Abstract:
Drawing methods and drawing furnaces for drawing an optical fiber with small non-circularity by simple drawing system are provided. An optical fiber preform is received into a muffle tube and heated by a primary heater placed to surround the muffle tube. The optical fiber preform is heated such that a starting position of a meniscus portion is higher in its position than the top of the primary heater, wherein the meniscus portion is created at the bottom portion of the optical fiber preform.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority from Japanese Patent Application No. 2008-083512 filed Mar. 27, 2008, the entire contents of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to optical fiber drawing methods and drawing furnaces for drawing an optical fiber from an optical fiber preform. 
       BACKGROUND OF THE INVENTION 
       [0003]    An optical fiber is obtained by so-called “drawing”. During draw, the bottom portion of an optical fiber preform is softened by heating, and the tension is applied to the softened portion of the preform to reduce its diameter. The preform is typically made from material such as quartz glass. In general, a drawing system for such a process is equipped with a drawing furnace to heat the optical fiber preform; a cooling device to cool the drawn glass optical fiber; a coating device to coat a resin around the drawn glass optical fiber; and a spooling device to spool the coated optical fiber. In addition, the drawing furnace is placed at the highest position possible from the ground to increase the travel distance for cooling the optical fiber and to increase the drawing speed for improving productivity. 
         [0004]    However, when an optical fiber was drawn form a standard drawing system, sometimes cross section(s) of the optical fiber had non-isotropic circle (such as ellipse). Deviation of an optical fiber cross section from an isotropic circle is called “non-circularity”. Non-circularity of an optical fiber is defined as (difference in the maximum and the minimum diameters of an optical fiber cross section)/(average of the maximum and the minimum diameters). An optical fiber is non-circular if its non-circularity is other than zero. 
         [0005]    The cause of such non-circular optical fiber is unevenness in softness of an optical fiber preform in the circumferential direction due to unevenness of temperature distribution in the heated optical fiber preform, which happens because it is difficult to perfectly match the center axis of a drawing furnace to that of the optical fiber preform. Also, if the heat value of a heating element of the drawing furnace is uneven in the circumferential direction, the optical fiber could have a non-circular cross section. When an optical fiber has a large non-circularity, it causes problems such as an increase in polarization mode dispersion (PMD) and an increase in connection loss due to misalignment of the cores when the fiber is connected to another optical fiber. 
         [0006]    To prevent non-circular optical fibers, drawing furnaces include equalization mean to uniformly distribute temperature along the heater circumference direction has been disclosed in U.S. Pat. No. 654,760. On the other hand, an optical fiber drawing method, which sets a relationship L (mm)=5D (mm)-50 (mm), has been disclosed in Japanese Patent Application Laid-open No. 2004-224587. Where D is a diameter of an optical fiber preform and L is the length of the heat zone developed by the heater(s) along the drawing direction. 
         [0007]    However, even with the methods disclosed in the U.S. Pat. No. 654,760, it is difficult to uniformly distribute the temperature around the circumference of the heater, and the structure of electrode within a furnace becomes complex. And, when the structure of the electrode is complex, it causes unevenness in the circumferential direction of the heating element, which causes uneven distribution of the heat. Further, with the methods disclosed in the Japanese Patent Application Laid-open No. 2004-224587, as the heater(s) is extended, the furnace itself needs to be extended, and it becomes difficult to receive an optical fiber preform into the drawing furnace, or the height of the drawing furnace and the distance for cooling down the drawn fiber are reduced. Those above problems are especially visible in a large optical fiber perform, such as a preform with a diameter of 100 mm or more. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides optical fiber drawing methods and drawing furnaces, which are capable of drawing an optical fiber with a small non-circularity using a simple drawing system. 
         [0009]    To solve the above problem, one of the optical fiber drawing methods disclosed in the present invention is a method of drawing an optical fiber from an optical fiber preform, wherein the optical fiber preform is received into a muffle tube and heated by a primary heater placed to surround the muffle tube, the method comprising the steps of heating the optical fiber preform such that a starting position of a meniscus portion, a portion from a starting position where the outer diameter of the optical fiber preform starts to decrease its outer diameter to a point where the outer diameter reaches to 1 mm, is higher in its position than the top of the primary heater, wherein the meniscus portion is created at the bottom portion of the optical fiber preform. 
