JPH0948629A - Optical fiber and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical fiber capable of preventing residual stress in wire drawing and giving a fiber having excellent transmission characteristics. SOLUTION: This optical fiber is composed of a core having high refractive index and a clad existing at an outer periphery of the core and having a refractive index lower than the core. The core has a central region wherein at least an amount of GeO2 dope decreases from central part to an outer periphery and an amount of fluorine dope decreases from the outer periphery to center in an at least a part of the region from the outer periphery to inside of the central region, then glass viscosity in the fiber cross section is almost uniformly adjusted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber mainly used for communication and having excellent transmission loss, and a method for manufacturing the same, more specifically, an optical fiber having a substantially uniform glass viscosity in a cross section orthogonal to the fiber axis and a method for manufacturing the same. It is about the method.

[0002]

_2. Description of the Related Art Conventionally, in an optical fiber for communication, a core which is a light transmission region is doped with a dopant such as GeO ₂ , Al ₂ O ₃ or the like which enhances a refractive index, or a cladding located on the outer periphery of the core. The waveguide structure is formed by changing the refractive index of the glass so that the refractive index of the core becomes relatively higher than that of the cladding by doping a dopant such as fluorine or B ₂ O ₃ which lowers the refractive index. These differences in glass composition make the glass viscosity in the cross section of the optical fiber non-uniform and thus affect the transmission loss of the optical fiber. That is, since the glass viscosity depends on the content of the dopant, for example, when the core is GeO ₂ -doped glass and the clad is pure silica glass, the viscosity of the core is lower than that of the clad and the softening point temperature is lowered. For this reason, in the melting / small-diameter fiber forming process, the clad is solidified earlier when the fiber is formed. On the other hand, when the core is pure silica and the clad is fluorine-doped glass, the viscosity of the clad is lower than that of the core, and the core first solidifies during drawing. As described above, when the glass viscosity is non-uniform, the solidification rate at the time of fiber formation is different, and a compressive or tensile stress remains between the non-uniform layers. When this residual stress becomes large, the refractive index of the glass is subtly changed to affect the transmission characteristics, and the generation of defects in the glass is promoted, leading to an increase in transmission loss.
As a method for solving this problem, a viscosity matching fiber in which the glass viscosity in the fiber is substantially uniform is disclosed in Japanese Patent Laid-Open No. 5-30.
1736. This publication discloses a method of adjusting the glass viscosity and adjusting the refractive index by appropriately adjusting the dopant GeO ₂ for increasing the refractive index of the glass and the dopant fluorine (F) for decreasing the refractive index of the glass.

[0003]

The above-described viscosity matching fiber is good in the case of a so-called step type refractive index profile in which GeO ₂ is uniformly doped in the radial direction.
When the eO ₂ doping amount has a distribution in the radial direction, the viscosities cannot be perfectly matched, and there is a problem that only the average viscosities can be matched. Another problem is that uniform doping of GeO ₂ is difficult in the vapor phase synthesis method. Normally, fluorine is doped into the porous glass base material in the sintering process, so that it is possible to dope relatively uniformly, but GeO ₂ is supplied together with the glass raw material during the gas phase synthesis, and the doping amount There is a strong tendency for the amount of doping to increase in the central part as the value increases. This is because the doping amount of GeO ₂ depends on the synthesis temperature in the vapor phase synthesis method, and it is devised to keep the temperature of the entire synthesis region uniform. However, as the doping amount of GeO ₂ is increased, the concentration of the Ge raw material stream in the raw material stream in the burner flame for synthesizing glass is less likely to be constant,
It becomes difficult to dope uniformly. In such a situation, the glass viscosity cannot be made completely uniform, and when the GeO ₂ doping amount of the core has a distribution, the average viscosity or the viscosity of the central portion can be matched with that of the outer peripheral portion. There wasn't. In particular, it was not possible to match the viscosities at the outer peripheral portion where the influence of the viscosity was considered to be the greatest. The present invention solves such a problem, and an optical fiber structure capable of effectively matching the viscosity of the core part with the viscosity of the clad part and the like even if the distribution of the doping amount exists in GeO _2. It is an object of the present invention to provide a manufacturing method, and thereby to provide an optical fiber having a good transmission characteristic and a manufacturing method thereof.

