Optical fiber preform manufacturing method

Abstract

To provide a method for manufacturing an optical fiber preform, capable of suppressing variation in a fluoride addition amount in the longitudinal direction of the optical fiber preform.SOLUTION: A method for manufacturing an optical fiber preform comprises: relatively moving an untreated base material including a core rod and a soot layer, and a heater to each other in the longitudinal direction of the untreated base material; and adding fluoride to the soot layer by heating the untreated base material by the heater while introducing fluorine based gas into a furnace core tube. When setting the absolute value of the relative speed of the untreated base material viewed from the heater to a traverse speed V [mm / hr], the thickness of the soot layer in the diameter direction of the untreated base material to d [mm] and the capacity ratio of the fluorine based gas in the furnace core tube to r [%], the following expressions a and b are satisfied, a:V=A×r / d, and b:4808≤A[mm2 / hr]≤17500.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing an optical fiber preform. [Background technology]

[0002] Patent document 1 describes that in a method for manufacturing an optical fiber preform in which fluorine (dopant) is added to a soot layer (trench portion) of an untreated preform using a fluorine-based gas, the fluorine should be added uniformly to the soot layer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6310378 Summary of the Invention [Problem to be solved by the invention]

[0004] Conventionally, studies have been conducted on making the amount of fluorine added to the soot layer uniform in the radial direction of the untreated preform. The amount of fluorine added to the soot layer depends on the bulk density of the soot layer. Therefore, if the bulk density of the soot layer is non-uniform in the longitudinal direction of the untreated preform, the amount of fluorine added to the soot layer is likely to be unintentionally non-uniform in the longitudinal direction. If the amount of fluorine added is non-uniform in the longitudinal direction, the desired optical properties (refractive index distribution) may not be obtained in an optical fiber melt-spun from an optical fiber preform obtained by sintering the untreated preform.

[0005] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a method for manufacturing an optical fiber that can suppress fluctuations in the amount of fluorine doping in the longitudinal direction of the optical fiber preform. [Means for solving the problem]

[0006] In order to solve the above problems, one embodiment of the present invention provides a method for manufacturing an optical fiber preform, which comprises moving a core rod and an untreated preform having a soot layer formed by depositing glass particles on the outer surface of the core rod, and a heater relative to each other along the longitudinal direction of the untreated preform, and heating the untreated preform with the heater while introducing a fluorine-based gas into a furnace tube in which the untreated preform is accommodated, thereby adding fluorine to the soot layer. When the fluorine-based gas is introduced into the furnace tube, the absolute value of the relative speed of the untreated preform as seen from the heater is a traverse speed V [mm / hr], the radial thickness of the soot layer of the untreated preform is d [mm], and the volume ratio of the fluorine-based gas in the furnace tube is r [%], the following formulas a and b hold: a:V=A×r / d b:4808≦A[mm 2 / hr]≦17500 [Effects of the Invention]

[0007] According to the above aspect of the present invention, it is possible to provide a method for manufacturing an optical fiber that can suppress fluctuations in the amount of fluorine doping in the longitudinal direction of the optical fiber preform. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a diagram showing an example of an optical fiber preform manufacturing apparatus used in the optical fiber preform manufacturing method of the present embodiment. FIG. [Figure 2] FIG. 2 is a cross-sectional view of the untreated base material taken along line II-II in FIG. [Figure 3] 2 is a diagram showing an example of a refractive index profile of an optical fiber according to the present embodiment. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, a method for manufacturing an optical fiber preform and an apparatus for manufacturing an optical fiber preform according to the present embodiment will be described with reference to the drawings. As shown in Fig. 1, the optical fiber preform manufacturing apparatus 1 includes a furnace tube 10 capable of accommodating an unprocessed preform M, a heater 20, a furnace body 30, and a heat insulating material 40. The furnace tube 10 is formed in the shape of a hollow container. The furnace tube 10 has a first end 10a and a second end 10b. The unprocessed preform M is supported within the furnace tube 10 by a support 50.

