Induction furnace with rotating susceptor for high precision waveguide glass drawing

Abstract

A drawing furnace for drawing glass elements is disclosed, comprising: a furnace body having an upper end and a lower end. The furnace body comprises: a top annular plate; an A / C induction coil capable of receiving an electric current and generating an oscillating electronic signal; a cylindrical susceptor capable of generating a heat output; a cylindrical quartz beaker; an insulating material disposed between the susceptor and the beaker; and a bottom annular plate accommodating and supporting at least one of the susceptor, the beaker, and the insulating material, wherein the furnace body comprises a central longitudinal axis; a rotation drive system operably connected to the bottom annular plate by an annular rotating gear system rotates the bottom annular plate together with the susceptor, the beaker, and / or the insulating material at a frequency between 0.01 Hz and 10 Hz. The current and the oscillation frequency determine the heat output of the susceptor.

Description

Background Art

[0001] The present disclosure relates to furnaces and methods for drawing optical fiber, optical fiber preforms, optical waveguide substrates / sleeves, and in particular to induction furnaces and methods for drawing optical fiber, optical fiber preforms, optical waveguide substrates / sleeves made from large glass preforms, cylinder-core rod assemblies, cylinders, and methods for drawing these glass blanks. More particularly, the present disclosure relates to devices and methods for circumferentially uniformly heating a pedestal to minimize or eliminate defects in drawn tubes, drawn preforms, drawn fibers, and particularly hollow fibers.

[0002] Optical fiber drawing is a process of drawing and shaping a fiber after melting an optical fiber preform at a high temperature through a fiber drawing furnace. The existing optical fiber drawing furnace heating device is mainly formed by a furnace shell, a heat release sleeve installed in the furnace cavity, an insulation layer installed around the heat release sleeve, and an induction coil. On the other hand, optical waveguide and preform drawing have a much larger shaping area than optical fiber drawing, which is usually provided by a resistance heater, and requires much higher power than optical fiber drawing.

[0003] The vertical drawing method using such a furnace is used to form a hollow cylinder of quartz glass into a tube with any desired cross-sectional profile without any tools. Here, the hollow cylinder is usually fed from above in a vertical orientation to a heating tube, where it softens zone by zone and a tube wire is drawn from the softened area, wherein a drawing ball is formed in the softened area.

[0004] On the one hand, the absence of tools in this forming process results in a low-damage surface of the drawn wire. On the other hand, however, the problem arises that the dimensional stability of the drawn wire must be ensured without any mechanical intervention. This is made particularly difficult by the already existing dimensional variations of the hollow cylinder, which tend to continue in the drawn tube wire or even intensify there. The most frequently found defects are high-frequency diameter variations and ovality or wall one-sidedness in the radial cross-sectional profile, i.e. radial irregularities in the thickness of the tube wall (also called "rolling" among experts).

[0005] The roll defect is largely formed due to the asymmetric circumferential temperature characteristics around the pedestal in the area where the ball is drawn. There are many reasons for the circumferential temperature asymmetry of the pedestal, such as, for example, the helical shape of the coil, the misalignment of the pedestal axis relative to the coil, and the uneven wall thickness of the pedestal. Circumferential temperature symmetry is particularly important when drawing hollow fibers that include internal geometries, because slight temperature variations around the pedestal will translate into temperature variations in the drawn fiber, which can deform or, worse, destroy the internal geometry of the hollow fiber.

[0006] Induction furnaces with pedestals as heating elements are widely used as drawing furnaces in the telecom fiber industry. But it is rarely used as a tube or preform drawing furnace, mainly due to its limited power, limited heating zone length, and the limited drawing production it can support. Recent technological advances in induction heating have made it possible to achieve sufficient power for prefabrication or tube drawing from a cylinder of about 200 mm OD. However, for tube (or preform) drawing with a much longer forming zone than standard telecom fiber drawing, symmetrical geometry accuracy (such as ellipticity, roll and eccentricity) is more sensitive to the circumferential temperature symmetry of the induction furnace. In addition, in the newly developed NANF HCF preform manufacturing and fiber drawing process, the precise tube geometry and the related symmetry inside the rod need to be well maintained to achieve the designed waveguide performance. Therefore, the drawing yield is also highly sensitive to the circumferential temperature asymmetry of the oven.

[0007] For the relevant temperature range (typically >2100°C), measuring the full temperature characteristics of an induction furnace is technically challenging and very difficult to measure accurately. Therefore, the temperature symmetry within the furnace is very difficult to determine by direct measurement.

[0008] Therefore, there is a need in the art for an apparatus and method for drawing an optical fiber from a glass preform that will impart circumferential temperature symmetry to the optical fiber as it is drawn to substantially eliminate the above-mentioned deficiencies. Summary of the invention

[0009] The present disclosure meets this need by providing a drawing furnace for drawing glass elements, the furnace comprising: a furnace body having an upper end including an opening and a lower end including an opening, wherein the furnace body comprises: i. a top annular plate, ii. an A / C induction coil capable of receiving a certain level of current and generating an oscillating electronic signal oscillating at a certain frequency, iii. a cylindrical base capable of generating a certain level of heat output, iv. a cylindrical quartz beaker, v. an insulating material disposed between the cylindrical base and the cylindrical quartz beaker component, and vi. a bottom annular plate, the bottom a bottom annular plate accommodating and supporting the cylindrical base, the cylindrical quartz beaker component and at least one of the insulating material, wherein the furnace body includes a central longitudinal axis; and a rotational drive system operably connected to the bottom annular plate through an annular rotation gear system, wherein when the rotational drive system is engaged with the annular rotation gear system, the bottom annular plate is caused to rotate together with the cylindrical base, the cylindrical quartz beaker component and at least one of the insulating material about the central axis at a frequency of 0.01 Hz to 10 Hz, and wherein the current level and the oscillation frequency determine the heat output level of the base.

