Fibre optic sensing represents a significant advancement in the utilisation of fibre-optic infrastructure. The same fibre technology that has revolutionised telecommunications can now be leveraged to continuously monitor and detect physical changes and events in the environment surrounding critical infrastructure. What is fibre optic sensing? The concept of fibre optic sensing is to treat fibre itself as the sensor by creating thousands of continuous sensing points along the fibre length. This phenomenon is called distributed fibre optic sensing, where the optical fibre itself acts as a distributed fibre optic sensor. Principle of operation Fibre sensing work is based on the phenomenon of backscattering that occurs due to interaction between light and the glass fibre. Different sensing technologies analyse different components of the backscattered optical signal. An instrument generally called fibre sensing interrogator is used to send optical pulses into a fibre and analyses the light returning from different points along the fibre. Changes in the characteristics of this returned light can reveal what is happening at specific locations. The main purpose is to use a standard or specific fibre for measuring the strain, temperature or vibrations along its length, using Raman, Brillouin or Coherent Rayleigh backscattering fibre sensing techniques. - DTS (Distributed temperature sensing): It incorporates a Raman OTDR. Raman scattering produces temperature-induced changes in photons scattered back to the source in the Stokes band. By measuring the difference between the intensity of backscattered light in the Stokes and anti-Stokes bands, the temperature can be accurately determined at any given location along the fibre length. - DTSS (Distributed temperature and strain sensing): It incorporates Brillouin OTDR. Brillouin scattering is a phenomenon where the backscattered light wavelength is influenced by the external temperature and acoustic stimulation in a predictable way. This data, when coupled with background knowledge of temperature at the