IEEE SENSORS JOURNAL, VOL. 11, NO. 6, JUNE 2011
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Ultra Sensitive Fiber-Optic Hydrogen Sensor Based on High Order Cladding Mode Young Ho Kim, Myoung Jin Kim, Byung Sup Rho, Min-Su Park, Jae-Hyung Jang, and Byeong Ha Lee, Member, IEEE
Abstract—We demonstrate a simple but sensitive hydrogen gas sensor composed of a palladium-coated long-period fiber grating (LPG). By writing an LPG in a low core index fiber, high-order cladding modes are excited. As the palladium thin layer absorbs hydrogen, the effective refractive indexes of the cladding modes are affected, thus the resonant wavelengths of the LPG are changed with a high sensitivity. With 70-nm-thick coating, 7.5 nm of the hydrogen-induced spectral shift was achieved. The spectral response of the proposed sensor to hydrogen gas and its recovery with nitrogen gas are presented. Index Terms—Fiber optic device, fiber sensor, hydrogen sensor, long-period fiber grating.
I. INTRODUCTION YDROGEN, which is considered as one of the future alternative energy resources, has been widely studied owing to its attractive merits including cleanness and plenty. On the other hand, hydrogen requires quite careful handling since it is quickly diffused and easily exploded in the condition of over 4% hydrogen concentration. Thus, detecting hydrogen leakage in advance is very important for safety. Recently, a variety of hydrogen sensors have been developed with the help of palladium metal that can absorb hydrogen up to 900 times of its volume at room temperature and atmospheric pressure [1]. Most of the hydrogen sensors utilize the properties of the palladium that vary with the absorption of hydrogen. Among various methods for hydrogen detection, fiber optic sensors have outstanding advantages such as safety from spark, immunity to ambient electromagnetic interference, and capability of long distance interrogation [2]. In prior works, Butler presented a micro-mirror formed at the end of a fiber with a palladium thin film [3]. A multimode fiber having a palladium coating over the fiber core exposed by chemical etching was demonstrated by Tabib-Azar, et al. [4]. Palladium-coated tapered
H
Manuscript received June 17, 2010; revised October 21, 2010; accepted November 07, 2010. Date of publication November 15, 2010; date of current version April 20, 2011. This work is supported in part by the Small & Medium Business Administration (SMBA) grants funded by the Korean government (No. S1068004). The associate editor coordinating the review of this paper and approving it for publication was Dr. M. Abedin. Y. H. Kim, M.-S. Park, and J.-H. Jang are with the Gwangju Institute of Science and Technology, Gwangju 500-712, Korea. M. J. Kim and B. S. Rho are with Nano-Photonics Research Center, Korea Photonics Technology Institute, Gwangju, 500-779, Korea. B. H. Lee is with the School of Information and Communications, Gwangju Institute of Science and Technology, Gwangju, 500-712, Korea (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2010.2092423
fibers, side-polished fibers, and integrated sensors based on surface Plasmon resonance (SPR) have been also reported as new sensing solutions [2], [5], [6]. However, they required cautious fabrication processes such as tapering and etching, which made the sensing part easy to be broken. As an effort to overcome those weak points, a simple structure employing a fiber Bragg grating (FBG) or long-period fiber grating (LPG) has been introduced, but its spectral response seems not enough for a practical use [7]–[9]. In this letter, a highly sensitive hydrogen sensor with a simple configuration is presented. As illustrated in Fig. 1, the proposed sensor is composed of a single LPG, which is fabricated by exposing UV laser on a specially designed low core index single mode fiber in order to excite high order cladding modes. The LPG has 400 m of grating period and 40 mm of grating length. Thin palladium layer (30, 50, and 70 nm) is coated over one side of the cladding surface. An amplified spontaneous emission (ASE) broadband light source was launched into the fabricated sensor. While flowing 4% of hydrogen gas into a gas chamber, the interaction of the LPG-induced cladding mode to the coated palladium was observed by an optical spectrum analyzer (OSA). 100% of nitrogen gas was poured by turns to restore it to the initial condition. It can be expected that the proposed sensor would present an enhanced spectral response because the higher order cladding modes are much more sensitive to external perturbations than the lower ones [10]. Furthermore, the fabrication procedure is simple and does not need any supplementary treatment like etching or tapering. While exposing the fiber to hydrogen and nitrogen in sequence, we measure the temporal dependency of the optical spectrum and confirm the feasibility as a practical hydrogen sensor. II. OPERATING PRINCIPALS An LPG induces the optical coupling between the fundamental core mode and the cladding modes at the resonant wavesatisfying the phase matching condition of lengths (1) where is the grating period. and are the effective indexes of the core and the th-order cladding modes, respectively [11]. The is readily affected by the refractive index of the material surrounding the cladding, which results in the shift of the resonant wavelength. From (1), in an ordinary case, to excite a high order cladding mode a short grating period is necessary since the effective index
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Fig. 1. The experimental setup for the proposed sensor. Hydrogen interacts with the cladding mode of the LPG through the palladium coated on the fiber. The transmission spectrum was measured while flowing 4% of hydrogen gas and 100% of nitrogen gas into the chamber by turns.
