Abstract:
Provided is an optical fiber glass base material manufacturing apparatus, including a furnace core tube that houses a porous glass base material; a movement mechanism that moves the porous glass base material in a longitudinal direction thereof in the furnace core tube; a first heating section that heats and dehydrates the porous glass base material in the furnace core tube; and a second heating section that is arranged downstream from the first heating section in a movement direction of the porous glass base material, and sinters the porous glass base material by heating a portion of the porous glass base material in the longitudinal direction.

Description:
[0001]    The contents of the following Japanese patent application are incorporated herein by reference: 
         [0002]    NO. 2014-227683 filed on Nov. 10, 2014. 
       BACKGROUND 
       [0003]    1. Technical Field 
         [0004]    The present invention relates to a manufacturing apparatus and a sintering method for a glass base material to be used for optical fiber. 
         [0005]    2. Related Art 
         [0006]    Manufacturing of an optical fiber glass base material includes forming a porous glass base material by depositing glass microparticles generated through hydrolysis. After this, the porous glass base material is heated and dehydrated in an atmosphere of inert gas and then the dehydrated porous glass base material is sintered through heating at a higher temperature. In this way, a transparent optical fiber glass base material is manufactured, as shown in Patent Document 1, for example. 
         [0007]    Patent Document 1: Japanese Patent Application Publication No. 2010-189251 
         [0008]    However, the method that includes passing the porous glass base material through a heater to achieve dehydration and passing the dehydrated porous glass base material through the heater again to achieve sintering after the porous glass base material has been drawn back through the heater requires a long time to move the porous glass base material, and this inhibits improvements to the producibility of the optical fiber glass base material. 
       SUMMARY 
       [0009]    According to a first aspect of the present invention, provided is an optical fiber glass base material manufacturing apparatus, comprising a furnace core tube that houses a porous glass base material; a movement mechanism that moves the porous glass base material in a longitudinal direction thereof in the furnace core tube; a first heating section that heats and dehydrates the porous glass base material in the furnace core tube; and a second heating section that is arranged downstream from the first heating section in a movement direction of the porous glass base material, and sinters the porous glass base material by heating a portion of the porous glass base material in the longitudinal direction. 
         [0010]    According to a second aspect of the present invention, provided is an optical fiber glass base material manufacturing method, comprising housing a porous glass base material in a furnace core tube; heating and dehydrating the porous glass base material with a heating section surrounding the porous glass base material housed in the furnace core tube; and sintering an entire length of the porous glass base material by sequentially heating portions of the porous glass base material in the longitudinal direction while the porous glass base material is being moved, with a heater arranged downstream of the porous glass base material in the movement direction of the porous glass base material. 
         [0011]    The summary clause does not necessarily describe all necessary features of the embodiments of the present invention. The present invention may also be a sub-combination of the features described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic structural view of an embodiment of the manufacturing apparatus  10  of the present invention used in the first embodiment. 
           [0013]      FIG. 2  shows a relationship between the base material position and the heating temperature of the multistage heater in the first embodiment. 
           [0014]      FIG. 3  shows a relationship between the base material position and the heating temperature of the multistage heater in the second embodiment. 
           [0015]      FIG. 4  is a schematic structural view of the manufacturing apparatus  20  used in the third embodiment. 
           [0016]      FIG. 5  shows a relationship between the base material position and the heating temperature of the multistage heater in the third embodiment. 
           [0017]      FIG. 6  is a schematic structural view of the manufacturing apparatus  30  used in the comparative example. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0018]    Hereinafter, some embodiments of the present invention will be described. The embodiments do not limit the invention according to the claims, and all the combinations of the features described in the embodiments are not necessarily essential to means provided by aspects of the invention. 
         [0019]    When manufacturing an optical fiber glass base material, first, using VAD or OVD, glass raw material is combusted in a flame to generate glass microparticles through hydrolysis. The generated glass microparticles are sequentially deposited on a rotating target rod in the axial direction or the radial direction to form a porous glass base material. 
