Title of Invention

APPARATUS AND METHOD FOR OVERCLADDING OPTICAL FIBER PREFORM ROD AND OPTICAL FIBER DRAWING METHOD

Abstract

There are provided an optical fiber rod overcladding and method, and an optical fiber drawing method. In the preform rod overcladding method, a preform rod (100) is clamped in a top chuck and leveled,and a glass tube (102) is mounted in a bottom chuck and leveled. The preform rod is coaxially inserted into a glass tube. Then, the glass tube is preheated by the furnace and heated by the burner until the glass tube reaches a softening point. The preform rod is completely sealed in the glass tube by sucking air in a clearance between the preform rod and the glass tube by application of a negative vacuum pressure. Thus, a preform is completed.

Full Text

APPARATUS AND METHOD FOR OVERCLADDING OPTICAL FIBER PREFORM ROD AND OPTICAL FIBER DRAWING METHOD BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a large-diameter optical fiber preform fabricating method, and in particular, to apparatus and method for overcladding an optical fiber preform rod, which reduces a preform fabrication time by preheating a glass tube with a furnace, heating the glass tube with an oxygen-hydrogen burner, and collapsing the glass tube onto the preform rod, and an optical fiber drawing method. 2. Description of the Related Art An optical fiber is generally fabricated in two processes: a preform rod is prepared and then a preform is fabricated by a rod-in-tube or overcladding method, in the first process; and an optical fiber having an outer diameter of 125 ,µ m is fusion-drawn from the fabricated preform, in the second process. The preform rod is fabricated by outside deposition or inside deposition. In the outside deposition process such as VAD (Vapor Phase Axial Deposition) process and OVD (Outside Vapor Deposition) process, SiO2 particles called soot are deposited onto a preform rod from the outside by hydrolyzing chemical gases -1A- such as SiCl4 and other dopants together with oxygen by flames, while supplying the gases to the preform rod. Then, this porous preform rod is fed into a furnace, dehydrated, and sintered, using Cl2 and He, thereby obtaining a transparent preform rod. On the other hand, in the inside deposition process including CVD (Chemical Vapor Deposition) process and MCVD (Modified CVD) process, a plurality of layers are deposited on the inner surface of a glass tube by providing SiCl2 and other dopants together with O2 into the tube, and the layer-deposited glass tube is heated at a high temperature and collapsed, while supplying Cl2 and He into the tube. Thus, a glass rod is obtained. The MCVD process is widely used and enables fabrication of a high-quality glass preform rod. The MCVD and CVD processes for fabrication of an optical fiber are limited in obtaining a preform rod having a diameter of about 23mm or above in view of their processing characteristics. Hence, to increase product yield, an overcladding method has been explored in which a glass tube is fusion-stuck to a preform rod prepared by the above inside deposition processes. To obtain a large-diameter preform, a prepared preform rod is inserted into a large-diameter glass tube, heated, and collapsed onto the glass tube in the rod-in-tube or overcladding method, as is well known and thus will not be described in detail. This is disclosed in detail in Korea Application No. 93-25712 entitled "Single-Mode Primary Overcladding Method and Apparatus), and U.S. Patent No. 4,820,322 entitled "Method of and Apparatus for Overcladding Glass Rod". Though there is no difficulty in inserting and overcladding a preform rod fabricated by the MCVD process into and with a glass tube having an outer - 2 - diameter of 70mm, the amount of heat required for overcladding increases with the outer diameter or thickness of the glass and, as a result, the overcladding rate of a burner for externally providing heat should slow down. The problem may be overcome by further lowering a vacuum pressure applied to the interface between the preform rod and the glass tube, but a very large negative pressure brings about degradation of concentricity and circularity in the cross section of a preform. On the other hand, the heat energy provided from outside can be increased simply by increasing the supply flow rate of a current oxygen-hydrogen burner. However, the outer surface of the glass tube is softened, resulting in a lower viscosity, while the inner surface thereof is rather slowly softened, keeping a predetermined viscosity. Therefore, the surface of the glass tube may be deformed by the flame pressure of the oxygen-hydrogen burner at the increased supply flow rate, or contaminating particles may adhere from the burner to the surface of the large-diameter glass tube. The oxygen-hydrogen burner cannot transfer heat sufficiently to the surface of the glass tube due to its relatively short hot zone, and brings about a non-uniform temperature distribution on the periphery of the glass tube. Hence, geometric irregularities such as ovality in the cross section of the glass tube occurs, and the difference between the viscosities of the outer and inner surfaces of the glass tube increases microbending loss. Moreover, product yield is remarkably lowered because about 2-4 hours is required to fabricate a preform. From the preform fabricated in the overcladding method, an optical fiber having an outer diameter of 125µ m is fusion-drawn at a predetermined linear velocity under a predetermined tension load. The keypoint of the drawing process is to increase productivity per unit time by increasing the linear velocity, and a -3- current linear velocity is usually 600-1200m/min. However, the above optical fiber drawing method has distinct drawbacks in that mass production of optical fibers is impossible due to the low linear velocity, and product yield decreases and optical