Frequently Asked Questions on Fiber Bragg Grating Technology & Systems

The values measured by the optical interrogator (data acquisition system) are the peak wavelengths of the narrow spectrum reflected by the fiber Bragg grating sensor. When strain at the optical strain sensor causes the wavelength to change, the interrogator detects a change in the peak wavelength that is proportional to the strain.
The gauge factor or the sensor sensitivity specified on the sensor packaging is used as the proportionality factor.

HBM FiberSensing interrogators can be used with the available graphical user interfaces, such as the BraggMONITOR, and powerful acquisition and data analysis software, i.e. catman®.

Fiber Bragg gratings are both sensitive to strain and temperature. This means that a strain measurement of a sensor that is subjected both to strain and temperature is also influenced by the temperature change. But this effect is well characterized and easy to compensate. There are several methods to compensate this effect:

• With an additional fiber Bragg grating sensor measuring only temperature and under the same temperature change;
• With an additional optical strain sensor as dummy (installed on the same material, but without any strain applied);
• With an additional strain sensor ensuring that the applied strain is the same in value, but with different signals (e.g. having one strain sensor at the top and one sensor at the bottom of a cantilever);
• With an additional bare fiber Bragg grating, not fixed and terminal;
• Using an athermal strain sensor with a mechanical design to compensate for the undesired temperature effect in strain measurement;
• Among others…

Fiber Bragg Grating (FBG) strain dependence

The strain dependence of a fiber Bragg grating is given by the expression:

Where:

k– k factor of the Bragg grating

Fiber Bragg Grating (FBG) temperature dependence

The temperature dependence of a fiber Bragg grating is:

Where:

 – coefficient of thermal expansion of the fiber

ζ –  thermo-optic coefficient (dependence of the index of refraction on temperature)

Temperature dependence of a fixed Fiber Bragg Grating (FBG)

If the optical strain gauge is fixed to a rigid strain free structure, the temperature may change the index of refraction of the fiber, but its expansion is fixed by the structure. This is equivalent to consider the thermal expansion of a fixed fiber as =0. The temperature dependence of a fiber Bragg grating measuring strain is:

When measuring strain this temperature induced wavelength change is confused with strain. The measured strain that is actually caused by temperature is:

The cross-sensitivity to temperature (TCS) is therefore given by:

The effective strain should be calculated from the strain sensor as the strain measured by the strain sensor minus the effect of temperature on the strain FBG:

This correction of the deformation does not take into account the effect of temperature on the deformation of the structure where the sensor is fixed on.

Temperature dependence of a Fiber Bragg Grating (FBG) fixed to a structure

To compensate also for the deformation of the structure due to temperature effects, the computation should be done considering the coefficient of thermal expansion (CTE) of the structure.

The total strain variation of a structure is:

The wavelength variation of a sensor fixed to a structure that is subjected to load and temperature is given by:

Meaning that to compensate the deformation of the structure due to temperature effect it is necessary to know the CTE value of the material of the structure where the sensor is fixed on.

In order to measure only stress, the effect of temperature needs to be compensated. There are several methods for that, which include the usage of a special mechanical package or the use of an additional Fiber Bragg Grating (FBG) sensor.

Strain can only be removed from the measurement if the casing of the grating can isolate the strain of the structure from the sensing element. This can be achieved by the sensor’s mechanical design (e.g. FS63 Optical Temperature Sensors or the Optical Temperature Compensation Sensor OTC) or by using a bare FBG, terminal and not fixed. 

Fiber Bragg sensors have a higher layer thickness than electrical strain gauges. When measuring bending strain of thin components, there is a measurement error that must not be neglected but is easy to compensate for:

With
ε_OF   strain on the component's surface
ε_Anz  strain measured by the fiber
h    thickness of the component
d    distance of the fiber from the component's surface

There is, however, another aspect that should be addressed. Optical fibers have minimum bending radius due to optical losses. HBM FiberSensing sensors from the OP Line offer the possibility of measuring even on curved surfaces. The distance to consider with these sensors is d=0.5 mm.

The Bragg wavelength of the optical sensors is defined on the instant the fiber Bragg grating is produced. To ease production, standard values were defined.

Historical reasons dictated that the standard wavelengths are currently different in the FS Line and the OP Line.

