In summary, the sensing mechanism of distributed optical fiber sensors is based on three scattering phenomena that occur within the optical fiber. Rayleigh scattering is a result of light interacting with microscopic irregularities in the fiber. This type of scattering is elastic, meaning the input light and scattered light have the same frequency. The primary application of Rayleigh scattering is to measure the loss of optical fiber, and this technology is known as Optical Time-Domain Reflectometer (OTDR). Additionally, when a highly coherent laser is available, and we can demodulate the phase of Rayleigh scattering, it becomes useful for strain change measurement, and this technology is called Distributed Acoustic Sensor (DAS).
Raman scattering, on the other hand, is a consequence of light interacting with optical phonons, and it is an inelastic scattering process. Since light can transfer a relatively large amount of energy during Raman scattering, it results in a significant frequency shift of around 13 THz. Thanks to Boltzmann’s law, Raman scattering’s intensity is sensitive to temperature. Hence, we can build a distributed temperature sensor based on Raman scattering.
Next, Brillouin scattering is the outcome of light interacting with acoustic phonons and is also a form of inelastic scattering. However, the energy transfer in Brillouin scattering is relatively small, leading to a smaller frequency shift of around 11 GHz. Brillouin scattering is closely related to the acoustic velocity in the optical fiber, which, in turn, is influenced by both temperature and strain. Therefore, we can develop distributed temperature and strain sensors based on Brillouin scattering.
By leveraging the unique characteristics of these scattering phenomena, distributed optical fiber sensors provide an effective means for measuring temperature, strain, and strain change over extended distances. These sensing technologies play a vital role in various applications, including structural health monitoring, industrial process control, and environmental monitoring. To see some of our past projects, you can check here.