JPH0453823B2 - - Google Patents

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to the structure of a base material for a quartz-based optical fiber for communications and a method for manufacturing the same, and particularly relates to a
The present invention relates to a structure of a base material suitable for manufacturing a single mode fiber (hereinafter referred to as "dispersion shifted fiber") with a zero dispersion wavelength shifted to the 1.5 μm band, and a method for manufacturing the same.

[Conventional technology]

Dispersion-shifted fibers, in which the zero-dispersion wavelength is shifted to the 1.5 μm band, which is the lowest loss wavelength region of silica-based optical fibers, are being put into practical use as long-distance, high-capacity optical communication transmission lines.

Among dispersion-shifted fibers, those with a step-type refractive index distribution as shown in Figure 1 have lower bending loss than dispersion-shifted fibers with a simple step-type refractive index distribution, and are considered for development due to their great practical advantages. (Reference 1: “Dispersion - Shifted Combinations -
“Index Single Mode Fibers (Dispersion Shifted Step Type Graded Index Single Mode Fiber)”, N. Kwaki et al., Electronics Letters, December 5, 1985, Vol. 21 No. 25/26, 1186-
1187 pages).

In the stepped refractive index distribution shown in FIG. 1, a central portion 1.1 having the highest refractive index (referred to as the inner core) and a portion 1 surrounding the inner core 1.1 having a lower refractive index than the inner core.・2 (referred to as outer core),
Further, a refractive index distribution structure is formed from cladding parts 1 and 3 having the lowest refractive index surrounding the outer cores 1 and 2.

Regarding the dispersion shifted fiber having such a stepped refractive index distribution, as shown in FIG. 6, the glass composition forming the refractive index distribution is GeO ₂ -SiO ₂ for the inner core 6. _SiO2 ,
It has been proposed that the cladding portions 6 and 3 are made of F-SiO ₂ [Reference 2: "Dispersion - Shifted Fibers with Fluorine Attached Cladding by the
VAD Method (Dispersion-Shifted Fiber with Fluorine-doped Cladding by Vapor-phase Axial Mounting)”, H. Yokota et al., Technical Digest on Topical Meeting on Optical Fiber Communication (Atlanta, 1986), Heper WF2].

The refractive index distribution of optical fiber is similar to that of SiO ₂ glass.
It is most commonly obtained by adding GeO ₂ as a refractive index increasing component. however,
When the amount of GeO ₂ added is increased, Raysor scattering of the glass increases and the transmission loss increases, or GeO ₂ →
Electronic transition absorption in the ultraviolet region, which is thought to be based on the reduction of GeO, increases, and its influence extends to the 1.5 μm band, which is the wavelength range used, and transmission loss also increases. Therefore, in the above composition, F is applied to the cladding parts 6 and 3.
By adding GeO 2 to lower the refractive index of the cladding portion, and adding GeO ₂ only to the inner cores 6 and 1, the amount of GeO ₂ added is lowered in an attempt to reduce transmission loss. Based on this idea, the present inventors have manufactured a dispersion-shifted fiber having the refractive index distribution and composition shown in Figure 6, and were able to reduce the transmission loss at a wavelength of 1.55 μm to 0.23 dB/Km. (Reference 3: Shigematsu et al., “Transmission characteristics of 1.5 μm band dispersion-shifted single mode fiber”, Institute of Electronics and Communication Engineers technical research report OQE86-99). In addition, in Fig. 6, the inner core diameter C ₁ is 3 μm, the outer core diameter C ₂ is 9 μm,
The fiber outer diameter C ₃ is 125 μm. In addition, in the figure below, the relative refractive index difference (△n) is the refractive index of pure SiO ₂ .
It is expressed as n=o. Regarding each numerical value, those shown in FIG. 6 or above are merely examples, and the present invention is not limited to these.

[Problem that the invention seeks to solve]

As mentioned above, in the dispersion-shifted fiber with a stepped refractive index distribution consisting of GeO ₂ -SiO ₂ in the inner core, SiO ₂ in the outer core, and F-SiO ₂ in the cladding part, the dispersion shift fiber has a dispersion shift of 0.23 dB/Km at a wavelength of 1.55 μm. Although transmission loss has been achieved, it has been difficult to further reduce the loss. The present invention has been made in view of this current situation, and an object of the present invention is to provide a new base material structure and a method for manufacturing the same, which can manufacture a step-type refractive index distribution dispersion shifted fiber with lower loss.

[Means and actions for solving problems]

As a result of repeated research, the present inventors found that the inner core of the present invention is a means to solve the above problems.
Provided is a base material for an optical fiber having a stepped refractive index distribution, characterized in that the outer core is made of F- _{SiO2 ,} and the _cladding part is made of F- _SiO2 .

