Valerio Marra, marketing director at COMSOL
Research at Brazil’s Universidade Estadual de Campinas (Unicamp) and Instituto de Estudos Avançados (IEAv) has revealed new designs for optical fibre pressure sensors. To the average person, optical fibres might conjure up an image of glowing hairs twisted artistically into a beautiful shape or fountaining out of a lamp holder, but these light-transmitting silica strands are much more than decoration.
Since their development in the 1950s, optical fibres have been used for power transmission, communication, imaging and sensing. Specifically, they are often used in situations where other sensing techniques can fail. Since optical fibres are dielectric and versatile, they can be used in environments such as vacuum chambers and the ocean floor.
From fibre optics to pressure sensors
Standard optical fibres are designed to act in telecommunications setups and, usually, are not useful for sensing purposes. In order to make optical fibres sensitive to a parameter of interest, processing procedures such as the imprinting of fibre gratings are necessary, or specialty microstructured optical fibres can be employed. Microstructured fibres show promise for obtaining highly sensitive pressure sensors used in activities such as petroleum exploration, where technicians and engineers can use them to detect fluid pressure. Figure 1 shows some examples of microstructured optical fibres able to act as pressure sensors.
Figure 1
Microstructured optical fibres used in pressure sensing measurements—(a) Photonic-crystal fibre; (b) Microstructured fibre with a triangular lattice of holes; (c) Side-hole photonic-crystal fibre.
Typically, microstructured optical fibres for pressure sensors are configured so that the application of an external load causes an asymmetric stress distribution within the fibre. This in turn causes variations in the fibre birefringence—a material property referring to an optically anisotropic refractive index—which can be measured for sensing purposes.
According to Jonas Osório, one of the researchers at Unicamp: “Advantages of optical fibre-based sensors include high sensitivity, electromagnetic immunity and the possibility of functioning in harsh environments. They are usually very compact, lightweight and provide great liberty when choosing a sensor’s characteristics.”
However, the fibres reported to date have very sophisticated microstructures and usually require several drawings and a delicate manual procedure for assembling the structure. At Unicamp and IEAv, work is being done to develop a different type of optical fibre—an embedded-core capillary fibre—which can act as a highly sensitive pressure sensor. This type of fibre requires a simpler fabrication process that involves a preform preparation method and direct fibre drawing.
A closer look at geometric characteristics
The embedded-core capillary fibre is a silica capillary tube endowed with a germanium-doped region (the fibre core) placed inside the capillary wall. Figure 2 shows representations of the fibre structure and cross-section. In contrast to the fibres shown in Figure 1, the embedded-core fibre is much simpler than the typical microstructured fibres employed in pressure sensing applications.
Figure 2
(a) Concept of embedded-core capillary fibre showing a cross-section of the tube with an embedded core, under hydrostatic pressure; (b) Embedded-core fibre cross-section.
Alongside Marcos Franco and Valdir Serrão from IEAv, Cristiano Cordeiro and Jonas Osório from Unicamp investigated pressure-induced birefringence in microstructured fibres in order to develop and validate a new design concept. Franco, Serrão, Cordeiro and Osório focused on fibres designed to sense hydrostatic pressure, namely pressure induced by a fluid at rest, such as a body of still water surrounding the sensor. However, they diverged from existing designs by using capillary fibres (very thin, hollow tubes) instead of solid fibres with a pattern of air holes that permit asymmetric stress distributions.
Their goal, ultimately, was to maximize the birefringence dependence on pressure variations, since this would improve the sensing capabilities of the fibre. Beginning with an analytical model, they studied pressure-induced displacements and mechanical stresses in the capillary walls (Figure 3).
Figure 3
Study of a pressurised capillary fibre without the embedded core, under pressure. The displacement profile was initially studied for an inner radius of rin = 40 µm, an outer radius of rout =80 µm, an inner pressure pin of 1 bar and an outer pressure pout of 50 bar.
The analytical model showed that applied pressure generates an asymmetrical stress distribution inside the capillary wall due to the capillary structure. Via the photoelastic effect, these stresses cause variations in the material refractive index that are different along the horizontal and vertical directions, generating the desired birefringence.
Maximising pressure-dependent properties
Using COMSOL Multiphysics software, Franco, Serrão, Cordeiro and Osório added the elliptical core, a germanium-doped region inside the silica capillary wall, to their mathematical model. Through their simulation, they obtained the change in modal birefringence as a function of the applied pressure and the location of the core in the capillary wall (Figure 4). Modal birefringence describes birefringence of the optical modes that can travel through the fibre core.
Figure 4
Changes in modal birefringence as a function of the position of the germanium core within the capillary wall. The case with the highest changes in birefringence due to pressure variations occurs when the core is very close to the inner radius of the fibre (top centre case).
The model calculated the effective refractive indices of the fundamental modes for different pressure conditions. These modes occur when incoming electromagnetic waves are guided through the fibre core. They discovered that to make the birefringence as dependent on pressure as possible and therefore maximize the sensitivity of the sensor, it was necessary to embed the core area completely within the capillary structure, close to the inner wall. As they analysed the changes in stress distribution for different geometries, they discovered that the birefringence derivative with respect to pressure values was higher for fibres with thinner walls and for positions closer to the inner radius of the capillary.
A new route to microstructured optical fibre sensors
Thanks to their research in exploring birefringence pressure dependence, Franco, Serrão, Cordeiro, and Osório laid out a new way to simplify the production of microstructured optical fibres and confirmed that their design would perform properly as a pressure sensor. They compared the sensitivity of their concept to existing, more complicated fibre structures and determined that their design produced similar results but required less assembly work. The embedded-core fibre provides a new route for obtaining highly sensitive optical fibre pressure sensors and will make it easier for petroleum explorers to evaluate the fluids they extract in real-time.
The research was originally published as an article titled ‘Simplifying the design of microstructured optical fibre pressure sensors’ in the journal Scientific Reports, by Nature Publishing Group.
References
1H. Y. Fu, et al. ‘Pressure sensor realized with polarization-maintaining photonic crystal fiber-based Sagnac interferometer,’ Applied Optics, 47, 15, 2835-2839, 2008.
2A. Anuskiewicz, et al., ‘Sensing characteristics of the rocking filters in microstructured fibers optimized for hydrostatic pressure measurements,’ Optics Express, 20, 21, 23320-23330, 2012.
3J. H. Osório, et al., ‘Photonic-crystal fiber-based pressure sensor for dual environment monitoring,’ Applied Optics, 53, 17, 3668-3672, 2014.
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