[0004]

As a means for solving the above problems, the present invention provides an optical fiber comprising a core having a high refractive index and a clad located on the outer periphery of the core and having a lower refractive index than the core. The core has at least a central region where the GeO ₂ doping amount decreases from the central portion toward the outer periphery, and the fluorine doping amount decreases from the outer periphery toward the center in at least a part of the inner region from the outer periphery toward the inner portion. Provided is an optical fiber characterized in that the glass viscosity in the cross section of the fiber is adjusted to be substantially uniform by having a distribution. Further, the present invention provides an optical fiber comprising a core having a high refractive index and a clad located on the outer periphery of the core and having a lower refractive index than the core, wherein the core has at least a GeO ₂ doping amount decreasing from the central portion toward the outer periphery. A region having a distribution in which the fluorine doping amount decreases from the outer periphery toward the center in at least a part of the region extending inward from the outer periphery of the central region, and the core has the maximum refractive index of the central region on the outer periphery of the central region. The outer peripheral region has a refractive index lower than the refractive index, and in the outer peripheral region, the doping amounts of GeO ₂ and fluorine are substantially uniform in the radial direction, so that the glass viscosity in the fiber cross section is adjusted to be substantially uniform. An optical fiber is provided. Furthermore, the present invention provides an optical fiber comprising a core having a high refractive index and a clad positioned on the outer circumference of the core and having a lower refractive index than the core, wherein the core has a GeO ₂ doping amount that decreases at least from the central portion toward the outer circumference. The glass viscosity in the cross section of the fiber is adjusted to be substantially uniform by having a distribution in which the fluorine doping amount decreases from the outer circumference toward the center in at least a part of the area from the outer circumference to the inner side of the central area. In the production of the optical fiber, the core of the central region is synthesized by (1) a step of synthesizing a porous glass preform having a composition distribution such that the GeO ₂ doping amount becomes high in the central portion by a vapor phase synthesis method, And (2)
After the porous glass base material is heated and contracted in an heating furnace in an inert gas atmosphere, it is heated in an inert gas atmosphere containing a fluorine raw material to be transparent, and the above method is provided. I will provide a. In the method of the present invention, the temperature at which the furnace atmosphere is changed to an inert gas atmosphere containing a fluorine raw material is 1250 to 1500 ° C. as a particularly preferred embodiment. Further, in the method of the present invention, a step of heating the porous glass base material in an inert gas atmosphere to shrink the porous glass base material in the heating furnace, and then heating the porous glass base material in an inert gas atmosphere containing a fluorine raw material to make it transparent. , The porous glass base material is made transparent while raising the temperature in a soaking furnace that can heat the entire length of the porous base material to a uniform temperature, and the atmosphere in the furnace at this time is initially an inert gas atmosphere and the temperature is raised. A particularly preferable embodiment is the above-mentioned method, which is characterized in that it is a step of forming an inert gas atmosphere containing a fluorine raw material from the middle of the step. In the method of the present invention,
A particularly preferred embodiment is the above method characterized in that the heating furnace is a zone furnace.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The structure of the optical fiber of the present invention will be described with reference to specific examples. FIG. 1A shows a composition distribution for making the glass viscosity uniform in the core having a roughly parabolic GeO ₂ doping distribution. The fluorine doping amount is adjusted to be high in the outer periphery and low in the center (indicated by the dotted line) in response to the GeO ₂ doping amount (indicated by the solid line) that increases in the center. Like this GeO
By adjusting the composition of ₂ and F in the radial direction, it is possible to adjust the viscosity in the glass. The refractive index distribution at this time becomes a distribution as shown in FIG.

