An optical fiber draw furnace muffle includes a body portion defining a substantially cylindrical cavity extending along a centerline axis of the muffle. A tapered portion has an interior surface which defines a first curved portion with a first radius of curvature and a second curved portion with a second radius of curvature. At least one of the first and second radii of curvature has a radius greater than a radius of the cylindrical cavity.

This application claims the benefit of priority to Chinese Patent Application No. 201810213262.4 filed on Mar. 15, 2018, the content of which is relied upon and incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The disclosure relates to furnace muffles, and more particularly to muffles for optical fiber draw furnaces

BACKGROUND

Graphite is widely used in muffles of fiber draw furnaces due to its high operating temperature. Graphite oxidation may take place at elevated temperatures. Oxidation of the graphite may result in grains and/or particles of graphite breaking free from the muffle and contacting a fiber being drawn. In addition to grains and particles traveling through the muffle, one or more gases which are typically destructive to the muffle may be generated. The motion of the particles and gases is affected by the shape and design of the muffle. Accordingly, new muffle designs may be advantageous.

SUMMARY OF THE DISCLOSURE

According to some embodiments of the present disclosure, an optical fiber draw furnace muffle includes a body portion defining a substantially cylindrical cavity extending along a centerline axis of the muffle. A tapered portion has an interior surface which defines a first curved portion with a first radius of curvature and a second curved portion with a second radius of curvature. At least one of the first and second radii of curvature has a radius greater than a radius of the cylindrical cavity.

According to some embodiments of the present disclosure, an optical fiber draw furnace muffle includes a body portion defining a substantially cylindrical cavity extending along a centerline axis of the muffle. A tapered portion has an interior surface which defines a curved portion having a radius of curvature of from about 10 cm to about 100 cm. The tapered portion defines an insert recess configured to receive a muffle insert. At least one of the muffle and the muffle insert includes graphite.

According to further embodiments of the present disclosure, an optical fiber production system includes an optical fiber preform including glass. A heater and a muffle including a body portion is positioned around the preform. The heater is positioned proximate the body portion and configured to heat the preform therein. A tapered portion has an interior surface defining a first curved portion having a first radius of curvature and a second curved portion having a second radius of curvature. At least one of the first and second radii of curvature is from about 10 cm to about 100 cm.

It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understanding the nature and character of the disclosure and the appended claims.

The accompanying drawings are included to provide a further understanding of principles of the disclosure, and are incorporated in, and constitute a part of, this specification. The drawings illustrate one or more embodiment(s) and, together with the description, serve to explain, by way of example, principles and operation of the disclosure. It is to be understood that various features of the disclosure disclosed in this specification and in the drawings can be used in any and all combinations. By way of non-limiting examples, the various features of the disclosure may be combined with one another according to the following embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Additional features and advantages will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description, or recognized by practicing the embodiments as described in the following description, together with the claims and appended drawings.

Referring toFIGS. 1 and 2, an optical fiber production system10is schematically shown. The system10includes a muffle14having a body portion18and a tapered portion22. The muffle14has an interior surface14A and a centerline axis A1. The interior surface14A of the muffle14extends between the body portion18and the tapered portion22. The muffle14defines a cavity26therein. The cavity26extends between the body portion18and the tapered portion22. A heater30is coupled to the muffle14and is configured to create a hot zone34within the cavity26. An upper muffle extension38is positioned above the muffle14. A gas screen42is coupled with the upper muffle extension38and is configured to inject a process gas into the upper muffle extension38and/or the muffle14. A downfeed handle46is positioned within the upper muffle extension38and is configured to support an optical fiber preform50.

The heater30is thermally coupled to the muffle14and is configured to create the hot zone34within the system10. Specifically, the hot zone34is generated within the cavity26of the muffle14. The heater30is positioned proximate the body portion18of the muffle14and configured to heat the optical fiber preform50therein. It will be understood that the heater30may additionally or alternatively be positioned proximate the tapered portion22. According to various examples, the heater30may be an induction heater. The hot zone34may have a temperature of from about 1700° C. to about 2100° C. For example, the hot zone34may have a temperature of about 1700° C., 1800° C., 1900° C., 2000° C., or about 2100° C. As will be explained in greater detail below, the heat of the hot zone34is sufficient to decrease the viscosity and/or melt the optical fiber preform50such that an optical fiber62may be drawn therefrom.

