Please cite this article in press as: S. Olyaee, et al., High sensitivity evanescent-ﬁeld gas sensor based on modiﬁed photonic crystal ﬁber for gas condensate and air pollution monitoring, Optik - Int. J. Light Electron Opt. (2013), http://dx.doi.org/10.1016/j.ijleo.2013.07.047 ARTICLE IN PRESS G Model IJLEO-53722; No. of Pages 5 Optik xxx (2013) xxx–xxx Contents lists available at ScienceDirect Optik jou rn al homepage: www.elsevier.de/ijleo High sensitivity evanescent-ﬁeld gas sensor based on modiﬁed photonic crystal ﬁber for gas condensate and air pollution monitoring Saeed Olyaee a,∗ , Alieh Naraghi a , Vahid Ahmadi b a Nano-photonics and Optoelectronics Research Laboratory (NORLab), Faculty of Electrical and Computer Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran b Faculty of Electrical Engineering, Tarbiat Modares University, Tehran, Iran a r t i c l e i n f o Article history: Received 4 March 2013 Accepted 5 July 2013 Available online xxx Keywords: Photonic crystal ﬁber Evanescent ﬁeld Gas sensor Oil rigs Sensitivity a b s t r a c t Flammable and/or toxic gas sensors can be used as safety measuring in gas production facilities, especially in oil rigs. The gas sensors detect gas leaks capable of causing ﬁre, explosion, and toxic exposure. Here we proposed an index-guiding photonic crystal ﬁber for gas sensing that have a broad spectral transmission band and so is capable to detect more gas condensate components. The dependence of relative sensitivity and conﬁnement loss on the ﬁber parameters is numerically investigated by ﬁnite element method (FEM). Introducing a hollow high index ring with an air hole in the center of ﬁber simultaneously enhances the relative sensitivity and achieves low conﬁnement loss. In addition, we prove that increasing the diameter of holes located in the inner rings, improve the relative sensitivity and increasing the ring diameter holes located in the outer rings, greatly reduces the conﬁnement loss. Placing hexagonal holes instead of circular holes in the innermost ring, the relative sensitivity is effectively enhanced. The relative sensitivity at wavelength of  = 1.33 m that is in the Methane absorption line is enhanced to value of 13.23%. The conﬁnement loss is also improved to 3.77 × 10 −6 . © 2013 Elsevier GmbH. All rights reserved. 1. Introduction Index-guiding photonic crystal ﬁbers consist of cladding includ- ing air holes that goes all along the ﬁber. These holes allow the interaction of light with gases or liquids located in the cladding holes through the evanescent ﬁeld [1–5]. Another type of photonic crystal ﬁbers are photonic band gap (PBG) ﬁbers that include large air core and guide light via photonic band gap mechanism [1,2]. Since in this type of ﬁbers, more light can be conﬁned inside the core ﬁlled with the gas sample, the interaction of light and sample is enhanced, so the sensitivity in photonic band gap is signiﬁcant [3–8]. However, photonic band gap ﬁbers have two fundamental limitations; a narrow transmission spectral band and the require- ment for an accurate control of the air-hole size, and periodicity of the air-hole is less stringent, thus increasing the fabrication toler- ance [4–9]. An important safety item for oil and gas production installa- tions (especially oil rigs) is the use of sensors to detect ﬂammable and/or toxic gases leakage. In oil rigs the most important toxic and ﬂammable gases are hydrogen sulﬁde and methane, respectively. Since index guiding PCFs have broader spectral band propaga- tion, more compounds may be detected. Here we interested in the ∗ Corresponding author. Tel.: +98 21 22 97 0006; fax: +98 21 22 97 0006. E-mail addresses: s olyaee@srttu.edu, s olyaee@yahoo.com (S. Olyaee). wavelength range from 0.8 to 2 m. This range is within the low loss window of silica ﬁber and covers the absorption lines of a number of important gases such as oxygen (O 2 ), nitrogen dioxide (N0 2 ), hydrogen ﬂuoride (HF), hydrogen bromide (HBr), acetylene (CZHZ), hydrogen iodide (H 1 ), ammonia (NH 3 ), carbon monoxide (CO), carbon dioxide (CO 2 ), hydrogen sulﬁde (H 2 S), methane (CH 4 ), and hydrogen chloride (HC 1 ) [10]. So far several studies have been reported to improve the sensing capability of index-guiding ﬁbers and varieties of methods have been proposed to improve their efﬁciency [4–11]. One of the pro- posed methods to enhance the sensitivity is introducing air hole with dimensions smaller than the cladding holes in the center of the ﬁber [6]. In these PCFs, by increasing the diameter of central hole the fraction of evanescent ﬁeld in the cladding holes and so the rela- tive sensitivity is increased. But, the diameter of central hole should be smaller than cladding holes to achieve the index guiding criteria. In addition, despite the improved relative sensitivity, the previous defected core PCFs show more conﬁnement loss and cause a critical trade-off between the sensitivity and the conﬁnement loss in PCF design for sensing applications. To improve the relative sensitivity and conﬁnement loss simul- taneously, an index-guiding PCF with a high index ring defect in the center was proposed by Park et al. [7]. In this ﬁber, the central air hole is surrounded with a hollow high index GeO 2 -doped sil- ica ring defect. Although the suspended core microstructure ﬁbers that have a small core surrounding with large air holes have very 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.07.047