Fundamentals of FBG optical sensing

Electrical sensors have, for decades, been the standard mechanism for measuring physical and mechanical phenomena. Despite their ubiquity, these sensors have inherent limitations such as transmission loss and susceptibility to electromagnetic interference (noise) that make their usage challenging or impractical in many applications.

Fibre-optic sensing is an excellent solution to these challenges, using light rather than electricity and standard optical fibre in place of copper wire.

The tremendous innovation over the past two decades in the optoelectronics and fibre-optic communication industries has significantly reduced optical component prices and improved quality.

By using these economies of scale, fibre-optic sensors and instruments have moved from experimental research applications in the lab to broad usage and applicability in field applications such as structural health monitoring.

Fundamentally, a fibre-optic sensor works by modulating one or more properties of a propagating light wave, including intensity, phase, polarisation and frequency, in response to the environmental parameter being measured.

Extrinsic (hybrid) optical sensors use the fibre only as a mechanism to transmit light to and from a sensing element, while intrinsic optical sensors use the optical fibre itself as the sensing element.

At the core of optical sensing technology is the fibre - a thin strand of glass that transmits light within its core. Fibre has three main components: the core, the cladding and the buffer coating. The cladding reflects stray light back into the core, ensuring the transmission of light through the core with minimal loss.

This is achieved with a higher refractive index in the core relative to the cladding, causing a total internal reflection of light. The outer buffer coating protects the fibre from external conditions and physical damage. It can incorporate many layers depending on the amount of ruggedness and protection required.

Figure 1: Cross-section of a typical fibre-optic cable.

One of the most commonly used and broadly deployed optical sensors is the fibre Bragg grating (FBG), which reflects a wavelength of light that shifts in response to variations in temperature and/or strain.

FBGs are constructed using holographic interference or a phase mask to expose a short length of photosensitive fibre to a periodic distribution of light intensity. The refractive index of the fibre is permanently altered according to the intensity of light it is exposed to.

The resulting periodic variation in the refractive index is called a fibre Bragg grating.

When a broad-spectrum light beam is sent to an FBG, reflections from each segment of alternating refractive index interfere constructively only for a specific wavelength of light, called the Bragg wavelength. This effectively causes the FBG to reflect a specific frequency of light while transmitting all others.

Figure 2: Operation of an FBG optical sensor.

Because the Bragg wavelength is a function of the spacing between the gratings, FBGs can be manufactured with various Bragg wavelengths, which enables different FBGs to reflect unique wavelengths of light.

Figure 3: An expanded view of an FBG.

Changes in strain and temperature affect both the effective refractive index and grating period of an FBG, which results in a shift in the reflected wavelength.

Because an FBG responds to both strain and temperature, the user needs to account for both effects and distinguish between the two. For sensing temperature, the FBG must remain unstrained. You can use packaged FBG temperature sensors to ensure the FBG inside the package is not coupled to any bending, tension, compression or torsion forces.

The expansion coefficient of glass is practically negligible; thus, changes in the reflected wavelength due to temperature can be primarily described by the change in the refractive index of the fibre.

FBG strain sensors are somewhat more complex because both temperature and strain influence the sensor’s reflected wavelength. For proper strain measurements, you must compensate for the temperature effects on the FBG.

This can be achieved by installing an FBG temperature sensor in close thermal contact with the FBG strain sensor.

The process of mounting an FBG strain gauge is similar to mounting conventional electrical gauges, and FBG strain gauges are available with a variety of form factors and mounting options including epoxy, weldable, bolt-on and embedded.

The ability to write FBGs with Bragg wavelengths lends itself well to wavelength division multiplexing (WDM) techniques. This provides the ability to daisy chain multiple sensors with different Bragg wavelengths along a single fibre over long distances.

WDM provides each FBG sensor its unique wavelength range within the light spectrum. Because of the wavelength nature of FBG, sensor measurements remain accurate even with light intensity losses/attenuations due to bending or transmission.

The number of sensors that can be incorporated within a single fibre depends on the wavelength range of operation of each sensor and the total available wavelength range of the interrogator.

Because wavelength shifts due to strain are typically more pronounced than temperature, FBG strain sensors are often given an ~5 nm range, while FBG temperature sensors require ~1 nm.

