Refractive-index dispersion mesuarements were performed using the minimum deviation method for bulk specimens at wavelengths from 0.4047 to 2.0581µm. The data were utilized to calculate an optimum index profile,αop, and zero-material-dispersion wavelength,λ0, for the optical fiber. For GeO2-SiO2 glass core fibers,αop values are 1.93 at 1.30µm, and 1.88 at 1.55µm. There are independent of GeO2 concentration. The wavelength λ0 shifts from 1.272 to 1.376µm, proportionally to the GeO2 concentration from 0-19.3 mol%. The data also show that the addition of P2O5 slightly increases material dispersion, and the value of αop for the P2O5-SiO2 core glass decreases monotonically from 2.16 at 0.4 µm to 1.91 at 2.0µm.

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.

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.

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.

Stimulated Raman scattering was investigated in optical fibers, pumped by a mode-locked and Q-switched Nd:YAG laser or a CW Nd:YAG laser. The critical powers for the first five Stokes, at which each Stokes begins to build up, decreased with an increase in the length of fiber and became constant values above 200 meter long in the case of a mode-locked and Q-switched pump. The first and the second Stokes were observed with 0.6 W and 2.1 W of CW pump power input, respectively, in a 35 km single-span fiber. Experiments showed that the pulse wave separation effect played an important role in stimulated Raman scattering characteristics of optical fibers.

To study material dispersion effects on transmission characteristics in optical fibers, material dispersion properties in GeO2-P2O5-doped silica optical fibers were determined as a function of GeO2 doping concentration. The material dispersion properties were evaluated from pulse delay measurement with nanosecond pulses in a 0.60-1.65 µm spectral region generated by a nanosecond optical pulse radiator. From the measured delay, refractive index of core glass, material group index, material dispersion, zero-material-dispersion wavelength and zero-dispersion wavelength in a single mode fiber have been determined as a function of GeO2 doping concentration in the range 4.1 to 19.2 mol %. By extrapolating present data in GeO2-P2O5-SiO2 ternaries, material dispersion properties in fused GeO2 glass have been predicted. The zero-material-dispersion wavelength in fused GeO2 glass can be estimated to be 1.68 0.03 µm.

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.