Patent Publication Number: US-2005132753-A1

Title: Optical fiber manufacture method, preform manufacture method, and preform manufacture apparatus

Description:
This patent application claims priority based on Japanese patent applications, H11-067366 filed on Mar. 12, 1999, H11-075129 filed on Mar. 19, 1999, H10-315856 filed on Nov. 6, 1998, H10-314564 filed on Nov. 5, 1998, H11-015293 filed on Jan. 25, 1999, H11-16840 filed on Jan. 26, 1999, H10-314574 filed on Nov. 5, 1998, H11-067199 filed on Mar. 12, 1999, H10-315849 filed on Nov. 6, 1998, H11-010197 filed on Jan. 19, 1999, H11-112354 filed on Apr. 20, 1999, H11-046141 filed on Feb. 24, 1999, H10-314553 filed on Nov. 5, 1998, H11-065819 filed on Mar. 12, 1999, H11-118094 filed on Apr. 26, 1999, H11-044902 filed on Feb. 23, 1999, and H11-064994 filed on Mar. 11, 1999, the contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of Invention  
      The present invention relates to an optical fiber manufacture method, a preform manufacture method and a preform manufacture apparatus that can manufacture a preform and an optical fiber with reduced variation in their diameters.  
      2. Description of Related Art  
       FIG. 1  shows a conventional glass base material first elongating apparatus  400 . A glass base material  102 , which is a base material of an optical fiber, is usually elongated by the glass base material first elongating apparatus  400 . This reduces the diameter of the glass base material  102 , to produce a glass rod  106 . The glass rod  106  has a diameter from 3 mm to 5 mm larger than the most convenient diameter to draw an optical fiber. The most convenient diameter for drawing an optical fiber is 30 mm to 80 mm.  
      A glass base material first elongating apparatus  400  comprises a heating furnace  100  that heats the glass base material  102  and a drawing chuck  104  that holds and elongates the heated glass base material  102 . To elongate the glass base material  102 , the glass base material first elongating apparatus  400  supplies the glass base material  102  to the heating furnace  100 . Here the glass base material  102  is heated to approximately 2000° C. The first elongating apparatus  400  then holds the glass base material  102  by the drawing chuck  104 , and draws the glass base material  102  from the heating furnace  100  downward continuously to form a glass rod  106 .  
       FIG. 2  shows a configuration of a conventional glass lathe  110 . The glass rod  106  made by the glass base material first elongating apparatus  400  undergoes secondary elongation by the glass lathe  110  to produce a preform  107 . At this time, the diameter of the glass rod  106  is reduced to prescribed diameter. The glass lathe  110  comprises chucks  118  and  119  that hold the glass rod  106 , a tail stock  116  which moves the chuck  119 , and a heating source  122  which heats the glass rod  106 . One side of the chuck  118  is fixed, and the other side of the chuck  119  movable. A traction force can be applied to the chuck  119 . The glass rod  106 , which is held by the chucks  118  and  119 , is heated by the heating source  122 . The heated glass rod  106  is elongated by moving the tail stock  116  which pulls the glass rod  106 . The result is, the diameter of the glass rod  106  reduces to become the prescribed diameter.  
      There was the possibility of manufacturing bent glass rods  106  when using a conventional glass base material first elongating apparatus  400  to elongate the glass base material  102 . Also, when using a conventional glass lathe  110  to elongate the glass rod  106  to manufacture the preform  107  further problems often arose. These problems included variation in the diameter of the preform  107  because the amount of gas provided to the heating source  122  and the speed of moving the tail stock  116  differed for each preform  107  produced.  
      When elongating a bent glass rod  106 , which is made by a conventional glass base material first elongating apparatus  400 , to make a preform  107  by the glass lathe  110 , the diameter of the preform  107  varied. When manufacturing optical fibers by drawing a preform  107  with a varying diameter, the diameter of the optical fibers produced also varies. This makes it difficult to manufacture an optical fiber of high quality.  
     SUMMARY OF THE INVENTION  
      As stated, it is an object of the present invention to provide an optical fiber manufacture method, a preform manufacture method and a preform manufacture equipment that can solve the problems outlined above. The object of the present invention can be achieved by the combinations of features described in the independent claims of the present invention. The dependent claims define further advantageous embodiments of the present invention.  
      According to the first aspect of the present invention, a method for manufacturing an optical fiber can be provided which comprises setting a heating condition for heating a glass rod, which is a parent material of the optical fiber, and an elongating speed of the glass rod based on a prescribed numerical value which changes with a progress of elongation of the glass rod; heating and elongating the glass rod to generate a preform based on the heating condition and the elongating speed which are set by the setting; and drawing the preform to a filament-like form by further heating the preform to generate the optical fiber.  
      A method for manufacturing an optical fiber can be provided such that the setting sets the heating condition and the elongating speed based on a progress time of the elongation as the numerical value. The heating and elongating may include end drawing for reducing a diameter of an end of the glass rod, and the end drawing end-draws the end of the glass rod with heat and elongation based on the progress time of the end drawing.  
      A method for manufacturing an optical fiber can be provided such that the setting sets a location of a burner, which heats the glass rod, and an amount of gas supplied to the burner as the heating condition based on the progress time of the elongation. The setting may set a moving speed of a chuck, which holds the glass rod, as the elongating speed based on the progress time of the elongation.  
      A method for manufacturing an optical fiber can be provided such that the setting sets the heating condition and the elongating speed based on an elongation length of the glass rod in the elongation as the numerical value.  
      A method for manufacturing an optical fiber can be provided such that the heating and elongating includes end drawing for reducing a diameter of an end of the glass rod, and the end drawing end-draws the end of the glass rod with heat and elongation based on the elongation length of the glass rod. The setting may set a moving distance of a burner, which heats the glass rod, and an amount of gas supplied to the burner as the heating condition based on the elongation length of the glass rod. The setting can further set a moving speed of a chuck, which holds the glass rod, as the elongating speed based on the elongation length of the glass rod.  
      A method for manufacturing an optical fiber can be provided such that the setting uses a encoder, which is provided on a motor that drives the chuck, to measure a moving distance of the chuck by measuring a rotation angle of the motor.  
      A method for manufacturing an optical fiber can be provided such that the setting sets the heating condition and the elongating speed based on a tensile stress generated on the glass rod in the elongation as the numerical value.  
      A method for manufacturing an optical fiber can be provided such that a heating source, which heats the glass rod, moves along a longitudinal direction of the glass rod with a progress of the elongation, and the heating and elongating controls the elongating speed so that the tensile stress before the heating source moves prescribed distance becomes substantially 110 percent or below an average value of the tensile stress after the heating source moves the prescribed distance.  
      A method for manufacturing an optical fiber can be provided such that the heating and elongating controls the tensile stress so that the tensile stress before the heating source moves the prescribed distance become substantially from 80 to 110 percent of an average value of the tensile stress after the heating source moves the prescribed distance.  
      The prescribed distance can be substantially between 50 mm to 150 mm. The heating and elongating may control the elongating speed to be a constant speed when the heating source moves the prescribed distance. The setting may set a moving speed of a chuck, which holds the glass rod, as the elongating speed based on the tensile stress.  
      A method for manufacturing an optical fiber can be provided such that the setting sets the heating condition and the elongating speed based on a location of a mark provided on a connection between the glass rod and each of dummy rods, which are welded to each of ends of the glass rod, as the numerical value.  
      A method for manufacturing an optical fiber can be provided such that the heating and elongating includes end drawing for reducing a diameter of an end of the glass rod, and the end drawing end-draws the end of the glass rod with heat and elongation based on the location of a mark. The setting can set the heating condition and the elongating speed based on a location of a cut provided on a connection between the glass rod and each of the dummy rods as the location of a mark.  
      A method for manufacturing an optical fiber can be provided such that the setting sets the heating condition and the elongating speed based on a location of a fluorescent paint applied on a connection between the glass rod and each of the dummy rods as the location of a mark.  
      A method for manufacturing an optical fiber can be provided such that the setting sets the elongating speed at a plurality of locations along axial direction of the glass rod based on a diameter at the plurality of locations along axial direction of the glass rod as the numerical value and the heating condition based on an average value of a diameter at the plurality of locations of the glass rod.  
      A method for manufacturing an optical fiber can be provided such that a end of the glass rod is end-drawn of which diameter is reduced, and the setting has detecting a location of an end-drawn region where the glass rod is end-drawn based on a diameter at a plurality of locations along axial direction of the glass rod and a change of a length of the glass rod along axial direction of the glass rod by the elongationas the numerical value, and setting a polishing range where the glass rod is polished by a flame based on the location of the end-drawn region and also setting a heating power condition of the flame based on a diameter of the end-drawn region, and the heating and elongating polishes the polishing range of the glass rod by the flame of the heating power condition.  
      According to the other aspect of the present invention, a method for manufacturing an optical fiber can be provided which comprises heating and elongating a glass rod, which is a parent material of an optical fiber, to generate a preform, drawing the preform with further heating to a filament-like form to generate an optical fiber, and the heating and elongating has pre-heating the glass rod until prescribed region of the glass rod softens, and end drawing the prescribed region for reducing a diameter of the prescribed region and for making an end of the glass rod by further heating and elongating the prescribed region.  
      A method for manufacturing an optical fiber can be provided such that the end drawing further includes second heating which heats by a flame a region which is more towards a middle side of the glass rod than a center of the prescribed region, a thickness of the flame being smaller than a thickness of the flame of the pre-heating.  
      According to the first aspect of the present invention, a method for manufacturing a preform, which is a parent material of an optical fiber, can be provided which comprises setting a heating condition for heating a glass rod, which is a parent material of the optical fiber, and an elongating speed of the glass rod based on a prescribed numerical value which changes with a progress of elongation of the glass rod, heating and elongating the glass rod to generate a preform based on the heating condition and the elongating speed which are set by the setting.  
      A method for manufacturing a preform can be provided such that the setting sets the heating condition and the elongating speed based on a progress time of the elongation as the numerical value.  
      A method for manufacturing a preform can be provided such that the heating and elongating includes end drawing for reducing a diameter of an end of the glass rod, and the end drawing end-draws the end of the glass rod with heat and elongation based on the progress time of the end drawing. The setting may set the heating condition and the elongating speed based on an elongation length of the glass rod in the elongation as the numerical value. The heating and elongating can include end drawing for reducing a diameter of an end of the glass rod, and the end drawing end-draws the end of the glass rod with heat and elongation based on the elongation length of the glass rod.  
      A method for manufacturing a preform can be provided such that the setting sets the heating condition and the elongating speed based on a tensile stress generated on the glass rod in the elongation as the numerical value.  
      A method for manufacturing a preform can be provided such that a heating source, which heats the glass rod, moves along a longitudinal direction of the glass rod with a progress of the elongation, and the heating and elongating controls the elongating speed so that the tensile stress before the heating source moves prescribed distance becomes substantially 110 percent or below an average value of the tensile stress after the heating source moves the prescribed distance.  
      A method for manufacturing a preform can be provided such that the heating and elongating controls the tensile stress so that the tensile stress before the heating source moves the prescribed distance become substantially from 80 to 110 percent of an average value of the tensile stress after the heating source moves the prescribed distance. The prescribed distance can be substantially between 50 mm to 150 mm. The heating and elongating may control the elongating speed to be a constant speed when the heating source moves the prescribed distance.  
      A method for manufacturing a preform can be provided such that the setting sets the heating condition and the elongating speed based on a location of a mark provided on a connection between the glass rod and each of dummy rods, which are welded to each of ends of the glass rod, as the numerical value. The heating and elongating can include end drawing for reducing a diameter of an end of the glass rod, and the end drawing end-draws the end of the glass rod with heat and elongation based on the location of a mark.  
      A method for manufacturing a preform can be provided such that the setting sets the elongating speed at a plurality of locations along axial direction of the glass rod based on a diameter at the plurality of locations along axial direction of the glass rod as the numerical value and the heating condition based on an average value of a diameter at the plurality of locations of the glass rod.  
      A method for manufacturing a preform can be provided such that a end of the glass rod is end-drawn of which diameter is reduced, and the setting has detecting a location of an end-drawn region where the glass rod is end-drawn based on a diameter at a plurality of locations along axial direction of the glass rod and a change of a length of the glass rod along axial direction of the glass rod by the elongation as the numerical value, and setting a polishing range where the glass rod is polished by a flame based on the location of the end-drawn region and also setting a heating power condition of the flame based on a diameter of the end-drawn region, and the heating and elongating polishes the polishing range of the glass rod by the flame of the heating power condition.  
      According to the other aspect of the present invention, a method for manufacturing a preform, which is a parent material of an optical fiber, can be provided which comprises pre-heating the glass rod until a prescribed region of the glass rod softens, and end drawing the prescribed region for reducing a diameter of the prescribed region and for making an end of the glass rod by further heating and elongating the prescribed region. The end drawing may further include second heating which heats by a flame a region which is more towards a middle side of the glass rod than a center of the prescribed region, a thickness of the flame being smaller than a thickness of the flame of the pre-heating.  
      According to the first aspect of the present invention, an apparatus for manufacturing a preform, which is a parent material of an optical fiber, can be provided which comprises a heating source which heats a glass rod, which is a parent material of the preform, an elongating unit which elongates the glass rod, a measurement device for measuring a numerical value which changes with a progress of elongation of the glass rod, and a control unit which controls a heating condition of the heating source and a elongating speed of the elongating unit based on the numerical value measured by the measurement device.  
      An apparatus for manufacturing a preform can be provided such that the measurement device measures a progress time of the elongation as the numerical value, and the control unit controls the heating condition and the elongating speed based on the progress time of the elongation measured by the measurement device.  
      An apparatus for manufacturing a preform can be provided such that the measurement device measures a moving distance of the elongating unit which changes with a progress of the elongation as the numerical value, and the control unit controls the heating condition and the elongating speed based on the moving distance of the elongating unit measured by the measurement device.  
      An apparatus for manufacturing a preform can be provided such that the measurement device measures a tensile stress generated on the glass rod by the elongation as the numerical value, and the control unit controls the heating condition and the elongating speed based on the tensile stress generated on the glass rod measured by the measurement device.  
      An apparatus for manufacturing a preform can be provided such that the heating source moves along a longitudinal direction of the glass rod with a progress of the elongation, and the control unit controls the elongating speed so that the tensile stress before the heating source moves prescribed distance becomes substantially 110 percent or below an average value of the tensile stress after the heating source moves the prescribed distance.  
      An apparatus for manufacturing a preform can be provided such that the control unit controls the tensile stress so that the tensile stress before the heating source moves the prescribed distance becomes substantially from 80 to 110 percent of an average value of the tensile stress after the heating source moves the prescribed distance. The prescribed distance can be substantially between 50 mm to 150 mm. The control unit may control the elongating speed to be a constant speed when the heating source moves the prescribed distance.  
