JP5242007B2 - Optical fiber manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing an optical fiber at a reduced manufacturing cost by making the large-size optical fiber without using expensive helium gas. <P>SOLUTION: The method comprises the cylindrical porous body making step S1 of making a cylindrical porous body 9C by depositing glass microparticles around the circumference of a mandrel and pulling the mandrel out, the dehydration step S2 of dehydrating the porous body 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 body 9C in a vacuum into a translucent glass cylinder 9B containing closed cells, the core rod insertion step S4 of inserting a core rod 7A into the translucent glass cylinder 9B, and the drawing step S5 of drawing the resultant assemblage under heating so as to integrate the core rod 7A and the translucent glass cylinder and so as to transparentizing the translucent glass cylinder 9B into a transparent glass. <P>COPYRIGHT: (C)2006,JPO&amp;NCIPI

Description

  The present invention relates to a method of manufacturing an optical fiber having a core layer and a clad layer surrounding the core layer and 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 quartz glass core tube is used in a heating furnace that performs heat treatment in order to prevent impurities from entering from the heating element. However, this quartz core tube softens and deforms easily at a temperature close to 1600 ° C. necessary for transparent vitrification of the porous preform, and particularly a large-diameter core for transforming a large optical fiber preform into transparent vitrification. Durability is a problem with tubes.

Furthermore, the helium gas, which has a high solubility, is dissolved in the base material that has been made into a transparent glass in helium gas, and this helium gas foams during drawing, creating 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, Is drawn as it is to make an optical fiber (for example, see Patent Document 2), after dehydration treatment, evacuated at a temperature below the start of sintering, and then densified at a high temperature of 1600 ° C. or higher to obtain a transparent glass body. (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 to solve 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 quartz furnace core tube is reduced. Thus, there is a strong demand for a low-cost optical fiber manufacturing method that can extend the service life and reduce the number of processes.

  The present invention has been made in view of the above, and a method for producing an optical fiber capable of producing a large-sized optical fiber preform without using expensive helium gas and reducing the production cost. With the goal.

  In order to solve the above-described problems and achieve the object, an optical fiber manufacturing method according to the present invention is an optical fiber manufacturing method including a core layer and a cladding layer surrounding the core layer, and an outer periphery of a mandrel. Forming a porous base material layer by depositing quartz-based glass fine particles on the cylindrical base material, and then pulling out the mandrel from the porous base material layer to prepare a cylindrical porous body; A dehydration step of dehydrating the cylindrical porous body under at least one of an inert gas and halogen gas atmosphere and an inert gas and halogen compound gas atmosphere; and the dehydrated cylindrical porous body And sintering a core rod made of quartz-based glass having a core shape into the semi-transparent glass cylinder. Inserting the core rod and inserting the heat into the translucent glass cylinder into which the core rod is inserted, so that the core rod and the translucent glass cylinder are fused and integrated, and the translucent glass cylinder is transparent A drawing step of drawing the clad layer made of glass.

  In the second method for manufacturing an optical fiber according to the present invention, the inside of the closed cell included in the translucent glass cylinder is substantially vacuum.

  Furthermore, in the third method for manufacturing an optical fiber according to the present invention, the dehydration step and the sintering step include a furnace core tube made of quartz glass in a pressure vessel, and a plurality of heaters are provided around the furnace core tube. The heating is performed by uniformly heating the entire porous base material layer in the arranged heating furnace.

Furthermore, in the fourth method for producing an optical fiber 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 base material layer of 1.8 g / cm ³ or more. The conditions are such that it is less than 2.2 g / cm ³ .

  Moreover, as for the manufacturing method of the 5th optical fiber 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, in the sixth method for producing an optical fiber according to the present invention, the drawing direction side end of the translucent glass cylinder into which the core rod is inserted is heated and melted between the core rod insertion step and the drawing step. It is assumed to be a state.