         [0010]    Also, one of the optical fiber drawing furnaces in the present invention comprises a muffle tube for receiving an optical fiber preform; a primary heater to heat the inside of the muffle tube, which is placed around the muffle tube concentrically with the muffle tube; and a furnace body to accommodate the muffle tube and the primary heater; wherein the optical fiber drawing furnace satisfies 2 E=L=1.5 E where E is an inner diameter of the muffle tube and L is a distance between the highest temperature point and a point, which is 400° C. lower than the highest temperature and located in the upper side of the muffle tube. 
         [0011]    According to the present invention, because uneven temperature distribution is reduced in the circumferential direction at a starting point of a meniscus portion, which causes the most impact in non-circularity of an optical fiber, an optical fiber with a small non-circularity can be drawn from a simple drawing system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Referring now to the drawings: 
           [0013]      FIG. 1  is a schematic longitudinal cross section of an optical fiber drawing furnace, which is related to the first embodiment of the present invention; 
           [0014]      FIG. 2  shows temperature profile inside of the muffle tube in  FIG. 1 ; 
           [0015]      FIG. 3  explains a method of drawing an optical fiber preform using the drawing furnace shown in  FIG. 2 ; 
           [0016]      FIG. 4  shows temperature profiles inside of the muffle tubes in an example 1 and a comparative example 1 of the present invention; 
           [0017]      FIG. 5  shows shapes of the meniscus portions of the optical fiber preform in an example 1 and a comparative example 1 of the present invention; 
           [0018]      FIG. 6  is a schematic longitudinal cross section of an optical fiber drawing furnace, which is related to the alternative embodiment 1 of the present invention; 
           [0019]      FIG. 7  is a schematic longitudinal cross section of an optical fiber drawing furnace, which is related to the alternative embodiment 2 of the present invention; and 
           [0020]      FIG. 8  is a schematic longitudinal cross section of an optical fiber drawing furnace, which is related to the alternative embodiment 3 of the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0021]    While various embodiments of the present invention are described below, it should be understood that they are presented by way of example, and are not intend to limit the scope of the presented invention. 
       First Embodiment of the Present Invention 
       [0022]      FIG. 1  is a schematic longitudinal cross section of an optical fiber drawing furnace, which is related to the first embodiment of the present invention. As shown in  FIG. 1 , the drawing furnace  10  has a furnace body  11 , a muffle tube  12 , a heater  13  as a primary heater, insulator  14 , a top cover  15 , and a cooling section  16 . 
         [0023]    The furnace body  11  is a cylindrical shape, made from heat-resistant material, and includes the muffle tube  12 , the heater  13  and the insulator  14 . The muffle tube  12  is a cylindrical shape and made from carbon for example. Also, the heater  13 , illustratively a carbon heater, is a cylindrical shape and is placed to cover the muffle tube  12 . Also, the insulator  14  is placed around the outside of the muffle tube  12  and the heater  13 . The top cover  15  is placed above the furnace body  11 . The cooling section  16  is placed under the furnace body  11  and includes the bottom portion of the muffle tube  12 . The top cover  15  and the cooling section  16  have a first opening  15   a  for receiving an optical fiber preform, and a second opening  16   a  for removing a drawn optical fiber. Also, in the first embodiment of the present invention, the inner diameter E of the muffle tube  12  is 150 mm, and the length I of the heater  13  is 250 mm. 
         [0024]      FIG. 2  shows temperature profile inside the muffle tube  12  of  FIG. 1 . In  FIG. 2 , the x-axis shows distance in the longitudinal direction of the muffle tube  12  as measured from the top end of the muffle tube, and the y-axis shows temperature at the center axis of the muffle tube  12 . As shown in  FIG. 2 , in the first embodiment of the present invention, the highest temperature of the center portion of the heater  13  in respect to the longitudinal direction of the muffle tube  12  is approximately 2,000° C. Also, the distance L between the highest temperature point P 1  and a point P 2 , which is 400° C. lower than the maximum temperature and located in the upper side of the muffle tube  12 , is 250 mm. Therefore, in the first embodiment of the present invention, a relationship 2E=L is satisfied. To satisfy the relationship, the inner diameter and the length of the muffle tube  12 ; the length and height of heater  13 ; and the insulator are selected. 