      An apparatus for manufacturing a preform can be provided such that the measurement device measures a location of a mark provided on a connection between the glass rod and each of dummy rods, which are welded to each of ends of the glass rod, as the numerical value, and the control unit controls the heating condition and the elongating speed based on the location of a mark measured by the measurement device.  
      An apparatus for manufacturing a preform can be provided such that the measurement device measures a diameter at a plurality of locations along axial direction of the glass rod as the numerical value, and the control unit controls the elongating speed at the plurality of locations along axial direction of the glass rod based on a diameter at the plurality of locations along axial direction of the glass rod, and the heating condition based on an average value of a diameter at the plurality of locations. 
    
    
     BRIEF DESCRIPTION OF THE ELONGATINGS  
       FIG. 1  shows a conventional glass base material first elongating apparatus  400 .  
       FIG. 2  shows a configuration of a conventional glass lathe  110 .  
       FIG. 3  shows a system of an optical fiber manufacturing apparatus of present invention.  
       FIG. 4  shows an optical fiber manufacturing method of the present invention.  
       FIG. 5  shows a configuration of a glass base material first elongating apparatus  900 .  
       FIG. 6  shows a first elongating device  402  that holds a standard rod  138  by a base material fix unit  136  to adjust the axis for elongating a glass base material  102 .  
       FIG. 7  shows a detailed flow chart of a glass base material first elongating (S 204 ) shown in  FIG. 4 .  
       FIG. 8  shows the first elongating device  402  that holds the standard rod  138  by the elongating chuck  142 .  
       FIG. 9  shows the first elongating device  402 , which holds the standard rod  138  by both of the hanging mechanism  134  and the elongating mechanism  140 .  
       FIG. 10  shows an example using elongating rollers  144   a  and  144   b  instead of the elongating chuck  142  on the elongating mechanism  140 .  
       FIG. 11  shows an example using elongating rollers  144   a  and  144   b  instead of the elongating chuck  142  on the elongating mechanism  140 .  
       FIG. 12  shows the glass base material  102 , the bending degree of which is measured.  
       FIG. 13  shows a mechanism by which the first elongating device  402  controls the speed of rotation of the elongating roller  144   a  and  144   b.    
       FIG. 14  shows a relationship between the amount of deviation between the center position of the heat softened region of the glass base material  102  and elongating axis  154 , and the degree of bend of the glass rod  106 .  
       FIG. 15  shows a deformation of the surface of the elongating rollers  144   a  and  144   b.    
       FIG. 16  shows displacement of the metal pipe when the metal pipe is carried by the elongating rollers  144   a  and  144   b  of batch number  300  shown in  FIG. 15 .  
       FIG. 17  shows the displacement of the center position of the heat softened region by the first elongating device  402  of the embodiment.  
       FIG. 18  shows a fluctuation of the center position of the heat softened region when the rotation speed of the elongating rollers  144   a  and  144   b  are controlled at the same rotation speed.  
       FIG. 19  shows an another embodiment of the burner  176  used in the glass rod fusing apparatus  370  shown in  FIG. 5 .  
       FIG. 20  shows a configuration of a glass rod transportation device  380 .  
       FIG. 21  shows a storage container  224  of the first elongating device  402 .  
       FIG. 22  shows a movement of the glass rod transportation device  380  when transporting the glass rod  106 .  
       FIG. 23  shows an another embodiment of the glass rod transportation device  380 .  
       FIG. 24  shows a movement of the glass rod transportation device  380  shown in  FIG. 23  when the glass rod transportation device  380  transports the glass rod  106 .  
       FIG. 25  shows a configuration of a glass rod second elongating apparatus  111  of the present invention.  
       FIG. 26  shows a detailed flow chart of the glass rod second elongating (S 206 ) shown in  FIG. 4 .  
       FIG. 27  shows an example where a cooling device  330  is provided on the fixed chuck  118  and the movable chuck  119  of the glass rod second elongating apparatus  111 .  
       FIG. 28  shows the temperature of the fixed chuck  118  and the movable chuck  119  of the example and the comparative example.  
       FIG. 29  shows a relationship between the distance between the heating source  122  and the diameter measurement device  124 , and the percentage of the fluctuation of the diameter of the glass rod  106 .  
       FIG. 30  shows a glass rod second elongating apparatus  111  that has a tensile stress measurement device  282 .  
       FIG. 31  shows a detailed flow chart of the elongating (S 154 ) shown in the  FIG. 26 .  
       FIG. 32  shows the process of diameter fluctuation during the elongation of the glass rod  106 .  
       FIG. 33  shows a glass rod  106  that is elongated according to the elongating (S 154 ) shown in  FIG. 31 .  
       FIG. 34  shows the tensile stress of the glass rod  106  at the early stage of the elongation of the example.  
       FIG. 35  shows the fluctuation of the tensile stress of the glass rod  106  at an early stage of the elongation of the comparative example.  
       FIG. 36  shows fluctuation of the diameter of the glass rod  106  after the elongation of the glass rod  106 .  
       FIG. 37  shows a detailed flowchart of the end drawing (S 158 ) shown in  FIG. 26 .  
       FIG. 38  shows a cut  284  which is provided on the connection between the glass rod  106  and the dummy rod  108  at the end drawing position detecting (S 169 ) shown in  FIG. 37 .  
       FIG. 39  shows a marking  287  that is applied on the connection between the glass rod  106  and the dummy rod  108  as another example of a mark.  
       FIG. 40  shows the glass rod second elongating apparatus  111  that detects the cut  284  at end drawing position detecting (S 169 ).  
       FIG. 41  shows the movements of the heating source  122  and the tail stock  116  during the end drawing process of the glass rod  106  shown in flow chart of  FIG. 37 .  
       FIG. 42  shows an example of the settings of an another method of the end drawing process at the end drawing (S 158 ) shown in  FIG. 37 .  
       FIG. 43  shows another example of the settings of another method of the end drawing process at the end drawing (S 158 ) shown in  FIG. 37 .  
       FIG. 44  shows a configuration of the heating source  122  of the glass rod second elongating apparatus  111 .  
       FIG. 45  shows a plan view of the top of the heating source  122 .  
       FIG. 46  shows a relationship between the linear speed of the oxygen gas and the temperature of the top of the heating source  122 .  
       FIG. 47  shows a shape of a tip of the preform  107 , the diameter of which is reduced and fused at the end drawing (S 158 ).  
       FIG. 48  shows another shape of the tip of the preform  107  that was end elongated.  
       FIG. 49  shows damage of the preform  107  before the preform  107  is surface treated in the surface treatment (S 168 ) shown in the  FIG. 26 .  
       FIG. 50  shows the preform  107   a , which was treated by the hydrofluoric acid etching on the example shown in FIGS.  51  and  FIG. 52 .  
       FIG. 51  shows the number of hydrofluoric concaves generated on the preform  107  counted by visual inspection of the example and the comparative example.  
       FIG. 52  shows the unevenness of the surface of the preform  107  after treatment of the hydrofluoric acid etching of the example and the comparative example.  
       FIG. 53  shows another shape of the preform  107  which is surface treated.  
       FIG. 54  shows an ultrasonic cleaning apparatus  404 , which cleans the heating source  122 .  
       FIG. 55  shows a configuration of the preform drawing apparatus  500  that elongates the preform  107  to produce an optical fiber. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will be explained using embodiments of the present invention. The following embodiments however, do not limit the scope of the present invention described in the claims. Moreover, not all the features or their combinations described in the embodiments are necessarily essential for the present invention.  
      Although the present invention has been described with reference to specific embodiments, the scope of the present invention is not limited to these embodiments. Those skilled in the art can make various modifications and improvements to the embodiments of the present invention. It is clear from the appended claims that such modifications or improvements are also covered by the scope of the present invention.  
       FIG. 3  shows a system of an optical fiber manufacturing apparatus of the present invention. The system of the optical fiber manufacturing apparatus of present invention comprises a glass base material generating apparatus  600  which generates a glass base material  102  being a base material of an optical fiber; a glass base material dehydrating and sintering apparatus  700  which dehydrates and sinters the glass base material  102 ; a glass base material first elongating apparatus  900  which elongates the glass base material  102  to generate a glass rod  106 ; a glass rod transportation device  380  which transports the glass rod  106 ; a glass rod second elongating apparatus  111  which elongates the glass rod  106  a second time to generate a preform  107 ; and a preform drawing apparatus  500  which draws the preform  107  to generate an optical fiber.  
       FIG. 4  shows an optical fiber manufacturing method of the present invention. The glass base material  102  is generated by the glass base material generating apparatus  600  using the VAD method, vapor-phase axial deposition method, or the like (S 200 ). The glass base material  102  is then dehydrated within a chlorine gas atmosphere and sintered within an inert-gas atmosphere by the glass base material dehydrating and sintering apparatus  700  (S 202 ).  
      The diameter of the glass base material  102  is normally 110 mm to 200 mm, compared to a diameter of 30 mm to 80 mm which is most practical for drawing to an optical fiber. Therefore, the dehydrated and sintered glass base material  102  is elongated firstly by the glass base material first elongating apparatus  900  to produce a glass rod  106  (S 204 ). The glass rod  106  has a diameter 3 mm to 5 mm larger than the diameter convenient for drawing to an optical fiber, which is from 30 mm to 80 mm.  
      The glass rod  106  is transported by the glass rod transportation device  380  (S 205 ). The glass rod  106  is then heated and elongated by the glass rod second elongating apparatus  111  to a prescribed diameter, thus producing a preform  107  (S 206 ) The preform  107  is heated and drawn to a filament-like form by the preform drawing apparatus  500  to produce an optical fiber (S 210 ).  
       FIG. 5  shows a configuration of a glass base material first elongating apparatus  900 . The glass base material first elongating apparatus  900  comprises a first elongating device  402  which heats and elongates the glass base material  102  and a glass rod fusing apparatus  370  which fusing the glass rod  106 . The first elongating device  402  has a elongating furnace  130 , which has a heating furnace  100 , and a hanging mechanism  134  which is provided above the elongating furnace  130 . The hanging mechanism  134  supplies the glass base material  102  to the inside of the elongating furnace  130  at a prescribed speed.  
      The first elongating device  402  further has an elongating mechanism  140  which is provided under the elongating furnace  130  to hold the glass rod  106  of reduced diameter and to pull the glass rod  106  at a prescribed speed. The hanging mechanism  134  has a base material fix unit  136  that holds the glass base material  102 . The elongating mechanism  140  has an elongating chuck  142  that holds the glass rod  106 . The glass rod fusing apparatus  370  has a burner  176 , a rotating table  210 , a timing belt  214 , a motor  212 , a supporting leg  208 , a burner stand  216 , an elongating device  206 , and an elongating fusion chuck  218 .  
      The glass base material  102  is installed on the base material fix unit  136 , and sent into the heating furnace  100  at a prescribed speed. The glass base material  102  heated by the heating furnace  100  is held and pulled by the elongating chuck  142  to reduce the diameter thus producing a glass rod  106 . The glass rod  106  is pulled by the elongating device  206  at a speed which is suitable for the diameter to be obtained, so that the glass base material  102  is elongated to the desired diameter. At this time, the diameter of the glass rod  106  is measured by a diameter measuring device  152 . The feeder  204 , heating furnace  100 , and elongating device  206  are controlled based on this measurement in order to elongate the glass rod  106  to the desired diameter.  
      The glass rod  106 , which has been elongated to a prescribed diameter and length, is fused by the burner  176  at the part that does not include the bubble or does not include the bubble of which diameter is substantially 0.3 mm or above. A flame of oxygen and hydrogen is a desirable heating means of the burner  176 . A gas flame of based on hydrocarbon fuels such as propane and oxygen can also be used for the burner  176 .  
      The burner  176  is installed on the rotating table  210  via the supporting leg  208 . The rotating table is rotated by a driving device such as motor  212  via the timing belt  214 . The rotating table  210  is installed on the burner stand  216 . The glass rod fusing apparatus  370  fuses the glass rod  106  by heating the glass rod  106  with the rotating the burner  176  and elongates the glass rod  106  using the elongating fusion chuck  218  with a prescribed speed and pull strength.  
       FIG. 6  shows a first elongating device  402  which holds a standard rod  138  by a material fix unit  136  to adjust the axis for elongating a glass base material  102 . The hanging mechanism  134  has a mechanism not shown in the figure, that adjusts the vertical inclination of the base material fix unit  136 . The elongating mechanism  140  has a mechanism, also not shown in the figure, that adjusts the vertical inclination of the elongating chuck  142 . The elongating mechanism  140  further has a mechanism, again not shown in the figure, that adjusts the position of the elongating mechanism  140  within the horizontal phase in the directions back and forth and left and right.  
       FIG. 7  shows a detailed flow chart of a glass base material first elongating (S 204 ) shown in  FIG. 4 . The glass base material first elongating (S 204 ) has a process to adjust the elongating axis of the first elongating device  402 . First, a metal or ceramic rod is prepared as a standard rod  138 . The straightness of the standard rod  138  should be guaranteed. The standard rod  138  usually has a length of a glass base material  102  and dummy rod that is welded onto the glass base material  102 . The straightness of the axis of the standard rod  138  is guaranteed along the full length.  
      As shown in  FIG. 6 , the standard rod  138  is held by the base material fix unit  136  of the hanging mechanism  134  (S 110 ). Then, the inclination A of the hanging mechanism  134  is adjusted so that the direction of the standard rod  138  matches with the vertical direction (S 112 ). Following this, the standard rod  138  is removed from the base material fix unit  136  after finishing the adjustment (S 114 ).  
       FIG. 8  shows the first elongating device  402  that holds the standard rod  138  by the elongating chuck  142 . The standard rod  138  is held by the elongating chuck  142  of the elongating mechanism  140  ( FIG. 7 , S 116 ), Then the inclination B of the elongating mechanism  140  is adjusted so that the direction of the standard rod  138  matches with the vertical direction ( FIG. 7 , S 118 ). At this time, it is desirable that the elongating chuck  142  maintains the approximate center of longitudinal direction of the standard rod  138 . The procedure for adjusting the hanging mechanism  134  and the elongating mechanism  140  can be reversible. The elongating mechanism  140  can be adjusted first, and then the hanging mechanism  134  can be adjusted.  
       FIG. 9  shows the first elongating device  402 , which holds the standard rod  138  by both the hanging mechanism  134  and the elongating mechanism  140 . After finishing the adjustment of the hanging mechanism  134  and the elongating mechanism  140 , by holding the standard rod  138  by the base material fix unit  136 , the lower end of the standard rod  138  is held by the elongating chuck  142  ( FIG. 7 , S 120 ). Then, the horizontal direction position C of the elongating mechanism  140  or the horizontal direction position C of the hanging mechanism  134  is adjusted so that the difference in horizontal direction between the vertical axis and the standard rod  138  is less than 0.5 mm per 1 m length ( FIG. 7 , S 122 ).  
