IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 11, JUNE 1, 2014
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Active Fiber Gas Sensor for Methane Detecting Based on a Laser Heated Fiber Bragg Grating Bin Zhou, Zhuo Chen, Yebin Zhang, Shaorui Gao, and Sailing He, Fellow, IEEE
Abstract— An active optical fiber gas sensor was developed in a cobalt-doped single mode fiber. This fiber can be heated up to a few hundred degrees Celsius by a heating laser, and a fiber Bragg grating (FBG) was fabricated inside this fiber as a thermometer. The Bragg wavelength of the FBG is sensitive to the thermal conductivity of the gas surrounding the fiber and can be used to monitor the concentration of a special gas once we know in advance the kind of gas. The proposed sensor is ideal for detecting explosive or corrosive gases in some practical application scenarios, such as gas pipeline, coal mine, flow meters, and so on. We have measured the methane concentration ranging from 0% to 4.8% (5% is its explosion limit) as a demonstration. Index Terms— Thermooptic effects, optical fiber devices, gas detectors.
I. I NTRODUCTION
T
HE measuring of an inflammable gas such as methane always requires great care. People should comply with certain operating specifications, and the instruments applied should also be carefully selected. No additional explosion risks should be introduced during measurement. This requirement has always been a technical challenge as most of the gas sensors are electrical and may produce electric sparks. Take methane as an example, the electrical methane sensors includes: catalytic combustion sensors [1], [2], semiconductor sensors [3], [4] and so on. Although they have been widely studied and massively produced for a long period of time, they still have their own natural safety weaknesses. Optical sensors are well known as their safety in sensing inflammable gases. The laser absorption spectroscopy (LAS) based technologies [5], [6] dominate the market. In some
Manuscript received March 1, 2014; revised March 22, 2014; accepted March 29, 2014. Date of publication April 1, 2014; date of current version April 29, 2014. This work was supported in part by the National Natural Science Foundation of China under Grant 61307053, in part by the China Post-Doctoral Science Foundation under Grant 2013M531866, and in part by the Guangdong Innovative Research Team Program under Grant 201001D0104799318. B. Zhou and Z. Chen are with the South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510631, China (e-mail:
[email protected];
[email protected]). Y. Zhang and S. Gao are with the Centre for Optical and Electromagnetic Research, Department of Optical Engineering, Zhejiang University, Hangzhou 310027, China (e-mail:
[email protected];
[email protected]). S. He is with the South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510631, China, the Centre for Optical and Electromagnetic Research, Department of Optical Engineering, Zhejiang University, Hangzhou 310027, China, and also with the Division of Electromagnetic Theory, Alfven Laboratory, Royal Institute of Technology, Stockholm S-100 44, Sweden (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2014.2314692
sensing cases such as in a very small space, inside curved tunnels like coal mine or for remote sensing, light should be curved and guided to the sensing position and the sensors should be with tiny size, then the LAS technologies are no longer sufficient. In that cases we need optical fiber sensors to fix those problems. Actually, we have very few fiber gas sensors in the world (there’s almost no methane fiber sensor nowadays), as the interaction between the light inside a fiber and the gas outside the fiber hardly occur. In this letter we take methane as an example to demonstrate a novel fiber optical gas sensor based on thermal conductivity. The thermal conductivity sensor (TCS), also known as a katharometer, has a long history [7], [8]. Although the selectivity of the TCS is not as good as catalytic combustion sensors and semiconductor sensors, they suffer less from longterm drift and contaminated due to their physical transducer principle. Thermal conductivity sensors have their unique advantages [9]–[11] and could be developed for specific applications as long as the gas under test is known in advance. In some cases, such as monitoring the leakage of natural gas along the pipeline, measuring the flow of a certain concentration of a known gas in a flow meter or in monitoring the methane inside the coal mine (up to 90% of the explosive gases in a coal mine is methane), TCS is widely used. The only problem of this technology remained is the risk of the electrical sparks. In order to overcome the shortage mentioned above, we took a cobalt doped fiber to replace the heating cord in the traditional TCS, and took an FBG inside this fiber to replace the thermometer, and then we developed our thermal conductivity fiber sensor (TCFS). Cobalt doped fiber is one of the active fibers, and it can transform optical power into heat effectively due to nonradiative processes [12]–[16]. Inside the fiber core the temperature could be several hundred degrees Celsius. The heat generated in the fiber core ultimately transfers to the environment, thus the temperature in the fiber core is sensitive to the conductivity coefficient of the surrounding gas. Due to its self-heating characteristics, the cobalt doped fiber can be used as flow sensors, gas sensors, ultra-low temperature sensors and so on [13]–[16]. The advantages of the thermal conductivity fiber sensor are obvious: (i) safety. The risk of electronic spark has been overcame by the combination of TCS and fiber sensing technology; (ii) quasi-distributed sensing capability. FBG is the key component of the quasi-distributed sensing network. The proposed TCFS is able for sensing network; (iii) remote monitoring capability. This is the natural feature of fiber sensors.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 11, JUNE 1, 2014
Fig. 1. The setup of the proposed all-optical methane concentration sensor based on laser heated fiber Bragg grating. SLED, superluminescent LED; HLS, heating laser source; OSA, optical spectrum analyzer; SMF, single mode fiber; FBG, fiber Bragg grating, l is the position in the Co-doped fiber starting from the left splicing point. Fig. 2. Temperature distribution in the Co-doped fiber (simulation results). The power of HLS is set to 150 mW, 275 mW, 400 mW, 525 mW and 650 mW respectively.
