JP2000203860A - Large-sized quartz glass reform - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a large-size quartz glass preform of high accuracy that can attain the mass production of optical fibers of high quality at low cost. SOLUTION: A high-purity natural quartz glass tube or a synthetic quartz glass tube having the outer diameter of 50-300 mm, the ratio of the outer diameter (Do) to the inner diameter (Di) of 1.1-7, the wall thickness of >=10 mm and the wall thickness error of <=2% and the inner surface roughness of <=20 μm and the core glass rod for optical fiber are integrated by fusing and the refractive index of the high-purity natural quartz glass tube or the synthetic quartz glass tube is adjusted to <=0.02% to the designed value. In addition, the difference in the refractive index is <=±0.01 on the boundary between the high-purity natural quartz glass tube or the synthetic quartz glass tube and the core glass rod for optical fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a large-sized quartz glass preform for an optical fiber which has excellent transmission characteristics and can be mass-produced and reduced in cost, and more specifically, a quartz glass tube and an optical fiber core for a single mode. The present invention relates to a large quartz glass preform in which a glass rod is welded and integrated.

[0002]

2. Description of the Related Art In recent years, with the practical use of optical fibers, particularly single mode optical fibers, a large amount of optical fibers have been used. The main production methods include the VAD method (gas-phase shaft attaching method), the OVD method (external method), and the MCVD method (internal method). Most of them are occupied. However,
It is expected that the range of use of optical fibers will be expanded from long-distance trunk lines to general subscriber systems, and it is expected that a larger amount of optical fibers will be required in the future. Alone is considered to have reached the limit both in terms of productivity and cost.

Research on silica glass-based optical fibers has already been performed for 20 years, and transmission characteristics and practical reliability have already been studied to the maximum. Therefore, mass production and cost reduction can be performed while maintaining these characteristics. It is difficult to develop new possible manufacturing methods.

One of the ways to achieve mass productivity and cost reduction is to increase the size of the preform and reduce it to 300 m / min.
It is considered that mass production and cost reduction can be obtained by drawing at a high speed as described above and increasing productivity per apparatus, and at the same time, cost reduction by preventing evaluation cost and random size can be expected. However, since the above three methods were started from a small laboratory scale and were examined with emphasis on characteristics, they were excellent in optical fiber characteristics, but had problems in mass productivity and cost reduction, and had a problem in one process. The fiber length that can be manufactured by reforming is 15 to 30 km in the MCVD method, VAD method,
In the OVD method, the limit is 100 km to 200 km.

Certainly, the above three manufacturing methods are suitable for manufacturing a transmission section of an optical fiber, but simultaneously manufacturing a clad section is a method which is never suitable in terms of mass productivity and cost reduction. Absent. For example, in a graded-index fiber or a single-mode fiber, a clad occupying 80% or more of the cross-sectional area of an optical fiber is manufactured by another method capable of reducing the cost with high efficiency, and combining it with the above three manufacturing methods. It is considered to be an excellent manufacturing method if it is used. For example, it is already practiced to attach a clad portion on a core glass rod made by the VAD method by the OVD method and use it as a core glass rod for an optical fiber. Have been. However, in this conventional method, a thin and short core glass rod is used, so that the synthetic adhesion efficiency of the clad is low, and since each core glass rod is synthesized, there is a limit in mass productivity and cost reduction.

As a result of further study of the above-mentioned conventional method, the present inventors separated the core glass rod and the clad portion, and made the core glass portion by the conventionally known high-performance core glass manufacturing method. The clad portion was independently formed by another efficient method, and it was thought that the above-mentioned problems could be solved by combining the clad portions, and it was concluded that the rod-in-tube method was optimal.

However, the conventional rod-in-tube method has a problem. First, there was a problem with the dimensions of the quartz glass tube. Conventionally, the size of a quartz tube used is a small diameter (outer diameter 15 to 30 mmφ, thickness 1 to 6 mm),
The dimensional accuracy varied about 10% in outer diameter and 20 to 30% in thickness. When the core glass rod was inserted into such a tube by the rod-in-tube method, a clearance of several mm was required for the purpose of preventing contact with the inner wall of the glass tube, depending on the length, thickness and skill. In this way, the small diameter, large dimensional error of the tube, and the need for wide clearance overlap, and the eccentricity occurs in the preform integrated by the rod-in-tube method. The rod-in-tube method has been a manufacturing method that has no merit, as it appears as a large eccentricity of the fiber, and especially when a coupling loss is assumed in a single-mode fiber multi-core connection work.

On the other hand, even if the core glass rod that is rod-in into the quartz glass tube is made under the same conditions, the characteristics vary.
The characteristics also vary depending on fiber specifications, user characteristics, and manufacturing methods. To cope with such conditions, high-precision quartz glass tubes of various dimensions are required. Making each of these high-precision quartz glass tubes of various dimensions by mechanical grinding etc. was excellent in dimensional accuracy, but required a lot of work time, mass production and cost reduction were difficult, and When it is produced by the method, mass productivity and cost reduction can be achieved, but if the dimensional accuracy of the original tube is poor, it is greatly amplified at the time of heating and stretching, and it has been difficult to accurately produce a quartz glass tube having a target size.

