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The telecomunications industry routinely uses optical components for the transmission
of data. Optical filters are sensitive to changes in environmental conditions such as
temperature and strain and these optical filters make ideal sensing elements which can
be used to monitor the structural integrity of a multitude of civil engineering conditions.
The departement of Civil Engineeeting at the University of Southampton has developed
a new breed of structural strain sensor using optical fibre Bragg gratings. A novel method
of epoxy resin encaspsulastion is presented here providing both support for the Bragg gratings
and protection from the rugged civil environment.
The sensors are primarilly for embedment into concrete and include multiplexing capability
and temperature isolation.
Optical fibre sensors possess many advantages over conventional electrical sensors including amoung others: Immunity to EMF, RMF and surges caused by lightening strikes - multiplexing capability and smallness of physical size.
A fibre Bragg grating is an optical filter which is produced in the core of an optical fibre. When a broadband optical light source travelling through an optical fibre impinges on a Bragg grating, a narrow band of the propogating light is back reflected. The wavelength of the reflected signal is sensitive to environmental conditions such as temperature and strain. A typical Bragg reflection viewed on an optical spectrum analyser is shown in the figure below.
The reflected signal is not a single wavelength, but rather a narrow band of wavelengths centred arond a peak wavelength called the Bragg wavelength. The Bragg reflection is given by the familiar Bragg equation:
where n = effective refractive index and Λ = the Grating Pitch.