  Furthermore, in the seventh method for manufacturing an optical fiber according to the present invention, as a heating method for preheating and melting the drawing direction side end of the translucent glass cylinder into which the core rod is inserted, oxyhydrogen, methane, etc. One of flammable gas flame radiation, plasma flame radiation and electric furnace heating is used.

  Moreover, the manufacturing method of the 8th optical fiber which concerns on this invention makes the pressure reduction state at least between the said core rod and the said semi-transparent glass cylinder in the said drawing process.

According to the present invention, in the dehydration step of dehydrating the cylindrical porous body, under reduced pressure, an atmosphere of an inert gas and a halogen gas, or an atmosphere of an inert gas and a halogen compound gas is used. The cylindrical porous body is dehydrated, and in the sintering step, the dehydrated cylindrical porous body is sintered until it becomes a translucent glass cylinder containing closed cells under reduced pressure. Furthermore, a core rod made of quartz glass having a core layer and having a core layer is inserted into the translucent glass cylinder, and heat is applied to the translucent glass cylinder into which the core rod is inserted, while translucent with the core rod. An optical fiber is manufactured by drawing so that the glass cylinder is fused and integrated, and the translucent glass cylinder is a clad layer made of transparent glass. Therefore, a large-sized optical fiber preform can be manufactured in a short time without using expensive helium gas, and the manufacturing equipment can be extended in service life and the process can be omitted, so that the manufacturing cost of the optical fiber can be reduced. it can.
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 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 structural features of the optical fiber manufacturing method according to the present invention will be described as an embodiment, and then, an actual optical fiber manufacturing method will be described as an example.

[Embodiment]
FIG. 1 is a flowchart showing the order of steps of an embodiment of an optical fiber manufacturing method according to the present invention. The optical fiber manufacturing method of the present embodiment includes a cylindrical porous body manufacturing step (step S1), a dehydration step (step S2), a sintering step (step S3), and a core rod insertion step (step S4). And a drawing process (step S5).

  In the cylindrical porous body manufacturing step, quartz glass fine particles are deposited on the outer periphery of the mandrel 53 to form the porous base material layer 9D, and then the mandrel 53 is pulled out from the porous base material layer 9D to form the cylindrical porous body. A material 9C is produced.

  In the next dehydration step, any one of the three environments (conditions) in a predetermined reduced pressure, in an atmosphere of an inert gas and a halogen gas, or in an atmosphere of an inert gas and a halogen compound gas ( Under the condition), the cylindrical porous body 9C is dehydrated.

  Further, in the next sintering step, the dehydrated cylindrical porous body 9C is sintered under reduced pressure until it becomes a translucent glass cylinder 9B in a translucent glass state containing substantially vacuum closed cells. 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 transparent in appearance in a state in which it does not contain closed cells substantially uniformly as a whole except for minute closed cells remaining in some defective portions. Here, the “closed cell” refers to a bubble or space formed inside the translucent glass cylinder 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”.

  In the next core rod insertion step, a rod-shaped core rod 7A having the core layer 3A is inserted into the translucent glass cylinder 9B.

  Further, in the next drawing step, the core rod 7A and the translucent glass cylinder 9B are melted and integrated while applying heat to the translucent glass cylinder 9B in which the core rod 7A is inserted. The optical fiber 51 is produced by drawing the clad layer 9A made of transparent glass.

  According to 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, and the life of the manufacturing equipment can be extended and the process can be omitted. Therefore, 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 view showing a refractive index profile of the optical fiber 51 manufactured in Example 1. FIG. As shown in FIG. 2, the optical fiber 51 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, an optical fiber 51 has a laminated structure formed concentrically in cross section, a core layer 3A is formed along a central axis, and thereafter, a first cladding is formed radially outward from the center. Each of the layers is formed in the order of the layer 5A and the second cladding layer 9A. Note that a coating layer applied to the outside of the second cladding layer 9A is omitted.

