IEEE SENSORS JOURNAL, VOL. 11, NO. 6, JUNE 2011 1423 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 hy- drogen gas and its recovery with nitrogen gas are presented. Index Terms—Fiber optic device, fiber sensor, hydrogen sensor, long-period fiber grating. I. INTRODUCTION H 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, immu- nity 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 demon- strated by Tabib-Azar, et al. [4]. Palladium-coated tapered 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 Sci- ence 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: leebh@gist.ac.kr). 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 ex- posing 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 ini- tial 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 perturba- tions than the lower ones [10]. Furthermore, the fabrication pro- cedure 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 funda- mental core mode and the cladding modes at the resonant wave- lengths satisfying the phase matching condition of (1) where is the grating period. and are the effective indexes of the core and the th-order cladding modes, respec- tively [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 1530-437X/$26.00 © 2010 IEEE