2.1 Construction

Commercially available silica fibres are widely used in the development of optical fibre temperature sensors owing to their small cross-sectional area and consequentially their ability to be implemented in restricted areas. However, the relatively low cost of silica fibre compared to more exotic fibres such as those manufactured from fluoride or synthetic sapphire may be a factor to consider. As is reported, optical fibre sensors are particularly suited to environments where an electronic sensor may not have sufficient protection from EMI, and where line of sight to the measurement point is obstructed [21]. Furthermore, optical fibre sensors do not represent a potential source of ignition in an explosive environment; can be biocompatible; and can be made to work over very long distances. To date, optical fibre temperature sensors have been implemented in applications ranging from in-vivo biomedical sensing [22], to structural and geothermal engineering [23]. for the fibre to be used as a point measurement device in applications where transmission is impractical, i.e. biomedical [28], automotive [29], and pharmaceutical [30] sensing. As the modus operandi of any particular optical fibre sensor is modulation of the light source, this can be carried out via intensity, frequency, or phase modulation. The latter of which will be the main focus of the review as this is

Fibre Bragg gratings are created by periodically modifying the refractive index

of an optical fibre core. At each change of refractive index the reflected light constructively interferes producing a high intensity narrowband signal. This effect is described by Eq. (2) where λ<sup>B</sup> is the Bragg wavelength, neff is the effective refractive index, and Λ is the pitch between each of the modified refractive indices. Figure 1 provides a schematic of an FBG inscribed on a fibre core. As is evident from Eq. (2) care must be taken to eliminate, or account for, mechanical straining of the fibre as this will artificially modify the grating period. Once mechanical strain has been determined the change in Bragg wavelength with temperature is given by

Eq. (3). A review on the packaging of FBGs is provided by Hong et al. [31].

dΛ dT <sup>þ</sup> <sup>Λ</sup> �

Owing to the simple nature of their construction FBGs have been utilised to great success as a means of sensing temperature, however they are not without inherent issues such as damage due to exposure to excessive temperatures [22], and grating orientation to the heat source [32]. Zhang et al. [22] reported the assessment of cylinder head temperature and mixture flow within maritime diesel engines, where significant steps were taken to protect the fibre coating from excessive cylinder head temperatures (873.15 K). Gassino et al. [32] examined the use of FBGs in the presence of strong temperature gradients (�10 K/cm) during thermal ablation of tumours. Several key design factors were discussed from which it was determined the largest sources of error were caused by the temperature gradient along the length of the

dλ<sup>B</sup>

dT <sup>¼</sup> <sup>2</sup> neff �

FBG, and FBG orientation with respect to the temperature gradient.

Schematic of FBG, highlighting grating pitch (Λ) in fibre core.

λ<sup>B</sup> ¼ 2neff � Λ (2)

dT (3)

dneff

the method typically employed by LiF-OFTS.

DOI: http://dx.doi.org/10.5772/intechopen.80653

Review of Liquid-Filled Optical Fibre-Based Temperature Sensing

3.1 Fibre Bragg gratings

Figure 1.

99
