The present invention relates to a furnace and a method for drawing an optical fiber from heated and molten preform.
Various techniques have been known relating to a furnace for drawing an optical fiber from heated preform formed with silica glass as a main component, and they are described in Japanese Patent No. 2,542,679, Japanese Patent Laid-Open No. 147969/1993 and Japanese Patent Laid-Open No. 2832/1997. Since the figures and terminology in these prior arts differ partially from those used in this invention, descriptions will be made below where they are interpreted according to the figures and terminology used in this invention in order to clarify the difference between the prior arts and this invention.
A main part of the furnace for drawing an optical fiber disclosed in Japanese Patent No. 2,542,679 is shown in FIG. 8, in which numeral 21 denotes a preform, 21a denotes an optical fiber, 22 denotes an inner tube, 22a denotes a gas blowing inlet, 23 denotes an outer tube, 24 denotes a gas supplying inlet, 24a denotes a gas passage, 25 denotes a dummy rod, 25a denotes a connecting part, 26 denotes a retainer, 27 denotes a seal piston, 28 denotes a muffle tube, and 29 denotes a heater.
In the furnace for drawing an optical fiber, the dummy rod 25 and the preform 21 are arranged inside the muffle tube 28 and the inner tube 22 arranged to be connected to an upper end of the muffle tube 28, while they are connected via the connecting part 25a and are descended together. The vicinity of a lower end of the preform 21 is melted through heating by the heater 29 arranged outside the muffle tube 28, and the optical fiber 21a is drawn downward from the lower end of the preform 21. The inner tube 22 arranged to be connected to the upper end of the muffle tube 28 is to contain the long preform 21 on starting the drawing.
The gas passage 24a is formed between the inner tube 22 and the outer tube 23 arranged outside the same, and an inert gas is supplied from the gas supplying inlet 24 to the gas passage 24a, so as to blow the inert gas into the inside of the inner tube 22 from the numerous gas blowing inlets 22a provided circumferentially and in the height direction on the wall surface of the inner tube 22. The inert gas is flowed inside the inner tube 22 and the muffle tube 28 to prevent oxidation deterioration of the muffle tube and the like, and when the temperature distribution of the inert gas by heating and the flow of the inert as are not uniform, the fluctuation of the diameter of the optical fiber drawn from the preform is liable to occur.
Therefore, in this example of the furnace for drawing an optical fiber, the seal piston 27 connected to the dummy rod 25 via the retainer 26 and moved with the dummy rod 25 is provided at the dummy rod 25 arranged on the upper part of the preform 21. In the beginning of drawing, the dummy rod 25 and the seal piston 27 are in the upper part because the preform is long. With proceeding the drawing, the preform 21 is shortened from the lower end, and is descended, and thus, the dummy rod 25 and the seal piston 27 are also descended.
In this case, if the seal piston 27 were not present, the space between the dummy rod 25 and the inner tube 22 would gradually increase, but because the seal piston 27 is present, the volume of space above the preform 21 is substantially constant. Therefore, it has been said that turbulence of the stream of the inert gas in the space between the preform 21 and the seal piston 27 occurs only scarcely due to the provision of the seal piston 27.
In the furnace using the seal piston for drawing an optical fiber, when the preform has a length of 1.5 m or more, the seal piston necessarily has an proportionate length, and the weight thereof becomes also heavy. Because a supporting member for supporting them at the upper part must withstand the weight of the preform and the seal piston, the supporting member becomes also necessarily large. Because the seal piston must resist to high temperature, a heat resistant material, such as carbon, quartz and the like, is necessarily used for the seal piston, and it becomes costly when it is of large scale.
Furthermore, because the seal piston moves as sliding on the inner wall surface of the inner tube, dust is liable to come from the sliding part, which may adversely affect the strength of the drawn optical fiber.
Furthermore, as the seal piston descents, the numerous gas blowing inlets provided in the wall surface of the inner tube are sealed one by one from the upper part with the seal piston, a precise controller for continuously controlling the gas flow rate is necessary to maintain the constant flow rate of the stream of the inert gas.
A furnace for drawing an optical fiber disclosed in Japanese Patent Laid-Open No. 147969/1993 will then be described. A main part of the furnace for drawing an optical fiber is shown in FIG. 9. In FIG. 9, the same references as in FIG. 8 show the same components. Numeral 30 denotes a separating plate, 30a denotes a gap, 30b denotes pores, 31a denotes an upper space, 31b denotes a lower space, and 32 denotes an upper lid. The furnace for drawing an optical fiber shown in FIG. 9 is different from that shown in FIG. 8 in the following points. There is no member corresponding to the seal piston in FIG. 8, and an upper end of an inner tube 22 is closed with the upper lid 32 except for the part through which a dummy rod 25 penetrates.
In the furnace for drawing an optical fiber, a preform 21 is arranged inside a muffle tube 28 and the inner tube 22 connected to an upper end thereof, and the preform 21 is supported by hanging by the dummy rod 25 through a connecting part 25a. The vicinity of the lower end of the preform 21 is heated and melted by a heater 29 arranged outside the muffle tube 28, and an optical fiber 21a is drawn downward.
