Researchers from Bar-Ilan University succeeded in developing an innovative chemical sensing method based on optical fibers that use the light passing through them to create sound waves outside of them, while receiving indirect information about the environment surrounding the fiber. The new method could improve sensing capabilities for a wide variety of applications, including industrial processes and remote chemical detection
[Translation by Dr. Nachmani Moshe]
Researchers from Bar-Ilan University succeeded in developing an innovative chemical sensing method based on optical fibers that use the light passing through them to create sound waves outside of them, while receiving indirect information about the environment surrounding the fiber. The new method could improve sensing capabilities for a wide variety of applications, including industrial processes and remote detection of chemicals.
"A basic limitation that makes the sensing of chemicals a challenging task is that there must be an interaction between the light and the substance being tested and measured," explains Professor Avi Zadok from the Faculty of Engineering and the Nanotechnology Institute at Bar-Ilan University, who led the research team. "This limitation has held us back for many years, us researchers in the field of optical fiber-based detectors." The research findings have long been published in the scientific journal Optica. "We managed to harness the interplay between light and sound, and we used sound as our messenger to the outside world."
Optical fibers are an efficient and convenient means of chemical sensing due to their very small size, they can be used remotely (we can use them to measure chemicals that are miles away from us) and can be installed almost anywhere, including in dangerous environments such as oil wells where the use of electricity is prohibited. At the same time, existing sensing technologies based on optical fibers force the light passing through the fiber to come into direct contact with the material being tested, a result that contradicts the purpose of optical fibers: to ensure that the light is not emitted. In the past, attempts to overcome this limitation required the introduction of significant changes, for example, punching holes in the fiber or canning it to obtain an extremely narrow diameter that would force the light to be emitted. "Despite the fact that high levels of sensitivity can be achieved with these approaches, the need to make these changes makes it difficult to produce this type of detectors and even affects their stability," says Zadok.
Instead of using light directly, the researchers came up with an idea that makes it possible to use the light transmitted through the optical fiber to create sound waves, or acoustic vibrations, while taking advantage of a phenomenon known as 'stimulated Brillouin scattering'. Despite the fact that this mechanism is currently used in commercial optical fiber detectors, the existing detectors preserve both the light and the sound waves inside the fiber. "The crucial step was to find certain mechanical behaviors that manage to get outside the fiber, and to take advantage of these behaviors," explains the lead researcher. "We found that the scattering mechanism in the direction of the front can be used to obtain information regarding what is happening outside the fiber." The innovative approach is based on the use of optical waves powerful enough to create an acoustic vibration that reaches outside the fiber. These fluctuations gradually fade depending on the properties of the tested material around the fiber, a result that provides an indirect method for sensing the chemical content of the environment in which the fiber is located. Thanks to the fact that the light remains inside the fiber, the method allows the use of ordinary optical fibers that have not undergone any modification, when all the researchers have to do is to remove the plastic protective covering of the fiber.
The researchers demonstrated the ability of their method with ethanol and deionized water, while measuring the acoustic vibrations that indicate the density of the liquid and the speed of the sound waves moving through that liquid. The results obtained were close to known values with an accuracy of 1%. In addition, the researchers were able to use the method to differentiate between water samples with different concentrations of salinity. In the future, the method could be used to monitor the desalination of water, for applications of electrochemical processes, such as fuel cells, or to measure the changes in concentrations of ions or dissolved salts used in chemical processes in industry. It may be possible to adapt the new method also for the discovery of specific chemicals. The researchers believe that by changing the outer surface of the fiber they will be able to attract the desired material, and thus the acoustic waves will measurably change when the chemical material binds to the fiber. Despite the fact that further development and research is still needed, such an approach, if successful, could be useful for detecting explosives or disease-causing substances. "Although of course we would like to explore more possible applications in the future, in this work we focused on solving the problem that seemed like a paradox: it is not possible to be both inside the fiber and outside it at the same time", says the researcher. "We found a way around this paradox."
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