Fibre Optic Sensors
Fibre optic sensors can be broadly divided into distributed and discrete (or point based) sensing technology. Epsimon has expertise in the application of both these technologies for measuring strain and / or temperature of structures. These measurements can be used to derive other engineering parameters such as displacement, bending, heave, etc.
With our distributed fibre optic sensor systems, based on Brillouin or Raman scattering, we are able to take measurements at closely spaced intervals along a fibre optic cable, which can be anywhere from a few meters to tens of kilometres long. This technology enables us to measure complete profiles of strain and temperature of civil infrastructure over long distances.
With discrete fibre optic sensors, based on fibre Bragg grating (FBG) technology, we commonly deploy quasi-distributed systems where several sensors (gratings) are manufactured in line on a single fibre optic cable. This allows us to maintaining the versatility and ease of installation offered by fibre optic sensors while benefitting from the measurement precision and cost-effectiveness of FBG systems.
Advantages of Fibre Optic Sensors
Fibre optic (FO) sensors offer several advantages that make them a very versatile monitoring technology, suitable for a wide range of applications:
- Measurements can be obtained over several kilometres along a single FO cable without the need for signal amplification.
- Individual FO cables or sensors can be daisy-chained together in series and the equivalent of thousands of sensors can be measured from a single interrogator.
- FO cables are small, lightweight, and easy to integrate into structures.
- There is no need for a power and signal cable for each individual sensor, as is the case with many traditional instruments. The same optical fibre is used as a sensor and as a transmitter of the measurement signal.
- Multiple parameters (strain, temperature, displacement, bending, etc.) can be measured by, or derived from the same system, thus eliminating the need for multiple monitoring systems.
- Since the sensing element (the optical fibre) is passive, FO sensors require no maintenance and no repeated calibration. The measurements will not drift over time.
Selection of fibre optic sensing cables
- Optical fibres are intrinsically safe since electrical power is not required at the sensing location and there is no self-heating effect. They are therefore well suited to hazardous and confined environments where there could be a risk of fire or explosion.
- Optical fibres are immune to and do not create electromagnetic interference. Hence, they can be used in high voltage environments without the measurements being affected and they will not disturb neighbouring electrical devices.
- Optical fibres are resistant to corrosion and are not affected by water ingress, which is the main mode of failure of conventional electrical sensors.
- Optical fibres are made of pure silica, which is a very robust and inert material. This makes the sensing element even more stable than the host structural material in which it is embedded, and ideally suited for long-term monitoring and whole-life asset management.
Distributed Fibre Optic Sensing
Distributed fibre optic sensing (known as DFOS or DOFS) is based on Brillouin, Raman or Rayleigh scattering. It relies on the fact that when light travels in an optical fibre a small amount is backscattered due to molecular refractive-index or density fluctuations. The backscattered light spectrum has various components, which characteristics are affected by strain and temperature in the optical fibre.
Components of the backscattered light spectrum (Kechavarzi et al., 2016)
Well-established DFOS techniques based on spontaneous or stimulated Brillouin scattering are known as Brillouin Optical Time (or Frequency) Domain Reflectometry (BOTDR / BOFDR) or Analysis (BOTDA / BOFDA). With these techniques, the Brillouin peak frequency of an optical fibre is measured with a spectrum analyser at every point along the fibre and then used to calculate strain or temperature, to which it is proportional.
The distance along the fibre at which the scattered light is generated can be determined from a time-domain analysis by measuring the propagation times of the light pulse traveling in the fibre. This gives a continuous strain or temperature measurement distribution along the fibre.
Since both variations in either temperature or strain can cause the Brillouin frequency to change, it is necessary to distinguish between these two effects. To overcome this, a common solution is to use a separate temperature-sensing FO cable, which is immune to changes in strain. It is placed adjacent to the strain-sensing FO cable to allow temperature-compensated strain to be obtained.
These techniques provide fully distributed strain and temperature measurements along the fibre optic cables over lengths of several kilometres with spatial resolutions of up to 0.5 m and sampling intervals as close as 0.05 m, making them ideal for installation in long, linear structures. The best measurement resolution of DFOS systems is of the order of ± 5 µɛ or ± 0.25°C.
Current commercially available DFOS spectrum analysers only permit non-dynamic measurements to be made, with measurement times of a few minutes. They can be single-ended (BOTDR / BOFDR), requiring only one end of a FO cable to be connected to the analyser, or double-ended (BOTDA / BOFDA), where both ends of the FO cable are connected to the analyser to achieve better measurement resolution.
Distributed fibre optic sensing cables installed on the reinforcement of a raft foundation
Another DFOS method, based on Raman scattering, is used for measuring temperature only in industrial and civil engineering applications. It is known as Distributed Temperature Sensing (DTS) to distinguish it from Brillouin-based techniques.
The DTS method uses the intensity of the anti-Stokes Raman frequency peak of the backscattered light spectrum, which is a function of the optical fibre’s temperature. Hence, the temperature distribution everywhere along the optical fibre can be obtained by analysing the frequency spectrum.
The technique can be single- or double-ended and, in general, it is more precise in measuring temperature than Brillouin-based methods. However, the measurement distance is shorter (a few kilometres) and the spatial resolution lower (typically 1 to 2 m). Being an intensiometric method, optical losses due to repeated splices and connections between individual cables will affect the DTS measurement and are to be avoided, making the installation of Raman-based DTS systems on structures with complex geometries more difficult.
A third type of DFOS sensing technique for measuring strain and temperature, based on Rayleigh scattering, is known as Optical Frequency Domain Reflectometry (OFDR). Commercially available systems from Luna Inc. use a tunable or swept laser to measure the phase and amplitude of the Rayleigh backscatter signal. A Fourier transform is applied to obtain the signal in the optical frequency domain as a function of length. A cross-correlation is then performed at each sampling point along the fibre to determine the spectral shift between a reference measurement and that affected by strain and / or temperature changes, to which it is linearly related.
The Luna Inc. technology provides dynamic (a few Hz) distributed measurements at very high spatial resolution (of the order of millimetres) and high precision but only over very short distances of tens of metres and is only suitable for small scale testing or monitoring of small structures.
Fibre Bragg Grating (FBG) Sensing
A fibre Bragg grating (FBG) is a wavelength-dependent reflector formed by physically writing a periodic refractive index structure of a few mm within the core of an optical fibre using a laser. Whenever a broad-spectrum light beam impinges on the grating, it will have a large portion of its energy transmitted through while a narrowband spectral component will be reflected by the FBG.
The reflected signal is centred on, and peaks at the Bragg wavelength of the FBG, which, in an optical fibre, is related to the effective refractive index and the periodicity of the grating. Since changes in strain and temperature both result in changes in refractive index and periodicity, they induce a shift in the Bragg wavelength to which they are linearly related.
Spectral response and basic structure of a fibre Bragg grating (FBG)
FBGs allow localised monitoring of strain, temperature and other derived measurands, at high sampling frequencies (kHz) and high resolution (~ ± 1 µɛ). FBGs consist of discrete sensors, however they can be daisy chained or manufactured in series in optical fibres at various wavelengths and at any required spacing (millimetres to metres) to form versatile quasi-distributed FBG arrays with tens of sensors on a single FO cable. Individual FBG sensor arrays can also be multiplexed on multiple channels and a single FBG interrogator / logger can measure simultaneously several hundred sensors.
As with distributed strain sensing systems, strain measurements have to be compensated for temperature effects. Specifically packaged FBG sensors (immune from strain) can be used to measure temperature only. Alternatively, conventional temperature sensors can be used for compensation.
FBG strain sensor spot-welded
onto a steel column