Patent Publication Number: US-2019169064-A1

Title: Optical fiber preform production method and optical fiber production method

Description:
TECHNICAL FIELD 
     An embodiment of the present invention relates to an optical fiber preform production method and an optical fiber production method. 
     BACKGROUND ART 
     Generally, an optical fiber preform is produced by a preform production method including a step of producing a core preform to be a core after drawing and a step of producing a cladding preform (outer peripheral portion) to be provided on an outer peripheral surface of the core preform and to be a cladding after the drawing. 
     The step of producing the core preform includes a glass synthesis step and an aftertreatment step such as dehydration, sintering (including collapsing), and elongation, to be performed subsequent to the glass synthesis step. Particularly, in the glass synthesis step, for example, a glass preform is produced by stacking a plurality of glass layers. As a method of producing the glass preform, an outside approach type of chemical vapor deposition (CVD) method in which a glass layer is formed on an outer peripheral surface of a glass deposition substrate and an inside approach type of chemical vapor deposition (CVD) method in which the glass layer is formed on an inner peripheral surface of the glass deposition substrate are known. 
     Particularly, an outside vapor phase deposition (OVD) method disclosed in Patent Document 1 is known as an example of the outside approach type of CVD method and a plurality of glass layers are stacked by causing glass raw material gas supplied to an outer peripheral surface of a core rod prepared as the glass deposition substrate to be subjected to a flame hydrolysis reaction by an oxyhydrogen burner and depositing synthesized glass particles on the outer peripheral surface of the core rod. 
     On the other hand, a modified chemical vapor deposition (MCVD) method disclosed in Patent Document 2 and a plasma-activated chemical vapor deposition (PCVD) described in Patent Document 3 are known as examples of the inside approach type of CVD method. In both the MCVD method and the PCVD method, a hollow glass tube is used as the glass deposition substrate and the glass raw material gas introduced into the glass tube is subjected to an oxidation reaction, so that the synthesized glass particles are deposited on an inner peripheral surface of the glass tube. In the case of the MCVD method, the oxidation reaction in the glass tube is accelerated by heating the glass tube by the oxyhydrogen burner and in the case of the PCVD method, the oxidation reaction is accelerated by generating plasma in the glass tube by a high-frequency cavity disposed outside the glass tube. 
     The core preform having a refractive index profile according to a desired α-profile is obtained via the above glass synthesis step and a multimode optical fiber (hereinafter, referred to as the “MMF”) having a desired optical characteristic is obtained by drawing the optical fiber preform including the core preform. 
     For example, Patent Document 4 discloses technology for slightly modifying a refractive index profile of the core according to the α-profile and obtaining the MMF having a wider bandwidth characteristic. Patent Document 5 discloses technology for controlling a deviation between the refractive index profile in the core and the α-profile to be less than 0.0015% and obtaining the MMF having a bandwidth characteristic of 5000 MHz·km or more at an arbitrary wavelength included in a wavelength range of 800 nm or more. Furthermore, Patent Document 6 discloses an MMF production method that adjusts cladding synthesis as well as adjustment of a drawing tension and a core diameter, on the basis of a shape (fitting shape) of the refractive index profile of the core preform along a radial direction. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: U.S. Pat. No. 8,815,103 
     Patent Document 2: U.S. Pat. No. 7,155,098 
     Patent Document 3: U.S. Pat. No. 7,759,874 
     Patent Document 4: U.S. Pat. No. 6,292,612 
     Patent Document 5: US Patent Application Laid-Open No. 2014/0119701 
     Patent Document 6: US Patent Application Laid-Open No. 2013/0029038 
     SUMMARY OF INVENTION 
     Technical Problem 
     As a result of examining the conventional optical fiber preform production method, the inventors have found the following problems. That is, all of the production methods disclosed in the above Patent Documents 1 to 6 require a long time to match the shape of the refractive index profile in the produced core preform with an ideal curve with high precision. Specifically, a preform producer frequently adjusts a doping amount of a refractive index adjusting agent depending on experience and the adjustment of the doping amount is ambiguous. Furthermore, if basic production conditions are different, it is necessary to accumulate a large number of data (experience) again to adjust the doping amount of the refractive index adjusting agent. 
     An embodiment of the present invention has been made to solve the above problems and an object thereof is to provide an optical fiber preform production method having a structure for matching a shape of a refractive index profile in a core preform with an ideal curve with high precision and in a short time and an optical fiber production method using an optical fiber preform. 
     Solution to Problem 
     In order to achieve the above object, an optical fiber preform production method according to the present embodiment comprises, at least, a glass synthesis step and a pretreatment step executed prior to the glass synthesis step, to produce a core preform. In the glass synthesis step, the core preform which extends along a center axis and constitutes a part of an optical fiber preform and in which a refractive index profile defined along a radial direction on a cross-section orthogonal to the center axis is adjusted to a predetermined shape, is produced. 
