JP2006193408A - Method for producing optical fiber preform and method for producing optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing an optical fiber preform at a reduced manufacturing cost by making the large-size optical fiber preform without using expensive helium gas. <P>SOLUTION: The method comprises the porous preform layer formation step S1 of forming a porous preform layer 9C by depositing glass microparticles around the circumference of a core rod 7A having a core layer 3A, the dehydration step S2 of dehydrating the porous preform layer 9C under at least one condition of a vacuum, an inert gas/halogen gas atmosphere, and an inert gas/halogen compound gas atmosphere, the sintering step S3 of sintering the dehydrated porous glass preform layer 9C in a vacuum into a translucent glass preform layer 9B containing closed cells, and the transparentization step S4 of transparentizing the translucent glass preform layer 9B containing the closed cells in an inert gas (except helium gas) atmosphere into a clad layer 9A. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing an optical fiber and a method for manufacturing an optical fiber preform, which have a core layer and a cladding layer surrounding the core layer, and are mainly used for optical communication.

In recent years, in addition to the improvement of optical transmission characteristics of optical fibers, there has been an increasing demand for price reduction of optical fibers.
Optical fiber production methods include vapor phase axial deposition (Vapor-phase Axial Deposition method: VAD), modified chemical vapor deposition (MCVD), and outer vapor deposition (Outside Vapor). Deposition method: OVD method), plasma chemical vapor deposition method (PCVD method), sol-gel method, rod-in-tube method (RIT method), and combinations thereof Manufacturing methods are known.

  In order to reduce the price of optical fibers, in any of the manufacturing methods, it is required to manufacture an optical fiber preform in a short time and reduce raw materials and processing costs. For example, in the formation of a cladding layer that occupies most of the optical fiber, a silica-based glass particle generated by vapor phase synthesis such as the OVD method is deposited on the outer periphery of the core rod to form a porous base material layer, which is heat treated. The method for producing the transparent glass preform layer is an excellent method for producing a high-quality and large-sized optical fiber preform.

  However, in order to obtain a transparent glass base material layer by heat-treating the porous base material layer, it is necessary to use expensive helium (He) gas. In addition, a furnace core tube made of quartz glass is used in a heating furnace that performs heat treatment for the purpose of preventing impurities from entering from the heating element. However, the core tube made of this silica glass is softened and easily deformed at a temperature close to 1600 ° C. necessary for the transparent vitrification of the porous preform, and particularly a large optical fiber preform is made into a transparent glass. Therefore, durability is a problem in a large-diameter core tube.

Furthermore, the helium gas, which has a high solubility, is dissolved in the base material that has been made into transparent glass in helium gas, and this helium gas is foamed during drawing to create a cavity in the optical fiber. It is necessary to heat-treat the transparent glass base material again in a low atmosphere, for example, in a gas atmosphere having a low solubility such as nitrogen (N ₂ ).

  As a method for solving these problems, for example, a method of heat-treating a porous base material under vacuum to form a transparent glass base material (see, for example, Patent Document 1), producing an opaque glass base material, The fiber is spun as it is to obtain an optical fiber (see, for example, Patent Document 2), after dehydration, exhausted under reduced pressure at a temperature below the start of sintering, and then densified at a high temperature of 1600 ° C. (For example, see Patent Document 3), a method in which the atmosphere gas is switched from helium to nitrogen during sintering, and sintering is completed and annealed in a nitrogen atmosphere (for example, see Patent Document 4).

Japanese Patent No. 2559395 Japanese Patent No. 2565712 JP-A-9-309735 Japanese Patent No. 3027509

  However, these conventional methods are not sufficient for solving the above-mentioned problems, and a large-sized optical fiber preform is heat-treated in a short time without using expensive helium gas, and damage to the core tube is reduced. Therefore, there is a strong demand for a low-cost optical fiber manufacturing method capable of extending the service life and reducing the number of processes.

  The present invention has been made in view of the above, and it is possible to manufacture a large-sized optical fiber preform and an optical fiber without using expensive helium gas, and to reduce the manufacturing cost. It aims at obtaining the manufacturing method and the manufacturing method of an optical fiber.

  In order to solve the above-described problems and achieve the object, an optical fiber preform manufacturing method according to the present invention is an optical fiber preform manufacturing method including a core layer and a cladding layer surrounding the core layer. A porous base material layer forming step of forming a porous base material layer by depositing silica-based glass particles on the outer periphery of a core rod made of silica-based glass having a core shape and having a core shape; and an inert gas under reduced pressure A dehydration step of dehydrating the porous base material layer under at least one of an atmosphere of an inert gas and a halogen compound gas, and dehydrating the porous base material layer under reduced pressure Sintering to a semi-transparent glass base material layer containing closed cells, and transparent glass forming the semi-transparent glass base layer containing closed cells in an inert gas (excluding helium gas) atmosphere. The cladding layer And a vitrification step of.

  In the second method for producing an optical fiber preform according to the present invention, the inside of the closed cell contained in the translucent glass preform layer is substantially vacuum.

  Furthermore, in the third method for manufacturing an optical fiber preform according to the present invention, the dehydration step and the sintering step include a core tube made of quartz glass in a pressure vessel, and a plurality of surroundings around the core tube The entire porous base material layer is uniformly heated in a heating furnace provided with a heater, and the transparent vitrification step is a zone heating type heating having a furnace core tube made of either quartz glass or carbon. Perform in the furnace.

