Optical Fiber Drawing Furnace System and Drawing Method

Abstract

An optical fiber drawing furnace system and method of operating the same includes positioning a lower feed handle for supporting an optical fiber preform within the furnace, the lower feed handle being movable within the furnace, and activating one or more heating elements to heat at least a portion of an upper muffle extension disposed within the furnace, the one or more heating elements being movable with the lower feed handle.

Description

Related Applications and Cross-References

[0001] This application claims priority to U.S. Provisional Patent Application No. 62 / 965,473, filed on January 24, 2020, under 35 U.S.C. § 119(e), the entire contents of which are hereby incorporated by reference herein as part of this specification.

Technical Field

[0002] The present invention generally relates to systems and methods for operating a draw furnace for an optical fiber, and more particularly, to systems and methods for operating a draw furnace for an optical fiber while heating a downward feed handle inside the draw furnace.

Background Art

[0003] Optical fibers are generally manufactured to have sufficient bending and damage resistance by providing an internal glass core surrounded by a glass cladding and a plurality of layers of coatings. Conventional techniques and manufacturing processes for manufacturing optical fibers include drawing the optical fiber from a preform. The preform is formed of consolidated silica glass and is provided with continuous concentric regions of silica glass having different levels or types of doping agents. By controlling the spatial distribution, concentration, type, or various combinations thereof of the doping agent in the preform, a plurality of regions having different refractive indices from each other are provided. These differences in refractive index define a plurality of regions having different functions in the manufactured optical fiber (for example, a core and a cladding, various low refractive index depressions, various profiles of refractive indices tailored for each condition, etc.).

[0004] The stretching process of the base material is typically carried out in a stretching furnace and involves melting and stretching the base material to achieve the target optical fiber diameter. To manufacture an optical fiber with a constant diameter, various characteristics such as the temperature of the furnace, the position of the base material, and the drawing speed are controlled. For example, the change in the furnace temperature over time may cause a change in the cooling rate of the base material during the stretching procedure, and as a result, the diameter of the optical fiber may become non-uniform and irregular.

[0005] Furthermore, the unsteady convection of various gases inside the stretching furnace may make the diameter of the optical fiber non-uniform and irregular. To prevent the atmosphere from entering the furnace, usually, an inert processing gas is introduced into the upper part of the stretching furnace. The atmosphere may react with various components of the stretching furnace and cause undesirable oxidation. However, the flow instability of the processing gas at the upper part of the furnace may affect the uniform stretching of the base material. More specifically, the flow instability in the unheated part at the upper part of the furnace propagates downward into the furnace towards the necked-down region of the base material. This may affect the heat transfer between the processing gas and the necked-down region of the base material, which in turn may cause fluctuations in the cooling rate of the base material and result in fluctuations in the diameter of the stretched optical fiber.

Summary of the Invention

Problems to be Solved by the Invention

[0006] Examples of conventional inert processing gases include nitrogen and argon. However, these gases may cause undesirable flow instabilities at the top of the drawing furnace. Helium gas is known to reduce any unsteady convection inside the drawing furnace. Therefore, by substituting nitrogen or argon with helium, the diameter of the drawn optical fiber has been made more uniform. However, helium is a non-renewable resource recovered as a by-product from natural gas wells. Since the price of helium is predicted to increase in the future, there is a growing need to use other gases in the drawing furnace. Therefore, there is a need to provide a system and method for maintaining stable gas convection inside the drawing furnace even when the drawing furnace is operated without necessarily using helium.

Means for Solving the Problems

[0007] According to one embodiment, a method for operating a drawing furnace for an optical fiber is provided. This method includes arranging a lower feed handle for supporting an optical fiber preform inside the furnace so that the lower feed handle is movable inside the furnace. This method also includes operating one or more heating elements for heating at least a part of an upper muffle extension provided inside the furnace, and performing the operation in a state where the one or more heating elements are movable together with the lower feed handle.

[0008] According to another embodiment, a drawing furnace system for an optical fiber is provided. The system has an upper muffle extension provided in a muffle to form an internal cavity. The system also includes a lower feed handle and an upper heating device. The lower feed handle is movably arranged inside the internal cavity. Further, in the upper heating device, one or more heating elements are provided in a state of being movable together with the lower feed handle inside the internal cavity.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6A

Figure 6B

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Best Mode for Carrying Out the Invention

[0010] Other features and advantages of the present disclosure are set forth in the following detailed description, will be apparent to those of ordinary skill in the art from that description, or may be recognized by practicing the present disclosure as set forth in the following description in conjunction with the claims and the accompanying drawings.

[0011] As used herein, the term "and / or" means that any one of two or more recited items may be employed alone or any combination of two or more recited items may be employed. For example, if a composition is described as containing "Component A, Component B, and / or Component C", the composition may contain Component A alone, Component B alone, Component C alone, a combination of Component A and Component B, a combination of Component A and Component C, a combination of Component B and Component C, or a combination of Component A, Component B, and Component C.

[0012] In this document, terms expressing relative relationships such as "first and second" and "top and bottom" are used only for the purpose of distinguishing one kind of entity or one kind of movement from another kind of entity or movement, and in the relationship between such two entities or two movements, no actual such relationship or order is necessarily required, nor is it necessarily implied.

[0013] It will be understood by those of ordinary skill in the art that the configurations of the described disclosure and the configurations of the other elements are not limited to any particular material. Unless otherwise specified in this document, the specific embodiments other than those described above of the content disclosed in this document can be formed from a wide range of materials.

[0014] As shown in the specific embodiments, it is also important to note that the configurations and arrangements of the elements of the present disclosure are merely illustrative. Although only a few embodiments are described in detail in the present disclosure, those skilled in the art who evaluate and consider the present disclosure can make many modifications without significantly departing from the novel and non-obvious teachings and advantages of the recited subject matter (for example, various modifications such as the size, dimensions, structure, shape, ratio, etc. of each component, and various modifications such as the values of each parameter, the mounting arrangement, the use of materials, color, orientation, etc.). For example, elements shown as being integrally formed may be composed of a plurality of parts, or elements shown as a plurality of parts may be integrally formed. The functions of each intermediate part may be reversed or otherwise transformed. In addition to being able to change the length and width of various structures, various members, various connection members, and other elements of the system, or various combinations thereof, the nature or number of each adjustment position set between the elements can also be changed. It should be noted that the elements of the system, each integrated body, or various combinations thereof can be composed of any variety of materials that provide sufficient strength or durability, in any variety of colors, textures, and combinations. Therefore, all such modification examples are intended to fall within the scope of the present disclosure. Substitutions, modifications, changes, and various omissions other than those described above can be made in the design, operating conditions, and arrangements of the preferred embodiments and other specific embodiments without departing from the essence of the present disclosure.

