Optical fibres for Brillouin sensing
Brillouin scattering derives from the interaction between light and acoustic waves that are generated by the same light. Due to the electrostriction in the fibre, the electrical field of the lightwave induces periodic longitudinal changes of the core refractive index that also move along the fibre as acoustic waves and interact with a small fraction of generating lightwave by Doppler-shifting its wavelength.
Thanks to the fact that such Doppler shift depends on physical parameters of the fibre such as the strain and temperature, it can be used for sensing.
Brillouin D-FOS requires identifying the peak intensity of Brillouin scattered light and measuring its wavelength with respect to that of the pump lightwave with great accuracy. The shape of Brillouin spectrum varies from one fibre type to another and it influences how easily the peak intensity can be identified and how accurately its wavelength shift can be measured. In addition, fibre coating, ageing and handling also influence its durability and sensing performance.
Due to this, a proper selection of the sensing fibre holds a role of primary importance.