Fiber’s expanding sensing opportunities

As fiber continues to proliferate throughout the nation through federal and private investment, new and exciting opportunities to utilize it are opening beyond delivering high-speed, low-latency broadband to unserved and underserved areas of the country. Technologies using fiber as a sensor have been around for decades, but the growth of fiber deployments worldwide is opening up new applications in everything from monitoring cicadas to locating new energy sources.

Fiber optic sensing measures changes in an optical fiber's naturally occurring light scattering. It can measure changes in the fiber's physical state through vibration, strain, and temperature, using the fiber as a sensor of its entire length. The fiber provides real-time information on its physical surroundings, and the information collected can pinpoint the precise location of events and conditions at or near the sensor cable.

Enter Distributed Acoustic Sensing (DAS)

Distributed Acoustic Sensing (DAS), defined by the Fiber Optic Sensing Association (FOSA), measures tiny fiber movements caused by acoustic vibrations, which cause changes in the light sent down an optical fiber.

Effectively, the entire length of the fiber becomes a length of thousands of susceptible virtual microphones that can be used to monitor physical assets and geophysical events.

Distributed Temperature Sensing (DTS) provides the ability to detect fires in tunnels in buildings, monitor the temperature of power cables, and even leak detection in pipelines and storage tanks along great distances, while Distributed Strain Sensing (DSS) uses the properties of light along the fiber to provide information on ground movement; deformation monitoring; elongation, bending and fatigue monitoring of cabling; and bridge and excavation work.

New applications emerge

Using one or more techniques with existing dark or in-use fiber already provides substantial information about the physical world around us at little additional cost without requiring installing and maintaining dedicated Internet of Things (IoT) sensors. Combining multiple techniques for different applications provides detailed data for protecting critical infrastructure such as bridges, pipelines, and power lines.

It’s odd yet amazing to think that an aerially-mounted fiber optic cable on a utility pole delivering broadband can provide insight into the activities of cicadas, as reported in the November 2023 Journal of Insect Science. According to the Journal, an existing fiber optic cable could easily cover and monitor a 50-kilometer route, providing a resolution of around one meter, the equivalent of 50,000 sensors along the same road, except without the need to install and power them.

Researchers monitored cicadas in their 2021 emergence cycle around Central New Jersey over a 16-day period, enabling them to easily listen to the patterns of their sounds, see a direct relationship between their calls and local temperatures, and monitor their activity during their lifecycle.

While fiber will be the communications backbone for the future of autonomous vehicles and connected cars, it is being used today to monitor physical events along the highways and byways across the land. For example, the Utah Department of Transportation (UDOT) uses over 3,250 miles of fiber buried alongside its roads to detect and monitor real-time events, including crashes, avalanches, vehicle speeds, and travel times. The derived information is transmitted to UDOT’s Traffic Operations Center, where it is used to assess traffic conditions, dispatch response vehicles, and warn motorists of hazardous conditions. DAS complements existing sensors, radar, and camera networks, providing affordable monitoring in lower population-density regions with lower populations and areas without straight-line roadways. A typical DAS-based road monitoring system based on fiber provides monitoring over a 50-mile stretch unaffected by weather conditions, can narrow down events to within 30 feet, and offers updates for conditions every second.

For ground transportation, fiber is the technology that keeps giving. Roadside fiber provides 5G backhaul connectivity for consumer convenience and safety today and will power connected vehicle technologies when ready for general use in the future. However, fiber also protects railway monitoring and security. DAS-based systems can detect vehicle incursions and more extensive animal incursions and work to detect trespassing for theft and other criminal activity.

Fiber sensing is playing an ever-increasing role in the power industry, both in terms of monitoring infrastructure and finding new resources through geological sensing technologies. Monitoring technology installed along electrical transmission lines can provide temperature, depth of burial, and monitoring for power cables. If a break or other damage occurs to power lines, fiber helps quickly pinpoint where the damage is so repair crews can be sent promptly to where needed.

A more recent innovation is using telecom fiber for large-scale geothermal seismic sensing. Berkley Labs, Rice University, and Zayo used existing dark fiber and DAS to record data near an active geothermal system, using the information to build subsurface models using the existing seismic noise, as well as cataloging local and regional ground movement associated with tectonic activity and geothermal operations.

Through their initial efforts, researchers have built larger, more detailed subsurface maps of the Imperial Valley area in Southern California. Rice University hopes to use fiber optic sensing with existing telecommunications infrastructure to better understand geothermal systems in the western part of the country more affordably than classic but costly surveying methods. More effective mapping using available technologies on existing telecommunication infrastructure will provide knowledge to find new geothermal resources and information on earthquakes.

Turkey is using existing fiber optic telecommunications cables to provide a better understanding of the seismic environment around Istanbul, a city of 15 million people in an area prone to earthquakes. Using DAS with existing fiber offers better resolution of ongoing ground motion for vast areas of the city, including detecting the February 6, 2023, Kahramanmaras earthquakes at around 850 kilometers away in south central Turkey and northern Syria, which took the lives of more than 55,000 people.

Using fiber to monitor more minor earthquakes and those farther away provides a better understanding of fault geometries and better mapping of where faults are moving and locked together. City planners can use the information for hazard forecasting and improvements in designing building and construction codes.

Today’s seismic monitoring projects with existing fiber are pilots providing valuable data for better models of subsurface geography that will save lives and reduce property damage, albeit at the relatively minimal cost of collecting petabytes of data annually for real-time analysis and storage. Collecting and leveraging this data is an unexpected scientific dividend from fiber optic networks deployed over the past thirty years and for the new networks built today through private and federal investments. A better understanding of our world, the ability to use and fix power lines more efficiently, and the ability to tap into new sources of geothermal energy will help reduce our carbon emissions.

Similarly, monitoring roadways at nominal cost by using existing fiber for middle-mile and future connected vehicle use improves public safety and the potential to optimize highway management to reduce carbon emissions further. While we can’t predict earthquakes today, more and improved data collected through existing telecom fiber optic lines can help us better prepare for them and could lead us to expect them in the future.

We’re just starting to fully grasp fiber optics' potential to monitor our civil infrastructure and environment. I have no doubt we will see new and innovative uses of telecom fiber in the future as researchers explore new ways to solve problems using this growing resource.