[0010] In another aspect, the present disclosure provides a method for uniformly distributing heat in a circumferential direction around a susceptor in a furnace during drawing of a glass element, the method comprising the steps of: inductively heating a drawing furnace by providing an A / C current, the drawing furnace comprising: a furnace body having an upper end including an opening and a lower end including an opening, wherein the furnace body comprises: i. a top annular plate, ii. an A / C induction coil capable of receiving a certain level of current and generating an oscillating electronic signal oscillating at a certain frequency, iii. a cylindrical susceptor capable of generating a certain level of heat output, iv. a cylindrical quartz beaker, v. an insulating material disposed between the cylindrical susceptor and the cylindrical quartz beaker component, and vi. a bottom annular plate that accommodates and supports at least one of the cylindrical base, the cylindrical quartz beaker component and the insulating material, wherein the furnace body includes a central longitudinal axis; and a rotational drive system that is operably connected to the bottom annular plate through an annular rotation gear system; and engaging the rotational drive system and the annular rotation gear system to rotate the bottom annular plate so that the bottom annular plate rotates circumferentially around the central axis at a frequency of 0.10 Hz to 10 Hz together with the cylindrical base, the cylindrical quartz beaker component and at least one of the insulating material, and wherein the current level and the oscillation frequency determine the heat output level of the base.

[0011] In another aspect, the present disclosure provides a method of forming an optical glass element in a furnace, the furnace comprising a furnace body having a top annular plate, an A / C induction coil, a cylindrical susceptor, a cylindrical quartz beaker, an insulating material disposed between the susceptor and the beaker, and a bottom annular plate; and a rotary drive system operably connected to the bottom annular plate through an annular rotary gear system, the method comprising: introducing a glass blank into the furnace body through an opening in the top annular plate; engaging the rotary drive system to the annular rotary gear system and rotating the bottom annular plate together with at least one of the cylindrical susceptor, the cylindrical quartz beaker or the insulating material about a central axis of the furnace body at a frequency of 0.01 Hz to 10 Hz while the glass blank is drawn through the furnace body; inductively heating the susceptor via a magnetic field generated by the A / C induction coil receiving an electric current; heating the glass blank in the susceptor; and supplying the produced optical glass element through the opening in the bottom annular plate. The glass blank may be one of a glass preform, a glass cylinder-core rod assembly, a glass cylinder, etc. The optical glass element may be one of an optical fiber (particularly a hollow optical fiber) or an optical fiber preform, an optical waveguide, a tube, etc. The rotation frequency may be more particularly 0.05 Hz to 0.15 Hz, and more particularly 0.1 Hz.

[0012] The embodiments of the present invention may be used alone or in combination with each other. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 shows a high level schematic cross-sectional view on a plane through the diameter of a furnace for drawing optical fiber according to the present disclosure;

[0014] Figure 2 shows a detailed cross-sectional view on a plane through the diameter of a furnace according to the present disclosure;

[0015] Figure 3 An embodiment of an induction coil to be placed in a furnace is shown;

[0016] Figure 4 A magnetic field density normative map is shown;

[0017] Figure 5 shows the overall temperature profile of the susceptor from Example 1;

[0018] Figure 6 The temperature characteristic diagram of the susceptor in the axial CSA of Example 1 is shown;

[0019] Figure 7 The temperature characteristic diagram of the susceptor when the axis of the susceptor deviates from the axis of the coil in Example 2 is shown;

[0020] Figure 8 The temperature characteristic diagram of the susceptor in the axial direction CSA when the susceptor axis deviates from the axis of the coil in Example 2 is shown;

[0021] Fig. 9 The temperature characteristic diagram of the susceptor of Example 3 when the wall thickness is uneven is shown.

[0022] Fig.10 shows a temperature characteristic diagram of a susceptor with 0.1 Hz rotation according to Example 4;

[0023] Fig.11 shows a temperature characteristic diagram on the axial CSA of a susceptor with 0.1 Hz rotation according to Example 4;

[0024] Fig.12 shows the temperature characteristic graph of the susceptor with 0.1 Hz rotation when the susceptor axis deviates from the axis of the coil;

[0025] Fig.13 shows a temperature characteristic graph of the axis of the susceptor with 0.1 Hz rotation toward the CSA when the axis of the susceptor deviates from the axis of the coil; and

[0026] Fig.14 Shown is the temperature profile of a susceptor having non-uniform wall thickness but with 0.1 Hz susceptor rotation added. DETAILED DESCRIPTION

[0027] The subsequent detailed description only provides preferred exemplary embodiments and is not intended to limit the scope, applicability or configuration of the present invention. On the contrary, the subsequent detailed description of the preferred exemplary embodiments will provide an enabling description for implementing the preferred exemplary embodiments of the present invention to those skilled in the art. Various changes may be made to the functions and arrangements of the elements without departing from the spirit and scope of the present invention as set forth in the appended claims.

