JP2013056787A - Method for manufacturing optical fiber preform - Google Patents

Abstract

An optical fiber preform manufacturing method capable of manufacturing a high-quality multimode optical fiber preform at low cost.
An optical fiber mother is provided with a trench layer 13 having a refractive index lower than that of pure quartz and having a thickness not exceeding the radius of the core portion 11 outside the core portion 11 having a graded index type refractive index distribution. A method for producing a material G, in which glass fine particles to be the core portion 11 are deposited by the VAD method, followed by dehydration and sintering to produce a transparent core glass body G2, and the core glass body G2 is stretched to obtain a core glass. The rod G3 is used, and glass particles are deposited by the OVD method using fluorine-containing gas outside the core glass rod G3, and then dehydrated and sintered, or the glass particles are deposited by the OVD method outside the core glass rod G3. After that, by dehydrating and sintering with a gas containing fluorine, the transparent glass body G4 including the trench layer 13 is formed, and the cladding layer 14 is formed on the outer periphery of the transparent glass body G4. To.
[Selection] Figure 1

Description

  The present invention relates to a method of manufacturing an optical fiber preform that is a preform of an optical fiber.

As an optical fiber used for connection in a data center, LAN connection, in-vehicle cable, etc., a multi-mode optical fiber (MMF) resistant to bending is used.
Such a multimode optical fiber includes a core region having a first radius and a profile alpha, an inner cladding extending radially from the first radius to the second radius, and from the second radius to the second radius. A trench extending radially to a radius of 3 and an outer cladding extending to a fourth radius, the maximum refractive index of the core region, the refractive index of the inner cladding, the refractive index of the trench, the refractive index of the outer cladding Has a specific value (for example, see Patent Document 1).

Also, a multimode optical fiber including an optical core surrounded by an outer optical cladding, the optical core being at the periphery of the central core exhibiting an alpha index type refractive index profile, and being refracted with respect to the outer optical cladding. A depressed core with a central core diameter of 50 ± 3 μm, a depressed trench width of 0.5 μm to 2 μm, and a depth with respect to the outer optical cladding. There is also known a multimode optical fiber in which the refractive index difference of the prestress trench is −4 × 10 ⁻³ to −1 × 10 ⁻³ (for example, see Patent Document 2).

  Furthermore, it has a core part having a higher refractive index than the cladding part, a surrounding cladding part, and a trench layer provided so as to surround the core part and having a lower refractive index than the cladding part, and the core part is higher than the cladding part. A multi-mode optical fiber comprising a central first core made of a material having a refractive index and a second core having a refractive index different from that of the first core and made of a material having a higher refractive index than that of the cladding portion around the first core. Is also known (see, for example, Patent Document 3).

  And a gradient refractive index glass core, and a clad surrounding the core and in contact with the core, wherein the clad has an annular portion with a reduced refractive index surrounding the core, and the annular portion with the reduced refractive index has a plurality of annular portions. A multimode optical fiber including a clad including glass having holes is also known (see, for example, Patent Document 4).

JP 2010-72647 A JP 2011-8248 A JP 2006-78543 A Special table 2010-515949

  As a method of manufacturing the optical fiber preform for the multimode optical fiber as described above, there is a method of synthesizing the center graded core to the trench layer by the PCVD method or the MCVD method (internal method). In this method, for example, the glass tube is internally added by increasing the addition amount of germanium or the like so that the refractive index gradually increases inside the glass tube having a refractive index to become a trench layer. A portion serving as a graded index (GI) core in which the refractive index distribution has a parabolic shape is formed. In this method, since the center hole remains at the end even if it is attached inside, the center hole is contracted by heating from the outside, so that the center hole is filled and solidified (collapsed).

  However, in such a manufacturing method using the internal method, the diameter of the glass tube to be attached cannot be increased so much that the size of the optical fiber preform cannot be increased. Therefore, when an optical fiber is manufactured by drawing one optical fiber preform, only an optical fiber having a conversion length of about 100 km to 200 km, which is shorter than the case of drawing from an ordinary optical fiber preform, can be manufactured. I'll be stuck.

In addition, as another manufacturing method of the optical fiber preform for the multimode optical fiber, there is a method of synthesizing the graded index type core portion to the trench layer by the OVD method. In this method, first, glass fine particles are deposited by supplying GeCl ₄ together with a glass raw material gas to a burner to prepare a glass fine particle deposit of a graded index core portion, and glass fine particles that become a trench portion are formed around it. Deposit.

