This invention relates to furnaces, and, more particularly, furnaces for manufacturing optical fibers and methods for using the same.
With the expansion of telecommunications services, there has been a great demand for optical fibers. Optical fibers are typically formed by drawing while heating and melting a transparent optical fiber preform in an optical fiber drawing furnace. Such furnaces typically draw the optical fiber while maintaining a flow of process gas around the optical fiber during processing. Such fiber drawing furnaces further conventionally flow the process gases from an end of the furnace adjacent the preform through to an opposite end of the draw furnace, which direction will generally be referred to herein as a downward flow. Examples of such a draw furnace are described, for example, in U.S. Pat. Nos. 5,848,093 and 5,637,130. It is also known, however, to use an upward flow of process gas in a draw furnace as illustrated in the above-referenced patents.
Draw furnaces generally provide for some form of outlet sealing means to prevent air intrusion into the draw furnace. For example, current optical fiber draw furnaces, which typically flow process gas in a downward direction, may provide a gas screen at the exit portion of the draw furnace as a sealing device. An example of a gas screen is a cylindrical, double wall mechanical device with flanges at both ends for mounting that may be used to supply, for example, inert gas to an optical fiber draw furnace. For a downward flow draw furnace, a single such gas screen may be attached to the bottom end of the draw furnace to introduce sealing gas, which will also flow downstream, to facilitate sealing of the lower end of the draw furnace. More particularly, the gas introduced at the single lower end gas screen need not be the same process gas as flowing through the system in the downward flow draw furnace as it is introduced downstream of the process region of the draw furnace. The gas from the single gas screen may be, for example, a sealing gas that is less expensive to utilize. This may provide for a reduced demand for process gas use during fiber draw in a downward flow optical draw furnace.
Such an approach may create difficulties when used in a process using upward gas flow of process gases during drawing of an optical fiber. More particularly, while a gas screen in such a system may be provided with sufficient flow of process gas to cause such gas to flow both upward through the treatment area as well as downwards to contribute toward sealing the lower end of the draw furnace, an adequate seal will generally not be provided due to the limited desired range of process gas flow rates through the fiber draw process chamber. In other words, it may not be possible to provide a sufficiently high flow rate for acceptable sealing without undesirably high flow rates in the process chamber. More particularly, for a graphite furnace, sealing quality is typically measured by carbon monoxide concentration that may result from reaction of oxygen (air) with carbon furnace components. Conventional graphite draw furnaces using a single gas screen have been shown to experience a carbon monoxide concentration measured at about 70 parts per million (ppm), which concentration level may not satisfy process requirements.
In various embodiments of the present invention, furnace assemblies are provided including a furnace defining an internal process chamber extending therethrough. A first gas screen is coupled to the furnace. The first gas screen is configured to introduce a first gas into the internal process chamber at a first end of the furnace. A second gas screen is positioned adjacent to the first gas screen at an opposite end of the first gas screen from the furnace. The second gas screen is configured to introduce a second gas to provide a seal for the first end of the furnace. The furnace may be a draw furnace and the process chamber may be an internal draw chamber.
In other embodiments of the present invention, the first end of the draw furnace is a downstream end of the draw furnace and the draw furnace has an upstream end opposite from the downstream end. The process gas from the first gas screen flows into the internal draw chamber and from the downstream end to the upstream end of the draw furnace so as to pass between a preform in the internal draw chamber and an inner wall of the draw furnace defining the internal draw chamber while the preform is heated. The sealing gas from the second gas screen may, in such embodiments, flow downstream from the downstream end of the draw furnace so as to reduce introduction of contaminant gases into the internal draw chamber while the preform is heated.
In further embodiments of the present invention, a flow controller is provided that controls a flow rate of the process gas from the first gas screen and a flow rate of the sealing gas from the second gas screen. The flow rates may be controlled to provide a desired flow rate of the process gas from the downstream end to the upstream end of the draw furnace (or a desired internal furnace pressure) and to provide a desired flow rate of the sealing gas from the second gas screen downstream from the downstream end of the draw furnace so as to reduce introduction of contaminant gases into the internal draw chamber while the preform is heated.