         [0020]    The porous glass base material is held by a support rod and hung into a furnace core tube. Furthermore, the porous glass base material is heated by a heater while being rotated and lowered through the inside of the furnace core tube. In this way, the porous glass base material is dehydrated and sintered inside the furnace core tube. When dehydrating the porous glass base material, inert gas necessary for dehydration is supplied from a gas supply nozzle provided in the lower portion of the furnace core tube, and gas is expelled from inside the furnace core tube through a gas exhaust tube provided in the upper portion of the furnace core tube. 
         [0021]    When dehydrating the porous glass base material, the temperature of the heating region of the furnace core tube is set to be from 900° C. to 1300° C. When sintering the porous glass base material, the temperature of the heating region of the furnace core tube is set to be from 1400° C. to 1600° C. 
         [0022]      FIG. 1  schematically shows the structure of an optical fiber glass base material manufacturing apparatus  10  used for the dehydration process and sintering process performed on a porous glass base material such as described above. The manufacturing apparatus  10  in the drawing includes a cylindrical furnace core tube  12  made of quartz glass housing a porous glass base material  11 , a multistage heater  13  in which the heaters are arranged along the longitudinal direction in a manner to surround the outer circumference of the furnace core tube  12 , a furnace body  14  that houses the multistage heater  13 , a gas induction opening  15  for introducing gas into the furnace core tube  12 , a support rod  16  for supporting the porous glass base material  11 , and a gas exhaust tube  17  for expelling the gas in the furnace core tube. 
         [0023]    The multistage heater  13  is formed by a first heater  13 A and a second heater  13 B that are arranged along the longitudinal direction of the furnace core tube  12 . Each heater is arranged to be able to be independently temperature controlled. The multistage heater  13  can form a heating region that is greater than or equal to the length of the porous glass base material, by having the total length of the multistage heater  13  be greater than or equal to the length of the porous glass base material. The number of stages in the multistage heater may be increased to reduce the cost of the apparatus, in consideration of the heater output, the power supply capacity, and the like. The following describes a method for manufacturing optical fiber glass base material by performing the dehydration process and the sintering process on the porous glass base material  11  using the manufacturing apparatus  10  shown in  FIG. 1 . 
         [0024]    (Dehydration Process) 
         [0025]    In the dehydration process, one end of the porous glass base material  11  is held by the support rod  16 . The porous glass base material  11  is inserted into the furnace core tube  12 , and a lid is placed on the furnace core tube  12 . After this, the porous glass base material  11  is moved to a prescribed heating position and held at this heating position. 
         [0026]    In the dehydration process, the multistage heater  13  increases the temperature in the furnace body  14  up to a prescribed temperature. The heating temperature realized by the multistage heater  13  is set to be a prescribed processing temperature for dehydrating the porous glass base material. The processing temperature is greater than or equal to 900° C. and less than or equal to 1300° C., for example. 
         [0027]    In the dehydration process, the gas necessary for the dehydration process is supplied from the gas induction opening  15 . The gas necessary for the dehydration process may be chlorine gas or a mixed gas containing chlorine gas and an inert gas such as He, Ar, or N 2 . The internal pressure within the furnace core tube  12  during the dehydration process is set to be a positive pressure of approximately 10 Pa to 5000 Pa relative to the atmospheric pressure. 
         [0028]    In the dehydration process, in the state described above, the porous glass base material  11  is rotated while being held in a heated state over a prescribed processing time. In this way, the dehydration process of the porous glass base material  11  is performed. 
         [0029]    (Sintering Process) 
         [0030]    The sintering process is performed after completion of the dehydration process. The temperature of the heater  13 A in the furnace body  14  is increased to a temperature at which the porous glass base material  11  can be sintered, e.g. a temperature greater than or equal to 1400° C. and less than or equal to 1650° C. In the sintering process, the inert gas such as He or Ar is introduced from the gas induction opening  15 . In the sintering process, the internal pressure of the furnace core tube  12  is set to be a positive pressure of approximately 10 Pa to 5000 Pa relative to the atmospheric pressure. 