In addition to the above-mentioned problems, the rod-in-tube method has a drawback that foreign matters are mixed into the fused surface between the inner surface of the quartz glass tube and the outer surface of the core glass rod and bubbles are generated. Although this depends on the atmosphere and cleaning method when the rod-in-tube method is performed, there is also a problem in the inner surface finishing of the quartz glass tube.

[0010]

The inventors of the present invention have conducted intensive studies on these problems, and as a result, in order to improve and increase the size of the above three methods which are currently used, a quartz glass tube having a large diameter and a large thickness is required. Is formed and fused with a core glass rod for an optical fiber by a rod intube method to form a large preform. The quality such as the eccentricity of a single mode optical fiber is good, It has been found that cost reduction is simultaneously satisfied. The problems with the rod-in-tube method are that a large-sized quartz glass ingot or a tubular body can be used to use a high-precision large-scale industrial machine. For example, a core drill hole of a drilling machine (trade name, manufactured by Ueda Giken) or the like can be used. Super-precision machining technology (edited by Ultra-Precision Processing Research Group, Industrial Research Committee, page 421, 1
984), the inner and outer surfaces are mechanically ground, drilled and polished with a precision honing device, etc., finished to precise dimensional accuracy, etched with hydrofluoric acid to remove surface contamination, and roughened cut surfaces. By reducing the processing strain, it is possible to obtain a substantially high-precision large quartz glass tube, which can be solved by combining this with a core glass rod for an optical fiber and fusing and integrating it. 300 in one large preform
It has been discovered that high-quality optical fibers of 0 km or more can be easily manufactured continuously. In particular, the best characteristics can be obtained when synthetic quartz glass capable of controlling the refractive index by removing foreign substances and impurities in quartz glass and dehydrating the material is used as a raw material. The present invention has been completed based on these findings.

An object of the present invention is to provide a large-sized quartz glass preform which is excellent in mass productivity and can manufacture an optical fiber at low cost.

Another object of the present invention is to provide a high-precision large-size quartz glass preform capable of producing a high-quality optical fiber.

[0013]

The present invention for achieving the above object has an outer diameter of 50 to 300 mm and a ratio of the outer diameter to the inner diameter of 1.1.
7, a high purity natural quartz glass tube or a synthetic quartz glass tube having a thickness of 10 mm or more, a thickness error of 2% or less, and an inner surface roughness of 20 μm or less, and a core glass rod for an optical fiber are integrally welded; The refractive index of the high-purity natural quartz glass tube or the synthetic quartz glass tube is adjusted within 0.02% with respect to a design value, and the high-purity natural quartz glass tube or the synthetic quartz glass tube and the core for the optical fiber are used. The present invention relates to a large quartz glass preform, wherein the refractive index step at the boundary between the glass rod and the cladding layer is ± 0.01 or less.

Here, the terms used in the present specification will be defined. 1) "Quartz glass tube" is a general term for tubular quartz glass, and in the present invention, includes a natural quartz glass tube or a synthetic quartz glass tube made for a rod tube.

2) The "quartz glass base material" is a long cylindrical ingot or a large cylindrical quartz glass made of a high-purity natural quartz glass or a synthetic quartz glass, which is roughly ground as required. This refers to quartz glass before being processed to the target dimensions. In particular, synthetic quartz glass includes those having OH group control and refractive index (n) control in accordance with the quality design of the optical fiber.

3) The term “quartz glass raw tube” refers to a processing method in which a cylindrical quartz glass preform is mechanically ground and opened with a core drilling machine or the like, or a carbon drill is press-fitted under heating. (Hereinafter referred to as the “carbon drill press-fitting method”) to make a tube or a large quartz glass base material in the form of a tube, and to roughly mechanically roughen the outer surface or inner and outer surface of the tube, and to grind it to almost the target dimensions. It refers to a quartz glass tube.

4) "Quartz glass raw tube" means that the inner and outer diameters of the raw quartz glass tube are accurately determined, the thickness error is set to 2% or less, and the inner and outer surfaces are polished and etched with hydrofluoric acid. Refers to a large quartz glass tube having an inner surface roughness of 20 μm or less.

5) The "thickness error" means that a large quartz glass tube having a predetermined length is, for example, five points or more in the longitudinal direction.
The tube is rotated at intervals of 0 to 100 mm, and the value of the following expression when the maximum value (t _max. ) And the minimum value (t _min. ) Of the tube thickness (t) at that position are defined as: [(t _{max .} -T _min. ) / {(T _max. + T _min. ) / 2}]
× 100 (%) means the value of the maximum value in the total length expressed in%.

6) "Core glass rod for optical fiber"
Is a transmission part of light, which is composed of a core part and an optical cladding part, and is used for public communication such as single mode and multi-mode for high quality. In addition, it refers to a glass rod in which a synthetic clad by the OVD method and / or a quartz glass tube is jacketed thereon, and a fiber that meets the standard cannot be obtained just by drawing it alone.