Furthermore, in producing the above-mentioned base material of the present invention,
After creating a soot body for a core having an inner core made of SiO ₂ doped with GeO ₂ and an outer core made of pure SiO ₂ using a plurality of burners for synthesizing glass fine particles by the VAD method, the soot body for the core was F
A step of creating a transparent glass body for the core by heating in an atmosphere containing soot for the core, adding F to the soot body for the core, and performing a heating dehydration treatment and a heating transparency treatment; A transparent glass body for cladding is created by heating a soot body for cladding made of SiO ₂ in an atmosphere containing F, adding F to the soot body for cladding, and performing heating dehydration treatment and heating transparency treatment. After that, the method comprises the steps of: drilling a hole in the center of the transparent glass body for the cladding; and inserting the core glass body into the hollow part of the glass body for the cladding, and heating and integrating the two. The inner core is GeO ₂ −F−SiO ₂ and the outer core is F−
A method for manufacturing an optical fiber base material having a stepped refractive index distribution made of SiO ₂ and a cladding portion of F-SiO ₂ is provided.

The refractive index distribution of the optical fiber preform of the present invention is step-shaped as shown in FIG. 1, and the composition of the preform of the present invention is such that _the inner cores ₁ . The outer cores 1 and ₂ are made of F-SiO2, and the cladding parts 1 and 3, which have a lower refractive index than the outer core, are made of F-SiO2.
It is made of _SiO2 . FIG. 3 is a diagram showing the refractive index distribution and glass composition of a glass base material for a dispersion-shifted fiber obtained in an example of the present invention to be described later.
In the figure, 3.1 is the inner core, 3.2 is the outer core,
3.3 is the cladding part.

Before explaining that the base material structure of the present invention enables further reduction in loss, consideration will be given as to why it was difficult to further reduce loss in the conventional structure shown in FIG. 6 described above. In other words, the transmission loss deterioration factor of the conventional structure shown in Figure 6 is that due to the GeO ₂ contained in the inner core, tetravalent Ge is usually reduced and changed to a divalent state during the high temperature heating process such as wire drawing. However, the outer core made of SiO ₂ sandwiched between the inner core containing GeO 2 and the cladding containing F becomes the absorption center for electronic transitions that have absorption in the ultraviolet region, and its influence extends to the wavelength band _{of 1.5} μm. The core part has a higher viscosity than other parts during the high-temperature heating process during wire drawing, so the tension applied during wire drawing concentrates on the outer core part, causing defects in the outer core part. , it is thought that this may also cause absorption in the ultraviolet region.

Based on the above considerations, it was considered that adding F of the present invention to both the inner core and the outer core is more effective in reducing loss. In other words, for the above transmission loss deterioration factor, by allowing F to coexist in the part of the inner core where GeO ₂ is added, even if Ge is reduced, further Ge-F bonds are created, so that divalent It can be expected that the ultraviolet absorption characteristic of Ge can be reduced, or that the Ge-F bond itself can suppress the reduction of Ge. Also, regarding the factors,
By adding F to the outer core, its viscosity can be lowered to approach that of the inner core and cladding, thereby reducing absorption in the ultraviolet region. Furthermore, since the refractive index of the inner core decreases by adding F to the inner core, it is necessary to lower the refractive index of the outer core in order to compensate for this decrease without increasing the amount of GeO ₂ added. , Also for this purpose, it is necessary to add F to the outer core.

However, in order to maintain the required refractive index difference between the outer core and the cladding, when F is added to the outer core, it is necessary to increase the amount of F added to the cladding to further reduce the refractive index of the cladding. There is.

Based on the above considerations, the base material of the present invention was manufactured and its properties were investigated as an optical fiber, and it was confirmed that a reduction in transmission loss, which had been difficult in the past, could be achieved.

Note that the VAD method (Vaporphase
Axial Deposition Method (vapor phase axial deposition method) is a method commonly used in the _production of optical fiber base materials. Gas, combustion auxiliary gas such as O ₂ , and frit gas such as SiCl ₄ are supplied, and a flame hydrolysis reaction occurs in the flame of the burner to generate glass particles, and the glass particles are rotated near the tip of the starting material. As the glass fine particle deposit grows, the starting material is moved in the axial direction to form a glass fine particle deposit in the axial direction, and then the glass fine particle deposit is subjected to heating dehydration treatment and heating transparentization treatment. This is a method to obtain a transparent glass body.

The base material of the present invention is a soot body produced by the VAD method.
Addition, dehydration, and clarification to form a glass body.
A glass body for the core and a glass body for the cladding corresponding to each composition are obtained, and the glass body for the cladding is made into a glass pipe for the cladding by drilling holes.
It can be obtained by stretching the core glass body and/or the cladding glass pipe as necessary, and then heating and integrating the core glass body inserted into the hollow part of the cladding glass pipe. can.