[0010] (direction definition) In this embodiment, the direction parallel to the central axis O of the furnace tube 10 is referred to as the Z direction or axial direction Z. The direction from the second end 10b toward the first end 10a along the axial direction Z is referred to as the +Z direction or upward, and the direction opposite to the +Z direction is referred to as the -Z direction or downward. A cross section perpendicular to the axial direction Z is referred to as a transverse cross section. A direction perpendicular to the central axis O of the furnace tube 10 is referred to as a radial direction. A direction approaching the central axis O along the radial direction is referred to as a radially inner direction, and a direction away from the central axis O is referred to as a radially outer direction. A direction going around the central axis O as viewed from the axial direction Z is referred to as a circumferential direction.

[0011] (Optical fiber preform manufacturing equipment) As shown in Fig. 2, the untreated preform M has a core rod R and a soot layer (porous glass body) M1. The soot layer M1 is formed by depositing glass particles on the outer peripheral surface of the core rod R. An optical fiber preform is obtained by subjecting the untreated preform M to a heat treatment such as sintering. An optical fiber F is obtained by melt-spinning the optical fiber preform in a spinning furnace.

[0012] In this embodiment, the core rod R has an untreated core portion MC and an untreated inner cladding portion M0. The untreated inner cladding portion M0 covers the outer peripheral surface of the untreated core portion MC. The untreated core portion MC is the portion that will become the core portion FC of the optical fiber F (see also FIG. 3). The untreated inner cladding portion M0 is the portion that will become the inner cladding portion F0 of the optical fiber F. The core rod R may be manufactured using a soot method, such as a VAD method or an OVD method.

[0013] When manufacturing a core rod R using the soot method, first, oxygen gas, hydrogen gas, inert gas, etc. are flowed from a burner installed in a reaction vessel, and glass raw material gas such as SiCl4 is introduced into the flame that causes these gases to react. This generates glass particles. These glass particles are attached to a quartz rod rotating in the reaction vessel, and soot is deposited on the outer periphery of the quartz rod. The quartz rod with the deposited soot is heated (sintered) to form a transparent glass, thereby manufacturing the core rod R. More specifically, the quartz rod becomes the untreated core portion MC, and the soot becomes the untreated inner cladding portion M0.

[0014] By further depositing glass particles on the core rod R manufactured by the above-mentioned method or the like using a soot method such as the VAD method or the OVD method, an untreated preform M having the core rod R and the soot layer M1 is manufactured.

[0015] The furnace muffle tube 10 according to this embodiment is formed in a substantially circular shape in cross section. Note that the term "substantially circular shape" also includes cases where the shape can be considered circular if manufacturing errors are removed. The material of the furnace muffle tube 10 can be, for example, quartz glass. As shown in FIG. 1, a furnace muffle tube lid 11 is provided at a first end 10a of the furnace muffle tube 10. By opening the furnace muffle tube lid 11, an unprocessed preform M can be inserted into the furnace muffle tube 10, or a processed preform (optical fiber preform) can be removed from the furnace muffle tube 10.

[0016] A gas inlet 12 is provided at the second end 10b of the muffle tube 10. Gas can be introduced from the gas inlet 12 into the space (inside the muffle tube 10) in which the untreated base material M is accommodated. The muffle tube 10 may also be provided with a gas outlet (not shown). The gas outlet is used to exhaust gas from the space (inside the muffle tube 10) in which the untreated base material M is accommodated.

[0017] The support portion 50 penetrates the core tube cover portion 11. In this embodiment, the untreated base material M is supported so that the longitudinal direction of the untreated base material M is parallel to the axial direction Z. Note that the untreated base material M may be supported in a state in which the longitudinal direction of the untreated base material M is not parallel to the axial direction Z.

[0018] The support part 50 is in slidable contact with the muffle tube cover part 11. The optical fiber preform manufacturing apparatus 1 may also include a lifting device (not shown) outside the muffle tube 10. The lifting device grips the support part 50 and moves it along the axial direction Z. With this configuration, the untreated preform M can move relative to the muffle tube 10 and the heater 20 along the axial direction Z.

[0019] The furnace body 30 is formed in the shape of a hollow vessel. The furnace body 30 according to this embodiment is formed in a substantially circular shape in cross section. Note that the term "substantially circular shape" also includes cases where the shape can be considered circular if manufacturing errors are removed.