Furthermore, in the fourth method for manufacturing an optical fiber preform according to the present invention, the dehydration step is performed at 1300 ° C. or less, and the sintering step has an average density of the porous preform layer of 1.8 g / cm. ^{It is} performed under the condition of ^{3 to} 2.2 g / cm ³ .

  Moreover, as for the manufacturing method of the 5th optical fiber preform which concerns on this invention, the said sintering process is performed by the pressure of 2000 Pa or less as the conditions under the said pressure reduction.

  Furthermore, an optical fiber manufacturing method according to the present invention is an optical fiber manufacturing method having a core layer and a clad layer surrounding the core layer, the core rod being made of quartz glass having the core layer and forming a rod shape. Porous matrix layer forming step of depositing quartz glass fine particles on the outer periphery of the substrate to form a porous matrix layer, an atmosphere of inert gas and halogen gas, an atmosphere of inert gas and halogen compound gas under reduced pressure A dehydration step of dehydrating the porous base material layer under at least one of the conditions, and sintering the dehydrated porous base material layer to a translucent glass base material layer containing closed cells under reduced pressure And a drawing step of drawing a translucent glass base material composed of the core rod and the translucent glass base material layer so that the translucent glass base material layer becomes a transparent glass layer.

According to the present invention, a porous base material layer is formed by depositing glass fine particles on the outer periphery of a rod-shaped core rod having a core layer, and an atmosphere of inert gas and halogen gas, inert gas under reduced pressure. The porous base material layer is dehydrated under at least one of an atmosphere of a halogen-containing compound gas, and the dehydrated porous base material layer becomes a translucent glass base material layer containing closed cells under reduced pressure. The semi-transparent glass base layer containing closed cells is made transparent into a cladding layer in an inert gas (excluding helium gas) atmosphere to form a cladding layer. A large-sized optical fiber preform can be manufactured in time, and the manufacturing equipment can be extended in life and the process can be omitted, so that the manufacturing cost of the optical fiber can be reduced.
The optical fiber manufacturing method according to the present invention is beneficially applied to a method of manufacturing an optical fiber having a core layer and a clad layer and having various refractive index distribution characteristics, and the increase in loss is particularly small. It is useful when applied to an optical fiber manufacturing method suitable for broadband WDM transmission. Specifically, it is suitable for an optical fiber manufacturing method such as a single mode fiber (hereinafter referred to as SMF). is there.

  Embodiments of an optical fiber preform manufacturing method and an optical fiber manufacturing method according to the present invention will be described below in detail with reference to the drawings. The present invention is not limited to the embodiments. In the following, the outline of the structural features of the optical fiber preform manufacturing method according to the present invention will be described as an embodiment, and then the optical fiber preform manufacturing method and the optical fiber manufacturing method will be implemented. This will be described in detail as an example.

[Embodiment]
FIG. 1 is a flowchart showing a process sequence according to an embodiment of an optical fiber preform manufacturing method and an optical fiber manufacturing method according to the present invention. The method for manufacturing an optical fiber preform according to the present embodiment includes a porous preform layer forming process, a dehydrating process, a sintering process, and a transparent vitrification process. Moreover, the manufacturing method of the optical fiber of this Embodiment has a drawing process instead of the transparent vitrification process of the manufacturing method of an optical fiber preform.

  First, in the porous base material layer forming step of step S1, silica-based glass fine particles are deposited on the outer periphery of a core rod 7A made of silica-based glass having a core layer 3A on the central axis and forming a rod shape. Layer 9C is formed.

  Next, in the dehydration process of step S2, any one of the three environments (conditions) in an atmosphere of an inert gas and a halogen gas or an atmosphere of an inert gas and a halogen-based compound gas under a predetermined reduced pressure. The porous base material layer 9C is dehydrated in one environment (condition).

  Furthermore, in the sintering process of step S3, the porous base material layer 9C dehydrated in the above dehydration process is sintered under reduced pressure, so that the semi-transparent glass state semi-transparent glass containing substantially vacuum closed cells is obtained. The transparent glass base material layer 9B is changed. Here, the “semi-transparent glass state” refers to a state in which closed cells are contained almost uniformly as a whole and are cloudy and opaque in appearance. On the other hand, the “transparent glass state” is a state that is transparent in appearance in a state that does not contain closed cells almost uniformly except for the minute closed cells that remain in some defective portions. Say. Here, “closed bubbles” refer to bubbles or spaces formed inside the translucent glass base material layer 9B and physically separated from the surrounding atmosphere. Furthermore, “vacuum” refers to the following definition in JIS Z 8126, that is, “a state of a specific space filled with a gas having a pressure lower than atmospheric pressure”.

  And in the manufacturing method of the optical fiber preform of this Embodiment, it transfers to the transparent vitrification process of step S4 next, and the semi-transparent glass preform layer 9B containing a closed cell is made into inert gas (except helium gas). ) Transparent glass is formed in the atmosphere to form the clad layer 9A. Thereby, an optical fiber preform is produced.

  On the other hand, in the manufacturing method of the optical fiber of this Embodiment, step S4 is not performed but it transfers to the drawing process of step S5. And in this drawing process, it draws so that the semi-transparent glass base material layer 9B may become a transparent glass layer.

  According to the optical fiber preform manufacturing method and the optical fiber manufacturing method according to such a procedure, a large-sized optical fiber preform can be manufactured in a short time without using expensive helium gas. Since the lifetime can be shortened and the process can be omitted, the manufacturing cost of the optical fiber can be reduced.

