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Sensors used on Tsing Ma Bridge

Research on miniature photonics sensors, known as fibre Bragg grating sensors, is being put to test on Hong Kong’s landmark Tsing Ma Bridge.
Fibre Bragg grating (FBG) sensors are created inside fibres that are commonly used in optical communication systems and can be used to measure temperature and strain on structures. A feature of the tiny sensors is that many can be created at points along a single length of optical fibre up to 50km long and measurements can be taken from either end of the fibre link.

An FBG interrogation system using sensors to measure strain at five positions along a railway track.

The number of sensors required for Tsing Ma Bridge is about 100. Researchers at The Hong Kong Polytechnic University under Principal Investigator Prof Hwayaw Tam, have designed and built two innovative systems; one can interrogate 12 sensors located more than 25 km away and has a sensitivity to stress of 0.3 microstrains.
FBG interrogation systems have a broadband light source that illuminates the FBG sensors, detects the reflected light and measures its wavelength to determine the induced strain. The sensors behave like mirrors, reflecting narrow bands of wavelength.
Changes in strain cause the sensors to reflect different wavelengths. Another significant result of the research was the development of FBG sensors that can withstand temperatures up to about 600OC, an important advance for systems measuring temperature.
Previously, this type of sensor could only be used to measure temperatures to about 300OC before itself being destroyed.

Principal Investigator
Prof Hwayaw Tam: