Distributed cladding mode fiber optic sensor

The analysis of substances outside the cladding has challenged the optical fiber sensors community for decades. A common solution relies on the propagation of light in cladding modes. However, the coupling of light to/from these modes is typically based on permanent gratings in specific locations, which restrict the sensors to point-measurements only. In this work, we present dynamic, random-access coupling of light between core and cladding modes of standard fibers, in arbitrarily-located short sections. Coupling is based on the stimulation of Brillouin dynamic gratings by two coded pump waves, and inter-modal scattering of a third optical probe wave. All waves are launched and collected in the core mode. No permanent gratings are inscribed. Distributed sensing of surrounding media is demonstrated with 2 meters range and 8 centimeters resolution. Measurements distinguish between water and ethanol outside the fiber. The measurement accuracy of local index outside the cladding is 0.004-0.0004 refractive index units.