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We have been developing a sensing system for checking the health of aircraft structures made of composite materials.
In this system, lead zirconium titanate (PZT) actuators generate elastic waves that travel through the composite material
and are received by embedded fiber Bragg grating (FBG) sensors. By analyzing the change in received waveforms, we
can detect various kinds of damage. The frequency of the elastic waves is several hundred kHz, which is too high for a
conventional optical spectrum analyzer to detect the wavelength change. Moreover, a conventional single-mode optical
fiber cannot be used for an embedded FBG sensor because it is so thick that it induces defects in the composite material
structure when it is embedded. We are thus developing a wavelength interrogator with an arrayed waveguide grating
(AWG) that can detect the high-speed wavelength change and a small-diameter optical fiber (cladding diameter of 40µm) that does not induce defects. We use an AWG to convert the wavelength change into an output power change by
using the wavelength dependency of the AWG transmittance. For this conversion, we previously used two adjacent
output ports that cover the reflection spectrum of an FBG sensor. However, this requires controlling the temperature of
the AWG because the ratio of the optical power change to the wavelength change is very sensitive to the relationship of
the center wavelengths between an FBG sensor and the output ports of the AWG. We have now investigated the use of a
denser AWG and six adjacent output ports, which covers the reflection spectrum of an FBG sensor, for detecting the
elastic waves. Experimental results showed that this method can suppress the sensitivity of the power change ratio to the
relationship of the center wavelengths between an FBG sensor and the output ports. Although our improved small-diameter
optical fiber does not induce structural defects in the composite material when it is embedded, there is some
micro or macro bending of the fiber, which causes propagation loss. To suppress this embedment loss, we adjusted the
refractive index difference of the fiber to have larger value. Experimental result showed that this reduced the embedment
loss by about 0.3 dB/cm. These enhancements make our sensing system more practical and should promote the use of
composite materials in a wider range of applications.
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