Sensors and Actuators B 166–167 (2012) 761–765 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journa l h o mepage: www.elsevier.com/locate/snb Photonic crystal fiber refractive index sensor based on a fiber Bragg grating demodulation Wenwen Qian a,b , Chi Chiu Chan b,∗ , Chun-Liu Zhao a , Yang Liu a , Tao Li a,b , Limin Hu a,b , Kai Ni a,b , Xinyong Dong a,b a Institute of Optoelectronic Technology, China Jiliang University, Hangzhou 310018, China b Division of Bioengineering, School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637457, Singapore a r t i c l e i n f o Article history: Received 2 December 2011 Received in revised form 1 March 2012 Accepted 23 March 2012 Available online 1 April 2012 Keywords: Fiber Bragg grating (FBG) Photonic crystal fiber (PCF) Intermodal PCF interferometer Refractive index sensor a b s t r a c t A refractive index (RI) sensor is proposed which consists of a combination of an intermodal photonic crystal fiber (PCF) interferometer as a sensing head and a fiber Bragg grating (FBG) as a demodulating element. The intermodal PCF interferometer is formed by using a short PCF which is spliced with two single-mode fibers (SMFs) at both ends. The air-holes in the splice regions are fully collapsed and it forms the intermodal PCF interferometer. The corresponding interference phenomenon is found to be sensitive to the change of external RI. A FBG is connected serially after the intermodal PCF interferometer which is used to reflect the optical power at the Bragg wavelength of the interference transmission spectrum. With different external RI values, the corresponding reflective power of the FBG will change, which is resulted from the shift on the interference spectrum of the intermodal PCF interferometer. The resolution of the proposed RI sensor can be obtained as high as 1.5 × 10 -5 at around RI of 1.360 with a combination of a 2.5 cm length intermodal PCF interferometer and a FBG at 1566.8 nm with 99% reflectivity. Due to a temperature compensation between the FBG and the intermodal PCF interferometer, the proposed RI sensor has a low temperature sensitivity and it is suitable for practical applications. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Refractive index (RI), a fundamental material property, has been considered as one of most significant parameters in chemical, bio- logical and physical fields. A compact and high sensitive RI sensor is a key for applications in biochemical sensing and industrial pro- cessing. Various kinds of optical fiber RI sensors have been attracted much attention in recent years, due to their small size, good per- formance and convenient operation. One kind of them is based on fiber gratings, such as fiber Bragg gratings (FBGs) [1,2], long period gratings (LPGs) [3,4] and titled fiber Bragg gratings (TFBGs) [5,6], in which the resonant wavelengths of gratings depend on RI of exter- nal environment due to the mode coupling. Another kind of them is based on interferometry, for example, Febry–Perot structures [7,8], abrupt taper Michelson structures [9] and single-mode multi-mode single-mode (SMS) fiber structures [10,11]. Photonic crystal fibers (PCFs), characterized by the fiber struc- ture with numerous air-holes running along the length of fibers, show remarkable and incomparable optical properties that sup- port to develop various optical fiber devices including optical fiber ∗ Corresponding author. E-mail address: eccchan@ntu.edu.sg (C.C. Chan). sensors and optical fiber communication components [12,13]. A large variety of PCF-based optical fiber sensors for RI, strain and temperature measurement have been investigated [14]. Especially, PCF-based refractometer as a vital sensing systems, could be real- ized by a FBG inscribed on PCFs [15], a TFBG inscribed on PCFs [16], a LPG fabricated on PCFs [17] and intermodal PCF interfer- ometers [18], etc. Intermodal PCF interferometers based on SMS structures for RI sensing are usually formed by a fully collapsed splicing or an offset splicing between the PCF and two single-mode fibers (SMFs), which are easy for fabrication and practical compare to the refractometers based on the inscribed FBG, TFBG and LPG on PCFs. In previous research works on intermodal PCF interferome- ters for RI sensing, the sensing information was usually obtained by monitoring the wavelength shift of the interference transmission spectrum, in which an expensive optical spectrum analyzer (OSA) was necessary. FBGs are considered as an important optical device in the optical fiber technology areas, which are compact in size, cost-effective and easy for manufacturing [19]. FBGs as sensing heads, have been widely applied for various measurements by monitoring the Bragg wavelength shift [20]. At the same time, FBGs also can be used as a demodulation element in optical sensing system by measuring the change of the reflective optical power at the Bragg wavelength. In this paper, a normal FBG is used as a demodulating element which 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.03.066