[0028] For purposes of this specification and the appended claims, the term "drawing furnace" refers to a furnace that can be used to draw a glass element (e.g., an optical preform) into an optical fiber or to stretch and collapse a cylinder-core rod assembly into a preform or to stretch a glass cylinder into a tube.

[0029] In the context of describing the present invention (especially in the context of the appended claims), the use of the terms "one", "a kind of", "the" and similar referents should be understood to cover the singular and plural, unless otherwise specified herein or clearly contradicted by the context. Unless otherwise specified, the terms "comprise", "have", "include" and "contain" should be interpreted as open terms (that is, meaning "including but not limited to"). Unless otherwise specified herein, the reference to the value range herein is only intended to serve as a convenient method for individually pointing out each independent value falling within the range, and each independent value is included in the specification as if quoted separately in this article. Unless otherwise specified herein or clearly contradicted by the context, all methods described herein can be performed in any suitable order. Unless otherwise specified, the use of any and all examples or exemplary language (e.g., "such as") provided herein is only intended to better illustrate the present invention, rather than to impose limitations on the scope of the claimed invention. Any language in the specification should not be interpreted as indicating that any element not protected by the claims is necessary for the implementation of the present invention.

[0030] Hollow core fibers (HCF) such as NANF HCF require excellent circumferential temperature symmetry to maintain the tube geometry in the rod. The desired tube geometry tolerances have become significantly tighter, with ovality targets of <= 0.1% of tube OD, roll targets of <= 1% of tube wall thickness, and CSA variations of less than 1% - all of which significantly reduce our tube drawing yields.

[0031] To meet or exceed such requirements, a drawing furnace for drawing glass elements is disclosed herein, the furnace comprising: a furnace body having an upper end including an opening and a lower end including an opening, wherein the furnace body comprises: i. a top annular plate, ii. an A / C induction coil capable of receiving a certain level of current and generating an oscillating electronic signal oscillating at a certain frequency, iii. a cylindrical base capable of generating a certain level of heat output, iv. a cylindrical quartz beaker, v. an insulating material disposed between the cylindrical base and the cylindrical quartz beaker components, and vi. a bottom annular plate, The bottom annular plate houses and supports at least one of the cylindrical base, the cylindrical quartz beaker component and the insulating material, wherein the furnace body includes a central longitudinal axis; and a rotational drive system, which is operably connected to the bottom annular plate through an annular rotation gear system, wherein when the rotational drive system is engaged with the annular rotation gear system, the bottom annular plate is caused to rotate with at least one of the cylindrical base, the cylindrical quartz beaker component and the insulating material around the central axis at a frequency of 0.01 Hz to 10 Hz, and wherein the current level and the oscillation frequency determine the heat output level of the base.

[0032] Figure 1 A high level schematic cross-sectional view on a plane through the diameter of a furnace for drawing optical fiber according to the present disclosure is shown. Figure 1 The drawings are not to scale and not all components of the furnace are shown in detail. Figure 2 A more detailed cross-sectional view is shown on a plane through the diameter of the furnace (relative to Figure 1 ). Typically, the furnace comprises a furnace body F, a top flue (not shown) and a bottom flue B. The preform is introduced into the furnace body through the top flue, and when the preform is drawn into fibers, the fibers leave the furnace through the bottom flue. The furnace body has a central axis CA running longitudinally through the center.

[0033] exist Figure 2 A preferred embodiment of a furnace according to the present invention is shown in . The furnace body F comprises a cylindrical base 2 housed within the interior of a cylindrical insulator 3, which is capable of producing a certain level of heat output. The insulator 3 is in turn preferably housed within a cylindrical quartz beaker 4. Two annular quartz plates 25 and 27 maintain the alignment of the quartz beaker 4 around the base 2. In some embodiments, a gap of preferably about 3mm-5mm may be provided between the outer surface of the base 2 and the inner surface of the insulator 3. In certain embodiments, a similar gap may also be provided between the outer surface of the insulator 3 and the inner surface of the quartz beaker 4. These gaps, if present, allow the components of the furnace body to be installed more easily and their individual components to be easily removed and replaced without interfering with other components of the assembly.

[0034] The height of the furnace is sized to contain the entire length of the preform together with the top and bottom flues. The height of the furnace should be sufficiently extended to heat the lower end of the preform to draw optical fiber therefrom, but should be sufficiently reduced to avoid unnecessary overheating of the remainder of the preform.

[0035] The coil 6 is an A / C induction coil capable of receiving a certain level of current and generating an oscillating electronic signal oscillating at a certain frequency. The coil 6 surrounds the quartz beaker 4, being positioned at a distance of about 5 mm-6 mm from the outer surface of the quartz beaker 4. The coil 6 generates an electric field, which in turn generates eddy currents and thus heats up to a temperature of about 2200° C. in the susceptor 2. All of these main components, namely the coil 6, the quartz beaker 4, the insulator 3 and the susceptor 2 are enclosed in the housing 1.

[0036] The housing 1, which may be made of metal (e.g. copper, brass or preferably aluminium), is closed at the top and bottom ends by annular end plates 15 and 16, each with a centrally located aperture. These end plates and the housing 1 may be made of, for example, aluminium. The apertures in the end plates cooperate with the top flue T and the bottom flue B. The end plates and the inner wall 1b of the housing 1 define the furnace interior.