     Particularly, in the glass synthesis step, as a glass preform to be the core preform, glass particles synthesized while a doping amount of a refractive index adjusting agent M is adjusted are sequentially stacked on an inner peripheral surface or an outer peripheral surface of a glass deposition substrate extending along a direction matched with the center axis. As a result, the glass preform having a cross-section in which a plurality of glass layers are concentrically arranged so as to be matched with the cross-section of the core prefoini and surround the center axis is produced. Further, in the pretreatment step, setting of a division section to be an unit of doping amount control for the refractive index adjusting agent M, creation of glass synthesis actual-result data, calculation of a correlation, and determination of a theoretical doping amount of the refractive index adjusting agent M in the glass synthesis step are performed for an arbitrarily set adjustment region of a core preform sample produced in the past. In the setting of the division section, for one of a cross-section of an i-th (=1 to m) core preform sample among in (an integer of 2 or more) core preform samples produced in the past and the number of glass layers constituting an i-th glass preform sample having become the i-th core preform sample, the adjustment region is divided into n (an integer of 2 or more) sections along the radial direction and for the other, a region corresponding to the adjustment region is divided along the radial direction to correspond to the n division sections divided as described above on one-to-one basis. The glass synthesis actual-result data includes actual measurement data of a relative refractive index difference of a k-th (=1 to n) division section in the i-th core preform sample as refractive index profile data and includes doping amount data of the refractive index adjusting agent M doped to the k-th division section in the i-th glass preform sample as production condition data. In the calculation of the correlation, a correlation between a deviation of the actual measurement data of the relative refractive index difference with respect to a target value and the doping amount data of the refractive index adjusting agent M is calculated from glass synthesis actual-result data of the k-th division section of each of the m core preform samples. In the determination of the theoretical doping amount, a theoretical doping amount of the refractive index adjusting agent M in which an absolute value of the deviation is minimized is obtained from the correlation in the k-th division section of each of the m core preform samples. 
     In the glass synthesis step, one or more glass layers belonging to a k-th glass synthesis section corresponding to the k-th division section of each of the m core preform samples are sequentially formed on the inner peripheral surface or the outer peripheral surface of the glass deposition substrate, in a state in which the doping amount of the refractive index adjusting agent M to be supplied at the time of synthesizing the glass particles is adjusted to the theoretical doping amount. 
     Each embodiment of the present invention can be more fully understood by the following detailed description and the accompanying drawings. These embodiments are merely exemplary and should not be considered as limiting the present invention. 
     An additional application range of the present invention will be apparent from the following detailed description. However, it should be understood that the detailed description and specific examples showing the preferred embodiments of the invention are merely exemplary and various modifications and improvements within a scope of the present invention will be obvious to those skilled in the art from the detailed description. 
     Advantageous Effects of Invention 
     According to the present embodiment, it is possible to match a shape of a refractive index profile in a core preform with an ideal curve with high precision and in a short time. Further, since variations of desired optical characteristics are suppressed between produced optical fibers, a production yield of the optical fibers can be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a diagram showing a structure of an optical fiber preform. 
         FIG. 1B  shows a refractive index profile along a radial direction of the optical fiber preform of  FIG. 1A . 
         FIG. 1C  is a diagram showing a drawing step of the optical fiber preform of  FIG. 1A . 
         FIG. 1D  is a diagram showing a cross-sectional structure of an optical fiber obtained through the drawing step of  FIG. 1C . 
         FIG. 2  is a flowchart illustrating a core preform production step ST 100  in an optical fiber preform production method according to the present embodiment. 
         FIG. 3  is a flowchart illustrating an aftertreatment step ST 130  in the core preform production step ST 100  shown in  FIG. 2 . 
         FIG. 4A  is a diagram showing a structure of an OVD production apparatus to execute a glass synthesis step ST 120  by an OVD method as an outside approach type of CVD method for obtaining a glass preform for a core preform. 
         FIG. 4B  is a diagram showing a structure of a material gas supply system in the OVD production apparatus of  FIG. 4A . 
         FIG. 5A  is a diagram showing a correspondence relation between a cross-section of the glass preform after the glass synthesis step ST 120  and a cross-section of the core preform obtained by performing the aftertreatment step ST 130  on the glass preform. 
         FIG. 5B  is a diagram showing an example of a correspondence relation between a division section in the cross-section of the glass preform of  FIG. 5A  and a division section in the cross-section of the core preform of  FIG. 5A . 
         FIG. 6A  is a diagram showing a structure of an inside approach type of CVD production apparatus for producing the optical fiber preform, particularly, the glass preform for the core preform by an inside approach type of CVD method (an MCVD method or a PCVD method). 
         FIG. 6B  is a diagram showing a structure of a material gas supply system in the inside approach type of CVD production apparatus of  FIG. 6A . 