A specific method for manufacturing the optical fiber base material of the present invention will be described in detail in the following examples, but this is merely an example and is not limited thereto.

〔Example〕

Example (1) Creation of a transparent glass body for a core A soot body for a core was created with the configuration shown in FIG. 4.1 is a burner for synthesizing glass particles for the inner core (referred to as the burner for the inner core);
4.2 is a burner for synthesizing glass particles for the outer core (referred to as an outer core burner), and the inner core burners 4 and 1 contain GeCl ₄ , SiCl ₄ , H ₂ ,
O ₂ and inert gas are supplied, and GeCl ₄ and SiCl ₄ are reacted in an acid/hydrogen flame to contain GeO ₂
SiO ₂ glass particles are generated and a soot body for the inner core is deposited on the tip of the starting material 5. The starting material 5 is rotated and pulled upward in accordance with the growth of the soot body for the inner core. On the other hand, SiO ₂ , H ₂ , O ₂ , and inert gas are supplied to the burners 4 and 2 for the outer core, and an outer core soot body made of SiO ₂ glass particles is formed to surround the soot body for the inner core. go. In this example, H ₂ 3.0/min and O ₂ 10 were applied to the inner core burners 4 and 1.
/min, SiCl ₄ 85c.c./min, GeCl ₄ 4.2cc/min,
Supply Ar3.5/min, H ₂ 80/min, O ₂ 5.0/min, SiCl ₄ 300 to burners 4 and 2 for the outer core.
By supplying cc/min and Ar2/min, a core soot body with an outer diameter of 80 mmφ (inner core diameter of 25 mmφ) and a length of 500 mm was obtained at a pulling speed of 50 mm/hour.

This core soot body was first inserted into a furnace equipped with a ring-shaped carbon heater, heated to 1050° C., and heated and dehydrated with the atmosphere in the furnace set to Cl ₂ :He=3:100. Next, the soot body for the core is heated to 1200°C, and the atmosphere inside the furnace is changed to SiF ₄ :He=
F was added to the soot body for the core at a ratio of 5:1000, and finally in an atmosphere of SiF ₄ :He = 5:1000.
Transparent vitrification was achieved by heating to 1600°C. As a result, the outer diameter is 35mmφ, and the inner core diameter is 12mmφ.
A transparent glass body for a core was obtained. FIG. 2 shows the refractive index distribution of this transparent glass body for the core. In Figure 2, 2.1 is the inner core equivalent part 2,2
are the parts corresponding to the outer core, and a ₁ and a ₂ are 12 mm and 35 mm, respectively.
It is.

The transparent glass body for the core obtained in this way was heated to about 1900℃ in an electric resistance furnace, and the diameter was 3.8℃.
It was stretched to mmφ. In this case, heating with a flame containing an OH component such as an acid/hydrogen burner during stretching is not preferable because the surface of the transparent glass body for the core will be contaminated with OH groups and the transmission loss after fiberization will be significantly degraded.

(2) Preparation of transparent glass body for cladding A soot body for cladding consisting only of SiO ₂ was prepared by the VAD method using one burner for synthesizing glass fine particles. _H2 30/ for burner
min, O ₂ 25/min, Ar15/min, SiCl ₄ 1600c.c./
A soot body for cladding with an outer diameter of 110 mmφ and a length of 550 mm was obtained.

This soot body for cladding is Cl ₂ :He=3:
SiF ₄ :He=
F addition treatment was carried out by heating to 1200° C. in an atmosphere of 8:100 ratio, and transparent vitrification was performed by heating to 1600° C. in an atmosphere of SiF ₄ :He=8:100.
As a result, a cylindrical transparent glass body for a cladding with an outer diameter of 50 mmφ and a length of 270 mm was obtained. Using an ultrasonic perforator, a hole of 8mm is made in the center of the transparent glass body for the cladding.
After drilling a φ hole and making it into a pipe shape, it was stretched to 22 mmφ (at this time, the inner diameter was approximately 3.5 mmφ). Next, by extending SF ₆ inside the transparent glass body for the cladding and heating it from the outside with an acid/hydrogen burner, the inner surface was heated to an inner diameter of approximately 7 mmφ.
Gas etched until This gas etching eliminated the scratches and irregularities that occurred on the inner surface during drilling, resulting in a smooth inner surface.

(3) Integration of the transparent glass body for the core and the transparent glass body for the cladding The transparent glass body for the core obtained in (1) above (3.8
mmφ) is inserted into the hollow part of the pipe-shaped glass body for cladding (outer diameter 22 mmφ, inner diameter 7 mmφ) obtained in (2) above, and the surface temperature of the cladding glass body is heated to 1700 to 1800℃ using an acid/hydrogen burner from the outside. By heating to a certain degree, the glass body for the cladding was contracted, and the inner wall of the glass body for the cladding and the surface of the glass body for the core were fused and integrated. The refractive index distribution of the glass base material of the present invention thus obtained is shown in FIG. In this case, the inner core diameter b ₁ is 1.3 mmφ, and the outer core diameter b ₂ is 3.8 mm.
φ and the outer diameter _b3 of the glass base material were 19 mmφ.

