Patent Publication Number: US-2018044221-A1

Title: Method and apparatus for producing optical fiber preform

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This non-provisional application claims priority under 35 U.S.C. §119(a) from Japanese Patent Application No. 2016-156518, filed on Aug. 9, 2016, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a method and apparatus for producing an optical fiber preform which stabilizes a burner flame to allow a high quality large-sized preform to be produced at a low burner load. 
     Background Art 
     In a VAD method, which is a well-known method for producing an optical fiber preform, a starting material is attached to a shaft which moves upward while rotating, and hung in a reaction chamber. Glass fine particles produced by a core deposition burner and clad deposition burner set at a predetermined angle with respect to the axial direction of the starting material in the reaction chamber are adhered and deposited on the tip of the starting material to produce a porous glass preform including a core layer and a cladding layer. The VAD method is suitable for growing the preform in size and for producing a low water peak fiber (LWPF). 
       FIG. 1  schematically shows an optical fiber preform producing apparatus  100  according to a VAD method. The optical fiber preform producing apparatus  100  includes a reaction vessel  110 , a core deposition burner  121 , a first clad deposition burner  122 , and a second clad deposition burner  123 . 
     The reaction vessel  110  includes a deposition chamber  111 , and an intake port  111   a  and exhaust port  111   b  formed in the deposition chamber  111 . A starting material (not shown) is inserted into the deposition chamber  111 . A core deposition burner  121  is disposed at a predetermined angle with respect to the pulling axis of the starting material toward the tip of the starting material. A first clad deposition burner  122  and a second clad deposition burner  123  are disposed at a predetermined angle with respect to the pulling axis of the starting material toward the side surface of the starting material. 
     The starting material is made to move upward while being rotated, and a reaction gas was supplied to each burner and hydrolyzed in an oxyhydrogen flame, to synthesize glass fine particles. The glass fine particles are sprayed onto the starting material and deposited to produce a porous glass preform  10 . The produced porous glass preform  10  is dehydrated and transparently vitrified in an electric furnace (not shown), thereby providing a preform for optical fiber. 
     For each burner of such a producing apparatus, a concentric multiple tube burner made of quartz glass has been generally used. However, in the burner having a concentric multiple tube structure, a glass raw material gas, a combustible gas, and a combustion supporting gas are insufficiently mixed, which provides insufficient production of glass fine particles. This causes poor deposition efficiency, which makes it difficult to produce the preform at a high speed. 
     As the structure of each port outlet of a burner for solving this problem, a multi-nozzle burner  120  having a structure as shown in  FIG. 2  is disclosed in JP 2010-215415 A. The multi-nozzle burner  120  includes a small diameter combustion supporting gas ejection port disposed in a combustible gas ejection port so as to surround a raw material gas ejection port located at the central part of the combustible gas ejection port. The multi-nozzle burner  120  includes a raw material gas ejection port  120   a  provided in a central part and ejecting a raw material gas, a first seal gas ejection port  120   b  annularly provided on the concentric outer side of the raw material gas ejection port  120   a  and ejecting a seal gas, a combustible gas ejection port  120   c  annularly provided on the concentric outer side of the first seal gas ejection port  120   b  and ejecting a combustible gas, a plurality of small diameter combustion supporting gas ejection ports  120   d  provided so as to surround the first seal gas ejection port  120   b  in the combustible gas ejection port  120   c  and ejecting a combustion supporting gas, a second seal gas ejection port  120   e  annularly provided on the concentric outer side of the combustible gas ejection port  120   c  and ejecting a seal gas, and a combustion supporting gas ejection port  120   f  annularly provided on the concentric outer side of the second seal gas ejection port  120   e  and ejecting a combustion supporting gas. 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In recent years, as a preform is grown in size for the purpose of cost reduction, the feed rate of a gas to a burner is increased, the life time of the burner is shortened by the fixing of glass fine particles to the burner, and the cracking of the preform due to the instability of a burner flame, and the variation of the diameter of the preform are aggravated. 
     An object of the present invention is to provide a method and apparatus for producing an optical fiber preform which stabilizes a burner flame to allow a high quality large-sized preform to be produced at a low burner load. 
     Means for Solving the Problems 
     A method for producing an optical fiber preform of the present invention using a multi-nozzle burner, 
     the multi-nozzle burner including: 
     a raw material gas ejection port provided in a central part and ejecting a raw material gas; 
     a seal gas ejection port annularly provided concentrically on an outer side of the raw material gas ejection port and ejecting a seal gas; 
