Development of high-speed optical wavelength interrogation system for damage detection in composite materials

We have been studying optical sensing technologies that use fiber Bragg gratings (FBGs) for health monitoring of aircraft structures made of carbon fiber reinforced plastic (CFRP) composite materials. The sensing system is composed of a piezoelectric transducer (PZT) actuator, which generates an elastic wave of several hundred kHz, and FBG sensors that receive the elastic wave. When some damage occurs in the composite materials, the elastic wave that propagates through those materials changes. Therefore the damage can be detected by analyzing the elastic waveform to be received by FBG sensors. For detecting this wave, we developed a high-speed optical wavelength interrogator for FBG sensors, and FBG sensor modules that can be embedded in the composite materials. In this interrogator, we employed an arrayed waveguide grating (AWG) as an optical filter that can convert the wavelength shift of the FBG sensors into optical power change. Using this interrogator and FBG sensor modules, we detected elastic waves of 300 kHz in frequency. We determined the required characteristics of FBG sensor both through simulation and experiments for improving the sensitivity of this health monitoring system.