JPH0717395B2 - Manufacturing method of base material for dispersion shift fiber - Google Patents

Description

The present invention relates to a structure of a silica-based optical fiber base material for communication and a method for manufacturing the same, and particularly to a zero dispersion wavelength shifted to a wavelength band of 1.5 μm. The present invention relates to a method suitable for manufacturing a single mode fiber (hereinafter referred to as "dispersion shift fiber").

[Prior Art] A dispersion-shifted fiber in which the zero-dispersion wavelength is shifted to the 1.5 μm band, which is the minimum loss wavelength region in a silica-based optical fiber, has been put to practical use as an optical communication transmission line with a long distance and a large transmission capacity. I'm out. The one having a graded refractive index distribution as shown in FIG. 1-A has a smaller bending loss and a large practical advantage as compared with a dispersion shift fiber having a simple step type refractive index distribution, and its development and study are required. (Reference 1: "Dispersion-Shifted Conveyx-Index Single Mode Fibers" N. Kwaki et al., Electronics Letters 1985
December 5, 21 , No. 25/26, p1186-1187). In the graded refractive index profile shown in FIG. 1-A, a portion 1.1 having the highest refractive index in the central portion (referred to as an inner core) and a portion 1.2 having a lower refractive index than the inner core surrounding the inner core 1.1 (the outer portion) (Hereinafter referred to as the core), and the refractive index distribution is formed from the cladding portion 1.3 surrounding the outer core 1.2 and having the lowest refractive index.

As an example of the dispersion shift fiber having such a graded refractive index distribution, as shown in FIG. 6, the inner core 1.1 is made of GeO ₂ -SiO ₂ , and the glass composition forming the refractive index distribution is
It has been proposed that the outer core 1.2 is made of SiO ₂ and the cladding 1.3 is made of F-SiO ₂ (reference 2: “dispersion.
Shift Tefde Ivors With Fluorine Attended Cladding By The Wave Aperture Axial Deposit Method, H. Yokota and others,
Technical Digest on Topical Meeting on Optical Fiber Comunication (Atlanta, 1986) Paper WF2). With the dispersion shift fiber with this configuration, the transmission loss at 1.55 μm could be reduced to 0.23 dB / km, but it was difficult to further reduce the loss.

[Problems to be solved by the invention]

The refractive index distribution of the optical fiber is generally obtained by adding GeO ₂ as a refractive index increasing component to the SiO ₂ glass of the core. However, if the amount of GeO ₂ added is increased,
The Rayleigh scattering loss of glass increases and the transmission loss increases, or the electron transition absorption in the ultraviolet region, which is considered to be due to the reduction of GeO ₂ → GeO, increases, and the effect is increased to the wavelength range of 1.5 μm. However, there is a problem that the transmission loss becomes high.

The outer core made of SiO ₂ sandwiched between the inner core containing GeO ₂ and the cladding containing F has a higher viscosity at high temperature heating than other parts, and the tension applied during drawing is applied to the outer core. There was a problem in that, when concentrated, defects were generated here, which also caused absorption in the ultraviolet region.

From the above consideration, the inventors of the present invention, as the composition of the glass for obtaining the refractive index distribution of the type shown in FIG. 1-A, use GeO ₂ --SiO ₂ for the inner core and F for the outer core as shown in FIG. 1-B. and -SiO _2, by the a F-SiO ₂ having a low refractive index than the outer core Kuratsudo can take a refractive index difference without increasing the GeO ₂ added amounts, moreover the tension as F-SiO ₂ outer core Since we can prevent concentration, we thought that we could solve the above problems and proceeded with the study. The present invention provides a novel method of manufacturing a base material for a dispersion shift fiber having such a configuration, and realizes the manufacture of a dispersion shift fiber with extremely low loss.

[Means for solving problems]

The present invention forms a Kuratsudo portion made of SiO ₂ with the addition of F to the transparent glass body outer peripheral core having an outer core portion made of SiO ₂ added with the inner core portion and F made of SiO ₂ with the addition of GeO ₂ In the method of manufacturing an optical fiber preform, a plurality of burners for synthesizing glass particles by the VAD method are used to produce GeO ₂
Consisting inner core soot body and pure SiO ₂ made of SiO ₂ was added to prepare a core soot body formed by an outer core soot body, after heating the dehydration process the soot body for the core, F
Of F is added to the outer core soot body by heating the core soot body at a temperature lower than the clearing temperature of the core soot body and then heating at a temperature at which the soot body for core becomes transparent in an atmosphere containing F. At the same time, the soot body for core is made transparent to produce the transparent glass body for core, which is a method for producing a base material for dispersion shift fiber.