         FIG. 7A  is a diagram showing a structure of a heating system for executing the MCVD method in the inside approach type of CVD production apparatus of  FIG. 6A . 
         FIG. 7B  is a diagram showing a structure of a heating system for executing the PCVD method in the inside approach type of CVD production apparatus of  FIG. 6A . 
         FIG. 8  is a flowchart illustrating a pretreatment step ST 110  in the core preform production step ST 100  shown in  FIG. 2 . 
         FIG. 9A  is a (first) diagram showing a structure of glass synthesis actual-result data created in the pretreatment step ST 110 . 
         FIG. 9B  is a (second) diagram showing a structure of glass synthesis actual-result data created in the pretreatment step ST 110 .  FIG. 10  is a diagram illustrating calculation of a theoretical doping amount of Ge based on the glass synthesis actual-result data of  FIG. 9B . 
         FIG. 11  is a flowchart illustrating the glass synthesis step ST 120  in the core preform production step ST 100  shown in  FIG. 2 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Description of Embodiments of Present Invention 
     First, contents of embodiments of the present invention will be individually enumerated and described. 
     (1) As one aspect, an optical fiber preform production method according to the present embodiment comprises at least a glass synthesis step and a pretreatment step executed prior to the glass synthesis step, to produce a core preform. In the glass synthesis step, a glass preform to be the core preform which extends along a center axis and constitutes a part of an optical fiber preform and in which a refractive index profile defined along a radial direction on a cross-section orthogonal to the center axis is adjusted to a predetermined shape, is produced. 
     Particularly, in the glass synthesis step, as the glass preform, glass particles synthesized while a doping amount of a refractive index adjusting agent M is adjusted are sequentially stacked on an inner peripheral surface or an outer peripheral surface of a glass deposition substrate extending along a direction matched with the center axis. As a result, the glass preform having a cross-section in which a plurality of glass layers are concentrically arranged so as to be matched with the cross-section of the core preform and surround the center axis is produced. Further, in the pretreatment step, setting of a division section to be an unit of doping amount control for the refractive index adjusting agent M, creation of glass synthesis actual-result data, calculation of a correlation, and determination of a theoretical doping amount of the refractive index adjusting agent M in the glass synthesis step are performed for an arbitrarily set adjustment region of a core preform sample produced in the past. In the setting of the division section, for one of a cross-section of an i-th (=1 to m) core preform sample among m (an integer of 2 or more) core preform samples produced in the past and the number of glass layers constituting an i-th glass preform sample having become the i-th core preform sample, the adjustment region is divided into n (an integer of 2 or more) sections along the radial direction and for the other, a region corresponding to the adjustment region is divided along the radial direction to correspond to the n division sections divided as described above on one-to-one basis. For the adjustment region, an entire range of the core preform sample along the radial direction may be set or a part thereof may be set. 
     The division sections in the set adjustment region may be sections divided equally or sections with different sizes along the radial direction. Further, a plurality of adjustment regions may be set in a state of being continuous or separated. A division section size of a certain adjustment region among the plurality of adjustment regions does not need to be matched with a division section size of other adjustment region. In this case, rough doping amount adjustment (a division size is set to be large) can be performed at the side of the center axis of the core preform to be produced, whereas fine doping amount adjustment (the division size is set to be small) can be performed at the outer side. 
     The glass synthesis actual-result data includes actual measurement data of a relative refractive index difference of a k-th (=1 to n) division section in the i-th core preform sample as refractive index profile data and includes doping amount data of the refractive index adjusting agent M added to the k-th division section in the i-th glass preform sample as production condition data. In the calculation of the correlation, a correlation between a deviation of the actual measurement data of the relative refractive index difference with respect to a target value and the doping amount data of the refractive index adjusting agent M is calculated from glass synthesis actual-result data of the k-th division section of each of the m core preform samples. In the determination of the theoretical doping amount, a theoretical doping amount of the refractive index adjusting agent M in which an absolute value of the deviation is minimized is obtained from the correlation in the k-th division section of each of the in core preform samples. 
     In the glass synthesis step, one or more glass layers belonging to a k-th glass synthesis section corresponding to the k-th division section of each of the m core preform samples are sequentially formed on the inner peripheral surface or the outer peripheral surface of the glass deposition substrate, in a state in which the doping amount of the refractive index adjusting agent M to be supplied at the time of synthesizing the glass particles is adjusted to the theoretical doping amount. 
     (2) As one aspect of the present embodiment, an outer periphery radius r k  of the k-th division section to be an index representing the k-th division section in the i-th core preform sample and a k-th glass synthesis section l k  in the i-th glass preform sample preferably satisfy a relation of the following expression (1) by a predetermined function f. 
     