  Hereinafter, detailed embodiments will be described using examples. It should be noted that the drawings are shown to the extent that there is no problem in understanding the contents, and the shape is not necessarily to the actual scale. In this example, the characteristics of the optical fiber are ITU-T G. It shall conform to the definition specified in 650.

Example 1
FIG. 2 is a diagram showing a refractive index profile when the optical fiber preform manufactured in Example 1 is finally made into an optical fiber. As shown in FIG. 2, the optical fiber has a step index type refractive index distribution, has a zero dispersion wavelength in the 1.3 μm band, and is a so-called SMF. In FIG. 2, the optical fiber has a laminated structure formed concentrically in cross section, a core layer 3A is formed along the central axis, and thereafter, the first cladding layer extends radially outward from the center. Each layer is formed in the order of 5A and the second cladding layer 9A.

  The portion composed of the core layer 3A and the first cladding layer 5A is a portion corresponding to a core rod 7A described later. Looking only at the portion of the core rod 7A, the outer diameter ratio between the core layer 3A and the first cladding layer 5A (hereinafter referred to as the cladding / core ratio) is 4.8 / 1. In the present embodiment, the outer diameter of the core layer 3A refers to the diameter of a half of the maximum value of the relative refractive index difference of the core layer 3A with respect to the refractive index of the first cladding layer 5A.

-Production of core rod In the present example, first, a core soot 7B, which later becomes the core rod 7A, was produced by the VAD method. FIG. 3 is a schematic diagram for explaining a process of producing the core soot 7B by the VAD method of the present embodiment, and a portion of the core soot 7B is a longitudinal section. In FIG. 3, the VAD method includes vaporized silicon tetrachloride (SiCl ₄ ), germanium tetrachloride (GeCl ₄ ), oxygen (O ₂ ), and hydrogen (H ₂ ) through a core burner 21 having a multi-tube structure. Gas 23 is fed and ignited and burned. And it is made to hydrolyze in a flame and synthetic glass microparticles are obtained. The synthetic glass fine particles are sprayed on the seed rod 11 to adhere to the seed rod 11.

  The sprayed synthetic glass fine particles form the core layer soot 3B. The core layer soot 3B is a part that will later become the core layer 3A. Then, the seed bar 11 is slowly pulled upward in FIG. 3 while rotating.

A similar cladding burner 22 is disposed on the upper portion of the core burner 21, and a gas 24 composed of silicon tetrachloride (SiCl ₄ ), oxygen (O ₂ ), and hydrogen (H ₂ ) is fed into the core burner 21 and reacted to react with the core soot 3B. A clad layer soot 5B to be the first clad layer 5A later is formed on the outer periphery. As a result, a core soot 7B having a rod shape with a predetermined thickness composed of the core layer soot 3B and the clad layer soot 5B is obtained.

  Next, a dehydration process and a sintering process are performed on the core soot 7B. This process is the same as that of the prior art, and there is no special feature, so the illustration is omitted. By this dehydration / sintering process, the core soot 7B is made into a transparent glass, and becomes a core rod 7A composed of the core layer 3A and the first cladding layer 5A.

-Production of core rod Next, the core rod 7A composed of the transparent vitrified core layer 3A and the first cladding layer 5A is heated and stretched by a vertical electric furnace stretching apparatus 41 shown in FIG. Rod. FIG. 4 is a side view of the electric furnace stretching device 41 showing a state in which the core rod 7A is heated and stretched, and the heating furnace 42 is a longitudinal section. In FIG. 4, an electric furnace stretching apparatus 41 is installed in a heating furnace 42 having openings 42 a and 42 b penetrating in the vertical direction, an upper grip 43 disposed above the heating furnace 42, and a lower part of the heating furnace 42. And a lower holding portion 44.

  The heating furnace 42 has a cylindrical heater 45 as a heating element inside. The core rod 7A is set so as to extend in the vertical direction along the central axis of the heater 45, and the upper and lower ends extend through the openings 42a and 42b to the outside of the heating furnace 42. The upper end portion of the core rod 7A is gripped by the upper chuck 46 provided in the upper grip portion 43, while the lower end portion of the core rod 7A is gripped by the lower chuck 47 provided in the lower grip portion 44. . The upper grip portion 43 and the lower grip portion 44 are guided by the guide rail 48 and the guide rail 49, respectively, and are supported so as to be movable in the longitudinal direction of the core rod 7A.

The operation of the stretching device 41 will be described. While heating the large diameter portion of the core rod 7A with the heater 45, the upper chuck 46 is moved relatively closer to the heating furnace 42, and the lower chuck 47 is moved relatively away from the heating furnace 42. Thus, the core rod 7A is stretched to have a predetermined thickness.
The heat source in the heating / stretching process is not limited to the heating furnace 42, and a flame such as an oxyhydrogen flame or a plasma flame may be used.

-Porous preform | base_material layer formation process Next, the quartz-type glass fine particle was deposited using the OVD method on the outer periphery of the extended core rod 7A, and the porous preform | base_material layer 9C of diameter 300mm was produced. The porous base material layer 9C is a portion that later becomes the translucent glass base material layer 9B, and finally becomes a transparent glass to become the second cladding layer 9A.

