The dependence of a linear electro-optic (LEO) coefficient induced into boron-codoped germanosilicate fibre on thermal poling conditions (poling voltage, poling temperature and poling time) has been systematically carried out using a Mach-Zehnder interferometer. The LEO coefficient increases as a 2.7 power law with the poling voltage; it can be maximally induced into the silica fibre within a temperature range from 250 to 300; it exponentially increases with poling time until saturation but after that it then decreases. Possible mechanisms of thermal poling are discussed in the light of the experimental results.

The dependence of a linear electro-optic (LEO) coefficient induced into boron-codoped germanosilicate fibre on thermal poling conditions (poling voltage, poling temperature and poling time) has been systematically carried out using a Mach-Zehnder interferometer. The LEO coefficient increases as a 2.7 power law with the poling voltage; it can be maximally induced into the silica fibre within a temperature range from 250 to 300; it exponentially increases with poling time until saturation but after that it then decreases. Possible mechanisms of thermal poling are discussed in the light of the experimental results.

The problem of TM-mode scattering from the finite number of rectangular notches in a parallel plate waveguide is considered. The Fourier-transform is employed to obtain simultaneous equations and the simultaneous equations are solved to obtain an analytic solution in rapidly-convergent series. Numerical computations are performed to investigate the scattering behavior in terms of frequency and notch sizes. The presented theory is applicable to the analysis of scattering from the E-plane stubs in the rectangular waveguide.

A folded short in parallel-plate waveguide is investigated using the Fourier transform and the mode-matching. A fast-converging series solution for scattering from the folded short is obtained and its characteristics are presented. Our solution for the E-plane short agrees well with measurements and is numerically more efficient than the existing moment-method solution. The presented scattering characteristics are useful for the design of the E- and H -plane shorts in rectangular waveguide.

A simple solution for the right-angle H-plane waveguide double bend is obtained in analytic series form. The simultaneous equations are solved to obtain the transmission and reflection coefficients in fast convergent series forms. The numerical computations are performed to show the behaviors of the transmission coefficient versus frequency.

We describe a new water sensing system for optical fiber cable networks. This system consists of optical fibers, water sensors and an OTDR (Optical Time Domain Reflectometer). The water sensor contains material which swells on contact with water and bends the optical fiber. The OTDR monitors the optical loss increase caused by this fiber bending and determines its location. In this system it is very important to determine the loss increase caused by the water sensor in terms of the OTDR performance. Therefore, we clarified the relationship between the water sensor structure and the increase in loss. Based on this study, we fabricated a sensor which causes a 5dB loss increase. The measured value is very close to the calculated value.

By optimizing the structure of erbium-doped fibers, high efficiency such as a gain coefficient of 6.3dB/mW, or a slope efficiency of 92.6% have been realized with very flat wavelength dependence. Though the optimized structure has high NA, the splice loss with standard fibers can be lowered by the additional arc technique. The carbon coated fiber with a fatigue parameter over 150 guarantees the reliability, even when wounded on a small coil. In-line isolators and WDM couplers have been also developed. An amplifier module has been assembled, resulting in an output power more than +16dBm owing to the high performance of each component.

Improvement of fatigue behavior of a fusion spliced portion on a carbon-coated fiber is achieved by recoating carbon using a thermal-CVD process with a CO2 laser as a local heat source. The fatigue parameters, so-called n-values, of 121 and 94 are obtained on the non-spliced portion and the spliced portion, respectively. Assuming a life time prediction model, these high values have been proved to have an advantage in a long-term reliability and to be sufficient in a practical submarine cable use.