[0037] exist Figure 1 and Figure 2 In the embodiment shown, the housing 1 comprises two walls, an outer wall 1a and an inner wall 1b, which together define an annular cooling chamber 12. The diameter and length of the walls depend on the application of the furnace and are by no means limiting. As an example, in one embodiment, the outer wall 1a has an outer diameter of 620 mm diameter, and the inner wall 1b has an outer diameter of 580 mm, and the height of the housing 1 is about 740 mm. In order to cool the outer housing 1, a cooling fluid flows through the cooling chamber 12. For example, the space between the walls 1a and 1b is cooled by a water flow. The cooling water enters the chamber 12 via a plurality of water supply lines 11. For example, three supply lines 11 may be distributed at 120° intervals around the periphery of the housing 1. The water is then discharged from the chamber 12 through the discharge line 13. Preferably, the number of supply lines 11 and discharge lines 13 is equal, and the supply lines 11 and discharge lines 13 are located on opposite sides of the housing 1 so that the cooling water cools the housing 1 uniformly.

[0038] The coil 6 is electrically connected to a high frequency current source (not shown) via passage 23. The current level in the coil 6 can result in a significant level of resistive heating. In order to control this heating, the coil 6 can be made of, for example, copper piping so that a cooling fluid such as water can circulate through the coil 6 during operation of the furnace. The electric field generated by the current flowing through the coil 6 induces eddy currents in the base 2. A conditioning gas (e.g., nitrogen) is preferably flowed into the space 43 in which the coil is accommodated to protect the insulator and / or the base from possible oxidation. The typical flow rate of the gas is about 20 to about 30 l / min. A supply line 44 and an exhaust line 45 are provided to the housing 1, for example, in the same manner as described for the lines 11 and 13, for allowing the gas to flow into the housing 1.

[0039] The structure supporting the coil 6 within the housing 1 should not conduct significant current, if any. Figure 3 In the embodiment shown, three legs 18 made of insulating ceramic material are distributed in the housing 1 at intervals of 120° to support the adjacent quartz beaker 4 ( Figure 3 1 ). After the coil 6 has been centered in the furnace by fixing the coil support 18, the central axis of the coil 6 is precisely aligned with respect to the central axis of the furnace by acting on the pin 70, which is inserted into the corresponding vertical slot 71 provided on the support 18; once the correct alignment has been achieved, the coil is fixed in such position by fixing each pin to the support 18 (for example, using a nut). Alternatively, the vertical slots in the support 18 can be replaced by a series of horizontal slots, each of which is provided at an appropriate height where the coil turn pin should rest. With respect to this latter embodiment, the mounting and alignment operations are generally easier and the centering of the coil is considerably more precise and constant in time.

[0040] The susceptor 2 of the furnace according to the present disclosure is generally made of graphite. In particular, the graphite susceptor material should have a relatively high thermal conductivity, preferably above about 50 W / m°K (e.g., about 100 W / m°K) and a relatively low resistivity, preferably about 1.010 -5 Ohms or less. In one embodiment, the base 2 is about 6 mm thick, with an inner diameter of 150 mm, making it compatible with large preforms (up to about 130 mm in diameter).

[0041] A conditioning gas is usually provided from a top flue in the cylindrical base to provide a controlled heating atmosphere in the furnace and to prevent the surface of the base from being oxidized due to accidental entry of air from outside the furnace. Any inert gas may be suitable as a conditioning gas, including but not limited to helium, argon and nitrogen, with helium being preferred. Typically, the conditioning gas flows in the furnace at a flow rate of about 15 l / min to about 20 l / min. The conditioning gas diffuses through the base into the gap between the base and the insulator and then penetrates the insulator. Due to the reduced width of the gap, the conditioning gas disposed in the gap is substantially stationary, i.e., no significant forced flow of gas is caused in the gap.

[0042] The insulating material suitable for the furnace according to the present disclosure may also be a graphite-based material for improving the compatibility between the base and the insulator. The insulating material differs from the graphite-based material used for the base mainly in terms of density (or porosity), thermal conductivity and resistivity. Specifically, the density of the graphite insulating material is less than about 0.4 g / cm 3 , preferably less than about 0.2 g / cm 3 (with graphite base material of at least 1.5g / cm 3 ). Associated with the lower density of the insulating material is its higher apparent porosity, which is higher than about 70%, preferably higher than about 85% (compared to a typical value of the porosity of the base material of about 20%). Also due to the relatively high porosity, the graphite insulating materials have good insulating properties. The thermal conductivity of these materials is relatively low compared to the thermal conductivity of the base material. In particular, the thermal conductivity is typically lower than about 1.5 W / m°K, preferably lower than about 1.2 W / m°K in an argon atmosphere at 2000°C, and lower than about 0.6 W / m°K, preferably lower than about 0.4 W / m°K in an argon atmosphere at 400°C. In order to limit possible electromagnetic coupling with the coil, the graphite insulating material should have a much higher specific resistivity than the specific resistivity of the base, preferably about 1.010 -3 ohms or higher.

[0043] Commercial carbon or graphite felts may be used as insulating materials in furnaces according to the present disclosure, such as those commercialized by SGL (e.g., Sigatherm KFA5 or KFA10) or by Union Carbide (e.g., National Grade VDG, Grade WDF, or Grade GRI-1). As previously mentioned, these felt materials are in the form of flexible sheets that need to be coupled to a rigid support. Typically, a sheet of such felt material has a thickness of about 5 mm-10 mm and is wound around a base a sufficient number of turns to impart the desired insulating properties, the base thus serving as a support for the insulator.