         [0031]    In the sintering process, the porous glass base material  11  is lowered into the furnace core tube  12  while being rotated around the center axis. In this way, the porous glass base material  11  is sequentially sintered from the bottom end thereof while the heating region of the porous glass base material  11  being heated by the heater  13 A moves at a prescribed speed. As a result, the porous glass base material  11  becomes transparent optical fiber glass base material. 
         [0032]    In the sintering process, the heating region of the heater  13 A with a temperature from 1400° C. to 1650° C. may be shorter than the length of the porous glass base material  11 . Furthermore, the sintering of the porous glass base material  11  may include transparent vitrification of the porous glass base material  11  as a result of gradual sintering from one end to the other end in the longitudinal direction of the porous glass base material  11  or from a central portion to an end portion in the longitudinal direction of the porous glass base material  11 . By performing sintering in this manner, it is possible to form a gas escape opening within the porous glass base material  11  during the sintering process, and therefore gas bubbles in the optical fiber glass base material obtained after the sintering process can be reduced, resulting in a base material with high transparency. 
         [0033]    In the sintering process, the remaining heater  13 B may have its setting temperature lowered to conserve power. In the sintering process, the remaining heater  13 B may have its temperature controlled to be a temperature that does not sinter the porous glass base material  11 , i.e. a temperature less than 1400° C., and the porous portion that is not yet sintered may be preheated to encourage an increase of the sintering speed. 
       First Embodiment 
       [0034]    Using the manufacturing apparatus  10  of the porous glass base material shown in  FIG. 1 , optical fiber glass base material was manufactured by performing the dehydration process and the sintering process on a porous glass base material obtained through deposition on an outer circumference of a starter core material using OVD. 
         [0035]    First, the porous glass base material  11  hanging from the support rod  16  was inserted from the opening at the top end of the furnace core tube  12 , the porous glass base material  11  having a length in the axial direction of 1600 mm and including a tapered portion at each end with a length of 200 mm was moved to a position relative to the multistage heater  13 , and a lid was placed on opening at the top end of the furnace core tube  12 . Next, each heater forming the multistage heater  13  was set to a temperature of 1200° C. and the porous glass base material  11  was heated. The relationship between the heating temperature resulting from the multistage heater  13  at this time and the temperature at each position on the porous glass base material  11  is shown as the circles plotted to form the solid line in  FIG. 2 . 
         [0036]    Here, the heaters  13 A and  13 B are each provided with a thermometer, and can be independently temperature controlled through PID control. The length of the heater  13 A in the longitudinal direction of the furnace core tube is 400 mm, and the length of the heat generating portion, which excludes the electrode portions and the like, is 300 mm. The length of the heater  13 B is 1300 mm, and the length of the heat generating portion, which excludes the electrode portions and the like, is 1200 mm. 
         [0037]    The heaters  13 A and  13 B are arranged adjacently with an interval of approximately 50 mm therebetween, and are both housed in the furnace body  14 . The total length of the multistage heater  13  is 1750 mm, and the heat generating portion spans 1650 mm from top to bottom. With this multistage heater  13 , the length of the heating region in the furnace reaching a temperature of at least 900° C. is approximately 1800 mm, and therefore it is possible to heat and perform the dehydration process on the entire porous glass base material  11  at the same time. 
         [0038]    In a state where the porous glass base material  11  was being held at the position described above, the porous glass base material  11  was rotated on the center axis at a speed of 5 rotations per minute. Chlorine gas with a flow rate of 0.5 liters per minute and He as the inert gas with a flow rate of 20 liters per minute were introduced from the gas induction opening  15 , and the internal pressure of the furnace core tube  12  was held at a positive pressure of 10 Pa to 5000 Pa relative to the atmospheric pressure. In the heating region within the furnace core tube  12 , the OH groups included in the porous glass base material  11  react chemically with the chlorine gas and enter into the atmospheric gas. The gas in which the OH groups have entered from the porous glass base material  11  is expelled to the outside of the furnace core tube  12  through the gas exhaust tube  17 . The dehydration process described above continued for 90 minutes. 