[0015] Here, the present disclosure will be discussed in detail with respect to the presently preferred embodiments thereof, and specific examples thereof are illustrated in the accompanying drawings. As much as possible, the same reference numerals are used throughout the drawings to refer to the same or similar parts.

Example

[0016] Referring now to FIG. 1, according to one embodiment, a specific fiber drawing furnace system is generally indicated by reference numeral 10. A muffle 20 is provided within an outer can 30 of the drawing furnace 10. A lower feed handle 40 is movably disposed within a handle cavity of the muffle 20 and supports an optical fiber preform 50. As will be further discussed later, an upper heating device, which is composed of one or more heating elements, is connected to the lower feed handle 40 and can move together with the handle. Each heating element acts to provide a uniform gas temperature and stable convection within the drawing furnace 10.

[0017] The muffle 20 includes a first end 24 and a second end 25 as illustrated in FIG. 1. The second end 25 forms an upper muffle extension 23 that extends downward along a predetermined length of the muffle 20. The top hat 21 is disposed above the upper muffle extension 23 and provides sealing performance and movement performance as is well known in the art. As illustrated in FIG. 1, the muffle 20 and the top hat 21 form an internal cavity 27 through which the lower feed handle 40 is movably disposed. As will be further discussed later, the cavity 27 is provided with a furnace cavity 22 at its first end. Further, a handle cavity 29 may be formed in a portion of the cavity 27 disposed between the lower feed handle 40 and the muffle 20. When the lower feed handle 40 moves within the cavity 27, the handle cavity 29 will include other plural portions of the cavity 27. For example, as a result of the lower feed handle 40 moving downward within the cavity 27 such that more portions of the lower feed handle 40 are disposed within the muffle 20, the length of the handle cavity 29 will also increase. An elastomeric seal 26 may be provided to establish an airtight connection between the lower feed handle 40 and the upper muffle extension 23.

[0018] The muffler 20, the upper muffler extension 23, or both of them may be composed of, for example, refractory materials such as graphite, zirconia, binders, alumina, mullite, quartz, silicon carbide, silicon nitride, or various combinations thereof, refractory metals, or both of them. Thus, the muffler 20, the upper muffler extension 23, or both of them are formed of carbon, and this may react with the atmosphere and burn. In addition to this, the muffler 20 and the upper muffler extension 23 may be a single component member, or may be formed from two or more separate component members. As illustrated in FIG. 1, the muffler 20 and the upper muffler extension 23 may have a generally uniform inner diameter. It is also conceivable that the muffler 20 and the upper muffler extension 23 may have different inner diameters from each other. Depending on the embodiment, the inner diameter of the muffler 20, the upper muffler extension 23, or both of them may vary along the longitudinal direction of the component member.

[0019] The lower heating device 60 is disposed inside the outer can 30 adjacent to the first end 24 of the muffler 20. The lower heating device 60 is heat transfer connected to the muffler 20, and a high temperature region can be provided inside the furnace cavity 22. The high temperature region is preferably at a temperature of about 1800 degrees Celsius to about 2100 degrees Celsius. Depending on the embodiment, the high temperature region may be at a temperature such as about 1800 degrees Celsius, about 1900 degrees Celsius, about 2000 degrees Celsius, or about 2100 degrees Celsius, or may be at an arbitrary range of temperatures with any two of the above values as the upper and lower limit points. As will be described in more detail later, the heat in the high temperature region is sufficient to lower the viscosity of the base material 50. Depending on the embodiment, the lower heating device 60 may be provided with an induction coil.

[0020] Furthermore, the muffler 20, the upper muffler extension 23, or both are also configured to protect other components from excessive temperatures after retaining heat within the stretching furnace 10. For example, the muffler 20, the upper muffler extension 23, or both may have sufficient heat insulation characteristics to maintain the high temperature in the high-temperature region inside the furnace cavity 22. For example, it is also conceivable that the heat insulating material 65 surrounds the muffler 20. As illustrated in FIG. 1, the heat insulating material 65 may be disposed between the muffler 20 and the induction coil of the lower heating device 60 and may also be disposed between the upper muffler extension 23 and the outer can 30. Thus, as a result of the heat insulating material 65 spreading over the length from the lower heating device 60 to the upper muffler extension 23, the heat insulating material is also provided around the upper muffler extension 23. Depending on the embodiment, there are also those in which the heat insulating material 65 is a graphite heat insulating material.

[0021] The muffler 20, the upper muffler extension 23, or both may generally be any various good heat insulating materials, but oxidation may still occur at high temperatures. Therefore, in order to prevent oxidation of these components, it is advisable to insert or inject one or more types of processing gases into the stretching furnace 10. As will be further discussed later, examples of the processing gas include inert gases such as nitrogen, argon, helium, or various combinations of these gases.

[0022] The outer can 30 may be provided with one or more gas inlet ports for injecting the processing gas into the cavity 27. For example, as illustrated in FIG. 1, the outer can 30 includes a first gas inlet port 70, a second gas inlet port 72, and a third gas inlet port 74. The first gas inlet port 70 is disposed in the upper muffler expansion portion 23, the second gas inlet port 72 is disposed in the outer can 30 near the lower heating device 60, and the third gas inlet port 74 is disposed at the bottommost part of the muffler 20. As illustrated in FIG. 1, the processing gas can also be injected into the cavity 27. As will be further discussed later, this processing gas is injected into the stretching furnace 10 to ensure that the atmosphere does not enter the stretching furnace 10 during the continuation of the stretching process. Thus, it is possible to prevent oxygen derived from the atmosphere from reacting with, for example, the carbon of the muffler 20 and the like.

