Demodulation of a tilted fibre Bragg grating transmission signal using α-shape modified Delaunay triangulation

Luigi Fazzi,Roger M Groves

doi:10.1016/j.measurement.2020.108197

Luigi Fazzi, Roger M Groves

Open Access

https://doi.org/10.1016/j.measurement.2020.108197

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Journal: Industrial Metrology	Publication Date: Jul 12, 2020
Citations: 15	License type: cc-by

Affiliation: Delft University of Technology

Abstract

Reflective Tilted Fiber Bragg Grating (TFBG) sensors have intriguing sensing capabilities due to the resonance-peaks present in their transmitted spectrum. Previous works measured the external refractive index (ERI) in which the TFBG sensor is placed, by considering the wavelengths or the envelope of the cladding-modes resonances. In this paper, primarily, we demonstrate the effectiveness of an alternative global technique, based on Delaunay triangulation, to analyze the TFBG spectrum for refractometer purposes. Hence, we performed the correlation between the area subtended by the upper and lower cladding-modes peaks and the ERI. An investigation on the goodness-of-fit correlation functions is also presented for TFBG sensors written in standard- and thin-optical fibres and considering different values of the fundamental triangulation parameter α.

Highlights

Short-period gratings with periodicity around 500 nm, otherwise called Fiber Bragg gratings (FBGs), are formed by a permanent modulation of the core of a single-mode (SM) optical fibre (OF)
The approach is based on the use of Delaunay triangulation to obtain the area of enclosed between the upper and lower peaks of the cladding resonances in the Tilted Fiber Bragg Grating (TFBG) transmission spectrum, which are correlated to the surrounding RI
We demonstrated that this approach can be applied for different kinds of optical fibre in which the TFBG

Summary

Introduction

Short-period gratings with periodicity around 500 nm, otherwise called Fiber Bragg gratings (FBGs), are formed by a permanent modulation of the core of a single-mode (SM) optical fibre (OF). The variation of the refractive index can be imposed with several refractive index profiles along the length of the Bragg grating which modify the propagation of electromagnetic waves inside and, the spectrum of the FBG. In the case of tilted FBG sensors, the previous profile scan is written with a so-called tilt angle with respect to the fibre axis, so that the Bragg gratings are single-sided or, even double-sided tilted [2]. Adjusting the tilt of the gratings, the TFBG transmission spectrum acquires unique features which make it suitable in several applications. The TFBG spectral bandwidth is composed of multiple resonance peaks, called cladding modes, that are more or less sensitive within a determined range of external refractive index (ERI) variations, based on the Bragg grating’s characteristics.

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