fiber cost increases by adding the overcladding process for fabrication a preform from a preform rod before drawing an optical fiber. SUMMARY OF THE INVENTION To solve the conventional problems, a first object of the present invention is to provide apparatus and method for overcladding an optical fiber preform rod, which prevents uniformless temperature distribution on the periphery of a glass tube by transferring sufficient heat to a glass tube by a furnace having a wide hot zone, and ensures concentricity in the cross section of a preform by stably collapsing the glass tube onto the preform rod through application of oxygen and hydrogen pressures, in fabrication of the optical fiber preform. A second object of the present invention is to provide apparatus and method for overcladding an optical fiber preform rod, which enables fabrication of a highly strong optical fiber by protecting the surface of the preform rod from contaminating particles with use of oxygen and hydrogen at low flow rates in fabrication of an optical fiber rod. A third object of the present invention is to provide apparatus and method for overcladding a preform rod, which can increase the collapsing rate of a large- -4- diameter glass tube by five times in maximum by increasing a total heat energy in fabrication of an optical fiber preform. A fourth object of the present invention is to provide an optical fiber preform rod overcladding method which can fabricate a preform regardless of the outer diameter of a glass tube. A fifth object of the present invention is to an optical fiber preform rod overcladding method for simply overcladding a preform rod with a glass tube. A sixth object of the present invention is to provide an optical fiber preform rod overcladding method which can increase the product yield of preforms. A seventh object of the present invention is to provide an optical fiber preform rod overcladding method which uniformly softens a glass tube from outer surface and inner surface. An eighth object of the present invention is to provide an optical fiber preform rod overcladding method which enables fabrication of a highly strong preform by protecting the surface of a glass tube from contaminants generated from a burner. A ninth object of the present invention is to provide an optical fiber preform rod overcladding method which can prevent the interface between a preform rod and a glass tube from stresses by controlling the viscosity therebetween. -5- A tenth object of the present invention is to provide an optical fiber drawing method which can reduce an optical fiber fabrication time. An eleventh object of the present invention is to provide an optical fiber drawing method which can draw an optical fiber successively without an overcladding process. To achieve the above object, there is provide an optical fiber rod overcladding apparatus. The overcladding apparatus includes a vertical lathe, a chuck installed in each end of the vertical lathe, a carriage in the vertical lathe, for vertically moving between both ends of the vertical lathe, an oxygen-hydrogen burner installed in the carriage, a furnace installed in the carriage, a vacuum pump provided at an end of the vertical lathe, a coupler for connecting the vacuum pump to the end of the vertical lathe, and a controller outside the vertical lathe, for controlling the vertical movement of the carriage, the flow rate of the oxygen-hydrogen burner, and the rotation of the chucks. Here, the furnace preheats or heats a glass tube to overclad a preform rod with the glass tube. According to another aspect of the present invention, there is provided an optical fiber preform rod overcladding method. In the overcladding method using the above overcladding apparatus, a preform rod is clamped in the top chuck and leveled, a glass tube is mounted in the bottom chuck and leveling the glass tube, the preform rod is coaxially inserted into a glass tube. The glass tube is preheated by the furnace and heated tube by the burner until the glass tube is softened. The preform rod is completely sealed in the glass tube by sucking air in a clearance between the preform rod and the glass tube by application of a negative vacuum -6- pressure, to thereby complete a preform. According to still another aspect of the present invention, there is provided an optical fiber drawing method. In the optical fiber drawing method, ends of a preform rod and a glass tube are sealed, the preform rod and the glass tube are clamped in a chuck provided to a feed module in an optical fiber drawing apparatus, and a vacuum pump is connected to the sealed ends thereof. The sealed ends of the preform rod and the glass tube is aligned in a hot zone of a furnace in the optical fiber drawing apparatus. Then, the glass tube is collapsed onto the preform rod by preheating the sealed ends of the preform rod by the furnace, heating the sealed ends thereof until the sealed ends thereof are softened, and sealing a clearance between the preform rod and the glass tube, thereby forming a preform. An optical fiber is drawn from the preform by the furnace, and the outer diameter of the drawn optical fiber is measured. Then, the optical fiber is cooled and coated with a curing resin. BRIEF DESCRIPTION OF THE ACCOMPAYING DRAWINGS The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: FIG. 1 is a perspective view of a furnace-having overcladding apparatus according to a first preferred embodiment of the present invention, for use in fabrication of an optical fiber preform; FIG. 2 is a view which depicts a process of making a preform out of a preform rod in the furnace-having overcladding apparatus according to the first -7 - preferred embodiment of the present invention; FIG. 3 A is a sectional view of FIG. 2, taken along line A-A; FIG. 3B is a sectional view of FIG. 2, taken along line B-B; FIG. 3C is a sectional view of a preform