for methane concentration of 1.5% was found to be 60 s, and the recovery time (t90) was nearly 180 s.
Fig. 9 (a) Calibration curve between the wavelength shift and methane concentration with the film thicknesses of 105 nm, 155 nm, and 210 nm, respectively. (b) Sensor signal with the film thickness of 210 nm when repeatedly exposed to pure nitrogen (N2) or 1.5% of methane (CH4).
The selectivity of the sensor was investigated by exposing it to N2, O2, CO, CO2, and H2S (hydrogen sulfide), respectively (Table 1). It was found that common potential interferents such as N2, O2, CO, CO2, and H2S did not interfere significantly with the response of the sensor, which implied perfect discrimination ability of methane among different analytes gas.
Table 1. Effect of potential interferents on the resonant wavelength of the PCF-LPG sensor
The temperature and humidity response characteristics of the sensors was studied by placing the sensor into a GDJS-50 temperature/humidity control chamber. When the temperature was in the range of 20 to 90 °C, the shift was only 2 pm/°C. Concurrently, the effect of the relative humidity (RH) on the sensor was also clarified. In the range of 20 to 90% RH, a wavelength red shift is about 0.19 nm. Because the temperature is less than 26 °C, and the humidity is in the range of 50 to 60% RH in the miner’s working environment, the influence of temperature fluctuation and humidity variation on the shift is not significant.
5. Conclusions
A high-sensitivity photonic crystal fiber long-period grating methane sensor with cryptophane-A-6Me absorbed on a PAA-CNTs/PAH nanofilm was studied, and the sensing films were coated onto the internal surface of the PCF cladding air holes. The methane-sensing principle was analyzed by utilizing the coupled local-mode theory. The influences of the sensing film’s RI and thickness on resonant wavelength shift were numerically investigated. Experimental results showed that, for the sensor with a film thickness of 210 nm, the sensitivity was 1.078 nm%–1 and the detection limit was 0.18%, which is a significant blue shift of the resonant wavelength of the transmission spectra. The proposed sensor has potential application prospects for coal-mine gas detection.
Acknowledgments
This work was carried out with the financial support of the National Natural Science Foundation of China (No. 61271059), the Fundamental Research Funds for the Central Universities (No.CDJZR14225502), and Chongqing Graduate Scientific Research Innovation Project (No. CXB14220).
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