The technology of fiber optic sensing
The fiber optic cables consist of a core, cladding, and an outer covering or a fiber jacket. The photons are transmitted through the fiber cable, bouncing and vibrating along the walls. The light travels through a core which is mostly made up of glass. The core acts like a mirror and its major function is to keep the photons inside the cable. It reflects the photon just like a mirror does.
The cladding is the outer part of the core which is also made up of glass. The cladding is made with glass with a less refractive index than the core. It allows the total internal reflection and thus the photons are not leaked out of the core. The outer cover is mostly plastic or some other effective material to protect the fiber from the outside environment.
There are a lot of possibilities for the cables to get damaged as they are fragile compared to the metallic wires. It also shields the cable from external shocks and pressures. The coating might be several layered as moisture might damage the optic cable if it is circuited underground. To increase the protection, a metallic coating is applied on the fiber jacket. To prevent data loss effectively, the engineering of the fiber optic cables must be aligned perfectly.
There are two types of fibers based on their structure and purpose. The first of the two is the single-mode fiber. The single mode fiber is thin compared to its counterpart. It has a core of less diameter that helps in transmitting the photons without letting them bounce around the peripheral region of the cable. As the signal travels straight without many hindrances, single mode fibers can be used for long distance asset monitoring and communication purposes.
Single mode fibers are bundled as a wrap to transmit Internet, Television, and telephone data. In contrast to single mode fibers, multi mode fibers have more than one propagation path. They have a wider core which enables multiple paths for the propagation of electrons. The diameter of the core will be greater in these type of multi mode fibers.
They are also used to connect data between computer networks. Fiber optic cables are relatively safer than their electrical counterparts. They are small in size, which makes it easier to implement and do maintenance. It also saves space and labor. The fiber optic cables are also chemically passive. They are also very compatible with the operational environments.
The technologies in Distributed Fibre Optic Sensing can be grouped under different titles based on the parameters they take into consideration. Some of them are:
Distributed Acoustic Sensing (DAS)
In Distributed Acoustic Sensing (DAS), the light is transmitted as short pulses through an optical fiber and the backscattered or reflected light is monitored. This technique is called Coherent Optical Time Domain Reflectometry or C-OTDR. When any activity happens around the pipeline, fiber gets acoustic or vibrates.
DAS works by monitoring the picostrain-level signatures created by these disturbances. The first use of the Distributed acoustic system was performed in 2009. Later many improvements have been made in this industry to suit different requirements. DAS is highly preferable because of its chemical resistance, adaptability, anti-electromagnetic interference, large sensing distance, and good concealment.
Compared to the old geophones used, Distributed Acoustic System has less cost of implementation and is easier to install along with other systems like Distributed Temperature Sensing (DTS) and Distributed Temperature Gradient Sensing (DTGS).
Distributed Temperature Sensing (DTS)
DTS is the abbreviation of Distributed Temperature Sensing, a fiber optic sensing technique to monitor the temperature through optical pulses. By deploying DTS, we will be able to gather the temperature profile throughout the length of the optical cable.
The temperature is measured by using the Raman effect. The Nobel Prize winning physicist Sri. Chandrasekhara Venkata Raman discovered that when light travels through a transparent medium, there will be a change of wavelength in the deflected light. The intensity of this scattering is measured to find the temperature variations.
The theory of Brillouin backscattering is also used in some other DTS technologies. They make use of information on strain and temperature. The systems that use Brillouin backscattering instead of the Raman effect are called Distributed Temperature and Strain sensing or DTSS.
But these systems have a great responsibility to keep the fiber isolated in order to extract the actual information about the temperature. Brillouin Optical Time Domain Reflectometry or B-OTDR has larger measurement dynamics than the OTDR system. But the sensitivity of attenuation changes measurement can lead to the breaking of fiber sometimes.
Distributed Temperature Gradient Sensing (DTGS)
Distributed Temperature Gradient Sensing is an amplitude-based detection of temperature variations using sensing fibers and coherent optical time domain reflectometry (C-OTDR). The fiber consists of strongly scattering dots that are equidistant. DTGS enables real-time monitoring without complex post-processing. Without calculating the absolute temperatures, DTGS measures the temperature gradient within no time in high resolutions.
Optical sensing fibers into the market
The applications of fiber sensing varies according to the needs. Even in the same domain or industry, the applications and requirements might be different and fiber sensing has to address it all. It is mainly used to measure physical properties such as temperature, strain, pressure, velocity, displacement, and acceleration. With fiber optic sensors, we can monitor structures in real-time and analyze their physical health and damage.
In buildings and bridges, we can monitor the concrete during its setting. The length and the propagation speed of the cracks can also be monitored. We can also analyze other aspects like prestressing, neutral axis evolution, shrinkage, and spatial displacement measurement. They will evaluate the damage rate induced by seismic activity. In tunnels, we can employ multipoint optical extensometers.
The sensors can evaluate prefabricated vaults, joints, and convergence of the tunnels. While building dams, fiber optic sensors can be used for distributed temperature monitoring, leakage, and foundation monitoring. In the oil and gas industries, fiber optic sensors are used to detect the ground movement and mechanical deformation of the pipelines.