         [0039]    After this, the gas introduced from the gas induction opening  15  was changed to only He with a flow rate of 20 liters per minute and the setting temperature of the heater  13 A was changed to 1560° C. The setting output of the heater  13 B was set to zero. After the temperature of the heater  13 A increased to the setting temperature, transparent vitrification was performed for the entire base material by rotating the porous glass base material  11  on the center axis at a speed of 5 revolutions per minute, moving the porous glass base material  11  downward at a speed of 10 mm per minute while introducing the He gas, and sintering from the bottom end to the top end of the base material. 
         [0040]    The relationship between the temperature at each position in the longitudinal direction of the porous glass base material  11  in the above sintering process and the heating temperature of the multistage heater  13  is shown in  FIG. 2  by the squares plotted to form the dashed line. As shown in the drawing, the heating region having at least a temperature need for sintering, i.e. a temperature of at least 1400° C., was approximately 250 mm. 
       Second Embodiment 
       [0041]    Using the manufacturing apparatus  10  shown in  FIG. 1 , optical fiber glass base material was manufactured by performing the dehydration process and the sintering process on a porous glass base material  11  obtained through deposition on an outer circumference of a starter core material using OVD. The length in the axial direction of the processed porous glass base material  11  was 1600 mm including a tapered portion at each end with a length of 200 mm. 
         [0042]    After performing the dehydration process on the porous glass base material in the same manner as in the first embodiment, the gas introduced from the gas induction opening  15  was set to only He with a flow rate of 20 liters per minute, the setting temperature of the heater  13 A was changed to 1560° C., and the setting temperature of the heater  13 B was set to 1200° C., which is the same as the temperature used for the dehydration process. After the temperature of the heater  13 A increased to the setting temperature, transparent vitrification was performed for the entire porous glass base material  11  by rotating the porous glass base material  11  on the center axis at a speed of 5 revolutions per minute, moving the porous glass base material  11  downward at a speed of 12 mm per minute while introducing the He gas, and sintering from the bottom end to the top end. 
         [0043]    The relationship between the base material position at this time and the heating temperature of the multistage heater  13  is shown in  FIG. 3  by the squares plotted to form the dashed line. The circles plotted to form the solid line in  FIG. 3  indicate the relationship between the position in the longitudinal direction of the porous glass base material  11  during the dehydration process and the heating temperature of the multistage heater  13 . 
         [0044]    As shown in the drawing, the heating region having at least a temperature needed for sintering, i.e. a temperature of at least 1400° C., was approximately 250 mm. Furthermore, a preheated region with a temperature greater than or equal to 900° C. and a length of approximately 1400 mm was provided above the heater  13 A, and therefore it was possible to obtain favorable glass base material without melt residue even though the movement speed of during the transparent vitrification was 12 mm per minute. 
       Third Embodiment 
       [0045]      FIG. 4  schematically shows the structure of another manufacturing apparatus  20  for optical fiber glass base material. Using the manufacturing apparatus  20 , optical fiber glass base material was manufactured by performing dehydration and sintering on a porous glass base material obtained through deposition on an outer circumference of a starting core base material through OVD. 
         [0046]    The manufacturing apparatus  20  has a different structure from the manufacturing apparatus  10  shown in  FIG. 1 , in that the multistage heater  23  includes three or more heaters, which are the heaters  23 A,  23 B,  23 C, and  23 D, arranged along the longitudinal direction of the furnace core tube  22 . The remaining structure of the manufacturing apparatus  20  is the same as that of the manufacturing apparatus  10  shown in  FIG. 1 , and therefore components of the manufacturing apparatus  20  are given reference numerals with the same last digit as corresponding components in the manufacturing apparatus  10 , and redundant descriptions are omitted. 
         [0047]    First, the dehydration process was performed using the manufacturing apparatus  20 . The porous glass base material  21  hanging from the support rod  26  was inserted through the opening at the top end of the furnace core tube  22 , and the porous glass base material  21  having a length in the axial direction of 1600 mm and including a tapered portion at each end with a length of 200 mm was moved to a position relative to the multistage heater  23  and held at this position. A lid was placed on the opening at the top end of the furnace core tube  22 . 
