  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
intermodal scattering of a third optical probe wave. All waves are launched and
collected inthe core mode. No permanent gratings are inscribed. Distributed
sensing of surrounding media is demonstrated with 2 m range and 8 cm
resolution. Measurements distinguish between water and ethanol outside the
fiber. The measurement accuracty of the local index outside the cladding is
0.004 - 0.0004 refractive index units.