[0044] Advantageously, for the insulator of the furnace according to the present disclosure, a rigid graphite insulating material is preferably used, the material having sufficient rigidity to be easily formed into a self-supporting cylinder. These materials are generally composed of graphite fibers, most of which are arranged parallel to each other along a preferred direction. In order to be hard enough and bear its own weight, the material should have a compression modulus of at least 0.1Mpa in the longitudinal direction of the fiber, preferably at least about 0.5Mpa and at most about 10Mpa, particularly preferably about 1Mpa. Typically, the insulating cylinder is made of a single sheet of desired thickness. The sheet is bent to form a cylinder and the two opposite ends of the bent sheet are kept in contact with each other (e.g., using welding or sewing). The insulating cylinder thus obtained can withstand its own weight without collapsing onto the base, thereby allowing a desired gap (preferably about 3mm-5mm) to be maintained between the insulator and the base. In case the susceptor cylinder needs to be replaced, for example due to its consumption, said susceptor can easily be removed from the furnace body without changing the entire geometry of the structure, thereby leaving the insulating cylinder in place.

[0045] Examples of suitable materials having the desired properties are Sigatherm PR-200-16, PR-201-16 or PR-202-16 (from SGL), (Carbon bonded carbon fiber) and UCAR graphite RIGID insulation manufactured by Union Carbide. Preferred materials for insulator 3 are It is rigid, has uniform porosity, is easy to machine and assemble. As previously mentioned, these materials consist of graphite fibers arranged parallel to each other along a preferred direction. In an embodiment of the present disclosure, the material is formed into a cylinder in such a way that the graphite fibers are arranged primarily parallel to the axis of the insulating cylinder.

[0046] The rigid graphite insulating cylinder according to the invention has a thickness of about 45 mm to 60 mm and an inner diameter of about 150 mm to about 160 mm. According to a preferred embodiment, the thickness is about 52 mm and the inner diameter is about 156 mm.

[0047] The quartz beaker 4 is a cylindrical quartz tube having an outer diameter of about 265 mm-285 mm (about 275 mm in the non-limiting illustrated embodiment) and is arranged to surround the insulator 3 and separate the conditioning gas within the cylindrical base 2 from the conditioning gas around the coil 6 (when these are different). A top annular plate 25 is arranged at the upper end of the beaker 4, the top annular plate having a central opening 26 and three concentric grooves in its lower surface. A bottom annular plate 27 is arranged at the lower end of the beaker 4, the bottom annular plate also having a central opening 28 and concentric grooves in its upper surface. The bottom annular plate accommodates and supports at least one of the cylindrical base, the cylindrical quartz beaker component and the insulating material, wherein the furnace body includes a central longitudinal axis. In a preferred embodiment, the top annular plate 25 and the bottom annular plate 27 include quartz. At least one of the ends of the base 2, the insulator 3 and the beaker 4 is accommodated in the concentric grooves in the plates 25 and 27 so that these parts are reliably and accurately positioned relative to each other and can be easily removed individually. This method of positioning at least one of the base 2, the insulator 3 and the beaker 4 ensures consistent performance from one furnace to another. The annular ridges 29 and 30 formed on the ends of the insulator 3 are inserted into the central grooves of the two plates 25 and 27. In one embodiment, the annular ridges 29 and 30 are aligned concentrically around the base 2 with the insulator 3. In one embodiment, the end of the base 2 is accommodated in the innermost concentric grooves of the two plates 25 and 27. Therefore, in one embodiment, the bottom annular quartz plate 27 accommodates and supports the cylindrical base 2, the cylindrical quartz beaker 4 and the insulator 3.

[0048] A hole 46 is usually made through the beaker 4, the insulator 3 and the housing 1 to enable the temperature of the outer surface of the susceptor 2 to be measured using a pyrometer set introduced into the furnace through the passage 39. Piercing the insulator 3 is very easy due to the machinability of the insulator 3. The temperature measured by the pyrometer is used as a feedback parameter to control the power supply to the furnace.

[0049] The drawing furnace disclosed herein includes a rotation drive system 50 operably connected to the bottom annular quartz plate 27 through an annular rotation gear system 52, wherein when the rotation drive system 50 is engaged with the annular rotation gear system 52, the bottom annular quartz plate 27 rotates with at least one of the cylindrical susceptor 2, the cylindrical quartz beaker 4, and the insulator 3 about the central axis CA at a frequency of 0.10 Hz to 10 Hz and preferably 0.05 Hz to 0.15 Hz, and wherein the current level and the oscillation frequency determine the heat output level of the susceptor 2. In one embodiment, the rotation frequency is 0.1 Hz.

[0050] In one embodiment, the rotation drive system 50 includes an annular rotation gear system 52, the rotation table including a gear for engaging with a gear of a drive motor. In some embodiments, the annular rotation gear system 52 includes a concentric groove that cooperatively receives and thereby accommodates at least the cylindrical base 2 and the cylindrical quartz beaker 4. In some embodiments, the annular rotation gear system 52 also includes a concentric groove for the insulator 3, so that the cylindrical base 2, the cylindrical quartz beaker 4 and the insulator 3 are integrally mounted on the annular rotation gear system 52. Preferably, the rotation drive system 50 includes a computer processing unit (not shown).