In other embodiments of the present invention, an orifice member is provided positioned between the first gas screen and the second gas screen. The orifice member includes a central opening having an area selected to provide a desired pressure drop across the orifice member so as to limit the flow of sealing gas through the first gas screen and to limit the flow of processing gas through the second gas screen. A muffle may also be coupled to the downstream end of the draw furnace and the first gas screen may be coupled to a downstream end of the muffle and the second gas screen may be coupled to a downstream end of the first gas screen.
In further embodiments of the present invention, draw furnace assemblies for manufacturing optical fiber are provided including a draw furnace defining an internal draw chamber extending therethrough. A first gas screen is positioned adjacent a downstream end of the draw furnace. The first gas screen is configured to introduce a process gas into the internal draw chamber at the downstream end of the draw furnace. A second gas screen is positioned adjacent to the first gas screen at an opposite end of the first gas screen from the draw furnace. The second gas screen is configured to introduce a sealing gas to provide a seal for the downstream end of the draw furnace. The sealing gas may be a heavier gas than the process gas. A flow controller is provided that controls a flow rate of the process gas from the first gas screen and a flow rate of the sealing gas from the second gas screen to provide a desired flow rate of the process gas from the downstream end to an upstream end of the draw furnace and to provide a desired flow rate of the sealing gas from the second gas screen downstream from the downstream end of the draw furnace so as to reduce introduction of contaminant gases into the internal draw chamber while a preform positioned in the internal draw chamber is heated.
In other embodiments of the present invention, methods are provided for providing a desired gas flow in a draw furnace for manufacturing optical fiber. A first gas screen is provided positioned adjacent a first end of the draw furnace. A second gas screen is provided positioned adjacent an end of the first gas screen opposite from the draw furnace. A process gas is injected into the draw furnace through the first gas screen at a process gas flow rate. A sealing gas is injected through the second gas screen at a sealing gas flow rate. The process gas flow rate and the sealing gas flow rate are selected to provide a desired flow rate of the process gas from the downstream end to an upstream end of the draw furnace and to provide a desired flow rate of the sealing gas from the second gas screen downstream from the downstream end of the draw furnace so as to reduce introduction of contaminant gases into the draw furnace while a preform positioned in the draw furnace is heated. For example, for a graphite furnace, the process gas flow rate and the sealing gas flow rate may be selected to provide a carbon monoxide concentration in the draw furnace while the preform is heated of less than about 50 parts per million (ppm) or an oxygen concentration of less than about 25 ppm.
The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of members, layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. It will be understood that when an element such as a member layer, region or substrate is referred to as being “on,” “connected to” or “coupled to” another element, it can be directly on, directly connected to or directly coupled to the other element, or intervening elements also may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to” or “directly coupled to” another element, there are no intervening elements present.
With reference to
The glass preform 102 is preferably formed of a doped silica glass. The preform 102 may be formed such that either the core or the cladding (if present) of the drawn fiber is doped or such that both the core and the cladding of the drawn fiber are doped. The silica glass may be doped with one or more of germanium and germanium and fluorine. Other suitable dopants may be used as well. Methods and apparatus for forming the preform 102 are well known and will be readily appreciated by those of skill in the art from the description herein.
The draw furnace 120 includes a housing 122 having a lower end 123 thereof serving as the exit wall of the draw furnace 120. An annular susceptor 126 (that may be, for example, formed of zirconium, graphite or other suitable metal, such as a refractory and/or precious metal that may have a high melting point (above about 2000° C.) and be electrically conductive) extends through the draw furnace 120 and defines an annular passage and an internal draw chamber 130. The internal draw chamber 130 includes an upper section adapted to receive and hold the glass preform 102 and a lower section through which the drawn fiber 110 passes as it is drawn off of the preform 102. The lower section of the passage 130 communicates with an opening 124 in the lower end 123 of the housing 122. An annular insulator 132 and an induction coil 136 surround the susceptor 126. The draw furnace 120, as described and illustrated, is merely exemplary of suitable furnaces and it will be appreciated by those of skill in the art that furnaces of other designs and constructions, for example, using other types of heating mechanisms, may be employed.