         [0025]    Next, a drawing method of an optical fiber preform using the drawing furnace  10  shown in  FIG. 1  is explained.  FIG. 3  is used to explain a method of drawing an optical fiber  2  from an optical fiber preform  1  using the drawing furnace  10  shown in  FIG. 1 . As shown in  FIG. 3 , at first, the optical fiber preform  1  is received from the top cover  15  on the first opening  15   a . The heater  13  heats and softens the received optical fiber preform  1 . The softened portion reduces its radius gradually to create a meniscus portion  1   a , and then by reducing its outer diameter further to the predetermined diameter, it becomes an optical fiber  2 . In this specification, the “meniscus portion  1   a ” means a portion from a starting position  1   b  where the outer diameter of the optical fiber preform  1  starts to decrease its outer diameter to a point where the outer diameter reaches to 1 mm. The meniscus portion  1   a  has an inflection point  1   c  where change of the outer diameter (in the longitudinal direction) becomes a concave shape from a convex shape. 
         [0026]    As shown in  FIG. 3 , in the first embodiment of the present invention, the optical fiber preform  1  is heated such that the starting point  1   b  of the meniscus portion  1   a  is higher in its position than top of the heater  13 . By doing so, the uneven temperature distribution in the circumference direction at the starting point  1   b  of the meniscus portion  1   a  is reduced. The starting point  1   b  has the most impact in the non-circularity of the optical fiber  2 . 
         [0027]    In other words, below the top  13   a  of the heater  13 , the temperature distribution of the meniscus portion  1   a  in the circumferential direction is easily affected by an uneven heating value of the heater  13  in the circumferential direction. However, above the top  13   a  of the heater  13 , the uneven heating value of the heater  13  is reduced. In addition, because the optical fiber preform  1  starts to soften at the starting point  1   b  of the meniscus portion  1   a , if non-circular shape exists at the starting portion of the meniscus, it affects the shapes of the meniscus portion  1   a  and the optical fiber  2  in a later process. 
         [0028]    Therefore, in the first embodiment of the present invention, an optical fiber with small non-circularity can be drawn because uneven temperature distribution in the circumferential direction at the starting point  1   b  of the meniscus portion  1   a  is reduced. Especially, when the diameter of the optical fiber preform is grater than 100 mm, the above effect is significant since the preform is easily affected by the unevenness of temperature distribution in the circumferential direction. 
         [0029]    In the specification, the diameter of the optical fiber preform means an outer diameter of a portion of the optical fiber preform where the outer diameter of the portion is approximately consistent. Also, in the first embodiment of the present invention, to achieve the starting point  1   b  of the meniscus portion  1   a , it satisfies a relationship 2E=L, where L is a distance, which defines a temperature distribution of the muffle tube  12 , and E is the inner diameter of the muffle tube  12 . 
         [0030]    Therefore, the drawing furnace  10  composes of simple construction without need for special devices. As for the relationship between the distance L and the inner diameter E, if 2E=L=1.5E, then the starting point  1   b  of the meniscus portion  1   a  can be positioned above the top  13   a  of the heater  13  as described in the examples below. 
       EXAMPLE 1 
       [0031]    As an example 1 of the present invention, an optical fiber preform, made from quartz glass, was drawn in a furnace at a drawing tension of 100 gf and a drawing speed of 1,500 m/min. The furnace has the same composition as the first embodiment of the invention, and comprises a muffle tube with an inner diameter E of 150 mm and a heater length I of 250 mm. An optical fiber preform having a diameter D of 125 mm and an average non-circularity (measured every 50 mm in length of the preform) of 0.08% was used. The average non-circularity of the drawn optical fiber (measured every 50 km in length of the fiber) was 0.10%. 
       EXAMPLE 2 
       [0032]    As an example 2 of the present invention, an optical fiber preform, made from quartz glass, was drawn in a furnace at a drawing tension of 100 gf and a drawing speed of 1,500 m/min. The furnace has the same composition as the first embodiment of the invention, and comprises a muffle tube with an inner diameter E of 180 mm and a heater length I of 350 mm (100 mm longer than the drawing furnace in example 1). An optical fiber preform having a diameter D of 150 mm and an average non-circularity (measured every 50 mm in length of the preform) of 0.07% was used. The average non-circularity of the drawn optical fiber (measured every 50 km in length of the fiber) was 0.10%. 
