Sensors and Actuators B 122 (2007) 289–294 Optimizing microstructured optical fibers for evanescent wave gas sensing H.L. Ho a,∗ , Y.L. Hoo a , W. Jin a , J. Ju a , D.N. Wang a , R.S. Windeler b , Q. Li c a Department of Electrical Engineering (EE), The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China b OFS Laboratories, USA c Beijing Institute for Glass Research, China Received 10 July 2005; received in revised form 29 May 2006; accepted 30 May 2006 Available online 11 July 2006 Abstract The relationships among the V parameter, confinement loss, relative sensitivity, operating wavelength and structural parameters of microstructured optical fibers for gas sensing applications are investigated theoretically. The V parameter of microstructured optical fibers and the confinement loss requirement set a limit on the maximum sensitivity achievable from index-guiding microstructured fibers. The maximum relative sensitivity is around 30–35%, corresponding to confinement losses of 0.1 dB/m and 1 dB/m, respectively. A novel fiber design to achieve faster sensor response is also proposed and analyzed. © 2006 Elsevier B.V. All rights reserved. Keywords: Microstructured optical fiber; Evanescent wave; Gas detection; Fiber optic sensor 1. Introduction Microstructured optical fibers (MOF) have a cladding region comprised of air-holes running along the full length of the fiber. These air-holes open up new possibilities for exploiting the inter- action of light with gases or liquids via evanescent fields in the holes [1]. The performances of these sensors are affected by a number of parameters including operating wavelength λ, diameter d and pitch Λ (separation between two holes) in the cladding. The relative sensitivity that is proportional to the frac- tion of power in the air-holes has been analyzed numerically as a function of relative hole diameter d/Λ and normalized wave- length λ/Λ [1,2]. However, the effect of the V parameter of MOFs and confinement loss that may comprise the sensitivity of the sensors has not been examined to our knowledge. In this paper, we report the results of such an investigation and present approximate formulas that relate the relative sensitivity to the relative hole-size d/Λ and the normalized operating wavelength λ/Λ, and formulas that determine the range of d/Λ and λ/Λ in order to keep the confinement loss below a certain level. These results will be useful for the design of evanescent wave gas sensors. ∗ Corresponding author. Tel.: +852 2766 6190; fax: +852 2330 1544. E-mail address: eehlho@polyu.edu.hk (H.L. Ho). 2. Relative sensitivity of MOF To calculate the relative sensitivity of MOF, the effective mode index and the mode field pattern of the MOF should be calculated first. Due to the mathematical difficulties inherent in Maxwell’s equations and the irregular cross-sectional geome- tries of the MOF, it is very complicated and impossible to obtain these quantities analytically. We here apply the full-vectorial finite element method (FEM) for the mode field analysis in MOF [2,3]. FEM is a powerful tool able to cope with any kind of geometry, and has been successfully used in a modal analysis of fibers with various geometries [4,5]. The evanescent field anal- ysis of MOF which consists of triangular arrangement of holes was demonstrated by using FEM and the results have a well agreement with the experimental results [2]. In our simulation study, the refractive index of the silica was taken as 1.45, and we consider here the fundamental mode in the MOF which consists of triangular arrangement of holes as shown in Fig. 1. The MOF is characterized by two important parameters, i.e., the pitch Λ and hole diameter d. The guided light power is confined within the solid core region with a part (evanescent field) of power extended into the holey region. Relative sensitivity coefficient (r) at a particular wavelength is represented by [6] r =  n r n e  f (1) 0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.05.036