Because typical interrogators provide a measurement range of 60 to 80 nm, each fibre array of sensors can usually incorporate anywhere from one to more than 80 sensors - as long as the reflected wavelengths do not overlap in the optical spectrum. Be careful when selecting the nominal wavelengths and ranges for the sensors in an array to ensure that each sensor operates within a unique spectral range.

Figure 4: Each FBG optical sensor in an array must occupy a unique spectral range.

With typical FBG sensor wavelengths operating within a few nanometres, optical interrogators must be capable of performing measurements with a resolution of a few picometres or less - a very small value to quantify.

You can choose from several methods for interrogating FBG optical sensors. Interferometers are often used in laboratory settings and can provide high-resolution optical spectrum measurements.

However, these devices are usually expensive, large and not rugged enough for field-monitoring applications involving a variety of structures including wind turbine blades, bridges, pipelines and dams.

A more rugged method involves the use of a charge-coupled device (CCD) and a fixed dispersive element, sometimes referred to as wavelength-position conversion.

With this method, a broadband source illuminates the FBG (or multiple FBGs in an array). The reflected light wave is passed through a dispersive element that distributes the various wavelength components of the reflection to different locations on a linear CCD sensor.

Figure 5: Wavelength-position conversion method of interrogating FBG optical sensors.

This method can yield fast, simultaneous measurements of all FBGs in the array, but it offers limited resolution and signal-to-noise ratio (SNR). For example, detecting an FBG peak shift of 1 pm over an 80 nm range requires a linear CCD with at least 20,000 pixels, more than 3x the pixel count of the linear CCDs currently available.

Additionally, the power of a broadband source is spread across a wide wavelength range, producing low-energy FBG reflections that can be difficult to detect.

The NI PXIe-4844 optical sensor interrogator uses a Fabry-Perot tunable filter to create a fast, high-power sweeping laser, replacing the traditionally weak broadband light source.

A tunable laser concentrates energy in a narrow band, providing a high-powered light source with an excellent SNR. The high-optical power generated by this architecture enables a single light source to be coupled with multiple fibre array channels, which reduces cost and complexity for multichannel interrogators.

Interrogators based on this tunable-laser architecture operate by sweeping a very narrow band of light across a wavelength range while synchronously using a photodetector to measure the reflections from the FBG(s).

When the wavelength of the tunable laser matches the Bragg wavelength of the FBG, the photodetector sees a corresponding response. The wavelength at which this response occurs corresponds to the temperature and/or strain of the FBG.

Figure 6: Tunable-laser approach for interrogating FBG optical sensors.

This method can deliver an accuracy of ~1 pm, which translates to typical FBG sensor accuracies of ~1.2 microstrain and ~0.1ºC (sensor dependent). The tunable laser approach also enables measurement over longer fibre lengths (more than 10 km) because of its high-optical power relative to alternatives.

FBG optical sensing overcomes many of the challenges associated with electrical sensing by using light rather than electricity and standard optical fibre in place of copper wire. Optical fibres and FBG optical sensors are nonconductive, electrically passive and immune to EMI-induced noise. Interrogation with a high-power tunable laser enables measurements over long distances with little or no loss in signal integrity. Also, unlike electrical sensing systems, each optical channel can measure dozens of FBG sensors, greatly reducing the size, weight and complexity of the measurement system.

Optical sensing is ideal for applications where conventional electrical sensors such as foil strain gauges, thermocouples and vibrating wires have proven ineffective or tough to use due to difficult environmental conditions and/or long distances. Because the installation and usage of optical sensors are similar to those of conventional electrical sensors, transitioning to an optical sensing solution is reasonably simple. A good understanding of optical fibres and the theory of FBG operation can help you quickly adopt FBG sensing technology and harness all of the benefits it has to offer.

Traditional optical measurement devices provide fixed software functionality and a fixed user interface. This lack of flexibility limits the system’s ability to meet many structural test and monitoring application needs. In addition, traditional optical sensing instruments are not designed for easy integration with electrical measurements or control systems, which is often required in real-world structural and environmental measurements.

The NI optical sensor interrogator (OSI) offers seamless integration with NI LabVIEW, a graphical development environment for customisable software and easy UI development. The NI PXIe-4844 is also based on the PXI platform, providing modular I/O for easy integration with a wide variety of PXI and PXI Express devices, including conventional thermocouple, strain and vibration devices, GPS synchronisation modules, as well as analog and digital outputs for control requirements.

Figure 7: NI PXIe-4844 opticdal sensor interrogator.