Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2001-08-31
2003-05-20
Hoffmann, John (Department: 2839)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S404000, C065S389000, C264S002100, C264S001240
Reexamination Certificate
active
06564587
ABSTRACT:
The present invention relates to an optical fiber and a process for producing the optical fiber, especially an optical fiber having properties surpassing those known in the prior art.
BACKGROUND OF THE ART
Predominantly metal halides are used for radiation conduction with optical fibers in the medium infrared spectral range, which covers the wavelength range between approx. 4 and 18 micrometers. One is dealing in this connection with polycrystalline materials as a rule.
Known fibers have high leakage based on a relatively rough structure and numerous impurities and the fibers' low mechanical load capacity.
Optical fibers normally have an fiber-optic channel or core and a cladding with a refractive index n that deviates from that of the core, in particular, usually being lower than that of the core. A light wave that is coupled into the core of the fiber is ideally fully reflected between the core and the cladding and held in the optical fiber in this manner. The cladding is primarily supposed to prevent the overflow of light, for example, from one core to another in a bundle of fibers.
A special type of optical fibers are so-called single mode fibers, which fulfill special known conditions as such with respect to the size of the core diameter with reference to the ratio of wavelengths to be coupled to the numeric aperture. These types of single-mode fibers have extraordinarily high requirements and are used for example for the heterodyne detection of objects, whereby the radiation emanated from the object of a specific wavelength is coupled into two laterally separated single-mode fibers and the phase or time displacement between the two fibers is evaluated from the signals picked up on their respective ends.
Another special type of optical fibers is represented by the so-called SELFOC fibers, which are self-focusing. Self-focusing means that an image coupled into such a fiber in one location recurs at certain intervals similar to the situation with a sequence of convergent lenses, which are at such intervals from one another that the focal point corresponds at both sides of the convergent lenses with one of the focal points of the next respective convergent lens. A parabola-like index of refraction profile of the optical fibers is required in this connection. As a result, an object of the present invention is to provide a process for producing an optical fiber with which high product quality can be reliably. In addition, it is an objective of the invention to disclose an optical fiber with the best possible fiber-optical characteristics, in particular in the medium infrared spectral range.
SUMMARY OF THE INVENTION
The objective is attained with respect to the process aspect for one by a process to produce a starting material containing metal halides with a specific index of refraction for an optical fiber with the following steps:
Mixing halogenated gases into a gas mixture with a partial pressure ratio that is a function of the index of refraction of the starting material,
Causing a chemical reaction of the gas mixture with one metal to at least one reaction product with an initial temperature above the melting temperature of the reaction product and;
Cooling the reaction product to a second temperature below the melting temperature.
In accordance with the invention, halogenated gases are first mixed. Then a direct chemical reaction between the halogen particles and a metal or a metallic mixture is brought about.
The mixing ratio of the halogenated gases is selected in accordance with the index of refraction desired for the starting material containing metal halides that is being produced. Any index of refraction can be set with the process in accordance with the invention due to the adjustable mixing ratio of the halogenated gases within a value interval prescribed by the respective pure metal halides.
Thus, silver halide has an index of refraction of approx. 2 in the spectral range between 4 and 14 micrometers, for example.
In many embodiments, the correct mixing ratio be attained by adjusting the partial pressures of the halogenated gases that are used. The partial pressure ratio can be adjusted, for example, by correspondingly controlling the opening of the gas valves at the outlet of the gas containers each of which contains one of the halogenated gases. Alternatively, an individual gas source in which the halogenated gases are already present in the required mixing ratio can be used in the production process.
The correct partial pressure ratio is set as a function of the desired index of refraction in accordance with the boundary conditions of the chemical reactions being caused. In this case, the halogens involved in the reaction, the metal or metals involved in the reaction, the reaction temperature and the reaction atmosphere (inert gas or vacuum, for example) can influence the setting of the partial pressure ratio.
The process in accordance with the invention overcomes the limitation that exists with known production processes of the mixing ratio of metal halides in the starting material being limited to a very limited value interval.
In the case of one embodiment of the invention, the reaction of the halogen particles with the metal or the metal mixture is brought about in a vacuum. Alternatively, the chemical reaction can take place in an inert gas atmosphere.
In at least one embodiment, the inventive process is designed to produce a solid solution of metal halides.
In these embodiments, alkali metals, in particular potassium, sodium or cesium, rubidium or thallium or silver are used as metals.
Gases containing bromine, iodine or chlorine are used as halogenated gases individually or in combination in different exemplary embodiments. Halogen gases with especially high purity are typically used. Commercially available high-purity halogen gases can be used for this or correspondingly designed purity steps can be added to the production process itself.
The process in accordance with the invention is suitable in particular for producing a solid solution of AgBr
x
Cl
l−x
over the entire range of the mixing ratio, i.e., 0≦x≦1. This starting material for optical fibers can be produced with the process in accordance with the invention with especially high mechanical ruggedness and optical quality.
The partial pressure ratio is set in another embodiment of the invention in such a way that the solid solution contains bromine and chlorine in a particle number ratio of 3 to 1.
The microstructure of the polycrystalline solid solution is especially homogenous with this ratio. As a result, the light scatter in the fibers is minimized.
In another embodiment of the process in accordance with the invention, the metal is purified electrolytically in an additional step before the chemical reaction with the halogens is brought about.
In order to produce especially pure starting material for optical fibers, the liquid reaction product is guided through a purification stage before the step of cooling to a temperature below the melting point. Plans can be made for several consecutive purification steps. The purification takes place by guiding the liquid reaction product through capillary vessels, for example.
Another purification effect is achieved in an exemplary embodiment by moving the reaction product along a temperature gradient in a furnace during cooling from the first temperature to the second temperature. A temperature gradient can be set for example in a reactor with an oblong furnace by providing heating elements that can be controlled separately along its longitudinal extension. Thus, the reaction product can be cooled bit by bit, wherein the crystallization front is stationary. The crystallization front is the area within the reactor in which the reaction production falls short of the melting temperature and crystallizes.
Another inventive idea independently worthy of protection concerns a process to produce a blank for optical fibers with the following steps:
Preparing a bath with liquid starting material,
Rotating a solid core that is partially submerge
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