Usually, the effect of decreasing the glass viscosity is that F is about three times as effective as GeO ₂ in terms of the added amount (specific refractive index difference ratio). For example, the viscosity of GeO ₂ added by 0.3% in terms of relative refractive index difference is the same as that of F added by 0.1% in terms of relative refractive index difference. Therefore, in order to match the glass viscosities of the core center and the outer periphery, it is sufficient to dope F with about 1/3 of the GeO _{2 at} the center (converted to the relative refractive index difference). As described above, even when the refractive index difference in the core has a distribution in which the center is high and the outer circumference is low, it is possible to uniformly adjust the glass viscosity, and the solidification rate during fiberization is It is possible to obtain a high-quality optical fiber because it can be made uniform within a cross section perpendicular to the axial direction. The relationship between the weight ratio of the added amount and the relative refractive index difference is approximately as follows. GeO ₂ : When the amount of Ge added is 17% by weight, the relative refractive index difference with respect to quartz (pure SiO ₂ ) is 1%. F: When the amount of F added is 1% by weight, the relative refractive index difference with respect to quartz is -0.33%.

FIG. 2 shows the refractive index distribution structure of a fiber in which the glass viscosities are matched at least at the center and the outer circumference of the fiber, even if the glass viscosities in the core do not completely match. In the case of FIG. 2, the center and outer circumference of the core can have the same viscosity as the clad formed outside, so that the solidification rate is almost the same as that of the clad at the outer circumference of the core, especially at the interface during fiberization. Therefore, it is possible to obtain the effect of reducing the factor of deterioration of transmission characteristics.

[0008] FIG. 3 is one in which the core showed a composition of viscosity uniform when two layers of less outer peripheral cores central core amount is larger doped with GeO ₂ doping amount of GeO _2. The solid line shows the GeO ₂ distribution and the chain line shows the fluorine (F) distribution. In the central core, for the roughly parabolic GeO ₂ distribution, F
The density is set to be high at the outer circumference and low at the center. With such a composition, the glass viscosity in the central core can be adjusted uniformly.

Next, a fluorine doping method for uniformly adjusting the glass viscosity of the porous glass base material having a composition distribution in which the GeO ₂ concentration is high in the center and low in the outer periphery will be described. Fluorine is generally doped by being heated in a fluorine (fluorine compound) -containing atmosphere in the heating / clearing process of the porous glass base material. The step of doping fluorine into the porous glass base material by sintering can be divided into three steps: diffusion / penetration into the porous glass base material, reaction / fixation with glass, and volatilization at high temperature. Volatilization is a reverse reaction of the reaction of reacting with the glass and being taken into the glass, and if the concentration of the fluorine raw material in the atmosphere gas at the time of transparency is kept constant, the reaction reaches equilibrium and the volatilization reaction can be suppressed. . On the other hand, diffusion into glass is caused by the bulk density of porous glass (a parameter indicating how particles are clogged) or the porosity (the ratio of pores surrounded by particles).
Have a close relationship with. That is, the fluorine source gas contained in the atmosphere permeates into the base material through the gaps between the particles of the porous glass base material. Therefore, it is difficult for the fluorine raw material to penetrate into the base material that has a low porosity and almost no gap,
It cannot be doped with fluorine.

From this, it is possible to adjust the permeation of fluorine in the radial direction by decreasing the porosity of the central portion of the porous glass base material and gradually increasing the porosity of the outer circumference, Fluorine distribution can be attached. As such a method, for example, in JP-A-60-161347, a layer having a high dopant concentration or a high bulk density (a layer having a low porosity) is formed on the outer peripheral portion of the core so that fluorine can penetrate into the core. A method of preventing this and doping only the clad with fluorine has been proposed. Further, in JP-A-62-182129, the bulk density is gradually reduced from the core center to the outer periphery, so that the concentration increases from the core center to the outer periphery. There has been proposed a method for increasing the fluorine concentration. These methods utilize the fact that the penetration of fluorine changes depending on the bulk density, and although they are effective methods, they have the problem that the bulk density (porosity) must be adjusted during gas phase synthesis. It is very difficult and not easy to adjust the GeO ₂ concentration distribution and also change the bulk density.