      Following this, a glass rod  106  is generated by elongating the glass base material  102  using the first elongating device  402 , the elongating axis of which is adjusted ( FIG. 7 , S 124 ). Finally, the glass rod  106  is fused by the glass rod fusing apparatus  370  ( FIG. 7 , S 126 ).  
       FIG. 10  and  FIG. 11  show examples that use elongating rollers  144   a  and  144   b  on the elongating mechanism  140  instead of the elongating chuck  142 . To adjust the vertical inclination of the axis connecting the hanging mechanism  134  and the elongating mechanism  140  in the case of using the elongating rollers  144   a  and  144   b , the following method is adopted. The standard rod  138  is held by the elongating rollers  144   a  and  144   b  as opposed to the holding of the standard rod  138  by the elongating chuck  142  ( FIG. 7 , S 116 ).  
      Following this, the inclination of the elongating mechanism  140  is adjusted by adjusting the horizontal inclination of the line F. The line F connects the two rotation axis between the elongating rollers  144   a  and  144   b . After the adjustment of the inclination of the elongating mechanism  140  ( FIG. 7 , S 118 ), the elongating rollers  144   a  and  144   b  can hold the standard rod  138  vertically.  
      Next, as shown in  FIG. 11 , the standard rod  138  is held by the base material fix unit  136  of the hanging mechanism  134  and the elongating rollers  144   a  and  144   b  of the elongating mechanism  140  at the step corresponding to holding the standard rod  138  by the base material fix unit  136  and the elongating chuck  142  ( FIG. 7 , S 120 ). Then, the vertical inclination E of the axis which connects the hanging mechanism  134  and elongating mechanism  140  is adjusted. This adjustment is made either by adjusting the position of the elongating mechanism  140  in the horizontal direction or adjusting the position of the hanging mechanism  134  in the horizontal direction at the step corresponding to adjustment of the horizontal direction position of the hanging mechanism  134  and the elongating mechanism  140  ( FIG. 7 , S 122 ).  
      The vertical inclination of the axis connecting the hanging mechanism  134  and elongating mechanism  140  can be readily adjusted using the adjusting method shown above. This method is suitable not only for elongating the straight glass base material  102  without any gap between the dummy rod and the glass base material  102 , but also for elongating a glass base material  102  with some bending, to obtain a glass rod  106  with reduced diameter within a desired range of straightness. This is possible, provided the glass base material  102  is welded onto the dummy rod without a gap between the axis of the glass base material  102  and the dummy rod.  
      The first elongating device  402  can adjust the vertical inclination of the elongating axis accurately for the methods of holding the glass base material  102  by either the hanging mechanism  134 , the elongating mechanism  140  or by both the hanging mechanism  134  and the elongating mechanism  140 . Therefore, the bending moment, which causes bending on the heat softened region of the glass base material  102  can be decreased. Bending is generated by the weight of the elongated glass base material  102  as it bears on the elongating mechanism  140 . The glass base material  102  can therefore be elongated within a desired range of straightness without causing a gap between the axis of the glass base material  102  and the dummy rod.  
       FIG. 12  shows the glass base material  102 , the bending degree of which is measured. The glass base material  102  is elongated by the first elongating device  402 , the vertical inclination of which is adjusted by the adjusting method shown above. Then, the degree of bending of the glass rod  106  is measured. First, the glass rod  106  is placed on two bearings  148  and  149 , which are installed horizontally so that the line connecting the top of bearings  148  and  149  can be a standard line. Next, the maximum or minimum value of the height from the standard line is measured by scanning the measuring device  150  along the glass rod  106  using a device such as a dial gauge.  
      Then, the glass rod  106  is rotated 180 degrees, and the maximum and minimum value of the height from the standard line is measured in the same way. The maximum value of the difference between the first measured maximum value and the next measured minimum value or the difference of the first measured minimum value and the next measured maximum value is set as “2h”. The value that divides the “h” by the length L 1 , which is a distance between two bearings  148  and  149 , represents the straightness of the glass rod  106  per unit of length.  
      5 pieces of the straight glass base material  102  without the gap of axis with dummy rod were elongated by the first elongating device  402  with an adjusted elongating axis to produce 5 of glass rod  106 . The straightness of each of the glass rods  106  was measured by the method shown in  FIG. 12 . The “h” of the glass rods  106  were all within 0.5 mm. Next, the glass rods  106  were elongated by the first elongating device  402  without adjustment of the elongating axis. An average of 90 percent of the glass rods  106  were bent which indicates that the glass rod  106  should be corrected through adjustment of the elongating axis.  
       FIG. 13  shows a mechanism by which the first elongating device  402  controls the speed of rotation of the elongating rollers  144   a  and  144   b . The first elongating device  402  controls the rotation speed of each of the elongating rollers  144   a  and  144   b  respectively. The glass base material  102  is hung by the base material fix unit  136  of the first elongating device  402  and sent to the heating furnace (not shown in the figure) at a prescribed speed. The glass rod  106 , which is heated and softened by the heating furnace, is taken by the pair of elongating rollers  144   a  and  144   b.    
      The center position of the heat softened region of the glass base material  102  is obtained by measuring the diameter of the heat softened region of the glass base material  102  using the diameter measuring device  152 . At the same time the center position of the measured diameter is calculated. A laser beam transmission type diameter measuring device is used as the diameter measuring device  152 . The laser beam is irradiated onto the heat softened region of the glass base material  102  through the window provided on the lower part of the heater in the heating furnace.  
      The measured diameter is input to the diameter control unit  156 , and the difference between the target diameter value and the measured diameter is calculated. The rotation speed of the elongating roller  144   a  is controlled based on the calculated difference of the diameter. Then, the information on the center position of the heat softened region is input to the position control unit  158 .  
      The position control unit  158  calculates the amount of deviation between the center position of the heat softened region and the elongating axis  154  of the first elongating device  402 . The position control unit  158  further calculates the correction value of the rotation speed, which can reduce the deviation between the center position of heat softened region and the elongating axis  154  to virtually zero. Then, the position control unit  158  controls the rotation speed of the elongating roller  144   b  based on the addition of the correction value and the rotation speed of the elongating roller  144   a.    
       FIG. 14  shows a relationship between the amount of deviation between the center position of the heat softened region of the glass base material  102  and the elongating axis  154 , and the degree of bend caused in the glass rod  106 . The larger the amount of deviation between the center position of the heat softened region of the glass base material  102  and elongated axis  154 , the larger the resultant bend in the glass rod  106 .  
      When the amount of deviation is large, the heat-resistant members on the surface of the elongating rollers  144   a  and  144   b  are deformed. The shapes of the elongating rollers  144   a  and  144   b  become slightly different to each other. The result is the rotation speeds of the surfaces of the elongating rollers  144   a  and  144   b  are different to each other. Since the deformation of the surface of the elongating rollers  144   a  and  144   b  is one of the causes of the bending of the glass rod  106 , the bend of the glass rod  106  can be reduced by controlling the rotation speed of each of the elongating rollers  144   a  and  144   b  respectively.  
      The surfaces of the elongating rollers  144   a  and  144   b  are formed from a heat-resistant material such as non-asbestos or asbestos. These materials are heat resistant and flexible, so that the elongating rollers  144   a  and  144   b  can easily elongate the glass rod  106  at high temperatures. The surface of the elongating rollers  144   a  and  144   b  that come into contact with the glass rod  106  are gradually deformed by the high temperature and pinching force or friction force of the glass rod  106 . Because the deformation of the elongating rollers  144   a  and  144   b  is slightly different to each other, the rotation speed of the surfaces of the elongating rollers  144   a  and  144   b  also differs.  
       FIG. 15  shows deformation of the surfaces of the elongating rollers  144   a  and  144   b . The outside shape of the elongating roller  144   a  and the elongating roller  144   b  is different. The number of batches is the number of glass base materials  102  which were elongated. As the number of batches is increased, the deformation and abrasion is progressed. The result is, the amount of elongation becomes different between the elongating rollers  144   a  and  144   b , which causes fluctuation in the position of the heat softened region of the glass base material  102  which in turn causes bending of the glass rod  106 .  
       FIG. 16  shows displacement of the center position of the heated region of the metal pipe when the metal pipe is taken by the elongating rollers  144   a  and  144   b  at batch number  300  shown in  FIG. 15 . The vertical axis shows the displacement of the center position of the heated region of the metal pipe, and the horizontal axis shows time. The curve A shows the fluctuation of the amount of deviation in the direction of rotation of the elongating rollers  144   a  and  144   b . The curve A shows that the displacement fluctuates largely during a single rotation of the elongating rollers  144   a  and  144   b . The curve B shows that the fluctuation of displacement is quite small for the axis direction of the elongating rollers  144   a  and  144   b.    
       FIG. 17  shows displacement of the center position of the heat softened region by the first elongating device  402  of the embodiment. The vertical axis shows the displacement of the center position of the heat softened region of the glass base material  102 , and the horizontal axis shows the time from the start of the elongation. The displacement of the heat softened region is controlled and maintained at a small level after 1500 seconds from the start of the elongation. Therefore, a glass rod  106  without a substantial bend can be manufactured by controlling the rotation speed of the each of the elongating rollers  144   a  and  144   b  respectively. This allows the center position of the heat softened region to be maintained at a relatively constant point.  
     COMPARATIVE EXAMPLE  
       FIG. 18  shows fluctuation of the center position of the heat softened region when the rotation speed of the elongating rollers  144   a  and  144   b  are controlled at the same rotation speed as each other. The vertical axis shows the displacement of the center position of the heat softened region of the glass base material  102 , and the horizontal axis shows the time from the start of the elongation.  
      A glass rod  106  having a prescribed diameter was manufactured by measuring the diameter of the heat softened region of the glass base material  102  using the same diameter measuring device  152  in  FIG. 17 . The rotating speeds of the elongating rollers  144   a  and  144   b  were controlled at the same rotation speed as each other. The fluctuation of the center position of the heat softened region was large so that a bend requiring correction was caused on the elongated glass rod  106 .  
       FIG. 19  shows another embodiment of the burner  176  used in the glass rod fusing apparatus  370  shown in  FIG. 5 . A ring burner  176  has a hydrogen gas supply pipe  190  and a ring-type gas inlet  194 , which are connected to an oxygen gas supply pipe  192 . The cooling pipe  196 , which is connected to the cooling water supply pipe  198  and cooling water drainage pipe  200 , is provided on the outer area of the ring burner  176 . The ring-type gas inlet  194  can be a single layer that ejects a mix of hydrogen gas and oxygen gas. The ring-type gas inlet  194  can also be multiple or triple layered which eject the hydrogen gas from the upper and lower layers and oxygen gas from the middle layer.  
      The glass rod  106  is set inside the ring of the ring burner  176 , after which the hydrogen and oxygen gases are supplied to the ring burner  176  and ignited. The surface of the glass rod  106  is fused by the flame  178 . The ring burner  178  can heat the glass rod  106  efficiently so that it is unnecessary to over heat the glass rod  106 . Therefore, the opaque region on the surface of the glass, generated when glass is heated to temperatures higher than 2000° C., cannot be seen on the fused surface of the glass rod  106 .  
      According to the embodiments shown above, the glass rod  106  was fused. The glass base material  102  with a diameter of 120 mm was heated by the ring burner  176  for ten minutes. Hydrogen gas was supplied to the ring burner  176  at a rate of 300 L/minute and oxygen gas at 120 L/minute. The glass rod  106  was fused by elongation when the glass rod  106  was melted. The fused surface of the glass rod  106  was shaped into a circular cone. The color of the surface of the glass rod  106  was transparent.  
       FIG. 20  shows a configuration of a glass rod transportation device  380 . The glass rod transportation device  380  is used for transporting the glass rod  106  generated by the first elongating device  402 . The glass rod  106  is held by the movable holding element  245  and the fixed holding element  246  installed on the air cylinder storage box  244 . When the air cylinder (not shown in the figure) provided inside the air cylinder storage box  244  is driven, the movable holding element  245  moves toward the fixed holding element  246  thereby holding the glass rod  106 .  
      The force with which the movable holding element  245  pushes the fixed holding element  246  can be modified by modifying the air pressure which flows into the air cylinder. The air pressure of the air cylinder can be modified by operating a switch during the transportation of the glass rod  106 . The switch is provided on the operating switch box  248 .  
      The present embodiment has a second level of pushing force for pushing the movable holding element  245  to the fixed holding element  246 . This is achieved by adjusting the air pressure which flows into the air cylinder to one of two possible levels. For example, the weak side of the pushing force, which pushes the movable holding element  245  to the fixed holding element  246 , is the first holding force, and the strong side of the pushing force is second holding force. The first holding force is set to 0.5 kg, and the second holding force is set to 80 kg.  
      The air pressure adjustment of the air cylinder does not have to have only two levels of adjustment. The air pressure adjustment can be a multiple level adjusting type which adjusts to more than three levels of air pressure or the continuous adjustment type that provides a gradual rather than stepped level change. A rotary actuator  250  rotates the glass rod  106  from the vertical condition to the horizontal condition by rotating the movable holding element  245  and the fixed holding element  246  through the air cylinder storage box  244 . A holding flame  252  holds the glass rod transportation device  380  by connecting the glass rod transportation device  380  to the first elongating device  402 . A handle  254  is used for operating the glass rod transportation device  380 . A rotation axis  256  rotates the air cylinder storage box  244 .  
       FIG. 21  shows a storage container  224  of the first elongating device  402 . The storage container  224  has a saucer  260 , a strut  262 , a pair of holding members  234   a  and  234   b  which hold the glass rod  106 , and a pair of holding members  236   a  and  236   b  which are provided under the holding members  234   a  and  234   b . The shapes of the holding members  234   a ,  234   b ,  236   a , and  236   b  are substantially semicircle, which is desirable to securely hold the glass rod  106  inside the storage container  224 . Together, each of the pair of holding members  234   a  and  234   b  and holding members  236   a  and  236   b  form circle shaped holding members.  
      One end of each of the holding members  234   a  and  234   b  and the holding member  236   a  and  236   b  is pin connected to strut  262 . The other end of each is connected to the corresponding pair of holding members by a pin  257  or a pin  258 . The holding members  234   a  and  234   b  are connected by the pin  257 , and the holding members  236   a  and  236   b  are connected by the pin  258 . The height of the strut  262  is 1,550 mm. The inside diameter of the saucer  260  is 300 mm. Each of the inside diameters of the holding members are 180 mm, formed by the pair of holding members  234   a  and  234   b  and the pair of holding members  236   a  and  236   b.    
      In the case of receiving inside the storage container  224 , a glass rod  106  with an outside diameter of 80 mm,  4 , the angle of inclination α between the strut  262  and the glass rod  106  in the front and rear direction can range from −3.1° to +8.1°. The angle of inclination β between the glass rod  106  and the strut  262  in the left and right directions can range from −5.9° to +5.9°. Here, The angle of inclination is a limited value, and the glass rod  106  can be received inside the storage container  224  in various angles within this limited value. The glass rod  106  is in a various angles inside the storage container  224 .  