Fig. 2. Comparison of the LPG-induced mode coupling in the fibers having a high core index (a) and a low core index (b). For the same effective index difference, the low core index fiber can excite higher order cladding modes.
of the cladding mode becomes smaller with the mode order. To excite high order cladding modes with a moderate grating period, however, we intend to reduce the effective index of the core mode. As described in Fig. 2, it is assumed that two refractive index profiles of fibers exist; one has a conventional high core index (a) and the other has a low core index (b) with the same cladding index. However, the effective indexes of the th-order cladding modes of (a) and (b) are almost the same owing to their larger cladding diameter than the core diameter. Therefore, we can easily excite the higher order cladding mode, , with the same introducing the same index difference grating period but with the low core index fiber [12]. The designed low index core fiber has about 0.13% of refractive index difference while the conventional single mode fiber has about 0.3% difference. When palladium is exposed to hydrogen gas, a hydrogen H at molecule is divided into two hydrogen atoms H the palladium surface with a very efficient dissociation rate. As the palladium layer absorbs hydrogen, its volume expands because hydrogen absorption converts palladium to palladium hydride (PdHx). At the same time, the density of free electrons decreases within palladium. Palladium hydride has different optical properties compared with those of hydrogen-free palladium. Both the real and imaginary parts of the permittivity are reduced. It results in the change in the boundary condition between the cladding surface and the palladium layer, which changes the effective index difference between the core and the cladding modes. Eventually, the variation of the transmission spectra around the LPG resonant wavelengths can be used as an efficient sensing parameter. III. EXPERIMENTAL RESULTS AND DISCUSSION The transmission spectrum of the fabricated sensor was measured. As Fig. 3 shows, it had dual resonant peaks within the
Fig. 3. Transmission spectra of the fabricated sensor without (black and solid line) and with (red and dotted line) hydrogen. With hydrogen, the dual resonant peaks of an LPG were moved to the opposite directions to each other; the inset is the mode field image of the 8th order cladding mode.
interesting wavelength range of 1300 1650 nm. Interestingly, the resonant peaks were shifted in a counter direction with hydrogen; the peak in the left (peak 1) moved to the left and the right one (peak 2) moved to the right. The counter movement is a typical behavior of the higher order cladding mode [12]. The inset in Fig. 3 is a near-field image of the excited high order cladding mode taken by an infrared camera and a tunable laser. The existence of seven rings verifies that the eighth-order is activated. cladding mode To investigate the response of the sensor to the thickness of the palladium coating, three samples were prepared. Each sample had the LPG of Fig. 3 but the coating thickness was different; 30, 50, 70 nm, respectively. The spectral movement of the right resonant peak of Fig. 3 was observed in terms of the hydrogen flowing time and the nitrogen flowing time in series. Fig. 4 exhibits that, with the flow of 4% hydrogen, the peak was shifted rapidly to the longer wavelength direction at first and then saturated slowly. The thicker palladium coating gave the wider spectral shift. It was also observed that the shifted peak returned back to its initial position under the inflow of 100% of nitrogen. Regardless of the coating thickness, the spectral behaviors of all samples were similar to each other and well fitted with exponential curves (red lines in Fig. 4). However, the decaying constant was proportional to the coating thickness as summarized in Table I. In Fig. 3, we can see that the intensity of resonant peak was also changed with the hydrogen. The amount of depth change was also proportional to the coating thickness, as presented in Table I.
KIM et al.: ULTRA-SENSITIVE FIBER-OPTIC HYDROGEN SENSOR BASED ON HIGH-ORDER CLADDING MODE
Fig. 4. The response of the resonant peak position to hydrogen and nitrogen. The sensor was exposed to 4% hydrogen and then 100% nitrogen in sequence. The peak position was shifted rapidly with hydrogen at first and then saturated slowly. With nitrogen, the peak returned exponentially. The sensor with thick palladium showed a big shift and also a big decaying constant.
