System measures strain, temperature and vibration


Researchers in China have shown how measurements of strain, temperature and vibration can be made simultaneously on a single optical fibre. This could be used to create a fibre-based system to monitor a number of different physical parameters in infrastructure, such as buildings, bridges and railways.

The work was carried out by Xinyu Fan and colleagues at Shanghai Jiao Tong University and their system uses two successive light pulses to measure three different types of light scattering in real time.

Distributed fibre-optic sensing (DFOS) is used for monitoring large, complex infrastructures. Optical fibres are attached to parts of the infrastructure and are subject to high-intensity light pulses. Some of this light will interact with the fibre and be scattered back to the source, where it is detected.

This backscattered light contains information about the state of the fibre, including its temperature, strain and mechanical vibrations. This information is extracted in real time with the help of specialised algorithms. A key benefit of this ‘single end’ approach is that it can monitor the entire length of an optical fibre, which can be km in length, using equipment installed at one end of the fibre.
However, current DFOS sensors tend to only measure a single parameter at a time, which limits their use. This limitation arises because DFOS involves three different types of scattering and each type needs to be monitored with a different approach, so combining the three had been a costly and complicated endeavour.

One scattering effect used in DFOS is Rayleigh backscattering, which occurs when the incident light scatters from microscopic density fluctuations in the fibre. This scattering arises because the refractive index of the fibre is related to its density. By monitoring the interference between this backscattered light and a reference beam, researchers can monitor mechanical vibrations in the fibre with extreme accuracy.

Another relevant effect is Brillouin scattering, which occurs when incident photons interact with acoustic phonons. The latter are quantised with long-wavelength vibrations in the fibre. This results in a shift (up or down) in the energy of scattered photons and is detected as a change in the wavelength of some of the detected light. Brillouin scattering provides information about strain and temperature in the fibre.

The third interaction is Raman scattering, which is similar to Brillouin scattering but involves interactions between photons and shorter-wavelength optical phonons in the fibre. This interaction is used to monitor the fibre’s temperature alone, without being affected by strain.

Fan and colleagues have now developed a hybrid DFOS system that monitors all three of these scattering processes at once. This is achieved by firing two successive light pulses into the fibre and analysing the light that comes back. The first pulse is optimised to measure Rayleigh backscattering, while the second is optimised for measuring Brillouin scattering. Raman scattering data can be extracted independently from measurements on both pulses.

The team has been able to achieve the simultaneous measurement of strain, temperature and vibration using a single fibre that was about 9 km in length.

With the early success of their new approach to DFOS, the team now hopes its technology could soon be adapted to monitor complex large-scale infrastructures in real time.

Such systems could be a great help in the development of ‘smart cities’, in which devices, systems and sensors are seamlessly interconnected to monitor the patterns, trends and needs that emerge in complex urban landscapes. In turn, this could help to boost the efficiency and sustainability of these communities, while improving the quality of life for the many people who call them home.