Distributed fibre-optic sensors (DFOS) are advanced measurement systems that use the optical fibre as a continuous sensor for measuring physical quantities along its entire length. These technologies are extremely valuable in non-destructive testing and structural health monitoring.
Distributed fibre-optic sensors (DFOS) are advanced measurement systems that use the optical fibre as a continuous sensor for measuring physical quantities along its entire length. These technologies are extremely valuable in non-destructive testing and structural health monitoring because they provide continuous measurements over long distances with high spatial resolution and are immune to electromagnetic interference.
1. Strain and stress monitoring
DFOS sensors can measure strain along the entire length of the optical fibre, allowing the stress and shape changes of structural elements — such as bridges, concrete beams or composite materials — to be tracked. Changes in the strain profile often signal the appearance and development of cracks before they become visually apparent.
Key benefits:
- Continuous strain measurement instead of point-wise readings
- Early detection of local anomalies
- Damage evolution from micro to macro level
2. Structural health monitoring (SHM)
DFOS technology is widely used in civil engineering for long-term monitoring of infrastructure — bridges, dams, tunnels and pipeline systems. Sensors provide continuous data on strain and temperature, helping to evaluate integrity and material degradation under operational loading.
Examples of monitored elements:
- Bridge girders and decks
- Concrete support columns
- Construction dams
- Tunnels and underground structures
3. Temperature measurement and compensation
Optical fibres can also measure temperature profiles along the sensor, which is critical for:
- Distinguishing mechanical strain from temperature changes
- Applications where temperature sensitivity matters
- Identifying overheating equipment
Some DFOS systems combine temperature and strain detection using different scattering mechanisms (e.g. Raman, Brillouin).
4. Embedding sensors in composite materials
In composite materials (e.g. aerospace or advanced engineering structures), DFOS sensors can be embedded directly within the material. This allows monitoring of internal stresses, curing processes, internal damage and structural changes without compromising the material’s integrity.
5. Detection of dynamic events and vibrations (DAS)
Some DFOS systems based on Distributed Acoustic Sensing (DAS) can detect dynamic changes caused by vibrations, acoustic signals or mechanical impacts. Such systems enable:
- impact and fatigue event detection
- tracking mechanical responses to external forces
- identifying unusual vibration patterns that may indicate damage
DAS uses the optical fibre as an acoustic sensor distributed along its full length.
6. Real-time monitoring and early-warning systems
DFOS provides continuous real-time data, ideal for in-service monitoring where you need to:
- track load changes immediately
- detect unusual structural behaviour continuously
- provide early warnings of possible failure
This is a major advantage over periodic manual measurements, because continuous sensing increases safety and enables automated decision-making.
7. Advanced active DFOS techniques
New research is developing advanced active DFOS methods such as distributed photoacoustic sensing, which integrates ultrasound generation and detection along the optical fibre. These approaches combine the benefits of ultrasonic NDT with distributed sensing and enable denser internal-defect detection at high spatial resolution.
Summary — main DFOS measured quantities
| Quantity | Measurement type | Role in NDT |
|---|---|---|
| Strain | Brillouin/Rayleigh | Detection of stress and structural change |
| Temperature | Raman/Brillouin | Separation of temperature effects |
| Dynamic signals / vibrations | DAS | Vibration and dynamic-event monitoring |
Conclusion
Distributed fibre-optic sensors (DFOS) are a highly flexible and powerful technology for non-destructive defect detection and structural health monitoring. They provide:
✔ continuous spatial measurements instead of isolated points ✔ high sensitivity to strain, temperature and dynamic signals ✔ ability to monitor long sensor routes (tens to hundreds of km) ✔ integration with operational systems for early damage detection and warnings
These properties make DFOS a highly suitable supplement to — or replacement for — traditional NDT methods.