This paper describes several novel designs of fiber-optic sensor made from standard silica fibers with plane oblique facets polished at the fiber tip, and various surface treatments. All use the phenomenon of total internal reflection (TIR) to distinguish drops, bubbles or other moieties of immiscible fluids in a multiphase flow, on the basis of contrast in refractive index. Surface treatments for control of wettability can be critical to some of the conceptions. Depending on the geometry chosen, sensors with quasibinary outputs may be made to distinguish various pairs of immiscible fluids (liquid/gas or oil/"not-oil"). Sensor pairs may then be used for discrimination of three mutually immiscible phases (oil, water, gas) in complex flows. Example outputs from prototype sensors in laboratory test flows are shown.
Rogerio T. Ramos and Edmund J. Fordham, "Oblique-Tip Fiber-Optic Sensors for Multiphase Fluid Discrimination," J. Lightwave Technol. 17, 1392- (1999)