Packaging methods of fiber-Bragg grating sensors in civil structure applications

Fiber-Bragg grating (FBG) sensors made on bare fibers are easily damaged when handled improperly during and after fabrication. As a protection from such damage, a novel technique for protecting and packaging FBG sensors has been developed and is presented in this paper. To characterize the strain transmission efficiency of the packaged FBG sensors, an analytical finite-element method is used, and the results are compared with the experiments. It is observed that the thickness and Young's moduli of glues have little influence on the strain transmission, especially when the thickness of the glue is less than the diameter of an optical fiber. However, recoating and steel-tube packaging will markedly affect the strain transmission rate. The strain transmission rates decrease with the increase in thickness of the packaging material. Also, the aging problem of the polymide or acrylate coating and epoxy glue must be considered, since the service life of most structures is usually designed for more than 50 years. The metallic recoated FBG sensor developed in this research uses different approaches, such as low-temperature solder welding, which shows no aging problem, to install the sensors in the structures. Based on the simulated and experimental results, the nickel recoating method is shown to have good strain transmission efficiency compared with other packaging methods.