An outer tube 23 is arranged concentrically outside the inner tube 22, and an inert gas is blown into the inside of the inner tube via a gas blowing inlet 22a through a gas passage 24a formed with the outer tube 23 and the inner tube 22. The inert gas is introduced to the gas passage 24a from a gas supplying inlet 24. The inert gas blown into the inner tube 22 descends through the space between (the inner tube 22 and the muffle tube 28) and (the preform 21 or the dummy rod 25), then it is exhausted through the vicinity of the optical fiber 21a. 
The space inside the inner tube 22 is separated by the separating plate 30 comprising a quartz plate or the like into an upper part and a lower part, and the inert gas flows from the upper space 31a of the separating plate 30 to the lower space 31b through the gap 30a between the separating plate 30 and the inner tube 22 or the pores 30b provided in the separating plate 30. Because the lower space 31b is of a relatively high temperature and turbulence of the gas stream is decreased owing to the presence of the separating plate 30, it has been said that an optical fiber having a small fluctuation in diameter can be drawn even in the case of a large preform.
However, in the case of this furnace for drawing an optical fiber, when the preform 21 becomes small with proceeding the drawing, the upper space 31a becomes large. On the other hand, the temperature near the upper end of the preform 21 is increased by heating the vicinity of the lower end of the preform 21, and said temperature becomes higher when the preform becomes small. Because of the presence of the separating plate 30 comprising a quartz plate, the temperature is somewhat decreased above the separating plate, but it becomes 550xc2x0 C. or more near the lower end of the upper space 31a. At this time, the temperature near the upper end of the upper space 31a is about 200xc2x0 C. to form a considerable difference in temperature between the upper part and the lower part inside the upper space 31a, therefore convection is liable to be caused. The convection causes fluctuation in volume of the inert gas, and the fluctuation is transmitted to the lower space 31b by the flow of the inert gas through the gap 30a or the pore 30b of the separating plate 30. Thus, fluctuation in flow of the inert gas near the optical fiber occurs. As a result, the amount of heat transmission is changed by the fluctuation of the inert gas to easily cause fluctuation in viscosity and softened amount of glass, and thus it is difficult to suppress fluctuation in fiber diameter to a small value less than the prescribed value.
A furnace for drawing an optical fiber disclosed in Japanese Patent Laid-Open No. 2832/1997 will then be described. The furnace for drawing an optical fiber is shown in FIG. 10. In FIG. 10, the corresponding references as in FIG. 8 show the same components. Numeral 33 denotes an upper space above the preform, and 34 denotes an auxiliary heater. In the furnace for drawing an optical fiber, a preform 21 is arranged inside a muffle tube 28 and the inner tube 22 connected to an upper end thereof, being supported by hanging by the dummy rod 25, and the vicinity of a lower end of the preform 21 is heated and melted by a heater 29 arranged outside the muffle tube 28, so as to draw an optical fiber 21a downward.
An outer tube 23 is arranged outside the inner tube 22 concentrically, and an inert gas is blown into the inside of the inner tube via a gas blowing inlet 22a through a gas passage 24a formed with the outer tube 23 and the inner tube 22. Because the vicinity of the upper end of the preform 21 becomes a high temperature of 1,000xc2x0 C. or more by heating the vicinity of the lower end of the preform 21, a large temperature difference formed between the upper part and the lower part of the space 33 above the preform.
In order to decrease the temperature difference, the auxiliary heater 34 is arranged outside the upper end of the inner tube 22 to heat the vicinity of the upper end of the inner tube 22. Thus, the vertical temperature difference of the space 33 between the inner tube 22 and the dummy rod 25 above the preform is decreased by the heating by the auxiliary heater 34 to prevent the convection from occurring at the space 33, and fluctuation in diameter of the optical fiber 21a drawn from the preform 21 is decreased.
However, in the case of this furnace for drawing an optical fiber, because the space 33 above the preform becomes large as the drawing of a large scale preform proceeds, it is difficult to make the temperature uniform inside the space solely by an auxiliary heater 34 arranged outside the upper part of the inner tube 22.
It is then considered that several auxiliary heaters are arranged vertically not only at the upper part but also at the lower part of the inner tube, and by controlling the temperature of each auxiliary heater, the temperature of the space above the preform is made uniform. However, there are problems that cost of the apparatus becomes increased, and the temperature control becomes complicated.
The invention is to provide a furnace for drawing an optical fiber and a method for drawing an optical fiber to solve the problems associated with the conventional art described in the foregoing.
The furnace for drawing an optical fiber according to the invention comprises a muffle tube and an inner tube connected to an end of the muffle tube. The preform is arranged inside the muffle tube and the inner tube and supported by a dummy rod at an upper part thereof in such a manner that the preform descends with the dummy rod. The preform is heated from outside of the muffle tube by a heater, and melted so as to draw an optical fiber from a lower end of the preform. One set or plural sets of separating plates separating a space inside the inner tube above the preform into plural parts in an advancing direction of the preform are arranged inside the space, and a gas blowing inlet for blowing an inert gas into the inner tube and the muffle tube is provided at the wall of the inner tube at a part under the separating plates.
