JP2019034865A - Production method of optical fiber preform - Google Patents

Abstract

To suppress deformation of a core in a sintering step of an optical fiber preform.SOLUTION: A production method of an optical fiber preform includes a bundling step for bundling a plurality of core-incorporated glass rods having a core part and a cladding part to cover the core part to obtain a rod cluster, a depositing step for depositing soot on the periphery of the rod cluster, and a sintering step for changing the soot deposited on the periphery of the rod cluster to transparent glass. The viscosity ηof the core part and the viscosity ηof the cladding part meet the relation η>ηin a temperature range in the sintering step.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a method for manufacturing an optical fiber preform.

  In recent years, as a technique for increasing the transmission capacity of an optical communication system, a multi-core fiber in which a plurality of cores are provided in one optical fiber has been studied. As a method for producing a multi-core fiber, an OBR (Over-Cladding Bundled Rods) method as disclosed in Patent Document 1 below is known. In the OBR method, cored glass rods having a core are bundled to form a rod assembly, and soot is deposited on the outer periphery of the rod assembly by an OVD (Outside Vaper Deposition) method. Thereafter, the rod assembly on which the soot is deposited is heated in an electric furnace to sinter the soot to form a transparent glass, thereby obtaining an optical fiber preform. A multi-core fiber can be manufactured by drawing this optical fiber preform.

JP-A-2015-178444

Usually, a dopant (GeO ₂ or the like) for increasing the refractive index is added to the core portion of the cored glass rod, while pure quartz is used for the cladding portion. By adding the dopant, the viscosity of the core portion becomes lower than the viscosity of the cladding portion.
Here, in the above-described manufacturing process of the optical fiber preform, in the sintering process in which the soot is sintered, when the soot is turned into a transparent glass, as shown in FIG. A contraction force is generated from the center toward the center. When the viscosity of the core portion is low, the core portion is deformed by the shrinkage force during the sintering process, and the shape of the core of the manufactured optical fiber preform is non-circular (elliptical as shown in FIG. 4B). In some cases, the shape may have changed. If the core is non-circular, the optical characteristics of the multi-core fiber are greatly affected.

  The present invention has been made in consideration of such circumstances, and an object thereof is to suppress the deformation of the core during the sintering process of the optical fiber preform.

In order to solve the above-mentioned problem, a method for manufacturing an optical fiber preform according to a first aspect of the present invention includes a rod assembly formed by bundling a plurality of cored glass rods having a core portion and a cladding portion covering the core portion. And bundling step to obtain a soot, an external step for depositing soot around the rod assembly, and a sintering step for transparent vitrification of the soot deposited around the rod assembly, in a temperature range in sintering step, the viscosity of the core portion and eta _{A_core,} when the viscosity of the cladding portion and eta _{A_clad,} satisfying η _{A_core>} η _{A_clad.}

According to the first aspect, since η _{A_core} > η _{A_clad} is _satisfied , even if a shrinkage force is generated in the optical fiber preform during the sintering process, the clad portion having a low viscosity is greatly deformed to cause the shrinkage. Can absorb power. Thereby, it can suppress that the core part of an optical fiber preform | base_material deform | transforms, and can suppress the deformation | transformation of the core of the manufactured multi-core fiber.

Here, in the temperature range in the sintering step, η _{A_clad} ≧ η _C or η _{A_clad} > η _C may be satisfied, where η _C is the viscosity of the soot that has become transparent vitrified.

  In this case, in addition to the deformation of the clad portion as described above, the soot located at a portion away from the core portion can also be deformed to absorb the contractile force. Thereby, a deformation | transformation of a core part can be suppressed more reliably.

Further, the rod assembly includes a filled glass rod having no core portion, and when the viscosity of the filled glass rod is η _B in the temperature range in the sintering step, η _{A_clad} ≧ η _B ≧ η _C may be satisfied.

  In this case, the viscosity at the sintering temperature is gradually reduced as the distance from the core portion increases, so that the portion farther from the core portion is greatly deformed by the shrinkage force, and the deformation of the core portion is more reliably suppressed. Can do.

  According to the said aspect of this invention, it can suppress that a core deform | transforms at the time of the sintering process of an optical fiber preform.