       COMPARATIVE EXAMPLE 1 
       [0033]    As a comparative example 1 of the present invention, the muffle tube of the drawing furnace used in the example 1 was exchanged with the one having an inner diameter of 180 mm, and the optical fiber preform used in the example 2 was drawn at a drawing tension of 100 gf and a drawing speed of 1,500 m/min. The average non-circularity of the drawn optical fiber (measured every 50 km in length of the fiber) was 0.21%. 
         [0034]      FIG. 4  shows temperature profiles inside of the muffle tubes in example 1 and comparative example 1 of the present invention. In  FIG. 4 , the x-axis shows distance in the longitudinal direction of the muffle tubes as measured from the top end of the muffle tube, and the y-axis shows temperature at the center axis of the muffle tube. Curve C 1  shows the case in example 1 and curve C 2  shows the case in comparative example 1. As shown in  FIG. 4 , in example 1, because the inner diameter E is 150 mm and the distance L is 250 mm, the relationship 2E=L=1.5E is satisfied. Contrary, in comparative example 1, since the inner diameter E was 180 mm and the distance L was 265 mm, the relationship 2E=L=1.5E is not satisfied. 
       COMPARATIVE EXAMPLE 2 
       [0035]    As a comparative example 2 of the present invention, an optical fiber preform, made from quartz glass, was drawn at a drawing tension of 100 gf and a drawing speed of 1,500 m/min. An optical fiber preform having a diameter D of 150 mm (as in example 2) and an average non-circularity (measured every 50 mm in length of the preform) of 0.08%, was used. As for the muffle tube, it has the same composition as the first embodiment of the invention, i.e., an inner diameter E of 180 mm and a heater length I of 250 mm. However, since the distance from the top of the heater to the top of the muffle tube is reduced, the distance L became 230 mm, and the meniscus starting point was approximately at the same location as the top of the heater. The average non-circularity of the drawn optical fiber (measured every 50 km in length of the fiber) was 0.25%. 
         [0036]      FIG. 5  shows shapes of the meniscus portions of the optical fiber preform in example 1 and comparative example 1. In  FIG. 5 , the x-axis shows distance in the longitudinal direction of the muffle tubes as measured from the top of the muffle tube, the left y-axis shows the outer diameter of the optical fiber preform, and the right y-axis shows the meniscus rate of change dD/dz of preform outer diameter. Curves C 3  and C 4  respectively represent the outer diameters D of example 1 and comparative example 1. Curves C 5  and C 6  respectively represent the meniscus rate of change dD/dz in example 1 and comparative example 1. When a distance N from a meniscus starting point to an inflection point was calculated, it was 200 mm. Since the diameter D of example 1 is 125 mm, example 1 satisfied the relationship N=1.5 D. On the other hand, the distance N in the comparative example 1 is 220 mm. Since the diameter D of comparative example 1 is 150 mm, this comparative example does not satisfy the relationship N=1.5 D. Also, lengths of the meniscus portions of example 1 and comparative example 1 are 490 mm and 580 mm, respectively. The absolute value of the meniscus rate of change dD/dz is maximized at the inflection point. The maximum values of the meniscus rate of change dD/dz in example I and comparative example 1 are 0.70 and 0.82, respectively. 
         [0037]    In accordance with the above results, as it was in example 1, if the maximum absolute value of a meniscus rate of change dD/dz in an optical fiber preform is less than 0.8, and the preform satisfies the relationship N=1.5 D, where D is a diameter of the preform in mm and N is a distance from a meniscus starting point to an inflection point in mm, then an optical fiber with small non-circularity can be obtained. Because the above conditions also reduce the length of the meniscus portion, the drawing furnace can be made smaller. 
         [0038]    Furthermore, the drawing furnace of the present invention does not limited to the configuration disclosed in above example 1. It has various alternative embodiments. Below, alternative embodiments of example 1 are disclosed. 
       Alternative Embodiment 1 
       [0039]      FIG. 6  is a schematic longitudinal cross section of an optical fiber drawing furnace, which is related to alternative embodiment 1 of the present invention. As shown in  FIG. 6 , the drawing furnace  20  has a furnace body  11 , a muffle tube  12 , a heater  13 , insulator  14 , a top cover  15 , which has a first opening  15   a  to receive an optical fiber preform, and a cooling section  16 , which has a second opening  16   a  to remove drawn optical fiber, as in the drawing furnace  10 . 
