What is fiber optic sensing?

Fibre optic sensing is an advanced technology that harnesses the physical properties of light as it travels along an optical fibre to detect changes in temperature, strain, and other environmental parameters. This innovative approach utilises the fibre itself as the sensor, creating thousands of continuous sensor points along its length. This technique is known as distributed fibre optic sensing, offering a level of detail and coverage that traditional point sensors cannot match.

The devices used to measure the fibre itself are generally called interrogators. These sophisticated instruments are designed to use either standard telecommunications fibre or specially designed fibre for measuring temperature and strain along its entire length. This is achieved using advanced Raman and Brillouin Distributed Fibre Sensor techniques, which we'll explore in more detail later.

 
The applications of fibre optic sensing are vast and varied. For instance, by using a fibre sensing interrogator, one can:

• Detect and precisely locate any hot spot along a power cable, allowing for proactive maintenance and preventing potential failures.
• Identify and pinpoint areas of excessive strain on optical cables, enabling operators to take preventive action before a break occurs.
• Monitor structural health in buildings, bridges, and other infrastructure, providing real-time data on structural integrity and safety.
• Detect leaks in pipelines, offering early warning systems for potential environmental hazards.

Below are examples of fiber sensing applications:

How does fibre sensing work?

Fibre optic sensing can be categorised into two main types: extrinsic and intrinsic sensing. Extrinsic sensing uses a fibre optic cable as a communication path between a test station and an external sensor. However, intrinsic fibre sensing, which is the focus of distributed temperature sensing systems (DTS) and DTSS, utilises the fibre itself as the fibre optic sensing system.

The primary advantage of intrinsic fibre sensing technology is that it eliminates the need for discrete interfaces between the fibre and external sensors. This reduction in complexity translates to lower costs and increased reliability. To make this possible, external stimuli such as temperature fluctuations and strain need to influence the light source within the cable in a measurable and predictable way, providing useful data for analysis.

The science behind fibre sensing relies on several scattering phenomena:

• Rayleigh Scattering: This occurs when light photons are scattered randomly after contacting particles within a fibre. Rayleigh scattering has proven invaluable in various fibre testing techniques, such as OTDR fibre testing. By analysing the volume, wavelength, and location of light backscattered to the detector, technicians can determine the magnitude and position of attenuation events within an optical fibre.
• Raman Scattering: This phenomenon produces temperature-induced changes in photons scattered back to the source in the Stokes band. By measuring the difference between the intensity of backscattered light in the Stokes and anti-Stokes bands, the temperature can be accurately determined at any given location along the fibre. This principle forms the basis of Distributed Temperature Sensing (DTS) systems.
• Brillouin Scattering: Similar to Raman scattering, Brillouin scattering involves backscattered light with a wavelength influenced by external temperature and acoustic stimulation in a predictable manner. When coupled with background knowledge of temperature at the same point, this data can be used to accurately determine the strain experienced by the fibre. By analysing this information, technicians can identify which areas (zones) of the fibre are impacted by strain.

Distributed fibre optic sensing

Distributed fibre optic sensing (DFS) effectively utilises both Raman and Brillouin scattering phenomena. Raman scattering is primarily used for Distributed Temperature Sensing (DTS), while Brillouin scattering is employed in Distributed Temperature and Strain Sensing (DTSS). A key advantage of these measurements is that they are not influenced by the optical loss of the fibre, allowing for accurate monitoring of temperature and strain over tens of kilometres.

In this context, the term "distributed" refers to fibre sensing technology that can measure continuously throughout the complete length of the fibre. Essentially, the fibre itself becomes the sensor, offering a level of coverage and detail that point sensors simply cannot match. Since these fibre sensing methods are completely intrinsic, standard telecommunications fibre can often be used as the sensing medium, provided the temperature remains below 100°C (212°F) and the fibre is not subjected to excessive chemical or mechanical disruption.

What are the applications of distributed fibre optic sensing?