[0020] A first insertion hole 31A is formed in the upper wall of the furnace body 30. A second insertion hole 31B is formed in the lower wall of the furnace body 30. The muffle tube 10 passes through the insertion holes 31A and 31B. An upper lid 32A that closes the first insertion hole 31A is provided on the upper surface of the furnace body 30. A lower lid 32B that closes the second insertion hole 31B is provided on the lower surface of the furnace body 30. The lids 32A and 32B also serve to fix the muffle tube 10 to the furnace body 30. The muffle tube 10 can be removed from the furnace body 30 by removing the lids 32A and 32B from the furnace body 30.

[0021] The heater 20 and the heat insulating material 40 are housed in the furnace body 30. The heater 20 and the heat insulating material 40 according to this embodiment are formed in a substantially circular shape in cross section. Note that the term "substantially circular shape" also includes cases where the shape can be considered circular if manufacturing errors are removed. The heater 20 and the heat insulating material 40 surround the furnace core tube 10 from the radial outside. The heater 20 heats the untreated base material M via the furnace core tube 10. The heater 20 may be, for example, an electric heater. The heater 20 faces the furnace core tube 10 in the radial direction. The heat insulating material 40 is a member that prevents heat from the heater 20 from escaping to the outside of the furnace body 30.

[0022] (Method of manufacturing optical fiber preform) Next, a method for manufacturing an optical fiber preform using the optical fiber preform manufacturing apparatus configured as above will be described.

[0023] FIG. 3 is a diagram showing an optical fiber F obtained by further melt-spinning the optical fiber preform manufactured by the optical fiber preform manufacturing method according to the present embodiment. In this embodiment, the optical fiber F has a core FC, an inner cladding F0, an outer cladding F1, and a second outer cladding F2. The inner cladding F0 covers the outer peripheral surface of the core FC. The outer cladding F1 covers the outer peripheral surface of the inner cladding F0. The second outer cladding F2 covers the outer peripheral surface of the outer cladding F1. The second outer cladding F2 is derived from glass particles (second soot layer M2) deposited on the outside of the sintered green preform M (details will be described later). The refractive index profile of the optical fiber F shown in FIG. 3 is an example and can be changed as appropriate. For example, the refractive indexes of the layers FC, F0, F1, and F2 of the optical fiber F do not need to be constant.

[0024] In this embodiment, the maximum refractive index n1 of the core FC is designed to be larger than the maximum refractive index n2 of the inner cladding F0, the maximum refractive index n3 of the outer cladding F1, and the maximum refractive index n4 of the second outer cladding F2. On the other hand, the maximum refractive index n3 of the outer cladding F1 is designed to be smaller than the n1 of the core FC, the maximum refractive index n2 of the inner cladding F0, and the maximum refractive index n4 of the second outer cladding F2. In this specification, the maximum refractive index refers to the maximum value of the refractive index in each layer FC, F0, F1, and F2 of the optical fiber F.

[0025] Furthermore, the refractive index profile of the optical fiber F may be designed so that the maximum value of the relative refractive index difference Δ of the outer cladding portion F1 is −0.186% or less, based on the maximum refractive index n4 of the second outer cladding portion F2. In this specification, the relative refractive index difference Δ is a value that represents how much the refractive index of each portion of the optical fiber F is smaller than the reference value (n4). For each portion of the optical fiber F, the relative refractive index difference Δ is defined by the following formula (1): In formula (1), n4 is the maximum refractive index of the second outer cladding portion F2, and n is the refractive index of that portion.

[0026]

number

[0027] The method for manufacturing an optical fiber preform according to this embodiment includes a fluorine-based gas diffusion step and a sintering step. The optical fiber preform manufactured through these steps is melt-spun to produce an optical fiber F having the refractive index profile described above.

[0028] First, the core rod R is prepared. As described above, the core rod R may be manufactured by the soot method. In addition, a dopant such as germanium or fluorine may be added to adjust the refractive index. Next, a green preform M having a core rod R and a soot layer M1 is manufactured. In manufacturing the green preform M, glass particles may be deposited on the core rod R by a soot method.

