Feeling with Optical Fibers

Illustration of optical fibers capable of detecting what sort of material or liquid they have come into contact with. (Source: Chow, EPFL)

In recent years optical fibers have served as sensors to detect changes in tempera­ture, like a thermo­meter, and pressure, like an arti­ficial nerve. This technique is particularly useful in structures such as bridges and gas pipelines. EPFL researchers have now come up with a new method that enables optical fibers to identify whether they are in contact with a liquid or a solid. This is achieved by simply gene­rating a sound wave with the help from a light beam within the fiber. This study was conducted by the Group for Fibre Optics (GFO) run by Luc Thévenaz within the School of Engi­neering.

No wider than a strand of hair, an optical fiber made of glass trans­mits light that varies according to four para­meters: intensity, phase, polarization and wave­length. These para­meters are altered when the fiber is stretched or the tempera­ture changes, enabling the fiber to act like a sensor by detecting cracks in structures or abnormal tempera­tures. But up to now it was not possible to determine what was happening around the fiber without having light escape the fibre, which disrupts its path.

The method developed at EPFL uses a sound wave generated inside the fiber. It is a hyper-frequency wave that regularly bounces off the fiber’s walls. This echo varies at different loca­tions depending on the material the wave comes into contact with. The echoes leave an imprint on the light that can be read when the beam exits the fiber, making it possible to map out the fiber’s sur­roundings. This imprint is so faint that it hardly disturbs the light propa­gating within the fiber. The method could be used to sense what is going on around a fiber and send light-based infor­mation at the same time.

The researchers have already immersed their fibers in water and then in alcohol, before leaving them out in the open air. Each time, their system was capable of correctly identi­fying the change in the sur­roundings. “Our technique will make it possible to detect water leakages, as well as the density and sali­nity of fluids that come into contact with the fiber. There are many potential appli­cations,” says Thévenaz.

These changes in the surroun­dings are located thanks to a simple time-based method. “Each wave impulse is generated with a slight time lag. And this delay is reflected upon the beam’s arrival. If there were any distur­bances along the way, we can both see what they were and deter­mine their location,” explains Thévenaz. “For the moment, we can locate distur­bances to within around ten meters, but we have the technical means to increase our accuracy to one meter.” The idea of using a sound wave in optical fibers ini­tially came from the team’s partner researchers at Bar-Ilan Univer­sity in Israel. Joint research projects should follow. (Source: EPFL)

Reference: D. M. Chow et al.: Distributed forward Brillouin sensor based on local light phase recovery, Nat. Commun. 9, 2990 (2018); DOI: 10.1038/s41467-018-05410-2

Link: Fiber Optics, Institute of Electrical Engineering, EPFL Swiss Federal Institute of Technology, Lausanne, Switzerland

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