We describe novel designs of sensor made from standard silica fibres with oblique facets polished at the tip, and various surface treatments. They use Total Internal Reflection to distinguish immiscible fluids in a multi-phase flow.
Rogerio T. Ramos,
"Fluid discrimination in multi-phase flow", Proc. SPIE 3746, 13th International Conference on Optical Fiber Sensors, 37462R (1 September 1999); doi: 10.1117/12.2302089; https://doi.org/10.1117/12.2302089