         [0048]    Next, the setting temperature of each heater forming the multistage heater  23  was increased to 1200° C. In the dehydration process, the relationship between the position in the longitudinal direction of the porous glass base material  21  and the heating temperature of the multistage heater  23  is shown by the circles plotted to form the solid line in  FIG. 5 . 
         [0049]    The heaters  23 A,  23 B,  23 C, and  23 D are each provided with a thermometer, and can be independently temperature controlled through PID control. The length of each of the heaters  23 A,  23 B,  23 C, and  23 D in the longitudinal direction of the furnace core tube  22  is 400 mm, and the length of each heat generating portion, which excludes the electrode portions and the like, is 300 mm. Adjacent heaters have intervals therebetween of approximately 50 mm, and are all housed in a single furnace body  24 . The total length of the multistage heater is 1750 mm, and the heat generating portion of the multistage heater spans 1650 mm from top to bottom. 
         [0050]    As shown in  FIG. 5 , the heating region where the temperature is at least 900° C. has a length of approximately 1800 mm. Accordingly, the multistage heater  23  can heat and perform the dehydration process across the entire length of the porous glass base material  21  at the same time. 
         [0051]    In the dehydration process, in a state where a position in the longitudinal direction of the porous glass base material  21  was being held at the position described above, the porous glass base material  21  was rotated on the center axis at a speed of 5 revolutions per minute. Chlorine gas with a flow rate of 0.5 liters per minute and He as the inert gas with a flow rate of 20 liters per minute were introduced from the gas induction opening  25 , and the internal pressure of the furnace core tube  22  was held at a positive pressure of 10 Pa to 5000 Pa relative to the atmospheric pressure. 
         [0052]    In the heating region within the furnace core tube  22 , the OH groups included in the porous glass base material  21  react chemically with the chlorine gas and enter into the atmospheric gas. The gas in which the OH groups have entered from the porous glass base material  21  is expelled to the outside of the furnace core tube  22  through the gas exhaust tube  27 . The dehydration process described above continued for 90 minutes. 
         [0053]    After the dehydration process described above, the sintering process was performed on the porous glass base material  21 . First, the gas being supplied from the gas induction opening  25  was changed to only He with a flow rate of 20 liters per minute and the setting temperature of the heater  23 B was changed to 1560° C. The setting output for each of the other heaters  23 A,  23 C, and  23 D was set to zero. The relationship between the position in the longitudinal direction of the porous glass base material  21  in this sintering process and the heating temperature of the multistage heater  23  is shown by the squares plotted to form a dashed line in  FIG. 5 . 
         [0054]    As shown in the drawing, the heating region having at least a temperature needed for sintering, i.e. a temperature of at least 1400° C., was approximately 250 mm in the longitudinal direction of the porous glass base material  21 . In the sintering process, after the temperature of the heater  23 B increased to the setting temperature, transparent vitrification was performed in a range from the bottom portion to the top end of the base material by rotating the porous glass base material on the center axis at a speed of 5 revolutions per minute, moving the porous glass base material downward at a speed of 10 mm per minute while introducing the He gas, and sintering from the bottom portion to the top end of the base material. 
         [0055]    The sintering of the tapered portion at the bottom end of the base material was incomplete and had melting residue. On the other hand, the trunk portion exhibited sufficient transparent vitrification, and no melting residue was seen. 
       COMPARATIVE EXAMPLE 
       [0056]      FIG. 6  schematically shows the structure of an optical fiber glass base material manufacturing apparatus  30  having a single heater, which a comparative example for comparison to the apparatus shown in  FIG. 1 . The structure of the manufacturing apparatus  30  differs from the structures of the manufacturing apparatus  10  shown in  FIG. 1  and the manufacturing apparatus  20  shown in  FIG. 2  by including a single heater  33 . The remaining structure of the manufacturing apparatus  30  is the same as that of the manufacturing apparatus  10  and the manufacturing apparatus  20 , and therefore components of the manufacturing apparatus  30  are given reference numerals with the same last digit as corresponding components in the manufacturing apparatus  10  and manufacturing apparatus  20 , and redundant descriptions are omitted. 
