Silica glass article and manufacturing process therefor

Details Silica glass article and manufacturing process therefor Silica glass article and manufacturing process therefor

1999-08-06

2001-12-25

Sample, David R (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S037000, C501S905000

Reexamination Certificate

active

06333283

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a silica glass article and a manufacturing process therefor. More particularly, the invention relates to a silica glass article including an optical fiber suitable for use in an ultraviolet region, having an excellent initial transmission characteristic and capable of preventing increase in a transmission loss occurring due to ultraviolet ray irradiation, and to a manufacturing process therefor. The industrial utility of ultraviolet rays having a wavelength of 160 nm to 300 nm has increased in industrial fields of photolithography, a laser process, sterilization, disinfection and the like. A silica glass article according to the present invention is free from substantial deterioration caused by ultraviolet ray irradiation and can be advantageously be used in such fields.
2. Description of Related Art
Silica glass articles have been used as light transmitting mediums, such as optical fibers and various optical elements. In particular, use of the optical fibers having advantages of light weight, small diameter and no induction, has recently been widened in various industrial fields including communication, image transmission and energy transmission. As one of the fields, use of the optical fiber to transmit ultraviolet rays has been expected in the medical and precise processing fields. However, when glass is irradiated with ultraviolet rays, it deteriorates and its transmission loss increases. That is, there arises a problem in that deterioration takes place because of ultraviolet ray irradiation. Since the transmission loss of a silica optical fiber having the silica glass as the core thereof is smaller than that of an optical fiber made of multicomponent type glass, the silica optical fiber is a preferred element to transmit ultraviolet rays. However, the problem of the deterioration which takes place because of ultraviolet ray irradiation remain unsolved.
It is possible for light transmission in silica glass to be superior to that in air if the wavelength is not longer than 200 nm. The reason for this is that dissociation of oxygen gas takes place because of ultraviolet ray irradiation in air. Therefore, a high transmission can be expected if the deterioration which takes place because of ultraviolet ray irradiation can be reduced in the wavelength region not longer than 200 nm.
It has been considered that the deterioration which takes place because of ultraviolet ray irradiation is mainly attributed to a defect in glass. In the present invention, the “defect in glass” means a broken portion of the glass network structure or a portion of the glass network structure that is stretched due to a distortion of the glass and is apt to break easily.
FIG. 4
shows a plurality of examples of reported defects in glass of silica glass. As representative defects in glass, defects related to E′ center (≡Si•) and oxygen-deficient type defects (≡Si—Si≡) are exemplified. The above-mentioned defects in glass absorb ultraviolet rays at wavelengths of 163 nm, 215 nm and 245 nm. It has been considered that the foregoing defects in glass occur in a glass synthesized in an atmosphere somewhat lacking in oxygen, or in a glass having a low concentration of OH groups.
As a technique for reducing deterioration due to ultraviolet ray irradiation of silica glass, a technique has been disclosed in JP-A-5-147966 (hereinafter referred to as Document (1)) (The term “JP-A” used herein means an unexamined published Japanese patent application), in which the content of OH groups in a pure silica core is adjusted to from 10 ppm to 1000 ppm, the contents of F (fluorine) is adjusted to from 50 ppm to 5000 ppm and the contents of Cl (chlorine) is adjusted to substantially zero. An optical fiber thus obtained has an excellent initial characteristic of transmitting ultraviolet rays and is capable of reducing deterioration due to ultraviolet ray irradiation because fluorine is contained in a specific amount.
There are several known techniques that are not aimed to improve deterioration due to ultraviolet ray irradiation but are related to an improvement in radiation resistance of a fiber for transmitting visible rays or near infrared rays. For example, JP-A-60-90853 (hereinafter referred to as “Document (2)), has suggested a process in which any one of a glass soot body, a transparent glass preform and an optical fiber is processed in a hydrogen atmosphere to delete defects in the glass so as to improve the radiation resistance of the optical fiber. In the foregoing document, only a result of measurement of an increase in the loss experienced with respect to a near infrared rays having a wavelength of 1.3 &mgr;m is described. In addition, the effect of improving the ultraviolet ray resistance obtained by the above-mentioned process disappears within about two months.
In “Improvement in Radiation Resistance of Optical Fiber by Hydrogen Treatment and &ggr;-Ray Irradiation”, Tomon, Nagasawa, et al. pp. 1-213, Vol. 1, papers for lectures in National Conference of Semiconductor and Its Material Section of Electronic Communication Society, 1985, issued in 1985 by Electronic Communication Society (hereinafter referred to as “Document (3)”), a process has been reported for the purpose of preventing increase in light absorption of a pure silica-core optical fiber at a wavelength of 630 nm (visible ray) occurring due to &ggr;-ray irradiation. In this document, two-step treatment for an optical fiber is performed. In the first step, an optical fiber is doped with hydrogen molecules and then, in the second step, is irradiated with &ggr;-rays. Thus, seeds (precursors) of defects in the glass are converted into defects that absorb photon energy of a 2 eV band. Then, hydrogen previously dispersed in the fiber in the previous step and the defects in glass are chemically bonded to each other so as to improve the radiation resistance in the visible ray region. Also in the Document (3), there is no description about the characteristic of the fiber against ultraviolet rays.
U.S. Pat. No. 5,574,820 (hereinafter referred to as “Document (4)”), suggests an optical fiber and its manufacturing process that serves as a means for preventing increase in a loss in a visible ray region when a pure silica core fiber is used as an image fiber for transmitting visible rays in a radiation field. The proposed optical fiber is manufactured by previously irradiating pure silica core fiber with radiation in a large dose of 10
5
Gy or greater, so that increase in the loss in a visible ray region having a wavelength of from 400 nm to 700 nm does not exceed 30 dB/km. Moreover, a process for manufacturing the optical fiber has been suggested, but the characteristic in the ultraviolet ray region has not been described.
JP-A-5-288942 (hereinafter referred to as “Document (5)), as in Document (4), has suggested a process for improving radiation resistance of an image fiber for transmitting visible rays. In the process, an image fiber is irradiated with g-rays in a large dose of 10
7
Roentgen to 10
9
Roentgen (10
5
Gy to 10
7
Gy) and then is heated in a hydrogen atmosphere. Also no description about the characteristic in the ultraviolet ray region has been made in the above-mentioned document.
In the Document (2), hydrogen is added so that the radioactive resistance of the optical fiber in the near infrared rays is improved. Recently there have been disclosed several processes in which hydrogen molecules are added to silica glass in an attempt to improve ultraviolet ray resistance. For example, JP-A-3-23236 (hereinafter referred to as “Document (6)”) suggests silica glass in which OH groups are contained in an amount of 100 ppm or higher, substantially no oxygen defect exists and hydrogen gas is contained, so that ultraviolet ray resistance is improved. JP-A-5-32432 (hereinafter referred to as “Document (7)”) suggests a process, in which deterioration due to ultraviolet ray irradiation is prevented by controlling the concentration of hydrogen i

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