[0029] Next, a fluorine-based gas diffusion step is carried out. Note that before the fluorine-based gas diffusion step is carried out, the untreated base material M may be subjected to a dehydration treatment as appropriate.

[0030] In the fluorine-based gas diffusion process, a fluorine-based gas such as CF4, SiF4, or SF6 is introduced into the furnace muffle tube 10. The fluorine-based gas introduced into the furnace muffle tube 10 reacts with the glass particles, thereby doping the soot layer M1 with fluorine (F). Doping the soot layer M1 with fluorine can reduce the refractive index of the outer cladding portion F1. Note that, while the fluorine-based gas is being introduced into the furnace muffle tube 10, an inert gas (carrier gas) such as argon may also be introduced into the furnace muffle tube 10.

[0031] In the fluorine-based gas diffusion process, the untreated base material M is heated by the heater 20. Heating the untreated base material M promotes the reaction between the glass particles and the fluorine-based gas, thereby increasing the efficiency of fluorine addition. Furthermore, the untreated base material M and the heater 20 are moved (traverse) relative to each other along the axial direction Z (the longitudinal direction of the untreated base material M) so that fluorine is added to the entire longitudinal direction of the untreated base material M. Particularly in this embodiment, the heater 20 is fixed in the furnace body 30, and the untreated base material M is moved along the axial direction Z by an elevator or the like. Hereinafter, in this specification, the absolute value of the relative speed of the untreated base material M as viewed from the heater 20 may be referred to as the traverse speed V [mm / hr]. In this embodiment, the moving speed of the untreated base material M is equal to the traverse speed V.

[0032] The inventors of the present invention have considered that the slower the traverse speed V, the more the fluctuation in the amount of fluorine added in the longitudinal direction of the optical fiber preform (untreated preform M) can be suppressed. This is because the slower the traverse speed V, the longer the time that each part of the untreated preform M is heated by the heater 20, ensuring the time for the glass particles and the fluorine-based gas to react in that part.

[0033] The inventors of the present application also considered that the time required for fluorine to be added to the entire soot layer M1 becomes longer as the thickness d of the soot layer M1 increases and as the concentration r of the fluorine-based gas in the furnace core tube 10 decreases. This is because it is believed that the greater the thickness d of the soot layer M1, the greater the amount of glass particles, and the lower the concentration r of the fluorine-based gas, the slower the reaction rate between the glass particles and the fluorine-based gas. According to this consideration, it is believed that the greater the thickness d of the soot layer M1 and the lower the concentration r of the fluorine-based gas in the furnace core tube 10, the slower the traverse speed V needs to be set.

[0034] On the other hand, if the traverse speed V is set unnecessarily slow, the time required to move the unprocessed base material M within the furnace tube 10 will be longer, which may reduce the production efficiency of the optical fiber base material (optical fiber F).

[0035] Therefore, in the manufacturing method of the optical fiber preform according to this embodiment, the traverse speed V is determined by the following formula (2). In formula (2), r [%] is the volume ratio (concentration) of the fluorine-based gas in the furnace tube 10. d [mm] is the radial dimension (thickness) of the soot layer M1. A [mm 2 / hr] is a coefficient set by the operator as appropriate.

[0036]

number

[0037] In formula (2), the traverse speed V is proportional to the coefficient A. As discussed above, the smaller the coefficient A is set, the more the fluctuation in the fluorine doping amount in the longitudinal direction of the optical fiber preform can be suppressed. However, if the coefficient A is set unnecessarily small, the production efficiency of the optical fiber preform will decrease. In contrast, in the manufacturing method of the optical fiber preform according to this embodiment, the coefficient A is selected from the range of 4808≦A≦17500. In this way, by setting an upper limit for the coefficient A, the fluctuation in the fluorine doping amount in the longitudinal direction of the optical fiber preform can be suppressed. Furthermore, by setting a lower limit for the coefficient A, the decrease in the production efficiency of the optical fiber preform can be suppressed.