By the way, in order to form the trench layer, it is necessary to add fluorine at a high concentration, but it is extremely difficult to locally add fluorine to the trench layer at the time of depositing the glass fine particles. Further, after depositing the glass particles of the core and the trench layer, when trying to add fluorine in a gas atmosphere containing fluorine at the time of dehydration sintering, GeO ₂ that increases the refractive index in the graded index core Co-addition of fluorine that lowers the refractive index offsets each other, increasing Rayleigh scattering loss and increasing transmission loss, and diffusion of fluorine and germanium results in a refractive index profile at the interface between the core and the trench layer. It becomes blurred and deviates from the intended refractive index distribution. In addition, fluorine must be added from the graded index core to the entire trench layer, which increases the manufacturing cost.

  The objective of this invention is providing the manufacturing method of the optical fiber preform which can manufacture the preform | base_material for high quality multimode optical fibers at low cost.

The method of manufacturing an optical fiber preform of the present invention that can solve the above-described problem is that the refractive index is lower than that of pure quartz and the radius of the core portion is outside the core portion having a parabolic refractive index distribution. A method of manufacturing an optical fiber preform comprising a trench layer having a thickness not exceeding,
A core glass body manufacturing step of manufacturing a transparent core glass body by dehydrating and sintering after depositing glass fine particles to be the core portion by a VAD method;
A stretching step of stretching the core glass body to form a core glass rod;
The glass particles are deposited on the outside of the core glass rod by the OVD method and then dehydrated and sintered, or the glass particles are deposited on the outside of the core glass rod by the OVD method. A trench layer forming step to form a transparent glass body including a trench layer by dehydrating and sintering with a gas containing
A cladding layer forming step of forming a cladding layer on the outer periphery of the transparent glass body;
It is characterized by including.

  In the method of manufacturing an optical fiber preform according to the present invention, the weight of the core glass rod is measured when the glass particles are deposited in the trench layer forming step, and the glass particles are measured when the measured weight reaches a predetermined weight. It is preferable to finish the deposition.

  In the manufacturing method of the optical fiber preform of the present invention, during the deposition of the glass fine particles in the trench layer forming step, always measure the outer diameter at each position in the longitudinal direction of the core glass rod, based on the measurement results, It is preferable to control the supply amount of the glass raw material gas to the burner that generates the glass fine particles so that the glass fine particles are uniformly deposited in the longitudinal direction.

  In the method for manufacturing an optical fiber preform of the present invention, a pure silica thin layer having a predetermined thickness that effectively contains no additive is formed on the outer periphery of a portion having a parabolic refractive index distribution in the core glass body manufacturing step. It is preferable to form it.

In the method for producing an optical fiber preform of the present invention, it is preferable that the bulk density of the glass fine particles deposited in the trench layer forming step is 0.2 g / cm ³ or more and 0.5 g / cm ³ or less.

  According to the present invention, glass fine particles that become the core portion are deposited by the VAD method, and then dehydrated and sintered to produce a transparent core glass body, and the core glass body is stretched to form a core glass rod. The trench layer can be easily formed by the OVD method. In other words, according to the present invention, when an optical fiber is used, a high-quality optical fiber preform having an intended refractive index distribution can be suppressed as much as possible without increasing the Rayleigh scattering loss and increasing the transmission loss. It can be manufactured easily.

It is a figure explaining the structure and refractive index distribution of the optical fiber preform manufactured with the manufacturing method of the optical fiber preform concerning this embodiment. It is a schematic block diagram which shows the apparatus which deposits glass particulates by VAD method. It is a schematic block diagram which shows the apparatus which deposits glass particulates by OVD method.

Hereinafter, an example of an embodiment of a method for manufacturing an optical fiber preform according to the present invention will be described with reference to the drawings.
A multimode optical fiber (MMF) that propagates a plurality of modes is used as an optical fiber for communication. There are two main types of refractive index distribution of the multimode optical fiber: a step index (SI) type and a graded index (GI) type.
In the method for manufacturing an optical fiber preform according to the present embodiment, an optical fiber preform that is the graded index type multimode optical fiber is manufactured.

  As shown in FIG. 1, the optical fiber preform G has a core portion 11. The core portion 11 has a refractive index distribution of a graded index type (parabolic shape) in which the central portion has the highest refractive index, and gradually decreases toward the outer side in the radial direction. The optical signal spread (mode dispersion) is suppressed. Further, the core portion 11 is provided with a thin layer 12 on the outer peripheral portion thereof. The thin layer 12 is made of pure quartz that effectively contains no additive, and is formed to have a predetermined thickness.

