What is Distributed Fiber Optic Sensing?

Fiber optic distributed sensing saw the light of day in the 1980s as a breakthrough technology providing uninterrupted, EMI-immune monitoring over long distances from a single interrogator.

Techniques have been developed to monitor temperature, strain, and vibration over distances of more than 50 km with incredible accuracy and long-term reliability.

Distributed Brillouin sensing (DTSS)

The nerves of your structure – Traditional measurement systems comprise a device plus copper wires or IoT communications that transmit the data. fibrisTerre’s system performs the measurements and transmits the data with a single optical fiber sensor. The fiber optic sensor detects and locates changes and transmits this data to the interrogator unit for processing. No individual sensors, just one long, flexible, robust one, continuously sensitive all along its length. A fibrisTerre system monitors continuously: It is ‘always on.’

How it works

Little known fact: optical fibers are intrinsically sensitive to certain physical parameters of their environment. Two of these are temperature and strain.

Light is injected into the optical fiber. External influences affect the properties of the light signal as it travels along the fiber. fibrisTerre’s fTB 5020 system (interrogator and sensing cable) converts standard optical fiber into continuous long-range strain and temperature sensors. The optical fiber itself, usually in a protective cable, is the sensor.

In turn, these cables are embedded or attached to the object to be monitored.

The signal is processed by the interrogator unit and analysis software to extract the measurement information. Brillouin-based sensing read-out units provide a continuous profile of the strain and temperature distributions along the entire length of the sensing cable.

So several tens of kilometers of sensing cable may be measured, with spatial resolution down to less than one meter, and a spatial resolution of 20 cm (over a shorter distance)

Measurement accuracy results in measurement repeatability of 2 μm/m for strain and 0.1°C for temperature, in ideal conditions.

Two light waves injected into an optical fiber from both ends meet and make the fiber shiver (literally) creating an acoustic wave inside the fiber. Influenced by this acoustic wave, the light that arrives at the instrument carries information on the fiber’s speed of sound and, consequently, on its strain and temperature.

Using pump and probe lightwaves interacting with molecular vibrations (phonons) is the secret to ensuring sensitivity while overcoming common fiber attenuation issues. Small strain and temperature events are detected and located.

Application-specific fiber optic sensing cables ensure reliable and versatile deployment into earth and rigid structures.

Typical installation

  • Loop configuration can be used for temperature compensation
  • Fiber-optic switches for channel multiplexing
  • Data transfer to control room by Ethernet or wireless connection
  • Cloud-based data storage service

Fiber optic sensing cables for fibrisTerre Distributed Sensing

Both strain and temperature can be measured using a fibrisTerre system. The structure of the sensing cable provides the differentiation.
A dedicated strain sensing cable encapsulates a tight-buffered fiber that couples mechanical deformation of the structure/material into strain to the sensing fiber.

A dedicated temperature sensing cable, in turn, contains a loose-tube optical fiber, moving freely with some excess fiber length so that it is mechanically decoupled from external strain. The optical fiber will sense the environment’s temperature as heat transfers through the cable’s protective layers and elements.

a strain sensing cable is tight buffered

Strain sensing cable

  • Tight-buffered design for strain
  • Metallic and non-metallic designs
  • Round or rectangular cross-sections
temperature sensing cable has fiber overlength

Temperature sensing cable

  • Loose-tube design for strain decoupling
  • Excessive fiber length inside tube
  • Metallic and non-metallic designs

Separating strain and temperature in DTSS

The choice of cables and system configuration ensures strain and temperature events are detected, as shown below:

Monitoring strain and temperature in a tunnel

Performance parameters

fibrisTerre’s interrogator units are designed to provide measurement data you can rely on.
Specific parameters for your application or project are available from fibrisTerre.

Contact us


Thanks to the Brillouin frequency-based sensing technique, once the instrument is calibrated, the measurements provide excellent long-term stability


The high signal-to-noise ratio means that optical signals can be detected, even when the fiber condition is challenging.

Accuracy and Sensitivity

Brillouin sensing is extremely sensitive. With calibrated sensing fibers, it provides measurement accuracy of better than 2 με (micro-meter per meter) for strain and better than 0.1°C for temperature.

Spatial resolution

Defines the size of the smallest change or event the system can detect. It is specified for a fiber as the minimum distance between two-step transitions of the fiber’s strain/temperature condition.
A higher spatial resolution resolves more details.

Imagine being able to pinpoint a strain or temperature event to within 50 cm at a distance 25 km from where you are sitting or standing right now. A fibrisTerre system can do just that – and at 50 km distance, the resolution will be 1 m.

fibrisTerre’s resolution enhancement

Besides the selectable spatial resolution and spatial accuracy, fibrisTerre’s fTB 5020 instruments offer a unique selectable post-processing technique to significantly increase the system’s ability to detect small strain and temperature events.

With this enhancement, a spatial resolution of 20 cm can be achieved for fibers up to 2 km length – ideal for geotechnical monitoring or civil engineering projects.

Distance range

Quite simply, how far do you want to measure? More than 50 km can be monitored with a spatial resolution of 1 m.

For very long distances or where you need highly accurate measurements, additional interrogator units can be connected. Up to 32 channels can be monitored by one interrogator using the fT X switch (see Products page).
Now just add to this performance an interrogator unit that weighs just 7 kg.

The instrument’s performance criteria are classified using the following key parameters (defined in EC 61757-2-2:2016
Fibre optic sensors – Part 2-2: “Temperature measurement – Distributed sensing).
Specific parameters for your application or project are available from fibrisTerre contact fibrisTerre.

Acquisition time

The time it takes to obtain and process a measurement. For example, 2 km of fiber sensor is profiled in 1 minute. The measurements are continuous. This means that an event happening 2 km away from the interrogator will be ‘seen’ by the user or trigger an alarm 2 minutes later. Events can be tracked as they evolve.

Attenuation budget

The sensing cable has performance parameters too, which affect distributed sensing. The most common is attenuation, defined as “The decrease in optical power of a signal, or light wave, from interaction with the propagation medium, for example, absorption, reflection, diffusion, scattering, deflection, dispersion or resistance. “ (Standard Terminology Relating to Optical Fiber Sensing Systems Designation: F3092 − 14).

Thanks to Brillouin Optical Frequency Domain Analysis developed and patented by fibrisTerre, the fTB 5020 range of instruments overcome a level of attenuation which would leave other measurement techniques in the dark.

Instrument reliability

The fibrisTerre unit is fan-less, fully sealed, and dust-proof, suitable for 19’’ instrument racks and on-site assignments.

Field friendly

Transportable, low power.
Weighing only 7 kg and consuming 40 W, (the computer uses around 150 W), the fTB 5020 range is energy-efficient and transportable. We call this ‘field friendly’!