[0023] The base material 50 is attached to the lower feed handle 40 and further suspended therefrom using the support member 80. It is also contemplated that the support member 80 is a component of the lower feed handle 40 or a separate component connected to the lower feed handle 40. The support member 80 may have substantially the same outer diameter as the lower feed handle 40. Thus, the gap between the outer diameter of the lower feed handle 40 and the inner diameter of the muffler 20 may be substantially equal to the gap between the outer diameter of the support member 80 and the inner diameter of the muffler 20. The support member 80 is configured to support the base material 50. In some embodiments, the support member 80 is a glass piece welded to the lower feed handle 40. In addition to or as an alternative to this, the support member 80 may be provided with slots and the base material 50 may be attached thereto. However, it is also contemplated that any suitable configuration may be employed for attaching the base material 50 to the lower feed handle 40.

[0024] The lower feed handle 40 can be made of, for example, quartz glass, graphite, silicon nitride, silicon carbide, or graphite coated with silicon carbide, and the outer diameter of the lower feed handle 40 is smaller than the inner diameter of the muffle 20. Therefore, the lower feed handle 40 (together with the support member 80) can move (e.g., move up and down) along the longitudinal axis direction of each of these components inside the muffle 20 and the top hat 21. In addition to this, the lower feed handle 40 (together with the support member 80) may be movable (e.g., move left and right, move back and forth) in the radial direction of each of these components inside the muffle 20 and the top hat 21, or may be rotatable inside each of these components. When the base material 50 is attached to the lower feed handle 40, it can move together with the lower feed handle 40 inside the muffle 20 and the top hat 21. For example, during the continuation of the drawing process, as the base material 50 is consumed, the lower feed handle 40 can move in the longitudinal axis direction inside the cavity 27. As illustrated in FIGS. 1 to 4, during the continuation of the drawing process, the lower feed handle 40 moves along the length of the muffle 20.

[0025] As the base material 50 moves inside the muffle 20 together with the lower feed handle 40 and descends toward the lower heating device 60, an optical fiber is drawn from the base material. The base material 50 may be composed of any well-known glass or other material, but may also be doped so as to be suitable for the production of optical fibers. Depending on the embodiment, the base material 50 may include a core and a cladding. As the base material 50 reaches the high-temperature region of the lower heating device 60, the viscosity of the base material 50 decreases, and an optical fiber can be drawn from the base material 50. As the base material 50 is continuously consumed during the continuation of the drawing process, the lower feed handle 40 continuously descends, and a new portion of the base material 50 is exposed to the high-temperature region generated by the lower heating device 60. The optical fiber is drawn from the base material 50, passes through the bottom of the drawing furnace 10, and is then wound around a spool. Depending on the embodiment, the optical fiber may have a diameter of about 125 micrometers.

[0026] As described above, the processing gas is injected into the muffler 20 during the continuation of the stretching process of the base material 50. More specifically, during the stretching process period, the door 76 is opened, and the processing gas is injected into the cavity 27 through the gas inlet port 70, the gas inlet port 72, or both. The processing gas injected into the first gas inlet port 70 passes through the cavity 27, flows downward along the longitudinal direction of the base material 50 through the furnace cavity 22, and flows into the lower muffler expansion part 90. Next, the processing gas exits through the door 76. Such a flow path of the processing gas is used to prevent the atmosphere from entering the muffler 20 during the continuation of the stretching process.

[0027] The gas inlet port 72 is used when the lower heating device 60 is powered on and the base material 50 is heated in the on state. The processing gas injected into the gas inlet port 72 can flow upward inside the outer can 30 and flow out of the stretching furnace 10 near the second end 25 of the muffler 20. The processing gas injected into the gas inlet port 72 is used as a purging gas to ensure that there is no air that may react with the heat insulating material 65 inside the outer can 30.

[0028] Furthermore, the processing gas is also injected into the gas inlet port 74 during the continuation of the loading procedure and the removal procedure of the base material 50. During the continuation of each such procedure, the door 76 is closed, and the processing gas injected into the gas inlet port 74 flows upward inside the cavity 27. This prevents air from entering the uppermost part of the stretching furnace 10 during the continuation of the loading procedure and the removal procedure.

[0029] In traditional various stretching furnace systems, during the continuation of the stretching procedure, the flow of the processing gas becomes unstable while flowing inside the stretching furnace. As described above, due to such flow instability in the processing gas, the diameter of the stretched optical fiber may become non-uniform and irregular. The flow instability results from unsteady natural convection, and its cause lies in density stratification within the muffle cavity and the flow of the inert gas, which propagates downward through the muffle. Such various flow instabilities ultimately affect the heat transfer between the processing gas and the stretching root of the optical fiber base material. More specifically, the flow instability appears as temperature change, pressure change, and mass flow rate change, which transfer to the stretching root and fluctuate the viscosity of the base material. Due to the temperature change, pressure change, and mass flow rate change, the heating and cooling conditions of the stretching root fluctuate, and as a result, the diameter of the optical fiber stretched from the base material fluctuates (for example, because the amount of material that can be drawn from the optical fiber base material at a given speed and tension changes).

[0030] Various flow instabilities of the processing gas, that is, flow unsteadiness, can be quantified by the Grashof (Gr) number. Physically interpreting the Grashof number, it can be said that it is the ratio of buoyancy to viscous force in the gas system. When the buoyancy becomes much larger than the viscous force, the flow becomes unstable and changes over time. The Grashof number is numerically expressed as follows in Equation (1).

[0031] [Number]

[0032] Here, g is the acceleration due to gravity, β is the thermal expansion coefficient of the processing gas, L C is the representative length (for example, the length of the space where the gas is located), ΔT is the temperature difference (such as measured in the vicinity of the stretching root of the optical fiber base material), and ν is the kinematic viscosity of the processing gas.

[0033] As described above, in some stretching furnaces, helium is used because of its high kinematic viscosity. As is clear from Equation (1), when the kinematic viscosity of the processing gas is high, the Grashof number becomes low, and as a result, stable and invariant natural convection occurs. In other words, a processing gas with a high kinematic viscosity resists the unsteady flow generated by buoyancy. Furthermore, the less likely the processing gas is to cause convection, the less likely it is to cause unsteady flow behavior within the cavity of the muffle. Therefore, the higher the kinematic viscosity of the processing gas, the higher the resistance to convection generated by buoyancy in the processing gas, thus reducing or preventing the unsteady flow instability within the muffle. Generally, when the Grashof number is about 7,000 or less, about 8,000 or less, about 9,000 or less, about 10,000 or less, about 11,000 or less, or about 12,000 or less, a stable and invariant flow occurs, but when the Grashof number exceeds about 13,000, an unsteady and time-varying flow results.

