# Optical Fibres for Condition Monitoring of Railway Infrastructure--Encouraging Data Source or
## 2 Optical Fibres--A Brief Overview
### Fibre Bragg Grating  
FBGs have been significantly more frequently applied than DFOS. This is mainly due to the easy handling and integration, simplicity and small size of the FBG. Furthermore, their linear sensing, high resolution and accuracy and wavelength multiplexing capability make FBGs the ideal sensor for local measurements. In general, FBGs are point sensors and for longer sensing distances, many FBGs arrays must be connected to realize a quasi-distributed sensing element. Theoretically, it is possible to monitor distances up to 100 km, but the maximum number a standard FBG interrogator can address is approximately 50, where such sensors are predominantly used for distances less than 1 km. In 2004, [PERSON] and [PERSON] [16; 17] used two FBGs for axle counting and derailment detection whereby eight tailormade FBG sensors were glued onto the rail beam. The measurements proved the ability of FBGs to count axles and thus derailment detection, identification of different train types and speed detection. Since then, a myriad of activities on utilizing FBGs in railways for different purposes has started. [PERSON] [18] installed FBG sensors in the rails in a straight section of a high-speed line in order to detect defective wheels. The experimental setup enabled the identification of wheels with little defects and, in the case of several flatted wheels, to identify and classify them. The obtained results were confirmed by visual inspections. [PERSON] et al. [19] carried out a similar experiment where they installed 60 FBG sensors in total (four lines, each line has 15 sensors) along a track of the subway in Milan. The sensors were placed near the sleeper to detect the strain deformation of the rail which is considered to be an indicator for the health status of both rails and wheels. The results presented the complexity in terms of distinguishing between the information coming from wheels and rails as a frequency overlap of the signal components emerges. However, the authors set up a sophisticated processing technique enabling the extraction of the individual information about the condition of rails and wheels. [PERSON] et al. [20] bonded FBG sensors to rails, aiming to demonstrate the usability of FBGs for railway monitoring and train tracking. Dynamic strain measurements were used to analyse the characteristic frequency response of the rail and for train tracking. The results confirmed the assumption that FBGs are suitable for these applications. In contrast to the studies mentioned before, [PERSON] [21] designed an array of FBG sensors in passenger cars, wheels, bogies, brake blocks, axles and a nine metre long track section in order to set up a composite evaluation system. The variation in the wavelength of the FBGs due to brake-induced temperature was considered to be an indicator of the health of brakes and axles. [PERSON] et al. [22] measured the speed of high-speed trains with a novel FBG interrogator where two FBG sensor arrays were equipped on the foot of two rail tracks. The measurements of the FBG sensors fit well together with values obtained by a digital image correlation. [PERSON] et al. [23] examined the amount of FBG sensors necessary to monitor the train speed, to determine the train direction and to measure the axle load. As a result, they stated that one FBG is able to measure the train speed but knowledge about the train configuration is mandatory. The train direction can be identified using two FGBs and at least four are necessary to determine the axle load. In order to assess the longitudinal force in continuously welded rails, [PERSON] [24] attached two FBGs on the rail track in vertical and longitudinal directions. In the experimental setup on a Chinese high-speed railway, the FBGs worked as bi-directional sensors measuring the temperature variations in the rail. The measurements were in good agreement with theoretical considerations as the measurement accuracy was over 95%. An experimental set up in was arranged by [PERSON] et al.