Since their development in the 1950s, optical fibers have been used for power transmission, communication, imaging, and sensing. They are often used in situations where other sensing techniques can fail. Because optical fibers are dielectrics and versatile, they can be used in a variety of environments from vacuum chambers to the ocean floor.
From Fiber Optics to Pressure Sensors
Standard optical fibers are designed to act in telecommunications setups, and usually, are not useful for sensing purposes. There are different methods for creating optical fibers that are sensitive to a parameter of interest. One technique is to imprint fiber gratings. Another approach is to use specialty microstructured optical fibers. Microstructured fibers show promise for highly sensitive pressure sensors in applications such as petroleum exploration, where technicians and engineers can use them to detect fluid pressure. Some examples of optical fibers able to act as pressure sensors are shown in Figure 1.
Typically, microstructured optical fibers for pressure sensors are configured so that the application of an external load causes an asymmetric stress distribution within the fiber. This, in turn, causes variations in the fiber birefringence — a material property referring to an optically anisotropic refractive index — which can be measured for sensing purposes.
“Advantages of optical fiber-based sensors include high sensitivity, electromagnetic immunity, and the possibility of functioning in harsh environments,” said Jonas Osório from the Universidade Estadual de Campinas (Unicamp). “They are usually very compact, lightweight, and provide great liberty when choosing a sensor's characteristics.”
But the fibers reported to date have very sophisticated microstructures and usually require several drawings and a delicate manual procedure for assembling the structure. At Unicamp and at the Instituto de Estudos Avançados (IEAv) in Brazil, work is being done to develop a different type of an optical fiber — an embedded-core capillary fiber — which can act as a highly sensitive pressure sensor. This type of fiber is fabricated with a simpler process, which involves a preform preparation method and direct fiber drawing.
A Closer Look at Geometric Characteristics
An embedded-core capillary fiber is a silica capillary tube endowed with a germanium-doped region (the fiber core) placed inside the capillary wall (Representations of the fiber structure and cross-section are shown in Figure 2). Embedded-core fiber is much simpler than the typical microstructured fibers employed in pressure sensing applications, as seen in Figure 1.
Alongside Marcos Franco and Valdir Serrão from IEAv, Jonas Osório and Cristiano Cordeiro from Unicamp investigated pressure-induced birefringence in microstructured fibers in order to develop and validate a new design concept. Franco, Serrão, Cordeiro, and Osório focused on fibers designed to sense hydrostatic pressure — 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 fibers (very thin, hollow tubes) instead of solid fibers with a pattern of air holes, which can 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 fiber. Beginning from an analytical model, they studied pressure-induced displacements and mechanical stresses in the capillary walls (Figure 3).
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.
Maximizing 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 fiber core.
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 fiber core. They discovered that to make the birefringence as dependent on pressure as possible and therefore maximizing 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 analyzed the changes in stress distribution for different geometries, they discovered that the birefringence derivative with respect to pressure values was higher for fibers with thinner walls and for positions closer to the inner radius of the capillary.
A New Route to Microstructured Optical Fiber 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 fibers and confirmed that their design would perform properly as a pressure sensor. They compared the sensitivity of their concept to existing, more complicated fiber structures and determined that their design produced similar results but required less assembly work. The embedded-core fiber provides a new route for obtaining highly sensitive optical fiber pressure sensors and will make it easier for petroleum explorers to evaluate the fluids they extract in real time.