  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 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) was 3.4 / 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 manufacturing the core soot 7B by the VAD method of the present embodiment, and a portion of the core rod base material 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 are deposited on the seed rod 11 to 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 clad 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 and reacted to react. A clad layer soot 5B, which will later become the first clad layer 5A, is formed on the outer periphery of 3B. In this way, a core soot 7B composed of a core layer soot 3B and a clad layer soot 5B having a rod shape with a predetermined thickness 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.

-Core rod stretching 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 device 41 shown in FIG. Rod. FIG. 3 is a side view of the electric furnace stretching device 41 showing how the core rod 7A is heated and stretched, and the heating furnace 42 is a longitudinal section. In FIG. 3, 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. Yes. 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 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.

  In addition, the heat source in this process is not restricted to the heating furnace 42, You may use flames, such as an oxyhydrogen flame, or a plasma flame. When contamination of hydroxyl (OH) groups is a problem, an oxyhydrogen flame is generally not desirable, and it is desirable to use an electric furnace or a plasma flame. However, if the clad / core ratio is about 4 times, even an oxyhydrogen flame is not a special problem.

-Cylindrical porous body manufacturing process A mandrel 53 having a diameter of 36 mm made of high-purity alumina or high-purity carbon is inserted into a tube-shaped quartz glass handle 55, and the outer periphery of the mandrel 53 is shown in FIG. In addition, quartz glass fine particles were deposited by using the OVD method to produce a cylindrical porous body 9C having an outer diameter of 300 mm. The cylindrical porous body 9C is a portion that later becomes a translucent glass cylinder 9B, and finally becomes a transparent glass to become the second cladding layer 9A.

FIG. 5 is a schematic diagram for explaining the formation of the porous base material layer 9D by the OVD method, and a portion of the porous base material layer 9D is a longitudinal section. In FIG. 4, 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 on the rotating mandrel 53 and deposited around the mandrel 53. Since the thickness of the synthetic glass fine particle layer deposited at one time is not so thick, it is repeated until the porous base material layer 9D having a sufficient thickness is obtained while reciprocating the burner 31 repeatedly.

The average density of the porous base material layer 9D (that is, the value obtained by subtracting the volume of the mandrel 53 from the total volume divided by the weight of the porous body) was about 0.7 g / cm ³ . The mandrel 53 was pulled out from the porous base material layer 9D formed to have a predetermined thickness, thereby producing a cylindrical porous body 9C having a through hole formed on the central axis.

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の外装表面なす炉体６７と、石英炉心管６２及びヒータ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, this cylindrical porous body 9C is dehydrated and sintered in the dehydration / sintering furnace 61 shown in FIG. The translucent glass cylinder 9B including

In FIG. 6, a dehydration / sintering furnace 61 includes a quartz furnace core tube 62 that is a sealable container made of quartz glass, and an annular heater 63 that is a plurality of heating elements provided around the quartz furnace core tube 62, Furthermore, the quartz furnace core tube 62 and the heater 63 are entirely covered and a furnace body 67 formed on the exterior surface of the dehydration / sintering furnace 61, and a heat insulating material 66 filled between the quartz furnace core tube 62, the heater 63 and the furnace body 67. And have.

A cylindrical porous body 9 </ b> C is installed inside the quartz furnace core tube 62. A gas 34 made of chlorine gas (Cl ₂ ) and nitrogen gas (N ₂ ) is introduced through a gas introduction pipe 68 connected to a handle 55 that supports the cylindrical porous body 9C, and a gas discharge port (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 cylindrical porous body 9 </ b> C is dehydrated and sintered in the quartz furnace core tube 62 to become a translucent glass cylinder 9 </ b> B containing closed cells that are substantially vacuum inside.

At the stage where the dehydration process and the sintering process are completed, the translucent glass cylinder 9B is in a state of containing closed cells 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 cylinder 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, 2.09. Gram / cm ³ .