Therefore, in the present invention, it was considered to utilize the change in bulk density when the porous glass base material contracts by the heat treatment for making it transparent. The shrinkage of the porous glass base material depends on the temperature and the GeO ₂ doping concentration. At the same temperature, the shrinkage is faster as the GeO ₂ concentration is higher. Therefore, in the porous glass base material having a high GeO ₂ concentration in the center, contraction occurs early in the center. In the present invention, after starting the heat treatment for clearing, the introduction of the fluorine raw material is started at the time when the shrinkage progresses moderately and the bulk density distribution is formed corresponding to the distribution of GeO _2. It is possible to realize appropriate penetration of fluorine. The means for adding fluorine during this sintering step in the present invention is hereinafter referred to as fluorine-doped sintering. According to the fluorine-doped sintering,
Fluorine does not penetrate to the center, and doping with high fluorine concentration can be realized in the outer peripheral portion. Moreover, the contraction is G
Because it depends on the eO ₂ concentration, GeO ₂
There is an advantage that fluorine doping can be performed according to the distribution.

The fluorine doping according to the present invention can be easily carried out by changing the heating temperature and the composition of the atmospheric gas while heating the porous glass base material in a soaking furnace as shown in FIG. In FIG. 4, 1 is a porous glass base material, 2 is a core tube, 3 is a heater, 4 is a gas inlet, and 5 is a gas outlet. When the temperature of the porous base material in the configuration of FIG. 4 is increased under the temperature conditions shown in the temperature and atmosphere condition graphs of FIG. 5, the porous base material starts to shrink according to the temperature. At this time, the internal bulk density distribution is high at the center and low at the outer peripheral portion according to the GeO ₂ distribution. A desired fluorine concentration can be realized by adding a fluorine raw material into the atmosphere when the shrinkage has progressed to an appropriate point. If the temperature to add the fluorine raw material is low,
Fluorine penetrates to the center, and when the temperature is high, only the outer peripheral portion can be doped with fluorine. Therefore, if the temperature is too low, the distribution cannot be provided and the distribution becomes uniform.

According to the results of various experiments conducted by the present inventors,
It was found that when the temperature was lower than 1250 ° C, fluorine was uniformly doped, and when the temperature was lower than 1300 ° C, the central portion was also slightly doped with fluorine. Further, it has been found that even if the temperature exceeds 1500 ° C., fluorine is not doped even in the outer periphery. In order to effectively perform viscosity matching, it is preferable to add fluorine at a temperature lower than 1450 ° C.

The atmosphere at the start of heating is He gas,
An inert gas atmosphere such as Ar gas is preferable. The fluorine source gas to be charged is preferably SiF ₄ , SF ₆ , CF _4, etc., and the fluorine source gas concentration is 0.01 to 100%.
(Volume ratio) and can be applied in a wide range.

Further, the preferable range of the bulk density at the start of heating of the porous base material of the present invention is 0.1 to 0.5 g / c.
c, the preferable range of the bulk density at the start of fluorine addition is 0.3 to 1.0 g / cc.

In the case of a soaking furnace, of course, the internal contraction affects not only the temperature range but also the temperature rising rate, and the distribution of fluorine can be adjusted by adjusting the temperature rising rate. If the rate of temperature rise is high, the internal temperature is unlikely to rise, so it has the effect of suppressing shrinkage, and if it is slow, it has the effect of promoting shrinkage. If the rate of temperature rise is too fast, internal contraction does not proceed and fluorine easily penetrates to the center, which is not preferable. It is preferably 10 ° C./minute or less. In consideration of obtaining stable shrinkage, 8 ° C./min or less is more preferable. There is a problem that it is too late from the viewpoint of productivity
It can be said that the range of / minute or more is preferable.