[0038] Furthermore, in formula (2), the set value of the traverse speed V is proportional to the concentration r of the fluorine-based gas in the furnace core tube 10 and inversely proportional to the thickness d of the soot layer M1. In other words, the traverse speed V is set smaller (slower) as the thickness d of the soot layer M1 increases and the concentration r of the fluorine-based gas in the furnace core tube 10 decreases. In this way, by determining the traverse speed V according to the thickness d of the soot layer M1 and the concentration r of the fluorine-based gas in the furnace core tube 10, it is possible to more reliably suppress fluctuations in the amount of fluorine added and to more reliably suppress decreases in the production efficiency of the optical fiber preform.

[0039] In the fluorine-based gas diffusion step, fluorine may be added to the soot layer M1 so that the maximum value of the relative refractive index difference Δ of the outer cladding portion F1 is −0.186% or less. In this case, the coefficient A and the concentration r of the fluorine-based gas in the furnace tube 10 may be appropriately adjusted. As described above, the outer cladding portion F1 is a portion where the soot layer M1 is sintered and further melt-spun (drawn).

[0040] After the fluorine-based gas diffusion process, a sintering process is performed. In the sintering process, the untreated base material M is heated to a glass transition temperature by the heater 20. This causes the soot layer M1 to become transparent and vitrified. Note that in the sintering process, the untreated base material M and the heater 20 may move relative to each other (traverse).

[0041] After the sintering process, glass particles may be deposited on the outer periphery of the sintered green base material M to form a second soot layer M2 (see FIG. 2). A soot method may be used to form the second soot layer M2. Note that the second soot layer M2 does not necessarily have to be formed.

[0042] The second soot layer M2 is a portion that will become the second outer cladding portion F2 of the optical fiber F. As described above, the maximum refractive index n4 of the second outer cladding portion F2 is greater than the maximum refractive index n3 of the outer cladding portion F1. In order to achieve such a refractive index profile, for example, fluorine need not be doped into the second soot layer M2. Alternatively, the amount of fluorine doped into the second soot layer M2 may be less than the amount of fluorine doped into the soot layer M1. Alternatively, a dopant (such as germanium) that improves the refractive index of the doped portion may be doped into the second soot layer M2.

[0043] Finally, the untreated preform M on which the second soot layer M2 has been formed is subjected to a sintering process again, thereby vitrifying the second soot layer M2 into a transparent glass. This results in an optical fiber preform. By melt-spinning the optical fiber preform, an optical fiber F having a refractive index profile as shown in the example of FIG. 3 can be obtained.

[0044] Next, the operation of the method for manufacturing the optical fiber preform configured as above will be described.

[0045] Conventionally, efforts have been made to make the amount of fluorine added to the soot layer uniform in the radial direction. The amount of fluorine added to the soot layer depends on the bulk density of the soot layer. Therefore, if the bulk density of the soot layer is non-uniform in the longitudinal direction, the amount of fluorine added to the soot layer is likely to be unintentionally non-uniform in the longitudinal direction. If the amount of fluorine added is non-uniform in the longitudinal direction, the desired optical properties (refractive index distribution) may not be obtained in an optical fiber melt-spun from an optical fiber preform obtained by sintering the untreated preform.

[0046] However, it is difficult to control the deposition of glass particles so that the bulk density of the soot layer is completely uniform in the longitudinal direction. Furthermore, for the convenience of manufacturing optical fiber, the soot layer may be deposited so that the bulk density is non-uniform.

[0047] In contrast, in the optical fiber manufacturing method according to this embodiment, the traverse speed V is determined using formula (2), and the value of the coefficient A is limited to 17,500 or less. This configuration ensures the reaction time between the soot layer M1 and the fluorine-based gas, and can suppress fluctuations in the amount of fluorine added in the longitudinal direction of the untreated preform M. Furthermore, by setting a lower limit (4,808≦A) for the value of the coefficient A, it is possible to suppress a decrease in the production efficiency of the optical fiber preform.

[0048] Furthermore, by determining the traverse speed V using the formula (2), it is possible to select the traverse speed V according to the thickness d of the soot layer M1 and the concentration r of the fluorine-based gas in the furnace tube 10. This makes it possible to more reliably suppress the fluctuation in the amount of fluorine added in the longitudinal direction of the untreated preform M, while more reliably suppressing the decrease in the production efficiency of the optical fiber preform. [Example]

[0049] The above embodiment will be described below using specific examples, but the present invention is not limited to the following examples.

