  Optical fibres constitute an exceptional sensing platform. However, standard
fibres present an inherent sensing challenge: they confine light by design to
an inner core. Consequently, nearly all distributed fibre sensors are
restricted to the measurement of conditions that prevail within the core. This
work presents distributed analysis of media outside unmodified, standard fibre.
Measurements are based on stimulated scattering by guided acoustic modes, which
allow us to listen where we cannot look. The protocol works around a major
difficulty: guided acoustic waves induce forward scattering, which cannot be
mapped directly based on time-of-flight. The solution relies on mapping the
Rayleigh backscatter contributions of two optical tones, which are coupled by
the acoustic wave. Analysis is demonstrated over 3 km of fibre with 100 m
resolution. Measurements distinguish between air, ethanol and water outside the
cladding, and between air and water outside polyimide-coated fibres. The
results establish a new sensor configuration: opto-mechanical time-domain
reflectometry, with several potential applications.
