A novel multiplexing scheme of anti-resonant reflecting optical waveguides (ARROW) for temperature sensing based on quartz capillary is firstly proposed and experimentally demonstrated. Each ARROW-based sensor is manufactured by fusion splicing a short section of capillary between two standard single-mode fiber (SMF). Experiments were demonstrated on a three cascaded ARROW-based sensing network chopped from a fiber array with multiple sensors. Simulation and experimental results demonstrate that the positions of sharp periodic transmission dips are independent of the capillary length, but dependent on the refractive indexes and thickness of the silica cladding. The temperature sensitivities of 18.77 pm/°C, 18.19 pm/°C, and 17.94 pm/°C are achieved by wavelength demodulation of selected dips of Gaussian fitting algorithm, and the tunability of sensitivity can be realized by selecting different dips. Furthermore, the merits of low strain cross-sensitivity (−0.71 pm/με), easy fabrication capability and high spectral extinction ratio (about 30dB) make the proposed multiplexing scheme an excellent candidate for performing multi-point and multi-parameter measurements in harsh environments.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Fiber-optic temperature sensors have been extensively investigated in fields of food processing, chemical industry, environmental condition with salient merits over traditional temperature sensors, such as low cost, high sensitivity, anti-electromagnetic and capability for multiplexing [1–3]. In the past decades, various optical fiber configurations, such as fiber Bragg gratings (FBGs) , long-period fiber gratings (LPFGs) , surface plasmon resonance (SPR) fiber optic sensors , Fabry-Perot interferometers (FPIs) , Mach–Zehnder interferometer (MZIs)  and Sagnac interferometers (SI)  have been developed for temperature sensing application. In general, grating based sensors are suitable for distributed remote sensing, but they have low temperature sensitivity and serious cross-sensitivity to strain or bending. The SPR-based sensors show great superiority in sensing resolution and sensitivity, but they faces the challenges to realize the multichannel distributed sensing in a single fiber link, because it is too hard to divide multi parallel zones in a such small fiber core to obtain multi independent sensing zones. The interferometer-based sensors have the advantages of high sensitivity and simple fabrication process. However, these sensors are difficult to be multiplexed and demodulated for their disadvantages of high insertion loss, low fringe visibility and the complex signal superposition, which limit the practical applications.
Antiresonant reflecting optical waveguides (ARROW) have attracted much attention for their properties of low dispersion, low nonlinear response and high damage threshold since its first demonstration . The ring cladding of the waveguide is considered as a Fabry–Pérot etalon, which results in periodic lossy dips in the transmission spectrum corresponding to the resonant condition. In the past few decades, various structures of anti-resonant-based fibers have been extensively investigated, including photonic crystal fibers (PCFs) , negative curvature fiber , hollow-core fiber (HCF)  and Kagome fibers . The anti-resonant mechanism is in widespread use for single-point or single parameter measurement, such as liquid level sensing , magnetic field sensing , gas pressure sensing , and humidity sensing . However, to the best of our knowledge, there are few reports about the study of multiplexing scheme for ARROW-based sensing network.
In this work, we present an innovative multiplexing scheme based on three serial anti-resonant reflecting optical waveguides (ARROW) sensors with different thickness of ring cladding. The unambiguous identification of the sharp transmission lossy dips through their unique resonance wavelengths and large free spectral ranges (FSRs) simplify the wavelength demodulation system, and they greatly improves the accuracy of detection. Furthermore, this scheme cannot only achieve the temperature sensing application, but also implement simultaneous measurement of multi-point and multi-parameters.
2. Theoretical model and principle
To further deepen the understanding of principle about the sensing network. Figure. 1 depicts the theoretical model of anti-resonant reflecting optical waveguides (ARROW) , which provides an analytical description of the optical guidance mechanism in quartz capillary. The schematic diagram of capillary cross-section is shown in Fig. 1(a), where the reflective indexes of core and silica cladding are represented (and ) as well as the inner diameter and the cladding thickness (and ). In this ARROW model, the high reflective index ring cladding is considered as a F-P etalon. Figure. 1(b) gives the sketch of the optical path of multiple beams at the interfaces within the silica cladding. The incident light from the SMF will be reflected at both the interface between the inner air/cladding and outer air/cladding. When the working wavelengths cannot satisfy the resonant condition of FP etalon, the guided light will be reflected back by the FP resonator and confined in the capillary’s air core as the guided core mode. On the contrary, when the working wavelengths satisfy the resonant condition of FP etalon, the light will transmit through the FP resonator and leaks out of the capillary cladding as leaky mode.