     a combustible gas ejection port annularly provided concentrically on an outer side of the seal gas ejection port and ejecting a combustible gas; and 
     a plurality of small diameter combustion supporting gas ejection ports provided so as to surround the seal gas ejection port in the combustible gas ejection port and ejecting a combustion supporting gas, 
     wherein when a gas flow rate of the raw material gas ejection port is V 1  and a gas flow rate of the seal gas ejection port is V 2 , the gas flow rates are controlled so that 1&gt;V 2 /V 1 &gt;0.05 is set. 
     An apparatus for producing an optical fiber preform of the present invention, 
     the apparatus including a multi-nozzle burner, 
     the multi-nozzle burner including: 
     a raw material gas ejection port provided in a central part and ejecting a raw material gas; 
     a seal gas ejection port annularly provided concentrically on an outer side of the raw material gas ejection port and ejecting a seal gas; 
     a combustible gas ejection port annularly provided concentrically on an outer side of the seal gas ejection port and ejecting a combustible gas; and 
     a plurality of small diameter combustion supporting gas ejection ports provided so as to surround the seal gas ejection port in the combustible gas ejection port and ejecting a combustion supporting gas, 
     wherein when a gas flow rate of the raw material gas ejection port is V 1  and a gas flow rate of the seal gas ejection port is V 2 , the gas flow rates are controlled so that 1&gt;V 2 /V 1 &gt;0.05 is set. 
     The method and apparatus for producing an optical fiber preform of the present invention optimize the flow rate ratio between the gas flow rate of the raw material gas ejection port and the gas flow rate of the seal gas ejection port to provide difficult fixing of the glass fine particles to the burner, and to stabilize a burner flame, thereby allowing a high quality large-sized preform to be produced at a low burner load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows an apparatus for producing an optical fiber preform according to a VAD method; and 
         FIG. 2  shows an example of a multi-nozzle burner used in a method and apparatus for producing an optical fiber preform according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described. 
       FIG. 1  shows an optical fiber preform producing apparatus  100  used for carrying out a method for producing an optical fiber preform according to the present invention. The optical fiber preform producing apparatus  100  includes a reaction vessel  110 , a core deposition burner  121 , a first clad deposition burner  122 , and a second clad deposition burner  123 . 
     The reaction vessel  110  includes a deposition chamber  111 , and an intake port  111   a  and exhaust port  111   b  formed in the deposition chamber  111 . A starting material (not shown) is inserted into the deposition chamber  111 . A core deposition burner  121  is disposed at a predetermined angle with respect to the pulling axis of the starting material toward the tip of the starting material. A first clad deposition burner  122  and a second clad deposition burner  123  are disposed at a predetermined angle with respect to the pulling axis of the starting material toward the side surface of the starting material. 
     All the burners are generally made of quartz glass, and a seal gas ejection port is provided on the concentric outer side of a raw material gas ejection port provided in a central part. Raw material gas of glass fine particles, Ar and O 2  are ejected from the raw material gas ejection port, but in the present specification, they are collectively referred to as a raw material gas. 
     To the core deposition burner  121 , for example, a concentric four-tube burner is applied, and a raw material gas (for example, SiCl 4 , O 2 ), a combustible gas (for example, H 2 ), a combustion supporting gas (for example, O 2 ), and a seal gas (for example, N 2 ) are supplied. To the first clad deposition burner  122  and the second clad deposition burner  123 , a multi-nozzle burner  120  as shown in  FIG. 2  is applied. 
     The multi-nozzle burner  120  includes a raw material gas ejection port  120   a  provided in a central part and ejecting a raw material gas (for example, SiCl 4 , O 2 ), a first seal gas ejection port  120   b  annularly provided on the concentric outer side of the raw material gas ejection port  120   a  and ejecting a seal gas (for example, N 2 ), a combustible gas ejection port  120   c  annularly provided on the concentric outer side of the first seal gas ejection port  120   b  and ejecting a combustible gas (for example, H 2 ), a plurality of small diameter combustion supporting gas ejection ports  120   d  provided so as to surround the first seal gas ejection port  120   b  in the combustible gas ejection port  120   c  and ejecting a combustion supporting gas (for example, O 2 ), a second seal gas ejection port  120   e  annularly provided on the concentric outer side of the combustible gas ejection port  120   c  and ejecting a seal gas, and a combustion supporting gas ejection port  120   f  annularly provided on the concentric outer side of the second seal gas ejection port  120   e  and ejecting a combustion supporting gas. 
