Since light carried by optical fibers cannot reach outside the inner core, it is difficult to use these cheap and flexible tools for the analysis of surrounding media. Fortunately, the same fibers also support the transfer of ultrasonic waves, and the interactions between light and sound waves can be exploited for probing the properties of liquids outside the protective coating.
Building on their previous research, Diamandi et al. extended their model of these light-ultrasound opto-mechanical sensors to include polyimide-coated fibers, which are readily available commercially. The coating gives the fiber some protection, and at the same time provides connectivity for the ultrasonic waves that actually perform the sensing task.
In their experiment, spectra of interaction between light and ultrasound were measured for stretches of fibers in air, ethanol and water. To push the experiment further, spatial mapping of liquids was carried out over a mile-long fiber that was coated in polyimide for its entire length. Results correctly identified and located segments of fiber that were immersed in ethanol and water, with a resolution of 100 meters. In all cases, excellent agreement was achieved between model and experiment.
The authors hope their results can help this new concept of optical fiber sensors move toward practical applications, such as in chemical processing, desalination plants, oceanography, and oil and gas industries. The results mean that it is possible to perform sensing tasks through a coating, which opens the door to possibilities for sensing through thicker coatings and in a broader variety of chemicals.
Source: “Distributed opto-mechanical analysis of liquids outside standard fibers coated with polyimide,” by Hilel Hagai Diamandi, Yosef London, Gil Bashan, and Avi Zadok, APL Photonics (2019). The article can be accessed at https://doi.org/10.1063/1.5067271.