Influence of high GeO2 concentration on the optical-loss in high numerical-aperture optical fibers has been investigated. It is shown that a large amount of OH ions are permanently trapped near Ge sites like near Si sites in binary GeO2-SiO2 glasses. The full Ge-OH absorption spectrum, including overtones, is very similar to the Si-OH spectrum but shifts to longer wavelength with increasing GeO2 concentration. Rayleigh scattering increases proportionally to GeO2 concentration and raises the transmission loss of the fibers, especially in shorter wavelength region. The minimum loss is, nevertheless, expected to be less than 1 dB/km even in the fibers containing GeO2 up to several tens of mol% because of a slight reduction of infrared absorption. The transmission loss of 0.8 dB/km at 1.55 µm was achieved by a 27 mol% GeO2-doped core fiber made by the VAD method.

Sintering process of porous performs made by a vapor-phase axial deposition (VAD) method has been investigated. SEM observation indicates that the final stage of sintering is the collapsing process of closed pores in the transparent glass body. A bubble-free transparent perform is easily obtained by sintering in the helium gas atmosphere, but hard in the argon gas atmosphere under the usual zone-sintering condition. An interpretation of the experimental results is presented based on the elementary model for final stage of the sintering process; the closed pore collapsing depends on the balance between gas permeation rate into the surrounding glass and pore expansion rate during temperature increase.

Several long (11.0-30.4 km) graded-index fibers with good transmission properties in the wavelength bands of 1.3 and 1.55 µm have been fabricated by the VAD method. Special attention was paid during the deposition process to improve the longitudinal uniformity in transmission characteristics of the fibers. The transmission loss uniformities of the VAD fibers along axial direction have been measured by the back scattering technique with use of Nd-YAG laser at 1.06 µm. The bandwidth uniformities of these fibers were also ascertained to be satisfactory. Preliminary transmission experiments on spliced 65.1 km and 116.5 km fiber systems were also made by use of 1.3 µm and 1.55 µm laser diodes.

A refractive-index profile formation mechanism in the VAD method for optical fiber preform fabrication was investigated and practical techniques for precisely controlling the preform index profile are proposed. The GeO2 dopant distribution in the preform is found to be mainly caused by the porous VAD preform surface temperature distribution and by the raw material vapors mixing effect. By applying the surface temperature effect, the index profile can be controlled in the wide profile parameter range of d1-10. Further, by utilizing the maxing effect, it is possible to adjust the profile parameter precisely around a desired value. Transmission characteristics for graded-index fibers obtained with the above control technique are also presented.

A thermo-optic phase shifter consisting of a thin film heater deposited on a bridge-suspended silica waveguide on a silicon substrate is proposed for reducing electric power consumption of the phase shifter. A successful application to a guided-wave 22 optical switch is demonstrated. The electric power consumption has been reduced to 1/2 to 1/10 depending on the bridge structure.

One of the main loss factors in the optical fibers fabricated by the vapor phase axial deposition process is at present residual OH ions which are originated mainly from an oxy-hydrogen flame. Residual OH ions show an influence on loss increase, especially, in the long wavelength region. OH-ion behaviors in the porous perform, transparent perform and drawn fibers are studied with use of an infrared spectroscopy and fiber transmission measurement. In order to realize low OH content VAD fibers, the dehydration effect of halide vapor on the porous perform is investigated experimentally. Dehydration effect of thionyl chloride on the porous perform is studied by various heat treatment temperature. As a result, residual OH ion content in the drawn fiber was reduced from 30 ppm to 20 ppb and the attained minimum loss of dehydrated fiber was 0.5 dB/km and 0.28 dB/km at 1.3 µm and 1.6 µm wavelengths, respectively. Present investigation clarified that the VAD fibers are applicable to an optical transmission system in the long wavelength band. We compare the loss peak values at the wavelengths of 1.39, 1.24 and 0.95 µm among several optical fibers fabricated by different method. Especially, the VAD fibers dehydrated with SOC12 show abnormal peak ratios at the wavelengths.

The hybrid integration of light sources and a photodetector with high-silica channel waveguides on a silicon substrate was successfully demonstrated for optical transmitting-receiving module use. The loss characteristics were experimentally examined and its individual losses were investigated in detail based upon theoretical and experimental results.

A strictly nonblocking 88 matrix switch was designed and fabricated using silica-based planar lightwave circuits (PLC) on a silicon substrate. The average insertion loss was 11 dB in the TE mode and 11.3 dB in the TM mode. The average switch element extinction ratio was 16.7 dB in the TE mode and 17.7 dB in the TM mode. The accumulated crosstalk was estimated to be 7.4 dB in the TE mode and 7.6 dB in the TM mode. The driving power of the phase shifter required for switching was about 0.5 W and the polarization dependence of the switching power was 4%. The switching response time was 1.3 msec. The wavelength range with a switch extinction ratio of over 15 dB was 1.31 µm30 nm.

Single-mode fibers are prospective candidates for future high-capacity and long distance transmission media. In particular, low loss single-mode fibers, operating around 1.27 µm wavelength, where the material dispersion of silica glass falls to zero, are very attractive because of their huge bandwidth capability. Until now, however, the attainment of very low loss in the long wavelength region has been confined to only multimode fibers. Single-mode fibers with GeO2 -SiO2 glass system, which have the broadest low loss window ant the long wavelength region, were fabricated. The minimum loss of 0.5 dB/km at 1.3 µm wavelength, where dispersion of the fiber is negligibly small, was obtained by suitably designing waveguide parameters and reducing OH contamination. Also, practical structural features and the OH ion behavior are clarified in relation to reduction of OH absorption, which is the major problem in reducing loss at the wavelength of interest.

Low-loss and long-length single-mode fibres have been fabricated by the VAD (Vapour phase axial deposition) method. The most important technique was a new torch development in order to form a thick low-loss cladding region in the thin porous preform. The best loss values for the VAD single-mode fibres were 0.35 dB/km at 1.3 µm and 0.2 dB/km at 1.55 µm. The OH absorption loss at 1.39 µm in the fibre was reduced to less than 0.1 dB/km. Moreover it turned out that VAD fibres had less tendency of the loss increase with increasing Δn than MCVD fibres.