Abstract:
An optical fiber base material drawing method for forming a constricted shape having a reduced diameter along a longitudinal direction of the optical fiber base material in a predetermined constricted shape segment in the longitudinal direction of the optical fiber base material at one end of the optical fiber base material, including: starting heating of the optical fiber base material with a heating source from a heating start position positioned in the constricted shape segment; then heating the optical fiber base material with the heating source in another segment having a predetermined length adjacent to the constricted shape segment; then returning the heating source to the heating start position and softening the optical fiber base material in the constricted shape segment by heating the optical fiber base material; and then forming the constricted shape by reducing the diameter of the optical fiber base material by applying a tensile force thereto.

Description:
[0001]    The contents of the following Japanese patent application is incorporated herein by reference: 
         [0002]    NO. 2014-235246 filed on Nov. 20, 2014. 
       BACKGROUND 
       [0003]    1. Technical Field 
         [0004]    The present invention relates to a drawing method of an optical fiber base material. 
         [0005]    2. Related Art 
         [0006]    An optical fiber base material undergoes primary elongation so that its outer shape conforms to a drawing machine, using an elongation apparatus that includes an electric furnace, and then both ends or one end is machined to have a constricted shape suitable for drawing, using a glass lathe. The constricted shape is formed by drawing. After this the optical fiber base material is finished by flame polishing the entire surface thereof. When drawing the optical fiber from the optical fiber base material, a dummy glass rod for hanging is fused to one end of the optical fiber base material, and the optical fiber is drawn forth from the thin diameter portion formed by the drawing. 
         [0007]    If there is an irregularity in the surface state caused by a scratch or the adhesion of impurities on the surface of the optical fiber base material, when drawing the optical fiber, there are various negative effects such as breaking, outer diameter fluctuation, and degradation of the characteristics. Therefore, the surface of the optical fiber base material is preferably smooth and has few impurities. 
         [0008]    However, there is a trend in recent years for the outer diameter of the optical fiber base material to become larger, and this has led to an increase in the heating amount provided by the preheating before the drawing and an increase in the expended gas amount and work time. As a result, a large amount of glass microparticles known as a silica cloud adheres to a region on the surface of the optical fiber base material near the region heated by the flame, and this worsens the surface state of the optical fiber base material. 
       SUMMARY 
       [0009]    According to a first aspect of the present invention, provided is an optical fiber base material drawing method for forming a constricted shape having a reduced diameter along a longitudinal direction of the optical fiber base material in a predetermined constricted shape segment in the longitudinal direction of the optical fiber base material at one end of the optical fiber base material. The method comprise starting heating of the optical fiber base material with a heating source from a heating start position that is positioned in the constricted shape segment; then heating the optical fiber base material with the heating source in another segment having a predetermined length that is adjacent to the constricted shape segment; then returning the heating source to the heating start position and softening the optical fiber base material in the constricted shape segment by heating the optical fiber base material; and then forming the constricted shape by reducing the diameter of the optical fiber base material by applying a tensile force to the optical fiber base material. 
         [0010]    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 
         [0011]      FIG. 1  is a schematic view of a stage for preheating the optical fiber base material  10 . 
           [0012]      FIG. 2  is a schematic view of a stage for flame polishing the optical fiber base material  10 . 
           [0013]      FIG. 3  is a schematic view of a stage for preheating the optical fiber base material  10 . 
           [0014]      FIG. 4  is a schematic view of a stage for starting drawing of the optical fiber base material  10 . 
           [0015]      FIG. 5  is a schematic view of a stage after the drawing of the optical fiber base material  10 . 
           [0016]      FIG. 6  is a schematic view of a stage for flame polishing the optical fiber base material  10 . 
           [0017]      FIG. 7  is a schematic view of a stage for thermal cutting of the optical fiber base material  10 . 
           [0018]      FIG. 8  is a schematic view of a stage for preheating the optical fiber base material  10 . 
           [0019]      FIG. 9  is a schematic view of a stage for starting drawing of the optical fiber base material  10 . 
           [0020]      FIG. 10  is a schematic view of a stage for starting drawing of the optical fiber base material  10 . 
           [0021]      FIG. 11  is a schematic view of a stage for flame polishing the optical fiber base material  10 . 
