3. Theory and working principles

In the use of optical fibres for temperature sensing, measurements can be conducted using both intrinsic and extrinsic techniques. The most common technique for assessing temperature changes using optical fibres is the use of interferometry. Examples being fibre Bragg gratings (FBGs), extrinsic Fabry-Perot interferometers (EFPIs), Mach-Zehnder interferometers (MZIs) and Sagnac interferometers (SIs). As the fibre requires a light source to conduct the measurement, various types have been explored such as; broadband light source [26] monochrome laser [21] and swept laser [27]. Measurements can be carried out by observing either the transmitted spectrum, or the reflected spectrum. When recording the reflected spectrum losses occur in the back reflections through the fibre and at the coupler between the light source and spectrometer. That said however, this method allows

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 the method typically employed by LiF-OFTS.

## 3.1 Fibre Bragg gratings

2.1 Construction

Applications of Optical Fibers for Sensing

2.2 Measurement stability

environment.

98

2.3 Distributed sensing

temperature sensing performance [25].

3. Theory and working principles

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].

In civil engineering applications, such as bridges, use of chirped fibre Bragg gratings (FBGs) provide excellent through life performance as fibre degradation is in excess of 25 years [24] with data transmission losses being minimal [23]. Furthermore, stability of measurements are generally quite good due to performance being driven predominantly by the wavelength stability of light source used. This being an easy factor to account for, as said light source may be kept in a controlled

In addition to the EMI resistance and dimensional advantages of using optical fibre sensors, their ability to provide multipoint sensing with minimal use of fibre is a desirable characteristic. One such method is the use of chirped FBGs whereby multiple FBGs are inscribed on a single fibre, with the grating period modified at each location thus providing high spatial resolution. Wave division multiplexing (WDM) is another commonly used method in multi-point sensing with optical fibres, however this method narrows the usable bandwidth of light proportionally to the number of fibres used. Historically, Raman scattering has been used as an efficient means of multi-point temperature measuring; however, Brillouin sensors reported have shown exceptional strain measurement capability for an equivalent

In the use of optical fibres for temperature sensing, measurements can be conducted using both intrinsic and extrinsic techniques. The most common technique for assessing temperature changes using optical fibres is the use of interferometry. Examples being fibre Bragg gratings (FBGs), extrinsic Fabry-Perot interferometers (EFPIs), Mach-Zehnder interferometers (MZIs) and Sagnac interferometers (SIs). As the fibre requires a light source to conduct the measurement, various types have been explored such as; broadband light source [26] monochrome laser [21] and swept laser [27]. Measurements can be carried out by observing either the transmitted spectrum, or the reflected spectrum. When recording the reflected spectrum losses occur in the back reflections through the fibre and at the coupler between the light source and spectrometer. That said however, this method allows

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].

$$
\lambda\_B = 2n\_{\rm eff} \cdot \Lambda \tag{2}
$$

$$\frac{d\lambda\_B}{dT} = 2\left(n\_{\ell\overline{\ell}} \cdot \frac{d\Lambda}{dT} + \Lambda \cdot \frac{dn\_{\ell\overline{\ell}}}{dT}\right) \tag{3}$$

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

Figure 1. Schematic of FBG, highlighting grating pitch (Λ) in fibre core.