The present invention can also be implemented in a zone furnace having a short heater. In this case, first, 125
After allowing the shrinkage to proceed once at an appropriate temperature of 0 ° C to 1500 ° C, and then allowing fluorine to penetrate into the porous glass base material,
This is a method of making transparent at high temperature. In this case, the shape of the fluorine concentration distribution can be adjusted by the degree of shrinkage of the porous glass base material, that is, the shrinking temperature.

As a method for forming a composition distribution, as shown in FIG. 3, a central core portion having a high refractive index, an outer peripheral core portion on the outer periphery of the central core portion having a lower refractive index and a cladding layer outside the outer peripheral core portion is formed. First, the central core portion is manufactured by the above-described method of the present invention, the sintered glass is once stretched, and then the outside is further subjected to a vapor phase synthesis method (for example, VAD method, OVD method).
Method) to form a porous layer of the outer peripheral core portion containing GeO ₂ ,
It is possible to manufacture by a multi-step process in which fluorine-doped sintering is performed, then the cladding is formed by a vapor phase synthesis method, and further fluorine-doped sintering is performed. Such a complex refractive index distribution structure can also be manufactured by repeating the method of the present invention. Although the method of forming the porous glass layer on the outer periphery of the glass sintered body after stretching is illustrated, a method of synthesizing the porous glass layer on the outer side of the glass sintered body without stretching and performing similar processing is naturally also possible. It is possible.

Further, the central core, the outer peripheral core and the clad are manufactured separately, and the outer peripheral core and the clad are processed by forming holes such as pipes, and then assembled by rod incolap. A method of going is also possible. Furthermore, instead of processing into a pipe, it is possible to create a pipe-shaped base material from the beginning.

Generally, the raw material used in the VAD method is SiCl ₄ as a glass raw material and GeC as a dopant raw material.
l ₄ is used, but not the effect of the present invention varies with the type of these materials. Therefore, for example, Al ₂ O ₃ , B ₂
The present invention can be applied to a base material doped with O ₃ or the like. Further, as the fluorine raw material, SiF ₄ , SF ₆ ,
CF ₄ or the like is used. The fluorine raw material is the same as the raw material of the VAD method. The GeO ₂ addition amount range is as follows.
0 to 1.5 in terms of relative refractive index difference with pure silica (SiO ₂ ).
%, And the addition amount range of fluorine is similarly about 0 to -0.8% in terms of the relative refractive index difference from pure silica as a desirable range in the present invention.

Further, in the present invention, dehydration treatment can be performed prior to the fluorine-doped sintering. The dehydration treatment in the present invention may be a general method according to a known technique in this kind of technical field. Specifically, in the case of a soaking furnace, Cl ₂ , SiCl ₄ , CCl ₄ , in an inert gas atmosphere such as He, SOCl ₂
1000 ℃ ~ 1200 ℃ in an atmosphere containing dehydrating gas such as
It is heated for about 0.5 to 2 hours. In the case of a zone furnace, the porous glass base material is fed into a heat zone heated to about 1000 ° C to 1200 ° C at a feeding rate of about 2 to 20 mm / min in the same atmosphere.

[0022]

【Example】

 [Example 1] SiO₂As the main component, and GeO at the center₂Concentration
High and GeO on the periphery₂GeO with low concentration₂Dark
90 mm outer diameter with a squared distribution of degrees and a bulk density of 0.2
A 0 g / cc porous glass base material was prepared by the VAD method.
Was. At this time, GeO₂The maximum value of the relative refractive index difference is 0.6
The raw material concentration was adjusted to be%. The porous glass mother
The material is placed in a soaking furnace as shown in FIG.₂including
He atmosphere (Cl ₂: He = 1: 50) from 500 ° C
Heating up at a rate of 8 ° C / min up to 1100 ° C
After watering, the shrinkage proceeded while raising the temperature at 5 ° C./min. 135
When it reaches 0 ° C, SiF is used as a fluorine raw material._FourThrow in
did. SiF_FourRegarding the concentration, fluorine (F) has a relative refractive index in the outer peripheral portion.
The difference was adjusted to be 0.2% (SiF_FourAs concentration
Is 3%). As it is, raise the temperature to 1550 ° C and hold for 20 minutes.
After holding it, the temperature was lowered. As a result, a good transparent sintered body (gas
A lath base material) was obtained. Perform elemental analysis of the glass base material
Where is GeO₂Is a square distribution type with 0.6% at the center
And F has a distribution as shown in FIG.
The relative refractive index difference is -0.2% at the outermost circumference,
It turned out that it was gradually decreasing. Also fluorine
The permeation range of (F) is from the center with the outer diameter ratio r / R.
Confirmed to be doped up to 0.2
Was.

