Fibre optic sensing is used to measure the temperature, strain, and acoustics at points along a fibre optic cable. Light is reflected or backscattered as it propagates through an optical fibre in response to a change in temperature, a bending or pulling force, or mechanical waves in the fibre’s proximity. The backscattered light is detected at the source, and the location and cause of the backscatter event can be determined.
The two main techniques used for fibre optic sensing include “distributed sensing” and “discrete sensing (fibre Bragg grating sensing)”.
Distributed sensing is used to determine the location and cause of a backscatter event by launching a series of laser pulses into standard optical fibre at recorded times. The pulse is backscattered if the fibre is strained or subjected to a localised change in temperature. Backscattered pulses return to the source and are detected.
The distance to the backscatter event relative to the source is inferred through the pulse optical travel time (“time-of-flight”).
Several different scattering processes can occur, broadening the spectrum of the returned pulse. The cause of the backscatter event is therefore determined by measuring the spectrum. If a photon loses energy in the scattering process, its wavelength increases (“Stokes shift”). If a photon gains energy in the scattering process, its wavelength decreases (“anti-Stokes shift”). Photons can also be scattered with no transfer of energy. This is known as elastic scattering (“Rayleigh scattering”).
The intensity of the anti-Stokes component of backscattered light increases with fibre temperature. Distributed temperature sensors (DTS) therefore locate hot or cold spots by measuring the intensity of anti-Stokes backscattered light as a function of distance along the fibre. The intensity of the Stokes signal is typically used as a reference value as it does not vary significantly with temperature.
Rayleigh scattering occurs when the fibre is strained. The straining may be caused by bending, pulling, or mechanical waves (as pressure variation propagates through the material). Distributed acoustic sensors (DAS) therefore locate straining events by measuring the intensity of Rayleigh scattered light as a function of distance along the fibre.
Discrete sensing (fibre Bragg grating sensing)
Discrete sensing uses a fibre Bragg grating (FBG) to measure variations in temperature and strain. An FBG is an optical fibre that reflects a specific wavelength, the Bragg wavelength, and transmits all others.
Embedded in the core of an FBG are periodic segments of a different refractive index to that of the surrounding core, effectively generating discrete surfaces upon which light can reflect. The Bragg wavelength is dependent on the separation between these segments, which can change due to thermal expansion, stress, or strain on the glass fibre. The Bragg wavelength is therefore measured as a function of fibre distance to locate changes in temperature and strain.
As both temperature and strain can influence the Bragg wavelength, the FBG must be unstrained when sensing temperature alone. Special packaging is used to insulate the FBG from strain by eliminating bending, tension, compression, or torsion. The change in Bragg wavelength is then entirely due to thermal expansion of the glass fibre.
When measuring strain, the temperature effects on the FBG must be compensated by installing an FBG temperature sensor in thermal contact with the FBG strain sensor. A temperature-compensated strain value is calculated by subtracting the wavelength shift due to temperature from the wavelength shift due to strain.
Oil and gas
Oil and gas pipelines and vessels can leak as a result of fatigue, corrosion, or poor installation. Fibre optic sensors can be installed along the pipeline or vessel exterior to locate hot spots or acoustic signatures generated by leakages.
Pipelines are also at risk of third-party intrusion. Human footsteps, animals, vehicles, or machinery are sources of acoustic waves that exert very small straining forces when propagating through a material. Fibre optic sensors can therefore be installed to help protect pipelines from accidental or malicious damage by identifying acoustics.
Wind turbine blades can be damaged by excessive wind pressure, resulting in failure of a turbine blade or decreased turbine efficiency. Fibre optic sensors can be installed to detect the stress and strain as result of deflections, twists, and vibrations. The blades may then be adjusted to decrease deflections, or to dampen vibrations.