Light is largely used to investigate matter using several basic principles, in the same way a driver uses car headlights to obtain information of the road in the darkness. Often light is used for monitoring the condition of the leading communication medium, the optical fiber. For example, an infrared light pulse is used to measure fiber losses through backscattering or to evaluate the temperature and strain stress along the fiber through Brillouin scattering.
The project DIVAS (DIstributed Vibrational and Acoustic Sensing technology) developed a system that turns the standard communication optical fiber in a distributed, point resolved, acoustic sensor through the Phase-OTDR (i.e. Phase-sensitive Optical Time Domain Reflectometry) principle. This method uses highly coherent pulses of light that turn each infinitesimal segment of the optical fiber into an interferometric device that provides an incredible sensitivity to vibrations through light backscattering phenomena. We can imagine the optical fiber as a km-long and extremely sensitive distributed microphone with a very wide bandwidth. Each infinitesimal segment contributes to the creation of an interferometric signal, the amplitude of which is proportional to the change of stress or temperature in the corresponding segment of the optical fiber. The change of stress occurs due to the propagation of vibrations through the fiber. Therefore, if the fiber is attached to a long structure, such as a bridge or a wind turbine, we can use it to monitor the vibrations through this structure and interrogate its integrity in order to monitor its condition and prevent damages.
The high level of sensitivity of the DIVAS system renders it capable of the detection of very low amplitude acoustic and ultrasonic vibrations, which is useful for the detection of seismic events, high-speed train location and identification, security systems for intruder detection, fluid flow monitoring in pipelines, or the structural health monitoring of bridges, wind turbines, nuclear plants, dams, and electric power transmission lines, among others. The system was developed by a series of innovative engineering solutions, which decreased dramatically its cost to below 10 k€ with a spatial resolution in vibration detection below 6 m and a detection range over 5 km. The combination of the system’s performance and cost will help distribute the, so far uncommon, Phase-OTDR technology widely, in order to take advantage of its capabilities.