       FIG. 22  shows a movement of the glass rod transportation device  380  when transporting the glass rod  106 . The glass rod  106  inside of the storage container  224  is held by the movable holding element  245  and fixed holding element  246  with the first holding force (b). Then, the glass rod  106  is moved so that the glass rod  106  stands vertical to the ground within the holding member  234   a  and  234   b  (C). Because the first holding force is very weak, the movable holding element  245  will be opened when a force larger than the first holding force is applied to the movable holding element  245  during movement of the glass rod  106 . Moreover, the friction force acted between the movable holding element  245  and glass rod  106  and between the fixed holding element  246  and glass rod  106  is very small compared to the weight of the glass rod  106 . Therefore, glass rod cannot be lifted by raising the glass rod transportation device  380 , which holds the glass rod  106  by the first holding force.  
      After confirming that the glass rod  106  stands vertical, the holding force of the glass rod transportation device  380  is changed to the second holding force (d). Following this, the pins  257  and  258  are removed, and each of the holding members  234   a  and  234   b  and the holding member  236   a  and  236   b  are opened. Next, the glass rod transportation device  380  takes the glass rod  106  out of the storage container  224  for transportation. The glass rod  106  taken from the storage container  224  is rotated to a horizontal position and placed on the keeping place. During horizontal placement of the glass rod  106  on the keeping place, air pressure larger than a constant value is applied to the air cylinder to raise and lower the glass rod transportation device  380 . Therefore, the weight of the glass rod transportation device  380  is not applied to the glass rod  106  which prevents damage to the glass rod.  
       FIG. 23  shows an another embodiment of the glass rod transportation device  380 . The glass rod transportation device  380  of this embodiment has two rotation mechanisms A and B. Each of the rotation mechanisms A and B has a rotary actuator. The rotation mechanism A rotates the glass rod  106  by rotating a rotation axis  256  through the rotary actuator  250 . The rotation mechanism B moves the glass rod  106  up and down or left and right through the coupling axis  266  by rotating a rotation axis  268  through the rotary actuator  264 . The rotation axis  268  lies at right angles to the rotation axis  256  horizontally or vertically.  
       FIG. 24  shows the movement of the glass rod transportation device  380  shown in  FIG. 23  when the glass rod transportation device  380  transports the glass rod  106 .  FIG. 24 ( a ) shows a plan view of the glass rod transportation device  380 , which holds the glass rod  106 .  FIG. 24 ( b ) shows the cross sectional view of the glass rod transportation device  380 , which transports the glass rod  106  to the V block  240 . As shown in  FIG. 24 ( a ), the movable holding elements  245  and  246 , which hold the glass rod  106  vertically, are rotated from the vertical to horizontal position by operating the rotary actuator  250 . Next, as shown in  FIG. 24 ( b ), the movable holding element  245  and the fixed holding element  246  are rotated downward by activating the rotary actuator  264 .  
      The direction of opening and closing of the movable holding element  245  changes from a vertical direction to horizontal direction by activating the rotary actuator  264 . Therefore, the movable holding element  245  and the fixed holding element  246  can release upward after placing the glass rod  106  on the V block  240  by opening the movable holding element  245 . By including not only the rotation mechanism A, which rotates the glass rod  106  from a vertical to horizontal position, but also the rotation mechanism B, which has another rotation axis  268  that lies at right angles to the rotation axis  256 , the transportation efficiency of the glass rod  106  is increased.  
       FIG. 25  shows a configuration of a glass rod second elongating apparatus  111  of the present invention. The glass rod second elongating apparatus  111  comprises a mounting  112 , a fixed chuck  118 , a movable chuck  119 , a heating source  122 , a mass flow controller  278 , tail stocks  114  and  116 , a tail stock driving motor  275 , a tail stock driving encoder  273 , a diameter measurement device  124 , a moving stand  120 , a sliding screw  270 , a moving stand motor  274 , a moving stand encoder  272 , a chain  276 , and a control unit  280 .  
      The fixed chuck  118  and the movable chuck  119  hold the glass rod  106  which has been weld at both ends to a dummy rod  108 . The heating source  122  heats the glass rod  106 , which is held by the fixed chuck  118  and movable chuck  119 . The mass flow controller  278  adjusts the amount of gas supplied to the heating source  122 . The tail stock  116  elongates the glass rod  106  by moving the movable chuck  119 . The tail stock driving motor  275  drives the tail stock  116 . The tail stock driving encoder  273  detects the amount of the rotation and controls the speed of the tail stock driving motor  275 . The moving distance of the tail stock  116  can be assessed from the amount of the rotation of the tail stock driving motor  275  detected by the tail stock driving encoder  273 .  
      The diameter measurement device  124  measures the diameter of the glass rod  106  corresponding to the position along the axial direction of the glass rod  106 . The heating source  122  and the diameter measurement device  124  are provided on the moving stand  120 . The moving stand  120  moves the heating source  122  and diameter measurement device  124 . The moving stand  120  is provided on the mounting  112 . The moving stand  120  can move along the sliding screw  270 , which is installed parallel to the axis that connects the fixed chuck  118  and movable chuck  119 . The moving stand  120  is driven by the moving stand motor  274  through the sliding screw  270  and the chain  276 . The moving stand encoder  272  controls the speed of the moving stand motor  274 .  
      The control unit  280  controls the moving distance of the heating source  122  by controlling the moving stand encoder  272 , the moving stand motor  274 , the chain  276 , the sliding screw  270  and the moving stand  120 . The control unit  280  controls the amount of gas provided to the heating source  122  by controlling the mass flow controller  278 . The control unit  280  controls the moving speed of the tail stock  116  by controlling the tail stock driving encoder  273  which controls the rotation speed of the tail stock driving motor  275 . The control unit  280  controls the elongating speed of the glass rod  106  by controlling the moving speed of the tail stock  116 .  
      The tail stock  114  and  116 , fixed chuck  118 , movable chuck  119 , tail stock driving motor  275 , and tail stock driving encoder  273  constitute an elongating unit which elongates the glass rod  106 .  
      The data on the measured diameter and position of measurement as measured by the diameter measurement device  124 , and the data on the changes in length of the glass rod  106  obtained from the moving distance of the tail stock  116  are input to control unit  280 . The control unit  280  controls the heating condition based on input data by controlling factors such as moving distance of the heating source  122 , the amount of gas provided to the heating source  122 , and also controls the elongation speed of the tail stock  116  based on input data.  
       FIG. 26  shows a detailed flow chart of the glass rod second elongating (S 206 ) shown in  FIG. 4 . First, the dummy rods  108  are held by the fixed chuck  118  and the movable chuck  119 . Following this, both ends of the glass rod  106  are welded to the dummy rods  108  (S 146 ) so that the glass rod  106  is set on the glass rod second elongating apparatus  111 . Next, a cut  284  of 3 mm depth is made around the connection of the glass rod  106  and the dummy rods  108  as a marker.  
      The starting and finishing position of the diameter measurement of the glass rod  106  and the target diameter are then set (S 150 ). The diameter of the glass rod  106  is measured corresponding to the location along the axial direction of the glass rod  106  (S 152 ). The elongating speed at a plurality of locations along the axial direction of the glass rod  106  is set based on the measured diameter and the location corresponding to the measured diameter. The heating conditions including the amount of gas supplied to the heating source  122  and the moving distance of the heating source  122  are set based on the average value of the diameter of the glass rod (S 153 ). The glass rod  106  is heated by the heating source  122  with a preset heating condition and elongated gradually by the tail stock  116 , which moves with a preset elongating speed (S 154 ).  
      The location of the cut  284 , which is provided around the connection of the glass rod  106  and the dummy rods  108 , are then detected by the diameter measurement device  124  in order to detect the location of both ends of the glass rod  106 . The moving distance of the tail stock  116  is measured by the tail stock driving encoder  273  in order to measure changes in the length of the glass rod  106  along the axial direction.  
      The diameter of the glass rod  106  is then measured at a position approximately 50 mm away from the cut  284  towards the center of the glass rod  106  (S 156 ). The heating position of the heating source  122  is set based on the position of the cut  284  and the changes in length of the glass rod  106  along the axial direction. The amount of gas supplied to the heating source  122  is set based on the measured diameter. The moving speed of the tail stock  116  is also set based on the measured diameter (S 157 ). The glass rod  106  is end-drawn which heats and elongates the glass rod  106  with a preset heating condition and elongating speed. The shape of the end of the glass rod  106  therefore becomes similar to a circular cone shape so that the diameter of end of the glass rod  106  reduced (S 158 ).  
      The position of the end-drawn part is then detected by measuring the diameter of the end-drawn part and the part elongated by the end drawing at the corresponding position. These measurements are undertaken by the diameter measurement device  124 . The change in length of the glass rod  106  along the axial direction resulting from end drawing is measured by the tail stock driving encoder  273  (S 160 ). The starting and finishing position of the fire polishing, which polishes the glass rod  106  with fire, and the heating power of the fire are then set. This setting is based on the detected position of the end-drawn part and the change in length of the glass rod  106  along the axial direction (S 161 ).  
      The position of starting and finishing the fire polishing is set based on the position of the cloud on the glass rod  106 . A cloud is generated in a region that is heated strongly during the end drawing process. The glass rod  106  is fire polished by the heating source  122  as per the preset fire condition from the set fire polishing starting position to the set fire polish finishing position (S 162 ). After fire polishing, the shape of the glass rod  106  is checked by measuring the finished diameter and length of the glass rod  106  (S 164 ). The dummy rod  108  is then removed from the glass rod  106  (S 166 ). Finally, the glass rod  106  is surface treated to produce a preform  107  (S 168 ).  
      As shown above, before each elongating (S 154 ), end drawing (S 158 ) and fire polishing (S 162 ) process, the diameter is measured in the corresponding location along the axial direction of the glass rod  106 . From this data, the heating condition and elongating speed for the next process can be accurately set. Therefore, a glass rod  106  of consistently high quality can be manufactured.  
       FIG. 27  shows an example which provides a cooling device  330  on the fixed chuck  118  and the movable chuck  119  of the glass rod second elongating apparatus  111 . The cooling device  330  protects the fixed chuck  118  and movable chuck  119  from the radiant heat generated from the heating source  122 . This is achieved by circulating cooling water around the fixed chuck  118  and the movable chuck  119 . The cooling device  330  uses a gas or liquid as a cooling medium.  
      The deformation of the fixed chuck  118  and the movable chuck  119  can be controlled by providing the cooling device  330  on the fixed chuck  118  and the movable chuck  119 . This allows control of the degree of temperature rise of the fixed chuck  118  and the movable chuck  119 . Therefore, the accuracy of transfer of the driving force that rotates the glass rod  106  is maintained, and the heating of the glass rod  106  becomes more even. Therefore, fluctuation of the diameter of the glass rod  106  decreases.  
     EXAMPLE  
      A glass rod  106  of 50 mm diameter and 1000 mm length was fire polished by a fixed chuck  118  and movable chuck  119  that has a cooling device  330  and a heating source  122  shown in  FIG. 27 . Oxygen (O 2 ) of 150 SLM and hydrogen (H 2 ) of 300 SLM are supplied to the heating source  122  as combustion gas. The glass rod  106  is rotated at a speed of 15 rpm. The glass rod  106  is fire polished by moving the heating source  122  relative to the glass rod  106  at a speed of approximately 20 mm/min.  
       FIG. 28  shows the temperature of the fixed chuck  118  and movable chuck  119  of the above example and the comparative example shown below. The vertical axis shows the temperature of the fixed chuck  118  and movable chuck  119 , and the horizontal axis shows the processing time of the fire polishing. The temperature of the fixed chuck  118  and movable chuck  119  of the example was maintained at a low temperature of about 45° C. The resultant fluctuation of the driving force that rotates the glass rod  106  caused by the deformation of the fixed chuck  118  and movable chuck  119  was small. Therefore the fluctuation of the diameter of the fire polished glass rod  106  was only 0.02%.  
     COMPARATIVE EXAMPLE  
      The glass rod  106  was fire polished under the same conditions as the above example except for the removal of the cooling device  330  from the fixed chuck  118  and movable chuck  119  shown in  FIG. 27 . As shown in  FIG. 28 , the temperature of the fixed chuck  118  and movable chuck  119  reached approximately 100° C. The fixed chuck  118  and movable chuck  119  were deformed as a result, so the driving force that rotates the glass rod  106  fluctuates. The fluctuation of the diameter of the glass rod  106  after fire polishing increased to 1.0%, which is larger than the degree of fluctuation of the above example.  
       FIG. 29  shows a relationship between the distance between the heating source  122  and the diameter measurement device  124  and the percentage of the fluctuation of the diameter of the glass rod  106 . The fluctuation rate (%) of the diameter of the glass rod  106  represents the (maximum value of the diameter of the glass rod  106 −minimum value of the diameter of the glass rod  106 )/(average diameter)×100.  
      The diameter measurement device  124  of the glass rod second elongating apparatus  111  shown in  FIG. 25  is provided on a location which is a constant distance, from 10 mm to 50 mm, away from the heating source  122 . Therefore, the diameter of the glass rod  106  can be accurately measured allowing accurate control of the diameter of the glass rod  106 .  
      When elongating the glass rod  106 , the position of highest temperature in the glass rod  106  is slightly different to the position that the heating source  122  is heating because the heating source  122  is moving. The elongating speed per unit length becomes largest at the location where the temperature of the glass rod  106  is highest.  
      It is desirable to control the heating power of the heating source  122  and the moving speed of the movable chuck  119  based on the diameter at the position of the largest elongating speed and the target value of the diameter. The moving speed of the movable chuck  119  is controlled based on the difference between the target value of the diameter and the diameter that is measured at the position that the elongating speed of the glass rod  106  is largest. This can be done by providing the diameter measurement device  124  on a position that is a constant distance away from the heating source  122 .  
      The position, which is a constant distance away from the heating source  122 , ranges from 10 mm to 50 mm away from the position where the heating source  122  is provided in the opposite direction to the moving direction of the heating source  122 . Therefore, the diameter measurement device  124  is provided on a position 10 mm to 50 mm away from the heating source  122  in the opposite direction of the moving direction of the heating source  122 .  
      If the heating source  122  used to heat the glass rod  106  is an oxygen hydrogen burner, the flow rate of the hydrogen gas supplied to the heating source  122  is set from 30 liters/minute to 500 liters/minute. The ratio of the flow rate of the hydrogen gas to the oxygen gas is set from 1.5 to 3.0. The moving speed of the heating source  122  is controlled within the limits of 2 mm/minute and 65 mm/minute. The heat quantity will be insufficient if the flow rate of the hydrogen gas is less than 30 liters/minute, and the fuel will be wasted if the flow rate of the hydrogen gas is more than 500 liters/minute. It is difficult to elongate the glass rod  106  if the ratio of the flow rate of the hydrogen gas to the oxygen gas is out of the range shown above because the heat quantity becomes insufficient.  