Distributed fibre optic sensing, including DTS and DTSS technologies, has a wide range of applications across various industries. Here are some key areas where these innovative technologies are making a significant impact:

• Power Cable Monitoring: DTS and DTSS systems can continuously monitor the temperature and strain along power cables, helping to identify potential hotspots or areas of excessive strain before they lead to failures. This proactive approach to maintenance can significantly reduce downtime and extend the lifespan of critical infrastructure.
• Pipeline Leak Detection: By monitoring temperature changes along pipelines, DTS systems can quickly detect and locate leaks, allowing for rapid response and minimising environmental impact. This is particularly crucial in oil and gas industries where early leak detection is vital.
• Structural Health Monitoring: DTSS technology can be used to monitor the structural integrity of buildings, bridges, dams, and other large structures. By detecting changes in strain and temperature, engineers can identify potential weaknesses or damage before they become critical issues.
• Fire Detection: DTS systems can provide early warning of fires in tunnels, industrial facilities, and other high-risk areas by detecting rapid temperature increases along the fibre optic cable.
• Geothermal Energy Production: In geothermal wells, DTS technology helps optimise energy production by providing detailed temperature profiles along the entire well depth.
• Environmental Monitoring: DTS and DTSS systems can be used to monitor soil temperature and moisture content in agriculture, as well as track temperature changes in rivers, lakes, and oceans for environmental research.
• Railway Infrastructure Monitoring: These systems can detect track deformations, monitor the temperature of critical components, and even provide early warning of landslides near railway lines.
• Data Centre Cooling: DTS technology can help optimise cooling systems in data centres by providing detailed temperature mapping, ensuring efficient operation and preventing equipment failures due to overheating.
• Perimeter Security: By detecting vibrations and temperature changes, DTSS systems can be used as part of advanced perimeter security solutions for critical infrastructure and high-security facilities.
• Wind Turbine Monitoring: DTSS technology can monitor the structural health of wind turbine blades, detecting strain and temperature changes that might indicate potential failures.

These applications demonstrate the versatility and importance of distributed fibre optic sensing technologies in modern industry and infrastructure management. As the technology continues to advance, we can expect to see even more innovative applications emerge, further cementing the role of DTS and DTSS systems in ensuring safety, efficiency, and reliability across a wide range of sectors.

• Optical Network Sensing: protect, inspect, or monitor optical fibre networks
• Infrastructure Monitoring Sensing: A fibre can be used to conduct infrastructure monitoring by using the fibre as a probing device. In this use case, one can deploy a fibre along critical infrastructure such as a bridge, pipeline, secure aperture or dam wall to set off an alarm if the fibre demonstrates sudden strain, movement, or the temperature of the fibre puts the infrastructure at risk of damage or failure. This can be used to secure openings such as doors or manhole covers to generate an alarm if the opening is breached.

Fibre optic sensing technology has revolutionised the way we monitor and protect critical infrastructure and networks. By leveraging the unique properties of optical fibres, we can now detect and locate issues with unprecedented accuracy and speed. This technology is particularly useful in various applications, including optical network sensing and infrastructure monitoring.

Several infrastructure monitoring applications are available with VIAVI fibre sensing interrogators, each designed to address specific challenges in different sectors:

• Detection of ground movement along a pipeline: This application is crucial for maintaining the integrity of oil and gas pipelines, water mains, and other underground conduits. By using distributed temperature and strain sensing (DTSS) technology, even slight shifts in the earth can be detected, allowing for early intervention and prevention of potential disasters.
• Detection of mechanical deformation of the pipeline: DTSS systems can identify areas where pipelines are experiencing unusual stress or strain, which could indicate potential weak points or areas at risk of failure. This early warning system enables operators to conduct targeted maintenance, significantly reducing the risk of leaks or ruptures.
• Detection and location of any leakage along a pipeline, dike, dam etc.: Distributed temperature sensing systems (DTS) are particularly effective in this application. By monitoring temperature changes along the length of the infrastructure, these systems can quickly identify and pinpoint the location of leaks, allowing for rapid response and minimisation of environmental impact.
• Detection and location of any critical point in a telecom optical network: In the telecommunications industry, fibre optic sensing can identify areas of excessive strain or temperature fluctuations in optical networks. This capability is invaluable for maintaining network integrity and preventing service interruptions.
• Detection and location of any hot spot along a power cable: Power cable monitoring is a critical application of DTS technology. By continuously monitoring temperature along the entire length of power cables, operators can identify potential hotspots before they lead to cable failure, thereby preventing power outages and extending the lifespan of the infrastructure.