Research Article High Sensitivity Refractive Index Sensor by D-Shaped Fibers and Titanium Dioxide Nanofilm Chuen-Lin Tien , 1,2 Hong-Yi Lin, 2 and Shu-Hui Su 2 1 Department of Electrical Engineering, Feng Chia University, Taichung 40724, Taiwan 2 Ph. D. Program of Electrical and Communications Engineering, Feng Chia University, Taichung 40724, Taiwan Correspondence should be addressed to Chuen-Lin Tien; cltien@fcu.edu.tw Received 17 November 2017; Accepted 7 December 2017; Published 5 February 2018 Academic Editor: Shuan-Yu Huang Copyright © 2018 Chuen-Lin Tien et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Tis paper presents a high sensitivity liquid refractive index (RI) sensor based on lossy mode resonance (LMR) efect. Te D- shaped fbers coated with nanosized titanium dioxide (TiO 2 ) thin flm as a sensing head were submerged into diferent refractive index solutions. Te variations in the optical spectrum of the proposed RI sensor with diferent refractive index solutions were measured. Te LMR resonance peaks were used to determine the wavelength shifs with diferent refractive index solutions. Te results show that the optical spectrum peaks shifed towards the longer wavelength side with increasing the refractive index. For the proposed fber sensing head with a polishing residual thickness of 72 m, the maximum shif of the absorption peak was 264 nm. Te sensitivity of the proposed RI sensor was 4122 nm/RIU for the refractive index range from 1.333 to 1.398. 1. Introduction Many fber-optic sensors for refractive index (RI) sensing have been developed due to some advantages such as small size, high sensitivity, light weight, and immunity to external electromagnetic interference. Various fabrication methods for fber-based RI sensors have been reported in the literature [1], including surface plasmon resonance (SPR) [2, 3], evanes- cent feld [4], fber gratings [5, 6], and optical fber interfer- ometry [7]. However, the sensitivity of the traditional sensing techniques needs to be improved in practical applications. Iadicicco et al. [5] reported an etched fber Bragg grating RI measurement, while etching can also be applied to a long period of grating to increase sensitivity to external refractive indices [6]. Optical fber sensors based on evanescent wave generate a strong interaction between the guided wave and the surrounding materials. However, most of the reported sensors showed a low-sensitivity or low-detection range. To overcome this problem, we proposed a D-shaped optical fber coated with a high refractive index titanium dioxide (TiO 2 ) nanoflm to enhance the sensitivity and to extend the detection range of the sensor. In the last decades, thin flm coated onto a fber-optic has become a widely explored technique in the feld of sensors. Te efects of depositing a thin, highly refractive index (RI) layer onto the cladding over the grating region have been reported [8, 9]. Since titanium dioxide (TiO 2 ) is one of the highly refractive index materials, TiO 2 is known to be a good photocatalytic material under UV radiation. In this study, we present the fabrication of high sensitivity refractive index fber-optic sensors based on D-shaped fbers and nanosized TiO 2 coatings. From the optical point of view, TiO 2 nanoflm deposited on D-shaped fbers can generate lossy mode reso- nance (LMR) efect under particular conditions [10]. In this work, we fabricate a new liquid refractive index fber-optic sensor called a D-LMR (D-shaped fber with lossy mode resonance) sensor that combines with the D-shaped fbers and nanosized TiO 2 coatings to achieve a high sensitivity sensor with a wide range of liquid refractive indices. We also utilized a DC magnetron sputtering technique to deposit TiO 2 nanoflms on the polished surface of D-shaped fbers for the fabrication of liquid refractive index fber sensors. Te proposed D-shaped fber sensor structure includes a low index upper cladding (about 3–5 m thick) between the fber core and the TiO 2 layer that is diferent from an uncladded fber structure in the literature [10–12]. Our fber structure design will help in giving the D-shaped fber good mechanical support afer side-polishing. Tis paper demonstrates the Hindawi Advances in Condensed Matter Physics Volume 2018, Article ID 2303740, 6 pages https://doi.org/10.1155/2018/2303740