[0051] Preferably, the rotary drive system 50 rotates the annular rotary gear system 52 and thus the cylindrical base 2, the cylindrical quartz beaker 4 and the insulator 3 around the central axis CA at a rotary frequency (at a frequency of 0.01 Hz to 10.0 Hz or 0.05 Hz to 0.15 Hz). In some embodiments, the rotary frequency is 0.1 Hz.

[0052] An example of such a device without a rotary drive system is disclosed in US Pat. No. 7,814,767, which is incorporated herein by reference in its entirety.

[0053] Also disclosed herein is a method for uniformly distributing heat in a circumferential direction around a susceptor in a furnace during drawing of a glass element, the method comprising the steps of: inductively heating a drawing furnace by providing an A / C current, the drawing furnace comprising: a furnace body having an upper end including an opening and a lower end including an opening, wherein the furnace body comprises: i. a top annular plate, ii. an A / C induction coil capable of receiving a certain level of current and generating an oscillating electronic signal oscillating at a certain frequency, iii. a cylindrical susceptor capable of generating a certain level of heat output, iv. a cylindrical quartz beaker, v. an insulating material disposed between the cylindrical susceptor and the cylindrical quartz beaker, cup member, and vi. a bottom annular plate that accommodates and supports at least one of the cylindrical base, the cylindrical quartz beaker member, and the insulating material, wherein the furnace body includes a central longitudinal axis; and a rotational drive system that is operably connected to the bottom annular plate through an annular rotation gear system; and engaging the rotational drive system and the annular rotation gear system to rotate the bottom annular plate so that the bottom annular plate rotates circumferentially around the central axis at a frequency of 0.10 Hz to 10 Hz together with the cylindrical base, the cylindrical quartz beaker member, and at least one of the insulating material, and wherein the current level and the oscillation frequency determine the heat output level of the base.

[0054] The first step of the method is to inductively heat a draw furnace by providing an A / C current, the draw furnace comprising a furnace body having an upper end including an opening and a lower end including an opening, wherein the furnace body comprises the features described above. As discussed above, heating occurs inductively due to providing the A / C current to the coil 6, which inductively heats the susceptor 2 via the magnetic field created by the current.

[0055] The second step of the method is to engage the rotation drive system 50 and the annular rotation gear system 52 to rotate the bottom annular plate 27, the cylindrical base 2, the cylindrical quartz beaker 4 and the insulator 3 circumferentially around the central axis at a frequency of 0.01 Hz to 10.0 Hz. The step of engaging the rotation drive system 50 can be performed via a computer processor or by manually engaging the electric motor of the rotation drive system 50. The second step of engaging the rotation drive system 50 and the annular rotation gear system 52 can occur simultaneously or the rotation gear system can be engaged before the heating step.

[0056] The base, insulating material and glass beaker are capped and aligned by two precisely machined annular plates. In one embodiment, the annular plates are quartz plates. The bottom plate is precisely connected and aligned with the annular rotating gear system under the oven jacket. When the oven jacket is water-cooled and the empty space inside is purged with inert gas, the rotating unit should be able to rotate the base and insulating material with precise axial alignment.

[0057] The disclosed method may also include the steps of inserting an optical fiber preform and drawing the optical fiber, or inserting a cylinder-core rod assembly followed by preform drawing, or inserting a cylinder for tube drawing. The optical fiber may be a solid or hollow core optical fiber. The disclosed method is particularly suitable for drawing hollow core optical fibers because the method results in a more uniformly distributed circumferential temperature signature, as shown below. Thus, the method significantly reduces (if not eliminates) imperfections in the geometry of the drawn hollow core optical fiber.

[0058] Example

[0059] The following example was performed to understand the temperature symmetry of an induction furnace through FEM multiphysics modeling. Considering the key physics involved, including electromagnetic heating (induction heating), surface-to-surface radiation exchange, and water cooling of the coils, a full 3D model was constructed as follows.

[0060] Assumptions for Coil and Heater Design

[0061] Coil geometry

[0062] oDiameter = 360 mm

[0063] Height = 320 mm

[0064] oLine width = 15mm

[0065] o line height = 20mm

[0066] oCooling channel width = 8mm

[0067] Cooling channel height = 13mm

[0068] Graphite base geometry

[0069] o ID=280mm

[0070] Height = 420 mm

[0071] oWall thickness = 14mm

[0072] Key physical setup assumptions

[0073] ●Nonlinear temperature dependence of graphite properties (electrical and thermal conductivity, specific heat) is taken into account

[0074] Heat conduction in solids

[0075] ●Electromagnetic heating on both coil and graphite tube

[0076] ● Surface-to-surface radiation BC on graphite tube and coil, with background temperature of 1000K

[0077] (Simulating the thermal insulation of a heater)

[0078] ● Simulate actual cooling water flow in a fully coupled model. Inlet temperature is 293K and flow rate is 7.95m / s (internal channel CSA 8mm×13mm), equivalent to 13.1 gallons / minute of current connector cooling capacity

[0079] ●15kHz, 1.7kA excitation

[0080] Magnetic flux density standard curve (see Figure 4 )

[0081] ●Solution of Maxwell's equations

[0082] • An oscillating excitation current in the coil (15kHz, 1.7kA excitation) generates an oscillating magnetic field which in turn creates a current flow in the graphite tube. The graphite tube creates a current flow and heats up by Joule heating from the induced current.