fabricated by the furnace-having overcladding apparatus according to the first preferred embodiment of the present invention; FIG. 4 is a view which depicts a process of fabricating the preform, while controlling the viscosity between a preform rod and a glass tube by injecting a glass forming material therebetween, in a furnace-having overcladding device according to a second preferred embodiment of the present invention; and FIG. 5 is a view which depicts a process of drawing an optical fiber successively using a furnace, according to a third preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail referring to the attached drawings. Like reference numerals denote the same components in the drawings, and a detailed description of related known function and structure of the present invention will be avoided if it is deemed to obscure the subject matter thereof. FIG. 1 is a perspective view of a furnace-having overcladding apparatus according to a first preferred embodiment of the present invention, and FIG. 2 is a view which depicts a process of making a preform out of a preform rod in the furnace-having overcladding apparatus shown in FIG. 1. FIGs. 3A, and 3B are sectional views of FIG. 2, taken along lines A-A and B-B, respectively. FIG. 3C is a sectional view of a preform fabricated by the furnace-having overcladding apparatus shown in FIG. 1. Referring to FIG. 1, the overcladding apparatus according to the first embodiment of the present invention has a vertical lathe 10 including chucks 20 and 30 for vertically supporting a glass tube 102 and a preform rod 100, a carriage 60 installed in the vertical lathe 10, for moving up and down, an oxygen-hydrogen burner 40 fixed to the carriage 60, for heating the preform rod 100 and the glass tube 102, a furnace 50 disposed under the burner 40 in the carriage 60, for heating or preheating the preform 100 and the glass tube 102, a vacuum pump 114 connected to one end of the vertical lathe 10 by a coupler, a controller (not shown) for controlling rotation of the glass tube 102 gripped in the chuck 30, the vertical movement speed of the carriage 60, the flow rate of the burner 40, and the pressure of the vacuum pump 114, and a power supply connected to the furnace 50 via bus bars 53 and cables 55, for supplying a power supply voltage to the furnace 50. Components of the optical fiber preform rod overcladding apparatus as constituted above will be described in more detail. A guide rod 11 and a transfer means (not shown) are provided to the vertical lathe 10, for vertically moving the carriage 60, and the top and bottom chucks 20 and 30 are disposed in both ends of the vertical lathe 10. The top chuck 20 functions to rotatably fix the preform rod 100, while the bottom chuck 30 functions to rotatably fix the glass tube 102. The carriage 60 having the burner 40 fixed -9- thereto vertically moves along the guide rod 11 in the vertical lathe 10. The furnace 50 is positioned under the burner 40, and a flexible ventilating duct 42 is provided above the burner 40. That is, the duct 42, the burner 40, and the furnace 50 are integrally stacked in the carriage 60, and their vertical movements are controlled by the controller. The furnace 50 has a heat emitting body of graphite therein and emits heat generally in the range of 2000-2500 °C upon receipt of a power supply voltage from the power supply. The heat transfers to the glass tube 102 and the preform rod 100 by radiation and forms hot zones on them. A manipulation unit 54 is installed along the longitudinal axis of the furnace 50, to facilitate user manipulation. The furnace 50 has a vertically longer heat emitting portion and a smaller thickness than that used in an optical fiber drawing process, in order to maximize transfer of radiated heat. The furnace 50 can be thinner by partially decreasing the thickness of a liner (not shown) in the furnace 50. Graphite (C) of an electrical resistance type or zirconia (ZrO2) of an induction type is used for the heat emitting body of the furnace 50. A plurality of tubes 58 are connected to the furnace 50 to introduce He, Ar, or a mixed gas thereof (He+Ar) into the furnace 50. On the top and bottom of the furnace 50, cover flanges 52 and conductor flanges 51a and 51b are assembled, respectively. The conductor flanges 51a and 51b are connected to a plurality of bus bars 53, for receiving a power supply voltage via the cables 55 from the power supply, and firmly fixed to the furnace 50 by engaging tie bars 56 in corners thereof. The He or Ar gas being an inert gas is injected into the furnace 50 heated at -10- a high temperature to prevent oxidation of the graphite from occurring on the outer surface of the preform rod 100 or the glass tube 102, and makes heat temperature distribution uniform on the outer surface of the preform rod 100 and the glass tube 102 due to its excellent thermal conductivity. A pyrometer 57 having a temperature sensor therein is installed in the body of the furnace 50, to sense the internal temperature of the furnace 50. A cooling line (not shown) is also provided to the furnace 50 to cool down the furnace 50 heated at a high temperature. The above preform rod overcladding apparatus should be installed in such aplace that keeps its ambient temperature between 0-40 °C and its ambient humidity within 50%. In the thus-constituted overcladding apparatus, the prepared preform rod 100 is clamped in the top chuck 20 using a handle rod and vertically leveled. Then, one end of the large-diameter glass tube 102 is connected to a dummy tube (not shown), the dummy tube is fixedly mounted in the bottom chuck 30, and the glass tube 102 is vertically leveled. Subsequently, the preform rod 100 fixed to the top chuck 20 is moved down to be coaxially inserted into the glass tube 102 under the control of the controller. A hot zone of the furnace 50 is positioned around a predetermined