         [0040]    Furthermore, the drawing furnace  20  has a sub-heater  23 , which is placed above the heater  13  and covers the muffle tube  12 . The sub heater  23  has less power compared to the heater  13 , which for example, is used to increase maximum temperature to approximately 1,600° C. within the muffle tube  12 . It has simple construction and provides heat value evenly in the circumferential direction. Also, the heater  13  and the sub-heater  23  are separated by insulator  14  so that they can be controlled independently. Because the drawing furnace  20  has a sub heater  23 , it can satisfy the relationship 2 E=L=1.5 E, where E is the inner diameter of the muffle tube  12  and L is a distance defined the temperature distribution of the muffle tube  12 . Therefore, when an optical fiber preform  1  (shown by dotted line in  FIG. 6 ) is drawn by the drawing furnace  20 , an optical fiber  2  with small non-circularity can be drawn. 
       Alternative Embodiment 2 
       [0041]      FIG. 7  is a schematic longitudinal cross section of an optical fiber drawing furnace, which is related to alternative embodiment 2 of the present invention. As shown in  FIG. 7 , a drawing furnace  30  has a shorter body  31   a  than the body  11  of furnace  10 ; a muffle tube  32 ; and a heater  13 ; insulator  14 ; a top cover  15 , which has a first opening  15   a  to receive an optical fiber preform; and a cooling section  16 , which has a second opening  16   a  to remove drawn optical fiber, as in furnace  10 . Furthermore, the drawing furnace  30  has another furnace body  31   b  above the top cover  15  and outside of the furnace body  31   a . The furnace body  31   b  is concentric with the heater  13 , and it includes a sub-heater  33 . The sub-heater  33  has less power than heater  13 , and it can be used in ambient atmosphere by using a Kanthal heater. For example, it is used to increase maximum temperature to approximately 1,600° C. within the muffle tube  32 . It has a simple construction and provides heat value evenly in the circumferential direction. Drawing furnace  30  (as in alternative embodiment 1) can satisfy the relationship 2E=L=1.5E, where E is the inner diameter of the muffle tube  32  and L is a distance defined the temperature distribution of the muffle tube  32 . Therefore, an optical fiber  2  with small non-circularity can be drawn from an optical fiber preform  1 . 
       Alternative Embodiment 3 
       [0042]      FIG. 8  is a schematic longitudinal cross section of an optical fiber drawing furnace, which is related to the alternative embodiment 3 of the present invention. As shown in  FIG. 8 , the drawing furnace  40  has a furnace body  11 ; a heater  13 ; insulator  14 ; a top cover  15 , which has a first opening  15   a  to receive an optical fiber preform; and a cooling section  16 , which has a second opening  16   a  to remove a drawn optical fiber, as in furnace  10 . A muffle tube  42  is similar to muffle tube  12 . However, the muffle tube  42  has a thick tube portion  42   a , which is thicker than the adjacent portion of the heater  13  that is positioned on top of the heater  13 . Furthermore, the drawing furnace  40  has insulator  44 , which has higher thermal conductivity than the insulator  14 , on top of the heater  13 . Because of the thick tube portion  42   a  and the insulator  44 , the drawing furnace  40  (as in the alternative embodiments 1 and 2) can satisfy the relationship 2E=L=1.5E, where E is the inner diameter of the muffle tube  42  and L is a distance defined the temperature distribution of the muffle tube  42 . Therefore, an optical fiber  2  with low non-circularity can be drawn from an optical fiber preform  1 . 
       EXAMPLE 3-5 
       [0043]    As examples 3-5, optical fibers were drawn from the above alternative embodiments 1-3 at a drawing tension of 100 gf and a drawing speed of 1,500 m/min. The inner diameters E of the muffle tubes are 180 mm. The optical fibers were drawn from an optical fiber preform, which had a diameter D of 150 mm and an average non-circularity (measured every 50 mm in length of the preform) of 0.07%. A table below shows distance L; distance N; maximum absolute value of meniscus rate of change (dD/dz) max ; and non-circularity measured every 50 km in length of the drawn fibers. As shown in the above table, in all examples 3-5, (dD/dZ) max  were less than 0.8, N=1.5D, and non-circularities were small. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 L (mm) 
                 N (mm) 
                 (dD/dz) max   
                 Non-circularity (%) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 3 
                 300 
                 290 
                 0.77 
                 0.12 
               
               
                 Example 4 
                 330 
                 300 
                 0.76 
                 0.09 
               
               
                 Example 5 
                 270 
                 270 
                 0.8 
                 0.11