II. E XPERIMENTAL S ETUP Fig. 1 is the experimental setup of the proposed TCFS based on a laser heated FBG. The key component is an 8 mm long cobalt doped single mode fiber (CorActive Inc., attenuation: 1.1dB/cm from 1250 nm to 1620 nm, cobalt is doped in the fiber core) which was spliced to the communication single mode fiber. The photosensitivity of this cobalt doped fiber was pre-enhanced by loading hydrogen at 110 °C under pressure of 10 MPa for three days. Then the FBG was fabricated inside this fiber using the phase mask technology by a pulsed 193 nm ArF excimer laser. The temperature sensitivity of the FBG is measured to be 11.8 pm/°C The heating laser source (HLS) supplies a high power pump laser which can be absorbed by element Co and transformed to heat [12] effectively and then realizes “all optical heating.” The core and cladding diameters of the Co-doped fiber are 7 μm and 125 μm, respectively. In order to enhance the sensitivity, the coating material was removed and the heat can transfer out from the fiber core to the surrounding medium in a short period of time. A superluminescent LED (SLED) was employed as a broadband source, the reflected spectra by FBG was monitored by optical spectrum analyzer (OSA). It should be mentioned that the operating wavelength of the HLS is 1560 nm, which is far away from the Bragg wavelength of the FBGs used in our experiments. In addition, the transformation efficiency of the absorbed light to heat by cobalt is about 38% in its attenuation spectral range [12], [15]. In order to realize an effective and stable opto-thermal conversion, we should make sure the HLS’ wavelength is between 1250 nm and 1620 nm. The methane and the standard air are mixed in the gas mixer. By controlling the flow ratio, we can get different methane concentration (volumetric) ranging from 0% to 4.8% (5% is its explosion limit). The heat Q(l) generated inside the core of the Co-doped fiber can be described as: Q(l) = kα P0 e−αl
(1)
where l is the fiber position starting from the splicing point shown in the inset of Fig. 1, α is the absorption coefficient of light, k is the converting coefficient of the absorbed optical power to heat by the dopant [12], P0 is the injected HLS power into the Co-doped fiber. The power of the heating laser
decays exponentially along the fiber due to the absorption, and P0 e−αl is the heating power at l. The generated heat is ultimately conducted to the air and the spliced SMF. The thermal conductivity of the surrounding gases is the key factor to determine the temperature of the sensing FBG whose reflection wavelength is sensitive to the temperature. By monitoring the FBG’s wavelength we get the gas conductivity which corresponds to its concentration. Moreover, as the flow of the gas can also take the heat away, the velocity of gas is controlled at a very low level in our experiment in order to overcome the influence of the gas flow. When we change the flow ratio of the gas mixture, it takes several minutes before the gas of new concentration is stable and ready for test. So it is difficult to measure the sensor’s response time in our setup. However, the thermal transfer process in fiber has been studied both theoretically and experimentally [12], [19], and they showed the thermal equilibrium could be reached in 10 seconds. We believe the response time of this fiber based thermal conductivity sensor is at the magnitude of several seconds, as the space between the heat source (fiber core) and the air is only 62.5 μm. III. T HE D ESIGN OF THE FBG’ S L ENGTH AND I TS P OSITION IN THE C OBALT-D OPED F IBER From Eq. (1) the generated heat is uneven along the Co-doped fiber, which leads to the uneven temperature distribution. So the length of the FBG sensor and its position in the Co-doped fiber are important factors to the Bragg wavelength. In this section we will give the design of these two parameters. Fig. 2 shows the temperature distribution in the core of the Co-doped fiber at different heat laser power (simulated using COMSOL, α is 1.1 dB/cm and k is 0.38 [12] in simulation). The fiber core is heated to a few hundred degree C. The highest temperature doesn’t appear at the position of the highest laser power. Actually it is a slight offset to the middle of the Co-doped fiber. That’s because the heat conductivity between the silica–silica interface at the splicing point is much higher than the silica–air interface. In Fig. 2 the temperature varies due to different heating laser power, while the position of peak temperature won’t change once the components of the surrounding air retains. We settled our FBG round the
ZHOU et al.: ACTIVE FIBER GAS SENSOR FOR METHANE DETECTING
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Fig. 3. The reflection spectra distortion of the FBGs with different grating length and heating power levels (experiment results). (a) 3 mm FBG, (b) 5 mm FBG heated using 0 mw, 100 mw, 200 mw and 300 mw heating laser respectively.