portion of the glass tube 102 having the preform rod 100 inserted therein by moving the carriage 60 under the control of the controller. The furnace 50 preheats the predetermined portion of the glass tube 102 for 10-30 minutes with an inert gas and a power supply voltage supplied thereinto, while the combined preform rod 100 and glass tube 102 are being rotated at 20-30rpm by driving both chucks 20 and 30 under the control -11 - of the controller. At this time, the oxygen-hydrogen burner 40 above the furnace 50 turns on at an initial gas flow rate. When the preheated portion of the glass tube 102 is lowered in viscosity and softened, the preform rod 100 is entirely sealed in the glass tube 102 by sucking air in the clearance therebetween using the vacuum pump 114 under the control of the controller. The interface between the preform rod 100 and the glass tube 102 can be stress-relieved by flowing SiCl4 and O2 through the clearance therebetween and enabling deposition of a contact material such as a glass forming material POC14 thereinto. Then, the carriage 60 is moved down, and the flow rates of the oxygen-hydrogen burner 40 are increased to 75LPM for oxygen and 150LPM for hydrogen. The carriage 60 moves down at a slightly increasing velocity from 1CPM to 3-5CPM, and thus the glass tube 102 is integrally collapsed onto the whole length of the preform rod 100. Then, the furnace 50 is tuned off, and the oxygen-hydrogen burner 40 is positioned around a connection portion between the glass tube 102 and the dummy tube and softens the connection portion by heating it for 3-5 minutes with oxygen at 75LPM and hydrogen at 150LPM. The top chuck 20 is moved up slowly at l-3mm/min and the softened connection portion becomes thin. When the outer diameter of a preform under fabrication reaches a 2/3th of a completed preform, the preform collapsed in the dummy tube is completely removed from the dummy tube by rapidly moving up the top chuck 20 using the manipulation unit 54. The completed preform is extracted from the chucks 20 and 30 and cooled down in a retainer for a predetermined time. Thus, the preform rod overcladding process is completed. -12- Referring to FIGs. 2 and 3A-3C, an overcladding process for fabrication of a preform using the above furnace-having overcladding apparatus will be described. The preform rod 100 is fabricated by outside deposition or inside deposition, and the glass tube 102 is a natural quartz tube or a synthesized quartz tube having an inner diameter of 10mm or above and a limitless outer diameter. Then, the preform rod 100 is clamped in the top chuck 20 and vertically leveled. One end of the large-diameter glass tube 102 is connected to a dummy tube (not shown), the dummy tube is mounted in the bottom chuck 30, and the glass tube 102 is vertically leveled. Here, there is a clearance 108 between the preform rod 100 and the glass tube 102 as shown in FIG. 3B. Subsequently, the preform rod 100 fixed to the top chuck 20 is moved down to be coaxially inserted into the glass tube 102 under the control of the controller. A hot zone of the furnace 50 is positioned around a predetermined portion of the glass tube 102 having the preform rod 100 inserted therein by moving the carriage 60 under the control of the controller. A power supply voltage is supplied to the furnace 60 and an inert gas such as Ar, He, or N is injected into the clearance 108 at 5-101/min, while the preform rod 100 and the glass tube 102 are being rotated in the chucks 20 and 30. When the surface of the glass tube 102 reaches 1700°C, the burner 40 decreases its heat temperature with oxygen at 51/min and hydrogen at 101/min and heats the surface of the glass tube 102, while moving the carriage 60 down at a velocity of 3-5cm/min. Then, the inner surface of the glass tube 102 is heated by heat transfer from the outer surface thereof and the inert gas, thus burning away microdusts stuck on the inner surface thereof. Microdusts are also burnt away from the outer surface of the preform rod 100 which is heated by heat transfer from -13- the inner surface and the inert gas. Subsequently, the predetermined portion of the glass tube 102 is preheated for 10-30 minutes by supplying an inert gas into the furnace 50 while the glass tube 102 and the preform rod 100 are rotated at 20-30rpm in the both chucks 20 and 30. At this time, the burner 40 above the furnace 50 is lighted with hydrogen at 30LPM and oxygen at 15LPM. Then, the preform rod 100 and the glass tube 102 are configured as shown in FIG. 3B. When the preheated portion of the glass tube 102 drops in viscosity and is softened, air is sucked away from the clearance 108 between the preform rod 100 and the glass tube 102 by driving the vacuum pump 114 under the control of the controller, completely sealing the preform rod 100 in the glass tube 102. Then, the carriage 60 is moved down at a slightly increasing velocity from 1CPM to 3-5CPM and, simultaneously, the oxygen and. hydrogen flow rates of the burner 40 are increased to 75LPM and 150LPM, respectively, so that the glass tube 102 is collapsed onto the whole length of the preform rod 100, while they are rotated at a predetermined peripheral velocity. The furnace 50 is turned off, and the burner 40 is disposed around the connection portion between the glass tube 102 and the dummy tube and softens the connection portion at 75LPM of oxygen and 150LPM of hydrogen for 3-5 minutes. When the connection portion between the dummy tube and the glass tube 102 is softened, it becomes thin by slowly moving up the top chuck 20 at l-3mm/min. When the outer diameter of a preform under fabrication reaches a 2/3th of the outer diameter of a completed preform 112, the preform 112 collapsed in the dummy tube is completely removed from the dummy tube by rapidly moving up the top chuck 20 using the manipulation unit 54, and cooled -14- down in a retainer for a