FIG. 5 is a schematic diagram for explaining the production of the porous base material layer 9C by the OVD method, and the portion of the porous base material layer 9C is a longitudinal section. In FIG. 5, in the OVD method, a gas 32 composed of vaporized silicon tetrachloride (SiCl ₄ ), oxygen (O ₂ ), and hydrogen (H ₂ ) is fed through a burner 31 and ignited and burned. And it is made to hydrolyze in a flame and synthetic glass microparticles are obtained. The synthetic glass fine particles are sprayed onto the rotating core rod 7A and are deposited around the core rod 7A. Since the thickness of the synthetic glass fine particles deposited at one time is not so thick, it is repeated until the porous base material layer 9C having a sufficient thickness is obtained while reciprocating the burner 31 repeatedly.
The average density of the porous base material layer 9C (that is, the weight of the porous base material layer 9C obtained by subtracting the weight of the core rod 7A from the total weight, and the volume of the porous base material layer 9C obtained by subtracting the volume of the core rod 7A from the total volume) The value obtained by dividing by ³ was about 0.7 g / cm ³ .

図６において、脱水・焼結炉６１は、石英ガラス製の密閉可能な容器である石英炉心管６２と、この石英炉心管６２の周囲に複数設けられた発熱体である環状のマルチヒータ６３と、さらに石英炉心管６２及びヒータ６３を全体的に覆うとともに脱水・焼結炉６１の外装をなす炉体６７と、石英炉心管６２及びマルチヒータ６３と炉体６７との間に充填された断熱材６６とを有している。 Dehydration step and sintering step Next, the porous base material layer 9C is dehydrated and sintered in the dehydration / sintering furnace 61 shown in FIG. A semi-transparent glass base material layer 9B containing

In FIG. 6, a dehydration / sintering furnace 61 includes a quartz furnace tube 62 that is a sealable container made of quartz glass, and an annular multi-heater 63 that is a plurality of heating elements provided around the quartz furnace core tube 62. Further, a furnace body 67 that covers the entire quartz furnace core tube 62 and the heater 63 and forms the exterior of the dehydration / sintering furnace 61, and a heat insulation filled between the quartz furnace core tube 62, the multi-heater 63, and the furnace body 67. Material 66.

Inside the quartz furnace core tube 62, a core rod 7A having a porous base material layer 9C on the outer periphery is installed. In the dehydration process, chlorine gas (Cl ₂ ) and nitrogen gas (N ₂ ) are introduced into the quartz furnace core tube 62 from a gas inlet (not shown) at a predetermined flow rate shown in Table 1, and a gas outlet (not shown). The pressure in the quartz furnace core tube 62 is maintained at a predetermined value by discharging an appropriate amount of gas from the chamber. A vacuum pump 65 is connected to the quartz furnace core tube 62, and the inside of the quartz furnace tube 62 is decompressed by using this vacuum pump in the sintering process. The porous base material layer 9 </ b> C is dehydrated and sintered in the quartz core tube 62 to become a translucent glass base material layer 9 </ b> B containing closed cells having a substantially vacuum inside.

At the stage where the dehydration process and the sintering process are completed, the translucent glass base material layer 9B is in a state containing closed cells that are physically separated from the surrounding atmosphere. In this example, this state was referred to as a “translucent glass state”. In this “translucent glass state”, closed cells, which are air bubbles physically separated from the surrounding atmosphere, were contained almost uniformly, and the appearance was cloudy and opaque. The surface was smooth and glossy. Further, the density of the translucent glass base material layer 9B at this time is 95% of the density (2.2 g / cm ³ ) of the second cladding layer 9A which finally becomes completely transparent glass, that is, 2 0.09 grams / cm ³ .

  In the conventional method of making the porous base material layer completely transparent, first, it is heated once at a temperature of 1200 ° C. or less where sintering does not proceed, and then sufficiently dehydrated, and then exposed to high temperature conditions to be transparent. I do. That is, it becomes transparent through two steps. This conventional method requires expensive helium gas. Furthermore, heating energy costs, equipment maintenance costs, and the like increase.

  Therefore, in this example, after dehydration, a method was introduced in which sintering was performed in a temperature range that would achieve a semi-sintered state under reduced pressure. In the sintering process of the glass porous body, the bonding between the fine particles is increased by heating, the pores are decreased, the density is increased, and finally, the glass is changed to a transparent glass containing no bubbles.

The progress rate of sintering varies depending on conditions such as temperature and time, the particle size and composition of the glass fine particles, but the surface of the porous body is more likely to be sintered. As a result of dehydrating and sintering the porous base material layer 9C at various temperatures and heating times, the average density is set so that the porous base material layer 9C has closed cells substantially isolated from the surrounding atmosphere. Has been found to be 1.8 g / cm ³ or more, preferably 2.0 g / cm ³ or more.

Further, from the viewpoint of preventing residual bubbles in the subsequent transparent vitrification step or drawing step, there is an upper limit for the pressure in the sintering step performed under reduced pressure. In order for the residual gas in the closed cells to permeate into the quartz glass in the next transparent vitrification process or drawing process and not remain as bubbles, the total amount of the residual gas in the closed cells is in the quartz glass at the transparency temperature. Must be below saturation solubility. For example, when the residual gas is nitrogen gas (N ₂ ), the solubility S at the ambient temperature T of N _{2 in} quartz glass is described in the document “GC Beerkens, Advances in the Fusion and Processing of Glass 2nd, 1990 Vol63K, pp222-242”. According to the following formula.