       
         
           
             
               
                 
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     Where the doping amount of the refractive index adjusting agent M in the k-th division section of the i-th core preform sample to be the glass synthesis actual-result data of the i-th core preform sample is set to M(r k ) i  and a deviation of the relative refractive index difference in the k-th division section of the i-th core preform sample is set to ε(r k ) i , a theoretical doping amount M(r k ) opt  of the refractive index adjusting agent M in the k-th division section of the core preform to be produced is preferably given by the following expression (2), and a theoretical doping amount M(l k ) opt  of the refractive index adjusting agent M in the k-th glass synthesis section l k  to be produced in the glass preform to be the core preform is preferably given by the theoretical doping amount M(r k ) opt  of the refractive index adjusting agent M in r k  associated with l k  by the above expression (1). 
     
       
         
           
             
               
                 
                   
                     
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     (3) As one aspect of the present embodiment, the refractive index adjusting agent M preferably includes one kind of dopant. Further, as one aspect of the present embodiment, the refractive index adjusting agent M preferably includes germanium. 
     (4) As one aspect of the present embodiment, the refractive index adjusting agent M may include one kind of first dopant and one or more kinds of second dopants. In this case, in the glass synthesis step, a doping amount of the first dopant is preferably adjusted for each glass synthesis section to be formed, in a state in which doping conditions of the second dopants are fixed during a period where n glass synthesis sections are formed. As one aspect of the present embodiment, the refractive index adjusting agent M preferably includes two or more kinds of dopants selected from germanium, phosphorus, fluorine, and boron. As one aspect of the present embodiment, the first dopant preferably includes germanium. 
     (5) As one aspect of the present embodiment, the optical fiber preform production method may further include a sintering step of sintering the glass preform to cause the glass preform produced by the glass synthesis step to be transparent. 
     (6) As one aspect, an optical fiber production method according to the present embodiment produces a desired optical fiber by preparing the optical fiber preform including the core preform produced by the optical fiber preform production method and drawing one end of the optical fiber preform while heating one end. In this case, the optical fiber to be produced includes a core extending along the center axis and a cladding covering an outer peripheral surface of the core along the center axis. In addition, a deviation of a refractive index profile in the core of the optical fiber from a target refractive index profile is preferably 0.002% or less as a relative refractive index difference with respect to a refractive index of pure silica glass. 
     (7) As one aspect, an optical fiber production method according to the present embodiment may produce an MMF by preparing the optical fiber preform produced by the optical fiber preform production method and including a core preform having a refractive index profile according to an α-profile along the radial direction orthogonal to the center axis and drawing one end of the optical fiber preform while heating one end. In this case, the MMF to be produced includes a core extending along the center axis and a cladding covering an outer peripheral surface of the core along the center axis. To guarantee broadband optical transmission, in the MMF, an a value defining the shape of the α-profile is preferably in a range of 1.9 to 2.3. In addition, an effective bandwidth EMB(λ) at an arbitrary wavelength λ(nm) included in a range of 800 to 1000 nm is preferably −20·λ+21700 MHz·km or more. 
     Each aspect enumerated in the “description of embodiments of present invention” can be applied to all of the remaining aspects or all combinations of these remaining aspects. 
     Details of Embodiments of Present Invention 
     Specific examples of the optical fiber preform production method and the optical fiber production method according to the embodiments of the present invention will be described in detail below with reference with the accompanying drawings. It should be noted that the embodiments of the present invention are not limited to these examples, but are indicated by claims and it is intended to include all changes in meanings and ranges equivalent to the claims. In the description of the drawings, the same elements are denoted by the same reference numerals and redundant explanations are omitted. 
       FIG. 1A  is a diagram showing a structure of an optical fiber preform,  FIG. 1B  shows a refractive index profile along a radial direction of the optical fiber preform of  FIG. 1A ,  FIG. 1C  is a diagram showing a drawing step of the optical fiber preform of  FIG. 1A , and  FIG. 1D  is a diagram showing a cross-sectional structure of an optical fiber obtained through the drawing step of  FIG. 1C . 
     An optical fiber preform  100  shown in  FIG. 1A  is configured to include a core preform  10  extending along a center axis AX and having a radius a and a cladding preform (outer peripheral portion)  20  provided on an outer peripheral surface of the core preform  10 . The core preform  10  corresponds to a core  110 A ( FIG. 1D ) of an optical fiber  110  obtained by drawing the optical fiber preform  100  and the cladding preform  20  corresponds to a cladding  110 B ( FIG. 1D ) of the optical fiber  110 . 
     Further, as shown in  FIG. 1B , a refractive index profile  150  of the core preform  10  of which shape is defined on a cross-section orthogonal to the center axis AX has a shape according to an α-profile. In the following description, a relative refractive index of a certain region is set to n, a refractive index of pure silica glass is set to n 0 , and a relative refractive index difference Δ of the region is represented by the following expression (3). 
       Δ={1−( n   0   /n ) 2 }/2   (3)
 
     Further, the α-profile refers to a refractive index profile where a radius with the center axis AX as an origin is set to r, a core radius is set to a, a relative refractive index difference on the center axis AX is set to Δ 0 , a relative refractive index difference in a core outer edge is set to A 0e , a relative refractive index difference in the cladding  110 B is set to Δ 1 , and a relative refractive index difference Δ between the core  110 A and the cladding  110 B is represented by the following expression (4). Even if there are variations in additive concentrations caused by production and variations in refractive indexes due to mixing of impurities, the refractive index profile may be regarded as the α-profile roughly in accordance with the expression (4). 
     
       
         
           
             
               
                 
                   
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     In an example of  FIG. 1B , the refractive index of the core  110 A on the center axis AX is n 1 , the refractive index of the cladding  110 B is n 0 , and the refractive indexes of the core outer edge and the cladding  110 B are matched. Therefore, in the example of  FIG. 1B , Δ oe Δ 1 =0 is satisfied. 
     One end of the optical fiber  110  having the above structure is heated by a heater  300  and softened, as shown in  FIG. 1C . At this time, the softened one end is drawn in a direction shown by an arrow S 1 , so that the optical fiber  110  including the core  110 A extending along the center axis AX and the cladding  110 B provided on the outer peripheral surface of the core  110 A is obtained. At this time, a deviation of the refractive index profile in the core  110 A of the optical fiber  110  from a target refractive index profile is 0.002% or less as the relative refractive index difference with respect to the refractive index of the pure silica glass. Further, the obtained optical fiber  110  is an MMF having a graded-index (GI) type refractive index profile according to the  60  -profile as shown in  FIG. 1B . At this time, to guarantee broadband optical transmission, an a value that defines the shape of the α-profile is preferably in a range of 1.9 to 2.3. In addition, an effective bandwidth EMB(λ) at an arbitrary wavelength λ(nm) included in a range of 800 to 1000 nm is preferably −20·λ+21700 MHz·km or more. It should be noted that a preferable effective bandwidth depends on a wavelength because material dispersion is considered. At the wavelength of 800 to 1000 nm, because the material dispersion decreases almost linearly as the wavelength increases, the effective bandwidth may be narrower when the wavelength is longer. 
     The bandwidth of the MMF depends on how a plurality of waveguide modes of the MMF are excited by a light source. As an index representing a typical bandwidth when the mode is excited by a surface emitting type semiconductor laser (VCSEL: Vertical Cavity Surface Emitting Laser) widely used as a light source in short distance information communication, an effective modal bandwidth (EMB) is defined. The EMB is obtained by the following expression (5) by calculating a calculated minimum effective modal bandwidth (minEMBc) from a measurement result of a differential mode delay (DMD) of the MMF. The details of this calculation method are defined in IEC 60793-1-49:2006 and IEC 60793-2-10:2011. 
       EMB=1.13×min EMBc   (5)
 
