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The development of an optical fiber refractometer by 5 hydrogen fluoride etching and sputtering deposition of a thin-film 6 of indium tin oxide on a single-mode-multimode-single-mode fiber 7 structure has been analyzed with the aim of improving the sen-8 sitivity to the changes of the refractive index (RI) of the external 9 medium. The device is sensitive to the RI changes of the surround-10 ing medium, which can be monitored by tracking the spectral 11 changes of an attenuation band or with a fast Fourier transform 12 (FFT) analysis. By using an optical spectrum analyzer combined 13 with a simple FFT measurement technique, the simultaneous real-14 time monitoring is achieved. The results show that the sensitivity 15 depends on the thin-film thickness. A maximum of 1442 nm/RIU 16 (refractive index unit) in the 1.32-1.35 RIU range has been attained. 17 In addition, a theoretical analysis has been performed, where simu-18 lations matched with the experimental results. As a practical appli-19 cation of the developed optical fiber structure, a°Brix (°Bx) sensor 20 has been implemented with a sensitivity of 2.13 nm/°Bx and 0.25 21 rad/°Bx respectively for wavelength and phase shift detection. 22 Index Terms-Etching, optical fiber sensor, refractive index, 23 single-mode-multimode-single-mode (SMS), thin-films. 24 I. INTRODUCTION 25 O PTICAL fiber refractometers have been extensively stud-26 ied for chemical, medicine, and biological applications 27 due their multiple advantages such as compact size and high 28 resolution. They can be used in harsh environments and allow 29 for minimally invasive procedures to be performed [1]. 30 Up until now, several technologies have been used to de-31 velop optical fiber refractometers. Some of these technologies 32

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This work presents a refractive index sensor based on a long period fiber grating (LPFG) made in a reduced cladding fiber whose low order cladding modes have the turning point at large wavelengths. The combination of these parameters results in an improved sensitivity of 8734 nm/refractive index unit (RIU) for the LP_0,3 mode in the 1400-1650 wavelength range. This value is similar to that obtained with thin-film coated LPFGs, which permits to avoid the coating deposition step. The numerical simulations are in agreement with the experimental results.