The mode field intensity distribution of the guided light was simulated with full-vector finite element method (FEM)  by using the commercial COMSOL Multiphysics software. Figure. 1(c) shows the mode field distribution for resonant wavelength ( = 1555nm, = 1.4482) which confirms that the resonant mode would transmit through the cladding of quartz capillary. Figure. 1(d) shows the mode field distribution for anti-resonant wavelength, which manifest that the anti-resonant mode can be confined in the hollow core of quartz capillary. The simulation results are in good agreement with the theoretical analysis. The position of the transmission dips in the spectrum corresponding to the resonant condition can be described by the following Eq :Eq. (1), we can derive the free spectral range ():Eq. (1) with respect to temperature (), we get the temperature sensitivity as following Eq.:
Numerical simulations and experiments based on above analysis are carried out. Here, we assume the RI of capillary core and cladding are 1.000 and 1.4482, respectively. The influence of length and cladding thickness about the capillary on the transmission spectrum has been researched. Firstly, the capillary has different length (L1 = 7mm, L2 = 14mm) and same thickness (d = 47.5μm). By taking these values into Eq. (1), as can be seen in Fig. 2(a), the resonant wavelengths can be calculated at 1530.9nm, 1554.9nm, 1579.6nm and 1605.1nm, which are very close to the experimental results in Fig. 2(c). Furthermore, we can see from Figs. 2(a) and 2(c) that sensors with different length and same thickness have the same transmission dips location but different transmission intensities (accumulation of energy leakage along the capillary length). Analogously, Figs. 2(b) and 2(d) describe the simulated and measured transmission spectra of the capillary with same length (L = 7mm) and different thickness (d1 = 37.5μm, d2 = 47.5μm), and it is obvious that the positions of sharp periodic transmission dips are dependent on the thickness of cladding. All these unique characteristic make the fabrication of ARROW-based sensing network more repeatable and convenient, and they will contribute to the implementation of multi-parameter or multi-point multiplexing measurements in quasi-distributed or distributed sensing network.
Additionally, we can know that the influence of temperature on the physical properties of capillary is manifested by changes in refractive index and changes in cladding thickness, whose values are finally determined by the thermo-optic coefficient () and the thermal expansion (). Assuming the capillary has the reflective index () and the cladding thickness (), the effect of temperature variation () on these two parameters can be described as following Eqs.:22]. Figure. 3(a) demonstrates the simulated transmission responses of sensor under different temperatures, and it is apparently that the transmission dip experiences a wavelength red-shift. As is shown in Fig. 3(b), the calculated temperature sensitivity of 18.6 pm/°C is achieved within the range from 25°C to 75°C. Moreover, the following conclusions can also be obtained through simulation: The length of the capillary has no effect on the temperature sensitivity. The resonance order m will affect the sensitivity. The higher the order is, the higher the sensitivity is achieved, which can be proved by Eq. (4). So we can achieve tunability of sensitivity by selecting different dip positions. It should be noted that thermal coefficient of the air refractive index in the hollow core will hardly contribute to the temperature dependence of the resonant wavelength .
3. Experimental setup results and discussions
To analyze the practical temperature sensing performance of the multiplexing system, we carry out the relevant experiments by the experimental equipment shown in Fig. 4. The proposed sensor is fabricated by splicing a segment of quartz capillary (Polymicro Technologies, TSP) between two standard SMFs with a Fujikura 80S Fusion splicer. The splicing parameters (fusion time and fusion power) are optimized to minimize the deformation of the hollow core at the splicing joints. A super-wideband light source (ASLD-CWDM-5-B-FA, Amonics) with a wavelength range from 1250 to 1650 nm is used to illuminate the 3 serial single-mode fiber (SMF)-capillary-SMF structures sensing link. The transmission spectrum of the sensing link is measured by an optical spectrum analyzer (AQ6370C, YOKOGAWA) with the resolution of 0.02 nm. A high-precision thermostatic controlling platform controlled by a programmable DC current controller is served as heating device, whose measurement accuracy and range are 0.1°C and 200°C, respecitively.