In any case, other customized wavelengths between 1500 nm and 1600 nm are available upon request.

Some examples of harsh environments where HBM FiberSensing optical systems have been successfully deployed are: high temperature, high radiation, high vacuum, high-voltage and cryogenic environments.

The following applications have also been successfully conducted:

• Vibration and temperature monitoring in high power generators;
• Hot spot monitoring in power transformers;
• Wind blade monitoring;
• Stress monitoring in airplane fuel tanks;
• Strain, temperature and displacement monitoring in thermonuclear reactor;
• Spacecraft monitoring, etc.

The attenuation with distance is very small in optical fibers. In combination with HBM FiberSensing optical interrogators, optical fiber lengths can go up to tens of km.

There is a wavelength shift on the reflected Bragg peak when the grating is subjected to pressure. The wavelength variation is approximately:

This effect is, however, very small when compared with the wavelength variations induced by strain or by temperature changes being, therefore, commonly neglected.

When instead of pressure the FBG is subjected to a point lateral loading, a birefringence phenomenon occurs. This means that a new peak will appear (two peaks will coexist at the same time) and its shift can also be quantified.

Optical sensors, in particular, Fiber Bragg Grating (FBG) sensors are a choice of election if the requested number of sensors is relatively high or if the distances to and between the sensors are long. Also, for particular environments the technology may be one of the existing alternatives to conventional sensing.

Take the example of an application where distances are suited for measuring with classical strain gauges. If this application requires more than 30 sensors, it becomes cost beneficial to use optical sensors rather than conventional ones.

Moreover, the advantages inherent to the technology here may turn FBG/optical sensors as the only available or the best possible solution for certain applications.

If long distances (in the order of km) or very specific application scenarios (for example, high magnetic fields, intense EMI/RFI, risk of explosion, etc.) are to be considered, optical sensors may be the only available solution, since electrical sensors would simply fail or carry numerous problems.

Such is the case of applications in cryogenic environments which require immunity to electromagnetic effects (EMI, RFI, sparks…) and electrical isolation.

Among other long recognized advantages of FBG based sensors are:

• safe operation in potentially explosive atmospheres;
• high multiplexing capability allowing the connection of a large number of different types of sensors to a single optical fiber, reducing network and installation complexity;
• small size and weight making them suitable to hard-to-reach locations and measurement points;
• remote sensing: large distance between sensors and interrogator (several kilometers);
• no mechanical failure and high resistance to fatigue;
• ability to provide absolute measurements without the need for referencing: based on the measurement of an absolute parameter - the Bragg wavelength - independent of power fluctuations.

One main benefit provided by optical fiber Bragg measurement technology is that several sensors can be integrated in a single optical fiber. It is a prerequisite that these sensors hold different Bragg wavelengths.

The Bragg wavelength varies as a function of the temperature and the strain affecting the sensor. Therefore, clearance distances need to be guaranteed between the sensors' wavelength peaks so that overlapping does not occur. These are required to enable the interrogator to allocate the sensors on the basis of the reflected wavelengths within the available measurement spectrum.

Another characteristic that can affect the number of sensors is the available power at the reception of the fiber Bragg grating reflection. This depends on the emission power of the interrogator, on the losses along the way (bending, connectors, splices, fiber length…) and on the reflectivity of the fiber Bragg grating.

There are so many details that can influence the number of sensors that it is difficult to state a number. However, a recommended value is 13 or 14 sensors per fiber, which corresponds to the standard central wavelengths available at HBM FiberSensing.

The Fiber Bragg Grating (FBG) wavelength is defined during the sensor’s production and can be tuned to be any value between 1500 nm and 1600 nm. All types of sensors (temperature, strain, tilt, displacement, etc.) can be produced with any wavelength. Nevertheless, there are pre-defined wavelengths that ease the production process by making it repetitive. However, these are transversal and coexisting in all sensor types. The constraint in selecting the sensors wavelengths is that it is not possible to have two FBG sensors in series (in the same fiber) reflecting the same wavelength. If multiple FBG sensors are in the same optical fiber, the only requirement in terms of their Bragg wavelength is that they do not overlap (each FBG should have a unique Bragg wavelength and should not overlap with each other within the measuring range). Sensors may reflect the same wavelengths as long as they are being measured in different optical channels of the interrogator (data acquisition system). Usually, the wavelength of the sensor is defined by the customer on request or by the engineering team during the project’s design.