     Next,  FIG. 2  is a flowchart illustrating an optical fiber preform production method according to the present embodiment. 
     The optical fiber preform production method according to the present embodiment includes a core preform production step ST 100 , an actual measurement step ST 200  of acquiring refractive index profile data of the core preform  10  obtained through the core preform production step ST 100 , an outer peripheral portion production step (cladding preform production step) ST 300  of forming the cladding preform  20  to be the cladding  110 B on an outer peripheral surface of the obtained core preform  10 , and a drawing step ST 400  of drawing the optical fiber preform  100  obtained through the outer peripheral portion production step ST 300  as shown in  FIG. 1C . The outer peripheral portion production step ST 300  includes a soot deposition step ST 310  of depositing glass particles on the outer peripheral surface of the core preform  10  through the actual measurement step ST 200  and an aftertreatment step ST 320 . 
     The core preform production step ST 100  includes a pretreatment step ST 110 , a glass synthesis step ST 120 , and an aftertreatment step ST 130 . In the pretreatment step ST 110 , setting of n (an integer of 2 or more) glass synthesis sections which are divided in advance and of which each section functions as an unit of doping amount control for a refractive index adjusting agent in the glass synthesis step ST 120 , creation of glass synthesis actual-result data 500 to determine the doping amount of the refractive index adjusting agent to be doped with each glass synthesis section, calculation of a correlation between past doping amount data and a deviation (error of the doping amount with respect to a target value) thereof, and determination of a theoretical doping amount of the refractive index adjusting agent for each glass synthesis section are performed. The glass synthesis actual-result data  500  includes refractive index profile data  520  measured in the actual measurement step ST 200  for each of m (an integer of 2 or more) core preform samples produced in the past and production condition data  510  of m glass preform samples that have become the m core preform samples. In the present specification, the m glass preforms which are produced in the past and of which the production condition data is already stored in a memory (refer to  FIG. 4A  and the like) of a controller to be described later are referred to as the “glass preform samples” and the m core preforms which are obtained by performing the aftertreatment step to be described later on the m glass preform samples and of which the refractive index profile data is already stored in the memory of the controller are referred to as the “core preform samples” as core preforms produced in the past. 
     In the glass synthesis step ST 120 , as the glass preform to be the core preform  10 , the glass particles synthesized while the doping amount of the refractive index adjusting agent is adjusted are sequentially stacked on an inner peripheral surface or an outer peripheral surface of a glass deposition substrate extending along a direction matched with the center axis AX. As a result, the glass preform having the cross-section in which a plurality of glass layers are concentrically arranged so as to be matched with the cross-section of the core preform  10  and surround the center axis AX is produced. It should be noted that each glass synthesis section to be an unit of doping amount control for the refractive index adjusting agent includes one or more glass layers. In addition, the doping amount of the refractive index adjusting agent in each glass synthesis section in the glass synthesis step ST 120  is added to the production condition data  510  together with past data. 
       FIG. 3  is a flowchart illustrating the aftertreatment step ST 130  in the core preform production step ST 100  shown in  FIG. 2 . In the aftertreatment step ST 130 , as shown in  FIG. 3 , the glass preform  200  obtained through the glass synthesis step ST 120  is dehydrated. The dehydrated glass preform  200  is sintered so as to be transparent. Specifically, the glass preform  200  is heated while the heater  350  is moved in a direction shown by an arrow S 2 . When the glass preform  200  has a hollow structure, collapse (solidification) is also performed. When an inside approach type of CVD method is applied to the glass synthesis step ST 120 , in the glass synthesis step ST 120 , every time a glass layer is deposited, the deposited glass layer is caused to be transparent, so that the dehydration step is unnecessary after the glass synthesis step ST 120 . Further, the transparent preform is extended to have a desired outer diameter, so that the core preform  10  is obtained. The refractive index profile of the obtained core preform  10  is measured by the actual measurement step ST 200  and the measurement data is added to the refractive index profile data  520  together with the past data. Also in the aftertreatment ST 320  in the outer peripheral portion production step ST 300 , the same treatment as the aftertreatment step ST 130  is performed on the soot deposition layer (glass layer) formed on the outer peripheral surface of the core preform  10  through the soot deposition step ST 310  and the cladding preform  20  is obtained. 
     The glass preform  200  in the glass synthesis step ST 120  is produced by a production apparatus shown in  FIGS. 4A and 4B , for example.  FIG. 4A  shows a structure of an OVD production apparatus for executing the glass synthesis step ST 120  by using the OVD method as the outside approach type of CVD method for forming the glass layer on the outer peripheral surface of the glass deposition substrate and  FIG. 4B  shows a structure of a material gas supply system in the OVD production apparatus of  FIG. 4A . 
     The OVD production apparatus  600 A of  FIG. 4A  includes a workbench  610 A, a core rod  620 A, an oxyhydrogen burner  630 A, a material gas supply system  640 A, a fuel gas supply system  650 A, and a controller  660 A. The core rod  620 A is the glass deposition substrate. The oxyhydrogen burner  630 A deposits the glass particles synthesized in flames on a surface of the core rod  620 A, thereby forming an intermediate glass preform  200 A including a plurality of glass layers on an outer peripheral surface of the core rod  620 A. The workbench  610 A rotates the core rod  620 A in a direction shown by an arrow S 3 A while supporting the core rod  620 A and moves the oxyhydrogen burner  630 A in directions shown by arrows S 4 Aa and S 4 Ab while supporting the oxyhydrogen burner  630 A. The material gas supply system  640 A supplies glass raw material gas (SiCl 4 , GeCl 4 , or the like) to the oxyhydrogen burner  630 A. The fuel gas supply system  650 A supplies fuel gas (H 2  or O 2 ) for forming flames to the oxyhydrogen burner  630 A. The controller  660 A controls each of the workbench  610 A, the material gas supply system  640 A, and the fuel gas supply system  650 A. The controller  660 A has a memory  670 A to store the glass synthesis actual-result data  500  of the m core preform samples produced in the past. 
