This illustration demonstrates how the new method allows optical fibers to detect what sort of material or liquid they have come into contact with. Image: EPFL/Desmond Chow.In recent years, optical fibers have served as sensors for detecting changes in temperature and pressure, proving particularly useful in structures such as bridges and gas pipelines. Now, researchers at the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have come up with a new method that allows optical fibers to identify whether they are in contact with a liquid or a solid by using a light beam within the fiber to generate a sound wave. The study was conducted by the Group for Fibre Optics (GFO) run by Luc Thévenaz within the School of Engineering and is reported in a paper in Nature Communications.
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, allowing 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 from it, which disrupts the light’s path.
The method developed at EPFL uses a sound wave generated inside the fiber – 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, which means the method can 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 10m, but we have the technical means to increase our accuracy to 1m."
The idea of using a sound wave in optical fibers initially came from the team's partner researchers at Bar-Ilan University in Israel.
This story is adapted from material from EPFL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.