  A trench layer 13 having a refractive index lower than that of pure quartz and having a thickness not exceeding the radius of the core portion 11 is provided outside the core portion 11. By providing the trench layer 13, when an optical fiber is used, leakage of light from the core portion 11 to the outside is prevented, and transmission loss due to bending is also suppressed. The outer peripheral side of the trench layer 13 is a clad layer 14 made of pure quartz.

  As an example of the optical fiber preform G described above, the relative refractive index ΔN based on the refractive index of pure quartz constituting the thin layer 12 and the cladding layer 14 is + 0.97% or more and 1.30 in the core portion 11. %, And the trench layer 13 is −0.60% or more and −0.25% or less. Further, when the diameter of the optical fiber preform G is R, the diameter of the core portion 11 is 0.40R or more and 0.64R or less, the thickness of the thin layer 12 is 0.008R or more and 0.024R or less, and the trench layer 13 Is set to 0.02R or more and 0.10R or less.

The thickness of the trench layer 13 corresponds to a range of about 10% to 30% of the radius of the core portion 11. If the thickness of the trench layer 13 is less than 10% of the radius of the core portion 11, the effect of preventing light leakage and suppressing transmission loss during bending cannot be obtained sufficiently, and the thickness of the trench layer 13 is not sufficient. When it exceeds 40% of the radius of the core part 11, a numerical aperture (NA) will become large too much.
The thickness of the thin layer 12 is set to be 0.008R or more and 0.024R or less, but when the optical fiber preform G is drawn into an optical fiber, the thickness is 1 μm or more and 3 μm or less. It is set to be.

Next, the case where the optical fiber preform G having the above configuration is manufactured will be described.
(Core glass body manufacturing process)
First, a glass fine particle deposit G1 to be a core glass body is formed by the VAD method. Specifically, as shown in FIG. 2, the flame of the burner 24 is blown from below in the reaction vessel 23 against the starting material 22 supported by the support device 21 and rotating around the axis. The burner 24 is supplied with glass source gas (SiCl ₄ , GeCl ₄ ), combustion gas (H ₂ ), and auxiliary combustion gas (O ₂ ). Then, the glass fine particles synthesized by the hydrolysis reaction by the oxyhydrogen flame are blown from the burner 24 to the starting material 22 and moved in the axial direction. Thereby, glass particulates are deposited on the starting material 22, and the glass particulate deposit G1 is manufactured. At this time, the concentration distribution of GeO ₂ is controlled by increasing the temperature at the center of the burner 24 to obtain a graded index type refractive index distribution.

Further, a pure quartz layer having a predetermined thickness that does not contain an additive such as germanium or fluorine, which effectively becomes a thin layer 12, is provided on the outer periphery of a portion having a graded index type refractive index distribution in the glass particulate deposit G1. Form it. Note that the pure quartz layer to be the thin layer 12 is formed by the VAD method or the OVD method.
Thereafter, the glass fine particle deposit G1 is dehydrated and sintered over a period of about 16 hours to be transparent, thereby obtaining a core glass body G2 having a diameter of about 50 mm to 60 mm.

(Stretching process)
The core glass body G2 is stretched in the axial direction by applying tension while being softened by heating to obtain a core glass rod G3 having a diameter of about 20 mm to 40 mm.

(Trench layer formation process)
The trench layer 13 is formed on the outside of the core glass rod G3 by the OVD method, and this core glass rod G3 is a transparent glass body G4 having the trench layer 13. Specifically, as shown in FIG. 3, flames of a plurality of burners 34 are blown in the reaction vessel 33 against the core glass rod G3 that is supported by the upper and lower support devices 31 and rotates around the axis. These burners 34 are supplied with glass source gas (SiCl ₄ ), combustion gas (H ₂ ), and auxiliary combustion gas (O ₂ ). Then, the core glass rod G3 is reciprocally moved in the axial direction relative to the burner 34 while spraying glass fine particles synthesized from these burners 34 by hydrolysis reaction onto the core glass rod G3. As a result, glass particles are deposited in a layer around the core glass rod G3. Then, after the glass fine particles are deposited, the glass is dehydrated and sintered to be transparent, and a transparent glass body G4 in which the trench layer 13 is formed on the outer periphery of the core glass rod G3 is obtained.

In this trench layer forming step, fluorine is added to the trench layer 13 when depositing glass particles or dehydrating and sintering.
In order to add fluorine when depositing glass particles, carbon tetrafluoride (CF ₄ ) gas or the like is added to the glass raw material gas supplied to the burner 34.