[0050] Optical fiber preforms were manufactured using the manufacturing methods of the optical fiber preforms according to the comparative example and examples 1 to 7. Furthermore, optical fiber F was manufactured by melt-spinning the optical fiber preforms manufactured in the comparative example and examples 1 to 7, and the refractive index fluctuation in the longitudinal direction of the optical fiber F was evaluated. Table 1 shows various manufacturing parameters and the evaluation results of the refractive index fluctuation in the comparative example and each example.

[0051] [Table 1]

[0052] In Table 1, the core rod diameter is the outer diameter of the core rod R. The core rod cross-sectional area is the area of the core rod R in a cross section perpendicular to the longitudinal direction of the core rod R. The soot thickness d is the dimension (thickness) of the soot layer M1 in the radial direction. Average bulk density B e is the average bulk density B of the soot layer M1 (glass particles) in the longitudinal direction of the untreated base material M. The bulk density fluctuation is the bulk density B of the soot layer M1 and the average bulk density B e The maximum difference between The fluorine-based gas concentration r is the volume ratio (concentration) of the fluorine-based gas in the furnace muffle tube 10. Coefficient A is a coefficient set by the operator. The traverse speed V is the relative speed of the untreated base material M as viewed from the heater 20. The traverse speed V is determined by the above-mentioned formula (2). The processing temperature is the temperature inside the furnace tube 10 while the fluorine-based gas is being introduced. The gas flow velocity is the flow velocity of the fluorine-based gas and carrier gas introduced into the furnace core tube 10. More specifically, it is the average flow velocity of the gas in the region where the inner peripheral surface of the furnace core tube 10 and the outer peripheral surface of the untreated base material M face each other.

[0053] ΔMAX is the maximum value of the relative refractive index difference (the minimum absolute value of the relative refractive index difference) in the longitudinal direction of the outer cladding portion F1. ΔMIN is the minimum value of the relative refractive index difference (maximum absolute value of the relative refractive index difference) in the longitudinal direction of the outer cladding portion F1. In the refractive index fluctuation judgment, when the value of ΔMAX−ΔMIN was 0.015% or less, it was judged as good, and when it was larger than a predetermined value, it was judged as bad.

[0054] As can be seen from Table 1, in each of Examples 1 to 7 in which the coefficient A was set within the range of 4808≦A≦17500, the refractive index fluctuation judgment was good regardless of the magnitude of other manufacturing parameters such as the soot thickness d and the fluorine-based gas concentration r. On the other hand, in the comparative example in which the coefficient A was not set within this range, the refractive index fluctuation judgment was poor.

[0055] In this way, by determining the traverse speed V using the above-mentioned formula (2) and setting the value of the coefficient A to 17,500 or less, it is possible to suppress the variation in the refractive index in the longitudinal direction of the optical fiber F. In other words, it is possible to suppress the variation in the amount of fluorine doping in the longitudinal direction of the optical fiber preform. Furthermore, by setting the coefficient A to 4,808 or more, it is possible to suppress the decrease in the production efficiency of the optical fiber preform.

[0056] As described above, the manufacturing method of the optical fiber preform according to this embodiment is a manufacturing method of the optical fiber preform, in which the untreated preform M having the soot layer M1 formed by depositing glass particles on the core rod R and the outer surface of the core rod R and the heater 20 are moved relative to each other along the longitudinal direction of the untreated preform M, and the untreated preform M is heated by the heater 20 while introducing a fluorine-based gas into the furnace tube 10 in which the untreated preform M is accommodated, thereby adding fluorine to the soot layer M1. When the fluorine-based gas is introduced into the furnace tube 10, the absolute value of the relative speed of the untreated preform M as seen from the heater 20 is the traverse speed V [mm / hr], the radial thickness of the soot layer M1 is d [mm], and the volume ratio of the fluorine-based gas in the furnace tube 10 is r [%], the following formulas a and b hold. a:V=A×r / d b:4808≦A[mm 2 / hr]≦17500

[0057] This configuration can suppress fluctuations in the amount of fluorine added in the longitudinal direction of the optical fiber preform, and also suppress a decrease in the production efficiency of the optical fiber preform.

