Fundamentals of Optical Strain Sensors: Working Principle and Applications

04-05-2020

Structures such as bridges, tunnels, pipelines, roadways or dams are continuously exposed harsh environmental conditions that slowly degrade their integrity. The popular foil type strain gauges have been used as structural health monitoring tools. However, they have certain limitations when it comes to signal stability at monitoring in challenging conditions such as high-level vibration loads, longer distances and in environments with electromagnetic and radio frequency interference. The foil strain gauges also tend to fail when used for extended periods.

Optical strain gauges are designed based on Fibre Bragg Grating (FBG) technology. They exhibit an intrinsically high multiplexing capability which minimizes the noise generated from longer cables and reduces installation complexity. Therefore, they are viable alternatives to foil strain gauges in structural health monitoring applications. These fibre optic strain gauges also require no electrical power supply to operate. Instead, they rely on light that propagates through a fibre. This fibre bragg grating technology can also be used in other types of transducers for measuring other physical parameters such as displacement, angle, tilt and temperature.

Fibre Bragg grating measurement principle

Fibre-bragg grating is basically a microstructure or a pattern of material interference created by modifying a standard single-mode telecom fibre. Deployed along the length of an optical fibre, these interference are placed at certain intervals. The FBG creates a permanent periodic variation on the refractive index of the core of the optical fibre. When an incident spectrum of light propagates through the fibre grating, a very narrow range of specific wavelengths is reflected back, while the rest of the spectrum is transmitted unaffected. The centre of this band of reflected wavelengths is called Bragg’s wavelength.

When an external compression or stretching force is induced in the fibre, it is subjected to positive or negative strain. As a result, the interval at which the fibre-grating interference are placed will change. It will lengthen if the fibre is stretched and shortens if the fibre is compressed. This change will alter the time required for the reflected light to travel back which also changes the Bragg’s wavelength. The change in the Bragg’s wavelength is proportional to the strain induced in the fibre. The results can be transmitted to the Fibre-Bragg Interrogator for analysis and signal conditioning.

Applications of Optical Strain Gauge

As a single fibre can accommodate hundreds of sensors, the optical strain sensors are ideal for huge projects such as a tunnel or pipeline monitoring, structural health monitoring of bridges, buildings and even infrastructure monitoring in mining site and condition monitoring of machinery in industrial processing plants. The ease of installation reduces the plant complexity and installation costs.

Optical strain sensors are also widely used in applications where access to the electrical power supply is restricted such as in offshore oil and gas platforms and marine applications. Fibre optics sensors are capable of providing accurate and reliable measurements on permanently stressed structures for more than 20 years without drift. There is absolutely no risk of electrical short circuits that could damage its electronics. These sensors can also be employed for applications in adverse environments such as hull monitoring due to their insensitivity to electromagnetic interferences and resistance to water and fatigue.

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