Featured image: AI-generated illustration. The technical configuration shown is for illustrative purposes.
Fiber optics transmit more than just data. In a suitably designed sensor system, they can detect changes along a facility and map them spatially. This guide explains measurement principles, methods, fields of application, and the questions that should be clarified before starting a project.
A single sensor path can provide information from many locations without requiring an electrically powered device at every measurement point. This is particularly interesting for long routes, large areas, and hard-to-reach zones. However, the glass fiber alone does not decide which measured variable is actually available. Measurement methods, sensor cables, installation, evaluation units, and project configuration must all be matched.
What is fiber-optic sensor technology?
Fiber-optic sensor technology uses light and optical fibers to detect physical changes. A fiber-optic sensor converts an input, such as temperature, strain, or vibration, into a measurable change in an optical signal. Depending on the design, the glass fiber itself acts as the sensor element, or it guides light to and from a spatially separated sensor head.
Fiber-optic sensors therefore differ in design and spatial effect. Traditional electrical sensors usually measure at defined points and require electrical cabling or local electronics there. An optical sensor area, on the other hand, can be designed to be passive. The light source, detection, and signal evaluation are typically located in an evaluation unit. This is advantageous in electromagnetically demanding environments, but it does not replace the need to verify cable construction, protection class, temperature range, and installation.
How does a fiber-optic sensor work?
An evaluation unit couples light into the glass fiber. As the light propagates, a small portion is backscattered by irregularities in the glass material. Temperature, strain, or dynamic mechanical inputs can change the intensity, frequency, phase, or travel time of the evaluated light. The system compares this optical information with its measurement model and calculates location-based measured values or event characteristics from it.
In time-domain-based methods, the position is calculated simply from the time between the emitted light pulse and the received backscattered signal. Other systems evaluate frequency or correlation information. The details vary by method. For planning, it is more important to know that range, spatial resolution, measurement speed, and signal quality influence each other and must be matched to the task.
What role do Rayleigh, Raman, and Brillouin scattering play?
The three scattering mechanisms react differently to influences in the fiber. Rayleigh-based methods are frequently used for dynamic strain and vibration. Raman backscattering is a common foundation for distributed temperature measurement. Brillouin-based methods can provide information on strain and temperature. This classification is a guide, not a complete system description. The specific measured variable also depends on the device architecture, the fiber, and the evaluation method.
Point, quasi-distributed, and distributed sensor technology
Not every fiber-optic sensor measures continuously along the entire fiber. The spatial arrangement of the measurement locations is a key selection criterion.
| Sensor type | Measurement locations | Typical configuration | Classification |
|---|---|---|---|
| Point | A defined location | Fiber sensor or optical sensor head at a specific position | Targeted measurement when a few clearly known measurement points are sufficient |
| Quasi-distributed | Multiple discrete locations along a fiber | e.g., a chain of Fiber Bragg Gratings | Many addressable measurement points, but no gapless measurement between them |
| Distributed | Continuously evaluated sections along the fiber | Backscatter-based Distributed Fiber Optic Sensing methods | The fiber forms a linear sensor path with system-dependent spatial resolution |
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In distributed fiber-optic sensor technology, "continuous" does not mean that every arbitrarily small point is measured independently. The system aggregates the optical response over a finite spatial length. How finely events can be separated and localized is determined by spatial resolution, sampling, measurement method, and signal processing.
Which measured variables can be detected?
Vibration and acoustic events
DAS usually detects dynamic strain or its change in the fiber. From this, vibration or acoustic patterns can be obtained. The cause is identified through classification, not directly "heard" like with a microphone.
Temperature
DTS provides a temperature profile along the sensor path. Local heating, cooling, or spatial gradients can thereby be made visible.
Strain
DSS detects location-dependent changes in the fiber's length. Appropriate mechanical coupling is crucial if changes in a structure are to be derived from this.
Pressure and indirect events
Pressure can only be meaningfully measured with a dedicated sensor design or transmission element. Leaks, excavation work, or tampering are often indirect events that are classified based on thermal, acoustic, or mechanical patterns.
