Keisuke Fukuchi

Polyimide-coated small-diameter optical fiber sensors for embedding in composite laminate structures

We have studied health-monitoring systems using embedded optical fiber sensors for damage detection in FRP (Fiber Reinforced Plastics) composite laminates. For sensing of strain or temperature changes in these composite materials, we developed the FBG (Fiber Bragg Grating) sensors using the polyimide-coated small-diameter optical fiber with cladding diameter of 40 micrometers , which can be embedded in FRP composite laminates without inducing any structural defects. The FBG was subjected to high-temperature heat treatment at 300 degree(s)C in order to suppress the optical characteristics change of the FBG over a long period of time. Even after such high temperature treatment, the FBG retained sufficient mechanical strength and reflectivity. We embedded the FBG into FRP composite laminates and measured the optical characteristics against temperature and tensile strain. These experimental results show this type of the embedded FBG is very promising for practical health-monitoring systems. In addition, we have investigated various multipoint measuring systems and propose the time division multiplexing system using these FBG sensors.

Small and thin structure plate type wideband antenna (3 GHz - 6 GHz) for wireless communications

The wideband antenna that we developed has a small and thin structure (W 41 /spl times/ H 34 /spl times/ t 0.3 mm); and it is suitable to be installed in small spaces. This antenna has a resonance frequency from 3 GHz to 6 GHz, where VSWR was below 2.0. The radiation pattern is omnidirectional and the average gain curve is almost flat over this frequency range, and the values were around 0 dBi. The group delay is almost flat and its perturbation is within 1 ns over the above frequency range.

Development of multipoint strain measurement systems using small-diameter fiber Bragg gratings

We have been studying optical sensing technologies using fiber Bragg gratings (FBGs) for health-monitoring systems in the fields of constructions, civil engineering, aerospace and so on. In these fields, various kinds of sensing techniques such as strain, temperature, vibration and crack detection are required. To meet these needs, we have fabricated the FBGs by precisely controlling photo-induced refractive index modulation or the grating period along a fiber. Also, as embedded sensors, we developed small-diameter FBGs which are embedded in fiber reinforced plastics (FRP) composite materials without inducing any mechanical deterioration. In this paper, we present the progress of our small-diameter FBGs and propose a new wavelength detection technique for FBG sensors using a wavelength division multiplexing (WDM) coupler. The reflected light is divided at the ratio depending wavelength through the WDM coupler. Especially, we adopted a PLC-type WDM coupler which has the advantage in low polarization sensitivity and integration compared with a fused-optical fiber-type WDM coupler. This technique is useful in high-speed detection for vibration or impact damage. Another application is a multipoint measurement system by using together with optical time domain reflectometry (OTDR) method. In this system, pulses of light incident into an optical fiber return from the FBGs to a detector with different arrival time. As a result, we can use FBGs with the same Bragg wavelength, because measuring delay time of the pulses enables to distinguish each FBG location. In addition, using together with wavelength division multiplexing within wavelength region for optical communications can increase the number of FBG sensors. The results of the basic performance in this system showed that it is very promising for the multipoint measuring system. These wavelength detection techniques will expand sensing applications using small-diameter FBGs.

Low profile antenna for mobile devices

A topology for low profile antenna composed of two open stubs and a quarter wavelength transformer on a ground plane, and a prototype of low profile antenna for 2.45/5 GHz operation bands using the topology are presented. To keep bandwidth, the topology for low profile antenna is designed by focusing the input admittance of open stubs using “leaky loss transmission line theory” [1, 2]. The prototype of low profile antenna printed on FR4 substrate achieved as element height of 2 mm and high radiation efficiency over 50 % in 2.45/5 GHz bands.

Embedded small-diameter fiber Bragg grating sensors and high speed wavelenth detection

We have been studying optical sensing technology using embedded fiber Bragg grating (FBG) sensors. The FBG is inscribed in a small-diameter optical fiber with the cladding diameter of 40 μm. This technology is very promising for health monitoring in aviation components, because the diameter of the sensor optical fiber is so small that embedding of the sensor does not deteriorate mechanical properties of the composite materials. For practical use, we have also studied high reliable fused-splicing method between the small-diameter optical fiber and an ordinary optical fiber in order to improve handling. The embedded FBG sensors are useful for vibration or impact detection as well as static strain detection. For the purpose of detecting dynamic phenomena, we have developed a high speed wavelength detection unit for the FBG sensors which uses wavelength division multiplexing (WDM) coupler based on planar lightwave circuit (PLC) technique. WDM coupler converts wavelength of the light reflected from the FBG sensor into output powers. Since there is no mechanical moving part, this type of wavelength detection technique is suitable for high speed detection.

Optical water-level sensing systems using fiber Bragg grating

We have developed the all optical high-precision water level sensors based on fiber Bragg grating (FBG) technique, which are applied for actual fields such as rivers, lakes, sewage systems and so on. The sensor head consists of a diaphragm, a customized Bourdon tube and two FBGs, one for tensile measurement and other for temperature compensation. The FBG attached to the Bourdon tube is strained as the water level increases, and causes center wavelength shift of the reflected light from the FBG, which is detected by the wavelength interrogation equipment composed of a tunable Fabry-Perot filer. We have achieved the sensor accuracy of +/- 0.1% F.S., i.e. +/- 1 cm in case of full measurement range of 10 m. Several sensor heads can be connected in series through one optical fiber and each water level at different places can be measured simultaneously by one wavelength interrogation equipment.