A distributed clad mode optical fiber sensor is reported for the first time. Random-access coupling of light to a clad mode of a standard single-mode fiber is achieved using Brillouin dynamic gratings. Coupling is restricted to a single, few-centimeters-long section that is scanned along a fiber under test. No permanent gratings are required, and all optical fields are launched and detected in the core mode. The coupling spectrum is affected by the local refractive index of the substance outside the cladding. Distributed mapping of surrounding media is reported over 2 meters of fiber with 8 centimeters resolution. The sensitivity of outside index measurements is between 4e-4 and 4e-3 refractive index units.
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.
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