Civil & Geotechnical
Unlike "traditional" sensors, which are used to take a point measurement, fiber optic sensors allow the measurement to be distributed along the optical fiber. There are two ways of using fiber as a sensor.
The first is to inscribe Fiber Bragg Grating (FBG) onto the optical fiber. These Bragg gratings act as wavelength-selective mirrors inside the fiber. The property of these Bragg gratings is that their spectral selectivity depends on the structure of the grating. Depending on the mechanical stresses of the fiber, the deformations undergone or temperature variations, the spectral response of the grating varies. By studying this variation in the grating's spectral response, we can trace it back to the individual measurands. This method makes it possible to place different measurement points along an optical fiber over a very long distance (several kolimeters) for monitoring purposes. This is known as quasi-distributed measurement.
The example (on the right) illustrates the operation of a Bragg grating interrogator: Hyperion, with an optical fiber placed on the rail of a railroad to measure the local deformations of the rail induced by the train wheels. The optical fiber, on which a Bragg grating is inscribed, is laid directly and glued to the rail structure. Rail deformations are directly transferred to the optical fiber. As a result, the geometry of the bragg grating is altered, resulting in a change in the spectrum of the reflected light signal for our interrogator. Bragg gratings can be used to measure temperature, deformation, displacement, tilt, acceleration, vibration and pressure.
A second approach is to consider the entire optical fiber as a sensor. In this case, it is no longer necessary to inscribe bragg gratings on the fiber in order to define zones of interest; the use of simple telecom fiber is sufficient. In this case, we speak of a distributed measurand measurement. This method makes it possible to carry out static or dynamic measurements with very good spatial resolution (equivalent to the installation of thousands of conventional sensors). The example opposite illustrates the use of an optical fiber attached to a metal reinforcement designed to be immersed in concrete to form a pilot. With the optical fiber integrated into the structure, it is possible to evaluate the drying of the concrete as a function of the temperature released by the chemical reaction as the concrete solidifies. Once the structure is dry, the fiber integrated into the concrete-covered reinforcement will translate the stresses undergone by the assembly, as well as the deformations induced.
These distributed measurements are carried out using a Rayleigh interrogator called ODiSI, designed to measure deformations and temperatures with micrometric spatial resolution over several hundred meters!
Finally, there's a much wider field where it would be impossible to list every possible application, but civil engineering in general offers a wide range of applications. Indeed, the instrumentation of architectural structures, whether new or old, is one of the main applications of distributed fiber optic measurement in civil engineering.
One example is the famous Polcevera Viadic (more commonly known as the Morandi Bridge, in reference to its designer Riccardo Morandi), which collapsed in August 2018. Rebuilt after a year, the new bridge is now instrumented by a multitude of distributed optical fiber sensing systems that transcribe the aging of the bridge by measuring the deformation of the materials in order to monitor the bridge aging. In addition to deformation, it is also possible to measure temperature variations using DTS (Distributed Temperature Sensing) instruments. This type of instrumentation can also be found in tunnels, as in the case of the Mont-Blanc tunnel, where the ceiling has been fitted with a Ramand DTS dedicated to fire monitoring.
In geotechnical applications, it is common practice to use DAS (Distributed Acoustic Sensing) acoustic interrogators for train monitoring or soil analysis, for example.
By installing optical fibers along the tracks, the acoustic vibrations generated when a train passes can be used to determine its position in real time, as well as its speed and dimensions, enabling the train in question to be identified. This information helps to keep traffic flowing smoothly, and above all to detect any malfunctions caused by a train stopped on a track. At the same time, these interrogators can be used to image the ground under the tracks, to ensure that the soil is dense enough to support the track, which can sometimes be fragile or even contain underground cavities.
In the oil industry, DAS are also used for soil mapping, enabling the operator to determine the probability of the presence of geological formations likely to contain hydrocarbons. This is then used to plan "exploration" drilling and, during the operating phase, to assess changes in soil structure indicating where hydrocarbons may be found.