         [0057]    Using the manufacturing apparatus  30 , optical fiber glass base material was manufactured by performing the dehydration process and the sintering process on a porous glass base material  31  obtained through deposition on a core rod using OVD. First, the porous glass base material  31  having a length in the axial direction of 1600 mm and including a tapered portion at each end with a length of 200 mm hanging from the support rod  36  was inserted through the opening at the top end of the furnace core tube  32 , and the porous glass base material  31  was moved to a position relative to the heater  33  and held at this position. In this state, a lid was placed on the opening at the top end of the furnace core tube  32 . 
         [0058]    Next, the setting temperature of each heater forming the heater  33  was increased to 1200° C. The length of heater  33  in the longitudinal direction of the furnace core tube is 400 mm, and the length of the heat generating portion, which excludes the electrode portions and the like, is 300 mm. The heater  33  is housed in the furnace body  34 . The heating region where the temperature is at least 900° C. has a length of approximately 250 mm. 
         [0059]    Next, the porous glass base material  31  was moved downward at a speed of 10 mm per minute while being rotated on the center axis of the base material at a speed of 5 revolutions per minute. At this time, chlorine gas with a flow rate of 0.5 liters per minute and He as the inert gas with a flow rate of 20 liters per minute were introduced from the gas induction opening  35 , and the internal pressure of the furnace core tube was held at a positive pressure of 10 Pa to 5000 Pa relative to the atmospheric pressure. In the heating region within the furnace core tube, the OH groups included in the porous glass react chemically with the chlorine gas and enter into the atmospheric gas. The gas in which the OH groups have entered from the porous glass base material is expelled to the outside of the furnace core tube through the gas exhaust tube  37 . With this method, the dehydration process for the porous glass base material required 160 minutes. 
         [0060]    After the dehydration process described above, the sintering process was performed on the porous glass base material  31 . In the furnace core tube  32 , the porous glass base material  31  was moved upward at a speed of 100 mm per minute, and the position of the porous glass base material  31  was returned to the position at the time when the dehydration process was started. 
         [0061]    Next, the gas being supplied from the gas induction opening  35  was changed to only He with a flow rate of 20 liters per minute and the setting temperature of the heater  33  was changed to 1560° C. The heating region having at least a temperature needed for sintering, i.e. a temperature of at least 1400° C., was approximately 250 mm. After the temperature of the heater  33  increased to the setting temperature, the porous glass base material  31  was rotated on the center axis at a speed of 5 revolutions per minute and moved downward in the drawing at a speed of 10 mm per minute while introducing the He gas. As a result the porous glass base material  31  was sequentially sintered from the bottom end to the top end, until finally realizing transparent vitrification over the entire length of the porous glass base material  31 . 
         [0062]    In the manner described above, the comparative example using the manufacturing apparatus  30  including a single heater  33  required approximately twice as much time for the dehydration process as the first to third embodiments described above, and before beginning the dehydration process, time was also needed to raise the porous glass base material  31  to the original position. 
         [0063]    As described above, the dehydration process and the sintering process include heating the porous glass base materials  11 ,  21 , and  31  with different conditions. When performing the heating for the dehydration process, the time needed for the dehydration process can be shortened by heating the entire length of the porous glass base material  11  or  21  all at once. Furthermore, before the sintering process, no time is required to raise the porous glass base material  11  or  21 , and therefore the time needed before beginning the sintering process can be shortened. 
         [0064]    In this way, it is possible to shorten the time needed for the dehydration process and the sintering process, and to improve the throughput relating to the manufacturing of the optical fiber glass base material. Therefore, it is possible to improve the production efficiency of the optical fiber glass base material and reduce the manufacturing cost of the optical fiber glass base material.