Guided acoustic waves Brillouin scattering (GAWBS) processes in standard optical fibers allow for sensing of liquids outside the cladding boundary, where light does not reach. Optical waves stimulate the oscillations of elastic modes of the fiber. The linewidths of these modes, in turn, depend on the mechanical impedance of surrounding media. These linewidths are monitored through photo-elastic scattering of optical probe waves. However, acoustic guided waves scatter light in the forward direction. The timing of forward-scattering event cannot be identified directly, hence distributed analysis based on GAWBS could not be performed, and measurements were restricted to point-sensing only. In this work we show a way around the problem. The exchange of optical power among two optical tones that stimulate the acoustic wave is monitored through careful analysis of Rayleigh back-scatter. Distributed GAWBS analysis is performed over 3 km of fiber with resolution between 100-200 meters. The measurements successfully distinguish between water and ethanol based on mechanical impedance.
Gil Bashan, H. Hagai Diamandi, Yosef London, Eyal Preter, and Avinoam Zadok, "Distributed analysis of guided acoustic waves Brillouin scattering: a new class of fiber sensor (Conference Presentation)," Proc. SPIE 10528, Optical Components and Materials XV, 105280G (Presented at SPIE OPTO: January 30, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2297585.5751448249001.
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