2.2 Measurement stability

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 environment.

## 2.3 Distributed sensing

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 temperature sensing performance [25].