(4) Fiberization After stretching the glass base material obtained in the above (3) to an outer diameter of 16 mmφ, a soot body consisting of SiO ₂ was deposited on the glass base material using a VAD device, and then
A preform with an outer diameter of 55 mmφ was obtained by performing the same heating dehydration, F addition, and transparency treatment as in case (2). At this time, the central glass base material was also compressed by the contraction force of the soot body, and its diameter increased to approximately 21 mmφ. This preform was drawn to an outer diameter of 25 mmφ and then spun to an outer diameter of 125 μmφ.

(5) Transmission loss characteristics and comparison with conventional fiber Part 5
In the figure, the transmission loss spectrum of the dispersion-shifted fiber according to the present invention obtained in the above (4) is shown by the solid line A, and the spectrum of the conventional fiber shown in FIG. 6 as a comparative example is shown by the broken line B. As is clear from FIGS. 3 and 6, the relative refractive index difference between the fiber of the present invention and the conventional fiber is the same: 0.65% between the inner core and the outer core, and 0.2% between the outer core and the cladding. However, in the product of the present invention, F is added to the inner core and outer core, and F is also added to the cladding part.
Since the amount added has been increased, it is between 0.45 and -0.4%, while the conventional product is between 0.65 and -0.2%.
As is clear from Figure 5, the transmission loss of the conventional fiber is relatively low at 0.25 dB/Km at a wavelength of 1.55 μm, but that of the fiber of the present invention is as low as 0.205 dB/Km at a wavelength of 1.55 μm. Low loss has been achieved. Moreover, the difference in transmission loss between A and B widens as the wavelength range gets shorter. This fact indicates that the structure of the present invention can reduce absorption in the ultraviolet region.

[Effects of the Invention] As explained above, the optical fiber base material of the present invention can reduce the transmission loss deterioration factor that has absorption in the ultraviolet region by adding F to the inner core and outer core, so that step-type dispersion can be achieved. When used as a base material for shift fibers, it is very effective in reducing the loss of the fibers.

Furthermore, in the method for manufacturing an optical fiber base material of the present invention, the inner core is made of GeO ₂ -F-SiO ₂ and the outer core is made of F-
It is possible to realize the base material of the present invention consisting of SiO ₂ and F-SiO ₂ in the cladding part, and when the base material is drawn by this manufacturing method, a step-shaped refractive index distribution fiber of very low loss and high quality can be obtained. It is very effective in manufacturing shifted fibers.

[Brief explanation of drawings]

FIG. 1 is a diagram explaining the stepped refractive index distribution of the present invention and the conventional optical fiber base material, and FIGS.
The figure is a diagram showing the refractive index distribution of a transparent glass body for a core obtained in an example of the present invention and a glass base material of the present invention. FIG. 4 is an explanatory diagram of a method for producing a core soot body in an embodiment of the present invention, FIG. 5 is a diagram comparing the transmission loss spectra of a fiber according to the present invention and a conventional product, and FIG. 6 is a diagram of a conventional dispersion-shifted fiber. FIG. 2 is a diagram showing the refractive index distribution of

Claims (1)

[Claims] 1. The inner core is made of GeO ₂ -F-SiO ₂ and the outer core is made of F.
_- A base material for an optical fiber having a stepped refractive index distribution, characterized in that the cladding part is made of -SiO2 and F- _SiO2 . 2 After creating a soot body for a core having an inner core made of SiO _{2 doped with GeO 2 and} an outer core made of pure SiO ₂ using a plurality of burners for synthesizing glass fine particles by the VAD method, the soot body for the core is A step of creating a transparent glass body for the core by heating in an atmosphere containing F, adding F to the soot body for the core, and performing a heating dehydration treatment and a heating transparency treatment, and a step of creating a transparent glass body for the core by VAD method. A transparent glass body for a cladding is obtained by heating a soot body for a cladding made of pure SiO ₂ in an atmosphere containing F, adding F to the soot body for a cladding, and performing a heating dehydration treatment and a heating transparentization treatment. After creating the transparent glass body for the cladding, the method includes the steps of drilling a hole in the center of the transparent glass body for the cladding, and inserting the glass body for the core into the hollow part of the glass body for the cladding and heating and integrating the two. Characteristic: The inner core is GeO ₂ −F−SiO ₂ and the outer core is F−.
A method for manufacturing an optical fiber base material having a stepped refractive index distribution made of SiO ₂ and a cladding portion of F-SiO ₂ .