      If the heating source  122  to heat the glass rod  106  is a propane gas burner, the flow rate of the propane gas supplied to the heating source  122  is set from 1 liter/minute to 15 liters/minute. The ratio of the flow rate of the propane gas to the oxygen gas is set from 0.1 to 0.3. The moving speed of the heating source  122  is controlled within the limits of 2 mm/minute and 65 mm/minute. The heat quantity will be insufficient if the flow rate of the propane gas is less than 1 liter/minute, and the fuel will be wasted if the flow rate of the propane gas is more than 15 liters/minute. Furthermore, it is difficult to elongate the glass rod  106  if the ratio of the flow rates of the propane gas to oxygen gas is out of the range shown above because the heat quantity becomes insufficient. The moving speed of the heating source  122  would preferably be controlled within the limit of 2 mm/minute and 65 mm/minute. It takes too much time elongating the glass rod  106  if the moving speed of the heating source  122  is below 2 mm/minute. Alternatively, it is difficult to elongate the glass rod  106  if the moving speed of the heating source  122  is more than 65 mm/minute because the speed is too fast to heat the glass rod  106  to its core.  
     EXAMPLE 1  
      The elongation of the glass rod  106  was begun by setting the distance between the heating source  122  and the diameter measurement device  124  as 15 mm. During the elongation of the glass rod  106 , the moving speed of the heating source  122  and the tail stock  116  were controlled based on the difference between the measured diameter of the glass rod  106  and the target diameter. The burning conditions of the heating source  122  were set including the flow rate of the hydrogen gas at 224 liters/minute, the ratio of the flow rate of the hydrogen to oxygen as 2.5, and the moving speed of the heating source  122  as 11 mm/minute. The fluctuation rate of the diameter of the glass rod  106  after the elongating process was 0.9%.  
     EXAMPLE 2  
      The distance between the heating source  122  and the diameter measurement device  124  was set to 40 mm. The flow rate of the hydrogen gas was set to 199 liters/minute. The ratio of the flow rate of the hydrogen to oxygen was set to 2.5. The moving speed of the heating source  122  was set to 13 mm/minute. The fluctuation rate of the diameter of the glass rod  106  after the elongating process was 0.6%.  
     COMPARATIVE EXAMPLE 1  
      The distance between the heating source  122  and the diameter measurement device  124  was set to 5 mm. The flow rate of the hydrogen gas was set to 209 liters/minute. The ratio of the flow rate of the hydrogen to oxygen was set to 2.5. The moving speed of the heating source  122  was set to 12 mm/minute. Because the distance between the heating source  122  and the diameter measurement device  124  was too close, the fluctuation rate of the diameter of the glass rod  106  after the elongating process was 3.7%. This is larger than the fluctuation rate of example 1 and example 2 above.  
     COMPARATIVE EXAMPLE 2  
      The distance between the heating source  122  and the diameter measurement device  124  was set to 60 mm. The flow rate of the hydrogen gas was set to 237 liters/minute. The ratio of the flow rate of the hydrogen to oxygen was set to 2.5. The moving speed of the heating source  122  was set to 10 mm/minute. Because the distance between the heating source  122  and the diameter measurement device  124  was too far, the fluctuation rate of the diameter of the glass rod  106  after the elongating process was 2.5%. This fluctuation rate is larger than the fluctuation rate of example 1 and example 2 above.  
     COMPARATIVE EXAMPLE 3  
      The distance between the heating source  122  and the diameter measurement device  124  was set to 15 mm. The flow rate of the hydrogen gas was set to 215 liters/minute. The ratio of the flow rate of the hydrogen to oxygen was set to 1.0. The moving speed of the heating source  122  was set to 12 mm/minute. Because the ratio of the flow rate of the hydrogen to oxygen was 1.0, which was smaller than the recommended 1.5 minimum, the glass rod  106  could not be elongated.  
     COMPARATIVE EXAMPLE 4  
      The distance between the heating source  122  and the diameter measurement device  124  was set to 15 mm. The flow rate of the hydrogen gas was set to 195 liters/minute. The ratio of the flow rate of the hydrogen to oxygen was set to 4.0. The moving speed of the heating source  122  was set to 13 mm/minute. Because the ratio of the flow rate of the hydrogen to oxygen was 4.0, which was larger than the recommended 3.0 maximum, the glass rod  106  could not be elongated.  
     COMPARATIVE EXAMPLE 5  
      The distance between the heating source  122  and the diameter measurement device  124  was set to 15 mm. The flow rate of the hydrogen gas was set to 204 liters/minute. The ratio of the flow rate of the hydrogen to oxygen was set to 2.5. The moving speed of the heating source  122  was set to 70 mm/minute. Because the moving speed of the heating source  122  was 70 mm/minute, which was larger than the 65 mm/minute recommended maximum speed, the glass rod  106  could not be elongated.  
       FIG. 30  shows a glass rod second elongating apparatus  111  which has a configuration providing a tensile stress measurement device  282  on the glass rod second elongating apparatus  111  shown in  FIG. 25 . The glass rod second elongating apparatus  111  has a tensile stress measurement device  282 , which measures the tensile stress applied to the glass rod  106 , on the movable chuck  119 .  
      The glass rod second elongating apparatus  111  can detect the position of the heating source  122  on the moving stand  120  using the moving stand encoder  272 . The tensile stress measurement device  282  is connected to a control unit  280 . The control unit  280  controls the moving speed of the tail stock  116  based on the tensile stress of the glass rod  106 , provided from the tensile stress measurement device  282 . This is undertaken until the moving distance of the heating source  122  reaches a prescribed distance.  
       FIG. 31  shows a detailed flow chart of the elongating (S 154 ) shown in the  FIG. 26 . First, the glass rod  106  is pre-heated until the prescribed region of the glass rod  106  is melted and softened by the heating source  122 . This will allow elongation of the glass rod  106  (s 132 ). Next, the heating source  122 , which is provided on the moving stand  120 , is moved via the moving stand  120 . The moving speed of the heating source  122  would ideally be as slow as possible at the early stage of the elongation so that the fluctuation of the diameter of the glass rod  106  can be reduced. The movement of the heating source  122  would also be a constant speed. The amount of gas supplied to the heating source  122  can be constant.  
      Next, the moving speed of the tail stock  116  is controlled so that the tensile stress of the glass rod  106  measured by the tensile stress measurement device  282  lies within substantially 80% to 110% of the average value of the tensile stress at the steady state (S 136 ). The steady state will be explained below. The moving speed of the tail stock  116 , which was originally set based on the diameter at a plurality of locations of the glass rod  106  along the axial direction, is re-set based on the tensile stress of the glass rod  106 . The glass rod  106  is elongated by the tensile stress load shown above until the heating source moves substantially 50 mm to 150 mm (S 138 ).  
      If the control unit  280  detects that the heating source  122  has moved substantially from 50 mm to 150 mm (S 138 ), the moving speed of the tail stock  116  changes to the speed at the steady state, which will be explained below. This is done by controlling the tail stock driving encoder  273  (S 140 ). The diameter measurement device  124  measures the diameter of the glass rod  106  during the elongation of the glass rod  106  (S 142 ). The elongation of the glass rod  106  is finished when the glass rod  106  is elongated to the desired diameter and length (S 144 ).  
      The speed at the steady state is the speed where the material balance before the elongation and after the elongation is balanced. Here, the original diameter of the glass rod  106  before the elongation is represented as D 1 , the target diameter to be obtained as D 2 . the moving speed of the heating source  122  as V 1 , and the speed of the elongation of the glass rod  106  as V 2 .  
      For example, assume that the elongation takes place only at the region heated at that time, so the region heated and elongated is quite small. The V 2  is equal to the speed at the steady state when the following equation is valid. 
 
 D   1   2   V   1   =D   2   2 ( V   1   +V   2 ) 
 
      Therefore, the V 2  can be set by adjusting the V 1  and the moving speed of the tail stock  116  based on the D 1  and the D 2 . The tensile stress of the glass rod  106  at the steady state is the tensile stress when the glass rod  106  is elongated with the tail stock  116  moving speed at the steady state.  
       FIG. 32  shows a process where the diameter fluctuates during the elongation of the glass rod  106 . The glass rod  106  softens when heated. As shown in  FIG. 32 ( 1 ), it may happen that the glass rod  106  cannot be softened enough by the pre-heating only to be elongated. The tensile stress generated on the glass rod  106  increases from twice to triple the normal tensile stress when the heating source  122  and the tail stock  116  start to move at the prescribed speed. Then, the region which is pre-heated is elongated rapidly, and the diameter of the pre-heated region is reduced as shown in shaded portion of  FIG. 32 ( 2 ). The elongation of the glass rod  106  occurs almost entirely in the pre-heated region, and the region which is heated newly by the heating source  122 , is less elongated. Therefore, necking of the diameter has occurred on the glass rod  106  as shown in  FIG. 32 ( 3 ).  
      The fluctuation of the diameter of the glass rod  106  tends to occur at the region from the starting place of the elongation of the glass rod  106  to the place 50 mm away from the starting place. If the elongation is progressed further than this place, the speed of providing the heat to the glass rod  106 , the speed that the glass rod  106  softens, and the elongation speed of the glass rod  106  are balanced to be a steady state. Therefore, the fluctuation of the diameter of the glass rod  106  will not occur as shown in  FIG. 32 ( 4 ).  
      The glass rod  106  is elongated by controlling the moving speed of the tail stock  116 . The aim is to keep the tensile stress of the glass rod  106  at the early stage of the elongation at substantially 110% or less of the average value of the tensile tension at the steady state. The fluctuation of the diameter at the early stage of the elongation of the glass rod  106  can thus be decreased. This is because the heat supply to the glass rod  106 , the soften speed of the glass rod  106 , and the elongation speed of the glass rod  106  can be balanced.  
      If the tensile stress of the glass rod  106  at the early stage is lower than 80% of the steady state, the distance required for the diameter of the glass rod  106  to reach the target value becomes long. Therefore, the region of the elongated glass rod  106  that can be used as product becomes short. This decreases the yield factor of the process and increases the time taken for the glass rod  106  to reach the target diameter. Therefore, it is desirable to control the tensile stress of the glass rod  106  at the early stage of the elongation in the range of substantially from 80% to 110% of the average value of the tensile stress at the steady state.  
       FIG. 33  shows a glass rod  106  that is elongated according to the elongating (S 154 ) shown in  FIG. 31 . First, as shown in  FIG. 33 ( 1 ) and ( 2 ), the heating source  122  and the tail stock  116  start to move after the pre-heating of the glass rod  106  to start the elongation of the glass rod  106 . Because the tensile stress of the glass rod  106  is controlled to be 110% or less of the tensile stress at the steady state, excessive tensile stress is not applied to the glass rod  106 . No necking therefore occurs on the glass rod  106  due to rapid elongation. If the heating source  122  moves the prescribed distance under this balanced condition, the heat supplied to the glass rod  106 , the soften speed of the glass rod  106 , and the elongation speed of the glass rod  106  are balanced. Thus the fluctuation of the diameter of the glass rod  106  can be prevented.  
      Fluctuation of the diameter may occur if the moving speed of the tail stock  116  continues to be controlled based on the tensile stress. The tensile stress of the glass rod  106  will change with small changes in the heat quantity provided by the heating source  122 . The moving speed of the tail stock  116  then fluctuates to maintain the tensile stress of the glass rod  106  at a constant, resulting in fluctuation of the diameter of the elongated glass rod  106 . Therefore, fluctuations in the diameter of the glass rod  106  caused by subtle fluctuations of the tensile stress can be prevented by changing the moving speed of the tail stock  116  to the speed at the steady state after the heating source  122  moves a prescribed distance on commencement of elongation.  
      The change in moving speed of the tail stock  116  to the speed of the steady state is made when the heating source  122  has moved substantially from 50 mm to 150 mm from the point of the start of the elongation. Until the heating source  122  moves 50 mm from the point of commencement of elongation, the heat supplied to the glass rod  106 , the soften speed of the glass rod  106 , and the elongation speed of the glass rod  106  are not balanced. The result is, necking of the glass rod  106  will occur due to the fluctuation of the diameter if the elongation speed is changed to the speed of the steady state before the heating source  122  has moved 50 mm. The tensile stress of the glass rod  106  should thus be controlled to be substantially 110% or less of the steady state until the heating source  122  moves substantially 50 mm. It is desirable to change the moving speed of the tail stock  116  to the speed of the steady state before the heating source  122  moves more than substantially 150 mm.  
     EXAMPLE  
      The glass rod  106  was elongated by the glass rod second elongating apparatus  111 . The glass rod  106  had an outside diameter of 65 mm and length of 980 mm. The dummy rods  108 , which had outside diameters of −60 mm and lengths of 250 mm, were welded on both ends of the glass rod  106 . The rotation speed around the axis during the welding of the glass rod  106  and the dummy rod  108  was 30 rpm. An oxygen hydrogen burner was used for the heating source  122 . The oxygen gas and hydrogen gas provided to the heating source  122  was 96 liters/minute and 240 liters/minute respectively.  
      After pre-heating of the glass rod  106 , the elongation of the glass rod was started by moving the heating source  122  at a moving speed of 12.4 mm/min. When elongating the glass rod  106  to reduce the diameter of the glass rod  106  from 65 mm to 50 mm, the tensile stress at the steady state was about 100 kgf/cm 2 , and the moving speed of the tail stock  116  at the steady state was 8.6 mm/min. The moving speed of the tail stock  116  was controlled so that the tensile stress did not exceed 110 kgf/cm 2  until the heating source  122  had moved 100 mm from the starting point of the elongation. After the heating source  122  moved 100 mm, the glass rod  106  was elongated by controlling the moving speed of the tail stock  116  to 8.6 mm/min, which is the speed at the steady state.  
       FIG. 34  shows the tensile stress of the glass rod  106  at the early stage of the elongation of the example. The vertical axis shows the tensile stress generated in the glass rod  106  and the horizontal axis shows the moving distance of the heating source  122  after the start of elongation. The tensile stress of the glass rod  106  was 110 kgf/cm 2  or less at the early stage of the elongation while the heating source  122  moved forward 100 mm.  
       FIG. 36  shows the fluctuation of the diameter of the glass rod  106  after the elongation of the glass rod  106 . The vertical axis shows the distance along the radiant direction of the glass rod  106 , and the horizontal axis shows the distance along the longitudinal direction of the glass rod  106 . The glass rod  106  elongated by the method according to the example had few diameter fluctuations such as necking, and the diameter of the glass rod  106  could be reduced to the target diameter at about 100 mm of the longitudinal distance after the elongation started. The accuracy of the diameter of the glass rod  106  at the region which was elongated at the speed of the steady state by the method according to the example was about the same accuracy as the diameter of the glass rod  106  which was elongated by the conventional elongating method.  