  In the conventional method of making the porous body completely transparent, first, it is heated once at a temperature of 1200 ° C. or less at which sintering does not proceed and sufficiently dehydrated, and then subjected to high temperature conditions to be transparent. . 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 the temperature and time, the particle size and composition of the fine particles, but the surface of the porous body is more likely to be sintered. As a result of dehydrating and sintering the porous body at various temperatures and heating times, the porous base material region has closed cells substantially isolated from the surrounding atmosphere. The average density is 1 .8g / cm ³ or more, preferably it has been found that it is sufficient 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 11 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 process 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. 11, 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.

-Core rod insertion process Next, the above-mentioned core rod 7A was inserted in the translucent glass cylinder 9B. After completion of the insertion, as shown in FIG. 7, the end of the translucent glass cylinder 9B on the drawing direction side is heated and melted by an oxyhydrogen flame radiated from the burner 35 to melt and seal, and the translucent glass cylinder 9B The end portion and the end portion of the core rod 7A were fused and integrated. The purpose of performing this treatment is that when the translucent glass cylinder 9B is melt-sealed in a drawing furnace, the translucent glass cylinder 9B takes in impurities contained in the atmosphere in the drawing furnace, thereby causing the core rod 7A. In this case, the translucent glass cylinder 9B is melt-sealed before being put into the wire drawing furnace. Further, if the end of the translucent glass cylinder 9B and the end of the core rod 7A are fused and integrated, the time from the start of drawing to the transition to the steady state can be shortened.

  In this embodiment, the oxyhydrogen flame is radiated as a heating method for preliminarily heating and melting the drawing direction side end of the translucent glass cylinder 9B. However, the present invention is not limited to this. For example, it may be a flammable gas flame radiation, a plasma flame radiation, or heating by an electric furnace.

  In addition, the end of the translucent glass cylinder 9B is not melt-sealed after the core rod 7A is inserted, but the end of the translucent glass cylinder 9B on the drawing direction side is heated in advance before insertion as shown in FIG. It may be melt-sealed by fusing. By doing so, impurity contamination can be prevented as in the method shown in FIG. When fusing the translucent glass cylinder 9B, as shown in FIG. 8, an oxyhydrogen flame radiated from the burner 35 while applying a pulling force as indicated by a black arrow at the end of the translucent glass cylinder 9B. To melt by heating.

この段階で、筒状多孔質体９Ｃの平均密度は２．１ｇ／ｃｍ³であり、完全に透明化したガラス体の密度（２．２ｇ／ｃｍ³）の９５％であった。次に、前述のコアロッド７Ａを半透明ガラスシリンダ９Ｂに挿入し、線引き開始側を加熱して溶融封止し、図７に示すような状態とした。 (Example 2)
A cylindrical porous body 9C having a hole on the central axis produced in the same manner as in Example 1 was dehydrated and sintered under the conditions shown in Table 2, and the inside contained a semi-vacuum containing substantially closed cells. It was set as the transparent glass cylinder 9B.

At this stage, the average density of the cylindrical porous body 9C was 2.1 g / cm ³ , which was 95% of the density of the completely transparent glass body (2.2 g / cm ³ ). Next, the aforementioned core rod 7A was inserted into the translucent glass cylinder 9B, and the drawing start side was heated and melt-sealed to obtain a state as shown in FIG.

この段階で、筒状多孔質体９Ｃの平均密度は２．０ｇ／ｃｍ³であり、完全に透明化したガラス体の密度（２．２ｇ／ｃｍ³）の９１％であった。次に、前述のコアロッド７Ａを半透明ガラスシリンダ９Ｂに挿入し、線引き開始側を加熱して溶融封止し、図７に示すような状態とした。 (Example 3)
A cylindrical porous body 9C having a hole on the central axis produced in the same manner as in Example 1 was dehydrated and sintered under the conditions shown in Table 3, and the inside contained a semi-vacuum containing substantially closed cells. It was set as the transparent glass cylinder 9B.

