DISTRIBUTED FIBER OPTIC SENSING USING GUIDED ACOUSTIC MODES
Optical fibers serve as an excellent sensing platform. However, the sensing of chemicals using standard fibers faces an inherent challenge. Light is confined to the inner core, whereas the test media of interest are outside the boundary of the fiber cladding, and even its coating. Standard optical measurement principles, such as absorption, refraction, scattering or fluorescence, all require spatial overlap between the light and the substance under test. Hence, they are inapplicable to standard fibers. Fiber-optic sensors for chemical analysis involve non-standard geometries, non-standard materials, and/or significant structural modifications of the fiber.
Sensing protocol and system are proposed based on the Opto-Mechanical properties of the fiber. Light in standard fibers is used to stimulate the oscillations of guided acoustic modes of the entire fiber cladding. Unlike guided light, these modes span the entire cross-section of the fiber cladding and reach its outer boundary. Their oscillations are therefore affected by the properties of surrounding media. The acoustic waves are monitored by optical probe light, which is again confined to the inner core. The mechanical attributes of liquids outside the cladding can be inferred from the analysis of such opto-mechanical interactions.