[0083] As a result of the above, the inventors were able to identify the following key factors that can significantly affect circumferential temperature symmetry:

[0084] Example 1: Intrinsic temperature asymmetry caused by the helical shape of the coil

[0085] like Figure 5As shown, along the spiral path of the coil, there is an azimuthal temperature asymmetry on the susceptor, ie, the isotherms are tilted relative to the end face of the susceptor.

[0086] observe Figure 6 From the axial CSA temperature profile in Figure 1, it is clear that there is a spatial offset of the isotherms between the left and right sides of the wall, especially on the CSA perpendicular to the coil leads. This inherent and persistent temperature asymmetry can cause the glass to flow differently on both sides, which will result in a significant level of tube ovality or roll (using tube drawing as an example). This can also cause similar problems in terms of geometric accuracy in preform drawing or fiber drawing.

[0087] Example 2: Temperature asymmetry caused by misalignment of the susceptor axis relative to the coil axis

[0088] like Figure 7 and Figure 8 As shown, if the axis of the pedestal is offset from the axis of the coil, significant azimuthal temperature asymmetry can result, depending on the severity of the misalignment problem. This again has a large impact on the accuracy of the axisymmetric geometry of tube, preform and fiber drawing.

[0089] Example 3: Temperature asymmetry caused by uneven wall thickness of the susceptor

[0090] As the susceptor ages, it may become non-uniform due to maintenance efforts such as pipe cleaning (scrubbing away SiC growth on the ID surface). Fig. 9 As shown, in the case of non-uniform wall thickness, hot spots are created in the thinner portions of the wall, which is a significant temperature asymmetry that can also affect the glass drawing accuracy discussed above.

[0091] Example 4: Solution according to the present disclosure

[0092] A solution for eliminating or significantly reducing the above-mentioned temperature asymmetry is to rotate the susceptor together with the potential insulating material between the coil and the susceptor during induction heating. Simulations have shown that a rotation frequency of 0.1 Hz (relatively slow rotation speed) can significantly improve the temperature symmetry. Fig.10 and Fig.11 As shown, the inherent temperature asymmetry caused by the helical shape of the coil is almost completely eliminated.

[0093] like Fig.12 and Fig.13 As shown, Example 2 above was repeated except that the susceptor was rotated at 0.1 Hz and the temperature asymmetry was significantly reduced.

[0094] like Fig.14 As shown, Example 3 above was repeated except that the rotation of the susceptor was increased at 0.1 Hz and the temperature asymmetry was significantly reduced.

[0095] Also disclosed herein is a method of forming an optical glass element in a furnace, the furnace comprising a furnace body having a top annular plate, an A / C induction coil, a cylindrical susceptor, a cylindrical quartz beaker, an insulating material disposed between the susceptor and the beaker, and a bottom annular plate; and a rotary drive system operably connected to the bottom annular plate through an annular rotary gear system, the method comprising: introducing a glass blank into the furnace body through an opening in the top annular plate; engaging the rotary drive system to the annular rotary gear system, and rotating the bottom annular plate together with at least one of the cylindrical susceptor, the cylindrical quartz beaker or the insulating material around a central axis of the furnace body at a frequency of 0.01 Hz to 10 Hz while the glass blank is drawn through the furnace body; inductively heating the susceptor via a magnetic field generated by the A / C induction coil receiving an electric current; heating the glass blank in the susceptor; and supplying the produced optical glass element through the opening in the bottom annular plate. The glass blank may be one of a glass preform, a glass cylinder-core rod assembly, a glass cylinder, etc. The optical glass element may be one of an optical fiber (particularly a hollow optical fiber) or an optical fiber preform, an optical waveguide, a tube, etc. The rotation frequency may be more particularly 0.05 Hz to 0.15 Hz, and more particularly 0.1 Hz.

[0096] The first step of the method includes introducing a glass body into the furnace F through an opening in the top annular plate 25 (eg, via a top chimney coupled to the furnace F). The glass body may be a glass preform, a glass cylinder-core rod assembly, a glass cylinder, or the like.

[0097] The second step of the method includes coupling the rotary drive system 50 to the annular rotary gear system 52 and rotating the bottom annular plate 27 together with at least one of the cylindrical base 2, the cylindrical quartz beaker 4, or the insulator 3 about the central axis CA of the furnace body F at a frequency of 0.01 Hz to 10 Hz or 0.05 Hz to 0.15 Hz while the glass blank is pulled through the furnace body F. In some embodiments, the rotation frequency is 0.1 Hz.

[0098] The third step of the method comprises inductively heating the susceptor 2 via the magnetic field generated by the A / C induction coil 6 receiving the electric current. Using the heat output of the susceptor 2, the fourth step of the method comprises heating the glass blank in the susceptor 2. The heating step allows the glass blank to soften and be drawn into the produced optical glass element, which can be an optical fiber (especially a hollow optical fiber), an optical fiber preform, an optical waveguide, a tube, etc. The fifth step of the method comprises feeding the produced optical glass element through an opening in the bottom annular plate 27, which in the embodiment shown allows the optical glass element to be extracted from the furnace body F via the bottom flue B.

[0099] Certain steps of the method may be initiated in a different order than that described above. For example, the glass blank may be introduced into the furnace body F after the rotation by the rotary drive system 50 has already taken place. Similarly, the heating of the susceptor 2 may begin before the glass blank is inserted.