           [0022]      FIG. 12  is a schematic view of a stage for thermal cutting of the optical fiber base material  10 . 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0023]    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. 
         [0024]      FIGS. 1 to 7  are schematic views of an optical fiber base material drawing method according to an embodiment of the present invention, shown in multiple stages. In  FIGS. 2 to 7 , components that are the same as shown in  FIG. 1  are given the same reference numerals, and redundant descriptions are omitted. 
         [0025]    First, as shown in  FIG. 1 , an optical fiber base material  10  connected to one end of a dummy rod  20  made of quartz glass is gripped by a chuck of a glass lathe. Next, a position where the constricted shape is to be formed is set near the connection portion between the optical fiber base material  10  and the dummy rod  20 , and initial preheating begins from this position. In the example shown in the drawings, the initial preheating begins at a starting point within the segment to be drawn that is on the side of the connection portion between the optical fiber base material  10  and the dummy rod  20 . During the preheating, the optical fiber base material  10  is heated through contact with a flame  40  emitted from a burner  50 . 
         [0026]    As shown in  FIG. 1 , silica clouds  31  and  32  stick to the circumferential edge of the region contacted by the flame  40  used for the initial preheating on the surface of the optical fiber base material  10 . The decomposition reaction of the quartz glass at a high temperature is complicated, but can essentially be represented as SiO 2  decomposing into SiO and O 2 , as shown by Expression 1 below. 
         [0000]      2SiO 2 →2SiO+O 2    Expression 1:
 
         [0027]    Furthermore, the evaporated SiO causes a hydrolytic reaction in the atmosphere, as shown by Expression 2 below, to again become SiO 2  and be deposited on a region with a low temperature near the heated portion as glass microparticles referred to as a silica cloud. 
         [0000]      SiO+H 2 O→SiO 2 +H 2    Expression 2:
 
         [0028]    The amount of the silica clouds  31  and  32  adhering to the optical fiber base material  10  as a result of the fusion of the dummy rod  20  and the initial preheating increases according to increases in the time and the amount of gas used for the heating. However, as shown in  FIG. 2 , the adhered silica clouds  31  and  32  can be removed by partial flame polishing, i.e. applying the flame  40  to the surface of the optical fiber base material  10  across a predetermined movement distance  60  of the burner  50  by moving the burner  50  along the optical fiber base material  10  from a start position  51  of the preheating. 
         [0029]    During the partial flame polishing, the movement direction of the burner  50  is reversed when the burner  50  has moved the predetermined movement distance  60 , and the burner  50  then returns to the preheating start position  51 , as shown in  FIG. 3 . In this way, the silica clouds, impurities, and the like on the surface of the optical fiber base material  10  can be removed. 
         [0030]    The movement distance  60  of the burner  50  is preferably a range that is no less than 1 time and no greater than 3 times the outer diameter of the optical fiber base material  10 . The movement distance  60  of the burner  50  is more preferably no less than 1.5 times and no greater than 2 times the outer diameter of the optical fiber base material  10 . If the movement distance  60  of the burner  50  is less than 1 time the outer diameter of the optical fiber base material  10 , there are cases where the silica cloud  31  remains. If the movement distance  60  of the burner  50  is greater than 3 times the outer diameter of the optical fiber base material  10 , the flame polishing continues to regions where the silica cloud  31  is not adhered, and therefore the energy efficiency is decreased. 
         [0031]    In the preheating described above, the movement speed of the burner  50  while moving the movement distance  60  from the preheating start position  51 , i.e. the forward travel, is preferably a speed that does not cause thermal warping of the optical fiber base material  10 . Furthermore, the movement speed of the burner  50  while returning to the start position  51  after having moved the movement distance  60 , i.e. the return travel, may be greater than the movement speed of the burner  50  while moving the movement distance  60  from the preheating start position  51 , i.e. the forward travel. The optical fiber base material  10  is already heated during the forward travel, and therefore it is difficult for thermal warping to occur due to the heating during the return travel of the burner  50 . Accordingly, the movement speed of the burner  50  can be increased to shorten the work time required for the drawing of the optical fiber base material  10 . 