[0058] Furthermore, in the fluorine-based gas diffusion step, the temperature inside the furnace tube 10 may be 1100° C. or higher and 1170° C. or lower, in this case, the reaction between the soot layer M1 (glass particles) and the fluorine-based gas is promoted.

[0059] Also, the bulk density B [g / cm 3 ] in the longitudinal direction of the untreated base material M is B e [g / cm 3 ], at each position in the longitudinal direction, B e -0.2≦B≦B e +0.2 may be established. In this case, fluctuations in the amount of fluorine added in the longitudinal direction of the optical fiber preform can be more reliably suppressed.

[0060] The average flow velocity of the fluorine-based gas in the region where the inner circumferential surface of the furnace tube 10 faces the outer circumferential surface of the untreated preform M may be 112 to 206 mm / min. In this case, the fluorine-based gas is smoothly supplied, and fluctuations in the amount of fluorine added in the longitudinal direction of the optical fiber preform can be more effectively suppressed.

[0061] Furthermore, fluorine may be added to the soot layer M1 so that the maximum relative refractive index difference of the outer cladding portion F1 is −0.186% or less. In this case, the bending loss of the optical fiber F can be more reliably suppressed.

[0062] The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.

[0063] For example, the transparent vitrification of the soot layer M1 and the transparent vitrification of the second soot layer M2 may be performed in a single sintering step, in other words, the soot layer M1 and the second soot layer M2 may be sintered simultaneously.

[0064] Furthermore, a part of the fluorine-based gas diffusion step and a part of the sintering step may be carried out simultaneously, or the whole of the fluorine-based gas diffusion step and the whole of the sintering step may be carried out simultaneously.

[0065] In the fluorine-based gas diffusion step, the untreated base material M may be fixed in the furnace tube 10, and the heater 20 may move along the axial direction Z. Alternatively, both the untreated base material M and the heater 20 may move along the axial direction Z.

[0066] In addition, it is possible to replace the components in the above-described embodiments with well-known components as appropriate, and the above-described embodiments and variations may be combined as appropriate, without departing from the spirit of the present invention. [Explanation of symbols]

[0067] M...Untreated base material M1...Soot layer R...Core rod 10...Furnace tube 20...Heater

Claims

1. A method for manufacturing an optical fiber preform, comprising the steps of: moving a core rod and a heater relative to each other along a longitudinal direction of the untreated preform, the untreated preform having a soot layer formed by depositing glass particles on an outer peripheral surface of the core rod; and heating the untreated preform with the heater while introducing a fluorine-based gas into a furnace tube in which the untreated preform is housed, thereby doping the soot layer with fluorine, the absolute value of the relative velocity of the untreated base material as seen from the heater when the fluorine-based gas is introduced into the furnace tube is defined as a traverse velocity V [mm / hr]; The thickness of the soot layer in the radial direction of the untreated base material is defined as d [mm], When the volume ratio of the fluorine-based gas in the furnace tube is r [%], the following formulas a and b are established: The method for manufacturing an optical fiber preform, wherein the temperature inside the furnace tube is 1100° C. or higher and 1170° C. or lower when the fluorine-based gas is introduced into the furnace tube. a: V = A × r / d b:4808≦A[mm ２ / hr]≦17500

2. The bulk density B of the soot layer [g / cm ３ ] in the longitudinal direction is B ｅ [g / cm ３ ], At each position in the longitudinal direction, B ｅ −0.2≦B≦B ｅ 2. The method for manufacturing an optical fiber preform according to claim 1, wherein +0.2 holds true.

3. 3. The method for manufacturing an optical fiber preform according to claim 1, wherein an average flow velocity of the fluorine-based gas in a region where the inner peripheral surface of the furnace tube and the outer peripheral surface of the untreated preform face each other is 112 to 206 mm / min.

4. A method for manufacturing an optical fiber preform described in any one of claims 1 to 3, wherein the fluorine is added to the soot layer so that when the soot layer to which the fluorine has been added is sintered, the maximum value of the relative refractive index difference of the soot layer is -0.186% or less.

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