Brillouin scattering – Distributed Strain Sensing (DSS)

Brillouin scattering is another type of inelastic scattering, in which photons exchange energy with acoustic phonons in the fibre via coherent density waves propagating through the silica at the speed of sound. These acoustic waves are either pre-existing thermal vibrations (spontaneous Brillouin scattering) or, in most commercial DSS instruments, actively driven by the light itself: two counter-propagating optical waves interfere to create a moving intensity pattern that, through electrostriction, generates a moving density wave in the glass (stimulated Brillouin scattering). In both cases the back-scattered light is shifted in frequency relative to the incident pulse by the Brillouin frequency shift, a quantity set by the local acoustic velocity in the glass and varying linearly with both mechanical strain and temperature relative to an unstrained reference. 

Distributed Strain Sensing is based on the Brillouin frequency shift to detect slow, quasi-static strain fluctuations along the cable. By isolating the strain contribution from temperature effects, for example using collocated DTS, or a hybrid cable carrying both tight-buffered and loose-tube fibres, DSS provides a continuous mechanical-strain profile along the fibre. This technology is used, among others, for monitoring structural health of the fibre and surrounding infrastructure, geotechnical applications, and pipeline integrity assessment. For example in EGS and CCS, DSS plays a crucial role in tracking high-resolution geomechanical fluctuations such as reservoir compaction, stress changes along the borehole, and potential caprock failure or leakage pathways, ensuring structural integrity, operational efficiency and safety compliance. 

References 

[1] Tuinstra, K. (2024). Enhancing Seismic Monitoring for Geothermal and Carbon Storage Applications using Fibre-Optic Sensing, Doctoral thesis, ETH Zurich. doi: https://doi.org/10.3929/ethz-b-000742567