Frequently asked questions

A fiber optic sensor is a sensor that works using fiber optic technology. Such a sensor uses optical fiber as a sensing element to measure physical quantities such as temperature, acceleration, tilt, strain, pressure and many more. Fiber optic sensors have many advantages over electronic sensors!

An optical fiber is as thin as a human hair, and consists of two basic elements made of glass, the core, and the cladding. The core is the part of the fiber in which light is guided.

Fiber optic sensing uses an optical interrogator to send infrared light down the fiber. The light passes through a Fiber Bragg Grating or FBG for short. An FBG is a periodic modulation in the core of the fiber which returns a specific wavelength or colour of light. The wavelength which is reflected is determined by the reflective index and periodicity of the FBG. External factors such as heat and strain will cause a shift in the reflection wavelength, as seen in the picture below. These variations can be translated to physical engineering units, such as temperature, and strain.

Because each FBG sensor is reflecting a specific wavelength band, it is possible to put multiple FBG sensors on one fiber! One optical cable is enough to support over 80 fiber optic sensors!

Fiber optic sensors have a lot of advantages over electronic sensors! Fiber optic sensors are passive sensors:

• Intrinsically safe

• Immune to HV fields

• No power at sensing location

Fiber optic sensors can withstand harsh environments:

• Liquids/ moisture

• Extreme temperatures

• High robustness

Optical signal:

• Immune to EM interferences

• No pre-amps required

• Remote operation over several km lengths

Fiber optic sensors can withstand hazardous environments

• ​ATEX zones

• Chemicals

• Radiation hard

• Electromagnetic Immunity: perfect for microwave environment, immune to radio frequency interference (RFI) and electromagnetic interference (EMI

More advantages:

• Enable small sensor sizes, do not contaminate their surroundings and are not subject to corrosion.

• Compact and Light; perfect match for surface mounting and embedding applications. Require small cable sizes and weights.

• Wide Dynamic Range: ability to monitor a wide range of physical and chemical parameters and thus permit remote sensing

• Integrated telemetry: fiber itself is a data link

• Can be tailored to specific needs

• Multiplexing and distribution Capabilities of sensors are sole as they offer measurements at a greater number of points along a single optical cable: ideal for minimizing cable deployment and cable weight, or for monitoring extended structures like pipelines, dams etc.

The working principle of strain gauges is based on the change in electrical resistance of the foil material as a result of mechanical elongation. This principle has been used for a long time, but it has a few limitations in comparison to fiber optic strain sensing.

Firstly, one strain gauges needs 2/3- conductor wire connected to the DAQ unit or signal conditioner. This makes it very impractical to establish high-density distributed strain gauge systems. Fiber optics however do not have this issue. Fiber optic sensors use optical fibers to measure strain, but the fiber itself is also the data link. This means that to establish a high-density network of strain measurements, less cable is needed, and therefore less weight. These weight savings are substantial when high density-strain measurements are needed for large structures, as for example, building, bridges, or pipes.

A second limitation is that strain gauges require specific installation techniques which can generally only be performed by a professional. Furthermore, to ensure a good connection between the gauges and the wires, excellent soldering is needed. If not done correctly, the measurements may not be accurate.

Soldering in strain gauges is comparable to splicing in fiber optics. Nevertheless, splicing only has to be done ones for each fiber supporting thousands of strain measurement points.

Lastly, strain gauges are very sensitive to electromagnetic interference (EMI). This means that every electrical device that consumes, generates, or transmits power is likely to cause noise in strain gauge circuits.

Fiber optic sensors however are immune to electromagnetic interference.

Fiber optic sensors are known for their long life and high reliability. So far, fiber cables don’t seem to degrade like other infrastructure. It’s likely that the future of fiber optics will even outlast the next generation of devices and industrial requirements.

Most of Somni's fiber optic sensors consist of a glass fiber and a monolithic structure that translates the quantity to be measured into strain to the fiber. Therefore, the number of components in this type of sensor is very small. In failure mode analysis (FMEA) this leads to a very small failure rate of the sensor compared to for example electrical sensors. In other words, fiber optic sensors are very reliable sensors. This has also been proven in practice.

Yes, a fiber optic sensor can be used to measure pressure! Check out our pressure sensors on the products page!

Optical fiber measurands:

• Temperature

• Pressure

• Force

• Vibration

• Rotation/tilt

• Strain

• Acceleration

• Displacement

• Flow

• Load

Yes, a fiber optic sensor can be used to measure temperature! Check out our temperature sensors on the products page!

Optical fiber measurands:

• Temperature

• Pressure

• Force

• Vibration

• Rotation/tilt

• Strain

• Acceleration

• Displacement

• Flow

• Load

Yes, a fiber optic sensor can be used to measure strain! Check out our strain sensors on the products page!

Optical fiber measurands:

• Temperature

• Pressure

• Force

• Vibration

• Rotation/tilt

• Strain

• Hydrogen levels

• Acceleration

• Displacement

• Flow

• Load

Yes, a fiber optic sensor can be used to measure acceleration! Check out our acceleration sensors on the products page!