The upper muffle extension38may be an annular structure which is coupled to the muffle14. As such, process gases from the gas screen42are passed into the upper muffle extension38and pass or flow into the cavity26of the muffle14. The process gas may be helium, argon, nitrogen and/or other gases which are inert to the muffle14, upper muffle extension38and/or optical fiber preform50. It will be understood that the process gas may further include one or more impurities (e.g. O2) which may be reactive with components of the system10. The upper muffle extension38is sized and positioned to receive the downfeed handle46. The downfeed handle46may be hollow or solid. The downfeed handle46may have an outside diameter of from about 6 cm to about 15 cm, or from about 7 cm to about 13 cm, or from about 8 cm to about 12 cm. The downfeed handle46is coupled to a motor which allows the handle46to move in and out of the upper muffle extension38. As will be explained in greater detail below, the downfeed handle46may move through the upper muffle extension38such that as the optical fiber preform50is consumed (e.g., through the production of the optical fiber62), the handle46may continuously move the optical fiber preform50into the hot zone34. As the downfeed handle46is moved in and out of the upper muffle extension38, the optical fiber preform50is moved in and out of the hot zone34. The process gas moves through the upper muffle extension38and into the muffle14. As will be explained in greater detail below, the interaction with and flow of the process gas through the muffle14may have an effect on both the quality of the optical fiber62drawn from the optical fiber preform50and a lifespan of the muffle14.

In operation of the system10, the optical fiber preform50is at least partially positioned within the body portion18of the muffle14. As such, the body portion18is positioned around the preform50. The optical fiber preform50may be constructed of any glass or material and may be doped suitable for the manufacture of optical fibers62. According to various examples, the optical fiber preform50may include a core and a cladding. As the optical fiber preform50reaches the hot zone34, the viscosity of the optical fiber preform50is lowered such that the optical fiber62may be drawn therefrom. As the optical fiber preform50is consumed through the production of the optical fiber62, the downfeed handle46may continuously lower into the upper muffle extension38and/or muffle14such that new portions of the optical fiber preform50are exposed to the hot zone34. The optical fiber62is drawn from the optical fiber preform50out through an outlet68of the muffle14and may be wound onto a spool. The optical fiber62is generally drawn through the muffle14along the centerline axis A1. It will be understood that one or more lower muffle extensions, cooling systems, process gas reclamation systems and/or coating systems may be positioned below, or downstream, of the muffle14without departing from the teachings provided herein.

The muffle14and/or upper muffle extension38may be composed of a refractory material such as graphite, zirconia, binders and/or combinations thereof. As such, one or more of the muffle14and upper muffle extension38includes carbon. The muffle14and upper muffle extension38may be configured to retain heat within the system10as well as protect other components of the system10from excess temperatures. Although the materials of the muffle14and/or upper muffle extension38may generally be good insulators, oxidation may occur at elevated temperatures. Although described as separate structures, it will be understood that the muffle14and the upper muffle extension38may be a single component or composed of more than two components. Further, lower muffle extensions may also be combined with the muffle14without departing from the teachings provided herein.

As explained above, while graphite offers superior thermal resistance properties, oxidation of graphite examples of the muffle14may also increase the likelihood of draw induced point defects (“DIPDs”) being generated on the optical fiber62. Oxidation occurs when gases from the ambient atmosphere and/or impurities within the process gas react with the graphite muffle14at high temperature according to equations (1) and (2):
C+O2→CO2(1)
C+CO2→2CO  (2)
Additionally or alternatively, silica particles (e.g., from the optical fiber preform50) can oxidize carbon based materials based on reaction (3):
C+SiO2→SiC+2CO  (3)
These reactions at the muffle14with oxygen, silicon dioxide and oxide containing gases may cause the muffle14to be consumed, especially at elevated drawing temperatures for the optical fiber62.