In the present invention, the amount of GeO ₂ added to the inner core portion is 0.1 to 0.8% in relative refractive index difference, and the diameter ratio of the inner core diameter and the outer core diameter is 0.2 to 1.0, and in an atmosphere without F. Heating
It is particularly preferable to carry out at a temperature within the range of 1300 to 1600 ° C.

In the present invention, the inner core is GeO ₂ —SiO ₂ , the outer core is F—SiO ₂ ,
The cladding is F-SiO ₂ which has a lower refractive index than the outer core.
-For the purpose of obtaining an optical fiber base material having a refractive index distribution of the type shown in Fig. B, glass fine particles for the core (hereinafter referred to as soot body) are produced by the VAD method, and this is made into a transparent glass to be used for the core. A transparent glass body is formed, and a cladding portion is formed around this.

The present invention is particularly characterized by a method for producing a transparent glass body for a core, using a plurality of burners, a GeO ₂ -SiO ₂ soot body to be an inner core and a pure SiO ₂ soot to be an outer core. First, a composite soot body composed of a body, that is, a soot body for core is formed. That is, the outer core soot body has not yet been added with F at this stage. The conditions at this time may be according to the usual VAD method, and specifically, soot synthesizing burners include, for example, SiCl ₄ , GeCl _{4 and} other glass source gases, such as H ₂ , O _{2 and} other combustion gases and auxiliary combustion gases, For example, A
A soot is generated by flowing an inert gas such as r and He.

Further, in the present invention, the addition amount as GeO ₂ of the inner core is a relative refractive index difference (when the refractive index of the inner core is n ₁ and the refractive index of pure SiO ₂ of the outer core is n ₂ , the relative refractive index is Difference = n ₁ −n ₂ /
n ₂₎ that is in the range 0.1 to 0.8% and the ratio d ₁ / d ₂ of the inner core diameter d ₁ and the outer core diameter d ₂ is manufactured such that 0.2 to 1.0, particularly preferred. The reason for this limitation is that the relative refractive index difference is in the range of 0.1 to 0.8%, which is an essential condition for shifting the zero dispersion wavelength to 1.55 μm in the fiber according to the present invention, and d ₁ / d ₂ is 0.2 to 1.0
This range is limited from the viewpoint of dispersion controllability, and is essential in terms of suppressing bending loss and increasing MFD. It is particularly preferable to satisfy both the refractive index range and the diameter ratio. Conventionally, the relative refractive index difference was set to exceed 0.8% as shown in FIG. 6, but in the present invention, since the outer core is F-SiO ₂ , 0.8% or less is sufficient, and GeO ₂
It is possible to eliminate the influence of the above and the problem of the outer core tension.

The core soot body obtained above is prepared by a known method such as Cl ₂
Gas is dehydrated by heating in an inert gas atmosphere containing a gas having a dehydrating action such as a Cl compound gas. For example, the conditions are heating at 1050 ° C. in a He atmosphere containing 4% by volume of Cl ₂ .

The heat-dehydrated core soot body is first heated in an atmosphere not containing F at a temperature lower than the clearing temperature, for example, in an atmosphere of only He within a temperature range of 1300 to 1600 ° C.
Since the softening point of GeO ₂ --SiO ₂ glass is lower than that of pure SiO ₂ glass, the heat treatment in this temperature range causes the inner core soot body portion made of GeO ₂ --SiO ₂ to shrink and its bulk density to increase sharply. The outer core soot portion of pure SiO ₂ cannot shrink. Therefore, the inner soot body of the core soot body has a considerably higher bulk density than the outer core soot body.

The core soot body in such a state is then heated in an atmosphere containing F, for example, in an atmosphere composed of an F compound gas such as SF ₆ , SiF ₄ , CCl ₂ F ₂ and an inert gas such as He. , F
Add / clear.

Here, the bulk density of the soot (g / cm ³ ) and the amount of F added (△ ^-
%) Has the relationship shown in the chart of FIG. This was obtained from the experimental data by the present inventors in the course of research until now, and as is clear from FIG. 5, when the bulk density of the soot becomes 0.3 g / cm ³ or more, F becomes soot. It turns out that it is difficult to diffuse inside. Almost F at 0.8 g / cm ³ or more
It can also be seen that is not added.

Therefore, in the inner core soot body in which the bulk density rapidly increased due to the heat treatment in the atmosphere containing no F in the previous stage, F did not diffuse even by this F addition treatment, and F was added only to the outer core soot body. Thus, a glass composition structure of GeO ₂ —SiO ₂ inner core / F—SiO ₂ outer core and a refractive index structure of the core portion corresponding to the refractive index distribution of FIG. 1-B can be realized.