TABLE I THE SUMMARY OF THE MEASURED RESPONSE OF THE PROPOSED SENSOR. THREE SENSORS WERE MADE WITH DIFFERENT THICKNESSES OF PALLADIUM COATING; 30, 50, 70 nm, RESPECTIVELY
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leakage interrogation mean, is much more cost-effective than the spectral shift measurement. The former needs just a single photodiode, but the latter needs a rather costly spectral device such as an OSA or a spectrometer. However, it is well known that the spectral shift variation measurement is more accurate than the intensity measurement in general. Therefore, development of a dedicated cost-effective spectrometer with narrowing down the bandwidth is under study. As tabulated in Table I, the rate of wavelength shift was considerably enhanced compared to the previously reported LPGbased hydrogen sensor demonstrated by Maier et al. [8]. The wavelength shift was in the order of hundreds of picometers, which required fine measurement equipment, but the proposed device was in the order of a few nanometers. In order to get practical hydrogen sensors, both high sensitivity and short response time should be satisfied. Furthermore, detection of below 4% of hydrogen concentration is also required for some real applications. In general, as hydrogen concentration decreases, the sensitivity becomes low. Although the experiment was made only with the 4% hydrogen concentration, owing to the high sensitivity of the proposed sensor, it might effectively detect less than 4% concentration. As shown in Table I, there is inherent tradeoff between the sensitivity and the response time of the sensor. Therefore, it is necessary to find the optimal thickness of the palladium coating depending on the real applications. One of the most important factors for accurate operation of the sensor is the unpredictable change in external environment, such as temperature, strain, humidity, and so on. Since these parameters can directly influence the optical performance of the sensor, it is required to isolate the system or compensate the environment-induced variation. By inserting an auxiliary device, which is free from palladium but sensitive to the environment changes, the compensation can be easily implemented. However, as a gas leakage sensor, the amount of the spectral shift in Table I is huge enough to consider the variation induced by environment change as a second factor. IV. CONCLUSION
Fig. 5. Peak wavelength shift (red line with the left vertical axis) and intensity variation (blue line with the right vertical axis) of the sensor having a 30-nmthick palladium coating. The device was exposed to 4% hydrogen flowing (on) and 100% nitrogen flowing (off) in sequence.
For a repeatability test, the sample having a 30-nm-thick palladium coating was exposed to 4% hydrogen (on) and then 100% nitrogen (off) flow in sequence for three times. As shown with the red curve and the left vertical axis of Fig. 5, the resonant wavelength of the LPG moved back and forth according to the on-and-off with a good repeatability. The optical intensity variation of the resonant peak, at a wavelength of 1576.5 nm, was monitored also and displayed with the blue curve and the right vertical axis of the same figure. It showed the similar repeatability with the wavelength shift. Therefore, the optical intensity measurement can be utilized as another sensing parameter or used to confirm the peak wavelength shift. The optical intensity variation measurement, as a gas
A simple, firm, and high-sensitive hydrogen sensor based on high order cladding modes coupled by a palladium-coated LPG has been demonstrated. The low core index fiber was used to excite high order cladding modes which were quite sensitive to the external refractive index change. The LPG written in the low core index fiber showed dual resonant peaks in the wavelength 1650 nm. When the palladium-coated LPG range of 1300 was exposed to hydrogen, both resonant peaks were shifted in the opposite directions. It is thought that the refractive index of the palladium coated on the fiber cladding was affected by hydrogen, so that the effective indexes of the cladding modes were changed. The change in the resonant peaks of the mode was monitored with flowing 4% of hydrogen gas and 100% of nitrogen gas in sequence. The sensor showed high sensitivity, good recovery, and reliable repeatability. REFERENCES [1] A. D’Amico, A. Palma, and E. Verona, “Surface acoustic wave hydrogen sensor,” Sens. Actuators B, vol. 3, pp. 31–39, 1982.