In graphite examples of the muffle14, the graphite material of the muffle14is a composite of graphite grains bonded together by a graphitized carbon binder matrix. The binder material is more susceptible to oxidation than the graphite grains. Therefore, when the composite of the two materials is exposed to oxygen-containing compounds at high temperatures, the matrix binder material preferentially oxidizes. The graphite grains, having no binder left to hold them in place, are then free to fall away from muffle14. Without being bound by theory, it is believed that this mechanism causes graphite particulates to migrate from the muffle14to the optical fiber62during drawing thereby inducing DIPDs. DIPDs manifest themselves as sharp attenuation increases in the signal transmitted through the optical fiber62. The larger the graphite particulate, the greater the chance of the DIPD being formed on the optical fiber62.

Without being bound by theory, it is believed that conventional muffle designs result in the agglomeration of particles on the muffle14which form DIPDs in addition to retaining and/or recirculating gases which are deleterious to the muffle14. For example, drastic reductions (e.g., at the tapered portion22) in the diameter of the muffle14(e.g., 45° or greater as measured from the centerline axis A1of the muffle14) may cause particles to strike the interior surface14A of the muffle14and stick. The sticking of these particles results in agglomerations which increase the size of the particle. The agglomerations may break free from the interior surface14A of the muffle14based on the flow pattern of the process gas through the muffle14and contact the optical fiber62. In addition to collecting and agglomerating the particles, sharp decreases in the inner diameter of the muffle14may cause recirculation, or reversed flow, of gasses such as CO rather than being transported out of the furnace system10. Such a recirculation may result in a decreased life expectancy of the muffle14based on equations (1), (2) and (3) listed above. In other words, the gases which tend to deteriorate the muffle14may have an increased residency time within the muffle14due to recirculation, resulting in a decreased life expectancy of the muffle14.

As explained above, the muffle14is composed of the body portion18and the tapered portion22. It will be understood that the body portion18and the tapered portion22may be separate components from one another such that the muffle14is composed of multiple pieces. Further, the body portion18and/or the tapered portion22may each be composed of multiple separate components or pieces. The muffle14may have a length of from about 100 cm to about 140 cm. The body portion18of the muffle14may have a thickness of between about 0.6 cm to about 4 cm, or from about 1.25 cm to about 2.5 cm, or from about 1.8 cm to about 2.0 cm. In a specific example, the thickness of the body portion18(e.g., as measured from the interior surface14A to an exterior surface14B) of the muffle14may be about 1.9 cm. The body portion18is positioned above the tapered region22and generally defines the cavity26within which the optical fiber preform50is positioned. The cavity26is generally cylindrical in shape in the body portion18and extends along the centerline axis A1. The portion of the cavity26defined by the body portion18may have a substantially uniform radius and/or diameter. The radius of the cavity26may be from about 6 cm to about 10 cm, or from about 7 cm to about 9 cm. The body portion18may have a length of from about 64 cm to about 89 cm. The muffle14defines the interior surface14A which extends between the body portion18and the tapered portion22.

The tapered portion22of the muffle14is below the body portion18of the muffle14. The tapered portion22may generally be delineated from the body portion18by the start of an internal taper (e.g., narrowing of the internal diameter of the muffle14by the interior surface14A) and/or a tapering of the exterior surface14B of the muffle14. The tapered portion22generally tapers down, or necks down, to the outlet68of the muffle14. The outlet68of the muffle14is generally the exit (e.g., or narrowest portion) of the cavity26defined by the muffle14.

The interior surface14A of the tapered portion22is both curved around a circumference of the muffle14to define the cavity26(e.g., to form the circular cross-sectional shape of the muffle14), as well as curved toward and away from the centerline axis A1of the muffle14. In the depicted example, the interior surface14A defines a first curved portion80having a first radius of curvature and a second curved portion84having a second radius of curvature. Although depicted as including two curved portions, it will be understood that the interior surface14A of the muffle14may define one or more than two curved portions. The first curved portion80is generally positioned above, or upstream (e.g., relative to the process gas flow), of the second curved portion84. The first and second curved portions80,84may be in contact with one another or may be separated by a portion of the interior surface14A which is linear (e.g., including a linear taper or no taper at all). It will be understood that in examples where the first and second curved portions80,84are in contact, an inflection point, or flat portion may exist. The first and/or second curved portions80,84may extend around a portion, a majority, substantially all or all of a circumference of the cavity26. In examples where the tapered portion22is composed of multiple separate components, the first and second curved portions80,84may be defined on separate components of the tapered portion22.

