Fiber Optic based Sensors

March 6, 2011 | By More

Fiber Optic Sensors (FOS) is based on the principle that minute changes in the dimensions of a length of fiber optic can be accurately measured and quickly translated into digital signals. With the correct calibration, these dimensional changes can be correlated into parameters under measurement such as: temperature, strain, load, deformation, and other physical characteristics.

Compared with conventional electrical wire strain gauges, the readings taken using FOS are more stable and reliable because fiber optic signals are immune to electromagnetic and radio frequency interference. This translates to the potential for higher data resolution and accuracy as well as reduction of erroneous data points caused by spurious interference.

These unique FOS characteristics are possible because a special grating called Fiber-Bragg Grating is embedded in the fiber optic cable that detects the minute dimensional variations.

How FBG works

Fiber Optics and Fiber Bragg Gratings have been extensively used in telecommunications for many years. The first in-line Fiber Bragg Grating (FBG) was demonstrated in 1978. This is done by accurately writing a series of grating by a high powered laser to change the refractive index in a section of an optical fiber.

The grating blocks and reflects the wavelength specific to the grating, allowing the rest of the beam to pass further along the fiber unaffected. The blocked wavelength is reflected back to its source where a detector measures the central frequency. An expansion at that point of the grating will cause the reflected light to increase in wavelength, whereas a contraction will decrease the wavelength. This change can be calibrated to determine the temperature changes, strain, load, deformation, and so on, at the point where the light is reflected.

The rest of the beam continues along the fiber until it encounters another FBG calibrated at a different wavelength, which will reflect that assigned wavelength in the same way.

Up to 30 sensors or more can be embedded into the length of a single strand of fiber optic. Being optical, the sensors do not need any electrical power. This means the end of bulky bundles of wires in construction sites using Fiber Optic Sensors, making cabling of sensors much easier and less time consuming.

Since 2000, this type of sensors have been researched and successfully validated at Singapore’s Nanyang Technological University (NTU). A US patent was filed in 2001 and awarded in 2006.

A company was formed to develop this technology into industrial sensors – that are have higher resolution, can be temperature compensated, more stable and reliable than conventional vibrating-wire sensors currently used in most construction sites today.

Over the next few years, these sensors and systems were successfully deployed at several construction projects around Singapore: LaSalle College of the Arts, Pulau Tekong, Jurong Island, Fort Canning Tunnel.

FBG sensors used at LaSalle College of Art and Fort Canning Tunnel

Following the Nicole Highway MRT collapse (read more about The Need for Smart Sensors), they were invited by Singapore’s Land Transport Authority (LTA) to demonstrate a large scale test bed using their FBG fiber optic sensors at the construction of the Telok Blangah MRT Station for the soon to be Circle Line. In all, nearly 350 different sensors were installed including the complete instrumentation and reporting system as well as a wireless alert system.

The Telok Blangah MRT site above (before digging) and below ground

The company has been invited back to demonstrate an improved system for construction of the new MRT Station at Bukit Panjang as part of the new Downtown Line 2, known as Contract 921 Zone-2.

Some fiber optic sensors used at Telok Blangah MRT construction site

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Category: Sensor Systems

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About the Author ()

EngTong, pioneer and innovator. Graduated from Imperial College London with an MBA from Cranfield School of Management. Lived in Scotland, England, California, Beijing and led teams in Italy, France, Japan, Taiwan and Malaysia to do the impossible. Now based in Singapore and believes the future is to blend the sophistication of western management practices with the strength of Asian Values. Trained as a Chartered Engineer. Member of IET, Associate of City and Guilds and a certified SixSigma Champion.

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