Distributed Brillouin gain measurements along single mode fibres have demonstrated their potentiality for local temperature sensing and monitoring strains experienced by optical fibres. The results of such measurements is directly related to the acoustic properties of the fibre medium that are temperature- and strain-dependent. The Brillouin gain spectrum (BGS) is classically measured by launching a highly coherent pump lightwave into a test fibre and by observing the amplification of a weak probe signal propagating in the backward direction