Review of Chirped Fiber Bragg Grating (CFBG) Fiber-Optic Sensors and Their Applications

Abstract

Fiber Bragg Gratings (FBGs) are one of the most popular technology within fiber-optic sensors, and they allow the measurement of mechanical, thermal, and physical parameters. In recent years, a strong emphasis has been placed on the fabrication and application of chirped FBGs (CFBGs), which are characterized by a non-uniform modulation of the refractive index within the core of an optical fiber. A CFBG behaves as a cascade of FBGs, each one reflecting a narrow spectrum that depends on temperature and/or strain. The key characteristic of CFBGs is that their reflection spectrum depends on the strain/temperature observed in each section of the grating; thus, they enable a short-length distributed sensing, whereas it is possible to detect spatially resolved variations of temperature or strain with resolution on the order of a millimeter over the grating length. Based on this premise, CFBGs have found important applications in healthcare, mechanical engineering, and shock waves analysis, among others. This work reviews the present and emerging trends in CFBG sensors, focusing on all aspects of the sensing element and outlining the application case scenarios for which CFBG sensors have been demonstrated.

Keywords: Chirped Fiber Bragg Grating (CFBG); FBG sensors; Fiber Bragg Grating (FBG); fiber optic sensors; photosensitivity.

Figure 1

Schematic of a CFBG and its incoherent discretization method. (a) Sketch of a linearly chirped FBG; (b) correspondent discretization of the CFBG into M uniform FBGs.

Figure 2

Simulation of CFBG spectra using the CMT-based model. The chart shows spectra having different length L ranging between 20 mm and 50 mm, and chirp rate coefficient ξ equal to 1–2 nm/mm; the other grating parameters are δn_eff = 10⁻⁶, n_eff = 1.5, λ_B(0) = 1520 nm, kL_g = 0.4 and the discretization step is L_g = 0.2 mm.

Figure 3

Simulation of the variations of CFBG spectra with CMT model, exposed to different temperature pattern. (a) Temperature variations applied to a 50-mm long CFBG with 1 nm/mm chirp rate; (b) Obtained CFBG reflection spectrum for each temperature profile. Each profile is displayed with the same color in the two charts.

Figure 4

Schematic of phase mask inscription setup.

Figure 5

Schematic of the phase mask diffraction principle.

Figure 6

Schematic of phase mask inscription setup based on KrF pulsed laser for inscription of gratings on PMMA fibers, reported by Marques et al.; image from [70].

Figure 7

Schematic of CFBG interrogators, sketched as single-channel systems. (a) White light setup based on a spectrometer; (b) scanning laser based setup.

Figure 8

Method for spectral reconstruction proposed by Bettini et al. based on CFBG spectral analysis. Image from [57].

Figure 9

CFBG for in situ temperature detection in RFA; image adapted from [89]. (a) Positioning of the CFBG in the ablated tissue; (b) example of a measured thermal map (the colorbar reports temperature in °C).

Figure 10

Photograph of the CFBG embedded in a load system for 3-point strain detection proposed by Bettini et al. Image from [57].

Figure 11

Schematic of high-speed CFBG interrogation systems proposed by Rodriguez and Gilbertson: (a) InGaAs photodetector-based setup; (b) fs laser-based scanning setup. Image adapted from [77].

Figure 12

Schematic of the CFBG assembly proposed by Wei et al. (a) Assembly of top and bottom parts of the probe; (b) structure of the whole sensing probe, inclusive of reference pins. Image from [34].

Figure 13

CFBG for overhead transmission line sag detection reported by Wydra et al. Image from [78].

Figure 14

2D inclinometer reported by Chang et al. based on a pair of CFBGs. (a) Schematic of the setup; (b) working principle. Image adapted from [53].