Self-compensated microstructure fiber optic sensor ... - OSA Publishing

Self-compensated microstructure fiber optic sensor to detect high hydrogen concentration Shuijing Tang,1 Bo Zhang,1 Zhi Li,2 Jixiang Dai,2 Gaopeng Wang,2 and Minghong Yang2,3,* 1 Department of Physics, School of Science, Wuhan University of Technology, Wuhan, 430070, China National Engineering Laboratory for Fiber Optic Sensing Technology, Wuhan University of Technology, China 3 Key Laboratory of Fiber Optic Sensing Technology and Information Processing, Ministry of Education, Wuhan University of Technology, Wuhan, China * [email protected]

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Abstract: Dual-cavity microstructure fiber optic hydrogen sensor based on evaporated Pt/WO3 film was proposed and experimentally explored in this paper, which provides a novel solution to detect high hydrogen concentration (10-30% H2). Dual-cavity microstructure fabricated by splicer is composed of an inner air-cavity and a collapsed photonic crystal fiber cavity. The proposed sensor has the advantages of miniature structure, stable configuration, low cost. Based on three-beam interference model and verification experiments, the compensation function to the fluctuation of light source and fiber loss is proved from the theoretical simulation and experimental investigation. The sensor has a short response time (1min), good repeatability and reliability. Besides, the change of temperature affects the response value of the hydrogen sensor, but the impact can be neglected in 10-30% H2. ©2015 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (300.6300) Spectroscopy, Fourier transforms; (220.4000) Microstructure fabrication; (310.0310) Thin films.

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Received 5 Jun 2015; revised 28 Jul 2015; accepted 14 Aug 2015; published 21 Aug 2015 24 Aug 2015 | Vol. 23, No. 17 | DOI:10.1364/OE.23.022826 | OPTICS EXPRESS 22826

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1. Introduction As a new energy of abundant, high efficiency, non-pollution, hydrogen is presently one of the most effective solutions to solve the energy crisis. Hydrogen gas is widely used in many fields, such as new energy vehicles, aerospace, petro-chemical field. However, it is also rather dangerous due to its high diffusivity, low ignition energy and wide explosion concentration range (between 4% and 75% by volume). Thus, the detection of hydrogen concentration is becoming more and more important. Traditional electrochemical hydrogen sensor is not safe because of the possibility of generating electric sparks in service. Due to their outstanding properties of nature immunity, electromagnetic immunity, and remote-operation capability, fiber optic hydrogen sensor is the research hotpot in recent years. There are two major types of hydrogen sensitive materials, Palladium (Pd [1,2], Pd/Ag [3], Pd/Ni [4]) and tungsten oxide (WO3 [5, 6]). The embrittlement effect of Pd caused by the phase transition after hydrogen cycles,makes the fiber optic hydrogen sensor based on Pd difficult to detect high hydrogen concentration [7]. Doping some other metals in pure palladium can suppress this phase transition and therefore deduces the embrittlement effect to some extent, but the sensitivity and accuracy of these sensitive films will decrease [8]. Therefore, Palladium is difficult to fully meet the requirements of practical applications. WO3 is well-suited to aerobic environment (such as the air) because of antioxidant capacity, and Pt/WO3 has excellent gasochromic response. In our previous work [9, 10], Fiber brag grating (FBG) hydrogen gas sensor based on Pt-loaded WO3 coatings prepared by sol-gel method shows a high sensitivity and fast response especially for relatively low hydrogen concentration (down to 200 ppm) due to the nano-platelet structure of WO3. However, the energy released by exothermic reaction leads to the increase of ambient temperature up to hundreds °C under 4% hydrogen concentration [11], therefore, the FBG hydrogen sensors have the possibility of exploding in high hydrogen concentration atmosphere. The intensity reflecting micro-mirror sensor has many unique advantages of convenient fabrication, lower cost, good reliability and flexibility, but susceptible to the fluctuation of light source and fiber loss. To eliminate the noise, two types of compensation technologies have been proposed, including double light channels compensation [12–14], single light channels compensation [15, 16]. Double light channels compensation is difficult to ensure the symmetry of the double channels, and therefore to affect the measurement accuracy of sensor, also the large sensing probe and gas chamber adverse to miniaturization. However, single light channels compensation essentially solves this problem. Moreover, single light channels

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compensation [17] has a simpler structure, more easily to achieve miniaturization, which achieves self-referencing and the multiplexing capability. In this work, a self-compensated dual-cavity microstructure fiber optic hydrogen sensor based on evaporated Pt/WO3 film, which applies to detect high hydrogen concentration, was proposed and experimentally explored. The correlation between hydrogen concentration and response value were investigated and analyzed with Fourier transform demodulation. 2. Experiment The sensor probe has a dual-cavity microstructure, which is composed of an inner air-cavity and a collapsed photonic crystal fiber (PCF) cavity as shown in Figs. 1(a)-1(b). A FITEL S177 splicer, with a manual operation splice mode, was used to fabricate the air cavity. Dualcavity microstructure is formed by splicing a single-mode fiber and an endless single mode photonic crystal fiber (ESMPCF) to form a closed F-P air-cavity, the uncollapsed section of ESMPCF was cleaved, and the length of collapsed region can be adjusted by cutter [18]. SMF-HOF-SMF [17] hydrogen sensors have the similar structure with dual-cavity microstructure. However, the two fusion operations and the short length of hollow fiber (The fiber is cleaved by a specially designed in-house tool), makes the sensor difficult to manipulate. By contrast, the easier manufacture method and lower fabrication cost are beneficial to mass production of the dual cavity microstructure fiber optic hydrogen sensor. Pt/WO3 film, as hydrogen sensitive element is prepared. 350 nm WO3 thin films were deposited by thermal evaporation only on the fiber-tip of the collapsed PCF after ultrasonic cleaning for five minutes. Then 15 nm Pt was sputtered on surface of WO3 film in Ar atmosphere (5 × 10−3 mbar) with BESTECH sputtering system. WO3 and Pt films act as the hydrogen transducer and Catalysts, respectively. The hydrogen sensitive film is also investigated and verified by SEM.

Fig. 1. Schematic (a) and the physical map (b) of the sensor having a dual-cavity microstructure.

Figure 2 shows the configuration of hydrogen sensing experiment, which consists of a broadband light source (SLED, λ: 1310nm.), an optical spectrum analyzer (OSA, YOKOGAWA, AQ6370B), a chamber, a variable optical attenuator (VOA). Since both the optical source power fluctuation and the mechanical perturbation in the light path affect the light intensity accepted by the optical spectrum analyzer, the light intensity can be adjusted by variable optical attenuator (VOA) instead.

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Fig. 2. Configuration of hydrogen sensing experiment.

3. Operating principle As shown in Fig. 3, there are three reflection surfaces in the sensor probe (The reflection from the interface between WO3 and Pd is not taken into account, because the thick of WO3 and Pd are far less than the length of the inner air-cavity and the collapsed PCF cavity.), labeled ‘1’, ‘2’, ‘3′, respectively. The lengths of the air cavity and the collapsed PCF are d1 and d2, respectively. The refractive index of the single mode fiber, air cavity, Collapsed PCF, and evaporated Pt/WO3 film are n0, n1, n2 and ng, respectively. Transmission loss factors at air cavity and collapsed PCF are α1 and α2. According to Fresnel formula, the reflectivity R1 and R2 at surface ‘1’ and ‘2’ are both equal to (n0-n1)2/(n0 + n1)2 = 0.034