At this stage, the average density of the cylindrical porous body 9C was 2.0 g / cm ³ , which was 91% of the density of the completely transparent glass body (2.2 g / cm ³ ). Next, the aforementioned core rod 7A was inserted into the translucent glass cylinder 9B, and the drawing start side was heated and melt-sealed to obtain a state as shown in FIG.

この段階で、筒状多孔質体９Ｃの平均密度は１．８ｇ／ｃｍ³であり、完全に透明化したガラス体の密度（２．２ｇ／ｃｍ³）の８２％であった。次に、前述のコアロッド７Ａを半透明ガラスシリンダ９Ｂに挿入し、線引き開始側を加熱して溶融封止し、図７に示すような状態とした。 Example 4
A cylindrical cylindrical porous body 9C having a hole on the central axis produced in the same manner as in Example 1 was dehydrated and sintered under the conditions shown in Table 4, and the inside contained a semi-vacuum containing substantially closed cells. It was set as the transparent glass cylinder 9B.

At this stage, the average density of the cylindrical porous body 9C was 1.8 g / cm ³ , which was 82% of the density of the completely transparent glass body (2.2 g / cm ³ ). Next, the aforementioned core rod 7A was inserted into the translucent glass cylinder 9B, and the drawing start side was heated and melt-sealed to obtain a state as shown in FIG.

この段階で、筒状多孔質体９Ｃの平均密度は２．１ｇ／ｃｍ³であり、完全に透明化したガラス体の密度（２．２ｇ／ｃｍ³）の９５％であった。次に、前述のコアロッド７Ａを半透明ガラスシリンダ９Ｂに挿入し、線引き開始側を加熱して溶融封止し、図７に示すような状態とした。 (Comparative Example 1)
A cylindrical porous body 9C having a hole on the central axis produced in the same manner as in Example 1 was dehydrated and sintered under the conditions shown in Table 5, and a translucent glass cylinder 9B containing closed cells inside. did.
In Comparative Example 1, the interior of the quartz core tube 62 is not depressurized in both the dehydration process and the sintering process. In addition, helium gas was used as an inert gas for the sintering process.

At this stage, the average density of the cylindrical porous body 9C was 2.1 g / cm ³ , which was 95% of the density of the completely transparent glass body (2.2 g / cm ³ ). Next, the aforementioned core rod 7A was inserted into the translucent glass cylinder 9B, and the drawing start side was heated and melt-sealed to obtain a state as shown in FIG.

-Drawing process Next, the core rod 7A and the translucent glass cylinder 9B produced in Examples 1 to 4 and Comparative Example 1 are heated to a heating furnace (hereinafter referred to as a drawing furnace) attached to the drawing machine from the melt-sealed portion. Inserting the outer diameter of the core rod 7A and the semitransparent glass cylinder 9B while melting and integrating the core rod 7A and the semitransparent glass cylinder 9B while making the translucent glass layer transparent glass while reducing the space between the core rod 7A and the semi-sintered glass cylinder The glass optical fiber of about 125 μm was drawn. Here, the reduced pressure state was realized by connecting a vacuum suction pump to the open end of the quartz glass handle 55 connected to the translucent glass cylinder 9B and performing suction. In this case, the degree of vacuum was about 100 Pa.

At the time of drawing, two layers of UV curable resin were coated on the outer surface of the optical fiber, and the resin was cured by irradiating with ultraviolet rays, and then wound on a reel through a winding capstan. The inner side of the coating is referred to as the primary layer and the outer side is referred to as the secondary layer. The Young's modulus is selected such that the primary layer is small and the secondary layer is large.
The drawing speed in this example was 2000 m / min.

  Tables 6 and 7 show the results of measuring the incidence of bubble defects during wire drawing and the transmission characteristics of the SMF manufactured under the above conditions.

