Shinji Komatsuzaki

Evaluation of debonding progress in composite bonded structures using ultrasonic waves received in fiber Bragg grating sensors

We have developed a damage detection system that generates ultrasonic waves with a piezo-ceramic actuator and receives them in a fiber Bragg grating (FBG) sensor. In this research, this system was applied to evaluate the debonding progress in carbon fiber reinforced plastic (CFRP) bonded structures. First, small-diameter FBG sensors were embedded in adhesive layers of a double-lap-type coupon specimen consisting of CFRP quasi-isotropic laminates bonded with epoxy adhesive films. Then, an ultrasonic wave at 300 kHz was propagated through the debonded region, and the wavelet transform was applied to the received waveform. The obtained results showed clear differences depending on the debonding length. Hence, a new damage index was proposed using the difference in the distribution of the wavelet transform coefficient. The damage index increased with an increase in the debonded area. Furthermore, this system was applied to a skin/stringer structural element of airplanes made of CFRP laminates. In this case, a correlation coefficient was also calculated from the results of the wavelet transform. As a result, the damage index increased and the correlation coefficient decreased with an increase in the debonded area. Hence the length of the debonding between the skin and the stringer could be easily evaluated.

High-speed optical wavelength interrogator using a PLC-type optical filter for fiber Bragg grating sensors

We study a new sensing system for health monitoring of aircraft structures made of composite materials. This sensing system is composed of fiber Bragg grating (FBG) sensors and a piezoelectric transducer (PZT). The FBG sensors receive elastic wave generated by the PZT. For high-frequency vibration monitoring, or acoustic emission (AE) detection, we have developed high-speed optical wavelength interrogator for FBG sensors. In this FBG interrogator, we used an optical filter which converts wavelength shift of the reflected light from the FBG into the output optical power change. This system is suitable for high-speed wavelength detection because there is no mechanical moving part. We studied two types of optical filter. One is a Mach-Zender interferometer and the other is an arrayed waveguide grating (AWG). Both of them were fabricated using silica-based planar lightwave circuit (PLC) technology. The optical filters based on the PLC technology have the advantage in integration of optical components. By combining the FBG interrogator using the AWG optical filter with the PZT actuator, we succeeded to detect elastic wave propagating in the CFRP laminated plate. As a result, we found that this FBG/PZT hybrid sensing system is very promising for detection of internal defects in composite materials.

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.

Small-diameter optical fiber and high-speed wavelength interrogator for FBG/PZT hybrid sensing system

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.

A compact 25-Gbit/s × 4ch optical interconnect module with straddle-shaped optical and electrical interface

A compact 25-Gbit/s × 4-ch embedded optical module (EOM) composed of a straddle-shaped optical and electrical interface was fabricated and evaluated. The fabricated EOM provides highly efficient optical coupling and large optical-coupling tolerance, and it successfully demonstrated 25-Gbit/s error-free optical transmission. These optical performance results indicate that the fabricated compact EOM can be applied for on-board and rack-to-rack optical interconnections with high transmission density.

Evaluation of debonding progress in composite bonded structures by ultrasonic wave sensing with fiber Bragg grating sensors

The authors are constructing a damage detection system using ultrasonic waves. In this system, a piezo-ceramic actuator generates ultrasonic waves in a carbon fiber reinforced plastic (CFRP) laminate. After the waves propagate in the laminate, transmitted waves are received by a fiber Bragg grating (FBG) sensor using a newly developed high-speed optical wavelength interrogation system. In this research, this system was applied to the evaluation of debonding progress in CFRP bonded structures. At first, small-diameter FBG sensors, whose cladding diameter is about 1/3 of common optical fibers, were embedded in an adhesive layer of a double-lap type coupon specimen consisting of CFRP quasi-isotropic laminates, and the ultrasonic wave was propagated through the debonded region. After that, the wavelet transform was applied to the received waveforms and the results showed clear difference depending on the debonding length. Hence, a new damage index was proposed, which could be obtained from the difference in the distribution of the wavelet transform coefficient. As a result, the damage index increased with an increase in the debonded area. Furthermore this system was applied to the skin/stringer structural element of airplanes made of CFRP laminates. Both of the waves received by a bonded FBG and by an embedded FBG changed sensitively to the debonding progress. Also, the damage index could evaluate the length of the debonding between the skin and the stringer.

A high-density 300-Gbit/s parallel optical interconnect module with efficient optical sub-assembly techniques

A high-density 300-Gbit/s (25-Gbit/s × 12-ch) parallel optical-interconnect module (OIM) was fabricated by using an efficient, newly developed optical sub-assembly (OSA) techniques. The OIM module provides high heat dissipation and high optical-coupling efficiency. Furthermore, the fabricated OIM successfully demonstrated 25-Gbit/s error-free multimode transmission.

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.

A high-density 300-Gbit/s parallel optical interconnect module with efficient thermal dissipation characteristics

A high-density 300-Gbit/s (25-Gbit/s × 12-ch) parallel optical interconnect module (OIM) with a high efficiency heat radiation structure was fabricated using a thermal via structure. The prototype OIM successfully demonstrated 25-Gbit/s error-free multimode transmissions, and the optical device temperature was less than 80°C.

Improvement of FBG/PZT hybrid sensing system for composite materials

We have been developing a system for monitoring the health of aircraft structures made of composite materials. In this system, the Lamb waves that are generated by lead zirconium titanate PZT actuators travel through the composite material structures and are received by the embedded FBG sensors. To detect any Bragg wavelength change due to the reception of the Lamb wave, an arrayed waveguide grating (AWG) is used, which converts the Bragg wavelength change into an output power change. Since the conversion ratio is largely dependent on the initial Bragg wavelength, a temperture control was necessary for obtaining an optimum condition. However, we have developed a system that uses a denser AWG to eliminate the need for a temperature control. We suceeded in detecting 25 kHz to 1 MHz Lamb waves using our new system. We have also tried calculating the Bragg wavelength change of the obtained waveform, and confirmed that the Bragg wavelength change due to the reception of Lamb waves was less than 1 pico meter.