S [cc (STP) / cm ³ * atm] = 0.0252 × EXP (-6665 / T) (1)

Here, STP means standard temperature and standard pressure.
Figure 10 is independently as in the bubble that N ₂ that has been depressurized is left, the ambient temperature T = 1600 ° C. in a N ₂ saturated in the quartz glass solubility ^{7.18 × 10 -4 [cc (STP} ) / From cm ³ * atm], the relationship between the pressure of the sintering process and the density of the semi-transparent glass base material layer to prevent bubbles from remaining by the transparency is shown by calculation.

When the density of the semi-transparent glass preform layer is ^{_{ρ 1 [g / cm 3]}} , the volume of the closed cells contained in the translucent glass base material ^{_{layer, (1-ρ 1 /2.2)[cc/cm 3}} ] It is expressed. Since the gas in the reduced sintering atmosphere remains in the closed cells, when the sintering pressure is P [Pa], the gas volume in the closed cells is

(1-ρ ₁ /2.2)×P/(1.013×10 ⁵ ) [cc / cm ³ ] (1.013 × 10 ⁵ is atmospheric pressure)

It becomes. If the gas volume in this closed cell is below the saturation solubility in quartz glass at the transparency temperature, it can be made transparent,

(Saturated solubility)> (1-ρ ₁ /2.2)×P/(1.013×10 ⁵ )> 0 (2)

It is necessary to proceed with the sintering until The curve in FIG.

7.18 × 10 ⁻⁴ = (1−ρ ₁ /2.2)×P/(1.013×10 ⁵ )

The relationship between the pressure in the sintering step and the density of the semi-transparent glass base material layer is shown, and equation (2) shows the region indicated by the oblique line above the curve in FIG.

Further, as a result of experiments under various conditions, it was found that all the bubbles were closed cells when the density of the semi-transparent glass base material layer was 2.13 g / cm ³ or more. Therefore, at a pressure higher than 2000 Pa from FIG. 10, it is very difficult to form a translucent glass base material layer so as to satisfy the formula (2), and it is difficult to make it transparent so that there are no residual bubbles.

  In order to minimize the residual bubbles in the transparent vitrification step or the drawing step, the pressure in the sintering step is particularly preferably 1000 Pa or less.

図７において、ゾーン加熱炉７１は、石英炉心管７２と、この石英炉心管７２の周囲に設けられた発熱体である環状のヒータ７３とを有している。石英炉心管７２の内部にて、コアロッド７Ａと半透明ガラス母材層９Ｂとが長さ方向に移動可能に支持されている。石英炉心管７２内部には、窒素ガス（Ｎ₂）が図示しないガス導入口から表２に示す所定の流量導入されるとともに、図示しないガス排出口から適量のガスを排出することにより、石英炉心管７２内の圧力を所定の値に保っている。 -Transparent vitrification process Next, the core rod 7A and the semi-transparent glass base material layer 9B were heat-treated in a nitrogen atmosphere using the zone heating furnace 71 shown in FIG. 7 to form a second cladding layer 9A having a diameter of 170 mm. . Table 2 shows the heat treatment conditions at this time.

In FIG. 7, the zone heating furnace 71 includes a quartz furnace core tube 72 and an annular heater 73 that is a heating element provided around the quartz furnace core tube 72. Inside the quartz furnace core tube 72, the core rod 7A and the translucent glass base material layer 9B are supported so as to be movable in the length direction. Nitrogen gas (N ₂ ) is introduced into the quartz core tube 72 from a gas inlet (not shown) at a predetermined flow rate shown in Table 2, and an appropriate amount of gas is discharged from a gas outlet (not shown) to thereby produce a quartz core. The pressure in the pipe 72 is maintained at a predetermined value.

  The operation of the zone heating furnace 71 will be described. The core rod 7 </ b> A and the translucent glass base material layer 9 </ b> B move in the length direction and change the relative position with respect to the heater 73. And the part heated with the heater 73 turns into transparent glass. In this embodiment, first, the lower end portion of the translucent glass base material layer 9B is made into transparent glass, and as the translucent glass base material layer 9B moves as indicated by an arrow in the figure, the transparent glass is sequentially turned upward. Goes to become. In this manner, an optical fiber preform was produced.

この段階で、多孔質母材層９Ｃの平均密度は２．１ｇ／ｃｍ³であり、完全に透明化したガラス母材の密度（２．２ｇ／ｃｍ³）の９５％であった。
次に、コアロッド７Ａ及び半透明ガラス母材層９Ｂを、実施例１と同様な図７に示すゾーン加熱炉７１にて、実施例１と同様な表２の条件により熱処理し、直径１７０ｍｍの光ファイバ母材を作製した。 (Example 2)
The core rod 7A and the porous base material layer 9C produced by the same method as in Example 1 were dehydrated and sintered under the conditions of Table 3 in the same dehydration / sintering furnace 61 shown in FIG. A semi-transparent glass base material layer 9B containing closed cells having a substantially vacuum inside was used.
In the present embodiment, the quartz furnace core tube 72 is decompressed also in the dehydration process.

At this stage, the average density of the porous base material layer 9C was 2.1 g / cm ³ , which was 95% of the density (2.2 g / cm ³ ) of the completely transparent glass base material.
Next, the core rod 7A and the semi-transparent glass base material layer 9B were heat-treated in the same zone heating furnace 71 as shown in FIG. A fiber preform was prepared.