In principle, the same fiber can be used for different optical evaluations depending on the system. However, this does not result in automatic multi-variable measurement. Sensor cables, fiber allocation, coupling, interrogator, and software must support the respective measurement task.
DAS, DTS, DSS, and DTGS in comparison
| Method | Typical measured variable | Typical events or applications | Important note |
|---|---|---|---|
|
DAS Distributed Acoustic Sensing |
Dynamic strain, vibrations, and acoustically induced mechanical effects | Footsteps, vehicles, digging, fence events, route and machine activity | DAS does not automatically measure the cause. Event classes must match the installation and environment. |
|
DTS Distributed Temperature Sensing |
Temperature and temperature profile along the fiber | Hotspots, thermal load, fire and leak warnings, cable and tunnel monitoring | The sensor cable must be thermally coupled appropriately and designed for environmental conditions. |
|
DSS Distributed Strain Sensing |
Static or slowly changing strain, depending on the method | Structural Health Monitoring, settlement, deformation, geotechnical changes | Temperature and strain influences can be coupled. Compensation and referencing are part of project planning. |
|
DTGS FOTAS: Distributed Thermal Gradient Sensing |
Thermal trends, change rates, and gradients based on location-specific temperature information | Conspicuous thermal profiles on power cables, pipelines, storage tanks, or connections | DTGS is used here according to FOTAS terminology and not represented as a universally defined industry standard. |
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Abatec offers in-depth information on the shop pages for Distributed Acoustic Sensing and FOTAS-DTGS technology, where the respective technology and specific system designs are in the foreground.
Advantages of fiber-optic sensors
- Many location-based pieces of information: Distributed methods form numerous virtual measurement zones along a fiber.
- Monitoring of long distances: Linear facilities and large areas can be viewed from a central evaluation unit.
- Passive sensor path: Electrical sensor devices are not required at every measurement location along the fiber.
- Electromagnetic compatibility: The optical fiber itself is insensitive to electromagnetic interference fields.
- Access to difficult areas: A fiber can be routed along tunnels, routes, fences, or inside structures.
- Existing infrastructure as an option: Spare fibers can be used under certain technical conditions.
These advantages do not apply generally to every fiber and every application. The achievable quality of the information depends significantly on how the sensor cable is coupled with the monitored object or environment.
Limitations and requirements
Fiber-optic sensor technology requires careful project planning. High sensitivity alone is no proof of quality. The decisive factor is whether relevant events can be distinguished from background signals under real environmental conditions.
- Fiber and cable construction: Fiber type, jacket, armoring, and temperature suitability must match the measurement method and installation location.
- Mechanical or thermal coupling: Vibration, strain, or temperature must reach the fiber in a defined way.
- Route planning: Loops, branches, reserve storage, and uncoupled sections influence spatial mapping.
- Attenuation and connection points: Fiber length, connectors, splices, and losses determine the available optical signal budget.
- Spatial resolution and response time: Both must be set appropriately for the event and the size of the zone to be monitored.
- Environment: Traffic, machines, wind, precipitation, or changing loads can generate relevant and irrelevant signals.
- Calibration and classification: Alarm rules or event models must be validated in the project and updated if necessary.
- System integration: Interfaces, alarm zones, timestamps, data storage, and response processes must be defined in advance.
Typical applications of fiber-optic sensor technology
Perimeter protection and industrial areas
Along fences or buried sensor paths, DAS can spatially detect mechanical activities. Whether a signal is classified as climbing, cutting, footsteps, or a vehicle must be trained or parameterized for the fence type, subsoil, and environment.
Pipelines, cable routes, and leak detection
Acoustic, thermal, or mechanical changes can provide indications of excavation activity, flow events, or escaping media. Leak detection is therefore not a single universal measurement function. The medium, pipe, sensor location, and evaluation method determine which physical signs can be utilized.
Energy facilities and ground-mounted PV
Fiber-optic temperature measurement can observe thermal profiles on cables, connections, or linear power systems. For ground-mounted PV, DAS can additionally be relevant for perimeter and route events. The specific benefit depends on cable routing, terrain, security concept, and available fibers.
