Optical strain gauges: What you need to know

Optical strain gauges (also called fiber optical (strain) sensors or FOS, optical (strain) sensors or Fiber Bragg Grating sensors) are used to measure strain, but can easily be integrated into different types of transducers, such as in temperature, acceleration or displacement. Compared to traditional electrical strain gauges, optical strain gauges do not need electricity. Instead, the technology is based on light that propagates through a fiber. Therefore, the sensors are completely passive and immune, for example, to electromagnetic interference. This is just one of the reasons why optical strain gauges are superior to electrical ones in certain applications.

By the way

For this article, Cristina Barbosa, Product Manager for HBM optical business, explains how optical strain gauges work.

She says: “When people need to measure strain, they first think of electrical strain gauges. Optical strain gauges can come in handy, though, where electrical ones won’t work, for example due to environmental conditions.”

Schematic image: The optical strain gauge consists of a silica core and cladding that channel the light in the fiber and an outer coating (usually plastic) for protection. — The optical strain gauge consists of a silica core and cladding that channel the light in the fiber and an outer coating (usually plastic) for protection.

Design

In this article, we will focus on intrinsic fiber optic strain sensors, where the fiber itself is the sensor. Other types of optical fiber sensors use the fiber itself to transmit light, not to measure with it.

An optical fiber usually consists of a glass or silica fiber and a plastic coating. It is much like regular telecommunication fiber and can be up to several kilometers long with many measuring points along its length. The fiber itself consists of two layers: the core and a surrounding cladding with lower density. A plastic coating is wrapped around the silica fiber for protection.

So, why is this density difference between the core and the cladding important? A laser is used to send light through the fiber. The two different fiber material densities create a barrier that channels the light inside the fiber so that it doesn’t scatter. For this to work, it’s important that the fiber is not bent too much. “It is flexible and won’t break, however, light could escape at the bends,” explains product manager Cristina Barbosa.

Method of operation

The fiber as a sensor

To create the actual strain sensor, the optical fiber is inscribed during production with a so-called Fiber Bragg Grating (FBG). This is basically a pattern of material interferences, which reflects the light differently from the rest of the fiber. For better understanding, you can visualize the fiber as a cylindrical length of transparent material, with a number of thin slices in it. When the light from the laser hits this pattern, certain wavelengths are reflected, while others pass through.

The material interferences—the “slices” —are placed at certain intervals. When the fiber is stretched or compressed—and is therefore subjected to positive or negative strain—these intervals change. When the fiber is stretched, it lengthens and the spaces get bigger and vice versa.

Not only does the reflected light take a little longer or shorter to travel back when the FBG is under strain, but the wavelength that is reflected also changes. In scientific terms, the FBG has a certain refractive index. The refractive index of a material describes how much light is bent or refracted when passing through the material. When the grating changes shape due to strain, its refractive index changes as well.

“One Fiber Bragg Grating as a whole is approximately 5 millimeters long, though the individual material interferences cannot be seen with the naked eye, only under a microscope”, explains Cristina Barbosa. Many Fiber Bragg Gratings can be inscribed in one long fiber—each working as an individual strain sensor.

When the optical fiber is applied to a material, it will be strained along with this material. The measured strain will in turn allow an analysis of the mechanical stress in the material, which is the aim of most strain measurements.

To give a practical example, when the fiber is applied to the walls of a long tunnel, it is strained when there is stress in the material of the walls. This can be, for example, due to the vibrations of trains rushing by. When the walls settle or even develop weak points or fissures over the years, this becomes visible from the information about strain and thus mechanical stress acquired by the sensors—a useful early indication as to where maintenance is needed.

The role of the interrogator

For measurements, the optical fiber needs to be connected to a so-called interrogator; it continuously sends out light in different wavelengths, one at a time, thus covering a wide spectrum. This is called “sweeping laser”. Light propagates through the fiber, is reflected at some point by a FBG and returns to the interrogator.

Thanks to the different periods of individual FBGs, it is possible to distinguish between the signals of different sensors. The rest of the light is refracted when reaching the end of the fiber so that it doesn’t interfere with the measurement. The actual strain and, in turn, the material stress can be deduced from the raw light signals which return from the FBGs.

“While there are different methods of measuring strain with different kinds of fiber optic sensors, what they all have in common is that they rely in some way on the properties of light.”

- Cristina Barbosa

Temperature compensation is crucial

Fiber Bragg Gratings-based optical fiber sensors are extremely susceptible to temperature. Obviously, the fiber—as any other material—expands when the temperature rises and contracts when the temperature drops. The refractive index changes as well. Without compensation, this would lead to the measurement of strain that has not been caused by material stress, but by temperature variations. There are several techniques for compensation, including:

Installing a temperature sensor next to the strain sensor; this allows for mathematical compensation by comparing the data and subtracting the temperature effects.
Placing two FBGs in a push-pull-configuration so that, when under strain, one is compressed and the other one is stretched. Temperature effects are identical for both (e.g. lengthening), yet the influence of mechanical stress differs: For one FBG the strain is “positive” as it is under traction, but for the other, the strain is “negative” because it is being compressed. Thus, mathematical compensation becomes possible.
Encapsulating the fiber in a mechanical apparatus that expands in the opposite direction of the material under test in such a way that stress applied to the FBG cancels the temperature effect and no mathematical compensation is needed.

Image showing possible applications for optical strain gauges in structural health monitoring, e.g. bridges, wind turbines and railways.

Applications for optical fiber sensors

“As part of the ITER project in France, our sensors need to cope with a huge temperature range starting from approximately -270°C up to 300°C while under intense electromagnetic fields. This is something that no electrical strain gauge could manage”, says Cristina Barbosa, naming one of her favorite applications for optical strain sensors.

Less exotic applications can be found, for example, in structural health or infrastructure monitoring. As a single fiber can accommodate hundreds of sensors, optical technology offers itself to huge projects like tunnel or pipeline monitoring, as cabling and installation costs are lower in comparison to traditional strain gauges.

Moreover, optical measurement technology is the first choice for all applications where the electricity needed for traditional strain gauges would be a problem, including environments with a lot of electromagnetic interference (like in space) or a high danger of explosion (like oil refineries). Or in Cristina Barbosa’s words:

“Where electrical strain gauges don’t work, optical ones might do the job.”