For purposes of this disclosure, the radius of curvature of a curved surface (e.g., the first and/or second curved portions80,84) equals the radius of the circular arc which best approximates the curve of that surface where the curve remains constant. In other words, the first and second radii of curvature are the radii of the circular arcs which best approximates the entirety of the first and second curved portions80,84, respectively. The first and second radii of curvature may be different, substantially the same or the same as one another. Further, the first and second curved portions80,84may have one or more radii of curvature. The radius of curvature for the first curved portion80may range from about 50 cm to about 100 cm, or from about 60 cm to about 90 cm, or from about 70 cm to about 80 cm and all values therebetween. In a specific example, the first radius of curvature may be about 70 cm. The second radius of curvature of the second curved portion84may range from about 10 cm to about 40 cm, or from about 20 cm to about 30 cm and all values therebetween. In a specific example, the second radius of curvature may be about 25.4 cm. In other words, the tapered portion22of the muffle14defines the interior surface14A as having a curved portion (e.g., the first or second curved portions80,84) having a radius of curvature of from about 10 cm to about 100 cm. Further, at least one of the first and second radii of curvature has a radius greater than a radius of the cylindrical cavity26. The first and second curved portions80,84may be concave or convex relative to the centerline axis A1of the muffle14. For example, the first curved portion80may be concave relative to the centerline axis A1of the muffle14. In such an example, the radius of curvature would extend generally toward the centerline axis A1. In another example, the second curved portion84may be convex relative to the centerline axis A1of the muffle14such that the radius of curvature of the second curved portion84may generally point away from the centerline axis A1.

The first and second curved portions80,84of the tapered portion22of the muffle14are configured to streamline the flow of the process gas through the muffle14. As explained above, conventional muffle designs having sharp decreases in the inner diameter of the muffle14(e.g., through fast tapering of the interior surface14A toward the centerline axis A1) may lead to pressure increases, particle agglomeration and/or gas recirculation. By utilizing an interior surface with high radii of curvature, the first and second curved portions80,84allow for a gradual and smooth decrease in the diameter of the cavity26. The gradual and smooth decrease in cavity diameter allows for a “soft landing” of the particles on the interior surface14A such that agglomeration of the particles is decreased and/or eliminated. In other words, the particles pass through the outlet68and away from the optical fiber62. Additionally, as the first and second curved portions80,84streamline the flow of the process gas more than conventional designs, gases which tend to deteriorate the muffle14may be efficiently expelled without undue recirculation. Such features may be advantageous in reducing DIPDs and increasing muffle lifetime.

The tapered portion22of the muffle14may define the exterior surface14B which is also tapered. For example, the exterior surface14B may be tapered at an angle of between about 8° and about 16° relating to the body portion18. In a specific example, the exterior surface14B may be tapered at an angle of about 12° relative to the body portion18. The tapered portion22of the muffle14may be thicker than the body portion18due to the first and second curved portions80,84decreasing the diameter of the cavity26. For example, the thickness of the tapered portion22as measured from the interior surface14A to the exterior surface14B may be from about 5 cm to about 15 cm while the body portion18may have a thickness of from about 1 cm to about 3 cm. According to various examples, the tapered portion22may be thicker than conventional muffle designs. As such, a greater amount of muffle14may need to be deteriorated before cracks and/or breakage occurs. In practical terms, the increased amount of muffle14may lead to a longer usable life of the muffle14as well as an increase in the efficiency of the muffle14to retain heat.

Referring now toFIGS. 2-4, the tapered portion22of the muffle14defines the outlet68of the cavity26. The outlet68opens to an insert recess90which is configured to receive an insert94and a gasket98. Although depicted with a single gasket98, it will be understood that the muffle14may include no gaskets98or a plurality of gaskets98. The gasket98may be formed from a graphite laminate (e.g., graphite with stabilizing inserts of stainless steel, carbon steel or wire mesh), woven and/or compressed graphite foil, ceramic fiber blankets and/or combinations thereof. The gasket98ensures a seal between the insert94and the muffle14.

