A low-cost intensity-based plastic optical fiber sensing system for permanent structural health monitoring

Fiber optic sensors are attracting an increasing interest in many applications because of their high sensitivity combined with other unique properties such as immunity to electrostatic discharges, intrinsic fire safety and minimum invasiveness. However, their wide diffusion in structural permanent monitoring is still hampered by costs, especially those related to the interrogators and installation: hence the necessity of providing Municipalities and other Authorities with low-cost fiber-based measuring setups to widen their deployment also in structures those are relevant for the local community only. This is particularly important in cultural heritage preservation applications, such as the monitoring of the crack evolution of ancient buildings. Among the solutions that have been proposed in the literature [1], this paper discusses a simple and cost effective fiber displacement measurement system. This system, which employs plastic optical fibers (POF) for the displacement transducer has already been successfully employed to monitor the evolutions of cracks in different cases. In particular, the results of an almost two-year in-situ test in a prestigious monument in Venice are given, highlighting the impact of perturbing effects in long term measurements and suggesting compensation techniques. The usage of POF accounts for having most of the advantages of fiber sensors without their usual costs and complexities, since it is possible to use cheap sources and to simplify the connectors, given the larger core diameter and the much higher numerical aperture typical of POF.