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[2] K. T. Kim, H. S. Song, J. P. Mah, K. B. Hong, K. Im, S.-J. Baik, and Y.-I. Yoon, “Hydrogen sensor based on palladium coated side-polished single-mode fiber,” IEEE Sensors J., vol. 7, no. 1, pp. 1767–1770, 2007. [3] M. A. Butler, “Micromirror optical-fiber hydrogen sensor,” Sens. Actuators B, vol. 22, pp. 155–163, 1994. [4] M. Tabib-Azar, B. Sutapun, R. Petrick, and A. Kazemi, “Highly sensitive hydrogen sensors using palladium coated fiber optics with exposed cores and evanescent field interactions,” Sens. Actuators B, vol. 56, pp. 158–163, 1999. [5] J. Villatoro, A. Diez, J. L. Cruz, and M. V. Andes, “In-line highly sensitive hydrogen sensor based on palladium-coated single-mode taper fibers,” IEEE Sensors J., vol. 3, no. 4, pp. 533–537, 2003. [6] P. Tobiska, O. Hugon, A. Trouillet, and H. Gagnaire, “An integrated optic hydrogen sensor based on SPR on palladium,” Sens. Actuators B, vol. 74, pp. 168–172, 2001. [7] M. Buric, K. P. Chen, M. Bhattarai, P. R. Swinehart, and M. Maklad, “Active fiber Bragg grating hydrogen sensors for all-temperature operation,” IEEE Photon. Technol. Lett., vol. 19, no. 5, pp. 255–257, 2007. [8] R. R. J. Maier, B. J. S. Jones, J. S. Barton, S. McCulloch, T. Allsop, J. D. C. Jones, and I. Bennion, “Fibre optics in palladium-based hydrogen sensing,” J. Opt. A, Pure Appl. Opt., vol. 9, pp. S45–S59, 2007. [9] Y. H. Kim, M. J. Kim, M.-S. Park, J.-H. Jang, and B. H. Lee, “Hydrogen sensor based on a palladium-coated long-period fiber grating pair,” J. Opt. Soc. Korea, vol. 12, no. 4, pp. 221–225, 2008. [10] X. Shu and L. Zhang, “Sensitivity characteristics of long-period fiber gratings,” J. Lightw. Technol., vol. 20, no. 2, pp. 255–266, 2002. [11] A. M. Vengarkar, P. J. Lemaire, J. B. Judkins, V. Bhatia, J. E. Sipe, and T. E. Ergodan, “Long-period fiber gratings as band-rejection filters,” J. Lightw. Technol., vol. 14, no. 1, pp. 58–65, 1996. [12] M. J. Kim, Y. M. Jung, B. H. Kim, W.-T. Han, and B. H. Lee, “Ultrawide bandpass filter based on long-period fiber gratings and the evanescent field coupling between two fibers,” Opt. Express, vol. 15, no. 17, pp. 10855–10862, 2007.
Young Ho Kim received the B.S. degree in mechanical engineering from Hannam University, Daejeon, Korea, in 2007 and the M.S. degree from Gwangju Institute and Science and Technology (GIST), Gwangju, Korea, in 2009. Currently, he is working towards the Ph.D. degree at the Department of Information and Communications, GIST. His research interests include integrated fiber optic sensors and fiber grating devices.
Myoung Jin Kim received the Ph.D. degree in Information and mechatronics engineering from Gwangju Institute of Science and Technology (GIST), Korea, in 2008. He is currently a Senior Researcher in the Korea Photonics Technology Institute (KOPTI), Gwangju, Korea, working in the area of optical sensor devices and systems for practical field applications.
Byung Sup Rho received the Ph.D. degree in Materials Science and Engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2000. He was a Research Professor in the Information and Communications University (ICU), Daejeon, Korea. He is currently a Project Leader in the Korea Photonics Technology Institute (KOPTI), Gwangju, Korea, and he has been studying the optical device packaging and various types of optical modules. Also, he has been studying terabus-level optical interconnection for chip-to-chip and board-to-board data link, including optical material characterization, design, and demonstrations of optical interconnection using electrooptical printed circuit board.
Min-Su Park received the B.S. degree in electrical engineering from Hongik University, Seoul, Korea, in 2005 and the M.S. degree from Gwangju Institute and Science and Technology (GIST), Gwangju, Korea, in 2006. Currently, he is working towards the Ph.D. degree at the Department of Information and Communications, GIST. His research interests include optoelectronic converter, high speed photodiodes, single photon detectors, and biotechnology applications.
Jae-Hyung Jang received the B.S. and M.S. degrees in electrical engineering from the Seoul National University, Seoul, Korea, in 1993 and 1995, respectively, and the Ph.D. degree in electrical and computer engineering from the University of Illinois, Urbana, in 2002. He is currently an Associate Professor in the Department of Information and Communications, Gwangju Institute of Science and Technology (GIST), Gwangju, Korea. His research interests at GIST include the design, fabrication, and characterization of compound semiconductor devices and circuits, including InP high-electron mobility transistors, single photon detectors, ZnO-based transparent thin-film transistors, and highly efficient solar cells. He is also doing active research works on small antennas based on metamaterials and ring-resonator-coupled devices on silicon on insulator for photonic integrated circuits and the integrated biosensors.
Byeong Ha Lee (M’00) received the B.S. and M.S. degrees in physics from the Seoul National University, Korea, in 1984 and 1989, respectively, and the Ph.D. degree in physics from University of Colorado at Boulder. After working as an Science and Technology Agency (STA) fellow in the Osaka National Research Institute of Japan from 1997 to 1999, he joined Gwangju Institute of Science and Technology (GIST), Korea, where he is currently serving as a Full-Time Professor. He is specialized in areas related to fiber optic sensors, fiber gratings, photonic crystal fibers, and optical coherence tomography. His current research interests are related to developing fiber optic systems for biomedical applications.
