Motohiro Nakahara

Reaction property of diffused H 2 with defect Centers in GeO 2 -doped fiber

This paper presents the reaction property of diffused molecular hydrogen (H 2 ) with irradiation-induced defect centers in a GeO 2 -doped fiber. When H 2 diffuses into an irradiated fiber, an OH loss increases even at room temperature. This loss increase is thought to be caused by chemical reaction between diffused H 2 and irradiation-induced nonbridging oxygen hole centers (NBOHCs). The hydrogen-associated defect centers having 11.9-mT hyperfine structure are formed by chemical reaction between diffused H 2 and irradiation-induced Ge-defect centers. Moreover, the irradiated fiber shows an OH loss increase at high temperatures even after diffused H 2 is removed from the fiber. The hydrogen-associated defect centers are considered to be the hydrogen source for the OH formation.

Fluorine and chlorine effects on radiation-induced loss for GeO 2 -doped silica optical fibers

Fluorine and chlorine effects on radiation-induced loss associated with the preexisting defects have been investigated. It is found that preexisting defects in fibers are closely related to radiation-induced loss. Fluorine and chlorine doping in GeO 2 -doped silica core optical fibers decreases GeO absorption at 0.24 μm and ESR signal intensity due to germanium-related paramagnetic defect. Fluorine and chlorine doping is effective in decreasing the radiation-induced losses in GeO 2 -doped silica core optical fibers.

Formation and disappearance of defect centers in GeO 2 -doped silica fibers with heat treatments

Electronic state changes of defect centers in Ge-doped SiO 2 fibers were investigated using ESR technique. The germanium associated defects, especially Ge(3) centers, are mainly produced during the drawing process and are quenched in the fibers. The heat treatment of the Ge-doped fiber causes a remarkable decrease in the Ge(3) ESR intensity, subsequently causing OH absorption loss increase. The mechanism of this OH loss increase is proposed, in relation to the interactions between the Ge(3) centers and hydrogen.

Silica-based Single-Mode Guided-Wave Devices

Silica-based single-mode waveguides, fabricated on silicon substrates by a combination of flame hydrolysis deposition and reactive ion etching, have low propagation loss and low fiber coupling loss. High controllability in the silica-based waveguide fabrication leads to directional couplers with an accurate coupling ratio can be fabricated. Based on these advantageous features, we have successfully constructed a variety of guided-wave devices such as multi/demultiplexers for 1.3 and 1.55 μm wavelength, 2 x 8 couplers, ring resonators, and Mach-Zehnder interferometers which operate as optical switches or multi/demultiplexers with various channel spacings from 0.008 nm to 0.25 μm.

Loss increases due to chemical reactions of hydrogen in silica glass optical fibers

Loss increases due to the chemical reactions of hydrogen are investigated for GeO 2 -doped, GeO 2 . F-doped, and pure silica core fibers. Dissolved hydrogen gas causes the hydroxyl loss increase as well as the short-wavelength loss increase in the first two fiber types at high temperatures. The short-wavelength loss increase is attributed to two kinds of ultraviolet absorption increases. Doping fluorine reduces both of these loss increases as well as enhances the activation energy of the hydroxyl formation.

Gamma-ray radiation effects on hydroxyl absorption increase in optical fibers

This paper presents gamma-ray irradiation effects on OH-loss increase due to the chemical reaction of H 2 for GeO 2 -doped and GeO 2 . P 2 O 5 -doped fibers. When H 2 diffuses into irradiated fibers (γ-H 2 treatment), OH-loss increase occurs even at room temperature. Moreover, OH-loss increases due to dissolved H 2 in irradiated fibers are larger than those in unirradiated fibers at high temperatures. These OH-loss increases are attributed to irradiation-induced defects with which H 2 reacts to form OH ions. When H 2 -dissolved fibers are irradiated, the OH-loss increases are larger than those in the γ-H 2 -treated fibers at room temperature. These larger loss increases are thought to be caused by the reaction of the unstable radical and dissolved H 2 , which are excited by the irradiation.

Fabrication of dispersion-free VAD single-mode fibers in the 1.5-µm wavelength region

Low-loss and low-dispersion single-mode fibers in the 1.5- μm wavelength region were fabricated by the VAD method. Causes for loss increase in these fibers were investigated. By improving uniformities in the refractive index, both in core section and along the core axis, minimum loss of 0.35 dB/km at 1.55 μm was obtained. Bending loss of the 1.5-μm optimized single-mode fiber was also discussed.

On-line monitoring technique of the refractive-index profile in the VAD process

A nondestructive on-line monitoring method to estimate the refractive-index profile in vapor-phase axial deposition (VAD) processing has been developed. This technique is developed on the basis of the deposition properties of a SiO 2 -GeO 2 glass particles in the flame hydrolysis reaction. The profile monitoring accuracy is \pm 7 \times 10^{-5} in refractive-index difference. The estimated profiles during the porous preform fabrication are well coincided with those of the consolidated preform. The average bandwidth properties of graded-index fibers produced by adjusting the fabrication conditions, to be optimized with using present method, is 2.5 GHz . km0.9at 1.3-μm wavelength.

Invited) Recent Progress in VAD Fiber Fabrication Process

Since the development of the Vapor-phase Axial Deposition (VAD) process in 1977, rapid progress has been made in the improvement of transmission properties by the analysis of the above process. This paper describes the progress in the fabrication process for attaining better transmission characteristics and future aspects of this process. Specific characteristics obtained in this process are: reduction in the transmission loss of fibers by the optimal material composition and dehydration technique, the realization of less than 1 ppb of OH content, a bandwidth that now exceeds 6 GHz-km at 1.3 µm, which was attained through the analytical studies of the profile formation mechanism and the controlling of the deposition condition, and the production of long low-loss graded-index fibers.

Germanium-dopant effect on hydroxyl loss increase in optical fibers

The germanium-dopant effect on hydroxyl loss increase in optical fibers is studied experimentally. The distribution profile of hydroxyl absorption which is caused by hydrogen diffusion is measured for GeO 2 -doped silica glasses. From the experiment, it is found that the distribution profile of induced hydroxyl absorption is similar to the GeO 2 concentration profile. Moreover, the absorption loss increases due to hydrogen diffusion are measured for GeO 2 -doped silica fibers. From the experiment, it is concluded that the induced molecular hydrogen loss as well as the induced hydroxyl loss increases with an increase in the GeO 2 concentration.