[0100] Many embodiments have been described as disclosed herein. However, it should be understood that various modifications may be made without departing from the spirit and scope of the embodiments disclosed herein. Therefore, other embodiments are within the scope of the following claims.

Claims

1. A drawing furnace for drawing glass elements, the furnace comprising: A furnace body having an upper end including an opening and a lower end including an opening, wherein the furnace body comprises: i. Top ring plate, ii. an A / C induction coil that is capable of receiving a certain level of current and generating an oscillating electronic signal that oscillates at a certain frequency, iii. a cylindrical base capable of producing a certain level of heat output, iv. Cylindrical quartz beaker, v. an insulating material disposed between the cylindrical base and the cylindrical quartz beaker, and vi. a bottom annular plate that receives and supports at least one of the cylindrical base, the cylindrical quartz beaker, and the insulating material, wherein the furnace body includes a central longitudinal axis; and vi. A rotation drive system, the rotation drive system being operably connected to the bottom annular plate through an annular rotation gear system, wherein when the rotation drive system is engaged with the annular rotation gear system, the bottom annular plate together with the cylindrical base and the insulating material or the bottom annular plate together with the cylindrical base, the cylindrical quartz beaker and the insulating material are rotated circumferentially around the central longitudinal axis at a rotation frequency of 0.01 Hz to 10 Hz, and the heat output level of the base is determined according to the current level and the oscillation frequency.

2. The drawing furnace of claim 1 wherein said bottom annular plate includes concentric grooves to accommodate at least said cylindrical susceptor and said cylindrical quartz beaker.

3. The drawing furnace of claim 1 wherein the susceptor comprises graphite.

4. The drawing furnace of claim 1, wherein the glass element is one of an optical preform, a glass cylinder-core rod assembly, and a glass cylinder.

5. The drawing furnace of claim 1 wherein the bottom annular plate comprises quartz.

6. The drawing furnace of claim 1 wherein the top annular plate comprises quartz.

7. The drawing furnace of claim 1, wherein the rotation frequency is 0.05 Hz to 0.15 Hz.

8. The drawing furnace of claim 1, wherein the rotation frequency is 0.1 Hz.

9. A method of evenly distributing heat in a circumferential direction around a susceptor in a furnace during drawing of a glass element, the method comprising the steps of: A draw furnace is inductively heated by providing A / C current, the draw furnace comprising: a furnace body having an upper end including an opening and a lower end including an opening, The furnace body comprises: i. Top ring plate, ii. an A / C induction coil that is capable of receiving a certain level of current and generating an oscillating electronic signal that oscillates at a certain frequency, iii. a cylindrical base capable of producing a certain level of heat output, iv. Cylindrical quartz beaker, v. an insulating material disposed between the cylindrical base and the cylindrical quartz beaker, and vi. a bottom annular plate that receives and supports at least one of the cylindrical base, the cylindrical quartz beaker, and the insulating material, wherein the furnace body includes a central longitudinal axis; and vi. a rotary drive system operably connected to the bottom annular plate via an annular rotary gear system; and The rotary drive system and the annular rotary gear system are engaged to rotate the bottom annular plate so that the bottom annular plate together with the cylindrical base and the insulating material or the bottom annular plate together with the cylindrical base, the cylindrical quartz beaker and the insulating material rotate circumferentially around the central longitudinal axis at a rotation frequency of 0.01 Hz to 10 Hz, and the heat output level of the base is determined according to the current level and the oscillation frequency.

10. The method of claim 9, wherein the glass element is one of an optical preform, an optical fiber, and a drawn tube. The method of claim 10 , wherein the optical fiber is a hollow optical fiber.

12. The method of claim 9, wherein the rotation frequency is 0.1 Hz.

13. The method of claim 9, wherein the bottom annular plate comprises quartz.

14. The method of claim 9, wherein the top annular plate comprises quartz.

15. The method of claim 9, wherein the rotation frequency is 0.05 Hz to 0.15 Hz.

16. A method of forming an optical glass element in a furnace, the furnace comprising a furnace body having a top annular plate, an A / C induction coil, a cylindrical susceptor, a cylindrical quartz beaker, an insulating material disposed between the susceptor and the beaker, and a bottom annular plate; and a rotational drive system operably connected to the bottom annular plate by an annular rotation gear system, the method comprising: introducing a glass body into the furnace body through an opening in the top annular plate; engaging the rotary drive system to the annular rotary gear system and rotating the bottom annular plate with the cylindrical susceptor and the insulating material or the bottom annular plate with the cylindrical susceptor, the cylindrical quartz beaker and the insulating material circumferentially about the central longitudinal axis of the furnace body at a rotational frequency of 0.01 Hz to 10 Hz while the glass body is drawn through the furnace body; inductively heating the susceptor via a magnetic field generated by the A / C induction coil receiving an electric current; heating the glass body in the susceptor; as well as The produced optical glass elements are fed through the openings in the bottom annular plate.

17. The method of claim 16, wherein the glass body is one of a glass preform, a glass cylinder-core rod assembly, or a glass cylinder.

18. The method of claim 16, wherein the optical glass element is an optical fiber preform or an optical waveguide.

19. The method of claim 16, wherein the optical glass element is an optical fiber.

20. The method of claim 16, wherein the optical glass element is a tube.

21. The method of claim 16, wherein the rotation frequency is 0.05 Hz to 0.15 Hz.

22. The method of claim 16, wherein the rotation frequency is 0.1 Hz.

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