[0034] Referring to FIG. 1, an upper heating device composed of one or more heating elements 46 is connected to a downward feed handle 40 to adjust the temperature difference within the muffle 20, thereby reducing the Grashof number and promoting a stable flow of the processing gas, whereby the variation in the diameter of the optical fiber to be stretched can be reduced. Therefore, even if a processing gas other than helium can be used by the one or more heating elements 46, a Grashof number as low as desired and a stable flow can be obtained. For example, when either argon or nitrogen is used as the processing gas, each heating element 46 of the stretching furnace 10 can provide a Grashof number in the range of about 800 to about 1200.

[0035] As is well known in the art, examples of each heating element 46 include, for example, a wound resistance heating device, a band heating device, an immersion heating device, a rod-shaped heating device, or various combinations thereof.

[0036] As shown in FIG. 1, when each heating element 46 is powered on (i.e., switched to the on position), a plurality of different heating regions can be formed on the lower feed handle 40. For example, by powering on a plurality of different heating elements, the first heating region 41, the second heating region 42, the third heating region 43, and the fourth heating region 44 of the lower feed handle 40 are formed. Although the embodiment of FIG. 1 illustrates four heating regions, it is also contemplated that more or fewer heating regions may be employed. For example, each heating element 46 may form, for example, one heating region, two, five, six, seven, eight, or ten heating regions. Each heating region can be heated independently of the other heating regions or powered on to be in the on state.

[0037] The heating regions 41, 42, 43, and 44 may be disposed on the inner surface of the lower feed handle 40 and may extend along the entire inner circumferential surface of the lower feed handle 40. However, it is also contemplated that each heating region may extend over a portion narrower than the entire inner circumferential surface of the lower feed handle 40. Further, the heating regions 41, 42, 43, and 44 may each have a length of about 8 inches (about 20.32 cm) to about 12 inches (about 30.48 cm). One or more regions may have the same length as or a different length from one or more of the other regions described above.

[0038] Each heating element 46 can heat one or more heating regions 41, 42, 43, 44 of the lower feed handle 40, and by extension, these regions heat the processing gas disposed within the handle cavity 29. As described above, the handle cavity 29 is part of the cavity 27 disposed between the lower feed handle 40 and the muffle 20. The handle cavity 29 may include a plurality of portions other than the above of the cavity 27 as the lower feed handle 40 moves inside the stretching furnace 10. By heating the processing gas disposed in the handle cavity 29, in turn, a part of the upper muffle extension 23 surrounding the heated lower feed handle 40 (and thus surrounding the handle cavity 29) can be heated. Accordingly, each heating element 46 heats the part of the upper muffle extension 23 that surrounds the heated lower feed handle 40.

[0039] By heating the processing gas disposed inside the handle cavity 29, the temperature of the processing gas is increased, thereby reducing the temperature gradients in both the vertical and radial directions and increasing the kinematic viscosity of the processing gas. As a result of such reduction in temperature gradient and increase in kinematic viscosity, the stability of the flow of the processing gas is improved. As described above, the flow instability in the stretching operation may result from the unsteady flow generated by the buoyancy of the processing gas. The heat from the lower feed handle 40 reduces or prevents such flow instability within the stretching furnace 10. Next, helium as the processing gas can be replaced with argon or nitrogen.

[0040] The temperature of the processing gas in the handle cavity 29 may be increased to about 450 degrees Celsius to about 750 degrees Celsius by each heating element 46. As illustrated in FIG. 1, the heat insulating material 65 around the upper muffle extension 23 serves to maintain the elevated gas temperature inside the handle cavity 29.

[0041] As illustrated in FIG. 1, each heating element 46 can be disposed inside the lower feed handle 40 (e.g., on the inner wall surface). Thus, each heating element 46 is disposed radially inside the muffle 20 (including the upper muffle extension 23) and radially inside the outer can 30. However, it is also contemplated that each heating element 46 may be disposed outside the lower feed handle 40, outside the muffle 20, or both. In some embodiments, each heating element 46 may be disposed on the outer wall surface of the lower feed handle 40 or may be embedded within the wall of the lower feed handle 40. Each heating element 46 may be disposed at any location in the stretching furnace 10 as long as it can heat the process gas in the annular space between the lower feed handle 40 and the upper muffle extension 23 to reduce the temperature gradient of the process gas. However, by providing each heating element 46 on the lower feed handle 40 rather than on the wall of the muffle 20, the benefit of reducing power consumption due to reduced heat loss to the upper part of the can is provided, and by simplifying various constraints on the power supply wiring and thermocouple wiring, by simplifying the sealing design of the muffle, and furthermore, by enabling the heating of the handle to be utilized without making any appropriate changes to the furnace, the benefit of relaxing various design constraints is also provided. Each heating element 46 may be composed of a plurality of heating elements disposed vertically along the longitudinal direction of the lower feed handle 40.

[0042] Each heating element 46 may be connected to the lower feed handle 40 so as to be movable together with the lower feed handle 40 inside the muffle 20. Each heating element 46 may also be configured to sequentially heat the heating regions 41, 42, 43, 44. For example, as further discussed later, as the lower feed handle 40 (and thus each heating element 46) moves in the long axis direction inside the muffle 20, each heating element 46 may be configured to sequentially heat the regions.

[0043] Depending on the embodiment, each heating element 46 may heat only a part of the lower feed handle 40 disposed inside the upper muffle extension 23. Therefore, even when a part of the lower feed handle 40 disposed outside the upper muffle extension 23 (for example, inside the uppermost hat 21) is powered on by at least some of the heating elements 46 and is in the on position, it may not be heated by each heating element 46. The portion of the lower feed handle 40 that was not heated initially because it was outside the upper muffle extension 23 can now be placed in the heating state by each heating element 46 as a result of the lower feed handle 40 moving downward within the muffle 20 (i.e., toward the lower heating device 60), and the portion being positioned inside the upper muffle extension 23.