The insert94is configured to be positioned within the insert recess90. As such, the insert94is positioned below, or downstream, of the optical fiber preform50while in the muffle14. The insert94may be formed of any of the above-noted materials described in connection with the muffle14and/or the gasket98. According to various examples, the insert94may include graphite. An inner insert surface94A of the insert94is configured to be substantially flush with the interior surface14A of the muffle14such that gas flow through the tapered portion22of muffle14is unimpeded by the insert94. Further, the optical fiber62may extend through the insert94while the optical fiber preform50is having the fiber62drawn therefrom. The insert94may have an internal diameter of from about 4 cm to about 10 cm, or about 5 cm to about 7 cm and all values therebetween. The insert94may have an external diameter of from about 5 cm to about 10 cm and all values therebetween. The insert94may have a length, as measured in the same direction as the centerline axis A1of the muffle14, of between about 8 cm and about 17 cm, or between about 10 cm and about 15 cm, or between about 12 cm and about 13 cm and all values therebetween. In a specific example, the insert94may have a length of about 12.7 cm. In some examples, the insert94can be fully positioned within the tapered portion22of the muffle14, while in other examples the insert94may only be partially disposed within the tapered portion22of the muffle14. In operation, the insert94is configured to collect large particles which are traveling through the gas stream of the muffle14. Such a feature may be advantageous in allowing the insert94to be removed from the muffle14, thereby removing particulates while not requiring full removal of the muffle14.

Referring now toFIG. 4, depicted is an example of the muffle14having a tapered region22without the first and second curved portions80,84(FIG. 2). In the depicted example, the interior surface14A of along the tapered portion is straight, or linear, and leads directly to the insert94. The interior surface14A may have an angle α relative to the centerline axis of about 45° or less. For example, α may be from about 1° to about 40°, or about 10° to about 30° or about 15° to about 25°. For example, α may be about 5°, about 6°, about 7°, about 8°, about 9°, about 10°, about 11°, about 12°, about 13°, about 14°, about 15°, about 16°, about 17°, about 18°, about 19° or about 20° and all values therebetween. Similarly to the curved portions80,84ofFIG. 2, by decreasing the angle of the interior surface14A, or decreasing the rate of cavity26diameter reduction, particles within the process gas do not stick to the interior surface14A and agglomerate. Further, as the flow path for the process gas has been streamlined relative to conventional designs, the reversed flows in the process gas are decreased and/or eliminated. As with the other examples of the muffle14, the tapered portion22may include the insert94.

Use of the presently disclosed muffle14may offer a variety of advantages. First, by streamlining the flow of process gases through the cavity26of the muffle14, the number and/or concentrations of DIPDs may be reduced compared to conventional designs. For example, use of the presently disclosed muffle14may result in a 37% or greater reduction of DIPDs as compared to conventional designs. As explained above, by decreasing the likelihood of particles sticking to the interior surface14A, agglomeration of particles is reduced which will reduce DIPDs. Further, as the process gases are streamlined through the muffle14, gases which tend to oxidize or damage the muffle14may be expeditiously removed from the muffle14. Second, as the tapered portion22of the muffle14is thicker than conventional muffle designs, the muffle14may have an increased lifetime before cracking. In other words, in addition to reducing the likelihood of oxidation, the greater volume of the muffle14in the tapered portion22increases the amount of muffle14which must be damaged before cracking and failure occurs. As such, a single muffle14may have a useable life of about twelve months or greater. Third, as the body portion18of the muffle14is decreased relative to conventional designs and the volume of the tapered portion22is increased, the heater30may more efficiently heat the hot zone34and the optical fiber preform50while also retaining more heat. The increase in efficiency of the muffle14may result in a decrease in the energy costs associated with powering the furnace system10. For example, an average of about 5 kW of power may be saved using the thinner body portion18as compared to conventional designs. Fourth, as the insert94is removable from the muffle14, the entirety of the muffle14may not need to be serviced in order to clear large particles from the muffle14. Such a feature is advantageous in decreasing damage from occurring to the muffle14while cleaning, in addition to reducing production downtime for cleaning the muffle14.

EXAMPLES

The following examples represent certain non-limiting examples of the muffle of the present disclosure, including the methods of making them.