         [0032]    Next, during the drawing of the optical fiber base material  10 , as shown in  FIG. 4 , the burner  50  that has returned to the start position  51  performs heating in order to soften the position where the constricted shape of the optical fiber base material  10  is to be formed. Furthermore, as shown in  FIGS. 4 and 5 , when the optical fiber base material  10  is softened, a tensile force is applied to the optical fiber base material  10  by increasing the space between the ends of the heated portion while adjusting the gas amount of the burner, thereby gradually reducing the diameter of the portion that has been softened by heating. In this way, the constricted shape is formed in the optical fiber base material  10 . 
         [0033]    During this stage of heating as well, silica clouds  33  and  34  are generated and adhere to the optical fiber base material  10  and the dummy rod  20 . However, since the optical fiber base material  10  has already been heated by the initial preheating, the heating time during this stage is short. Accordingly, the amount of the silica clouds  33  and  34  generated in this stage is low. 
         [0034]    A position near the position where the constricted shape is formed in the optical fiber base material  10  has already reached a high temperature due to the initial preheating and the flame polishing performed previously. As described above, the reaction by which SiO 2  is deposited according to the hydrolysis of SiO occurs in a region with low temperature. Therefore, near the position where the constricted shape is formed, a large percentage of SiO is expelled without causing SiO 2  deposition, thereby restricting the generation of the silica clouds  33  and  34 . 
         [0035]    Furthermore, it is difficult for impurities to adhere to the surface of the optical fiber base material  10  that has already become smooth due to the flame polishing. In the same way that the deposition of the silica clouds  33  and  34  is restricted, it is assumed that deposition of silicate compound impurities of metal components, which are impurities, is difficult in regions with high temperature. Furthermore, it is assumed that decreasing the surface area of the optical fiber base material  10  that has become smooth due to the flame polishing will also have an effect. 
         [0036]    The optical fiber base material  10  in which the constricted shape is formed as described above has its entire surface flame polished and finished, by moving the burner  50  along the optical fiber base material  10 , as shown in  FIG. 6 . In this way, the silica cloud  33  adhered to the surface of the optical fiber base material  10  is easily and reliably removed by the polishing. 
         [0037]    Furthermore, as shown in  FIG. 7 , at the constricted shape of the optical fiber base material  10 , a region near the portion having the smallest diameter is further heated to sever the dummy rod  20  from the optical fiber base material  10 . The optical fiber base material  10  having the constricted shape obtained in this manner prevents unevenness in the optical fiber base material  10  caused by the adherence of the silica clouds  33  and  34  and impurities. Furthermore, by performing flame polishing across the entire length of the optical fiber base material  10 , warping of the optical fiber base material  10  is reduced. Therefore, the quality of the optical fiber base material  10  is improved, and the yield for the optical fiber is also improved. 
       FIRST MANUFACTURING EXAMPLE 
       [0038]    An optical fiber base material  10  was drawn using a glass lathe including a burner  50 . The burner  50  was an oxyhydrogen flame burner including an oxygen nozzle that supplies oxygen as a combustion supporting gas. The drawn optical fiber base material had an average diameter of 85 mm, was connected to dummy rods  20  at both ends, and was set in a glass lathe via the dummy rods  20 . 
         [0039]    A position 20 mm toward the base material side from a connection portion between the optical fiber base material and a dummy rod was set as the preheating start position  51 . The initial flame polishing was performed with the movement speed of the burner  50  set to 30 mm/min in a direction toward the optical fiber base material  10  side. The movement direction was reversed when the movement distance of the burner  50  reached 150 mm, and the burner  50  returned to the preheating start position  51  at a speed of 60 mm/min, which is double the movement speed of the forward travel. 
         [0040]    Next, the optical fiber base material  10  was preheated from the start position  51  in order to form the constricted shape, and after the segment to have a reduced diameter was sufficiently heated, the diameter of the optical fiber base material  10  was reduced by increasing the space between the ends of the segment to have the reduced diameter. In this way, the constricted shape was formed in the optical fiber base material  10 . A constricted shape was also formed at the opposite end of the optical fiber base material  10  using the same process, and then the entire optical fiber base material  10  was flame polished, the thin diameter portions of the constricted shapes were thermally cut, and the optical fiber base material  10  was removed from the glass lathe. 
         [0041]    With the conditions described above, the drawing process was performed on  100  optical fiber base materials to form the constricted shapes, and an investigation of the finished surfaces was performed. Unevenness that is assumed to be caused by the silica clouds  33  and  34  was found on the end surfaces of 2 of the 100 optical fiber base materials. Accordingly, the rate of unevenness of optical fiber base materials  10  manufactured with this method was 2%. 
