Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – With measuring – controlling – sensing – programming – timing,...
Reexamination Certificate
2000-07-07
2002-07-30
Hoffmann, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
With measuring, controlling, sensing, programming, timing,...
C065S412000, C065S381000, C065S501000
Reexamination Certificate
active
06425270
ABSTRACT:
INDUSTRIAL FIELD OF APPLICATION
The present invention relates to a method for producing an optical fiber preform. It further relates to a method for producing an optical fiber preform which can provide high quality optical fibers at a low cost. It also comprises melt welding the quartz glass tube for use in the optical fiber preform with the core glass rod for use in the optical fiber preform to obtain a monolithic product by switching from a pressurized state to a reduced pressure state.
Prior Art
Recently, mass production and the cost reduction of optical fibers are in high demand with increasing use of optical fibers in larger quantity. The mass production and cost reduction in the production of optical fibers can be accomplished most simply by forming a large optical fiber preform and by then drawing it. In the conventional practical method for producing optical fibers such as the vapor-phase axial deposition process (VAD) or the outer vapor-phase deposition process (OVD process), the core portion and the clad portion are all produced by VAD or OVD process. Thus, in case of scaling up, there was a disadvantage that the productivity of the fiber optical preform may be lowered. It one attempts to form a larger soot body (that is a precursor for an optical fiber preform which is a porous body obtained before vitrifying it into a transparent body), there may occur troubles such as the generation of cracks or the drop off of the porous soot body as to greatly lower the productivity. As a method for producing an optical fiber which overcomes the problems above, in JP-A-7-109136 and the like is proposed a production method, i.e., the so-called rod in tube method, which comprises forming the quartz glass tube for use as the clad portion accounting for 80% or more of the cross section area by a method capable of forming a high performance quartz glass tube and yet, of reducing cost, and then monolithically integrating the resulting quartz glass tube with the core glass rod formed by, for example, the VAD process or the OVD method. In the rod in tube method, in general, a gas is flown in the interstices between the quartz glass tube for optical fiber preform and the core glass rod for the optical fiber preform to prevent impurities and the like from being incorporated therein, or to control the OH groups (see JP-A-52-200042, JP-A-4-42848, JP-A-6-42848, and JP-A-6-21725). On the other hand, because the melt welding into a monolithic body should be performed under a reduced pressure, one end of the quartz tube for preforms must be sealed and the pressure be reduced before the melt welding. The maintenance of the pressure-reduced state is exemplified in, for example JP-A-7-196332 or JP-A-8-277138.
For the cost reduction in the rod in tube method, it is essential to effectively use the quartz glass tube for optical fiber preforms without making any loss, and it is therefore preferred to initiate the heat melting of the quartz glass tube for optical fiber preform from a portion as near as possible to the edge end of the quartz glass tube for optical fiber preforms. Furthermore, because the quartz glass tube for optical fiber preforms is expensive, generally, a dummy tube is used after the melt welding is performed to obtain a monolithic body for the portion before the dimension becomes stable as the optical fiber preform. The dummy tube is a tube that is connected to the quartz tube for optical fiber preforms above, and is made of an inexpensive quartz glass of a low quality such as those containing impurities, pores etc., at a large quantity. However, concerning the increase in total weight and the cost of the dummy tube itself, it is preferred that the dummy is provided at a length as short as possible and that the heat melting is initiated from the edge portion thereof To satisfy this requirement, the lower end portion of the quartz glass tube for optical fiber preforms, or the lower end of the dummy tube in case of using the dummy tube, is placed in side the heating furnace during heating up to the initiation of heat melting. However, if heating is performed in this state, the atmosphere that is not clean inside the heating furnace enters into the quartz glass tube. Accordingly, it is necessary to supply a gas from the upper end portion of the quartz glass tube to maintain the inside of the tube clean. On the other hand, in initiating the heating for melt welding the quartz glass tube for optical fiber preforms to obtain a monolithic product with the core glass rod for optical fiber preforms, the gas supply must be switched to reduce the pressure. However, this can be accomplished by sealing the end portion of the side from which melting is initiated. If the pressure reduction is started before sealing, particularly when the open end is still remaining inside the heating furnace, the atmosphere that is not clean inside the heating furnace is sucked into the tube. This produces an optical fiber preform in which the residual impurities or generated bubbles remain at the melt welded interface between the quartz glass tube and the core glass rod, and this results in an optical fiber having impaired properties. Thus, it is necessary to reduce the pressure after the end portion of the side from which melting is initiated is molten, stretched by gravitational force, and drawn out to the outside of the heating furnace from the lower side of the furnace. On the other hand the aperture area of the lower open end of the quartz glass tube for optical fiber preforms gradually decreases with progressive melting of the quartz glass tube due to the surface tension and the stretching in the longitudinal direction exerted by the gravitational force. However, in case of maintaining a constant pressure by using an ordinary regulator and the like or a gas supply using a flow control, the gas supply does not change even if the aperture portion is reduced by the melting of the quartz glass tube. Then, it happens that the portion just above the reduced aperture portion expands by the gas pressure, and this finally causes explosion. If the molten portion is observable, the gas supply rate can be reduced depending on the observed results, but it is difficult to observe the inside of the furnace.
SUMMARY OF THE INVENTION
It has been found that the above problems can be overcome and that favorable optical fiber preforms can be produced at a low cost by measuring the pressure loss with respect to the gas supply in the interstices between the quartz glass tube and the core glass rod, and by then controlling the gas supply rate in such a manner that the pressure loss can be maintained constant. The present invention has been accomplished based on the above findings. An object of the present invention is to provide a method for producing a favorable optical fiber preform at a low cost, which comprises favorable changing from gas supply state to a reduced pressure state, and thereby acquiring a long effective length of the quartz glass tube.
The object above is accomplished by a method for producing an optical fiber preform, which is a production method for an optical fiber preform comprising inserting a core glass rod for use in the optical fiber preform into the quartz glass tube for the optical fiber perform and then melting them in a vertical heating furnace to melt weld them together into a monolithic body, wherein the melting is started in such a state that the lower open edge of said quartz glass tube is placed inside the heating furnace and a gas is supplied from the upper edge or the tube, and after the lower edge portion of the quartz glass tube is drawn out from said heating furnace by melt deformation and stretching by gravitational force, the gas supply is cut off and the pressure is reduced.
As described above, the method for producing an optical fiber preform according to the present invention comprises starting melting in such a state that the lower open edge of said quartz glass tube is placed inside the heating furnace and a gas is supplied from the upper edge or the tube, and after the lower edge
Kato Toshiyuki
Shimada Atsuhi
Suzuki Masanori
Watabe Yutaka
Heraeus Quarzglas GmbH & Co. KG
Hoffmann John
Milde & Hoffberg LLP
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