     COMPARATIVE EXAMPLE  
      A glass rod  106  with a diameter of 65 mm was elongated to a diameter of 50 mm. The conditions of the moving speed and the amount of gas to the heating source  122  were the same as the above example. The glass rod  106  was elongated by controlling the moving speed of the tail stock  116  to 8.6 mm/min from the start of the elongation. This is the speed at the steady state.  
       FIG. 35  shows a fluctuation of the tensile stress of the glass rod  106  at the early stage of the elongation of the comparative example. The vertical axis shows the tensile stress generated in the glass rod  106 , and the horizontal axis shows the moving distance of the heating source  122  after commencement of elongation. The tensile stress of the glass rod  106  increased to 300 kgf/cm 2  at the early stage of the elongating, which is 3 times greater than the tensile stress of the steady state. This occurred whilst the heating source  122  was moving the initial 100 mm.  
      As shown in  FIG. 36 , the glass rod  106  after the elongation of the comparative example had large necking at about 100 mm from the start of the elongation. Because the undulation continues until about 300 mm from the start of the elongation, this region cannot be used as product, and the yield rates decreased.  
       FIG. 37  shows a detailed flow chart of the end drawing (S 158 ) shown in  FIG. 26 . First, the position, of the glass rod  106  which has been end-drawn is detected (S 169 ). Next, the prescribed region of the glass rod  106  is pre-heated by the flame of the heating source  122  (S 170 ) until the prescribed region nearly softens. Then, the glass rod  106  is elongated by heating the prescribed region of the glass rod  106  with the heating source  122  and moving the tail stock  116  so that the diameter of the prescribed region is reduced (S 172 ).  
      The heating source  122  is moved from the center of the prescribed region to a region towards the middle side of the glass rod  106 . Then, the heating source  122  heats the glass rod  106  secondly (S 174 ) with a flame. The thickness of this flame is smaller than the thickness of the flame of the pre-heating (S 170 ). The prescribed region of the glass rod  106  is further elongated by moving the tail stock  116  so that the diameter of the prescribed region is reduced (S 176 ). Then, the prescribed region of the glass rod  106  is fused by the flame. Again the thickness of this flame is smaller than the thickness of the flame of the pre-heating (S 170 ).  
       FIG. 38  shows a cut  284  that is provided as a mark on the connection between the glass rod  106  and the dummy rod  108 . This allows the detection of the position of the end drawing at the end drawing position detecting (S 169 ) shown in  FIG. 37 . A mark is provided on the connection between the glass rod  106  and the dummy rod  108 . The device that recognizes the mark is installed on the glass rod second elongating apparatus  111  to detect the location of the mark.  
      The position of the start of the end drawing process is set based on the detected mark location. The elongation process of the glass rod  106  finishes at the set end drawing starting position, and the end drawing process of the glass rod  106  starts at the same time. The method shown in  FIG. 38  is used when the device that recognizes the mark is a device that measures the diameter. An example of such a device would be a diameter measurement device  124 .  
       FIG. 39  shows a fluorescent paint  287  that is applied on the connection between the glass rod  106  and the dummy rod  108  as another example of a mark. The method shown in  FIG. 39  is used when the device that recognizes the mark is an image processing apparatus.  
       FIG. 40  shows the glass rod second elongating apparatus  111  that detects the cut  284  at end drawing position detecting (S 169 ). First, the dummy rod  108  is welded on both ends of the glass rod  106 . The glass rod  106 , which has the dummy rod  108  on both sides, is fixed on the fixed chuck  118  and movable chuck  119 , not shown in the figure. The cut  284  having depth of 3 mm is provided all around the welded position. The welded position results from the connection between the glass rod  106  and the dummy rod  108 .  
      During the elongation of the glass rod  106 , the diameter measurement device  124  measures the diameter of the glass rod  106 . When the diameter measurement device  124  detects the position of the cut  284  by detecting a change in diameter of the glass rod  106 , the glass rod second elongating apparatus  111  starts the end drawing. The position of commencement of the end drawing is slightly towards the middle direction of the glass rod  106  from the connection between the glass rod  106  and the dummy rod  108 . Also, the position of commencement of the end drawing does not have a bubble or bubbles with a diameter of 0.3 mm or above. Then, the process is shifted from elongation to end drawing.  
      When a mark is the marking  287 , fluorescent paint is applied on the connection between the glass rod  106  and the dummy rod  108 . The camera of the image processing apparatus, which can detect the fluorescent paint, is installed on the position of the diameter measurement device  124 , which is provided on the moving stand  120 . The camera processes the picture of the glass rod  106  during the elongation of the glass rod  106 . If the camera detects the fluorescent paint, the glass rod second elongating apparatus  111  starts the end drawing. The position of commencement of the end drawing is slightly towards the middle direction of the glass rod  106  from the connection between the glass rod  106  and the dummy rod  108 . Also, the position of starting the end drawing does not have a bubble or bubbles with a diameter of 0.3 mm or above. Then, the process is shifted from elongation to end drawing.  
       FIG. 41  shows the movements of the heating source  122  and the tail stock  116  after detecting the position of the end drawing (S 169 ) during the end drawing process of the glass rod  106  shown in flow chart of  FIG. 37 . At the pre-heating for end drawing (S 170 ), the flame of the heating source  122  heats the glass rod  106  at the prescribed region until the glass rod  106  nearly softens. At elongating for end drawing (S 172 ), the heating source  122  heats the prescribed region of the glass rod  106 , and the tail stock  116  elongates the prescribed region of the glass rod  106 . This therefore reduces the diameter of the prescribed region.  
      At second heating (S 174 ), the tail stock  116  stops, and the heating source  122  moves in the direction towards the middle side of the region of the glass rod  106  (to the left in the figure), from the center of the prescribed region. Then, the heating source  122  heats the glass rod  106  by flame, the thickness of which is smaller than the thickness of the flame of the pre-heating (S 170 ). At the second elongating for end drawing (S 176 ), the heating source  122  moves further to the left side in the figure and heats the glass rod  106 . The tail stock  116  also moves to elongate the prescribed region of the glass rod  106 . At fusing for end drawing (S 178 ), the heating source  122  heats the glass rod  106  by flame, the thickness of which is smaller than the thickness of the flame of the pre-heating (S 170 ). The position of the heating source  122  is at the same position as the second elongating for end drawing (S 176 ). The tail stock  116  moves to fuse the glass rod  106 .  
       FIG. 42  shows an example of the settings of another method of the end drawing process at the end drawing (S 158 ) shown in  FIG. 37 . This method controls the gas amount, the moving distance of the heating source  122 , and the moving speed of the tail stock  116  based on the progress time of the end drawing process of the glass rod  106 .  
      The gas amount, the moving distance of the heating source  122 , and the moving speed of the tail stock  116  are set once. This setting is based on the location of the cut  284 , the changes of the length and the diameter of the glass rod  106  along the axial direction at the second heating condition and elongating speed setting (S 157 ). The glass rod second elongating apparatus  111  then resets the gas amount, the moving distance of the heating source  122 , and the moving speed of the tail stock  116  based on the progress time of the end drawing process of the glass rod  106  at the end drawing (S 158 ).  
      For example, at the pre-heating for the end drawing (S 170 ), which is undertaken for 300 seconds, the moving distance of the heating source  122  is set to 0 mm. The moving speed of the tail stock  116  is set to 0 mm/minute. The amount of hydrogen (H 2 ) gas for the heating source  122  is set to 250 cc/minute. The amount Oxygen (O 2 ) gas (inside) that is output from the inside nozzle of the heating source  122  is set to 30 cc/minute. The amount of oxygen (O 2 ) gas (outside) that is output from the outside nozzle of the heating source  122  is set to 100 cc/minute. The glass rod  106  is heated by the heating source  122 , which is set according to the above conditions.  
      At the elongating for end drawing (S 172 ), which is undertaken for 60 seconds, the amount of hydrogen (H 2 ) gas for the heating source  122  is set to 250 cc/minute. The amount of the oxygen (O 2 ) gas (inside) that is output from the inside nozzle of the heating source  122  is set to 30 cc/minute. The amount of oxygen (O 2 ) gas (outside) that is output from the outside nozzle of the heating source  122  is set to 100 cc/minute. The glass rod  106  is heated by the heating source  122 , which is set according to the above conditions. With the moving distance of the heating source  122  at 0 mm, the tail stock  116  is moved at the speed of 10 mm/minute to elongate the glass rod  106 .  
      At the second heating (S 174 ), which is undertaken for 20 seconds, the moving speed of the tail stock  116  is set to 0 mm/minute. The moving distance of the heating source  122  is set to 15 mm. The amount of hydrogen (H 2 ) gas for the heating source  122  is set to 130 cc/minute. The amount of oxygen (O 2 ) gas (inside) that is output from the inside nozzle of the heating source  122  is set to 15 cc/minute. The amount oxygen (O 2 ) gas (outside) that is output from the outside nozzle of the heating source  122  is set to 50 cc/minute. The glass rod  106  is heated by the heating source  122 , which is set according to the above conditions.  
      At the second elongating for end drawing (S 176 ), which is undertaken for 180 seconds, the moving distance of the heating source  122  is increased from 15 mm to 25 mm. The amount of hydrogen (H 2 ) gas for the heating source  122  is set to 130 cc/minute. The amount oxygen (O 2 ) gas (inside) that is output from the inside nozzle of the heating source  122  is set to 15 cc/minute. The amount of oxygen (O 2 ) gas (outside) that is output from the outside nozzle of the heating source  122  is set to 50 cc/minute. The glass rod  106  is heated by the heating source  122 , which is set according to the above conditions. The tail stock  116  is moved at a speed of 10 mm/minute to elongate the glass rod  106 .  
      Finally, at the fusing for end drawing (S 178 ), which is undertaken for 30 seconds, the heating source  122  does not move from the position at the second elongating for end drawing (S 176 ), so the moving distance remains at 25 mm. The amount of hydrogen (H 2 ) gas for the heating source  122  is set to 130 cc/minute. The amount of oxygen (O 2 ) gas (inside) that is output from the inside nozzle of the heating source  122  is set to 30 cc/minute. The amount oxygen (O 2 ) gas (outside) that is output from the outside nozzle of the heating source  122  is set to 20 cc/minute. The glass rod  106  is heated by the heating source  122 , which is set according to the above conditions. The tail stock  116  is moved at a speed of 120 mm/minute to fuse the glass rod  106 .  
      The glass rod  106  with a diameter of 60 mm was end-drawn by the glass rod second elongating apparatus  111  according to the setting condition shown in  FIG. 42 . The shape of the preform at the region that was end-drawn, was a well formed circular cone shape. The length and the diameter of the region were 61 mm and 60 mm respectively. The time that was required for the end drawing process was 12 minutes.  
       FIG. 43  shows another example of the settings of other method of the end drawing process at the end drawing (S 158 ) shown in  FIG. 37 . This method controls the gas amount, the moving speed of the heating source  122 , and the moving speed of the tail stock  116  based on the moving distance of the tail stock  116 .  
      The glass rod second elongating apparatus  111  detects the moving distance of the tail stock  116 . The moving distance of the heating source  122 , and the moving speed of the tail stock  116  are set once based on the location of the cut  284 , the change of the length of the glass rod  106  along the axial direction, and the diameter of the glass rod  106  at the second heating condition and elongating speed setting (S 157 ). The glass rod second elongating apparatus  111  resets the gas amount, the moving distance of the heating source  122 , and the moving speed of the tail stock  116  based on the detected moving distance of the tail stock  116  at the end drawing (S 158 ).  
      There is a case where the moving distance of the tail stock cannot be measured because the tail stock does not move. This might occur from lack of power of the tail stock driving motor  275  when the glass rod  106  is not heated sufficiently during the end drawing process. When the output of the tail stock driving motor  275  is not large enough, the AC servomotor, which can detect the torque of the output shaft, can be used for driving the tail stock  116 . A threshold value can be set for the torque generated in the tail stock driving motor  275 . When the torque exceeds the threshold value, the glass rod second elongating apparatus  111  can judge that the heating is insufficient. Then, the glass rod second elongating apparatus  111  can stop the driving of the tail stock  116  for a period of time and increase the gas amount supplied to the heating source  122 .  
      The settings shown in  FIG. 43  are the same as the settings shown in  FIG. 42  except that the “Progress Time” setting changes to the “Tail Stock  116  Moving Distance” setting. The end drawing method shown in  FIG. 43  also has the processes of pre-heating for end drawing (S 170 ), elongating for end drawing (S 172 ), the second heating (S 174 ), second elongating for end drawing (S 176 ), and fusing for end drawing (S 178 ). The gas amount and moving distance of the heating source  122 , and the moving speed of the tail stock  116  are set based on the moving distance of the tail stock  116  at each stage of the process.  
      For example, at the pre-heating for the end drawing (S 170 ), because the moving speed of the tail stock  116  is set to 0 mm/minute, the time after the commencement of the pre-heating for end drawing is measured for 300 seconds. That is, for 300 seconds the moving distance of the heating source  122  is set to 0 mm. The amount hydrogen (H 2 ) gas for the heating source  122  is set to 250 cc/minute. The amount of oxygen (O 2 ) gas (inside) that is output from the inside nozzle of the heating source  122  is set to 30 cc/minute. The amount of oxygen (O 2 ) gas (outside) that is output from the outside nozzle of the heating source  122  is set to 100 cc/minute. The glass rod  106  is heated by the heating source  122 , which is set according to the above conditions. When the time after the commencement of the pre-heating for end drawing passes 300 seconds, the process is shifted to next step.  
      At the elongating for end drawing (S 172 ), whilst the moving distance is changed from 0 mm to 30 mm, the amount hydrogen (H 2 ) gas for the heating source  122  is set to 250 cc/minute. The amount of oxygen (O 2 ) gas (inside) that is output from the inside nozzle of the heating source  122  is set to 30 cc/minute. The amount oxygen (O 2 ) gas (outside) that is output from the outside nozzle of the heating source  122  is set to 100 cc/minute. The glass rod  106  is heated by the heating source  122 , which is set according to the above conditions. With the moving distance of the heating source  122  as 0 mm, the tail stock  116  is moved at a speed of 10 mm/minute to elongate the glass rod  106 .  
      At the second heating (S 174 ), the moving speed of the tail stock  116  is set to 0 mm/minute so that the moving distance of the tail stock  116  remains at 30 mm. The moving distance of the heating source  122  is set to 15 mm. The amount of hydrogen (H 2 ) gas for the heating source  122  is set to 130 cc/minute. The amount of oxygen (O 2 ) gas (inside) that is output from the inside nozzle of the heating source  122  is set to 15 cc/minute. The amount of oxygen (O 2 ) gas (outside) that is output from the outside nozzle of the heating source  122  is set to 50 cc/minute. The glass rod  106  is heated by the heating source  122 , which is set according to the above conditions. After the heating source  122  has moved 15 mm, the process is shifted to next step.  