この段階で、多孔質母材層９Ｃの平均密度は２．０ｇ／ｃｍ³であり、完全に透明化したガラス母材の密度（２．２ｇ／ｃｍ³）の９１％であった。
次に、コアロッド７Ａ及び半透明ガラス母材層９Ｂを、実施例１と同様な図７に示すゾーン加熱炉７１にて、実施例１と同様な表２の条件により熱処理し、直径１７０ｍｍの光ファイバ母材を作製した。 (Example 3)
The core rod 7A and the porous base material layer 9C produced by the same method as in Example 1 were dehydrated and sintered under the conditions of Table 4 in the dehydration / sintering furnace 61 shown in FIG. A semi-transparent glass base material layer 9B containing closed cells having a substantially vacuum inside was used.
In the present embodiment, the quartz furnace core tube 72 is decompressed also in the dehydration process.

At this stage, the average density of the porous base material layer 9C was 2.0 g / cm ³ , which was 91% of the density of the completely transparent glass base material (2.2 g / cm ³ ).
Next, the core rod 7A and the semi-transparent glass base material layer 9B were heat-treated in the same zone heating furnace 71 as shown in FIG. A fiber preform was prepared.

この段階で、多孔質母材層９Ｃの平均密度は１．８ｇ／ｃｍ³であり、完全に透明化したガラス母材の密度（２．２ｇ／ｃｍ³）の８２％であった。
次に、コアロッド７Ａ及び半透明ガラス母材層９Ｂを、実施例１と同様な図７に示すゾーン加熱炉７１にて、実施例１と同様な表２の条件により熱処理し、直径１７０ｍｍの光ファイバ母材を作製した。 Example 4
The core rod 7A and the porous base material layer 9C produced by the same method as in Example 1 were dehydrated and sintered under the conditions shown in Table 5 in the dehydration / sintering furnace 61 shown in FIG. A semi-transparent glass base material layer 9B containing closed cells having a substantially vacuum inside was used.
In this embodiment, the inside of the quartz core tube 72 is depressurized only in the sintering process.

At this stage, the average density of the porous base material layer 9C was 1.8 g / cm ³ , which was 82% of the density of the completely transparent glass base material (2.2 g / cm ³ ).
Next, the core rod 7A and the semi-transparent glass base material layer 9B were heat-treated in the same zone heating furnace 71 as shown in FIG. A fiber preform was prepared.

この段階で、多孔質母材層９Ｃの平均密度は２．１ｇ／ｃｍ³であり、完全に透明化したガラス母材の密度（２．２ｇ／ｃｍ³）の９５％であった。
この半透明ガラス母材層９Ｂ（直径１７４ｍｍ）は透明ガラス化せず、直接線引きを行った。 (Example 5)
The core rod 7A and the porous base material layer 9C produced by the same method as in Example 1 were dehydrated and sintered under the conditions shown in Table 6 in the dehydration / sintering furnace 61 shown in FIG. A semi-transparent glass base material layer 9B containing closed cells having a substantially vacuum inside was used.
In this embodiment, the inside of the quartz core tube 72 is depressurized only in the sintering process.

At this stage, the average density of the porous base material layer 9C was 2.1 g / cm ³ , which was 95% of the density (2.2 g / cm ³ ) of the completely transparent glass base material.
This translucent glass base material layer 9B (diameter 174 mm) was not drawn into transparent glass, but was directly drawn.

この段階で、多孔質母材層９Ｃの平均密度は２．１ｇ／ｃｍ³であり、完全に透明化したガラス母材の密度（２．２ｇ／ｃｍ³）の９５％であった。
次に、コアロッド７Ａ及び半透明ガラス母材層９Ｂを、実施例１と同様な図７に示すゾーン加熱炉７１にて、実施例１と同様な表２の条件により熱処理し、直径１７０ｍｍの光ファイバ母材を作製した。 (Comparative Example 1)
The core rod 7A and the porous base material layer 9C produced by the same method as in Example 1 were dehydrated and sintered under the conditions in Table 7 in the dehydration / sintering furnace 61 shown in FIG. A translucent glass base material layer 9B containing closed cells inside was used. In this comparative example, the interior of the quartz core tube 72 is not depressurized in both the dehydration process and the sintering process. In the sintering process, helium gas is used as an inert gas.

At this stage, the average density of the porous base material layer 9C was 2.1 g / cm ³ , which was 95% of the density (2.2 g / cm ³ ) of the completely transparent glass base material.
Next, the core rod 7A and the semi-transparent glass base material layer 9B were heat-treated in the same zone heating furnace 71 as shown in FIG. A fiber preform was prepared.

-Drawing process Next, the optical fiber preform produced in Examples 1 to 4 and Comparative Example 1 and the core rod 7A and the translucent glass preform layer 9B produced in Example 5 were drawn. At the time of drawing, two layers of UV curable resin were coated on the outer surface of the glass optical fiber, and the resin was cured by irradiating ultraviolet rays, and then wound on a reel through a winding capstan. The inner side of the coating was called the primary layer and the outer side was called the secondary layer, and the materials were selected so that the Young's modulus of these layers was smaller in the primary layer and larger in the secondary layer. The drawing speed in this example was 2000 m / min.
About each SMF produced in each above-mentioned Example, the incidence rate and the transmission characteristic of the bubble defect during drawing were measured. The results are shown in Tables 8 and 9.