[0044] The portion (one or more locations) of the lower feed handle 40 heated by each heating element 46 may be heated to a temperature in the range of about 200 degrees Celsius to about 1200 degrees Celsius, about 400 degrees Celsius to about 1000 degrees Celsius, about 600 degrees Celsius to about 900 degrees Celsius, about 700 degrees Celsius to about 850 degrees Celsius, or about 700 degrees Celsius to about 800 degrees Celsius. Therefore, the regions 41, 42, 43, 44 of the lower feed handle 40 can each be heated separately and independently to a temperature within the range disclosed in this way. One or more regions may be heated to a different temperature than one or more other regions. It is also conceivable that all regions 41, 42, 43, 44 are heated to the same temperature.

[0045] Depending on the embodiment, one or more regions 41, 42, 43, 44 may be heated so that the temperature gradient becomes the temperature gradient within a specific region. Therefore, for example, the first region 41 may be heated such that the temperature due to heating of its uppermost part (the part farther from the lower heating device 60) is higher than its lowermost part (the part closer to the lower heating device 60) and a certain temperature gradient is provided between these two parts.

[0046] To monitor and adjust the temperatures of the respective regions 41, 42, 43, 44 and the processing gas within the handle cavity 29, a control unit (not shown) is connected to each heating element 46. For example, one or more sensors such as thermocouples may be connected to the control unit to monitor and adjust the temperature. This sensor serves to provide closed-loop temperature control and thermal temperature gradient management.

[0047] As the lower feed handle 40 moves inside the muffle 20 and approaches the lower heating device 60, the regions 41, 42, 43, 44 are sequentially heated by the respective heating elements 46. As the portion of the base material 60 is increasingly consumed by the stretching process, the sequential heating of each region proceeds. For example, when the lower feed handle 40 is in the first position, each heating element 46 is placed in the off position so that none of the regions 41, 42, 43, 44 are heated. The first position may be adopted, for example, during the loading or removal of the base material 50. In this first position, the regions 41, 42, 43, 44 are each disposed outside the upper muffle extension 23.

[0048] In one embodiment, after the lower feed handle 40 descends to the second position within the cavity 27 and the first region 41 is disposed at least partially inside the upper muffle extension 23, each heating element 46 can heat the first region 41. At this time, the remaining regions 42, 43, 44 are not heated by the respective heating elements 46 and are each disposed at least partially outside the upper muffle extension 23. FIG. 1 illustrates the second position of the lower feed handle 40, at which the first region 41 is in a heated state by the respective heating elements 46. As also illustrated in FIGS. 1 to 4, the first region 41 is disposed closest to the lower heating device 60 among all the regions (when the lower feed handle 40 is disposed within the muffle 20).

[0049] Subsequently, as the lower feed handle 40 moves from the second position to the third position by descending (e.g., so as to relatively approach the lower heating device 60), both the first region 41 and the second region 42 are at least partially disposed inside the upper muffler extension 23. At this point, the second region 42 is finally heated. Thus, the second region 42 is heated after the first region 41 is heated. As the stretching process consumes more of the base material 50, the lower feed handle 40 can move from the second position to the third position. When the lower feed handle 40 is in the third position, the remaining regions 43, 44 are each at least partially disposed outside the upper muffler extension 23 without being heated by the respective heating elements 46. FIG. 2 depicts the third position of the lower feed handle 40, at which position both regions 41, 42 are heated by the respective heating elements 46. As also illustrated in FIGS. 1 to 4, the first region 41 is relatively closer to the lower heating device 60 than the second region 42 (when the lower feed handle 40 is disposed inside the muffler 20).

[0050] Next, as a result of the downward feed handle 40 moving from the third position to the fourth position by descending (e.g., so as to approach the lower heater 60 relatively), the first region 41, the second region 42, and the third region 43 are each at least partially disposed inside the upper muffler extension 23. At this point, the third region 43 is finally heated. Thus, the third region 43 is heated after heating the first region 41 and the second region 42. At the same time as more base material 50 is consumed by the stretching process, the downward feed handle 40 can move from the third position to the fourth position. When the downward feed handle 40 is at the fourth position, the remaining region 44 is at least partially disposed outside the upper muffler extension 23 without being heated by each heating element 46. FIG. 3 depicts the fourth position of the downward feed handle 40, at which position regions 41, 42, 43 are all heated by each heating element 46. As also illustrated in FIGS. 1 to 4, the first region 41 and the second region 42 are (when the downward feed handle 40 is disposed inside the muffler 20) at a position relatively closer to the lower heating device 60 than the third region 43.

[0051] Next, as a result of the downward feed handle 40 moving from the fourth position to the fifth position by descending (e.g., so as to approach the lower heater 60 relatively), the first region 41, the second region 42, the third region 43, and the fourth region 44 are each at least partially disposed inside the upper muffler extension 23. At this point, the fourth region 44 is finally heated. Thus, the fourth region 44 is heated after heating the first region 41, the second region 42, and the third region. At the same time as more base material 50 is consumed by the stretching process, the downward feed handle 40 can move from the fourth position to the fifth position. FIG. 4 depicts the fifth position of the downward feed handle 40, at which position regions 41, 42, 43, 44 are all heated by each heating element 46. As also illustrated in FIGS. 1 to 4, the first region 41, the second region 42, and the third region 43 are (when the downward feed handle 40 is disposed inside the muffler 20) at a position relatively closer to the lower heating device 60 than the fourth region 44.

[0052] Next, the lower feed handle 40 can move from the fifth position to a predetermined position by descending inside the muffler 20 (for example, so as to approach relatively closer to the lower heater 60). At the same time as a further base material 50 is consumed by the stretching process, the lower feed handle 40 can move from the fifth position to a predetermined position. Depending on the embodiment, there are also those in which the fifth position is the predetermined position and the lower feed handle 40 does not need to move to proceed from the fifth position to the predetermined position. The predetermined position may be any position relative to the lower heating device 60. As illustrated in FIG. 4, when the lower feed handle 40 is in the predetermined position, the output of one or more heating elements 46 corresponding to the first region 41 can be reduced. For example, the output of one or more heating elements 46 corresponding to the first region 41 can be reduced by about 10%, about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80%, about 90%, or about 100%. Even when the lower feed handle 40 is in the predetermined position and the output of one or more heating elements 46 corresponding to the first region 41 is reduced, each heating element 46 corresponding to the remaining regions 42, 43, 44 can be maintained at its respective output level. Therefore, depending on the embodiment, there are also those in which the reduced output of the first region 41 is lower than the output applied to each of the second region 42, the third region 43, and the fourth region 44.

