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.

A continuous fabrication process for high-silica fiber preforms has been developed. Unlike the conventional fabrication process, the perform is grown in an axial direction. The process is essentially a soot process. A porous preform is grown in the axial direction by the deposition of fine glass particles synthesized with vapor reaction of raw materials, and is continuously consolidated into a transparent preform by zone-melting. The refractive index profile is controlled in the spatial domain, rather than as a function of time, as in conventional methods. The transmission loss of the fiber made by this method was 2.3 dB/km at 0.85 µm and 0.75 dB/km at 1.2 µm. The bandwidth graded index type fiber was 300 MHzkm as an average. The best value was 1.0 GHzkm. These fibers had good uniformity along the longitudinal direction, both in transmission characteristics and in dimensions.

Fundamentals and development of PDFAs are described. Spectroscopic data of Pr3+ in a fluoride glass are presented with a view to understanding the performance of PDFA. An amplification mechanism model which explains PDFA performance is established. On the basis of the model pump schemes which efficiently extract the potential in Pr3+-doped fluoride fiber are discussed in order to construct amplifier modules. Gain characteristics of Pr3+-doped fluoride fibers are clarified. Codoping effect on pump wavelength extension is investigated. LD-pumped PDFA construction and performance are described. PDFAs are shown to be attractive device to upgrade the performance of optical systems at 1.3µm. Furthermore future approaches to PDFA research are discussed.

We report the generation of deeply-modulated optical pulse trains at high-repetition-rates through modulational instability in optical fibers. The optical pulse train can be generated over the wide repetition-rate-range from sub-THz to a few THz by controlling the anomalous group velocity dispersion of optical fibers. Very-high-repetition-rate of more than 4 THz is attained by the minimization of the absolute value of the anomalous dispersion. The deep modulation of the high-repetition-rate optical pulse train is achieved by inducing modulational instability with pump-probe mixing or external optical feedback. Both inducing methods also enable us to tune the pulse repetition rate precisely. The high repetition rate optical pulse trains seem to go far toward applications such as high-bit-rate optical communication and high-speed optical processing.

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.