An optical sensing system uses light scattered from a sensing fibre to
sense conditions along the fibre, and has a receiver with a frequency to
amplitude converter to obtain a frequency of a Brillouin component of the
received scattered light, to deduce the conditions. This converter can
avoid time consuming scanning of frequencies to obtain the Brillouin
frequency spectrum, and avoids the heavy processing load of deducing a
peak or average frequency from the spectrum. The converter can be
implemented in the optical domain using a grating or interferometer, or
in the electrical domain using a diplexer or electrical interferometer.
It can generate complementary signals, having opposite signs, a ratio of
these signals representing the frequency. This can avoid sensitivity to
amplitude changes in the received scattered signals and provide common
mode rejection of noise.