     In the case of the concentric multi-tube burner, the degree of mixing of the combustible gas forming a burner flame with the combustion supporting gas is largely influenced by the relationship between the gas flow rate of the combustible gas and the gas flow rate of the combustion supporting gas. Therefore, when the raw material gas is reacted with the combustible gas and combustion supporting gas separated by a seal gas, the simple control of the relationship between the gas flow rate of the raw material gas and the gas flow rate of the seal gas does not allow the control of the reaction of the raw material gas with the combustible gas and the combustion supporting gas to be completed. On the other hand, in the case of the multi-nozzle burner in which the plurality of small diameter combustion supporting gas ejection ports are provided in the combustible gas ejection port, the combustible gas and the combustion supporting gas are stably and sufficiently mixed. Therefore, the control of the relationship between the gas flow rate of the raw material gas and the gas flow rate of the first seal gas completes the control of the reaction among the raw material gas, the combustible gas, and the combustion supporting gas to allow both the gases to be reacted at an appropriate position. 
     Then, in the method for producing an optical fiber preform according to the present invention, when the gas flow rate of the raw material gas ejection port  120   a  is V 1  and the gas flow rate of the first seal gas ejection port  120   b  provided on the concentric outer side thereof is V 2 , the flow rates are controlled so as to satisfy 1&gt;V 2 /V 1 &gt;0.05. Therefore, the flow rate ratio between the gas flow rate of the raw material gas ejection port and the gas flow rate of the seal gas ejection port is optimized, which allows the raw material gas, the combustible gas and the combustion supporting gas to react with each other at an appropriate position. This causes difficult fixing of the glass fine particles to the burner, and stabilizes the burner flame, which makes it possible to produce a high quality large-sized preform at a low burner load. 
     In particular, since the first clad deposition burner  122  generally has a lower raw material gas feed rate than that of the second clad deposition burner  123 , the raw material gas has poor straight-running stability. Therefore, by adopting the method of the present invention for at least the first clad deposition burner  122 , the straight-running stability of the raw material gas is improved, which largely contributes to prevention of occurrences of preform cracking and variation of a preform diameter. 
     When 1&lt;V 2 /V 1  is set, the flow rate of the seal gas is higher than the flow rate of the raw material gas, and the raw material gas on the concentric inner side of the seal gas and the combustible gas and combustion supporting gas on the concentric outer side of the seal gas react with each other at a point more distant from the tip of the burner. Therefore, the burner flame becomes unstable, which causes problems such as preform cracking and variation of a preform diameter. 
     When V 2 /V 1 &lt;0.05 is set, the raw material gas, the combustible gas, and the combustion supporting gas react in the vicinity of the tip of the burner, which causes the fixing of the glass fine particles to the burner and the burning of the burner. In such a case, the burner is damaged or blocked, which makes it necessary to discard the burner and replace the burner with a new burner. 
     It should be noted that the present invention is not limited to the above embodiment. The above embodiment is just an example, and any examples that have substantially the same configuration and exhibit the same functions and effects as the technical concept described in claims according to the present invention are included in the technical scope of the present invention. 
     EXAMPLES 
     By a VAD method, a porous glass preform  10  was produced using an optical fiber preform producing apparatus  100  shown in  FIG. 1 . As a core deposition burner  121 , a concentric four-tube burner was used, and appropriate amounts of a raw material gas (SiCl 4 , O 2 ), combustible gas, combustion supporting gas, and seal gas were supplied. As shown in  FIG. 2 , for a first clad deposition burner  122  and a second clad deposition burner  123 , a multi-nozzle burner  120  having a nozzle focal length of 100 mm was used. The multi-nozzle burner  120  included eight small diameter combustion supporting gas ejection ports  120   d  having the same diameter and provided at equal intervals so as to surround a first seal gas ejection port  120   b  in a combustible gas ejection port  120   c . Here, the raw material gas (SiCl 4 , O 2 ), the combustible gas, the combustion supporting gas, the first seal gas, and the second seal gas were supplied to the first clad deposition burner  122  in a state where the flow rate ratio V 2 /V 1  of the gas flow rate V 1  of the raw material gas ejection port  120   a  to the gas flow rate V 2  of the first seal gas ejection port  120   b  was adjusted by changing only the flow rate of the first seal gas as shown in Table 1 among the raw material gas (SiCl 4 , O 2 ), the combustible gas, the combustion supporting gas, the first seal gas, and the second seal gas. The flow rate was varied by changing the feed rate of the seal gas. As the second clad deposition burner  123 , suitable amounts of the raw material gas (SiCl 4 , O 2 ), combustible gas, combustion supporting gas, first seal gas, and second seal gas were supplied. A deposition time is 24 hours. Next, sintering vitrification was carried out to produce 10 transparent glass preforms. Table 1 shows the deposition conditions due to the first clad deposition burner  122  and the production results of the transparent glass preform. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 Comparative 
                 Comparative 
               