Moreover, it was confirmed using the optical fiber defect detection apparatus that the closed cells which existed in the translucent glass preform layer 9B did not remain in the glass optical fiber in the drawing process.
Specifically, a light beam such as a laser beam is irradiated from the lateral direction with respect to the axis of the glass optical fiber during the drawing process, forward scattered light from the glass optical fiber is received by an image sensor, and the scattered light By detecting an abnormality in the intensity distribution pattern, a cavity defect such as a bubble was detected, and the bubble in the glass optical fiber was monitored.

表８、９から解るように、実施例１から５の各光ファイバは、いずれもカットオフ波長λｃｃが１３１０ｎｍ以下となっており、１３１０ｎｍ以上の波長領域で、シングルモード動作が保証される。
尚、ここでいうカットオフ波長とは、ＩＴＵ−Ｔ Ｇ．６５０規格で定義されるケーブルカットオフ波長λｃｃのことである。 The occurrence rate of bubble failure during drawing was inspected using the following optical fiber defect detection device. FIG. 8 is a perspective view showing a schematic configuration of the optical fiber defect detection apparatus. FIG. 9 is an explanatory diagram showing scattered light input to the image sensor of the translucent elongated body defect detection apparatus shown in FIG. 8 and a scattered light intensity distribution pattern obtained based on the scattered light. As shown in FIG. 8, this optical fiber defect detection device continuously irradiates a glass optical fiber 51 that is traveling in an uncoated state immediately after drawing from an optical fiber preform from the lateral direction, The forward scattered light 84 is received by a light receiving image sensor 85 such as a CCD line sensor or a photodiode array, the output is processed by a signal processing unit 86, and the scattered light intensity distribution pattern obtained from the signal processing unit 86 is determined. At 87, the processing result of the processing unit 86 is displayed on the monitor unit 88. If an abnormality is determined, an alarm is issued from the alarm unit 89, and the determination result is recorded by the recording unit 90.

As can be seen from Tables 8 and 9, in each of the optical fibers of Examples 1 to 5, the cutoff wavelength λcc is 1310 nm or less, and single mode operation is guaranteed in a wavelength region of 1310 nm or more.
The cut-off wavelength referred to here is ITU-TG. This is the cable cutoff wavelength λcc defined by the 650 standard.

The optical fibers of Examples 1 to 5 are all optical fibers having a loss at 1385 nm of 0.40 dB / km or less and a sufficiently small absorption loss of hydroxyl groups (OH).
Further, the strength of each optical fiber was tested by winding it to another reel while applying a tension corresponding to an elongation of about 2% with respect to the total length of the optical fiber after drawing. As a result, it was confirmed that the optical fiber had no problem with no breakage.

  As described above, the optical fiber according to the present embodiment has a strength due to contamination by a contaminant from a heating furnace particularly concerned when there are open pores or irregularities on the surface of the semi-transparent glass base material layer 9B. Deterioration and other problems do not occur. That is, the semi-transparent glass base material layer 9B is sintered until there are no open bubbles or irregularities that take in contaminants on the surface, that is, until the internal bubbles become closed cells. Because.

  On the other hand, in Comparative Example 1 (SMF (6)) in which helium gas was used in the sintering process without reducing the pressure in the dehydration and sintering process atmosphere of the porous base material layer 9C, there was a problem with transmission characteristics. Although there is no, the bubble defect rate is high. This is because helium, which is an atmospheric gas at the time of dehydration sintering, remains in the closed cells contained in the semi-transparent glass base material layer 9B, and the helium gas that has permeated into the glass during the clearing treatment is sufficiently It is presumed that the glass continued to dissolve without detaching from the outside of the glass and foamed during drawing to form a cavity in the optical fiber.

この段階で、多孔質母材層９Ｃの平均密度は２．１ｇ／ｃｍ³であり、完全に透明化したガラス母材の密度（２．２ｇ／ｃｍ³）の９５％であった。
次に、コアロッド７Ａの周囲に半焼結状態の半透明ガラス母材層９Ｂが形成されたものを、別のゾーン加熱方式の加熱炉で表２の条件により熱処理し、直径１７０ｍｍの透明ガラス母材とした。フッ素添加したクラッド層の屈折率は純石英系ガラスの屈折率に比べて０．４％低かった。以下、他の実施例と同様の手順を行い、線引き中にコーティングを施し、線引き後の光ファイバ中に気泡の残留がなく、強度にも問題がない被覆外径が約２５０μｍの光ファイバを得た。
本実施例のように、クラッド層の一部に屈折率の小さな領域を設ける事も可能である。 (Example 6)
In each of the above embodiments, fluorine may be added to the cladding layer. In this example, when the core rod 7A and the porous base material layer 9C produced in the same manner as in Example 1 were dehydrated and sintered, fluorine was added under the conditions shown in Table 10, and the inside was substantially vacuum. It was set as the semi-transparent glass base material layer 9B containing a closed cell.

At this stage, the average density of the porous base material layer 9C was 2.1 g / cm ³ , which was 95% of the density (2.2 g / cm ³ ) of the completely transparent glass base material.
Next, a semi-sintered semi-transparent glass base material layer 9B formed around the core rod 7A is heat-treated in another zone heating type heating furnace under the conditions shown in Table 2 to obtain a transparent glass base material having a diameter of 170 mm. It was. The refractive index of the fluorine-added cladding layer was 0.4% lower than that of pure silica glass. Thereafter, the same procedure as in the other examples is performed, coating is performed during drawing, and there is no residual bubble in the optical fiber after drawing, and an optical fiber with a coated outer diameter of about 250 μm that has no problem in strength is obtained. It was.
As in this embodiment, it is possible to provide a region having a small refractive index in a part of the cladding layer.

