Forward stimulated Brillouin scattering (FSBS), because of its resonating transverse acoustic waves, has the potential to facilitate detections in an optical fiber’s surroundings. Researchers have now demonstrated a technique to measure the distributed FSBS spectrum of an optical fiber and retrieve the local acoustic impedance of the surrounding materials. Researchers at Ecole Polytechnique Fédérale de Lausanne (EPFL) observed that the hyper-frequency wave that regularly bounces off the fiber’s walls creates an echo that varies, depending on the material the acoustic 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. According to researchers, the method could be used to sense what is going on around a fiber and send information at the same time. The optical fibers are capable of detecting what sort of material or liquid they have come into contact with. Courtesy of EPFL/Desmond Chow. To test the technique, researchers immersed optical fibers first in water and then in alcohol, before leaving them out in the open air. Each time, the fiber was able to correctly sense the change in its surroundings. According to researchers, the acoustic impedances of water and ethanol obtained by their technique agree well with the reported standard values. Changes in the fiber’s surroundings are located using a time-based method. “Each wave impulse is generated with a slight time lag," said professor Luc Thévenaz. "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. For the moment, we can locate disturbances to within around 10 meters, but we have the technical means to increase our accuracy to one meter.” According to researchers, until now it has not been possible to determine changes and events in a fiber’s surroundings without light escaping from the fiber and disrupting its path. The team’s experimental results show that the distributed acoustic impedance measurements of a material are possible using the FSBS optoacoustic interaction, without direct interaction between light and the external material. “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,” said Thévenaz. "There are many potential applications." The research was published in Nature Communications (doi: 10.1038/s41467-018-05410-2).