The optical sensor may be attached to the specimen in different ways. Optical sensors produced by HBM FiberSensing may be glued, spot welded to metallic structures, embedded (into concrete, for instance), inserted in composite material, fixed with screws…

The influence of temperature on the sensor is immediate. It only depends on the heat transfer through the material.

All temperature sensors are delivered with a calibration sheet including polynomial calibration function.

An optical strain sensor does not have an active grid length as it can be defined for electrical strain gauges.

The gauge length depends on the sensor encapsulation and is stated on the sensors datasheets. In case a bare FBG is used, or a sensor where the full length of the grating is attached, the gauge length corresponds to the grating length that is approximately 6 mm.

Cold-curing adhesives used in the bonding of optical sensors do not offer long-term stability at increased relative humidity. This applies for cyanoacrylate adhesive (Z70) in particular.

Epoxy resin systems (X280), however, are resistant against the influence of humidity.

Please note that humidity affecting optical sensors results in swelling of the materials used. In the case of the optical strain gauges, this generates forces affecting the Bragg grating. This has a negative effect on the measuring point's stability.

At any rate, we recommend using a covering agent similar to those used with electrical strain gauges.

The tilt sensor includes two FBGs and both are needed to get the angle measurement without temperature influences. The two fiber Bragg gratings operate in a push-pull configuration which means that when one sensor is being tensioned due to the sensor position the other is being compressed with the same strain value. With this configuration we can identify the angle variation by the wavelength variation that is equal in value but with opposite signals. The temperature effect causes equal wavelength variations to the FBGs and is, therefore, removed.

The sensitivity of the tilt does not depend on the initial angle of the sensor. However, the sensor operates as a pendulum, so the angle measurement range of ±5 deg is centred at the vertical and the sensor will not operate outside these boundaries.

The temperature limitation of -20ºC to 80ºC is true for FS line sensors. The limits are given by the acrylate coating of the fiber.

There are many reasons to select FBG sensors for the monitoring of wind turbine blades. The following table compares FBG sensors to conventional strain gauges and to other types of optical technology.

You may find more information on HBM FiberSensing optical system for monitoring and testing applications on Wind turbines here.

We can say that the fiber Bragg Grating alone (bare fiber) has “no limit”. The limitation on the sampling rate is on the interrogator side. The interrogation technology will dictate the maximum samples per second that can be measured. HBM FiberSensing standard interrogators are currently limited to 1000 S/s, but the company has in house technology for acquisition rates up to 10 kS/s. For encapsulated sensors, i.e. with some mechanical apparatus around, the virtual limit might not be true as the mechanical interface limits the strain transfer to the grating.

The fundamental difference between static (SI) and dynamic (DI) interrogators lies in their sampling rate: while the first are used for quasi-static applications, such as temperature measurements, the second – with higher acquisition rates are suitable for dynamic applications-(with higher sampling rates, such as strain measurements). Please check the HBM FiberSensing interrogators page and the available options.

There are also some differences in terms of design. The static interrogators include an embedded traceable wavelength reference for continuous calibration, thus providing better accuracy and resolution. On the other hand, the dynamic interrogators are most suited for higher frequency measurements - due to their higher sample rates. Additionally, the SI have an internal memory to store data without an additional PC. and can operate as stand-alone.

One feature that is unique to the static interrogators is SPD (Smart Peak Detection). Learn more about it here.

Although the interface is quite similar, HBM FiberSensing has available two distinct applications: BraggMONITOR SI for the static interrogator FS22 Industrial BraggMETER SI and BraggMONITOR DI for the dynamic interrogator FS22 Industrial BraggMETER DI.

The central wavelength value that must be filled during the sensor edition corresponds to the wavelength (λ0 in nm) value from which the wavelength variation (x=Δλ, in nm) is calculated. This means that the result for the wavelength variation of a sensor in an instant t (xt, Δλt) is:

Where λt is the wavelength of the sensor measured at the instant t.

If the user’s purpose is to “zero” the measurements from an instant, the value to insert on the CWL field should be the value measured at that particular instant. On the other hand, if the user needs absolute measurements (as, for instance, on temperature sensors) the CWL to use should be the one defined on the calibration Sheet that is provided with the sensor.