[0053] When the lower feed handle 40 is in the predetermined position, the reduction in the output to the first region 41 is compensated for by the fact that the region is proximal to the lower heating device 60. For example, as the first region 41 moves and approaches due to the heat of the lower heating device 60, the output of each heating element 46 corresponding to the first region 41 may be reduced. Therefore, depending on the embodiment, there are also those in which, due to receiving extra heat from the lower heating device 60, even if the output of each heating element 46 is reduced, the temperature of the first region 41 does not decrease.

[0054] It is considered that the lower feed handle 40 may move in any order among the first position, the second position, the third position, the fourth position, the fifth position, and a predetermined position. Therefore, for example, according to an embodiment, the lower feed handle 40 may be moved from the third position to the predetermined position.

[0055] According to some embodiments, when the lower feed handle 40 is in the predetermined position, there are some that can reduce the output of each heating element 46 corresponding to both the first region 41 and the second region 42. Also, when the lower feed handle 40 is in the predetermined position, it is also considered to reduce the output of each heating element 46 corresponding to the first region 41, the second region 42, and the third region 43. Furthermore, in various other embodiments, when the lower feed handle 40 is in the predetermined position, the output of all the heating elements 46 may be reduced.

[0056] For the purpose of maintaining a desired heat profile along the length of the muffle 20 when the lower feed handle 40 is in the predetermined position, the output of each heating element 46 corresponding to one or more of the regions can be reduced. As the base material 50 is increasingly consumed by the stretching process and as the lower feed handle 40 approaches the lower heating device 60, there is a potential risk that the temperature inside the muffle 20 may rise due to the combined heat from each heating element 46 and the lower heating device 60. If the internal temperature of the muffle 20 becomes too high, the lower feed handle 40 may overheat, which may cause undesirable secondary effects such as the handle stretching. Therefore, for the purpose of preventing such overheating when the lower feed handle 40 reaches the predetermined position, the output of one or more of the regions 41, 42, 43, 44 is reduced.

[0057] By adjusting the temperature of one or more of the above regions provided on the lower feed handle 40, the temperature of the upper muffle extension 23 can be maintained, and the Grashof number of the stretching furnace 10 can also be maintained below the critical value required to obtain stable convection. For example, the Grashof number may be maintained within the range of about 800 to about 1200.

[0058] As described above, each heating element 46 is connected to the lower feed handle 40 to mitigate the instability of the flow of the processing gas, and thus, gases such as nitrogen and argon can be used. Further, by providing a plurality of heating regions, overheating of the lower feed handle 40 can be prevented. The separate heating regions also help to better regulate the temperature within the muffle 20, for example, enabling the use of relatively low-temperature sealing materials. For example, the sealing portion 26 may be composed of various relatively low-temperature sealants such as silicone, polyurethane, rubber, or various other elastomeric materials.

[0059] Also as described above, if the regions 41, 42, 43, 44 are each at least partially disposed within the upper muffle extension 23, each heating element 46 will continuously heat each region. However, it is also contemplated that each region will be heated if each is at least partially disposed inside the uppermost hat 21. Thus, for example, the second position of the lower feed handle 40 may be the case where the first region 41 is at least partially disposed inside the uppermost hat 21.

[0060] FIG. 5 depicts yet another embodiment where each heating element 46 is replaced by a (one or more) non-contact heating element. In this embodiment, for example, non-contact heating is achieved by induction heating to heat the regions 41, 42, 43, 44. As illustrated in FIG. 5, the induction coil 100 surrounds the muffle 20 and generates a magnetic field that effects magnetic coupling with the lower feed handle 40. The magnetic field is transmitted to the material of the upper muffle extension 23 to heat the lower feed handle 40. For example, in the embodiment of FIG. 5, the lower feed handle 40 may be composed of graphite, and the upper muffle extension 23 may be made of quartz.

[0061] Figures 6A to 13 present the results of numerical fluid dynamics (CFD) simulation experiments regarding the gas flow and temperature graphs inside a furnace assembly (e.g., the elongation furnace 10). In validating the numerical fluid dynamics simulation experiments, past observation results based on the current actual operating configuration were used to confirm the validity of each.

[0062] Referring to FIGS. 6A and 6B, various models of numerical fluid dynamics are depicted, exemplifying the temperature distribution and gas flow patterns during the elongation procedure for three examples. In particular, Comparative Example A shows a model of an unheated handle when using helium as the processing gas, Comparative Example B shows a model of an unheated handle when using argon as the processing gas, and Example C shows a model of a heated handle when using argon as the processing gas. In all three examples, a downward feed handle with an outer diameter of 4.87 inches (about 12.3698 cm) and an inner diameter of 4.49 inches (about 11.4046 cm) was used. Further, for Example C, each heating element 46 was used to heat the downward feed handle to about 800 degrees Celsius, and a heat insulating material with a thickness of 2.565 inches (about 6.5151 cm) was applied to the outer wall of the upper muffle extension over a length of 47 inches (about 119.38 cm).

[0063] As illustrated in FIG. 6A, the comparison between Comparative Example A and Comparative Example B and Example C shows that the purification gas in Example C becomes hotter in the annular space (e.g., region X) between the downward feed handle and the upper muffle extension. For example, the purification gas in the annular space of Comparative Example A and Comparative Example B has a temperature of about 75 to about 100 degrees Celsius. Conversely, the purification gas in the annular space of Example C has a temperature of about 600 degrees Celsius. The high temperature in Example C is due to the heating element provided on the downward feed handle.