(A) is sectional drawing of the state which deposited soot on the rod assembly. (B) is sectional drawing of the optical fiber preform after a sintering process. (A) is a figure which shows distribution of the relative refractive index difference in this embodiment. (B) is a figure which shows distribution of the viscosity in the sintering temperature in this embodiment. (A) is a figure which shows distribution of the relative refractive index difference in the conventional manufacturing method. (B) is a figure which shows distribution of the viscosity in the sintering temperature in the conventional manufacturing method. (A) is a figure explaining the shrinkage force which acts on an optical fiber preform at the time of a sintering process. (B) is a figure which shows a mode that the core part deform | transformed after the sintering process.

  Hereinafter, the manufacturing method of the optical fiber preform according to the present embodiment will be described with reference to FIGS. In each drawing used in the following description, the scale is appropriately changed to make each member a recognizable size.

  In the method for manufacturing an optical fiber preform according to the present embodiment, for example, a cored glass rod A and a filled glass rod B as shown in FIG.

The glass rod A with a core has a core part 1 and a clad part 2 that covers the core part 1. The core part 1 becomes a core of an optical fiber. The core part 1 is formed in a circular shape in a cross sectional view. The clad part 2 becomes a part of the clad of the optical fiber. The filled glass rod B does not have a core part. The filled glass rod B becomes a part of the clad of the optical fiber together with the clad portion 2 of the cored glass rod A and the soot C described later.
A dopant for adjusting the refractive index and viscosity of the glass is added to the core portion 1, the cladding portion 2, and the filled glass rod B, respectively.

  Here, in the present embodiment, a dopant that increases the refractive index, such as germanium (Ge), is mainly added to the core portion 1. On the other hand, the cladding portion 2, the filled glass rod B, and the soot C described later, such as a soot C of the optical fiber, together with a dopant that decreases the refractive index such as fluorine (F), chlorine (Cl) and the like A dopant that increases the refractive index is added. In addition, you may change suitably the kind of dopant added.

  Next, the manufacturing process of the optical fiber preform will be described.

(Bundle process)
When manufacturing an optical fiber preform, first, in a bundle process, a plurality of cored glass rods A and a plurality of filled glass rods B are bundled to form a rod assembly (see FIG. 1A). In this embodiment, one filled glass rod B is disposed at the center of the rod assembly, and four cored glass rods A are disposed around the filled glass rod B. Further, a total of four filled glass rods B are arranged on the outer periphery of the rod assembly so as to be in contact with two adjacent cored glass rods A, respectively.

  In addition, you may change suitably the kind, number, arrangement | positioning, etc. of the glass rod contained in a rod assembly. For example, the filled glass rod B may not be included in the rod assembly. A plurality of types of filled glass rods B having different diameters may be used. Further, three or less or five or more cored glass rods A may be included in the rod assembly.

(Welding process)
Next, in the welding step, dummy rods for rotating the rod assembly are fixed to both ends in the longitudinal direction of the rod assembly. From the viewpoint of preventing impurities from adhering to the rod assembly or the like, the dummy rod is preferably fixed to the rod assembly by welding.

(External process)
Next, in the external process, soot C made of glass fine particles is deposited around the rod aggregate by the OVD method or the like. The soot C becomes a part of the clad of the optical fiber.
When the OVD method is used, this flame is applied to the outer periphery of the rod assembly while vaporized SiCl ₄ is introduced into the flame of the oxyhydrogen burner. At this time, the dummy rod is fixed to a lathe and the oxyhydrogen burner is reciprocated in the longitudinal direction while rotating the rod assembly. As a result, as shown in FIG. 1A, soot C is deposited around the rod assembly.

(Sintering process)
In the sintering process, the rod assembly on which the soot C is deposited is sintered in an electric furnace. Thereby, as shown in FIG.1 (b), while the glass rod A with a core, the filling glass rod B, and the soot C are mutually sintered, the soot C becomes a transparent glass body. In the sintering process, the clad portion 2 of the cored glass rod A, the filled glass rod B, and the soot C are melted and integrated into a clad of the optical fiber preform. In FIG. 1B, the broken line indicates which part of the clad after the sintering process corresponds to which part before the sintering process. The temperature range in the furnace in the sintering process (hereinafter simply referred to as “sintering temperature”) can be changed as appropriate, and is, for example, about 1500 ° C. An optical fiber preform is obtained by the sintering process, and an optical fiber (multi-core fiber) is obtained by drawing the optical fiber preform.