  The embodiments described herein are examples for explaining the present invention, and various modifications such as an optical fiber having a more complicated refractive index profile can be included in the scope of the present invention. Is well understood by those skilled in the art.

It is a flowchart which shows the order of the process of embodiment of the manufacturing method of the optical fiber preform which concerns on this invention, and the manufacturing method of an optical fiber. It is a figure which shows the refractive index profile of the optical fiber produced in the Example. It is the schematic diagram which made the core soot part the longitudinal cross-section explaining the preparation process of the core soot by VAD method. It is a side view which made the part of the heating furnace of the electric furnace extending | stretching apparatus which shows a mode that a core rod is heated and extended | stretched to the longitudinal cross-section. It is the schematic diagram which made the part of the porous base material layer the longitudinal cross-section explaining the formation of the porous base material layer by OVD method. It is a longitudinal cross-sectional view of the spin-drying | dehydration and sintering furnace used in order to make a porous base material layer into a semi-transparent glass base material layer in a spin-drying | dehydration process and a sintering process. It is a longitudinal cross-sectional view of the zone heating furnace used in order to make a semi-transparent glass base material layer into a 2nd cladding layer in a transparent vitrification process. It is a perspective view which shows the schematic structure of an optical fiber defect detection apparatus. It is explanatory drawing which shows the scattered light intensity distribution pattern obtained based on the scattered light input into the image sensor of the translucent long body defect detection apparatus of FIG. 9, and this scattered light. It is the graph which calculated | required the relationship between the pressure of the sintering process for the bubble not remaining by transparency, and the density of a semi-transparent glass base material layer by calculation.

Explanation of symbols

3A Core layer 3B Core soot 5A Clad layer 5B Clad soot 7A Core rod 7B Core soot 9A Clad layer 9B Translucent glass base material layer 9C Porous base material layer 11 Seed rod 21 Core burner 22 Clad burner 23 Gas 24 Gas 41 Furnace 32 Gas Drawing device 42 Heating furnace 42a Opening 42b Opening 43 Upper gripping part 44 Lower gripping part 45 Heater 46 Upper chuck 47 Lower chuck 48 Guide rail 51 Glass optical fiber 61 Dehydration / sintering furnace 62 Quartz furnace core tube 63 Multi-heater 65 Vacuum pump 66 Heat insulation Material 67 Furnace body 71 Zone heating furnace 72 Quartz furnace core tube 73 Heater 83 Parallel beam 84 Forward scattered light 85 Image sensor 86 Signal processing part 87 Judgment part 88 Monitor part 89 Alarm part 90 Recording part 91 Scattered light intensity distribution pattern

Claims (6)

A method of manufacturing an optical fiber preform having a core layer and a cladding layer surrounding the core layer,
A porous base material layer forming step of forming a porous base material layer by depositing silica glass fine particles on the outer periphery of a core rod made of a silica-based glass having the core layer and having a rod shape;
A dehydration step of dehydrating the porous base material layer under at least one of an atmosphere of an inert gas and a halogen gas, an atmosphere of an inert gas and a halogen-based compound gas under reduced pressure;
Sintering the dehydrated porous base material layer under reduced pressure until it becomes a translucent glass base material layer containing closed cells;
A transparent vitrification step of forming the translucent glass preform layer containing closed cells into a clad layer by transparent vitrification in an inert gas (excluding helium gas) atmosphere. Production method. The method for manufacturing an optical fiber preform according to claim 1, wherein the inside of the closed cells contained in the translucent glass preform layer is substantially vacuum. In the dehydration step and the sintering step, a furnace core tube made of quartz glass is provided in a pressure vessel, and a plurality of heaters are arranged around the furnace core tube, and the entire porous base material layer is made uniform. Done by heating,
The method for producing an optical fiber preform according to claim 1 or 2, wherein the transparent vitrification step is performed in a zone-heating heating furnace having a furnace core tube made of either quartz glass or carbon. The dehydration step is performed at 1300 ° C. or lower,
The sintering process is performed under conditions such that an average density of the porous base material layer is 1.8 g / cm ³ or more and less than 2.2 g / cm ³ . The manufacturing method of the optical fiber preform of item 1. The method for manufacturing an optical fiber preform according to any one of claims 1 to 4, wherein the sintering step is performed under a pressure of 2000 Pa or less as the condition under the reduced pressure. A method of manufacturing an optical fiber having a core layer and a cladding layer surrounding the core layer,
A porous base material layer forming step of forming a porous base material layer by depositing silica glass fine particles on the outer periphery of a core rod made of a silica-based glass having the core layer and having a rod shape;
A dehydration step of dehydrating the porous base material layer under at least one of an atmosphere of an inert gas and a halogen gas, an atmosphere of an inert gas and a halogen-based compound gas under reduced pressure;
Sintering the dehydrated porous base material layer under reduced pressure until it becomes a semi-transparent glass base material layer containing substantially vacuum closed cells;
And a drawing step of drawing a translucent glass preform composed of the core rod and the translucent glass preform layer so that the translucent glass preform layer becomes a transparent glass layer. Method.

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