predetermined time. Thus, the overcladding process is over and the preform 112 is configured as shown in FIG. 3C. FIG. 4 schematically illustrates an overcladding process for fabrication of a preform, controlling the viscosity between a preform rod and a glass tube by injecting a glass forming material into a clearance between them, in a furnace-having overcladding apparatus according to a second embodiment of the present invention. Referring to FIG. 4, the preform rod 100 is fabricated by outside deposition or inside deposition, and the glass tube 102 is a natural quartz tube or a synthesized quartz tube having an inner diameter of 10mm or above and a limitless outer diameter. Then, the preform rod 100 is clamped in the top chuck 20 and vertically leveled. One end of the glass tube 102 is connected to a dummy tube (not shown), the dummy tube is mounted in the bottom chuck 30, and the glass tube 102 is vertically leveled. Here, there is the clearance 108 between the preform rod 100 and the glass tube 102. Subsequently, the preform rod 100 fixed to the top chuck 20 is moved down to be coaxially inserted into the glass tube 102 under the control of the controller. A hot zone of the furnace 50 is positioned around a predetermined portion of the glass tube 102 having the preform rod 100 inserted therein by moving the carriage 60 under the control of the controller. A power supply voltage is supplied to the furnace 60 and an inert gas such as Ar, He, or N is injected into the clearance 108 at 5-101/min by a rotary union 106, while the preform rod 100 and the glass tube 102 are being rotated in the chucks 20 and 30. When the surface of the glass tube -15- 102 reaches 1700°C, the burner 40 decreases its heat temperature with oxygen at 51/min and hydrogen at 101/min and heats the surface of the glass tube 102, while moving the carriage 60 down at a velocity of 3-5cm/min. Then, the inner surface of the glass tube 102 is heated by heat transfer from the outer surface thereof and the inert gas, thus burning away microdusts stuck on the inner surface thereof. Microdusts are also burnt away from the outer surface of the preform rod 100 which is heated by heat transfer from the inner surface and the inert gas. Then, to control the viscosity at the interface between the glass tube 102 and the preform rod 100, a glass forming material 110 is injected into the clearance 108 together with SiCl4. That is, a mass flow controller 104 supplies SiCl4 at 500mg/min and POC14 at 30mg/min, or both gases and freon or/and boron at a predetermined rate to the rotary union 106. Then, the rotary union 106 mixes SiCl4 with the glass forming material 110 and injects the mixture into the clearance 108. SiCl4 and the glass forming material 110 are combined as in the following chemical formula, and controls the viscosity at the interface. (Chemical Formula) SiCl4 + POC13 + O2 - SiO2 + P2O5 + 2C12? (gas) SiCl4 + POCl3 + Freon + O2 - SiO2 + P2O5 + F + 2C12? (gas) SiCl4 + POC13 + Freon + Boron + O2 - SiO2 + P2O5 + F + B2O3 + 2C12 ? (gas) Simultaneously, a silica layer having a matching viscosity is slowly deposited in the clearance 108 by heating SiCl4 and the glass forming material 110 in the clearance 108. Here, the surface of the glass tube 102 is heated at 1800°C -16- and the velocity of the burner 40 is 1.5-2cm/min. Subsequently, the predetermined portion of the glass tube 102 is preheated for 10-30 minutes by supplying an inert gas into the furnace 50 while the glass tube 102 and the preform rod 100 are rotated at 20-30rpm in the both chucks 20 and 30. At this time, the burner 40 above the furnace 50 is lighted with hydrogen at 30LPM and oxygen at 15LPM. When the preheated portion of the glass tube 102 drops in viscosity and is softened, air is sucked away from the clearance 108 between the preform rod 100 and the glass tube 102 by driving the vacuum pump 114 under the control of the controller, completely sealing the preform rod 100 in the glass tube 102. Then, the carriage 60 is moved down at a slightly increasing velocity from 1CPM to 3-5CPM and, simultaneously, the oxygen and hydrogen flow rates of the burner 40 are increased to 75LPM and 150LPM, respectively, so that the glass tube 102 is collapsed onto the whole length of the preform rod 100, while they are rotated at a predetermined peripheral velocity. The furnace 50 is turned off, and the burner 40 is disposed around the connection portion between the glass tube 102 and the dummy tube and softens the connection portion at 75LPM of oxygen and 150LPM of hydrogen for 3-5 minutes. When the connection portion between the dummy tube and the glass tube 102 is softened, the connection portion becomes thin by slowly moving up the top chuck 20 at l-3rnm/min. When the outer diameter of a preform under fabrication reaches a 2/3th of the outer diameter of the completed preform 112, the preform 112 collapsed in the dummy tube is completely removed from the dummy tube by rapidly moving up the top chuck 20 using manipulation unit 54 and cooled down in a retainer for a predetermined time. Thus, the -17- overcladding process is over. In the overcladding method, the thus-fabricated preform 112 absorbs external impacts and the interface between the preform rod 100 and the glass tube 102 is stress-relieved. FIG. 5 is a view which depicts a method for drawing an optical fiber successively using a furnace without an overcladding process according to a third embodiment of the present invention. In the optical fiber drawing method, a preform rod 158 is inserted into a glass tube 156, while being leveled. With ends thereof sealed together, the combined preform rod 158 and glass tube 156 are clamped in a chuck 154 provided in a feed module 150 of the optical fiber drawing apparatus. Then, the sealed ends of the preform rod 158 and the glass tube 156 are connected to a vacuum pump 152, and the sealed ends thereof are