本実施例の光ファイバは、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ている。
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. 9 is a perspective view showing a schematic configuration of the optical fiber defect detection apparatus. FIG. 10 is an explanatory diagram showing scattered light input to the image sensor of the translucent elongated body defect detection apparatus shown in FIG. 9 and a scattered light intensity distribution pattern obtained based on the scattered light. As shown in FIG. 9, this optical fiber defect detection device continuously irradiates a parallel optical beam 83 from the lateral direction onto a traveling glass optical fiber 51 in an uncoated state immediately after drawing from the optical fiber preform. 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.

In all the optical fibers of this example, 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.

In addition, the optical fibers of this example all have an optical fiber with a loss at 1385 nm of 0.40 dB / km or less, and a sufficiently low hydroxyl (OH) group absorption loss.
Further, the strength of the optical fiber was investigated by winding it on another reel while applying a tension equivalent to about 2% of 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 of the present embodiment has a deterioration in strength due to contamination of contaminants from the heating furnace, which is of particular concern when there are open pores or irregularities on the surface of the translucent glass cylinder 9B. The problem does not occur. That is, the semi-transparent glass cylinder 9B is sintered to such an extent that there are no open bubbles or irregularities that take in contaminants on its surface.
On the other hand, in Comparative Example 1 (SMF (5)) in which helium gas was used in the sintering treatment without dehydrating and sintering the atmosphere of the cylindrical porous body 9C, there was a problem with transmission characteristics. Although there is no, the bubble defect rate is high. This is presumably because helium gas having a high solubility was dissolved in the translucent glass cylinder 9B, and this helium gas was foamed during drawing and a cavity was formed in the optical fiber.

この段階で、多孔質体の平均密度は２．１ｇ／ｃｍ³であり、完全に透明化したガラス体の密度（２．２ｇ／ｃｍ³）の９５％であった。
以下、他の実施例と同様にコアロッド７Ａを半透明ガラスシリンダ９Ｂに挿入し、線引き開始側を加熱して溶融封止した後、溶融封止した部分から線引き炉に挿入し、該コアロッド７Ａと線引き炉半焼結ガラスシリンダとの間の空間を減圧状態にしながら、半透明ガラスシリンダ９Ｂを透明ガラス化しつつ、該コアロッド７Ａと半透明ガラスシリンダ９Ｂとを溶融一体化させながら外径約１２５μｍのガラス光ファイバ５１に線引きした。線引き中にコーティングを施し、線引き後のガラス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。フッ素添加したクラッド部の屈折率は純粋なシリカガラスの屈折率に比べて０．４％低く、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 5)
In this example, fluorine was added to the glass cylinder 9B. When the cylindrical porous body 9C having a hole on the central axis produced in the same manner as in Example 1 was dehydrated and sintered, fluorine was added under the conditions shown in Table 8, and the inside was substantially independent of vacuum. A translucent glass cylinder 9B containing bubbles was obtained.

At this stage, the average density of the porous body was 2.1 g / cm ³ , which was 95% of the density of the completely transparent glass body (2.2 g / cm ³ ).
Thereafter, the core rod 7A is inserted into the translucent glass cylinder 9B in the same manner as in the other examples, and after the drawing start side is heated and melt-sealed, the core rod 7A is inserted into the drawing furnace from the melt-sealed portion. While reducing the space between the drawing furnace semi-sintered glass cylinder and making the translucent glass cylinder 9B transparent glass, the core rod 7A and the translucent glass cylinder 9B are melted and integrated to obtain a glass having an outer diameter of about 125 μm. The optical fiber 51 was drawn. Coating was performed during drawing, and an optical fiber having a coating outer diameter of about 250 μm with no bubbles remaining in the drawn glass optical fiber and no problem in strength was obtained. The refractive index of the cladding portion added with fluorine is 0.4% lower than that of pure silica glass, and it is possible to provide a region having a small refractive index in a part of the cladding layer as in this embodiment. .

