Optical waveguides – Having particular optical characteristic modifying chemical... – Of waveguide core
Reexamination Certificate
2001-08-09
2003-08-26
Nasri, Javaid H. (Department: 2839)
Optical waveguides
Having particular optical characteristic modifying chemical...
Of waveguide core
Reexamination Certificate
active
06611650
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a silica optical fiber, particularly to a silica optical fiber superior in resistance to high energy.
BACKGROUND OF THE INVENTION
In recent years, an optical fiber scope is frequently used for observation and checkup of places to which human or a camera cannot have access, such as a nuclear power plant, inside of a blast furnace, a boiler of power plants and the like. An optical fiber scope generally consists of an image fiber, an objective lens, an eyepiece, a light guide, and the like.
The image fiber consists of from thousands to tens of thousands of pixel fibers integrated by alignment, and transmits images formed on a fiber end by the lens to the other end upon decomposition of the images into each pixel fiber. The image fiber includes a multiple type fiber and a bundle type fiber. The multiple type fiber includes a number of aligned pixel fibers that are integrated by melting and drawing, thus forming a common clad. In contrast, the bundle type fiber includes a number of constituent fibers that are fixed by adhesion on both ends.
The optical fiber scope is used under the above-mentioned severe conditions. Particularly in a nuclear power plant where a high-energy electromagnetic wave irradiation of &ggr;-ray is inevitable, pure silica is generally used as the core material of an image fiber, because it is superior in resistance to high energy. Due to the superior characteristics of the resistance to high energy, moreover, a silica optical fiber is also used for transmitting electromagnetic waves such as UV light and waves having shorter wavelengths such as X-ray.
However, the resistance to high energy of conventional silica optical fibers has not been improved to a sufficient level, and there have been ongoing attempts to improve resistance of silica optical fibers to high energy.
The present inventors have tried to improve optical fibers in the resistance to ultraviolet light by making the core of a silica optical fiber contain F element and OH group and removing Cl element from the core, as disclosed in JP-A-5-147966 (JP-B-8-9489), though sufficient effect has not been achieved.
Based on such results, the present inventors have noted completely different effects provided by the F element and the Cl element in radiation resistance and resistance to UV light, despite the fact that these elements fall under the seventh group in the periodic table, and acquired a completely new conception that the period of the element contained in the pure silica core material may be deeply involved in the resistance to high energy.
It is therefore an object of the present invention to provide a silica optical fiber superior in radiation resistance and having resistance to high energy of the light in the ultraviolet region, particularly to the light in the shorter wavelength ultraviolet region, specifically in an X-ray region.
SUMMARY OF THE INVENTION
The present invention is based on the finding that a pure-silica containing Cl element is markedly inferior to that containing F element in the resistance to high energy of ultraviolet radiation and &ggr;-ray radiation because, in addition to the difference from Si element in the electronegativity and the difference in the atomic radius from the Si element or oxygen (O) element, the Cl element basically has a
3
d
orbital or
4
s
orbital that permits electrons in the ground state to easily transit to the
3
d
or
4
s
orbital upon application of an energy, thus producing an excited state, and easily becomes a valence state such as trivalence, pentavalence and heptavalence, producing many radicals. From this finding, the present inventors have considered that the changes in valence due to high energy irradiation may cause a greater effect on the high energy resistance of quartz by not only the F element and the Cl element, but by the elements belonging to the first period-second period and non-metallic elements belonging to the third period-seventh period.
Further studies based on the above-mentioned consideration have led to the following conclusion. That is, the major differences between the elements belonging to the third period-seventh period, as represented by the Cl element, and the elements belonging to the first period-second period, as represented by the F element, are that the elements belonging to the third period-seventh period have any of the factors of (1) a
3
d
orbital or
4
s
orbital that permits electron(s) in the ground state to easily transit to the
3
d
or
4
s
orbital upon application of a large external energy, such as &ggr;-ray and UV light, thus producing an excited state, (2) a small difference in the electronegativity from Si element, and (3) a greater difference in atomic radius from Si element or oxygen element. Because they possess any of these elements (1)-(3), the elements belonging to the third period-the seventh period are considered to be easily activated when subjected to a large external energy and easily destroy a quartz structure, as compared to the elements belonging to the first period-second period.
Because the elements belonging to the first period-second period, particularly C element, show an extremely small difference in the atomic radius from the other elements belonging to the first period-second period, when they are introduced into the quartz structure and an Si—C bond or C—O bond is produced upon supplementation of radicals, the occurrence of the defects of the quartz structure, such as distortion and breakage of bond by a large external energy (e.g., &ggr;-ray, UV light and the like), is suppressed, whereby resistance to the high energy of quartz is considered to be remarkably improved.
Based on these new findings, the present invention provides a silica optical fiber characterized by the following.
(1) A silica optical fiber comprising a pure-silica core and a cladding layer formed on the pure-silica core, wherein the pure-silica core comprises a C element and has a content of elements belonging to the third period-the seventh period of the periodic table, except an Si element that constitutes the quartz structure, of not more than 100 ppm.
(2) The silica optical fiber of the above-mentioned (1), wherein the content of the C element is 10 ppm-500 ppm.
(3) The silica optical fiber of the above-mentioned (1), wherein the pure-silica core further comprises an F element and/or an OH group.
(4) The silica optical fiber of above-mentioned (3), wherein the content of the F element and/or the OH group is not more than 5000 ppm.
DETAILED DESCRIPTION OF THE INVENTION
The present invention has been made based on the aforementioned new findings. In the silica optical fiber of the present invention, the content of the elements belonging to the third period-seventh period (except the Si element constituting the quartz structure) in the pure-silica core is set for not more than 100 ppm, and the C element is contained in the pure-silica core, thereby improving the radiation resistance, and the resistance to electromagnetic waves in the ultraviolet region and the region of shorter wavelengths (e.g., X-ray), namely, high energy resistance.
According to the present invention, an F element and/or an OH group are/is preferably contained in the pure-silica core. When, for example, an F element is contained, it reacts with radicals to produce a relatively stable chemical structure such as Si—F. When an OH group is contained, it reacts with radicals to produce a relatively stable chemical structure such as Si—OH. When the F element and/or OH group are/is contained, therefore, the high energy resistance can be further improved due to the mutual action with the aforementioned stable structure provided by the introduction of the C element.
When one of the OH group and F element is to be contained along with the C element, it is preferably the F element, which becomes more effective when combined to the C element. In particular, when the radiation resistance at a wavelength shorter than the visible region is desired, an OH group does not need to
Gozen Toshikazu
Hayami Hiroyuki
Iio Terunobu
Kuzushita Hirokazu
Mori Michitsugu
Leydig , Voit & Mayer, Ltd.
Mitsubishi Cable Industries Ltd.
Nasri Javaid H.
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