Therefore, the inert gas entered from the gas blowing inlet mainly flows downward, and thus the gas in the upper space of the separating plate above the preform less influences the conditions of the drawing of the preform. Accordingly, the drawing is stably continued, and fluctuation in fiber diameter is decreased. Therefore, an optical fiber produced by using this furnace for drawing an optical fiber has a substantially constant outer diameter in the longitudinal direction, and has small fluctuation in transmission characteristics. Owing to the use of a simple separating plate but not using a large member such as a seal piston, the cost of the apparatus is low, and since heavy sliding as in the seal piston does not occur, dust formed by sliding is so minimal that it doesn""t adversely affect the strength of the optical fiber.
While the separating plates comprise plural sets of separating plates arranged as being penetrated by the dummy rod, and the plural sets of separating plates descend with the descent of the dummy rod, the respective sets of the plural sets of separating plates are stopped on the inner wall surface of the inner tube one by one, so that the space inside the inner tube above the preform is respectively separated into an upper part and a lower part by the stopped separating plates. Thus, because the space inside the inner tube can be separated by plural positions into suitable sizes, the flow of the inert gas inside the inner tube can be further stabilized even when the preform becomes small and the space inside the inner tube is enlarged with proceeding the drawing of the optical fiber.
While the respective sets of separating plates may each comprise one disk-shaped plate, the sets of separating plates may each be constituted by two plate members of an outer member and an inner member. In this case, the outer member has an outer diameter that is the same as the inner diameter of the inner tube at a position where the separating plate is stopped, and has a center hole diameter that is larger than the outer diameter of dummy rod so as to absorb the deviation from the concentric condition of the inner tube and the dummy rod. The inner member has an outer diameter that is larger than the center hole diameter of the outer member and smaller than the outer diameter of the outer member, and has a center hole diameter larger than an outer diameter of the dummy rod by 2 to 10 mm.
When the dummy rod penetrates both the members with the inner member being placed at upper side and the outer member being placed at lower side, the upward inner member is supported by being placed on the outer member. By constituting each of the sets of separating plates of an outer member and an inner member, even when the concentric condition of the inner tube and the dummy rod is deviated by rolling of the preform to cause decentering, the upper member moves in the radial direction by sliding on the lower member, and no such phenomenon that the inner wall surface of the inner tube is damaged by the separating plate occurs.
Alternatively, one or plural sets of separating plates may be arranged near the lower end of the dummy rod or at the upper part of the preform by fixing to the dummy rod, the connecting part or the preform, and they descend with the descent of the preform without stopping on the way on the inner wall of the inner tube. Accordingly, the space between the separating plates and the preform can be unchanged and constant even when the preform is shortened, and thus the flow of the gas in the space above the preform can be stabilized. It is also possible in this case that the respective sets of separating plates are each constituted by an outer member and an inner member. In this case, however, the outer diameter of the outer member must be smaller than the inner diameter of the inner tube by 5 to 10 mm, and the center hole diameter of the outer member must be larger than the outer diameter of the dummy rod. The outer diameter of the inner member must be larger than the center hole diameter of the outer member but smaller than the outer diameter of the outer member. The center hole diameter of the inner member must be the same as or larger than the outer diameter of the dummy rod. The inner member is then supported by the dummy rod or the connecting part with the inner member being downward and the outer member being upward.
By constituting each of the respective sets of separating plates by the outer member and the inner member, even in the case where the concentric condition is deviated by rolling of the preform to cause decentering, the upper member moves in the radial direction by sliding on the lower member, and no such phenomenon that the inner wall surface of the inner tube is damaged by the separating plate occurs.
When plural protrusions are provided on an outer periphery of the separating plate, the sliding friction coefficient of the separating plate to the inner wall surface of the inner tube on descending can be reduced, so as not to damage the inner tube, because only the protrusions are in contact with the inner wall surface of the inner tube.
By forming the separating plate with a heat insulating material, heat transferred from the upper end of the heated preform to the upper space above the separating plate can be reduced, and thus the temperature of the upper space above the separating plate can be reduced. Therefore, the temperature difference between the upper part and the lower part in the upper space is also reduced, and the generation of convection of the inert gas due to the temperature difference can also be suppressed. As a result, the influence on fluctuation in gas flow near the lower part of the preform influenced by the convection of the inert gas in the upper space can be suppressed, and thereby fluctuation in heat transfer amount by the inert gas, and fluctuation in viscosity and softened amount of the glass are suppressed, so as to reduce fluctuation in diameter of the optical fiber. Furthermore, because the influence by the convection of the inert gas in the upper space is reduced, the inert gas atmosphere can be maintained and the fluctuation in fiber diameter can be suppressed by flowing only a small amount of the inert gas near the preform. Therefore, the amount of consumption of high cost inert gas, such as helium, is reduced, and thus an economical effect is also expected.
Furthermore, by arranging an auxiliary heater outside the vicinity of the upper end of the inner tube to heat the vicinity of the upper part of the inner tube, the temperature inside the upper space can be further uniform.