  Here, in the manufacturing method of the optical fiber preform of the present embodiment, the cored glass rod A is configured so that the viscosity of the core portion 1 is higher than the viscosity of the cladding portion 2 at the sintering temperature. This will be described in more detail below.

FIG. 2A is a graph showing the refractive index distribution of the sintered optical fiber preform in this embodiment. The horizontal axis in FIG. 2A corresponds to the r ′ axis shown in FIG. More specifically, 0 to R of the horizontal axis' range _{A_core} ranges corresponding to the core portion of the core containing glass rod A, r 'cladding portion 2 in the range of _{A_core} ~r' _{A_clad} is cored glass rod A R ′ A — _{clad to} r ′ _C is a range corresponding to the soot C. The vertical axis | shaft of Fig.2 (a) has shown the relative refractive index difference with respect to the pure quartz of each part. Hereinafter, the relative refractive index difference of each part with respect to pure quartz is _expressed as Δ _{A_core} or the like corresponding to the position on the r ′ axis.

FIG. 2B is a graph showing the viscosity distribution of each part at the sintering temperature in the optical fiber preform manufacturing method of the present embodiment. The horizontal axis in FIG. 2B is the same as the horizontal axis in FIG. The vertical axis | shaft of FIG.2 (b) has shown the viscosity of each part in sintering temperature. Hereinafter, the viscosity of each part at the sintering temperature is displayed as η _{A_core} or the like corresponding to the position on the r ′ axis.

  FIG. 3A is a graph corresponding to FIG. 2A for a conventional method for manufacturing an optical fiber preform. FIG.3 (b) is a graph corresponding to FIG.2 (b) about the manufacturing method of the conventional optical fiber preform | base_material.

In the conventional manufacturing method, pure quartz to which no dopant is added is often used as the clad portion 2 of the cored glass rod A, the filled glass rod B, and the soot C. For this reason, the relative refractive index difference with respect to pure quartz in the range of r ′ A — _{core to} r ′ _C in FIG. Further, since a dopant such as Ge for increasing the refractive index is added to the core portion 1, the value of Δ _{A_core} is larger than zero.

As shown in FIG. 3B, in the conventional optical fiber preform manufacturing method, η _{A_core} is smaller than η _{A_clad} and η _{C at the} sintering temperature. This is because when a dopant is added to pure quartz, the viscosity decreases according to the amount added. That is, in the conventional optical fiber preform manufacturing method, the core part 1 to which the dopant is added has a lower viscosity at the sintering temperature than the clad part 2 to which the dopant is not added. On the other hand, when soot C is made into a transparent glass in the sintering process, as shown in FIG. 4A, a contraction force acts from the outside of the optical fiber preform toward the center side. For this reason, when the viscosity of the core part 1 in the sintering process is smaller than the surrounding viscosity, the contraction force acts on the core part 1 and the core part 1 is deformed into a noncircular shape (FIG. 4B). )reference). And in the optical fiber obtained by drawing the optical fiber preform | base_material which the core part 1 deform | transformed in this way, desired optical characteristics may not be acquired.

On the other hand, in the optical fiber preform manufacturing method of the present embodiment, η _{A_core} is _larger than η _{A_clad as} shown in FIG. This is so eta _{A_core} is greater than eta _{A_clad,} because that adjusting the amount of dopant added to the core portion 1 and the cladding portion 2. In quartz glass, the viscosity usually decreases as the amount of dopant added increases. Therefore, for example, by _{reducing the} amount of dopant added to the core portion 1 and increasing the amount of dopant added to the cladding portion 2, η _{A_core} > η _{A_clad} can be _satisfied .

As described above, by _setting η _{A_core} > η _{A_clad} , the amount of deformation of the cladding portion 2 is made larger than that of the core portion 1 when a contraction force acts on the optical fiber preform as shown in FIG. be able to. That is, it is possible to reduce the influence of the contraction force on the core portion 1 and suppress deformation of the core portion 1.
Then, for example, by adding Ge to the core part 1 and adding F and Cl to the cladding part 2, the refractive index difference (Δ _{A_core} −Δ between the core part 1 and the cladding part 2 while η _{A_core} > η _{A_clad} is _{satisfied. A_clad} ) can be made equivalent to the conventional one. Thereby, deformation | transformation of the core part 1 can be suppressed, maintaining the optical characteristic equivalent to the past.