Although it is possible to deposit glass fine particles by adding fluorine, it is necessary to add fluorine at a high concentration in order to obtain the trench layer 13 having a refractive index low enough to realize the function of the trench. It is generally difficult to add fluorine at a high concentration. Therefore, as a method for forming the trench layer 13, a method is used in which glass fine particles made of pure quartz are deposited on the outside of the core glass rod G3 by the OVD method and then dehydrated and sintered with a gas containing fluorine. Is preferred. In this case, any gas of silicon tetrafluoride (SiF ₄ ), sulfur hexafluoride (SF ₆ ), or carbon tetrafluoride (CF ₄ ) is introduced into the reaction vessel 33. For example, when the gas to be added is silicon tetrafluoride (SiF ₄ ) gas, the atmosphere in the reaction vessel 33 is preferably set to a SiF ₄ gas concentration of about 5% to 15%.

In the process of manufacturing the transparent glass body G4 by forming the trench layer 13, the bulk density of the glass fine particles deposited on the core glass rod G3 is 0.2 g / cm ³ or more and 0.5 g / cm ³ or less. , Deposit glass particles. By adjusting the flow rate of the combustion gas or the like supplied to the burner 34, the temperature of the flame can be adjusted, and the bulk density of the portion where the glass particles are deposited can be adjusted. If the bulk density of the deposited glass fine particles is 0.2 g / cm ³ or more and 0.5 g / cm ³ or less, it is possible to suppress the occurrence of cracks and the like due to the bulk density being too low and improve the handleability. Moreover, the problem that the addition amount of fluorine is insufficient due to the bulk density being too high can be eliminated.

Further, in the process of manufacturing the transparent glass body G4 by forming the trench layer 13, the weight is measured when the glass fine particles are deposited, and the weight of the trench layer 13 is controlled to be the same between lots.
From the weight, length, and diameter of the core glass rod G3 before the trench layer 13 is formed, and the weight increased when the glass fine particles are deposited, the amount of the glass fine particles to be the trench layer 13 can be estimated. Specifically, a weight meter such as a load cell is provided in the support device 31 that supports the core glass rod G3, and when the measurement result of the weight meter reaches a predetermined weight, the deposition of glass particles is performed. finish.
As described above, by monitoring the weight of the core glass rod G3 and determining the end timing of the deposition of the glass particles, it is possible to achieve stability of the amount of the glass particles deposited between lots.

Further, in the step of forming the trench layer 13 to manufacture the transparent glass body G4, control is performed so that the glass particles are uniformly deposited over the longitudinal direction.
Specifically, an outer diameter measuring device for measuring the outer diameter of the core glass rod G3 is provided at a plurality of locations in the longitudinal direction, and gas is supplied to each burner 34 based on the measurement results of these outer diameter measuring devices. The flow rate adjusting device (MFC) 35 to be controlled is controlled, and the supply amount of the glass raw material gas to the burner 34 is controlled.
In this way, by monitoring the outer diameter at each position in the longitudinal direction of the core glass rod G3 and performing feedback control of the flow rate of the source gas to the burner 34, the trench layer 13 in the longitudinal direction of the core glass rod G3 is obtained. It is possible to improve the quality by uniformizing the amount of deposited glass particles.

(Clad layer forming process)
A clad layer 14 is formed outside the transparent glass body G4 to obtain an optical fiber preform G. Specifically, pure silica glass particles substantially free of additives are deposited on the outer periphery of the transparent glass body G4 by the OVD method, and then dehydrated and sintered to make it transparent to make the optical fiber preform G. And

  Also in the step of forming the clad layer 14, the target value of the weight is set in advance, and when the measurement result of the weighing scale of the support device 31 reaches a predetermined weight, the deposition of the glass fine particles is finished. Thereby, the thickness of the cladding layer 14 in the longitudinal direction can be controlled, and the stability of the amount of deposited glass particles between lots can be achieved.

Also in the step of forming the cladding layer 14, the supply amount of the glass raw material gas to the burner 34 is controlled based on the measurement result of each outer diameter measuring device. Thereby, the deposition amount of the glass fine particles to be the clad layer 14 in the longitudinal direction of the core glass rod G3 can be made uniform to improve the quality.
The formation of the cladding layer 14 is not limited to the OVD method, and may be performed by the VAD method.

  If the optical fiber preform G thus manufactured is drawn, the optical signal to be transmitted has a graded index-type refractive index distribution that gradually decreases in the radial direction toward the outer side in the core 11. In addition, it is possible to manufacture an optical fiber that suppresses the spread of light (mode dispersion), prevents leakage of light from the core portion 11 to the outside at the trench layer 13 portion, and suppresses transmission loss due to bending.

