Optimization of Cladding Diameter for Refractive Index Sensing in Tilted Fiber Bragg Gratings.

Sanzhar Korganbayev,Marzhan Sypabekova,Daniele Tosi,Christophe Caucheteur,Paola Saccomandi,Aida Amantayeva,Álvaro González-Vila

doi:10.3390/s22062259

Abstract

This work presents an experimental investigation of the effect of chemical etching on the refractive index (RI) sensitivity of tilted fiber Bragg gratings (TFBGs). Hydrofluoric acid (HF) was used stepwise in order to reduce the optical fiber diameter from 125 µm to 13 µm. After each etching step, TFBGs were calibrated using two ranges of RI solutions: the first one with high RI variation (from 1.33679 RIU to 1.37078 RIU) and the second with low RI variation (from 1.34722 RIU to 1.34873 RIU). RI sensitivity was analyzed in terms of wavelength shift and intensity change of the grating resonances. The highest amplitude sensitivities obtained are 1008 dB/RIU for the high RI range and 8160 dB/RIU for the low RI range, corresponding to the unetched TFBG. The highest wavelength sensitivities are 38.8 nm/RIU for a fiber diameter of 100 µm for the high RI range, and 156 nm/RIU for a diameter of 40 µm for the small RI range. In addition, the effect of the etching process on the spectral intensity of the cladding modes, their wavelength separation and sensor linearity (R2) were studied as well. As a result, an optimization of the etching process is provided, so that the best trade-off between sensitivity, intensity level, and fiber thickness can be obtained.

Highlights

Tilted Fiber Bragg Grating (TFBG) is a periodic modulation of the refractive index of the core of an optical fiber, photo-inscribed at a certain angle with respect to the fiber radial axis [1]
The response of the unetched TFBG to refractive index (RI) variations was studied, as can be seen in the increase in RI leads to the gradual disappearance of the modes since they are no longer totally internally reflected by the cladding boundary and reach the cut-off condition [1]
For minor RI variations, in this case less than 0.0203 RIU, only wavelength shift is observable, while amplitude change corresponds to higher RI variations

Summary

Introduction

Tilted Fiber Bragg Grating (TFBG) is a periodic modulation of the refractive index of the core of an optical fiber, photo-inscribed at a certain angle with respect to the fiber radial axis [1]. While a standard fiber Bragg grating (FBG) behaves as an optical narrow-band notch filter [2], the transmission spectrum of a TFBG reveals a collection of backwardpropagating cladding mode resonances. In addition to the typical temperature and strain sensitivities, these structures are able to detect variations in the refractive index (RI) of the medium surrounding the optical fiber [3]. Examples of applications in mechanical sensing include optical fiber accelerometers [9], cantilever-based micro-displacement sensors [10], and even three-dimensional shape sensors based on orthogonal TFBGs [4]. Optical fiber biosensing is the field where

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