     As shown in  FIG. 4B , the material gas supply system  640 A includes an O 2  tank, a SiCl 4  tank storing SiCl 4  to be a glass synthesis material, a GeCl 4  tank storing a Ge compound to be the refractive index adjusting agent, and the like and these tanks are connected via a mixing valve  641 A. The controller  660 A controls opening and closing of the mixing valve  641 A and a flow rate adjuster not shown in the drawings and adjusts a flow rate of the glass raw material gas, in particular, a flow rate (doping amount) of the refractive index adjusting agent. In an example of  FIG. 4B , although germanium (Ge) is shown as the refractive index adjusting agent, the refractive index adjusting agent may include two or more kinds of dopant selected from germanium (Ge), phosphorus (P), fluorine (F), and boron (B). In addition, Ge and the other refractive index adjusting agents (P, F, and B) may be prepared as first and second dopants, respectively, and the controller  660 A may adjust a doping amount of the first dopant for each glass synthesis section to be formed, in a state in which doping conditions of the second dopants are fixed during a period where each glass synthesis section is formed. 
     On the other hand, as shown in  FIG. 4B , the fuel gas supply system  650 A has the O 2  tank and the H 2  tank and the controller  660 A adjusts a flow rate of O 2  and a flow rate of H 2  through the mixing valve  651 A and the flow rate adjuster not shown in the drawings. 
     As shown on a left side of  FIG. 5A , the glass preform  200  produced by the OVD production apparatus  600 A having the above structure has a cross-sectional structure in which a space (space from which the core rod  620 A has been removed)  210  is provided in a center and a plurality of glass layers  201  are stacked on a concentric circle. By performing the aftertreatment step ST 130  on the glass preform  200  having the above cross-sectional structure, the core preform  10  having a cross-sectional structure shown on a right side of  FIG. 5A  is obtained.  FIG. 5B  is a diagram showing an example of a correspondence relation between a division period in the cross-section of the glass preform  200  after the glass synthesis step ST 120  and a division section in the cross-section of the core preform  10  obtained by performing the aftertreatment step ST 130  on the glass preform  200 . In the following description, an example in which an adjustment region to be section divided is set over an entire range of the core preform  10  along a radial direction is mentioned. 
     In the optical fiber preform production method according to the present embodiment, in the glass synthesis step ST 120 , the entire region of the doping amount adjustment section (glass synthesis section) of the refractive index adjusting agent is divided into the n sections as the adjustment region and optimization control of the flow rate (Ge doping amount) of GeCl 4  by the controller  660 A is performed for each of the divided glass synthesis sections. Each glass synthesis section corresponds to a layer region including one or more glass layers  201  in the cross-section of each of the m glass preform samples  200  produced in the past. In addition, the glass synthesis section may be a section obtained by equally dividing the number (for example, 500 layers) of glass layers  201  constituting the produced glass preform sample  200  by n along the radial direction or may be obtained by equally dividing the cross-section radius of the m core preform samples  10  produced in the past by n.  FIG. 5B  shows a graph showing a correspondence relation between a glass synthesis section l k  (k=1 to n) when the cross-section of the glass preform sample  200  is equally divided by n in the radial direction and a radial section r k  (k=1 to n) of the core preform sample  10  obtained from the glass preform  200 . It is considered that the correspondence relation between the glass synthesis section of the glass preform sample and the radial section of the core preform sample is not linear as shown in  FIG. 5B , because a shrinkage ratio at the time of sintering is different between a center portion and a peripheral portion of the glass preform sample. In addition, r k  is an outer diameter of each radial section of the core preform sample  10  and is also an index representing each radial section. Therefore, an outer periphery radius r k  representing a k-th (=1 to n) division section in an i-th (=1 to m) core preform sample among the m core preform samples  10  and a k-th glass synthesis section l k  in an i-th glass preform. In sample having become the i-th core preform sample satisfy the relation of the above expression (1) by a predetermined function f. Here, if easiness of mutual conversion between r k  and l k  is considered, the function f is preferably a function in which it is easy to find an inverse function. 
     The OVD production apparatus  600 A for executing the glass synthesis step ST 120  is an apparatus for producing the glass preform  200  by the so-called outside approach type of CVD method. However, the glass preform  200  for the core preform can be produced by the inside approach type of CVD method represented by an MCVD method or a PCVD method.  FIG. 6A  is a diagram showing a structure of the inside approach type of CVD production apparatus and  FIG. 6B  is a diagram showing a structure of a material gas supply system in the inside approach type of CVD production apparatus of  FIG. 6A . In addition,  FIG. 7A  is a diagram showing a structure of a heating system for executing the MCVD method in the inside approach type of CVD production apparatus of  FIG. 6A  and  FIG. 7B  is a diagram showing a structure of a heating system for executing the PCVD method in the inside approach type of CVD production apparatus of  FIG. 6A . 
     An inside approach type of CVD production apparatus  600 B of  FIG. 6A  includes a workbench  610 B, a hollow glass tube  620 B, a heating system  630 B, a material gas supply system  640 B, and a controller  660 B. The hollow glass tube  620 B is a glass deposition substrate in which a plurality of glass layers are stacked on an inner peripheral surface thereof. The heating system  6308  has different structures in the MCVD method and the PCVD method to be described later. However, even if any one of the MCVD method and the PCVD method is used, the glass particles synthesized in the hollow glass tube  620 B are deposited on the inner peripheral surface of the hollow glass tube  620 B, so that an intermediate glass preform  200 B including a plurality of glass layers is formed. The workbench  610 B rotates the hollow glass tube  620 B in a direction shown by an arrow S 3 B while supporting the hollow glass tube  620 B and moves the heating system  630 B in directions shown by arrows S 4 Ba and S 4 Bb while supporting the heating system  630 B. The material gas supply system  640 B supplies glass raw material gas (SiCl 4 , GeCl 4 , or the like) to the heating system  630 B. The controller  660 B controls each of the heating system  630 B, the workbench  610 B, and the material gas supply system  640 B. The controller  660 B has a memory  670 B to store the glass synthesis actual-result data  500  of the in core preform samples produced in the past. 