vertically fed into a furnace 162 to be aligned with a hot zone of the furnace 162. Here, to increase thermal conductivity, the furnace 162 is formed of graphite. Then, the furnace 162 operates by turning on the power supply, and Ar gas is injected at about 101/min into the furnace 162 to prevent oxidation of the graphite therein. The sealed ends of the preform rod 158 and the glass tube 156 are preheated by the furnace 162 for about 20 minutes or longer, heated, and softened. The feed module 150 having the preform rod 158 fixed thereto is operated, while applying a pressure of about -700mm Bar from the vacuum pump 152 to the interface between the glass tube 156 and the preform rod 158. Thus, an optical fiber 160 having an outer diameter of 125 µ m is drawn -18- from under the furnace 162, and a collapse process continues, while the preform rod 158 and the glass tube 156 are being sealed by heating the glass tube 156 in the hot zone of the furnace 162 and applying the negative pressure. The feed module 150 feeds down the preform rod 15 8 and the glass tube 156 which are not yet sealed into the furnace 162, as long as the optical fiber 160 is drawn from them. An outer diameter measuring instrument 164 measures the diameter of the optical fiber 160 to determine whether the diameter is a predetermined value, generally, 125 µ m, and notifies a diameter controller (not shown) of the measurement. Then, the diameter controller controls a capstan 116 to maintain the diameter of the optical fiber 156 to be 125 µ m and the capstan 176 controls the tension of the optical fiber 160 under the control of the diameter controller. To protect the optical fiber 160 rapidly cooled down in a cooler 166, the descending optical fiber 160 is coated with acrylic resin or silicon resin. The coated optical fiber 160 is cured by an ultraviolet curer 170, and wound around a spool 174 by the drawing force of the capstan 172. Thus, the optical fiber drawing process is completed. As described above, the optical fiber preform rod overcladding method and apparatus, and the optical fiber drawing method according to the present invention have the following advantages. (1) Non-uniform temperature distribution on the surface of a glass tube is prevented and the glass tube can be uniformly and stably collapsed by transferring sufficient heat to the surface thereof with a furnace having a wider hot zone than an oxygen-hydrogen burner in prior art. -19- (2) The viscosities of a preform rod and a large-diameter glass tube at their interface are equal to each other with sufficient heat energy, thereby remarkably reducing microbending loss. (3) The surface of the preform rod may be contaminated by contaminants of the oxygen-hydrogen burner during a process in the prior, whereas the surface thereof can be protected against the contaminants using oxygen and hydrogen at low flow rates (i.e., 1001/min and 2001/min, respectively. Therefore, a highly strong optical fiber can be fabricated. (4) A preform can be fabricated to be concentric in its cross section with a uniform temperature distribution in the furnace because the surface of the large- diameter glass tube is heated by the furnace and collapsed by the pressure of the oxygen-hydrogen burner. (5) The collapse rate of the glass tube may be increased by five times in maximum due to a larger amount of total heat supply than in the prior art, processing can be automated, and an optical fiber fabrication time can be remarkably reduced. (6) Any preform rod can be overclad regardless of its size. (7) The collapse process is facilitated by means of a vacuum pump, and the interface between the preform rod and the glass tube is stress-relieved by flowing SiCl4 & O2 and a glass forming material POC13, boron, or freon through a clearance therebetween and thus enabling deposition of a contact material. -20- (8) An optical fiber can be successively drawn without an overcladding process because the furnace of the present invention is provided in a location where a preform rod is installed in an optical fiber drawing apparatus. As a result, a fabrication time is remarkably reduced and product yield is increased. While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. -21 - 530/CAL/98 WE CLAIM: 1. An apparatus for overcladding an optical fiber perform rod, comprising: a vertical lathe (10); two chucks (20,30) each installed in an end of the vertical lathe; a carriage (60) in the vertical lathe, for vertically moving between both ends of the vertical lathe; an oxygen-hydrogen burner (40) installed in the carriage (60); a furnace (50) installed in the carriage; a vacuum pump (114) provided at an end of the vertical lathe; a coupler for connecting the vacuum pump to the end of the vertical lathe; and a controller outside the vertical lathe, for controlling the vertical movement of the carriage, the flow rate of the oxygen-hydrogen burner, and the rotation of the chucks; characterized in that, said furnace (50) is adapted to preheat or heat a glass tube (102) to overclad a perform rod (100) with said glass tube (102). 2. The apparatus as claimed in Claim 1 wherein the furnace is disposed under the burner in the carriage. 3. The apparatus as claimed in Claim 1 wherein the furnace receives a power supply voltage from a power supply and has a graphite heat emitting body. 4. The apparatus as claimed in......................................................... - 22- claim 1, wherein the furnace prevents oxidation of the preform rod and the glass tube, using an inert gas which is one of helium (He), argon ,(Ar), a mixture of helium and argon, and nitrogen (N2). 