Using the temperature sensor example

On the Calibration Sheet of an HBM FiberSensing temperature sensor, the temperature is described
as a second order polynomial of the wavelength variation:

Where:

S2 is the second order sensitivity,

S1 is the first order sensitivity,

S0 is the temperature offset.

The S0 value corresponds to the reference temperature at the calibration procedure, so to get absolute temperature values the x has to be calculated using the same central wavelength from the calibration: the used CWL in the measurements has to be the same as the stated on the calibration sheet of the sensor.

Using the strain sensor example

The FBG based strain sensors dependence to deformation is:

Where k is the k factor of the strain sensor, and S is the strain sensitivity indicated on the calibration sheet.

This deformation will always be registered from an instant that is defined as “zero” meaning that the x value will always be calculated relatively to the wavelength that the sensor was exhibiting at the “zero” moment after the sensor installation.

There is a limit to the number of sensors that can be addressed with an interrogator that has a limited range, let’s say [1500; 1600] nm. However, it is still possible to have multiple sensors in a single optical fiber (tens or even hundreds of sensors), as long as the Bragg wavelengths of each sensor are distinct and do not overlap within their measuring range.

As an example, the interrogator range is [1500; 1510] nm and one would like to have 3 sensors measuring in this range.

If the Bragg wavelengths of the sensors are as follows:

sensor1 = 1502 nm

sensor2 = 1505 nm

sensor3 = 1508 nm

and all the sensors have a wavelength shift of +/- 1 nm during measurement, there will be no overlap during measurement.

If for instance, the measurands cause the wavelength shift of the sensors to be +/- 3 nm, overlap of the Bragg wavelength within the measuring range would occur and one would be limited by that.

It is good practice to join a temperature sensor to a strain sensor in order to compensate for the temperature effect on the strain sensor.

However, there’s no need to spend two channels to measure temperature and strain.

FS22 Industrial BraggMETER interrogators from HBM FiberSensing (with up to 8 channels) may measure sensor arrays with several sensors.

HBM FiberSensing athermal strain sensors are able to measure strain regardless of the temperature with only one FBG.

HBM FiberSensing interrogators communicate via Ethernet. With this, it is possible to connect them via Wi-Fi or GSM using  dedicated equipment connected to the interrogator. Care has to be taken, though, to the reliability of the transferred data. For example, on the dynamic interrogators with higher acquisition rates the needed bandwidth to ensure flawless data transfer is high.

Optical sensors do not comprise any communication as the sensors do not have any electronics involved. Optical sensors are electrically passive and rely only on a bare optical fiber.

HBM FiberSensing interrogators are small and can easily be carried around. Nevertheless, FS22 Industrial BraggMETERs need power supply and a PC for an interface, which makes the equipment harder to move.

The FS42 Portable BraggMETER interrogators are designed for on-the-move applications. These are stand-alone interrogators that can provide measurements in disperse sites and be used with any type of FBG sensors (temperature, strain, tilt, etc.). They include batteries, a touch screen interface and a built-in software in the internal PC. It is very common to use these portable interrogators in the field during the installation of networks of sensors, or to perform in-situ live measurements.

The FS22 Industrial BraggMETER and the FS42 Portable BraggMETER interrogators are calibrated upon production and include a traceable internal reference that ensures their accuracy over time.

Nevertheless, it is possible to perform a certified calibration for all interrogators from time to time.

Yes, in the case of the FS22 Industrial BraggMETER interrogators. These interrogators communicate via Ethernet with TCP/IP interface.

In the case of the FS42 Portable BraggMETER interrogator the answer is no.

Drivers are supplied with the interrogators’ support material and can also be downloaded on our website.

The SPD is only embedded in the real time operating system of the FS22 Industrial BraggMETER SI static interrogator.

HBM FiberSensing FS22 Industrial BraggMETER interrogators need a PC for configuration, data management and visualization.

After a measurement is configured, the static FS22 Industrial BraggMETER SI interrogator can be left as stand-alone while data is being stored locally. Later on, a PC is needed for data retrieving.

The case of the dynamic FS22 Industrial BraggMETER DI interrogator is different, as it does not have storage capability and requires a PC to receive the measured data. The FS42 Portable BraggMETER interrogator has its own computer embedded, therefore it is completely independent.