The spectrum of Brillouin scattered light builds up from the interactions between the different optical and acoustic modes that are allowed in the fibre.
Single-mode fibres are preferable for Brillouin D-FOS due to their fewer allowed optical modes that in-general generate a simpler and stronger Brillouin spectrum, and also due to the longer measurement range that is achievable thanks to their lower attenuation. Brillouin scattering is also observed in multi-mode fibres but in this case the acoustic waves can interact with multiple optical modes raising the intensity threshold for Brillouin scattering.
The interaction between optical and acoustic waves derives from the intimate structure of the fibre and therefore varies between the different fibre types and brands. In following sections we analyze the key elements that influence the Brillouin properties and must be considered for a proper selection of a sensing fibre.
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