Example 2 A glass preform was prepared under the same conditions as in Example 1 except that the same porous preform as in Example 1 was used and the temperature of introducing the fluorine raw material was changed to 1300 ° C. As a result, a good transparent sintered body was obtained. When the elemental analysis of this glass base material was performed, GeO ₂ showed a distribution similar to that of Example 1 in a square distribution with 0.6% at the center. It was confirmed that the fluorine-doped range was from the center to 0.1 at the outer diameter ratio r / R.

Example 3 A glass base material was prepared under the same conditions as in Example 1 except that the same porous base material as in Example 1 was used and the temperature of the fluorine raw material was changed to 1400 ° C. A unity was obtained. Elemental analysis of this glass base material showed that GeO ₂ had a square distribution of 0.6% at the center and a distribution similar to that of Example 1, but the range doped with fluorine had an outside diameter ratio of It was confirmed that r / R was doped up to 0.6 from the center.

From the results of Examples 1 to 3 above, it was confirmed that the permeation degree of fluorine can be controlled by the method of the present invention.

[Examples 4 and 5] With the same constitution as in Example 1, the glass base material was prepared at two input temperatures of 1240 ° C and 1520 ° C for the fluorine raw material. As a result, a good transparent sintered body was obtained, and elemental analysis of this glass base material was performed.
Although the distribution of GeO ₂ does not change, fluorine is uniformly doped in the entire base material at 1240 ° C. (Example 4), 1
No fluorine doping at 520 ° C (Example 5)
I understand.

[Example 1] Using the sintered body prepared in Example 1 as a core preform, an optical fiber whose viscosity was matched was prepared as follows. The viscosity was confirmed by measuring the residual stress as described later. First, after stretching the sintered body to 15 mm, a porous glass preform for clad made of SiO ₂ was synthesized on the outer periphery of the stretched body by the VAD method, and then this preform was inserted into a soaking furnace. After dehydration under the same conditions as in No. 1, the temperature was raised to 1200 ° C.
After a He gas atmosphere (SiF ₄ concentration 3%) containing iF _4, the temperature was raised up to 1550 ° C. at a heating rate of 5 ° C. / min and vitrified. By performing the fluorine-doped sintering in this way, an optical fiber preform having a clad doped with F by -0.2% with a relative refractive index difference was prepared. When this was drawn into a fiber with an outer diameter of 125 μm and its characteristics were evaluated, the loss at a wavelength of 1.55 μm was 0.198 dB.
A fiber having excellent characteristics of / km was obtained.

[Example 2] Further, when the sintered body obtained in Example 2 was used as a core base material and a fiber was formed in the same manner as in Example 1 to evaluate the loss, a wavelength of 1.55 µm was obtained.
It was possible to obtain a good loss value of 0.199 dB / km, which was almost the same as that of the fiber of Example 1.

