| Research Abstract |
We are working to construct a structural health monitoring system using fiber-Bragg-grating(FBG) optical-fiber sensors for the purpose of determining soundness in the danger zone of bridges, high-rise buildings, dams and other man-made structures, of bedrock slippage, etc. A basic experiment on strain measurement by a differential method by which two fiber Bragg gratings(FBGs) are aligned so that their fibers intersect. It was shown that the measurement sensitivity of the differential method can be selected according to the intersection angle of the two fibers and that the method has sensitivity to strain in one direction only.
Further, we performed on detecting wavelength shift in dynamic strain measurements by combining a FBG and an unbalanced Mach-Zehnder interferometer(MZI) using serrodyne modulation. This equipment was shown to be capable of measuring Bragg wavelength shift of a fiber Bragg grating at a resolution of about 1×10^<-12>m.
To detect a wavelength shift of a FBG easily and quickly, unbalanced Mach-Zehnder interferometers(MZIs) - combining 2-by-2 and 3-by-3 light couplers - were implemented. In addition, to enable strain to be measured in real time, a real-time system utilizing a digital processing device, made up of a digital signal processor(DSP), was developed. It was demonstrated that using unbalanced MZIs combining light couplers and a real-time digital processing device made up of a DSP enables real-time measurement of dynamic strain every 0.1 ms. This result suggests that it is possible to measure dynamic strain with an oscillation frequency of several kilohertz.
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