     As shown in  FIG. 6B , the material gas supply system  640 B includes an O 2  tank, a SiCl 4  tank storing SiCl 4  to be a glass synthesis material, a GeCl 4  tank storing a Ge compound to be the refractive index adjusting agent, and the like and these tanks are connected via a mixing valve  641 B. The controller  660 B controls opening and closing of the mixing valve  641 B and a flow rate adjuster not shown in the drawings and adjusts a flow rate of the glass raw material gas, in particular, a flow rate (doping amount) of the refractive index adjusting agent. In an example of  FIG. 6B , although germanium (Ge) is shown as the refractive index adjusting agent, the refractive index adjusting agent may include two or more kinds of dopants selected from germanium (Ge), phosphorus (P), fluorine (F), and boron (B), similar to the example of  FIG. 4B . In addition, Ge and the other refractive index adjusting agents (P, F, and B) may be prepared as first and second dopants, respectively, and the controller  660 B may adjust a doping amount of the first dopant for each glass synthesis section to be formed, in a state in which doping conditions of the second dopants are fixed during a period where each glass synthesis section is formed. 
     When the inside approach type of CVD production apparatus  600 B of  FIG. 6A  produces the glass preform  200  by the MCVD method, the inside approach type of CVD production apparatus  600 B includes a heating system  630 Ba as shown in  FIG. 7A . That is, the heating system  630 Ba has an oxyhydrogen burner  652  that is moved in directions shown by arrows S 4 Ba and S 4 Bb while being supported by the workbench  610 B and an O 2  tank and an H 2  tank that supply fuel gas (H 2  and O 2 ) for flame formation to the oxyhydrogen burner  652 . The controller  660 B adjusts the flow rates of O 2  and/or H 2  through the mixing valve  651 B and a flow rate adjuster not shown in the drawings. Thereby, the glass particles synthesized in the hollow glass tube  620 B are deposited on the inner peripheral surface of the hollow glass tube  620 B and as a result, the intermediate glass preform  200 B is formed. 
     On the other hand, when the inside approach type of CVD production apparatus  600 B of  FIG. 6A  produces the glass preform  200  by the PCVD method, the inside approach type of CVD production apparatus  600 B includes a heating system  630 Bb as shown in  FIG. 7B . That is, the heating system  630 Bb has a high-frequency cavity  653  that is moved in the directions shown by arrows S 4 Ba and S 4 Bb while being supported by the workbench  610 B. The high-frequency cavity  653  is disposed to surround outer periphery of the hollow glass tube  620 B and can generate plasma  654  in the hollow glass tube  620 B, according to a control signal from the controller  660 B. Thereby, the glass particles synthesized in the hollow glass tube  620 B are deposited on the inner peripheral surface of the hollow glass tube  620 B and as a result, the intermediate glass preform  200 B is formed. 
       FIG. 8  is a flowchart illustrating the pretreatment step ST 110  in the core preform production step ST 100  shown in  FIG. 2 . The pretreatment step ST 110  is a step executed by the controller  660 A and  660 B. In the pretreatment step ST 110 , since the doping amount of the refractive index adjusting agent for each division section to be the unit of doping amount control for the refractive index adjusting agent is determined, creation of glass synthesis actual-result data (ST 111 ), calculation of a correlation (ST 112 ), and determination of a theoretical doping amount of the refractive index adjusting agent (ST 113 ) are performed. The division section is set according to the example of  FIG. 5B . In step ST 111 , the glass synthesis actual-result data  500  shown in  FIG. 9A  is created from the production condition data  510  (stored in the memories  670 A and  670 B) of the m glass preform samples  200  constituting a glass preform sample group  250  and produced in the past and the refractive index profile data  520  of the m core preform samples  10  constituting a core preform sample group  15  and produced in the past, obtained through the actual measurement step ST 200 . For example, the i-th glass synthesis actual-result data  500  is an example in the case where the refractive index adjusting agent added at the time of glass synthesis is Ge. The i-th glass synthesis actual-result data  500  includes a Ge flow rate (Ge(l k ) i ) at the time of glass synthesis as production condition data  510  of an i-th glass preform sample  200 , a Ge flow rate (Ge(r k ) i ) in a radial section r k  corresponding to the glass synthesis section l k , a target value (Δsp(r k )) of a relative refractive index difference Δ in the radial section r k , an actual measurement value (Δpv(r k ) i ) of the relative refractive index difference Δ in the radial section r k  measured by the actual measurement step ST 200 , and a deviation (ε(r k ) i =Δpv(r k ) i =Δsp(r k )) of the relative refractive index difference Δ in the radial section r k , for each glass synthesis section shown by partition No. and a symbol l k . A unit of the Ge flow rate is “slm”. 
     In step ST 112 , for each glass synthesis section, the glass synthesis actual-result data of the same glass synthesis section of each of the m glass synthesis actual-result data  500  created as described above is collected. For example, in the example of  FIG. 9B , the data is collected in the glass synthesis actual-result data of the k-th glass synthesis section in each of the m core preform samples  10 . In addition, in step ST 112 , a correlation of each data is calculated for a deviation (ε(r k ) i=1 to m ) of the relative refractive index difference Δ in the k-th radial section r k  in newly collected glass synthesis actual-result data and a Ge flow rate (Ge(r k ) i−1 to m  as in the example of  FIG. 9B .  FIG. 10  is a diagram in which each data with the Ge flow rate (slm) as an x coordinate component and the deviation ε(r k ) i  as a y coordinate component is plotted in a two-dimensional coordinate system. Points P 1  to P 5  shown in  FIG. 10  show respective data for correlation calculation. In the example of  FIG. 10 , a correlation in the case of m=5 (five core preform samples) is shown by five points P 1 (Ge(r k ) i=1 ,ε(r k ) i=1 ) to P 5 (Ge(r k ) i−5 ,ε(r k ) i−5 ). However, a correlation in the case of m&gt;5 is shown by m points P 1 (Ge(r k ) i=1 ,ε(r k ) i=1 ) to P m (Ge(r k ) i=m ,ε(r k ) i=m ). 
     In step ST 113 , the correlation shown in  FIG. 10  is linearly approximated, so that a theoretical doping amount of Ge in the k-th glass synthesis section l k  is determined. That is, a square sum S(A,B) of a difference between an arbitrary point (x i ,y i ) and a straight line y=Ax+B (G 1000  in  FIG. 10 ) is represented by the following expression (6). 
     