position of the peak temperature for two reasons: to increase the measuring sensitivity (which could be testified by the experimental result below) and to avoid the distortion of the FBG’s spectrum (or to avoid the uniform FBG becoming a chirped FBG). We also found as the quasi-uniform area of the temperature was limited (less than 1 mm in Fig. 2), and the length of the FBG should also be carefully considered in order to avoid the expansion and the distortion of the spectrum. We fabricated two FBG round the highest temperature region whose length were 3 mm and 5mm, and their Bragg wavelengths at room temperature were about 1550.1 nm and 1550.2 nm. Fig. 3 shows the broadened spectra after the FBGs of different length being heated. It seems the shorter the better. However, it is not necessary to make FBGs shorter than 1 mm, as it will also broaden the spectra and weaken the reflectivity of the gratings [16]. In our experiment we decided to make 3 mm long FBG for sensing application. IV. E XPERIMENTAL R ESULTS AND D ISCUSSION For testing and calibrating we fabricated a 3 mm long sensing FBG whose Bragg wavelength was 1544.95 nm at room temperature and it shifted to 1547.50 nm after turning on the heating laser. It was estimated the peak temperature in the Co-doped fiber was about 216 °C and the heating laser power at the splicing point was 400 mw. The thermal conductivity coefficients of a standard gas and methane are about 24 mW/(m.k) and 30 mW/(m.k), respectively [9]. We mixed those two gases by a mixer and the methane concentration could be controlled by controlling the flow ratio. The effective conductivity of the mixture increases with the methane concentration [18], leading to a decrease in temperature of the Co-doped fiber. The wavelength drift of the sensing FBG was recorded while the methane concentration increased from 0% to 4.8% [Fig. 4(a)]. The wavelength decreased from 1547.50 nm to 1547.43 nm which indicated the temperature had dropped
Fig. 4. Experiment results. (a) Wavelength drifting of the sensing FBG due to the increasing of the methane concentration. (b) The sensitivity of the methane sensor increases with the HLS power.
by about 6 degree. The fitted line matched the data well and a sensitivity of 14.6 pm/% was realized. By increasing the heating power the sensitivity of methane concentration sensor could be improved, and this phenomenon was analyzed experimentally [Fig. 4(b)]. The heating power injected into the Co-doped fiber was from 0 to 600 mw and the sensitivity could rise from 0 to 26.74 pm/%. V. C ONCLUSION An active fiber thermal conductivity sensor was presented and tested by measuring the methane concentration in a methane-air mixture to demonstrate its operating principle. The sensitivity of this sensor could be chosen by controlling the heating laser power. The FBG was used as thermometer here and had a potential ability to form a quasi-distributed sensing network using wavelength division multiplexing technology. The all-optical heating feature made it very safe for hazardous gas monitoring. Although the selectivity of this TCFS technology is not good enough, it is practical once the components of the mixture under tested are known in advance. R EFERENCES [1] N. Gunasekaran, A. Meenakshisundaram, and V. Srinivasan, “Kinetics and mechanism of CO oxidation on Ln2 NiO4 oxides (Ln = La, Pr or Nd),” Surf. Technol., vol. 22, no. 1, pp. 89–98, 1984. [2] B. de Collongue, E. Garbowski, and M. Primet, “Catalytic combustion of methane over bulk and supported LaCrO3 perovskites,” J. Chem. Soc. Faraday Trans., vol. 87, no. 15, pp. 2493–2499, 1991. [3] J. Watson and D. Tanner, “Applications of the Taguchi gas sensor to alarms for inflammable gases,” Radio Electron. Eng., vol. 44, no. 2, pp. 85–91, Feb. 1974.
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