[0064] As illustrated in FIG. 6B, in Comparative Example B using a non-heated lower feed handle and argon as the processing gas, a multi-cell flow pattern driven by buoyancy is formed in the annular space at the upper part of the furnace. More specifically, since the flow of the processing gas is unstable and changes over time, temperature fluctuations and pressure fluctuations occur in the annular space as a result. In Comparative Example A using helium as the processing gas, since the gas enters perpendicular to the wall when flowing into the furnace, only two small recirculating vortices are formed near the gas inlet, and a consistent gas flow is achieved. Therefore, Comparative Example A makes the flow of the processing gas stable without temporal changes. Similar to Comparative Example A, Example C also provides a stable flow of the processing gas. However, Example C can achieve a stable flow by heating the lower feed handle even when argon is used as the processing gas. More specifically, FIG. 6B shows that the buoyancy flow in Example C is stable and temperature fluctuations are suppressed.

[0065] FIG. 7 graphically illustrates the gas temperature at position X as a function of time for Comparative Example A, Comparative Example B, and Example C. As described above, the temperature graph of Comparative Example A using helium as the processing gas is relatively stable. Similarly, the temperature graph of Example C using a heated lower feed handle and argon as the processing gas is also relatively stable. However, Comparative Example B using a non-heated lower feed handle and argon as the processing gas has a relatively unstable temperature graph. Comparative Example B has large temperature fluctuations, in the range of approximately 150 degrees Celsius to approximately 400 degrees Celsius.

[0066] FIG. 8 graphically illustrates the gas pressure at position X as a function of time for Comparative Example A, Comparative Example B, and Example C. Similar to the aforementioned temperature graph, Comparative Example A and Example C have relatively stable gas pressures at position X. Conversely, Comparative Example B has a relatively unstable gas pressure at position X.

[0067] As described above, the temperature fluctuations and gas fluctuations near the upper muffler extension 23 may propagate downward inside the muffler 20 and reach the neck-shaped constriction region of the base material 50. FIG. 9 illustrates, as a function of time, the gas temperature at position Y (near the neck-shaped constriction region of the base material 50) for Comparative Example A, Comparative Example B, and Example C. In Comparative Example A, helium gas was used, and in Example C, a heated lower feed handle was used, resulting in a relatively stable temperature graph with no fluctuations. In Comparative Example B, the temperature graph is also relatively unstable, and temperature fluctuations occur in the range of approximately 1771 degrees Celsius to approximately 1816 degrees Celsius at intervals of 50 seconds.

[0068] FIG. 10 illustrates, as a function of time, the gas pressure at position Y for Comparative Example A, Comparative Example B, and Example C. Similar to FIG. 8, the gas pressures in Comparative Example A and Example C are relatively stable, while Comparative Example B has fluctuations in gas pressure.

[0069] FIG. 11 illustrates, as a function of time, the temperature at position Y for three different heating rates of the lower feed handle 40. More specifically, the first lower feed handle was heated to a temperature of approximately 400 degrees Celsius, the second lower feed handle was heated to a temperature of approximately 600 degrees Celsius, and the third lower feed handle was heated to a temperature of approximately 800 degrees Celsius. The temperature fluctuations at position Y were greatest at the temperature of the first lower feed handle and smallest at the temperature of the third lower feed handle. Therefore, heating the lower feed handle to 800 degrees Celsius makes the temperature at position Y (near the neck-shaped constriction) of the muffler more stable compared to heating the lower feed handle to 600 degrees Celsius or 400 degrees Celsius.

[0070] An FFT (Fast Fourier Transform) analysis of the data in FIG. 11 is illustrated in FIG. 12. As can be seen in FIG. 12, as the temperature of the lower feed handle increases, the amplitude of the temperature fluctuations decreases. FIG. 12 shows that the fluctuations are significantly suppressed when the temperature of the lower feed handle approaches approximately 800 degrees Celsius.

[0071] Similar to the graph of temperature versus time, as the temperature of the lower feed handle approaches approximately 800 degrees Celsius, the gas pressure fluctuations at position Y also become relatively more stable. FIG. 13 illustrates, in a graph, the gas pressure at position Y as a function of time for three different temperatures of the lower feed handle. That is, the first lower feed handle was heated to a temperature of approximately 400 degrees Celsius, the second lower feed handle was heated to a temperature of approximately 600 degrees Celsius, and the third lower feed handle was heated to a temperature of approximately 800 degrees Celsius. The gas pressure fluctuations at position Y were greatest at the temperature of the first lower feed handle and least at the temperature of the third lower feed handle. Therefore, heating the lower feed handle to 800 degrees Celsius makes the gas pressure at position Y (near the neck constriction region) of the muffle more stable compared to heating the lower feed handle to 600 degrees Celsius or 400 degrees Celsius.

[0072] As described above, the optical fiber drawing furnace disclosed in this specification is advantageous in that it can use a processing gas (such as nitrogen or argon) having a lower kinematic viscosity while still providing a constant and uniform diameter to the drawn optical fiber.

[0073] Hereinafter, preferred embodiments of the present invention will be described item by item.

[0074] Embodiment 1 A method of operating an optical fiber drawing furnace, the method comprising: When arranging a lower feed handle for supporting an optical fiber preform inside the furnace, making the lower feed handle movable inside the furnace, and Actuating one or more heating elements to heat at least a part of an upper muffle extension disposed inside the furnace with heat, the one or more heating elements being movable together with the lower feed handle.

[0075] Embodiment 2 The method according to Embodiment 1, further comprising injecting a processing gas around the lower feed handle.

[0076] Embodiment 3 The method according to Embodiment 2, wherein the processing gas is at least one of nitrogen and argon.

[0077] Embodiment 4 The method according to any one of Embodiments 1 to 3, further comprising operating a lower heating device inside the furnace, wherein the upper muffle extension is disposed above the lower heating device inside the furnace.

[0078] Embodiment 5 The method according to any one of Embodiments 1 to 4, wherein operating one or more heating elements includes heating a lower feed handle such that only a part of the lower feed handle disposed inside the upper muffle extension is heated.

[0079] Embodiment 6 The method according to any one of Embodiments 1 to 5, wherein operating one or more heating elements includes heating the lower feed handle to a temperature in the range of about 400 degrees Celsius to about 1000 degrees Celsius.

[0080] Embodiment 7 The method according to Embodiment 6, wherein operating one or more heating elements includes heating the lower feed handle to a temperature of about 800 degrees Celsius.

