Optical waveguides – Having particular optical characteristic modifying chemical... – Of waveguide core
Reexamination Certificate
1998-11-23
2001-09-11
Font, Frank G. (Department: 2877)
Optical waveguides
Having particular optical characteristic modifying chemical...
Of waveguide core
Reexamination Certificate
active
06289161
ABSTRACT:
The invention relates to an optical component having a cylindrical core glass of synthetic quartz glass which contains hydroxyl groups and a maximum 200 wt.-ppm of chlorine and no dopant in the form of a metal oxide, and which is axially enveloped by a glass cladding of doped quartz glass having a lower index of refraction than the core glass.
Subject matter of the invention can be, for example, an optical fiber or a preform for the production of an optical fiber. A preform with a core of synthetic quartz glass with low chlorine content and an optical fiber drawn from it is described for example in U.S. Pat. No. 4,650,511. To improve the long-term stability of the fiber in a moist or a hydrogen-containing atmosphere the chlorine content of the core glass is kept low. In an embodiment the chlorine content is set at 5 ppm and the hydroxyl group concentration of the core glass at 50 ppb. To increase the index of refraction the core glass is doped with germanium oxide. The known optical fiber is characterized by high resistance to the action of hydrogen and moisture. However it is suitable without limitation only for the transmission of light of specific wavelengths which are uninfluenced by absorption bands such as those caused, for example, by the dopant, germanium oxide.
An optical compound of this class is described in EP-A1 590 199. This is a so-called step index fiber with a cladding glass of fluorine-doped or boron-doped quartz glass surrounding a core glass of pure synthetic quartz glass. 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, but no dopants in the form of metal oxides such as germanium oxide. The known optical fiber is characterized by high resistance to high-energy UV radiation.
It is known that optical fibers of quartz glass with an undoped core can have low attenuation values. In the publication of Gotaro Tanaka et al. in “Fiber and Integrated Optics,” vol. 7, pp 47-56, a so-called single-mode fiber is described having a core of pure undoped quartz glass and with a jacket of fluorine-doped quartz glass. At light wavelengths of 1.5 &mgr;m, attenuation of less than 0.16 dB/km was measured. The theoretical attenuation minimum in such optical fibers is even slightly lower than this. For the transmission of optical radiation in the spectroscopy range, however, the absolute attenuation minima are less relevant; instead a low attenuation is desired over a wide wavelength range, typically from about 350 nm to 1000 nm and even up to 2100 nm. Such spectroscopic applications are combined under the term, “broad-band spectroscopy,” hereinafter. The known optical fibers of quartz glass, however, on account of molecular vibrations of the hydroxyl groups, display either absorption bands in the infrared wavelength range, or in the UV wavelength range they have absorption bands of molecular chlorine or defects due to intrinsic structural defects. On account of such absorptions, either in the one or in the other boundary area of the relevant wavelength spectrum, the possibilities for the use of quartz glass fibers in broad-band transmission, especially in broad-band spectroscopy, have heretofore been limited.
The present invention therefore is addressed to the problem of offering an optical component of quartz glass suitable for broad-band transmission, especially for broad-band spectroscopy, which is characterized by low attenuation over a wide range of wavelengths.
This problem is solved by the invention, setting out from the optical component of the kind described above, in that the core glass contains less than 5 wt.-ppm of hydroxyl groups.
The core glass of the optical component of the invention is distinguished by a combination of features which can be characterized qualitatively as “low chlorine-content” at a simultaneously “very low OH content,” and “freedom from metallic dopants.” In synthetic quartz glass the chlorine content and OH content, however, are usually correlated in such a way that a low chlorine content entails a high OH content, and vice-versa, a low OH content results in a high chlorine content. This correlation is to be attributed to the conditions involved in making synthetic quartz glass. The flame hydrolysis processes or sol-gel processes used as a rule for this purpose result in intermediates with a comparatively high OH content. An especially effective method for reducing the OH content consists in treating the OH-laden quartz glass with chlorine or chlorine-containing substances. This treatment, however, causes the installation of chlorine in the synthetic quartz glass.
With this background it is understandable that synthetic quartz glass, which is simultaneously low in chlorine and low in OH or even free of OH, is not easily available. For the reduction of the OH content, however, still other processes come under consideration as alternatives or supplements to the conventional treatment in a chlorine-containing atmosphere, such as treatment in a fluorine-containing atmosphere, or the subsequent removal of chlorine from the intermediate product by a temperature, vacuum or gas treatment. Surprisingly it has been found that synthetic quartz glass, which has a chlorine content of no more than 200 wt.-ppm and at the same time a hydroxyl content of less than 5 wt.-ppm, and which is free of metallic dopants, is outstandingly suitable as a core glass for an optical component for use in broad-band spectroscopy. For a synthetic quartz glass of this kind is characterized by a broader spectral range with lower absorption in comparison with the known quartz glasses. Furthermore, the component according to the invention assures a substantially “true-color” transmission of optical radiation, that is, without change of the transmitted wavelength spectrum. This is important when the optical component is used in spectroscopy if near-reality is necessary over a wide range of wavelengths, as for example in astronomy. The absorption of the optical component of the invention in the ultraviolet range is shifted toward shorter wavelengths, which is to be attributed to the comparatively low chlorine content. On account of the low OH content the absorption in the infrared spectral range is shifted toward longer wavelengths. Moreover, absorption bands in the wavelength range from about 700 nm to 2700 nm, which can impair “true-color” transmission, are either weak or non-existent.
The wavelength range between 350 and 2100 nm is relevant to suitability for broad-band transmission. For use in broad-band spectroscopy the optical component is usually in the form of a so-called multimode fiber with a step index profile. The core glass is free of metallic dopants which have absorption bands in the relevant wavelength range. The quartz glass in the cladding is provided with dopants such as fluorine and/or boron which lower the refractive index of quartz glass.
The concentration figures given above refer to a value obtained across the diameter of the core glass. The OH content is determined by spectroscopy based on the absorption losses, and the concentrations of chlorine and fluorine by chemical analysis.
An additional improvement in regard to the use of the optical component of the invention for broad-band transmission, especially in broad-band spectroscopy is obtained when the core glass contains less than 1 wt.-ppm of hydroxyl groups. The absorptions of the core glass, which falsify and impair image transmission, especially in the infrared spectral range from about 700 nm to 2700 nm, are thereby further reduced.
In regard to broad-band use of the optical component it has been found desirable to limit the chlorine content of the core glass to no more than 100 wt.-ppm. This has an absorption-reducing effect especially in the ultraviolet spectral range.
An embodiment of the optical component of the invention has proven especially good, in which the core glass contains fluorine, the fluorine content amount to at least 2000 wt.-ppm.
Prefer
Schötz Gerhard
Vydra Jan
Font Frank G.
Fulbright & Jaworski LLP
Heraeus Quarzglas GmbH & Co. KG
Nguyen Tu T.
LandOfFree
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