Simultaneously with or after the F addition treatment step, heat sintering is performed to obtain a transparent glass body for a core. If the heating temperature is set to about 1650 ° C. in the same atmosphere as when F is added, and F is added and the material is made transparent at the same time, the process becomes simple.

Forming a Kuratsudo consisting F-SiO ₂ on the outer periphery of the transparent glass body for obtained core above. Specifically, i) a method in which a glass tube for a cladding and a transparent glass body for a core made of F-SiO ₂ separately prepared by a known technique are united and solidified by heating, ii) a transparent glass body for a core As a starting material,
After forming a cladded soot body by sooting to the outer periphery thereof, a method of adding F to the cladded soot body to make it transparent in the same manner as described above, or any of the methods,
Alternatively, both may be combined. In the method (i), it is preferable to perform solidification in an atmosphere containing at least a halogen gas such as Cl ₂ , SOCl ₂ , CCl ₄ or the like because a fiber with less loss increase can be obtained.

The present invention uses a plurality of burners to perform sooting of the core portion at once, and then adds F to this and then sinters it, thereby forming a GeO ₂ —SiO ₂ inner core / F—SiO ₂ outer core structure in a simple process. The cost reduction effect can be expected in that the core material can be manufactured. A method of directly attaching SiO ₂ soot to the GeO ₂ —SiO ₂ glass rod is also conceivable. However, in this method, H is diffused from the oxyhydrogen flame into the rod, and removal of water is easy even in the subsequent dehydration treatment. No, it is not possible to reduce the transmission loss.

Then, in the present invention, by adding F to the outer core and lowering the refractive index thereof, the amount of GeO ₂ added to the inner core for increasing the refractive index of the inner core can be relatively reduced. As a result, in addition to reducing the effects of Rayleigh scattering loss and absorption loss in the ultraviolet region due to GeO ₂ , the problem of concentration of tension on the outer core can be solved, so a base material for dispersion-shifted optical fibers with excellent transmission characteristics can be used. Can be manufactured.

〔Example〕

A soot body for a core was produced with the structure shown in FIG. 2.
1 is a burner for synthesizing glass fine particles for inner core (called burner for inner core), 2.2 is a burner for synthesizing glass fine particle for outer core (called burner for outer core), burner for inner core 2.1 is GeCl ₄ , SiCl ₄ , H ₂ , O ₂ and an inert gas were supplied to hydrolyze GeCl ₄ and SiCl ₄ in an oxyhydrogen flame to generate SiO ₂ glass fine particles containing GeO ₂ , and an inner core was formed on the tip of the starting material 4. The soot body 1 for use is deposited. The starting material 3 is rotated and is pulled upward as the inner core soot body grows. Meanwhile, the outer core burner
SiCl ₄ , H ₂ , O ₂ and an inert gas are supplied to 2.2, and the outer core soot body 2 made of SiO ₂ fine particles is formed so as to surround the inner core soot body 1.

In this embodiment, the outer core burner 2.2 has SiCl ₄ 800 cc./min,
_{H 2 14l / min, O 2} 7l / min, flowing AR2L / min, to 2.1 to the burner for inner core _{H 2 3.5l / min, O 2} 7l / min, SiCl 4 270cc / mi
By supplying n, GeCl ₄ 8 cc / min and Ar ₂ l / min, a core soot body having an outer diameter of 100 mmφ (inner diameter core diameter of 30 mmφ) and a length of 450 mm was obtained at a pulling rate of 50 mm / hr.

The core soot body is first inserted into a furnace having a ring-shaped carbon heater and heated to 1050 ° C., and the atmosphere in the furnace is set to Cl ₂ /
A heat dehydration treatment was performed with He = 4/100. Next, the soot body for core was heat-treated in a He atmosphere at 1450 ° C. to shrink the soot for core. Finally, heat to 1650 ° C and change the atmosphere in the furnace.
SiF ₄ / He = 1/100 was added to the soot shrinkage body for the core, and F was added at the same time to form a transparent glass. The relationship between the refractive index distribution and the element concentration (Ge, F) distribution of the obtained transparent glass body for a core is summarized in FIG.

On the other hand, the soot body for the cladding made of pure SiO ₂ produced by the VAD method was dehydrated under the condition of Cl ₂ / He = 4/100 and 1050 ° C.
A transparent glass body for cladding was produced by heating at 1350 ° C. at iF ₄ / He = 2/100, adding the soot body for cladding, and then heating at 1650 ° C. in the same atmosphere to carry out a clearing treatment. A glass base material having a core / clad is formed by perforating the central part of the transparent glass body for cladding, inserting the glass body for core obtained in the above into the inner space of the glass body for cladding, and heating and integrating both. Obtained.