Example 3 Using the same method as in Example 1, G
The maximum value of eO ₂ is 0.83% in relative refractive index difference, and a GeO ₂ distribution having a square distribution is created.
The content was adjusted to 0.27% and fluorine-doped sintering was performed to obtain a sintered body. The fluorine doping method is the same method as in Example 1
Only the F ₄ concentration was set to 3.5%. An optical fiber of the type shown in FIG. 3 in which the viscosities were matched was prepared by the following steps using this sintered body as the central core preform. Separately from the central core base material, a porous glass base material in which GeO ₂ was uniformly doped by 0.18% with a relative refractive index difference was synthesized by the VAD method.
After dehydration in He atmosphere containing Cl ₂ at ℃, SiF
_In a He atmosphere containing ₄ (SiF ₄ concentration 3.5%), heat treatment was performed at 1250 ° C. so that F was uniformly doped with −0.23%, and then transparent vitrification was performed at 1550 ° C. This sintered body was processed into a pipe shape by punching, and then the central core preform was drawn and rod-in collapsed to prepare an intermediate preform having a central core and an outer peripheral core. After stretching the intermediate preform to 15 mm, to synthesize a porous glass layer of pure silica by the VAD method in this periphery, including the SiF ₄ which He
Fluorine-doped sintering was performed in an atmosphere ((SiF ₄ concentration 3.5%) to form a clad doped with F of −0.27% to obtain a preform for an optical fiber. The preform was drawn and obtained. When the transmission characteristics of the obtained optical fiber were evaluated, the loss at the wavelength of 1.55 μm was 0.19.
It had excellent characteristics of 8 dB / km.

The viscosity in the cross section of the fiber of the present invention is determined by measuring the residual stress in the cross section of the fiber.
I confirmed that they are almost consistent. This residual stress was measured by the photoelastic CT method. The fiber according to the conventional method has a fluctuation of about 200 MPa, but the fiber according to the present invention has a fluctuation of about 20 MPa, and it is considered that the viscosities are substantially matched.

Example 4 As in Example 1, a porous glass base material prepared so that the maximum value of GeO ₂ was 0.6% in relative refractive index difference was inserted into a zone furnace and the inside of the furnace was changed. atmosphere containing Cl ₂ while satisfying with _{(Cl 2:: He = 1} 50), 1
The base material was sent to the heat zone kept at 100 ° C. at a speed of 15 mm / min for dehydration. Next, change the atmosphere in the furnace to H
e The temperature of the heat zone was raised to 1300 ° C. as an atmosphere, and 1
After shrinking by feeding the base material at a speed of 0 mm / min, the furnace atmosphere is a He atmosphere containing SiF ₄ (SiF 4
₍₄ concentration 3%), the heat zone was heated to 1500 ° C., and the base material was fed at a speed of 8 mm / min to form a transparent glass. As a result, F has a relative refractive index difference of −0.2% at the outermost circumference, and it is confirmed that the range in which fluorine penetrates is doped from the center to 0.3 at the outer diameter ratio r / R. Was done. When this sintered body was used as a core and made into a fiber by the same method as in Example 1, the transmission characteristics were good at 0.198 dB / km at a wavelength of 1.55 μm.

[Comparative Example] A porous base material having a GeO ₂ distribution in which the maximum value of GeO ₂ is 0.83% in terms of relative refractive index difference and a square distribution of GeO ₂ is prepared, and no fluorine raw material is added thereto Dehydration and transparent vitrification were carried out in the same manner as in Example 1. as a result,
A sintered body having a GeO ₂ distribution with a squared distribution, to which fluorine was not added, was obtained. An optical fiber was obtained by the same method as in Example 3 using this sintered body as the central core. As a result, the transmission characteristics of the obtained optical fiber were large at a loss of 0.220 dB / km at a wavelength of 1.55 μm.

[0033]

As described above, according to the present invention, the fluorine concentration is high in the outer peripheral portion and the fluorine concentration in the central portion so that the glass viscosity is substantially constant in the core base material having the GeO ₂ concentration distribution of the square distribution type. Can be doped with fluorine so that the glass fiber has a low refractive index, and even if the optical fiber has a distribution with a high refractive index at the center, the glass viscosity can be made uniform, and an optical fiber with excellent low loss characteristics can be realized. .

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a relationship between a dopant amount distribution configuration (a) and a refractive index distribution (b) in a radial direction of a core portion in one specific example of the optical fiber of the present invention.

