Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – With measuring – controlling – sensing – programming – timing,...
Reexamination Certificate
2000-12-20
2003-06-10
Meeks, Timothy (Department: 1762)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
With measuring, controlling, sensing, programming, timing,...
C065S378000, C065S417000, C427S008000, C427S163200, C427S166000, C427S237000, C427S255130, C427S255180, C427S255190
Reexamination Certificate
active
06574993
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of applying glass layers, which may or may not be doped, to the interior of a substrate tube by means of a chemical vapour deposition (CVD) technique, using a reactive gas mixture, in order to obtain a preform that exhibits a precisely defined refractive index profile.
2. Description of the Related Art
A method of this kind is known per se from U.S. Pat. No. 4,292,341. According to said US patent, efficient transmission of optical wave energy along an optical fiber is achieved by grading the refractive index of the fiber core. Thus, accurate knowledge of the index profile is necessary in order to assess a fiber's transmission properties. To this end, various techniques have been developed for measuring the index profile of optical fibers and optical fiber preforms. According to the method known from the aforesaid US patent, a precursor, in the form of a gas containing glass forming material and suitable index modifying dopants, is caused to flow into a substrate tube. The substrate tube is then heated, causing a layer of glass to be deposited on the inner surface of the tube. Thus the thickness of each deposited layer, and the concentration of dopants within each layer can be considered to be functions of a number of parameters including the temperature of the hot zone created within the tube, the speed with which the hot zone moves along the tube, and the concentrations of the glass forming material and the index modifying dopants within the gas being fed to be tube. Thus the exact concentrations in the gaseous mixture being fed to the substrate tube are accurately measured and carefully checked during manufacture of the preform. To this end the gaseous components, for example GeCl
4
and SiCl
4
, are illuminated with ultraviolet radiation, after which the intensity of the radiation energy from said gaseous material is measured, which measurement is compared with a reference signal, whereby a control signal is generated in response to this comparison, which control signal causes the concentration of GeCl
4
and/or SiCl
4
in the gaseous material to change. Such a change will only take place until the difference between the reference signal and the control signal has reached value zero. When using such a method, it will be possible to manufacture a preform exhibiting a precisely computed refractive index profile. One drawback of a technique of this kind is the fact that in practice the measurements whereby ultraviolet radiation is used are not accurate and reproducible to a sufficient degree, so that a preform is obtained which exhibits an unacceptable variation in comparison with the desired refractive index profile of the preform to be manufactured. In addition, such a method is only suitable for supplying one dopant, because generally two or more dopants cannot be distinguished from each other to a sufficient degree and because they produce interference upon being illuminated with ultraviolet radiation.
A method of this kind is also known from U.S. Pat. No. 4,161,656, wherein the concentration and distribution of index-modifying dopants is determined by illuminating a length of the fiber or fiber preform with ultraviolet radiation. A drawback that occurs thereby is that such a measurement does not meet the requirements that are currently made with regard to accuracy. In addition to that, exact feedback between the process conditions during deposition and the measured values is difficult to realize in practice, so that the profile that is eventually obtained exhibits differences with the intended profile.
BRIEF SUMMARY OF THE INVENTION
In one embodiment, a method of applying glass layers, which may or may not be doped, to the interior of a substrate tube by means of a chemical vapour deposition (CVD) technique, using a reactive gas mixture, in order to obtain a preform that exhibits a precisely defined refractive index profile, includes but is not limited to: a) determining the desired refractive index profile of the preform to be manufactured, b)precisely adjusting both the composition and the supply rate of the reactive gas mixture for manufacturing the intended preform, in conformity with the refractive index profile determined in step a), c) introducing the reactive gas mixture, under the conditions adjusted in step b), into the substrate tube and effecting a reaction therein so as to achieve deposition of the glass-forming oxides onto the interior of the substrate tube, d)collapsing the substrate tube obtained from the deposition process of step c) into a preform and subsequently subjecting said preform to a refractive index profile measurement, e) comparing the refractive index profile determined in step a) with the refractive index profile measured in step d), and f) correcting the differences in refractive index profiles measured in step e) by adapting the composition of the reactive gas mixture as a function of time during the subsequent deposition process.
DETAILED DESCRIPTION OF THE INVENTION
Accordingly, an embodiment of the present invention provides a method of manufacturing a preform exhibiting a precisely defined refractive index profile by means of a chemical vapour deposition (CVD) technique.
Another embodiment of the present invention provides a method of manufacturing a preform by means of a chemical vapour deposition (CVD) technique wherein two or more dopants can be used for increasing or decreasing the refractive index value in a continuous deposition process.
Another embodiment of the present invention provides a method of manufacturing a preform by means of a chemical vapour deposition (CVD) technique wherein a preform exhibiting any desired refractive index profile can be obtained in a reproducible manner.
According to an embodiment of the present invention, the method referred to in the introduction is characterized in that said method comprises the following steps:
a) determining the desired refractive index profile of the preform to be manufactured,
b) precisely adjusting both the composition and the supply rate of the reactive gas mixture for manufacturing the intended preform, in conformity with the refractive index profile determined in step a),
c) introducing the reactive gas mixture, under the conditions adjusted in step b), into the substrate tube and effecting a reaction therein so as to achieve deposition of the glass-forming oxides onto the interior of the substrate tube,
d) collapsing the substrate tube obtained from the deposition process of step c) into a preform and subsequently subjecting said preform to a refractive index profile measurement,
e) comparing the refractive index profile determined in step a) with the refractive index profile measured in step d), and
f) correcting the differences in refractive index profiles measured in step e) by adapting the composition of the reactive gas mixture as a function of time during the subsequent deposition process.
Using the aforesaid steps a)-f), it is possible to obtain precisely defined refractive index profiles in a preform whereby there is feedback of the refractive index profile measurement in the final preform and the gas dosage used in the deposition process. The results of the refractive index profile measurement obtained in step d) are used for adjusting the production process that takes place in step c) for manufacturing the preform. On the basis of the refractive index profile measurement obtained in step d) it is thus possible according to the present invention to determine the extent to which the process, in particular the composition of the reactive gas mixture, is to be adjusted in order to approach the refractive index profile desired in step a) as best as possible. After both the composition and the supply rate have been precisely adjusted according to step b) of the present invention, the deposition process according to step c) is carried out. Once the deposition process according to step c) has been terminated, the preform thus obtained is subjected to a refractive ind
Breuls Antonius Henricus Elisabeth
de Fouw Marinus Jacob
Meeks Timothy
Plasma Optical Fibre B.V.
Seed IP Law Group PLLC
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