2(1)5 5. A method for overcladding an optical fiber preform rod in an optical fiber preform rod overcladding apparatus having a vertical lathe including top and bottom chucks, a ring-shaped oxygen-hydrogen burner, a furnace for heating or preheating a glass tube, and a carriage for vertically moving between both chucks, a vacuum pump disposed at one end of the vertical lathe, a coupler connected between the vacuum pump and one of the chucks, and a controller for controlling for the vertical movement of the carriage, the flow rate of the oxygen-hydrogen, and the rotation of the chucks, the method comprising the steps of: clamping the preform rod in the top chuck and leveling the preform rod; mounting a glass tube in the bottom chuck and leveling the glass tube; coaxially inserting the preform rod into a glass tube; preheating the glass tube by the furnace and heating the glass tube by the burner until the glass tube reaches a softening point; and completely sealing the preform rod in the glass tube by sucking air in a clearance between the preform rod and the glass tube by application of a negative vacuum pressure, to thereby complete a preform. 6. The method as claimed in claim 5, wherein the preform rod is fabricated by one of outside deposition and inside deposition. 7. The method as claimed in claim 5, wherein the glass tube is one of a synthesized quartz tube and a natural quartz tube. -23- 8. The method as claimed in claim 7, wherein the glass tube has an inner diameter of 10mm or larger. 9. The method as claimed in claim 5, wherein foreign materials are removed from the clearance between the preform rod and the glass tube by injecting an inert gas thereinto. 10. The method as claimed in claim 9, wherein foreign material stuck to the outer surface of the preform rod are removed by heat transferred from the surface of the glass tube and the inert gas. 11. The method as claimed in claim 9, wherein foreign material stuck to the inner surface of the glass tube are removed by heat generated from one of the furnace and the burner and the inert gas. r 12. The method as claimed in claim 9, wherein the inert gas is one of He, Ar, and N. 2(1)5 \ ' T> I 13. A method for overcladding an optical fiber preform rod in an optical fiber preform rod overcladding apparatus having a vertical lathe including top and bottom chucks, a ring-shaped oxygen-hydrogen burner, a furnace for heating or preheating a glass tube, and a carriage for vertically moving between both chucks, a vacuum pump disposed at one end of the vertical lathe, a coupler connected between the vacuum pump and one of the chucks, and a controller for controlling for the vertical movement of the carriage, the flow rate of the oxygen-hydrogen, and the rotation of the chucks, the method comprising the steps of: -24- clamping the preform rod in the top chuck and leveling the preform rod; mounting a glass tube in the bottom chuck and leveling the glass tube; coaxially inserting the preform rod into a glass tube; depositing a silica layer having a matching viscosity in a clearance between the preform rod and the glass tube, while controlling the viscosity of the preform rod and the glass tube by injecting SiCl4 and a glass forming material into the clearance between the preform rod and the glass tube; preheating the glass tube by the furnace and heating the glass tube by the burner until the glass tube reaches a softening point; and completely sealing the preform rod in the glass tube by sucking air in the clearance between the preform rod and the glass tube by application of a negative vacuum pressure, to thereby complete a preform. 14. The method as claimed in claim 13, wherein the glass forming material is one of POCl3, freon, and boron having a low viscosity. 15. The method as claimed in claim 13, wherein SiCl4 and POC13 are injected into the clearance between the preform rod and the glass tube. 16. The method as claimed in claim 13, wherein SiCl4, POC13, and freon are injected into the clearance between the preform rod and the glass tube. 17. The method as claimed in claim 13, wherein SiCl4, POC13, freon, and boron are injected into the clearance between the preform rod and the glass tube. 18. An optical fiber drawing method comprising the steps of: -25- sealing ends of a preform rod and a glass tube, clamping the preform rod and the glass tube in a chuck provided to a feed module in an optical fiber drawing apparatus, and connecting a vacuum pump to the sealed ends thereof; aligning the sealed ends of the preform rod and the glass tube in a hot zone of a furnace in the optical fiber drawing apparatus; collapsing the glass tube onto the preform rod by preheating the sealed ends of the preform rod by the furnace, heating the sealed ends thereof until the sealed ends thereof are softened, and sealing a clearance between the preform rod and the glass tube, thereby forming a preform; drawing an optical fiber from the preform by the furnace; and measuring the outer diameter of the drawn optical fiber, cooling the optical fiber, and coating the optical fiber with a curing resin. 19. The optical fiber drawing method as claimed in claim 18, wherein the furnace is formed of graphite. 20. The optical fiber drawing method as claimed in claim 19, wherein an argon gas is injected into the furnace. 21. The optical fiber drawing method as claimed in claim 19, wherein the feed module in the optical fiber drawing apparatus moves down the preform rod and the glass tube as long as the optical fiber is drawn. 21. -26- There are provided an optical fiber rod overcladding and method, and an optical fiber drawing method. In the preform rod overcladding method, a preform rod (100) is clamped in a top chuck and leveled,and a glass tube (102) is mounted in a bottom chuck and leveled. The preform rod is coaxially inserted into a glass tube. Then, the glass tube is preheated by the furnace and heated by the burner until the glass tube reaches a softening point. The preform rod is completely sealed in the glass tube by sucking air in a clearance between the preform rod and the glass tube by application of a negative vacuum pressure. Thus, a preform is completed.