[0081] Embodiment 8 The method according to any one of Embodiments 1 to 7, wherein operating one or more heating elements includes sequentially heating a plurality of regions of the lower feed handle.

[0082] Embodiment 9 The method according to Embodiment 8, wherein sequentially heating a plurality of regions includes heating one or more additional regions as the lower feed handle moves closer to the lower heating device.

[0083] Embodiment 10 The method according to Embodiment 9, wherein sequentially heating a plurality of regions further includes reducing the output applied to at least one region when the lower feed handle moves to a predetermined position relative to the lower heating device.

[0084] Embodiment 11 The plurality of regions includes a first region that is disposed closest to the lower heating device among all of those regions, The method according to Embodiment 8, wherein sequentially heating a plurality of regions includes heating the first region after moving the lower feed handle to be relatively closer to the lower heating device.

[0085] Embodiment 12 The plurality of regions further includes a second region, the first region is located relatively closer to the lower heating device than the second region, and The method according to Embodiment 11, wherein sequentially heating a plurality of regions includes heating the second region after heating the first region and after moving the lower feed handle to be relatively closer to the lower heating device.

[0086] Embodiment 13 The plurality of regions further includes a third region, the first region and the second region are located relatively closer to the lower heating device than the third region, and The method according to Embodiment 12, wherein sequentially heating a plurality of regions includes heating the third region after heating the first region and the second region and after moving the lower feed handle to be relatively closer to the lower heating device.

[0087] Embodiment 14 The method according to Embodiment 13, further including reducing the output applied to the first region after moving the lower feed handle to a predetermined position relative to the lower heating device while maintaining the output applied to the second region and the output applied to the third region.

[0088] Embodiment 15 suspending and supporting an optical fiber preform from a lower feed handle, and the method according to any one of Embodiments 1 to 14, further comprising stretching an optical fiber from the optical fiber preform.

[0089] Embodiment 16 the method according to any one of Embodiments 1 to 15, wherein one or more heating elements are connected to the lower feed handle.

[0090] Embodiment 17 the method according to Embodiment 16, wherein one or more heating elements are arranged on the lower feed handle.

[0091] Embodiment 18 The optical fiber drawing furnace system includes a muffle having an upper muffle extension and forming an internal cavity, a lower feed handle movably disposed inside the internal cavity, and an upper heating device composed of one or more heating elements that are movably together with the lower feed handle inside the internal cavity.

[0092] Embodiment 19 The drawing furnace system according to Embodiment 18, further comprising a lower heating device disposed inside the furnace, wherein the lower feed handle is movably relative to the lower heating device.

[0093] Embodiment 20 The drawing furnace system according to any one of Embodiments 18 and 19, wherein one or more heating elements are connected to the lower feed handle.

[0094] Embodiment 21 The drawing furnace system according to Embodiment 20, wherein one or more heating elements are arranged on the lower feed handle.

[0095] Embodiment 22 The drawing furnace system according to Embodiment 21, wherein one or more heating elements are arranged on the inner wall surface of the lower feed handle.

[0096] Embodiment 23 The drawing furnace system according to any one of Embodiments 18 to 22, wherein one or more heating elements are arranged radially inside the muffle.

[0097] Embodiment 24 The drawing furnace system according to any one of Embodiments 18 to 23, wherein one or more heating elements are composed of a plurality of heating elements arranged vertically along the longitudinal direction of the lower feed handle.

[0098] Embodiment 25 The drawing furnace system according to any one of Embodiments 18 to 24, wherein one or more heating elements are configured to sequentially and separately heat a plurality of regions of the lower feed handle.

[0099] Embodiment 26 The drawing furnace system according to any one of Embodiments 18 to 25, further comprising a gas inlet port arranged in the upper muffle extension portion and configured to inject a processing gas into the internal cavity.

[0100] Embodiment 27 The drawing furnace system according to any one of Embodiments 18 to 26, further comprising a heat insulating material arranged around the upper muffle extension portion.

[0101] Embodiment 28 The drawing furnace system according to Embodiment 27, wherein the heat insulating material extends in the long axis direction from the lower heating device, is arranged inside the furnace, and reaches the upper muffle extension portion.

Description of Reference Numerals

[0102] 10 Optical fiber drawing furnace 20 Muffle 21 Top hat 23 Upper muffle extension portion 26 Elastomer Sealing Part 27 Cavity 29 Handle Cavity 30 Outer Can 40 Lower Feeding Handle 46 Heating Element 50 Base Material 60 Lower Heating Device 65 Heat Insulation Material 70 Gas Inlet Port 72 Gas Inlet Port 74 Gas Inlet Port 90 Lower Muffle Expansion Part

Claims

1. A method of operating an optical fiber drawing furnace, comprising: disposing a lower feed handle for supporting an optical fiber preform inside the furnace, the lower feed handle being disposed so as to be movable inside the furnace, and operating one or more heating elements, the one or more heating elements being movable together with the lower feed handle, and operating to sequentially heat a plurality of regions of the lower feed handle to heat at least a part of an upper muffle extension disposed inside the furnace by heat The method comprising.

2. The method according to claim 1, further comprising injecting a processing gas around the lower feed handle.

3. The method according to claim 1 or 2, further comprising operating a lower heating device inside the furnace, wherein the upper muffle extension is disposed above the lower heating device inside the furnace.

4. Operating one or more heating elements includes heating only a part of the lower feed handle disposed inside the upper muffle extension when heating the lower feed handle. The method according to any one of claims 1 to 3.

5. Operating one or more heating elements includes heating the lower feed handle to a temperature in the range of about 400 degrees Celsius to about 1000 degrees Celsius. The method according to any one of claims 1 to 4.

6. Sequentially heating a plurality of regions includes heating one or more additional regions as the lower feed handle moves closer to the lower heating device. The method according to claim 1.

7. A muffle having an upper muffle extension and forming an internal cavity, A lower feed handle movably disposed inside the internal cavity, An upper heating device composed of one or more heating elements that are movable together with the lower feed handle inside the internal cavity. The one or more heating elements are configured to sequentially heat a plurality of regions of the lower feed handle, and the optical fiber drawing furnace system.

Images