# Optical Fibres for Condition Monitoring of Railway Infrastructure--Encouraging Data Source or
## 2 Optical Fibres--A Brief Overview
### Distributed Fibre Optical Sensing (DFOS)  
Distributed sensors are mainly based on optical scattering, such as Rayleigh, Brillouin and Raman scattering [36]. They are useful in the monitoring and measuring of long linear assets including areas such as (1) the leakage detection of pipelines; (2) embedded sensors in composite materials; (3) temperature monitoring in reactor systems and (4) stress monitoring of bridges and buildings [15]. A light wave, which travels through a medium, interacts with the individual atoms and molecules. As each medium has its resonance, the electric field will only induce a time-dependent polarization dipole if the light wave's wavelength is far from a medium resonance. A secondary, light-scattering electromagnetic wave is generated by the induced dipole. Compared to the wavelength of light, the distances between the scattering particles are smaller and lead to coherent secondary light waves. Consequently, the resulting intensity is the addition of scattered fields. Light scattering, however, is a random statistical process and occurs in all directions. Small inhomogeneities and impurities within the fibre are the reason for density variations and attenuate the forward propagating signal, which is proportional to 1/A\({}^{4}\) (\(\lambda\) = wavelength), as well as the creation of the backward propagating wave. This effect is called Rayleigh scattering and is known as an elastic and linear scattering without any frequency shift. Hence, the frequency of the incidental light is the same as that of the scattered light and the scattered power is proportional to the incidental power. The frequency spectrum of Rayleigh, Brillouin and Raman scattered light is illustrated in Figure 1. The scattering peaks on both sides of the central Rayleigh peak are termed Brillouin scattering components. They originate from light interactions with propagating sound waves. The interaction between the incident light and molecular vibrations is known as Raman scattering. In contrast to Rayleigh scattering, both Brillouin and Raman scatterings are associated with frequency shifts and are thus characterized as inelastic scatterings. If scattering is shifted down to lower frequencies, it is termed Stokes component. Otherwise, upshifted frequencies are termed anti-Stokes frequencies [37; 38]. In addition to the classification made previously, a further distinction is made between spontaneous and stimulated scattering. While for the former, the optical properties of the medium do not change during the process, for the latter they are altered by interaction with the incident light. Brillouin and Rayleigh scattering-based sensors have been widely used and examined. For diverse applications, various sensors have been developed and applied but, especially the latter sensors and those for railway monitoring, are of particular interest. The performance of DFOS is determined by various facets, some of which are considered below as they are decisive:  
1. operating range--total length of an optical fibre that can be monitored by the interrogator unit. 2. gauge length--the length a measurement is made over. 3. spatial resolution--the segment length along the fibre that can be measured as an output (later referred to as a channel). 4. sampling rate--numbers of samples obtained per seconds. The operating range is, in general, a highly theoretical value provided by the supplier that the maximum length a system is able to monitor. The spatial resolution is of utmost importance for a distributed sensing system as it refers to the spacing difference between the channels by which the measurands are given. The gauge length, spatial resolution and sampling rate differ, however, and are subject to changes affecting the sensing sensitivity of fibres. Longer gauge lengths result in an impaired spatial resolution but enable longer monitoring distances and vice versa. Shorter gauge lengths lead to an improved spatial resolution (down to 30 cm) but reduce the operating range significantly.