The muffle 140 is secured to the lower end of the housing 122, for example, by a plurality of threaded bolts or other fastening means. A rubber O-ring or other sealing means may be provided between the housing 122 and the muffle 140. The muffle 140 defines a passage 142 aligned with the internal draw chamber 130 of the draw furnace 120.
The assembly 100, as shown in
The second gas screen 160, as shown in the embodiments of
In particular embodiments of the present invention, the process gas is an inert gas. The process gas (PG) may be helium, nitrogen, argon, a mixture of helium, nitrogen or argon or other suitable gas or gas mixture selected for use during processing of an optical fiber in a process chamber. In alternative embodiments of the present invention, the process gas may be a reactive gas, such as CO, CO2, Cl2, O2, H2, D2 or other suitable reactive gas. The sealing gas (SG) may be nitrogen, argon, a mixture of nitrogen and argon or other suitable gas or gas mixture selected to seal a process chamber. The sealing gas may be a heavier gas than the process gas. In particular embodiments, the process gas is helium and the sealing gas is argon.
The sealing gas (SG) as illustrated in
Also shown in the embodiments of
The embodiments illustrated in
Also shown in
The tensioning station 180 may be any suitable device for controlling the tension in the drawn fiber 110. The tensioning device 180 may include a microprocessor that receives input from one or more fiber tension and/or diameter sensors and is operative to address the tension of the fiber 110 as needed. Other draw apparatus sub-components are not shown for clarity, such as diameter sensors, and coating and curing apparatus.
The assembly 100 may be used in the following manner to manufacture an optical fiber 110. The induction coil 136 or other suitable heating member (such as a resistance element) is operated to heat the tip 102A of the preform 102 to a selected draw temperature TD. Preferably, the draw temperature TD is in the range of between about 1800° C. and about 2100° C. More preferably, the draw temperature TD is in the range of between about 1850° C. and about 1950° C. The tip 102A is maintained at the selected draw temperature TD so that the drawn fiber 110 is continuously drawn off of the tip 102A in a downward direction D. The fiber 110 is maintained at a selected draw tension FD as described above by the tensioning device 180 or other suitable means. An upstream flow at a desired rate of a process gas (PG) is provided from the first gas screen 150. A downstream flow at a desired rate of a sealing gas (SG) is provided from the second gas screen 160.
Operations related to providing a desired gas flow in a draw furnace for manufacturing optical fiber according to embodiments of the present invention will now be further described with reference to the flowchart illustration of
A process gas flow rate and a sealing gas flow rate are selected to provide a desired flow rate of the process gas from the downstream end to an upstream end of the draw furnace and to provide a desired flow rate of the sealing gas from the second gas screen in a downstream direction so as to reduce introduction of contaminant gases into the draw furnace while a preform position in the draw furnace is heated (block 205). The process gas is injected into the draw furnace through the first gas screen at the process gas flow rate (block 210). The sealing gas is injected through the second gas screen at the sealing gas flow rate (block 215). In particular embodiments, operations at block 215 may include selecting the process gas flow rate and the sealing gas flow rate to provide a carbon monoxide concentration in the draw furnace while a preform is heated of less than about 50 parts per million (ppm) or an oxygen concentration of about 25 ppm. The process gas flow rate may be between about 10 and about 40 standard liters per minute (SLPM). The sealing gas flow rate may be between about 5 and about 20 SLPM.
Thus, as described with reference to
The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although a few exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as preforming the recited function and not only structural equivalents but also equivalent structures. Therefore, it is to be understood that the foregoing is illustrative of the present invention and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.
Number | Date | Country | |
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Parent | 09947306 | Sep 2001 | US |
Child | 10957061 | Oct 2004 | US |