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

The performance of E-SMS (Etched Singlemode-Multimode-Singlemode) optical fiber structures as immunosensors has been assessed by the implementation of antibody/antigen immunoassays. Through this procedure it has been proven that E-SMS structures are effective and suitable optical platforms for label-free biosensing. Using the phase shift and tracking the wavelength response it was found that the fabricated E-SMS devices exhibited limits of detection (LOD) down up to concentrations of 0.2 mg/L of antigens in solution. This was achieved by coating the E-SMS with an antibody-based biolayer (goat IgG) that is able to determine the presence of anti-goat IgG antigen. Both a wavelength detection and a fast Fourier transform (FFT) analysis technique were used to perform this analysis. The FFT method showed similar results to those observed with the most traditional wavelength analysis, but with the advantage of a simpler detection system that makes unnecessary the use of sophisticated optical interrogators.

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An optical fibre refractometer has been developed by etching and deposition of a thin film of indium tin oxide (ITO) on a single-mode-multimode-single-mode (SMS) fibre structure. The interference between modes in this structure is sensitive to the refractive index changes of the surrounding medium, achieving sensitivities of up to 7000 nm/RIU in the 1.333-1.338 RIU range. A salinity sensor has been implemented as a practical application of this proposed structure. Fast Fourier transform (FFT) analysis and tracking of an interference dip were used to monitor the interference between modes obtaining sensitivities of 0.99 nm/PSU and 0.025 rad/PSU, respectively.

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