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Fiber Optic Bragg-Grating Sensors

In-fibre Bragg gratings are sensor elements which are photo-written into optical fibre using intense ultra-violet laser beams and have the potential for the measurement of strain/deformation and temperature with applications reported including monitoring of highways, bridges, aerospace components and in chemical and biological sensors. The development of a fiber Bragg grating (FBG) measuring system plays a significant role in monitoring and recording the actual seismic responses of underground structures, rock mass and bridges etc.

The basic principle of a fiber Bragg grating (FBG)-based sensor system lies in the monitoring of the wavelength shift of the returned Bragg-signal, as a function of the measurand (e.g. strain, temperature and force). The Bragg wavelength is related to the refractive index of the material and the grating pitch. Sensor systems involving such gratings usually work by injecting light from a spectrally broadband source into the fiber, with the result that the grating reflects a narrow spectral component at the Bragg wavelength, or in transmission this component is missing from the observed spectrum. Fig.1 shows this simply and schematiclly.

Fig. 1  Funktional principle of a fiber optic Bragg grating

Fig.2 shows an example of a fibre Bragg grating based sensor system for dynamic strain measurement. The sensor-head consists of a glassfibre reinforced polymer (GRP) rockbolt in which the grating is glued by epoxy resin. A 3dBm distributed feedback (DFB) laser, with tunable wavelength in the range from 1548.75 nm to 1551.25 nm, sends an optical signal at an optimised wavelength to the fibre Bragg grating through a fibre optical circulator. A part of the optical signal is reflected from the Bragg grating, goes back through the circulator to a photodetector, and is converted into an electrical signal. The signal is amplified, filtered and then sampled with an oscilloscope. Finally, the sampled signal is processed in a PC system.
The intensity of the reflected optical signal is a function of the Bragg grating wavelength that relates to the applied strain on the fibre Bragg grating. Therefore, the dynamic strain can be derived from the intensity change measurement as function of the wavelength of the reflected optical signal.

This measuring system has a powerful laser source with tuneable and programmable wavelength, so that the system sensitivity can easily be improved by optimizing the laser wavelength. Furthermore, the wavelength sensitivity can be directly determined by using the dc photovoltage measurement and the wavelength change of the laser source without any extra calibration.

Fig.2  Measuring system of dynamic strain with a fiber optic Bragg grating sensor
(FBG-rockbolt is a patent of the German Research Center of Earth Science GFZ in Potsdam)

Fig.3  Seismic signal detected with the Bragg grating sensor system and calculated strain

This technique can be applied to the detection of dynamic strain variations down to 10 -9 in underground rock mass excavations and to the monitoring of bridge structures etc.

For further information please contact with the German Research Center of Earth Science GFZ in Potsdam

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