Fiber Bragg Grating Temperature Sensor: How It Works and Where It Is Used

Some temperature measurement jobs are difficult for conventional electrical sensors. The measuring point may be close to high voltage equipment, exposed to strong electromagnetic interference, or located far from the monitoring system. There may also be several points along the same structure or process that need to be checked.

A fiber bragg grating temperature sensor offers a different way to handle such applications. Instead of depending on an electrical signal at the sensing point, it uses changes in the wavelength of light travelling through an optical fiber. Several sensing points can also be placed along one fiber, which makes the technology useful when temperatures need to be checked at more than one location.

Tempsens offers the Bragg Senz Fiber Bragg Grating Sensor for this type of measurement. To understand where such a sensor fits, it helps to first look at what happens inside the fiber.

What Is a Fiber Bragg Grating Temperature Sensor?

A Fiber Bragg Grating, or FBG, is a small section created inside the core of a single-mode optical fiber. Within this section, the refractive index of the fiber is changed in a repeated pattern. This forms what is known as a grating.

When light travels through it, the grating reflects one particular wavelength while the remaining light continues through the fiber. The reflected wavelength is called the Bragg wavelength.

Temperature changes affect the grating. As it expands or contracts, the wavelength reflected by it shifts. By detecting and measuring this wavelength shift, the system can determine the corresponding change in temperature.

This is quite different from an RTD or thermocouple, where the measurement depends on an electrical property or voltage.

How Does an FBG Temperature Sensor Work?

The working process can be understood in three basic stages.

Light Travels Through the Optical Fiber

Light from the measurement system is sent through the fiber. When it reaches the Fiber Bragg Grating, a particular wavelength is reflected while other wavelengths continue along the fiber.

The reflected wavelength depends on the spacing of the grating and the refractive properties of the fiber.

Temperature Changes the Grating

When the temperature around the sensor changes, the optical fiber responds to that change. The grating spacing and refractive index are affected, causing the reflected Bragg wavelength to move.

An interrogation unit detects this shift and converts it into a usable measurement.

Tempsens’ technical work on FBG temperature sensing reports a temperature sensitivity of approximately 11.5 pm/°C for its experimental setup. This means temperature can be tracked through very small changes in the reflected wavelength.

Strain Must Be Considered

There is one important point when using FBG technology for temperature measurement: an FBG responds to both temperature and strain.

If the fiber is stretched or compressed, the Bragg wavelength can shift even when the temperature has not changed. A temperature sensor therefore needs to separate the temperature response from the effect of mechanical strain.

Tempsens explains that, for temperature-only measurement, the FBG can be placed inside a protective case with both ends left loose. This protects the sensing element while reducing the influence of strain on the temperature reading.

Why Use FBG for Temperature Measurement?

FBG sensors make sense in applications where the properties of optical fiber provide a practical advantage.

One such situation is an area with strong electromagnetic interference. Since the sensing method is optical, the sensor is not affected by electromagnetic interference in the same way as an electronic sensor.

The sensing element is also passive. Electrical power does not need to be supplied directly to each FBG sensing point. This becomes useful when sensors have to be installed along long pipelines, transmission systems or other extended assets.

The optical fiber itself is lightweight, and several measurements can be taken along the same sensor cable.

Single-Point and Multi-Point Temperature Sensing

One of the more useful features of FBG technology is its ability to support both single-point and multi-point measurement.

For a single-point application, a grating can be positioned at the exact location where temperature needs to be monitored.

For multi-point monitoring, several gratings can be written at known positions along the same optical fiber. Each grating is designed to reflect a particular wavelength. The monitoring system can identify the different reflected wavelengths and read the sensing points separately.

This reduces the need to run an individual electrical cable to every measurement point. Tempsens states that FBG sensing can be carried out at one or multiple points over several hundred metres of optical cable.

Where Are Fiber Bragg Grating Temperature Sensors Used?

Pipelines and Extended Assets

Long pipelines are a natural fit for optical sensing because monitoring may be required at several locations spread across a large distance.

Tempsens identifies gas pipelines and oil and gas exploration among the temperature applications for FBG sensors. Multiple sensing locations can be defined along the fiber according to where measurements are required.

The passive nature of the sensing points is also helpful where providing electrical power at every location would add complexity.

Electrical and High-EMI Environments

Electrical and electronic installations can contain high voltages and considerable electromagnetic interference. These conditions may create difficulties for conventional electronic sensors and their signal cables.

FBG sensors transmit measurement information optically rather than through an electrical sensing signal. This makes the technology relevant to temperature monitoring in areas where electromagnetic interference is a concern.

Research and Development

FBG sensors are also used in research and development where small changes need to be studied or several physical parameters need to be monitored.

Tempsens has tested a single-point bare FBG for temperature measurement up to 600°C in an experimental setup. An RTD was used as the reference sensor during this test.

The technology can go beyond temperature as well. Depending on how the sensor is designed and installed, FBGs can measure strain, load, pressure, vibration and tilt.

Structural Monitoring

FBG technology is well suited to structural health monitoring because sensing points can be installed at selected positions along a structure.

Temperature is relevant here because thermal expansion can influence strain measurements. Tempsens notes that an FBG temperature sensor can be installed alongside a strain sensor so the local temperature effect can be compensated.

FBG sensors can also be applied to vibration measurement in structures, heavy machinery, bridges and similar infrastructure.

FBG Temperature Sensors vs Conventional Temperature Sensors

FBG sensors do not replace RTDs and thermocouples in every application. Each technology has its place.

RTDs are widely selected where accurate contact temperature measurement is required within their working range. Thermocouples suit a broad range of industrial processes and can handle very high temperatures depending on their type and construction.

FBG sensors become particularly useful when electrical interference, multiple sensing points, long measurement distances or passive sensing are part of the application.

The decision should therefore be based on the measurement environment rather than treating one sensor technology as better in every situation.

What to Consider When Selecting an FBG Temperature Sensor

Start with the measurement points. A single location may only require one grating, while a long structure may benefit from several FBG points on the same fiber.

The operating temperature must also be considered. The sensor construction and protective arrangement need to suit the actual environment in which the fiber will work.

Mechanical strain is another factor that cannot be ignored. Since both temperature and strain can shift the Bragg wavelength, a temperature-only application needs a sensor arrangement that limits unwanted strain effects.

Finally, consider the complete measurement system. An FBG installation requires an interrogation unit capable of detecting the reflected wavelength shifts and turning them into temperature readings.

Final Thoughts

A fiber bragg grating temperature sensor measures temperature by tracking a shift in the wavelength of light reflected from a grating inside an optical fiber. That simple principle gives it some useful properties: the sensing point is passive, electromagnetic interference is less of a concern, and several measurement points can be placed along one fiber.

These advantages are particularly relevant for pipelines, electrical environments, research setups and structural monitoring.

The technology still needs to be selected with care. Temperature range, sensing locations, strain effects and the interrogation system all need to be considered. When the application calls for optical rather than conventional electrical sensing, FBG provides a practical way to monitor temperature at one point or across many points along the same fiber.

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Adam Regan: Deputy Editor

Features and account management. 7 years media experience. Previously covered features for online and print editions.

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