Referring now toFIGS. 5A-7B, depicted are contour plots and path line plots for conventional and exemplary refractory elements (e.g., muffle14). The left edge of the contour plots are positioned at a center of a furnace (e.g., the heater30) while the right edge is proximate an end (e.g., the outlet68) of the refractory element.FIG. 5Ais a contour plot andFIG. 5Bis a path line plot for a Comparative Example 1 of a conventional refractory element. In Comparative Example 1, a conventional refractory design is provided having an approximately 45° angle of reduction at a refractory reduction point (e.g., the tapered portion22) proximate the end of the refractory element. As can be seen, the path lines accumulate at the reduction point, indicating the building of gas pressure at the reduction point. The concentration of gas and gas pressure increase may lead to flow instability and reversed flow within the refractory element. Further, such a configuration may lead particles to strike and agglomerate. Such instability and reversed flow may result in the breaking free of larger agglomerated particles which are carried by the reverse flow and result in DIPDs in a fiber optic (e.g., optical fiber62) drawn in the conventional refractory.

Referring now toFIGS. 6A and 6B,FIG. 6Ais a contour plot andFIG. 6Bis a path line plot for Example 1 of a refractory element according to the present disclosure. In Example 1, the refractory element includes two curved areas (e.g., the first and second curved portions80,84) along an inner surface (e.g., the interior surface14A). A first, or upper, curved area (e.g., the first curved portion80) has a radius of curvature of about 70 cm and a second, or lower, curved area (e.g., the second curved portion84) has a radius of curvature of about 25.4 cm. As can be seen from the path lines ofFIG. 6B, the flow paths are less concentrated near the refractory walls and the flow of the gas is generally more streamlined than in Comparative Example 1. As explained above, the streamlined nature of the refractory of Example 1 prevents a pressure build-up from taking place which may result in less particle agglomeration and reversed flow. With the decreased agglomeration and reversed flow, fewer large particles may be carried toward a fiber optic being drawn and may prevent a reverse flow from carrying particles upwards towards the fiber optic. As such, a lower amount of DIPDs is expected in such a fiber optic.

Referring now toFIGS. 7A and 7B, depicted is a contour plot (FIG. 7A) and a path line plot (FIG. 7B) for Example 2. Example 2 includes a reduction point (e.g., tapered portion22) which is linear, or straight. In the depicted example, an angle of the reduction point relative to a refractory axis (e.g., the centerline axis A1) is between about 11° and about 12°. As can be seen inFIG. 7B, the path lines are highly streamlined through the refractory element indicating very little pressure build up and turbulence. Analysis of the results revealed that particles do not bounce off of the reduction point, but instead slide along the refractory element wall allowing particles to escape the refractory element and not agglomerate.

Referring now toFIG. 8, depicted is a plot showing the percentage of particles trapped vs particle size for Comparative Example 1 and Example 1. CFD modeling with particle tracking was used to predict the flow pattern and particle motion through the refractory elements. Various particle sizes ranging from 1 μm to 50 μm were used in the simulations. As can be seen from the plot, Example 1 which is streamlined accumulates about 20% fewer particles relative to Comparative Example 1. The decrease in particles trapped is advantageous as it is the trapped particles which agglomerate to generate DIPDs. As such, by decreasing the number of particles trapped (e.g., increasing the number of particles passing out of the system10), the likelihood of DIPDs forming is decreased.

Referring now toFIG. 9, depicted is a plot of DIPDs per 1000 km of fiber optic produced using Comparative Example 1 and Example 1. As can be seen, the flow pattern of Example 1 purges more particles out of the refractory element which reduces the incidents of particle attachment to the fiber optic. Use of Example 1 of the refractory element results in a 37% reduction of DIPDs as compared to Comparative Example 1 which is a significant performance improvement. With the decrease in particle agglomeration, it is also expected that an increased purge rate of SiO2and oxygen-containing gases will be removed from the refractory element thereby increasing an expected lifespan.

Referring now toFIG. 10, the thickness of the refractory element affects how much power is used to form the fiber optic. For example, it was determined that the furnace power usage dropped by about 5 kw when Example 1 of the refractory element was installed on the draw furnace (e.g., the system10) as compared to Comparative Example 1. The reason for the increase in efficiency of the furnace is due to reduced heat loss owing to the large refractory element volume and thicker element wall (e.g., in the tapered portion22).