       SECOND MANUFACTURING EXAMPLE 
       [0042]    Using the same method and conditions as in the first manufacturing example, drawing was performed on 100 optical fiber base materials  10  having an average diameter of 120 mm. Upon investigating the finished surfaces of the resulting optical fiber base materials  10  having constricted shapes, unevenness that is assumed to be caused by the silica clouds was found on the end surface of 1 of the 100 optical fiber base materials. Accordingly, the rate of unevenness was 1%. 
       COMPARATIVE EXAMPLE 
       [0043]      FIGS. 8 to 12  show each step in the drawing method of an optical fiber base material  10  as a comparative example. In these drawings, components that are the same as components shown in  FIGS. 1 to 7  are given the same reference numerals, and redundant descriptions are omitted. 
         [0044]    First, as shown in  FIG. 8 , an optical fiber base material  10  connected to a dummy rod  20  made of quartz glass is gripped by a glass lathe, and preheating is begun from a position where the constricted shape is to be formed that is set near the connection portion between the optical fiber base material  10  and the dummy rod  20 . 
         [0045]    Next, as shown in  FIG. 9 , the position where the constricted shape is to be formed in the optical fiber base material  10  is heated and softened by the burner  50 . Next, as shown in  FIGS. 9 and 10 , a tensile force is applied to the portion of the optical fiber base material  10  softened by the heating, by increasing the space between the ends of the heated portion while adjusting the gas amount of the burner, thereby gradually reducing the diameter. In this way, the constricted shape is formed in the optical fiber base material  10 . 
         [0046]    Next, as shown in  FIG. 11 , the optical fiber base material  10  is flame polished and finished by moving the burner  50  along the optical fiber base material  10  such that the flame  40  of the burner  50  contacts the entire surface of the optical fiber base material  10 . Furthermore, as shown in  FIG. 12 , a region near the portion with the smallest diameter in the constricted shape of the optical fiber base material  10  is further heated, thereby severing the optical fiber base material  10  from the dummy rod  20 . 
         [0047]    In this way, the optical fiber base material  10  having a constricted shape is manufactured. As shown in  FIG. 12 , even after the flame polishing, there are cases where deposition of a silica cloud  35  is found on the surface of the optical fiber base material  10 . 
         [0048]    According to the process described above, using a glass lathe including a burner  50  with the same specifications as the burner used for the drawing in the first and second embodiments,  100  optical fiber base materials  10  connected to dummy rods  20  at both ends and having an average diameter of 85 mm, which is the same diameter as in the first embodiment, were drawn. Upon investigating the finished surfaces of the resulting optical fiber base materials  10  having the constricted shapes, unevenness assumed to be caused by the silica clouds was found on the end surfaces of 10 of the 100 optical fiber base materials, indicating an unevenness rate of 10% and a low yield. 
         [0049]    In this way, during the drawing of an optical fiber base material  10  with a flame burner as a heating source using a glass lathe, by flame polishing a portion of the optical fiber base material  10  on the product side from the preheating position after the preheating and before beginning the process to reduce the diameter of the optical fiber base material  10 , it is possible to remove silica clouds, impurities, and the like, and to restrict irregularities in the surface state caused by unevenness or adherence of impurities on the base material surface during the process to reduce the diameter. As a result, it is possible to improve the yield for the drawing of the optical fiber base material, and to improve the producibility of the optical fiber. 
         [0050]    While the embodiments of the present invention have been described, the technical scope of the invention is not limited to the above described embodiments. It is apparent to persons skilled in the art that various alterations and improvements can be added to the above-described embodiments. It is also apparent from the scope of the claims that the embodiments added with such alterations or improvements can be included in the technical scope of the invention. 
         [0051]    The operations, procedures, steps, and stages of each process performed by an apparatus, system, program, and method shown in the claims, embodiments, or diagrams can be performed in any order as long as the order is not indicated by “prior to,” “before,” or the like and as long as the output from a previous process is not used in a later process. Even if the process flow is described using phrases such as “first” or “next” in the claims, embodiments, or diagrams, it does not necessarily mean that the process must be performed in this order.