       
         
           
             
               
                 
                   
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     The above expression (6) is expanded to find an inclination A and an intercept B of an approximation straight line G 1000  in which the square sum S(A,B) is minimized. At this time, two partial differential equations represented by the following expression (7) are established. One of these partial differential equations is a linear equation that is obtained by differentiating the expansion expression of the square sum S(A,B) with respect to the inclination A and has the inclination A as a variable and the other is a linear equation that is obtained by differentiating the expansion expression of the square sum S(A,B) with respect to the intercept B and has the intercept B as a variable. Therefore, from simultaneous linear equations having the inclination A and the intercept B as variables, the inclination A and the intercept B are obtained as shown in the following expression (8). 
     
       
         
           
             
               
                 
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     Particularly, as shown in  FIG. 10 , an x-axis component x y=0  at an intersection of the approximation straight line G 1000  and the x axis is given by the following expression (9). 
     
       
         
           
             
               
                 
                   
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     If the variables x i  and y i  in the above expression (9) are set to an doping amount Ge(r k ) i  and a deviation ε(r k ) i  of Ge in the k-th division section r k  in the i-th core preform sample among the m core preform samples produced in the past, respectively, for x y=0 , a theoretical doping amount Ge(r k ) opt  of Ge (the refractive index adjusting agent) in the k-th division section r k  of the core preform to be produced is given by the following expression (10) and a theoretical doping amount Ge(l k ) opt  of Ge in the k-th glass synthesis section l k  in the glass preform to be the core preform is given by a theoretical doping amount Ge(r k ) opt  of Ge in r k  associated with l k  by the above expression (1). 
     
       
         
           
             
               
                 
                   
                     
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       FIG. 11  is a flowchart illustrating the glass synthesis step ST 120  in the core preform production step ST 100  shown in  FIG. 2 . 
     As described above, if the theoretical doping amount of Ge in each glass synthesis section is determined in the pretreatment step ST 110 , in the glass synthesis step ST 120 , a counter showing the glass synthesis section to be a treatment target is initialized (ST 121 ) and flow rate control of Ge is performed for all the glass synthesis sections (ST 122  and ST 128 ). The controllers  660 A and  660 B respectively control the mixing valves  641 A and  641 B of the material gas supply systems  640 A and  640 B and the flow rate adjusters so that the doping amount becomes the theoretical doping amount Ge(l k ) opt  of the k-th glass synthesis section l k  to be the treatment target (ST 123 ). Then, a counter showing one or more glass layers belonging to the k-th glass synthesis section l k  is initialized (ST 124 ) and glass synthesis is performed (ST 125 ) while the number of glass layers deposited on the inner peripheral surface or the outer peripheral surface of the glass deposition substrate is counted (ST 126  and ST 127 ). The glass synthesis (ST 125 ) is performed for all the glass layers belonging to the k-th glass synthesis section l k  (ST 126 ). If the above steps ST 123  to ST 127  are executed for all the glass synthesis sections, the aftertreatment step ST 130  is performed subsequent to the glass synthesis step ST 120 . 
     In the case where there are a plurality of glass layers belonging to the k-th glass synthesis section l k , the theoretical doping amount of Ge in each glass layer belonging to the glass synthesis section l k  may be constant with Ge(l k ) opt . However, the theoretical doping amount may be changed linearly, for example, so as to gradually change toward the (k+1)-th glass synthesis section l k+1 , or may be changed in a curve shape using an arbitrary function so as to be smoothly connected. 
     In the above example, the adjustment region in which the equally divided division sections are set is set over the entire range of the core preform sample along the radial direction. However, the setting of the adjustment region in the present embodiment is not limited to this example. That is, a part of the core preform sample along the radial direction may be set to the adjustment region. The division sections in the set adjustment region may be sections with different sizes along the radial direction. Further, a plurality of adjustment regions may be set in a state of being continuous or separated. The division section size of a certain adjustment region among the plurality of adjustment regions may be different from the division section size of other adjustment region. 
     From the above description of the present invention, it is apparent that the present invention can be variously modified. Such variations cannot be regarded as departing from the spirit and scope of the present invention and improvements obvious to all those skilled in the art are included in the following claims. 
     REFERENCE SIGNS LIST 
       10  . . . core preform (core preform sample);  15  . . . core preform sample group;  20  . . . cladding preform (outer peripheral portion);  100  . . . optical fiber preform;  110 A . . . core;  110 B . . . cladding;  110  . . . optical fiber;  200  . . . glass preform (glass prefo m sample);  250  . . . glass preform sample group;  500  . . . glass synthesis actual-result data;  510  . . . production condition data; and  520  . . . refractive index profile data.