  According to the method for manufacturing an optical fiber preform according to this embodiment, glass fine particles to be the core portion 11 are deposited by the VAD method, and then dehydrated and sintered to manufacture a transparent core glass body G2, and this core glass. The body G2 is stretched to form the core glass rod G3, and then the trench layer 13 can be easily formed by the OVD method. That is, according to this embodiment, when an optical fiber is used, a high-quality optical fiber preform G having an intended refractive index distribution without increasing the Rayleigh scattering loss and increasing the transmission loss can be produced as much as possible. It can be manufactured easily while suppressing.

  Moreover, since the core portion 11 is manufactured by the VAD method, a large-sized optical fiber preform G can be manufactured at a lower cost than the MCVD method attached to the glass tube, and the structure of the trench layer 13 such as the refractive index. Can be controlled accurately. Moreover, it is possible to eliminate the need for solidification (collapse) to close the center hole, simplifying the work, and suppressing dip (disturbance of refractive index distribution) due to solidification (collapsing) to achieve high-bandwidth transmission characteristics. An excellent optical fiber can be manufactured.

  Moreover, in the manufacturing method using the glass tube used as the trench layer, when heating with the oxyhydrogen flame of the burner in the drilling process to the transparent glass when making the glass tube or the internal process when attaching to the glass tube , OH groups may be mixed into the glass tube. In this embodiment, since the trench layer 13 is formed by the OVD method on the core glass rod G3 having the core part 11 manufactured by the VAD method, compared with the method using the glass tube as the trench layer, the trench layer It is possible to suppress OH mixing into 13 or the like.

  Further, in the core glass body manufacturing process, a thin layer 12 of pure quartz having a predetermined thickness that does not effectively contain an additive is formed on the outer periphery of the core portion 11 having a graded index type refractive index distribution. The trench layer 13 can be formed without affecting the core portion 11 in the subsequent steps. That is, even if the outer peripheral portion of the core glass body G2 is etched by a flame or the like, the influence on the core portion 11 having a graded index type refractive index distribution can be suppressed as much as possible. Thereby, the malfunction that the transmission frequency band of the high band side of the light in the core part 11 is impaired can be eliminated, and a favorable transmission characteristic can be obtained.

11: Core part, 12: Thin layer, 13: Trench layer, 14: Clad layer, 34: Burner, G: Optical fiber preform, G2: Core glass body, G3: Core glass rod, G4: Transparent glass body

Claims (5)

A method of manufacturing an optical fiber preform including a trench layer having a thickness lower than that of pure quartz and having a thickness not exceeding the radius of the core part outside the core part having a parabolic refractive index distribution,
A core glass body manufacturing step of manufacturing a transparent core glass body by dehydrating and sintering after depositing glass fine particles to be the core portion by a VAD method;
A stretching step of stretching the core glass body to form a core glass rod;
The glass particles are deposited on the outside of the core glass rod by the OVD method and then dehydrated and sintered, or the glass particles are deposited on the outside of the core glass rod by the OVD method. A trench layer forming step to form a transparent glass body including a trench layer by dehydrating and sintering with a gas containing
A cladding layer forming step of forming a cladding layer on the outer periphery of the transparent glass body;
An optical fiber preform manufacturing method comprising: It is a manufacturing method of the optical fiber preform according to claim 1,
An optical fiber characterized by measuring the weight of the core glass rod during the deposition of the glass particulates in the trench layer forming step, and terminating the deposition of the glass particulates when the measured weight reaches a predetermined weight. A manufacturing method of a base material. It is a manufacturing method of the optical fiber preform according to claim 1 or 2,
During the deposition of the glass particulates in the trench layer forming step, the outer diameter at each position in the longitudinal direction of the core glass rod is always measured, and based on the measurement result, the glass particulates are deposited uniformly over the longitudinal direction. A method for producing an optical fiber preform, which controls the amount of glass raw material gas supplied to a burner that produces glass particles. An optical fiber preform manufacturing method according to any one of claims 1 to 3,
In the core glass body manufacturing step, a thin layer of pure quartz having a predetermined thickness that does not effectively contain an additive is formed on the outer periphery of a portion having a parabolic refractive index distribution. A method of manufacturing the material. A method for producing an optical fiber preform according to any one of claims 1 to 4,
The method for producing an optical fiber preform, wherein the bulk density of the glass particles deposited in the trench layer forming step is 0.2 g / cm ³ or more and 0.5 g / cm ³ or less.

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