Documents:

00530-cal-1998-abstract.pdf

00530-cal-1998-claims.pdf

00530-cal-1998-correspondence.pdf

00530-cal-1998-description(complete).pdf

00530-cal-1998-drawings.pdf

00530-cal-1998-form-1.pdf

00530-cal-1998-form-2.pdf

00530-cal-1998-form-3.pdf

00530-cal-1998-form-5.pdf

00530-cal-1998-pa.pdf

00530-cal-1998-priority document(other).pdf

00530-cal-1998-priority document.pdf

530-cal-1998-granted-abstract.pdf

530-cal-1998-granted-claims.pdf

530-cal-1998-granted-correspondence.pdf

530-cal-1998-granted-description (complete).pdf

530-cal-1998-granted-drawings.pdf

530-cal-1998-granted-examination report.pdf

530-cal-1998-granted-form 1.pdf

530-cal-1998-granted-form 2.pdf

530-cal-1998-granted-form 3.pdf

530-cal-1998-granted-form 5.pdf

530-cal-1998-granted-letter patent.pdf

530-cal-1998-granted-pa.pdf

530-cal-1998-granted-priority document.pdf

530-cal-1998-granted-reply to examination report.pdf

530-cal-1998-granted-specification.pdf

530-cal-1998-granted-translated copy of priority document.pdf