      Then, at the second elongating for end drawing (S 176 ), whilst the moving distance of the tail stock  116  is increased from 30 mm to 55 mm, the moving distance of the heating source  122  is increased from 15 mm to 25 mm. The amount hydrogen (H 2 ) gas for the heating source  122  is set to 130 cc/minute. The amount of oxygen (O 2 ) gas (inside) that is output from the inside nozzle of the heating source  122  is set to 15 cc/minute. The amount of oxygen (O 2 ) gas (outside) that is output from the outside nozzle of the heating source  122  is set to 50 cc/minute. The glass rod  106  is heated by the heating source  122 , which is set according to the above conditions. The tail stock  116  is moved at a speed of 10 mm/minute to elongate the glass rod  106 .  
      Finally, at the fusing for end drawing (S 178 ), whilst the moving distance of the tail stock  116  increased from 55 mm to 100 mm, the heating source  122  did not move from the position at the second elongating for end drawing (S 176 ). The moving distance therefore remains at 25 mm. The amount hydrogen (H 2 ) gas for the heating source  122  is set to 130 cc/minute. The amount of oxygen (O 2 ) gas (inside) that is output from the inside nozzle of the heating source  122  is set to 30 cc/minute. The amount of oxygen (O 2 ) gas (outside) that is output from the outside nozzle of the heating source  122  is set to 20 cc/minute. The glass rod  106  is heated by the heating source  122 , which is set according to the above conditions. The tail stock  116  is moved at a speed of 120 mm/minute to fuse the glass rod  106 .  
     EXAMPLE 1  
      A glass rod  106  having a diameter of 60 mm was end-drawn according to the setting values shown in  FIG. 43 . An AC servomotor of 200 W was used for the tail stock driving motor  275 . A rotary encoder that can detect the amount of rotation of the tail stock driving motor  275  was used as the tail stock driving encoder  273 . The rotation speed of the tail stock driving motor  275  was controlled by the output of the tail stock driving encoder  273 . The moving distance of the tail stock  116  was obtained by measuring the output of the tail stock driving encoder  273 . The time required for the end drawing was 15 minutes. The shape of the processed glass rod  106  at the region which was end-drawn was a well formed circular cone shape. The length and the diameter of the region were 61 mm and 60 mm respectively.  
     EXAMPLE 2  
      A glass rod  106  having a diameter of 60 mm was end-drawn according to the setting values shown in  FIG. 43 . A linear encoder that can detect the moving distance of the tail stock  116  was provided on the tail stock  116 . The gas amount and the moving distance of the heating source  122 , and the moving speed of the tail stock  116  were controlled based on the moving distance of the tail stock  116  detected by the linear encoder. The shape of the processed glass rod  106  at the region that was end-drawn was a well formed circular cone. The length and the diameter of the region were 65 mm and 60 mm respectively.  
       FIG. 44  shows a configuration of the heating source  122  of the glass rod second elongating apparatus  111 . The bottom end of the outside pipe  285  of the heating source  122  is closed. The outside pipe  285  is connected to a combustible gas channel  312 . This is a channel for hydrogen gas which is an example of a suitable combustible gas. The heating source  122  has a combustible gas flow rate control unit  314  placed in the combustible gas channel  312 . All of the inside pipes  286  are connected to an oxygen gas channel  308  through the branching tool  316 . The oxygen channel  308  is a channel for oxygen gas. An inert-gas channel  296  is connected to the oxygen gas channel  308  by the connecting element  302 . An oxygen gas flow rate control unit  310  is installed between the connecting element  302  and the entrance of the oxygen gas channel  308 .  
      The inert-gas channel  296  has a valve  300  and an inert-gas flow rate control unit  298 . The heating source  122  has a control element  304  which controls a driving source  306  based on the data of the flow rate that is output from the oxygen gas flow rate control unit  310 . The driving source  306  is connected to the valve  300 . The combustible gas flow rate control unit  314  and the oxygen gas flow rate control unit  310  control the flow rate of the hydrogen gas H 2  and oxygen gas O 2  shown in the  FIG. 42  and  FIG. 43 . A valve such as an electric valve or electromagnetic valve can be used as the valve  300 . A three directional pipe or a three directional valve can be used for the connecting element  302 .  
       FIG. 45  shows a plan view of the top of the heating source  122 . A plurality of the inside pipes  286 , each of which has an inside diameter of 1 mm and an outside diameter of 3 mm, is inserted into the outside pipe  285 , which has an inside diameter of 30 mm. The inside pipes  286  are placed around the center of the outside pipe  285  in a plurality of rows of concentric circles.  
      The inside pipes  286  are placed with regular spacing intervals for each row. The closer the rows are towards the outside of the outside pipe  285 , the higher the density of the intervals of the inside pipe  286  for the each row becomes. The inside pipes  286  can be installed inside the outside pipe  285  with a homogeneous density. Oxygen gas flows inside the oxygen gas outlet  288 , which is inside of the inside pipe  286 . A combustible gas flows inside the combustible gas outlet  290 , which is inside of the outside pipe  285 .  
      The movement of the heating source  122  will be explained below. Hydrogen gas flows into the outside pipe  285  through the combustible gas channel  312  from a hydrogen gas supply source, not shown in the figure. Oxygen gas is distributed to the inside pipe  286  by the branching tool  316 . Oxygen gas is supplied from an oxygen gas supply source (not shown in the figure) through the oxygen gas channel  308 . The hydrogen and oxygen gas are mixed at the top of the outside pipe  285 . A flame  294  can be obtained by igniting the mixed gas.  
      According to the purpose of the processing of the glass rod  106 , the quantity of the hydrogen and oxygen gas were adjusted by using the oxygen gas flow rate control unit  310  and the combustible gas flow rate control unit  314  to obtain the optimum flame condition. At this time, the signal that shows the flow rate of the oxygen gas is output from the oxygen gas flow rate control unit  310  to the control element  304 . The linear speed of the oxygen gas is a value derived by dividing the flow rate of the oxygen gas by the area of the inside of the inside pipe  286 .  
      If the linear speed of the oxygen gas is 1.0 m/sec or under, the control element  304  drives the driving source  306  and opens the valve  300 . Then, nitrogen gas, which is an inert gas, flows into the oxygen gas channel  308  with a linear speed of 0.5 m/sec and is mixed with the oxygen gas. When changing the flow rate of the oxygen, the control element  304  drives the driving source  306  and closes the valve  300  if the linear speed of the oxygen reaches 1.1 m/sec.  
      When reducing the flow rate of the combustible gas and oxygen gas to make the flame smaller, the region of high temperature near the top of the inside flame moves from the top of the heating source  122 . This is because the flame  294  diffuses as a result of mixing the inert-gas with oxygen gas. Therefore, the surface temperature of the top of the heating source  122  is maintained below 400° C., so that e oxidation of the heating source  122  can be prevented.  
      When strong heating power is needed, the valve  300  for the inflow of the inert gas is closed because the combustion temperature drops if inert gas is mixed. At this time, because the flame  294  is large owing to the increase of the flow rate of the combustible gas and oxygen gas, the region of high temperature of the flame  294  is no longer at the top of the heating source  122 . Therefore, the surface temperature of the top of the heating source  122  is maintained below 400° C. The generation of a pulse caused by the opening and closing of the valve  300  can be prevented by setting a different linear speed value for the oxygen gas at the time of opening and closing of the valve  300 . This should be set to 1.0 m/sec or below for opening and 1.1 m/sec or above for closing.  
      It is desirable that the inert gas has a linear speed of between 0.5 m/sec to 2 m/sec as it flows by the opening of the valve  300 . The linear speed of the inert gas is calculated by dividing the flow rate of the inert gas by the area inside the oxygen gas outlet  288  of the inside pipe  286 . If the linear speed of the inert gas is 0.5 m/sec or below, it is difficult to control the temperature of the top of the heating source  122 . On the other hand, if the linear speed of the inert gas is 2.0 m/sec or above, the hydrogen gas burns incompletely, and the temperature of the flame  294  decrease.  
      If using a heating source  122  to heat the glass rod  106  with the flame  294 , a metal oxide will not usually be generated at the top of the heating source  122 . This is because the temperature of the top of the heating source  122  is maintained at 400° C. or below. Therefore, a metal oxide does not attach to the glass rod  106 , and a glass rod  106  of high quality can be manufactured.  
      A glass rod  106  having an average diameter of 65 mm was elongated by a glass rod second elongating apparatus  111  that has heating source  122  controlling the flow rate of the inert gas. The ratio of the number of glass rods  106  having foreign matter such as metal oxide to the total numbers of processed glass rod  106  was 0.2%. This is a low value compared to the ratio of glass rods made by the conventional heating source  122 . For comparison, the ratio of the number of glass rods  106  having foreign matter such as metal oxide to the total numbers of the processed glass rods  106  became a high value of 15% when the glass rod  106  was elongated by always closing the valve  300 .  
       FIG. 46  shows a relationship between the linear speed of the oxygen gas and the temperature of the top of the heating source  122 . This is illustrated for the case of always mixing oxygen gas with nitrogen gas having linear speed of 0.5 m/sec and of not mixing the oxygen gas with the nitrogen gas. The temperature of the top of the heating source  122  does not exceed 400° C. when mixing the nitrogen gas. The temperature reached 400° C. to 700° C. at the region where the linear speed of the oxygen gas was 1 m/sec or under when the nitrogen gas was not mixed. Therefore, the surface temperature of the heating source  122  can be controlled by mixing the oxygen gas with nitrogen gas when the linear speed of the oxygen gas is 1 m/sec or below.  
       FIG. 47  shows the shape of a tip of the preform  107 , the diameter of which is reduced and which is fused at the end drawing (S 158 ). The D represents the diameter of the preform  107 . The O represents the location where the diameter of the preform  107  starts to be reduced. The P represents the location where the diameter D of the preform  107  is reduced to 1% or below the original diameter. The preform  107  has a taper shape, both ends of which can be shown by the equation 1/3D≦L≦0.3D. Here, L represents the length between the location O and the location P.  
      The time that the drawing reaches the steady state is the time from the setting of the preform  107  on the preform drawing apparatus  500  until the diameter and the drawn speed of the optical fiber reaches the prescribed value. When the preform  107  is drawn to an optical fiber, the original shape of the preform  107  influences the time it takes for the drawing to reach the steady state. This influence becomes larger as the diameter of the preform  107  becomes larger. Then, the time taken for the drawing to reach the steady state becomes longer.  
      The preform  107  having the shape of the equation 1/3D≦L≦3D can reduce the time taken for the drawing to reach the steady state. If L&lt;1/3D, the time taken for the diameter and the drawn speed of the optical fiber to reach the prescribed value increases because the time that the tip of the preform  107  drops down becomes longer. If L&gt;3D, the time taken for the tip of the preform  107  to drop down can be decreased, but the time taken for the taper shape of the preform  107  to become the shape of the steady state of the drawing takes longer. Then, the time taken for the diameter and the drawn speed of the optical fiber to reach the prescribed value becomes longer. Therefore, it is best to make the shape of the taper of the preform  107  as L=D.  
      In the case of fusing the preform  107  by heating part of the preform  107  by a flame, a residual strain remains on both ends of the taper part of the preform  107 . If the residual strain in the taper part is large, cracks can be generated on both ends of the preform  107  when a strong impact is applied on the preform  107 . The cracks can also be generated on both ends of the preform  107  by a thermal impact generated by the welding of the preform  107  and the dummy rod. The quantity of the strain on both ends of the preform  107  would ideally be 40 kgf/cm 2  or below. The cracks generated on the preform  107  can be prevented by controlling the quantity of the residual strain remaining in the preform  107  at 40 kgf/cm 2  or below.  
     EXAMPLE  
      A preform  107  with a diameter of 30 mm was drawn. The length L was set to 30 mm. The quantity of the strain remaining in the taper part of the preform  107  was 40 kgf/cm 2 , and cracks were not generated during the welding of the preform  107  and the dummy rod. When the set diameter of the optical fiber was 125 μm and the speed of the drawing was 100 mm/min, the time that the drawing took to reach the steady state was a total of 20 minutes. The time from the setting of the preform  107  on the preform drawing apparatus  500  to the dropping of the tip of the preform  107  was 10 minutes. The time taken for the diameter and the drawn speed of the optical fiber to reach the prescribed value was 10 minutes.  
     COMPARATIVE EXAMPLE 1  
      A preform  107  with a diameter of 30 mm was drawn. The length L was set to 5 mm. The quantity of the strain remaining in the taper part of the preform  107  was 40 kgf/cm 2 , and cracks were not generated during the welding of the preform  107  and the dummy rod. When the set diameter of the optical fiber was 125 μm and the speed of the drawing was 100 mm/min, the time that the drawing reached d the steady state was a total of 50 minutes. The time from the setting of the preform  107  on the preform drawing apparatus  500  to the dropping of the tip of the preform  107  was 20 minutes. The time taken for the diameter and the drawn speed of the optical fiber to reach the prescribed value was 30 minutes.  
     COMPARATIVE EXAMPLE 2  
      A preform  107  with a diameter of 30 mm was drawn. The length L was set to 100 mm. The quantity of the strain remaining in the taper part of the preform  107  was 40 kgf/cm 2 , and cracks were not generated during the welding of the preform  107  and the dummy rod. When the set diameter of the optical fiber was 125 μm and the speed of the drawing was 100 mm/min, the time taken for the drawing to reach the steady state was a total of 40 minutes. The time from the setting of the preform  107  on the preform drawing apparatus  500  to the dropping of the tip of the preform  107  was 10 minutes. The time taken for the diameter and the drawn speed of the optical fiber to reach the prescribed value was 30 minutes.  
     COMPARATIVE EXAMPLE 3  
      A preform  107  with a diameter of 30 mm was drawn. The length L was set to be 30 mm. The quantity of the strain remaining in the taper part of the preform  107  was 60 kgf/cm 2 . The preform  107  could not be drawn because cracks were generated during the welding of the preform  107  and the dummy rod.  
      As shown above, the time required for drawing the preform  107  to an optical fiber can be reduced by making the shape of the tip of the preform  107  as 1/3D≦L≦3D.  
       FIG. 48  shows another shape of the tip of the preform  107  that was end-drawn. The preform  107  shown in  FIG. 48  has a fused part  332  on one end formed by a flame, and a cutting face  334  on the other end, which is cut mechanically. The fused part  332 , which is shown in  FIG. 48 ( a ), is fused rapidly by a flame. The fused part  332 , which is shown in  FIG. 48 ( b ), is fused gradually by reducing the diameter to form a taper part  336 . A thin part  338  is provided on the tip of the fused part  332  shown in  FIG. 48 ( c ).  
