Fiber sensing explained

With fiber sensing, existing telecoms fiber-optic cables can be transformed into a network of sensors, through so-called Distributed Fiber Optic Sensing (DFOS) technologies. When DFOS is deployed on these cables, they do not only transport information but will also actively contribute to generating new information.

By using existing fiber infrastructure, there is no need to install measurement equipment in hard-to-reach places, such as on the seabed. Through the deployment of a fiber sensing device that interprets vibrations in the cable, a so-called interrogator, the fiber-optic cable becomes the measurement device. This provides valuable insights for a wide range of research fields, such as earth sciences, biology, and environmental sciences.

The difference between fiber sensing and conventional sensors

Fiber sensing distinguishes itself from conventional point sensors in its ability to provide continuous measurements along an entire optical fiber rather than only at predefined sensor locations. A single fiber can act as thousands of virtual sensing points, allowing researchers to observe how strain, temperature, vibration, or acoustic signals vary across large distances with high spatial resolution. This enables the detection and localization of events anywhere along the instrumented fiber, providing a more complete picture of the monitored environment than is typically possible with a limited number of discrete sensors. 

The benefits of fiber sensing

In many research fields, fiber sensing can complement or potentially replace conventional sensors such as geophones, seismometers, strain gauges, thermocouples, accelerometers, and geotechnical monitoring instruments. This makes fiber sensing particularly valuable for applications in geophysics, structural health monitoring, environmental science, subsurface research, and transportation monitoring, where understanding how conditions vary across large areas or structures is often as important as measuring conditions at individual points. 

The best-known fiber sensing technologies 

Fiber Optic Sensing (FOS) technologies can be broadly divided into 2 categories: single-point and distributed. At SURF we focus on distributed technologies (DFOS) as those can be easily applied to existing telecoms fiber infrastructure without the need for deploying specific reflectors along the cable. DFOS technologies also allow for broad support of research, can be deployed on existing unmodified telecoms fiber, and can even potentially co-exist with live telecoms traffic.

Distributed Fiber Optic Sensing (DFOS) technologies can be further subdivided into 2 categories: scattering and forward-transmission based sensing.  [link to Kurosh/Sikt] Current trials and projects SURF is involved in aim at deploying scattering based DFOS technologies such as Distributed Acoustic Sensing (DAS), Distributed Temperature Sensing (DTS), and Distributed Temperature and Strain Sensing (DTSS).

Distributed Acoustic Sensing (DAS): chain of thousands of microphones

DAS turns a fiber optic cable into a chain of thousands of microphones. It works using Rayleigh backscattering: tiny imperfections in the glass scatter a bit of light back toward the source. When sound or vibration disturbs the fiber, it slightly changes this reflected light, letting the system detect and locate noise, movement, or vibration anywhere along the cable.

Distributed Temperature Sensing (DTS): long thermometer 

DTS turns a fiber optic cable into a long thermometer. It relies on Raman backscattering, where scattered light shifts to different colors depending on temperature. By comparing these color shifts at each point along the fiber, DTS builds a full temperature map along the cable's length, useful for monitoring precise temperature across a long distance.

Distributed Temperature and Strain Sensing (DTSS): measures temperature and physical strain 

DTSS measures both temperature and physical strain (stretching or bending) along a fiber at the same time. It uses Brillouin backscattering, where light interacts with sound waves naturally traveling through the glass, shifting frequency based on both heat and strain. By analysing this shift, DTSS can tell the two apart, making it ideal for monitoring of physical structures like bridges and dams for both temperature changes and structural stress.

State of Polarization (SOP) and Optical Time Domain Reflectometry (SOP-OTDR)

SOP sensing detects disturbances along a fiber optic cable by tracking changes in the polarization of light traveling through it. Being a forward-transmission based  technique, it does not rely on backscattering. External forces like vibration, movement, or pressure on the cable slightly twist or rotate the light’s polarization as it travels. By continuously monitoring these polarization shifts at the receiving end, SOP sensing can detect when an event disturbs the cable, although it generally cannot pinpoint where along the cable the disturbance occurred.
 
SOP-OTDR enables point-based sensing in subsea cables and like regular SOP does not rely on backscattering. In addition, it periodically sends tone signals down the cable and uses the subsea cable high-loss loopbacks (HLLBs) at each repeater to bounce these tones back. Individual tones are analysed for polarization changes like in regular SOP. This way, SOP-OTDR lets operators track conditions at multiple points along the cable. It can also be used on land-based networks.