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Development of small-diameter optical fiber sensors and high-speed optical wavelength interrogator for damage detection in composite materials

We have been developing a sensing system for monitoring the structural health of aircraft structures made of composite materials. The sensing system is composed of fiber Bragg grating (FBG) sensors, a wavelength interrogator and piezoelectric actuators. The FBG sensors receive 100 kHz to 1 MHz elastic waves generated by the PZT actuators. For the FBG sensors, we previously developed a polyimide-coated optical fiber with a cladding diameter of 40 μm and core-cladding relative refractive index difference Δ of 0.65 %, that can be embedded in composite materials without inducing any mechanical defects. Since the cladding of that fiber is so thin, however, under embedded conditions, the transmission loss of the fiber is larger than that of a normal single-mode optical fiber. We therefore developed a new small-diameter optical fiber with an Δ of 1.8 %, in order to suppress the loss increase caused by micro-bending or transversely applied strain under the embedded condition. On the other hand, the small-diameter optical fiber needs to be connected to a normal optical fiber whose claddingding diameter is 125 μm, because it is fragile and difficult to handle. For practical use, we developed a small-diameter optical fiber module that has a special connector on both ends of the small-diameter optical fiber. The special connector can connect the small-diameter optical fiber to a normal optical fiber that has a standard MU connector. We also developed a high-speed optical wavelength interrogator that can detect the high-frequency vibration of the FBG sensors. It uses an arrayed waveguide grating (AWG) as an optical filter that converts the wavelength shift of the light reflected from the FBG into the output optical power changes. This wavelength interogator is suiatable for high-speed wavelength detection because it has no mechanical moving parts. The development of these components will help put this system to practical use and thus extend the use of composite materials to a wider range of applications.