FIG. 2 is a schematic diagram showing a relationship between a dopant amount distribution configuration (a) and a refractive index distribution (b) in a radial direction of a core portion in another specific example of the optical fiber of the present invention.

FIG. 3 shows a central core portion, an outer peripheral core portion and a cladding portion in still another embodiment of the optical fiber of the present invention.

FIG. 4 is a schematic explanatory view of one embodiment of the method of the present invention.

FIG. 5 is a graph showing temperature rise and atmospheric conditions after soaking the porous glass base material in one embodiment of the present invention.

[Explanation of symbols]

1 porous glass base material, 2 core tube, 3 heater,
4 gas inlets, 5 gas outlets.

 ─────────────────────────────────────────────────── ─── Continued front page (72) Inventor Motonobu Nakamura 1 Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Industries, Ltd. Yokohama Works (72) Inventor, Shoji Ohashi 1-6, Uchisaiwai-cho, Chiyoda-ku, Tokyo No. Japan Nippon Telegraph and Telephone Corporation (72) Inventor Katsusuke Tajima 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Japan Nippon Telegraph and Telephone Corporation

Claims (6)

[Claims] 1. An optical fiber comprising a core having a high refractive index and a clad located on the outer periphery of the core and having a lower refractive index than the core, wherein the core has at least a GeO ₂ doping amount decreasing from the central portion toward the outer periphery. The glass viscosity in the cross section of the fiber is adjusted to be substantially uniform by having a distribution in which the fluorine doping amount decreases from the outer circumference toward the center in at least a part of the area from the outer circumference to the inner side of the central area. An optical fiber characterized by 2. An optical fiber comprising a core having a high refractive index and a clad located on the outer periphery of the core and having a lower refractive index than the core, wherein the core has at least a GeO ₂ doping amount decreasing from the central portion toward the outer periphery. A region having a distribution in which the fluorine doping amount decreases from the outer periphery toward the center in at least a part of the region extending inward from the outer periphery of the central region, and the core has the maximum refractive index of the central region on the outer periphery of the central region. The outer peripheral region has a refractive index lower than the refractive index, and in the outer peripheral region, the doping amounts of GeO ₂ and fluorine are substantially uniform in the radial direction, so that the glass viscosity in the fiber cross section is adjusted to be substantially uniform. An optical fiber characterized by 3. An optical fiber comprising a core having a high refractive index and a clad located on the outer periphery of the core and having a lower refractive index than the core, wherein the core has at least a GeO ₂ doping amount decreasing from the central portion toward the outer periphery. The glass viscosity in the cross section of the fiber is adjusted to be substantially uniform by having a distribution in which the fluorine doping amount decreases from the outer circumference toward the center in at least a part of the area from the outer circumference to the inner side of the central area. In the manufacture of optical fiber
The core of the central region is synthesized by (1) the vapor phase synthesis method of GeO
_{Step 2} doping amount to synthesize a porous glass base material having a high the compositional distribution center, and (2) the porous glass preform is heated in an inert gas atmosphere is contracted in a heating furnace And then heating in an inert gas atmosphere containing a fluorine raw material to make it transparent, and a method for producing an optical fiber. 4. The temperature at which the atmosphere in the heating furnace is changed to an inert gas atmosphere containing a fluorine raw material is 1250 to 1500.
4. The method for manufacturing an optical fiber according to claim 3, wherein the temperature is in degrees Celsius. 5. The step of heating the porous glass base material in the heating furnace in an inert gas atmosphere to shrink the porous glass base material and then heating the porous glass base material in an inert gas atmosphere containing a fluorine raw material to make it transparent is porous. The porous glass base material is made transparent while the temperature is raised in a soaking furnace that can heat the entire length of the base material substantially uniformly, and the atmosphere in the furnace at this time is initially an inert gas atmosphere, The method of manufacturing an optical fiber according to claim 3 or 4, wherein is a step of setting an atmosphere of an inert gas containing a fluorine raw material. 6. The method for producing an optical fiber according to claim 3, wherein the heating furnace is a zone furnace.