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7 A Novel Approach to Evaluate the Sensitivities of the Optical Fiber Evanescent Field Sensors Xuye Zhuang1, Pinghua Li2 and Jun Yao1 1State Key Laboratory of Optical Technologies for Microfabrication, Institute of Optics and Electronics, Chinese Academy of Sciences 2Department of Electronic Information Technology, Sichuan Modern Vocational College China 1. Introduction Optical fiber evanescent field sensors(OFEFS) are playing more and more important roles in detecting chemical, bacilli, toxin, and environmental pollutants for their high efficiency, good accuracy, low cost, and convenience (Maria et al., 2007; Wolfbeis, 2006; Angela et al., 2007). In general, these sensors exploit the interactions between evanescent field of the sensing fibers and the surrounding analyte under investigation. Concentrations of analyte can be related to the power attenuation caused by these interactions, and the more energy interacted with the analyte, the higher the sensitivities of the sensors. In order to expose the evanescent filed of the fiber to the analyte, the fiber cladding is often removed and the fiber core is surrounded by the detecting material. To make clear the operation principle of the evanescent field interacting with the analyte and to estimate the sensitivity according to this principle are extremely important to the sensor’s designers. This field has attracted a lot of research groups extraordinarily and been widely discussed both based on geometric optics and optical waveguide theory (Yu et al., 2006; Messica et al.,1996; Gupta & Singh, 1994; Guo & Albin, 2006). However a big difference between the theoretical and experimental values is often observed especially when the sensor’s absorbency is low (Payne & Hale, 1993; Wu et al., 2007; Deng, et al., 2006). In this chapter, a thoroughly theoretical study of the OFEFS is presented. The reason why the sensor’s absorbency, A, shows nonlinear increase with the sensing fiber’s length, l, is given and analyzed. Also, a new method to estimate the sensitivity of the sensor is proposed and confirmed with experimental results, which shows a good agreement with the experimental results and is more accurate than the methods used before. The fabrication technologies of the sensing fibers are introduced. The practical importance of this study lies in helping for understanding, design and construction of optical fiber evanescent field sensors. This chapter is organized as follows. In section 2, the theory basis of the optical fiber evanescent field sensor is introduced and analyzed. The nonlinear relationship between A and l is discussed theoretically. The new method to estimate the sensitivity of the sensor is proposed in section 2.3. The fabrication method of the sensing fiber with acicular