Wavelength Shift Detection of Fiber Bragg Grating by Dynamic Strain using Serrodyne Modulation

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Abstract

To evaluate the stability of man-made structures such as bridges, high-rise buildings and dams and of regions prone to dangers such as bedrock collapse, we are planning the construction of a health monitoring system using a fiber Bragg grating (FBG). In this paper we discuss the use of an FBG and serrodyne modulation to perform dynamic strain measurements in order to detect Bragg wavelength shifts in combination with an unbalanced Mach-Zehnder interferometer. When a Bragg wavelength shift is measured as a phase difference, the pseudo-heterodyne effects of serrodyne modulation are represented in terms of the frequency ratio and serrodyne coefficient. As the frequency ratio shifts away from unity, it causes non-linear changes in electrically measured phase, as has been demonstrated by experimental measurements of the shift in wavelength of a swept narrowband signal reflected by a fiber Bragg grating. The shift coefficients of FBG Bragg wavelengths measured in this apparatus matched the values measured with a wavelength resolution of 1×10-12m using an optical spectrum analyzer and an external cavity tunable diode laser, showing that the resolution of wavelength shift detection was 1×10-12m or less.

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