# Optical Fibres for Condition Monitoring of Railway Infrastructure--Encouraging Data Source or
## 2 Optical Fibres--A Brief Overview
### Distributed Fibre Optical Sensing (DFOS)
The capabilities are defined by the sensing technique utilised and differ enormously. However, narrower optical pulses are received with a lower optical power and result in shorter sensing distances. A pulse duration 10 ns and a fibre group refractive index of 1.5 corresponds to a 1 m spatial resolution. In 2013, [PERSON] et al. [39] utilized a slope assisted Brillouin scattering optical time-domain analysis (BOTDA) interrogation technique for real-time traffic monitoring. The collected data identified the location of trains, counted the number of axles, measured the train velocity and monitored the dynamic loading conditions for dynamic strain measurements in order to calculate the rail bed modulus. Another test focussing on the monitoring of local strain and track displacement under the load of a locomotive and a passenger coach was performed by [PERSON] et al. [40] on a 34 m long rail track based on the principle of Brillouin backscattering. The results showed that in the case of the locomotive, the axles were hard to determine, and the vertical deformation measured was less than 2 mm. For the  
Figure 1: Typical spontaneous light scattering spectrum. passenger coach, the vertical deformation was over 5 mm. One of the first more extensive field tests was carried out by [PERSON] et al. [41], whereby a length of over 25 km strain and the temperature at the joint of rails based on BOTDA were monitored. The spatial resolution was \(\sim\)2 cm and the results concluded that the gage length of the sensor required should be at least 1.5 mm. However, for this study, a Rayleigh backscatter-based system is used whereby several interrogation techniques exist. Optical frequency domain reflectometry (OFDR) provides a short gauge length and notably improved spatial resolution (in the range of mm) but has shortcomings such as a limited sensing range and interrogation bandwidth. OFDR was applied by [PERSON] et al. [42] for measurements of rail displacements under low vibration conditions on a 2.8 m long rail section. The results were compared with the results obtained from digital image correlation cameras and had a difference of 0.3 mm. OFDR failed to monitor rail displacement under a high vibration induced by fast moving trains. In contrast to that [PERSON] et al. [43] focussed on a ballastless track and proposed an OFDR sensing system to monitor the long term strain distribution in the concrete slab. Another DAS technique with a longer gauge length, but a shorter lower spatial resolution, is phase-sensitive optical time-domain reflectometry (\(\phi\)-OTDR) and requires a coherent light source with direct phase detection. [PERSON] et al. [44] first proposed a phase-based system for intrusion sensing in 2005. In a field test, a cabled single mode fibre was buried along a monitored perimeter and detected people walking above the buried cable. Since then this class of sensor has been widely applied, including for the monitoring of train speed, detection of seafloor faults or monitoring of reservoirs. [PERSON] [45] carried out a 10.2 km field test with a spatial resolution of 20 m for the position monitoring and speed monitoring of trains. [PERSON] et al. were able to detect seafloor faults in the ocean over an 20-km-long seismic array [46]. [PERSON] et al. [47] concluded that DAS poses an alternative to 3D vertical seismic profiling (VSP), which is used for illuminating targets under complex overburden for exploration purposes and for the time-lapse monitoring of reservoirs. The condition of the railway subgrade influences the quality of a track enormously and thus the condition needs to be monitored. [PERSON] et al. [48] developed a vibration monitoring method based on phase-sensitive OTDR for railway safety and verified it by a field test over 20 km. With the developed system it is possible to monitor the subgrade vibration induced by passing trains.