  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 which concerns on this invention. 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 part of the core rod base material layer the longitudinal cross-section explaining the preparation process of the core rod base material layer by VAD method. It is the side view which made the longitudinal cross section the part of the heating furnace of the electric furnace extending | stretching apparatus which shows a mode that a core rod base material is heated and extended | stretched. 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 dehydration and sintering furnace used in order to make a cylindrical porous body into a semi-transparent glass cylinder in a dehydration process and a sintering process. It is process drawing which shows a mode that the edge part of the drawing direction side of a semi-transparent glass cylinder is heat-melted with the oxyhydrogen flame radiated | emitted from a burner, and the edge part of a semi-transparent glass cylinder is melt-sealed. It is process drawing which shows a mode that the edge part by the side of the drawing direction of a translucent glass cylinder is heat-melted and the end part of a translucent glass cylinder is melt-sealed prior to insertion of a core rod. 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 rod base material 9A Clad layer 9B Translucent glass cylinder 9C Cylindrical porous body 9D Porous base layer 11 Seed rod 21 Core burner 22 Clad burner 26 Gas 27 Gas 27 Gas 32 gas 34 gas 35 burner 41 electric furnace stretching device 42 heating furnace 42a opening 42b opening 43 upper gripping portion 44 lower gripping portion 45 heater 46 upper chuck 47 lower chuck 48 guide rail 51 glass optical fiber 53 mandrel 55 handle 61 dehydration and sintering Furnace 62 Quartz core tube 63 Heater 65 Vacuum pump 66 Heat insulating material 67 Furnace body 68 Gas introduction tube 83 Parallel beam 84 Forward scattered light 85 Image sensor 86 Signal processing unit 87 Judgment unit 88 Monitor unit 89 Alarm unit 90 Recording part 91 Scattered light intensity distribution pattern

Claims (6)

A method of manufacturing an optical fiber having a core layer and a cladding layer surrounding the core layer,
A cylindrical porous body preparation step of forming a porous base material layer by depositing silica-based glass fine particles on the outer periphery of the mandrel, and then extracting the mandrel from the porous base material layer to prepare a cylindrical porous body; ,
A dehydration step of dehydrating the cylindrical porous body under reduced pressure at least one of an atmosphere of an inert gas and a halogen gas and an atmosphere of an inert gas and a halogen compound gas;
Sintering the dehydrated cylindrical porous body until it becomes a translucent glass cylinder containing closed cells under reduced pressure;
A core rod insertion step of inserting a core rod made of quartz-based glass having the core layer into a rod-like shape into the translucent glass cylinder;
The clad layer in which the core rod and the translucent glass cylinder are fused and integrated while applying heat to the translucent glass cylinder into which the core rod is inserted, and the translucent glass cylinder is made of transparent glass; includes a drawing step of drawing so that the,
The sintering step is performed at a temperature of 1400 ° C. or less and a pressure of 2000 Pa or less as a condition under the reduced pressure inside a furnace core tube made of quartz glass, and the closed cell contained in the translucent glass cylinder An optical fiber manufacturing method, characterized in that the inside is substantially vacuum. The dehydration step and the sintering step, provided the furnace heart pipe within the pressure vessel, a heating furnace in which a plurality of heaters around the core tube, to uniformly heat the entire porous preform layer The method of manufacturing an optical fiber according to claim 1, wherein: The dehydration step is performed at 1300 ° C. or lower,
^3. The light according to claim 1, wherein the sintering step is performed under a condition 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. Fiber manufacturing method. Between the said core rod insertion process and the said drawing process, the drawing direction side edge part of the said semi-transparent glass cylinder which inserted the said core rod is heat-melted, and it is set as the state integrated. The manufacturing method of the optical fiber of any one of Claims. As a heating method when the end portion in the drawing direction of the translucent glass cylinder into which the core rod is inserted is heated and melted in advance, radiation of flammable gas flames such as oxyhydrogen and methane, radiation of plasma flames, and heating by an electric furnace Either is used. The manufacturing method of the optical fiber of Claim 4 characterized by the above-mentioned. The optical fiber manufacturing method according to any one of claims 1 to 5, wherein in the drawing step, at least a space between the core rod and the translucent glass cylinder is reduced.

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