      When drawing a preform  107  which has the taper part  336  as shown in  FIG. 48 ( b ), the time taken for the tip of the preform  107  to dropdown is short, and the quantity of preform  107  to be dropped is also small because the diameter of the fused part  332  is small. When drawing a preform  107  which has the taper part  336  and thin part  338  as shown in  FIG. 48 ( c ), the time taken for the tip of the preform  107  to drop down can be reduced to one third or less of the time required for the conventional shape of the preform  107 . The loss in material caused by the dropping of the preform  107  can be limited to the small quantity of the thin part  338 .  
      It is desirable that the shape of the thin part  338  occupies between 0.1 percent to 15 percent of the weight of the fused part  332 . If the weight of the thin part  338  is smaller than 0.1 percent of the weight of the fused part  332 , the effect produced by providing the thin part  338  cannot be obtained. On the other hand, if the weight of the thin part  338  is larger than 15 percent of the weight of the fused part  332 , the time taken for the tip of the preform  107  to drop becomes long, and the loss of preform  107  increases during the drawing.  
      It is desirable that the diameter of the thin part  338  be between ½ to {fraction (1/10)} of the diameter of the main body of the preform  107 . If the diameter of the thin part  338  is within this range, the time required for the dropping of the tip of the preform  107  at the early stage of the drawing can be short. If the length of the thin part  338  is approximately one to five times this diameter, the loss of the preform  107  can be limited to a small quantity.  
       FIG. 49  shows a preform  107  that is damaged, before the preform  107  is surface treated at the surface treatment (S 168 ) shown in the  FIG. 26 . The preform  107 , which is elongated by the glass rod second elongating apparatus  111 , is etched by hydrofluoric acid as a surface treatment. This cuts the cladding of the preform  107  chemically so that the preform  107  has the prescribed ratio of thickness of core to cladding.  
      The hydrofluoric acid etching treatment is a treatment that decomposes the bonds between the Silicon and oxygen of the glass. The hydrofluoric acid etching treatment cuts the surface of the preform  107  chemically at a speed of about 8 mm per one hour. However, if there is a crack or a concave on the surface of the preform  107 , the place having the crack or concave is cut further to form a larger concave than the concave made on the other parts of the preform  107 . This concave caused by the treatment of hydrofluoric acid etching is called a hydrofluoric concave. This hydrofluoric concave is the cause of the breaking of an optical fiber during the drawing of the preform  107  to an optical fiber.  
      A preform  107  without hydrofluoric concaves on its surface can be obtained by removing cracks and concaves on the preform  107  by polishing before the treatment of hydrofluoric acid etching. There is a method of fire polishing the preform  107  with the temperature above the strain point of the preform  107 . During the fire polishing, the preform  107  is fire polished so that the unevenness of the surface will be within a 0.3 mm range. The generation of the hydrofluoric concave can be prevented by fire polishing the preform  107  before etching the preform  107  with hydrofluoric acid. This is possible because the quantity of the strain in the preform  107  can be decreased and a smooth surface without cracks can be obtained. Not only is fire polishing suitable, but also mechanical polishing can be used for polishing the preform  107 .  
       FIG. 51  shows a number of hydrofluoric concaves generated in the preform  107  counted by visual inspection of the example and the comparative example.  FIG. 52  shows the unevenness of the surface of the preform  107  after the treatment with the hydrofluoric acid etching of the example and the comparative example. In the pre-treating  1  shown in  FIG. 51  and  FIG. 52 , the preform  107   a  having a diameter of 60 mm and a length of 1000 mm was damaged. First, the preform  107   a  and the other preform  107   b , which had the same shape as the preform  107   a , were placed on the floor.  
      Next, one end of the preform  107   a  was lifted to height of 10 cm while the other end remained on the floor. Then, the end of the preform  107  that was lifted was dropped onto the preform  107   b  so that the preform  107   a  had a crack. Each of a plurality of the preform  107   a  was damaged in 3 places at 20 cm intervals by the same method shown above. On the pre-treating  2  shown in  FIG. 51  and  FIG. 52 , the preform  107   a  was lifted to a height of the 20 cm. The other procedure of damaging the preform  107  was same as pre-treating  1 .  
      On the example shown in  FIG. 51  and  FIG. 52 , each of the preform  107   a  was treated by the pre-treating  1  and pre-treating  2 . Then, each of the preform  107   a  was fire polished with a burner that was provided with hydrogen gas at 250 ml/min and oxygen gas at 145 ml/min. Each of the fire polished preform  107   a  was treated by hydrofluoric acid etching at room temperature. The thickness of material etched from the exterior diameter of the preform  107  was one of 4 steps of 0.2 mm, 1.2 mm, 2.2 mm, and 3.2 mm. 10 pieces of the preform  107   a  were etched by hydrofluoric acid for each of the 4 steps of the etching thickness. The number of the hydrofluoric concaves was checked by visual inspection after the treatment by hydrofluoric acid etching.  
       FIG. 50  shows the preform  107   a , which was treated by the hydrofluoric acid etching in the example shown in the  FIG. 51  and  FIG. 52 . The unevenness of the surface of the preform  107   a  was obtained by measuring the difference of the diameter between the point which was shown by the mark X and the diameter of the point which was shown by the mark ◯. The point which was shown by the mark X was the place damaged by contacting with preform  107   b . The point which was shown by the mark ◯ was a place 10 cm away from the point of the mark X, which was not damaged by contacting with preform  107   b . The average value of the diameter of the 3 points shown by the mark X were used as the diameter of the each of the preform  107   a.    
      In the comparative example shown in  FIG. 51  and  FIG. 52 , each of the preform  107  treated by pre-treatment  1  and pre-treatment  2  were treated by hydrofluoric acid etching without fire polishing. The number of hydrofluoric concaves was assessed by visual inspection, and the unevenness of the surface was measured in the same way as the example. As shown in  FIG. 52  and  FIG. 53 , the unevenness of the surface of the pre-treatment  2  was larger than the unevenness of the surface of the pre-treatment  1 . This is because pretreatment  2  was lifted higher pre-treatment  1  in the damage process. Also, the number of hydrofluoric concaves generated by the hydrofluoric acid etching of the pre-treatment  2  was larger than the number of the hydrofluoric concaves of the pre-treatment  1 .  
      The larger the quantity of the etching, the larger the unevenness of the surface of the preform  107 . Also, the larger the quantity of the etching, the larger the number of hydrofluoric concaves generated by the hydrofluoric acid etching. The unevenness of the surface of the preform  107   a  of the example which was fire polished, was lower than the unevenness of the surface of the preform  107   a  of the comparative example, which was not fire polished.  
      The number of the hydrofluoric concave generated on the example is smaller than the number of the hydrofluoric concave generated on the comparative example as shown in  FIG. 51 . Therefore, the number of the hydrofluoric concave in the preform  107   a  and the unevenness of the surface of the preform  107   a  can be decreased by fire polishing the preform  107   a  before etching the preform  107   a  with hydrofluoric acid.  
       FIG. 53  shows another shape of the preform  107  which is surface treated. The preform  107  has a handle  340 . The handle  340  is made of a silica glass and is installed on the cutting face  334  of the surface treated preform  107  shown in  FIG. 48 ( c ) by welding or mechanical processing. The preform  107  with a handle  340  can be installed onto the preform drawing apparatus  500  promptly when drawing the preform  107  to an optical fiber. The diameter of the handle  340 , installed on the cutting face  334 , can be smaller than the diameter of the preform  107  as shown in  FIG. 53 ( b ).  
       FIG. 54  shows an ultrasonic cleaning apparatus  404 , which cleans the heating source  122 . The ultrasonic cleaning apparatus  404  comprises an ultrasonic oscillator  396 . A cleaning liquid  398  is contained inside of the ultrasonic cleaning apparatus  404 . The cleaning liquid  398  contains 10 percent hydrofluoric acid and 3 percent nitric acid. The hydrofluoric acid dissolves the metal oxide generated on the surface of the outside pipe  285  and inside pipe  286  of the heating source  122 . Oxidation of the surface of the outside pipe  285  and the inside pipe  286  does not readily occur if the outside pipe  285  and the inside pipe  286  are made of stainless steel. This is because iron, chromium, and nickel, which are contained in stainless steel, form a passive thin film on the surface of the stainless steel from the effect of the nitric acid, thus protecting the surfaces.  
      The cleaning liquid  398  can contain a soluble organic solvent. Examples of soluble organic solvents are alcohol, acetone, acetonitrile, and tetrahydrofuran. The heating source  122  can be soaked in the cleaning liquid  398  containing hydrofluoric acid and then soaked in the other cleaning liquid  398  which contains nitric acid. The ultrasonic oscillator  396  oscillates an ultrasonic wave of strength of 1 W/cm 2  to 2 w/cm 2 .  
      The heating source  122  to be cleaned is made of stainless steel. The heating source  122  has a plurality of inside pipes  286 , which have an internal diameter of 1 mm and an outside diameter of 3 mm. The inside pipes  286  are inside the outside pipe  285 , which has an internal diameter of 30 mm. Hydrogen gas flows inside the outside pipe  285 , and oxygen gas flows inside the inside pipe  286 . The outside pipe  285  is connected to a hydrogen inlet pipe  392 , and all the inside pipes  286  are connected to an oxygen inlet pipe  394 .  
      When the glass rod  106  is heated by the flame of the heating source  122 , the temperature of the top of the heating source  122  increases to a high temperature of between 400° C. to 700° C. Therefore, a metal oxide will be generated on the surface of the top of the heating source  122 . The metal oxides gradually dislodges to become free floating particles if the heating source is used for a long time.  
      Particles of metal oxide or foreign matter impurities such as glass particles attached to the heating source  122  may be dislodged during the heat treatment of the glass rod  106 . These particles can attach to the surface of the glass rod  106  in which case the surface layer of the glass rod  106  has to be polished. If the glass rod  106  is polished, the ratio of the diameter of the cladding and the core of the glass rod  106  will change. The characteristic of light transmission of an optical fiber made from the glass rod  106  will deteriorate as a result. Therefore, foreign matter impurities and metal oxides attached to the heating source  122  are removed from the heating source  122  by cleaning the heating source  122 .  
      To clean the heating source  122  using the ultrasonic cleaning apparatus  404 , first, the hydrogen inlet pipe  392  and oxygen inlet pipe  394  are opened to the outside. Then, the heating source  122  is soaked in the cleaning liquid  398  with the flame nozzle  390  directed downward. Any air remaining inside the outside pipe  285  and the inside pipe  286  is released through the hydrogen inlet pipe  392  and oxygen inlet pipe  394 . Following this, the outside pipe  285  and the inside pipe  286  are immersed and soaked in the cleaning liquid  398  to the top of the water level. The ultrasonic cleaning apparatus  404  then cleans the heating source  122  by oscillating the ultrasonic wave using the ultrasonic oscillator  396 . The vibration frequency of the ultrasonic waves is 10 kHz to 100 kHz.  
      The heating source  122  was cleaned using the ultrasonic cleaning apparatus  404 . Metal oxide was present around the stainless steel flame nozzle  390  of the heating source  122 , which is used for heating the glass rod. The area around the flame nozzle  390  of the heating source  122  was soaked in the cleaning liquid  398 . To clean the heating source  122 , an ultrasonic wave with a vibration frequency of 10 kHz to 100 kHz was oscillated for 30 minutes by the ultrasonic oscillator  396  having output of 500 W. Then, the heating source  122  was removed from the ultrasonic cleaning apparatus  404  and any cleaning liquid  398  remaining on the surface of the heating source  122  was cleaned with pure water. The heating source  122  was then dried.  
      The top of the outside pipe  285  and the inside pipe  286  were inspected, and metal oxides and foreign matter impurities were not found in the outside pipe  285  and the inside pipe  286 . The surface of the glass rod  106  was heat treated by the cleaned heating source  122 . The ratio of the number of glass rods  106 , which had foreign matter impurities attached, compared to the total number of treated glass rods  106  was 6 percent.  
      The surface of the glass rod  106  was heat treated by the heating source  122 , which was not cleaned, for a comparison. In this case, the ratio of the number of glass rods  106 , which had foreign matter impurities attached, to the total number of heat treated glass rods  106  was 15 percent. This is larger value than the ratio obtained by the cleaned heating source  122 .  
      As shown above, the metal oxide and attached foreign matter generated on the top of the heating source  122  can be removed by cleaning the heating source  122  with the ultrasonic cleaning apparatus  404 . A preform  107  of high quality can be obtained by heating the glass rod  106  with a heating source  122 , which is cleaned by the ultrasonic cleaning apparatus  404 , because less foreign matter is attached to glass rod  106 .  
       FIG. 55  shows a configuration of the preform drawing apparatus  500  that draws the preform  107  to an optical fiber. The preform drawing apparatus  500  comprises a chuck  346 , which holds a dummy rod  342  that is welded to the preform  107 ; a heating means  348  which heats the preform  107 ; movable support  344  which supplies the preform  107  to the heating means  348 ; a diameter measurement device  352  which measures the diameter of an optical fiber  350  drawn from the preform  107 ; a first coating device  354  which undertakes the first coating of the optical fiber  350 ; a first curing device  356  which cures the first coated optical fiber  350  by a ultraviolet rays; a second coating device  358  which coats the optical fiber  350  a second time; a second curing device  360  which cures the second coated optical fiber  350  by a ultraviolet rays; and a tractor  362  which winds the optical fiber  350 .  
      To draw the preform  107  into an optical fiber  350  using the preform drawing apparatus  500 , first, the dummy rod  342 , which is welded to the preform  107 , is held by the movable support  344  with the chuck  346 . The starting end of the preform  107  is then set to the prescribed position of the heating means  348 , and the preform  107  is heated. When the tip of the preform  107  softens and drops, the dropped tip of the preform  107  is caught and drawn out to be passed through the diameter measurement device  352 .  
      When the diameter of the optical fiber  350  reaches the desired diameter, the optical fiber  350  is first coated with resin by passing through the first coating device  354 . The first coated optical fiber  350  is then passed through the first curing device  356  to be cured. The optical fiber  350  is then second coated by the second coating device  358  and cured by the second curing device  360 . When the diameter and the speed of the drawing of the optical fiber  350  reaches a prescribed value, the optical fiber  350  is wound onto a bobbin, not shown in the figure, through the tractor  362 .  
      A preform  107  of high quality and little variation in diameter can be manufactured by the glass base material first drawing apparatus  900  and the glass rod second elongating apparatus  111  shown above. Therefore, optical fibers of high quality and reduced diameter variation can be manufactured by drawing the preform  107 , manufactured by the glass base material first drawing apparatus  900  and the glass rod second elongating apparatus  111 , using the preform drawing apparatus  500 .  
      Although the present invention has been described by reference to specific embodiments, the scope of the present invention is not limited to these embodiments. Those skilled in the art can make various modifications and improvements to these embodiments of the present invention. It is clear from the appended claims that such modifications or improvements are also covered by the scope of the present invention.