To demonstrate the operation of the multiplexing scheme, three ARROW-based sensors were fabricated by quartz capillary in single-mode optical fiber in series employing fusion technology, and the distance between each other is 1 m. The thickness of three ARROW-based sensors were selected at 25, 37.5, and 47.5, respectively, and they have the same outer diameter (125) and the refractive index (RI) of the cladding. In the experiment, the length of capillary is 1 cm with the core diameter of 50, and the insertion loss is about 6dB. The insertion loss can be further decreased by reducing the capillary length and optimizing the fusion parameters. Furthermore, the performance of the cascaded ARROW-based sensing network can be greatly improved by utilizing the tapered SMF technique . Figure. 5 shows the transmission spectra of three ARROW-based sensors and the cascaded results. Compared with other multiplexing structures, which require fast Fourier transform (FFT) algorithm , Hamming-windowed filtering , Time division multiplexing (TDM) technology  and Frequency-shifted interferometry , etc. Signal demodulation in our multiplexing scheme is very simple and straightforward. In our multiplexing scheme, the cascaded spectrum contains multiple characteristic dip wavelengths which represent the information of each individual sensor, and they have no disturbance to each other. Therefore, multi-point or multi-parameter multiplexing sensing measurement can be achieved by selecting multiple specific dips within a certain wavelength range.
The temperature response of the proposed ARROW-based sensing network has been investigated by placing each individual senor on the central heating zone of the thermostatic controlling platform and gradually increasing temperature from 25 °C to 75 °C with an increment of 10 °C. In order to monitor the wavelength variation more precisely, a local Gaussian fitting algorithm is applied to demodulate the selected resonant dips (As is shown by the purple dashed box in Fig. 5, dip1, dip2 and dip3 corresponds to sensor1, sensor3 and sensor2, respectively).
Figures. 6(a), 6(c) and 6(e) show the transmission spectra responses of the cascaded structure when heating sensor1 (d = 47.5μm), sensor2 (d = 37.5μm) and sensor3 (d = 25μm), respectively. It is obvious that the three selected typical dips shift toward the longer wavelength with the increase of temperature. The temperature sensitivity can be obtained by linear fitting of experimental data. Figures. 6(b), 6(d) and 6(f) illustrate the wavelength shifts corresponding to temperature of three dips (1578.25nm, 1581.32nm, and 1592.64nm), which exhibits the temperature sensitivity of 18.77 pm/°C, 18.19 pm/°C, 17.94pm/°C, respectively. Furthermore, the linear fitting curve of other two tracking dips are approximate a horizontal line, which means that the wavelengths are of no variation. Three sensors can achieve independent sensing measurements without interfering with each other.
In addition, the influence of strain on the sensing network has also been researched. In the experiment, the sensor head is placed in the middle of a two-dimensional translation stages, and the distance between the two stages on the smooth lead rail can be adjusted 20μm at one time by personal computer (PC). As can be seen Figs. 7(a) and 7(b), with the increasement of strain on sensor 3, the transmission dips shift to the shorter wavelength. And the low strain cross-sensitivity of −0.71pm/με has been achieved, which means that the strain change will slightly affect the correctness and feasibility of the proposed multiplexing scheme and it can be ignored in practical applications.
In this work, we firstly presented and experimentally demonstrated an original multiplexing sensing network based on ARROW-based sensor. Experimental results show that the temperature sensitivities of 18.77pm/°C, 18.19pm/°C, and 17.94pm/°C are achieved by wavelength demodulation of selected dips by Gaussian fitting algorithm, and the tunability of sensitivity can be realized by selecting different dip positions. It can be expected that the sensitivity and multiplexing capacity will be further increased if we use other types of anti-resonant-based fibers with higher thermal-optic coefficient material and low loss and optimizing the splicing process precisely. Furthermore, the proposed multiplexing scheme also exhibits the advantages of low strain cross-sensitivity, easy fabrication capability, and good repeatability.
National Natural Science Foundation of China (NSFC) (61675078).
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