               
                   
                   
                   
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4  
                 Example 1  
                 Example 2 
               
               
                   
               
             
            
               
                 Gas flow 
                 Gas ejection port 
                 Gas 
                   
                   
                   
                   
                   
                   
               
               
                 rate (m/s) 
                 Raw material (V1) 
                 SiCl 4   
                 4.71 
                 4.71 
                 4.71 
                 4.71 
                 4.71 
                 4.71 
               
               
                   
                 SiCl4:O2 = 1:1 
                 O 2   
                   
                   
                   
                   
                   
                   
               
               
                   
                 First seal (V2) 
                 N 2   
                 4.24 
                 3.20 
                 2.12 
                 1.08 
                 5.32 
                 0.24 
               
               
                   
                 Combustible 
                 H 2   
                 1.8 
                 1.8 
                 1.8 
                 1.8 
                 1.8 
                 1.8 
               
               
                   
                 Small diameter 
                 O 2   
                 10.6 
                 10.6 
                 10.6 
                 10.6 
                 10.6 
                 10.6 
               
               
                   
                 combustion support 
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 Second seal 
                 N 2   
                 1.0 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
               
               
                   
                 Combustion support 
                 O 2   
                 1.0 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Gas flow rate ratio V2/V1 
                 0.90 
                 0.68 
                 0.45 
                 0.23 
                 1.13 
                 0.05 
               
               
                 Production number 
                 10 
                 10 
                 10 
                 10 
                 10 
                 10 
               
               
                 Preform cracking, Number 
                 0 
                 0 
                 0 
                 0 
                 4 
                 0 
               
               
                 Variation of preform diameter, Number 
                 0 
                 0 
                 0 
                 0 
                 5 
                 0 
               
               
                 Production number until burner blocking 
                 No blocking 
                 No blocking  
                 No blocking 
                 No blocking 
                 No blocking 
                 7 
               
               
                   
               
            
           
         
       
     
     In the case of Comparative Example 1, the flow rate of the first seal gas was high and the reaction between the raw material gas and an oxyhydrogen flame occurred at a point distant from the tip of the burner, which caused an unstable burner flame, and the pulsation of the flame was observed. As a result, preform cracking and variation of a preform diameter occurred. In the case of Comparative Example 2, the flow rate of the seal gas was slow, and the reaction between the raw material gas and the oxyhydrogen flame occurred in the vicinity of the tip of the burner, so that the produced glass fine particles were fixed to the burner, to cause the burner to be clogged and become unusable. On the other hand, in the case of Examples 1 to 4 in which the flow rate of the first seal gas was adjusted so that the flow rate ratio V 2 /V 1  of the flow rate V 1  of the raw material gas to the flow rate V 2  of the seal gas was set to 1&gt;V 2 /V 1 &gt;0.05, the burner flame was stable and no preform cracking occurred. Furthermore, after ten preforms were produced, the glass fine particles were not fixed to the burners, and thereafter the burners could be used without problems.