Optical fiber measurands:

• Temperature

• Pressure

• Force

• Vibration

• Rotation/tilt

• Strain

• Acceleration

• Displacement

• Flow

• Load

Yes, a fiber optic sensor can be used to measure tilt! Check out our tilt sensors on the products page!

Optical fiber measurands:

• Temperature

• Pressure

• Force

• Vibration

• Rotation/tilt

• Strain

• Hydrogen levels

• Acceleration

• Displacement

• Flow

• Load

Yes, one of the main applications for fiber optic sensors is structural health monitoring. Structural health monitoring is the process of implementing a damage identification strategy by monitoring changes to the material and geometric properties of civil, aerospace, and mechanical engineering infrastructure. Structural health monitoring has many benefits.

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Structural health monitoring reduces uncertainties

Owners of a structure have little information about the real state of the materials, the real loads acting on the structure, and about its ageing. When making decisions, the owners have no other choice than to assume the worst to keep the structure on the safe side. Monitoring gives the owner the ability to make informed decisions based on factual data. By reducing the uncertainty associated with the insured risk it can also reduce the insurance costs.

Structural health monitoring discovers hidden structural reserves

Most structures are in far better condition than expected. When taking advantage of over-design, better material properties, and synergetic effect it is possible to extend the lifetime or load-bearing capacity of a structure safely without any intervention. By doing this, the repair and replacement costs can be delayed.

Structural health monitoring discovers deficiencies in time and increases safety

Some deficiencies cannot be found by visual inspection or modelling. In these cases, it is important to undertake appropriate remedial or preventive actions before it gets worse. Repairing such a deficiency will be much cheaper and will cause less disruption to the use of the structure if it is done at the right time. Monitoring a structure gives permanent and reliable data to improve the safety of the structure and its users.

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Structural health monitoring ensures long-term quality

Monitoring a structure provides continuous and quantitative data which helps in evaluating the quality of the structure during construction, operation, maintenance, and repair, thus eliminating the hidden costs of non-quality. Many of the damages and defects to a structure are build in during the process of construction. However, many of them will not be visible till years later, when repair is no longer covered by the contractor’s warranty. This makes repair much more expensive.

Structural health monitoring allows structural management

Monitoring data can increase the quality of decisions by providing reliable and unbiased information, therefore allowing owners to spend on maintenance and reconstruction when required and not only when budget allows it.

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Structural health monitoring increases knowledge

Knowing how a structure performs in real conditions, will lead to performance-based designs in the future. This will bring about structures with increased reliability and performance, which are also cheaper, safer, and more durable. Investing in monitoring will lead to savings later in the project by discovering weaknesses in time and optimizing the design.

Check out our applications and projects page for more information!

Yes definitely! Fiber optic sensors can be used for structural health monitoring and condition monitoring of civil engineering structures.

For example, Somni has installed low frequency fiber optic accelerometers to monitor the vibrations and movements of the tuned mass damper in the Taipei 101. By monitoring the vibrations, information is extracted to identify changes in the behaviour of the structure which is indicative of a developing fault. But monitoring the vibrations also gives information about the way the structure reacts to wind or for example an earthquake. Therefore, condition monitoring is a major part of predictive maintenance and increases knowledge about the behaviour of the structure.

Somni has done many more projects for structural health monitoring and condition monitoring. Check out our projects and applications page for more information!

Yes, fiber optic sensors can be used in biomedical instrumentation. Fiber optic sensors have some unique properties that make usage in the biomedical field possible:

• Small dimensions.

• Light weight.

• Non-electrical connection to patients.

• Ability to monitor multiple measurands.

• Biocompatibility with MRI and CT.

• Usage in high EM (Electromagnetic) or RF (Radiofrequency) environments, due to its high immunity to EM interferences.

• Minimally invasive or non-invasive technique, because of the inherent penetrating properties of light signal.

• Fast speed since light is used.

Our fiber optic sensors are compatible with the following interrogators (among others):

Your interrogator type or brand not listed? Contact us.

Need advice regarding interrogator selection? Contact us

This depends very much on the interrogator and the sensors that will be used. To clarify this, an example is given.

Example:

We want to connect as many Somni AC-8-NT accelerators as possible to a LUNA Hyperion interrogator. The AC 8 NT uses a bandwidth of 8 nm, and the Luna interrogator has an available bandwidth of 160 nm. This means that 20 AC-8-NT’s can be connected to one channel (so on one fiber). The LUNA interrogator has 4 channels, which means that 80 AC 8 NT sensors can be read simultaneously.

If an interrogator with less bandwidth or less channels is chosen, correspondingly less sensors can be connected to this interrogator. The AC 8 NT has a bandwidth of 8 nm, but when another sensor is required which for example only uses a bandwidth of 3 or 4 nm, more sensors can be connected to the same interrogator.

So, there is no simple answer to this question. The number of sensors that can be connected to an interrogator depends on the type of sensors and the type of interrogator used.

Yes, it is possible to place the sensors far away (up to 10 km) from the interrogator. If you want to know more about this, please contact us.

Yes, it is possible to connect multiple points along one fiber. FBG sensors can be installed in series by using FC/APC patch cords.

Yes, fiber optic sensors can be embedded in composite materials. Please go to our “embedding sensors in composite materials” page for more information.