Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2002-05-23
2003-09-23
Hoffmann, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S413000, C065S424000
Reexamination Certificate
active
06622527
ABSTRACT:
The present invention relates to a core glass for the preparation of a preform for an optical fiber, especially an optical fiber for the transmission of ultraviolet radiation, obtained by flame hydrolysis of a silicon compound, deposition of finely granular SiO
2
on a substrate with direct vitrification and formation of a synthetic quartz glass.
The invention furthermore relates to a preform for an optical fiber, especially for an optical fiber for the transmission of UV radiation, with a core glass of synthetic quartz glass, obtained by flame hydrolysis of a silicon compound, deposition of finely granular SiO
2
on a substrate with direct vitrification and formation of a synthetic quartz glass which is enveloped in a jacket glass.
The invention furthermore relates to a method for the production of a core glass for a preform for an optical fiber, especially for an optical fiber for the transmission of UV radiation, comprising the production of synthetic quartz glass by flame hydrolysis of a silicon compound, deposition of finely granular SiO
2
on a substrate with direct vitrification and formation of the synthetic quartz glass.
The invention furthermore relates to a method for the manufacture of an optical fiber, especially an optical fiber for the transmission of UV radiation, by drawing from a preform which comprises a core glass of synthetic quartz glass which is formed by flame hydrolysis of a silicon compound, deposition of finely granular SiO
2
on a substrate with direct vitrification and formation of the synthetic quartz glass.
Preforms for optical fibers generally have a core which is enveloped by a jacket of a material with a lower index of refraction. Preforms are also known which consist only of a so-called core rod of core material, the jacket or a portion of the jacket being applied to the core material when the fiber is drawn from the preform. For the manufacture of preforms for optical fibers from synthetic quartz glass, essentially three methods have become established which are referred to in the technical literature as VAD processes (vapor phase axial deposition), OVD processes (outside vapor-phase deposition), and MCVD processes (modified chemical vapor deposition). Furthermore, the production of preforms by the so-called rod-in-tube technique is known. In all these methods a core glass of synthetic quartz glass is generally made by the flame hydrolysis of a silica containing compound by producing SiO
2
particles and depositing them and vitrifying them on a substrate. The substrate can be, for example, a quartz glass tube which consists of a jacket glass. The vitrification of the SiO
2
particles can be performed directly during its deposition on the substrate (referred to hereinafter as direct vitrification), or in a separate sintering process as in the case of the so-called “soot process.” Both variant vitrifications result in a dense, transparent, high-purity quartz glass.
On account of their porosity, so-called soot bodies are simply cleaned, doped or otherwise treated prior to vitrification. On the other hand, directly vitrified synthetic quartz glass has advantages for some applications in connection with the transmission of short-wave UV radiation. Due to the presence of hydrogen and oxygen during the flame hydrolysis, quartz glass made by direct vitrification generally contains a relatively high OH content and a certain concentration of hydrogen.
Depending on the method, the jacket glass is produced in a separate process (OVD, MCVD, plasma process, rod-in-tube technique), or the jacket glass and the core glass are produced simultaneously as in the so-called VAD process. To change the index of refraction of the quartz glass, a dopant is usually added, such as germanium, for example, to increase the index, or fluorine and boron to lower the index of refraction.
The present invention sets out from these long-known processes for the production of a preform, the core glass of the preform being made by direct vitrification. By the heating and drawing of the preform thus prepared optical fibers are obtained from it.
Such optical fibers are used not only for the transmission of information in the form of optical signals in communication technology, but also increasingly for the transmission of high-energy UV radiation, as for example in medical technology, in material machining, in ultraviolet spectroscopy or in microlithography apparatus for the production of highly integrated circuits in semiconductor chips. The illumination systems of modern microlithography apparatus are equipped with excimer lasers which emit high-energy, pulsed UV radiation of a wavelength of 248 nm (KrF lasers) or 193 nm (ArF lasers). It is known that such short-wave UV radiation can produce structural defects in the quartz glass of the optical fibers and resultant absorptions. For example, an excess oxygen defect, in which a non-bridge building oxygen atom is present (a so-called NBOH center) results in a relatively broad absorption band at a wavelength of about 265 nm. A defect in which only three oxygen atoms (instead of four) are bound to a silicon atom, and which is called an E′ center, produces an absorption band around 215 nm. A review of structural defects in quartz glass is given by David L. Griscom in “Defect Structure of Glasses”, J. Non-Cryst. Solids, 73 (1985), pages 55-77.
The influence of the chemical composition of quartz glass on the damage that can be done by irradiation with high-energy UV light is described, for example, in European Patent Application A1 401 845. Accordingly, a high stability in radiation was found in high-purity quartz glass containing a relatively high OH content of 100 wt.-ppm to about 1000 wt.-ppm and at the same time a relatively high hydrogen concentration of at least 5×10
16
molecules per cm
3
(with respect to the volume of the quartz glass). The good influence of hydrogen on stability in radiation can be explained by the fact that it can contribute to the healing of defects and thus to a slower increase of the adsorption caused by radiation. On account of this action of hydrogen it is recommended in European Patent Application A1 401 845 that optical components which must satisfy stringent requirements in regard to stability under radiation be charged with hydrogen.
In European Patent Application EPA1 590 199 a description is given of core glass of the kind for the production of an optical fiber for the transmission of high-energy UV light a preform made with the use of the core glass, a method for the production of the core glass, and a generic process for the production of such an optical fiber. The prior-art core glass is a pure synthetic quartz glass which is made by the flame hydrolysis of methyl trimethoxysilane. The core glass is substantially free of chlorine, its hydroxyl group content (OH content) is between 10 and 1000 ppm, and it contains fluorine in a concentration ranging between 50 and 5000 ppm. To prepare a preform for a so-called graded index fiber a core glass rod is provided with a tube of core glass by the “rod-in-tube” technique wherein the jacket glass consists of fluorine-doped or boron-doped quartz glass. An optical fiber is drawn from the preform by heating the preform to about 2000° C.; it softens beginning at one end, and the fiber is drawn from the softened part. The known optical fiber shows, in comparison to other fibers, good stability under high-energy UV radiation. For applications in which an especially slow increase of the radiation-induced absorption, a low transmission loss, and a good long-term stability are important, the known optical fiber, however, is inadequate.
The invention is therefore addressed to the problem of optimizing the stability of a core glass for the production of a preform, obtained by direct vitrification, under ultraviolet radiation, especially under high-energy ultraviolet radiation of a wavelength of 250 nm and shorter, with a view to lower absorption, slower increase of absorption and greater long-term stability.
The invention is furthermore addressed to the p
Schötz Gerhard
Vydra Jan
Fulbright & Jaworski L.L.P.
Heraeus Quarzglas GmbH
Hoffmann John
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