Further, by adjusting the addition amount of the dopant to the filled glass rod B and the soot C, as shown in FIG. 2B, the viscosity at the sintering temperature gradually decreases as the distance from the core portion 1 increases. Viscosity distribution can be obtained. Further, by adjusting the balance between the addition amount of the dopant such as F that lowers the refractive index of the glass and the addition amount of the dopant such as Ge and Cl that increases the refractive index of the glass, FIG. As shown in (b), it is possible to make η _{A_clad} larger than η _C while making Δ _{A_clad} and Δ _C equal.

In this embodiment, as shown in FIG. 2A, the values of Δ _{A_clad} and Δ _C are negative. This is because the cladding part 2, the filled glass rod B, and the soot C are added with a dopant (F, etc.) that lowers the refractive index of the glass more than a dopant (Ge, Cl, etc.) that raises the refractive index of the glass. This is because.

As described above, in the method for manufacturing an optical fiber preform of this embodiment, at sintering temperature, the viscosity of the core portion 1 and eta _{A_core,} when the viscosity of the cladding portion and η _{A_clad,} η _{A_core>} η _{A_clad} Satisfied. With this configuration, deformation of the core portion 1 due to contraction force generated in the optical fiber preform during the sintering process can be suppressed, and the shape of the core of the optical fiber can be kept circular. Accordingly, an increase in transmission loss of the optical fiber can be prevented.

Further, when the viscosity of the soot C that has been vitrified at the sintering temperature is η _C , soot C separated from the core portion 1 can be clad by satisfying η _{A_clad} > η _C or η _{A_clad} ≧ η _C. The contraction force can be absorbed by being greatly deformed together with the portion 2. Thereby, the deformation | transformation of the core part 1 can be suppressed more reliably.
Furthermore, by satisfying η _{A_clad} ≧ η _B ≧ η _C , the viscosity at the sintering temperature gradually decreases as the distance from the core portion 1 increases, and the portion further away from the core portion 1 deforms more greatly. Can do. Thereby, the deformation | transformation of the core part 1 can be suppressed more reliably.

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

For example, in the viscosity distribution shown in FIG. 2B, η _{A_core} > η _{A_clad} > η _C is _satisfied , but η _{A_clad} = η _C may also be used. That is, the clad part 2 and the soot C may have the same dopant type and amount.
Similarly, η _{A_clad} = η _B or η _B = η _C may be used. That is, the kind and addition amount of the dopant added to the cladding part 2 and the filled glass rod B, or the filled glass rod B and the soot C may be the same.
Further, pure quartz may be used as the core part 1 without adding a dopant to the core part 1. In this case, it is possible to _satisfy η _{A_core} > η _{A_clad} and Δ _{A_core} > Δ _{A_clad} by adding a dopant that lowers the refractive index of the glass to the cladding portion 2. In this case, the value of Δ _{A_core} is zero, and the value of Δ _{A_clad} is negative. The same applies when a dopant that lowers the refractive index of the glass is added to the filled glass rod B and soot C.

  In addition, the constituent elements in the above-described embodiment can be appropriately replaced with known constituent elements without departing from the gist of the present invention, and the above-described embodiments and modifications may be appropriately combined.

  DESCRIPTION OF SYMBOLS 1 ... Core part 2 ... Cladding part A ... Glass rod with a core B ... Filled glass rod C ... Soot

Claims (4)

A bundle step of bundling a plurality of cored glass rods having a core part and a clad part covering the core part to obtain a rod assembly;
An external step of depositing soot around the rod assembly;
A sintering step of transparent vitrification of the soot deposited around the rod assembly,
In the temperature range in the sintering step, when the viscosity of the core portion is η _{A_core} and the viscosity of the clad portion is η _{A_clad} ,
An optical fiber preform manufacturing method that satisfies η _{A_core} > η _{A_clad} . In the temperature range in the sintering step, when the viscosity of the soot that has been made transparent glass is η _C
The method for manufacturing an optical fiber preform according to claim 1, wherein η _{A_clad} ≧ η _C is satisfied. The method for producing an optical fiber preform according to claim 2, wherein η _{A_clad} > η _C is satisfied. The rod assembly includes a filled glass rod having no core part,
In the temperature range in the sintering step, when the viscosity of the filled glass rod is η _B ,
The method for manufacturing an optical fiber preform according to claim 2, wherein η _{A_clad} ≧ η _B ≧ η _C is satisfied.

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