Further, the glass base material obtained above was stretched, and pure SiO ₂ soot body was deposited on the glass base material by the VAD method, followed by heat dehydration and F addition clarification treatment to obtain a preform. The refractive index distribution of the obtained preform is shown in FIG. 1-B. The outer diameter of the preform is 70 mm and the inner core diameter is 1.
It was 6 mmφ and the outer core diameter was 4.5 mmφ.

After stretching this preform to an outer diameter of 23 mmφ, an outer diameter of 125
It was spun to a diameter of μmφ.

In FIG. 4, the transmission loss spectrum of the dispersion shift fiber obtained by the present invention is shown by the solid line a, and as a comparative example, the spectrum of the conventional fiber having the structure of FIG. 6 is shown by the broken line b.

As is clear from the comparison between FIG. 1-B and FIG. 6, the fiber of the present invention has an increase of 0.5% in refractive index due to GeO ₂ from the pure SiO ₂ refractive index level, whereas that of the conventional fiber is 0.8%. %.

Further, as is clear from FIG. 4, the transmission loss (b) of the conventional fiber is 0.22 dB / km at a wavelength of 1.55 μm, which is a relatively low loss, but that of the fiber of the present invention (a) is a wavelength of 1.55 μm.
A low loss of 0.20 dB / km has been achieved.

〔The invention's effect〕

As described above, the present invention prevents diffusion of F into the inner core made of GeO ₂ —SiO _{2 as} much as possible, and by adding F to the outer core, GeO ₂ —SiO ₂ inner core / F—SiO. ₂ outer core / F
Can be realized a structure of -SiO ₂ Kuratsudo, since take a sufficiently relative refractive index difference by reducing the GeO ₂ content, ultraviolet absorption due to the Rayleigh scattering loss and GeO ₂ due to GeO ₂ is reduced This is a method that is extremely effective in reducing the loss of the dispersion shift fiber, because it can be reduced and the concentration of tension on the outer core can be suppressed to prevent an increase in bending loss. Further, in the present invention, the manufacturing process can be reduced and the economical effect is large as compared with the conventional method for obtaining the same fiber structure.

[Brief description of drawings]

FIG. 1-A is a diagram showing the refractive index distribution of a step-type dispersion shift fiber base material, and FIG. 1-B is a diagram showing the refractive index distribution and glass composition structure of the dispersion shift fiber base material according to the present invention. FIG. 2 is a diagram showing an embodiment of making a soot body for a core in the present invention, and FIG. 3 is a refractive index distribution and element concentration (Ge, F) of a transparent glass body for a core produced in an example of the present invention. diagram showing the relationship between distribution, Fig. 4 shows compares the transmission loss spectrum of the dispersion Shifutofuaiba and conventional product according to the invention Fig, Fig. 5 F amount (△ ^-) and soot bulk density ([rho ), And FIG. 6 is a view showing the refractive index distribution and the glass composition structure of the conventional base material for dispersion shift fibers.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Gotaro Tanaka, No. 1 Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Industries, Ltd. Yokohama Works (56) Reference JP-A-62-108746 (JP, A) Kai 61-31324 (JP, A) JP-A-63-222042 (JP, A)

Claims (3)

[Claims] 1. A form Kuratsudo portion made of SiO ₂ with the addition of F to the transparent glass body outer peripheral core having an outer core portion made of SiO ₂ added with the inner core portion and F made of SiO ₂ with the addition of GeO ₂ In the method for producing optical fiber preform by
By using multiple burners for synthesizing glass particles by method D, G
Inner core soot body consisting of SiO ₂ with eO ₂ added and pure SiO 2
_A core soot body having an outer core soot body consisting of ₂ is produced, and the core soot body is heated and dehydrated, and then heated at a temperature lower than the clearing temperature of the core soot body in an atmosphere in which F does not exist, Then, by heating in an atmosphere containing F at a temperature at which the soot body for core becomes transparent, F is added to the outer core soot body and the soot body for core is made transparent to produce the transparent glass body for core. A method for manufacturing a base material for a dispersion shift fiber, which comprises: _2. The amount of GeO ₂ added to the inner core portion is 0.1 to 0.8% in terms of relative refractive index difference, and the diameter ratio of the inner core diameter and the outer core diameter is 0.2 to 1.0. A method for producing a base material for a dispersion shift fiber according to the item. 3. Heating in an atmosphere free of F is 1300 to 16
The method for producing a base material for a dispersion shift fiber according to claim 1, which is carried out at a temperature within the range of 00 ° C.