Tunnels, rail, and transport infrastructure
Distributed temperature and vibration data can provide supplementary monitoring of long structures and transport routes. Separating normal operation, environmental influences, and conspicuous events is an essential part of the design.
Structural Health Monitoring
For buildings and geotechnical structures, distributed strain measurement can reveal changes along larger areas. To ensure the measurement reflects structural movement, planning for cable selection, fastening, embedding, reference status, and temperature compensation is required.
Data centers
A technically concrete use is linear temperature monitoring of power distribution, busbars, or cable routes. Perimeter areas can also be supplemented with fiber optics. The technology is therefore not a blanket sensor solution for all operational data in a data center.
Can an existing glass fiber be used as a sensor?
In principle, this may be possible, especially with suitable, freely available single-mode fiber. However, a blanket guarantee would not be technically sound. First, it must be clarified whether the fiber runs where the event is to act on the fiber, and whether the cable construction and coupling transmit the desired measured variable.
Additionally, allocation, accessibility of fiber ends, splice plans, connections, attenuation, route length, reserve storage, and possible interruptions must be checked. A telecommunications fiber might be optically accessible but still poorly positioned for the desired sensor task. Whether parallel use is possible depends on the specific system and network design.
For project-related verification and integration into security or control systems, the Abatec solution page on fiber-optic monitoring for critical infrastructure is the appropriate next step.
Which technology fits the task?
- Define measurement goal: Should dynamic events, temperature, strain, or thermal trends be recorded?
- Describe the event: What does a relevant condition look like, and what noise sources occur during normal operation?
- Determine route and spatial resolution: What length needs to be covered, and how precisely must an event be localized?
- Check fiber path: Is a suitable fiber available, free, and sufficiently coupled to the measurement object?
- Plan environment and installation: Consider temperature, humidity, mechanics, electromagnetic environment, and accessibility.
- Define data forwarding: Should the result be displayed, logged, or transmitted as an alarm to SCADA, a control center, or a security system?
- Plan for validation: Include reference measurements, alarm tests, calibration, and acceptance criteria in the project plan.
An overview of the offered acoustic and thermal system families can be found at FOTAS Sensor Technology in the Abatec Shop. Concrete system pages such as the FOTAS D-50 DAS System, the FOTAS DTS CH4 temperature sensor system, and the FOTAS DTGS DCH temperature gradient system show available versions. The appropriate selection remains project-dependent.
Frequently Asked Questions about fiber optic sensing
What is a fiber optic sensor?
A fiber optic sensor uses light to make a physical influence measurable. The glass fiber itself can be the sensor element, or it can transmit light to a separate sensor head.
How does fiber optic sensing work?
An evaluation unit sends light into the fiber and analyzes the returning or transmitted light. Depending on the method, changes in intensity, frequency, phase, or transit time are assigned to temperature, strain, or dynamic influences.
What is distributed fiber optic sensing?
With Distributed Fiber Optic Sensing (DFOS), the fiber is evaluated over its entire length as a linear sensor path. Measured values or events can be assigned to a position with system-dependent spatial resolution.
Can an existing fiber optic cable be used as a sensor?
In some cases, yes. Requirements include, among other things, a suitable spare fiber, appropriate routing and coupling, sufficient optical signal budget, and access to the fiber. A technical assessment is necessary.
What is the difference between DAS and DTS?
DAS monitors dynamic mechanical and acoustic influences on the fiber. DTS determines temperature values and temperature profiles along the fiber.
What is the range of fiber optic sensor systems?
There is no universal value. Range and spatial resolution depend on the method, device, fiber, attenuation, connection points, measurement speed, and the required signal quality. The specific system data sheet and project assessment are authoritative.
Which events can DAS detect?
DAS can detect signal patterns from vibrations and dynamic strain. Depending on the installation and classification, these can be used to identify, for example, footsteps, vehicles, excavations, or fence tampering.
For which facilities is fiber optic monitoring suitable?
Typical examples include long or extensive facilities such as fences, pipelines, cable routes, tunnels, railway lines, energy plants, and larger structures. Suitability depends on the measurement goal and the technical coupling.