Fundamental Experiment of Multiple-point Measurement for Strain by Fiber Bragg Gratings using Optical Frequency Sweeping

To evaluate the safety of man-made structures such as bridges, dams and high-rise buildings and of regions prone to dangers such as bedrock collapse, we are planning the construction of health monitoring system using fiber Bragg grating (FBG). The strain can be measured from the reflected wavelength of FBG, and the strain at multiple locations can be measured by using FBGs that have different Bragg wavelengths. Normally, however, it is not easy to make multi-point measurements using FBGs of the same Bragg wavelength. In this paper we describe foundational experiments on measurement of strain at multiple points using FBGs of the same reflection wavelength and optical frequency sweeping. The experiments demonstrate that it was possible to measure the reflection spectrum of each FBG independently and the measure reflection wavelengths were consistent with the measurements made with a commercial wavelength monitor