The large-sized high-purity natural quartz glass tube or synthetic quartz glass tube is made of quartz glass satisfying quality characteristics required for a general optical fiber clad tube, and has a large outer diameter of about 50 to 300 mm. It is a quartz glass tube.
The large diameter and thickening can reduce the dimensional error of the quartz glass tube, and the optical fiber obtained by drawing a large preform made by the rod-in-tube method using it can reduce the eccentricity and mass-produce it. It is effective for cost reduction. The larger the outer diameter, the larger the outer diameter / inner diameter, the thicker the pipe, the greater the absolute value, and the processing accuracy can be increased. However, if the outer diameter or the outer diameter / inner diameter is small, the error increases, and the MCVD closer to the core High precision is required for small-diameter, thin-walled tubes for reaction tubes and multiple jackets.

FIG. 1 is a cross-sectional view of a large quartz glass tube used in the present invention, and a large quartz glass preform of the present invention.
A cross-sectional view of the system is shown in FIG. In Figure 1, D _o is the outer diameter of the large quartz glass tube, D _i is the inner diameter of the large-sized quartz glass tube, also 1 in 2 core glass rod, 2 a cladding layer, 3 is coated large quartz glass tube is there. An optical fiber obtained by drawing the large quartz glass preform,
For example, a schematic cross-sectional view of a single-mode fiber is shown in FIG. In FIG. 3, 4 is a core, 5 is an optical cladding,
6 indicates over-cladding, "a" is a core diameter (d _core ), "b" is an optical cladding diameter (d _cladi ),
“C” indicates the outer diameter (d _clado ) of the optical fiber of 125 μm. FIG. 4 shows a conceptual diagram of a refractive index distribution and a power distribution of a single mode fiber. In FIG. 4, the optical cladding is located outside the core diameter (d _core ), and the power distribution of light is widened. Therefore, the optical cladding diameter (d
_clad ) is synthesized with the core simultaneously with the synthesis of the core. The thickness of the clad varies depending on the conditions such as the shape of the refractive index distribution of the core, the refractive index difference (Δn), and the usage of the fiber. The multiplied large value is adopted. The quartz glass core rod for an optical fiber referred to in the present invention is at least FIG.
Is a quartz glass rod including the optical cladding.

Incidentally, the optical fiber of the present invention comprises:
Since the large diameter preform is formed by drawing, the ratio d _clado / d _cladi of the outer diameter (d _clado ) to the optical cladding diameter (d _cladi ) of the optical fiber shown in FIG. Outer diameter (D _o ) and inner diameter (D _i ) of preform
_Is approximately proportional to the ratio D _o / D _i . Thus, hitting the optical fiber designs should be designed the D _o / D _i as an index. For example, a single mode fiber (1.
(3 μm wavelength) core diameter is 9 μm, GI multimode fiber core diameter is 50 μm, and optical fiber outer diameter is 12 μm.
When 5μm, D _o / D _i is a value shown in the following Table 1.

[0023]

[Table 1] Note) The example in the parenthesis is multi-mode.

According to Table 1, for example, in the case of a multimode fiber shown in parentheses as a representative example, D _o / D _i is 2.
5 or less, usually 5% ~
If there is a cladding layer of 30%, for example, 20%, the thickness becomes 60 μm, and D _o / D _i = 2.08. In the case of a single mode fiber, a practical optical fiber can be obtained if D _o / D _i is about 7 or less. That is, for 1.3 μm band (matched clad type, depressed type), 1.55 μm
Main power for obi, dispersion shift type, etc.
_Each distribution is estimated to be about 20 μm or less, and taking a safety factor, d _cladi / d _core ≒ 3 or more, ie, D _o / D _i ≒
4.63 or less is a practical range. When the jacket is double-layered or triple-layered, D _o / D _i is further reduced.
Therefore, the condition that it is to produce a practical optical fiber selected in the range of 1.1 to 7 and D _o / D _i. However, since D _o / D _i is the ratio of the diameter of the preform, in the case of a quartz glass tube for a rod tube, it is necessary to provide a slight gap between the quartz glass tube and the core glass rod. Needless to say.

Various methods known for the case of natural quartz can be used to make the large quartz glass tube of the present invention. A crucible melting drawing method and a molding method can also be used, but it is difficult to increase the diameter by the crucible melting method.
In the molding method, the heat-resistant material used for the container comes into direct contact with the quartz glass for a long time, and the impurities in the heat-resistant material are transferred and diffused into the inside and outside surfaces of the quartz glass base material. For this reason, the transmission loss of the optical fiber is increased. Therefore, when the jacket is brought close to the core, the contaminated portion must be largely removed. As a method for manufacturing the large quartz glass tube, a cylindrical quartz glass preform is prepared, and the center thereof is opened by mechanical grinding such as a core drilling machine shown in FIG. Or hot-hole drilling by hot carbon drill press-fitting method for short-time contact, or through a two-step process, or by depositing a porous silica soot material on a heat-resistant core material, dehydrating, and melt-vitrifying OVD method or direct VAD method A one-step method of producing soot with pores, dehydration, and melt vitrification can be used. In FIG. 5, 7 is a columnar quartz glass base material, 8 is a core drill, and 9 is a grindstone.

Incidentally, if the dimensional accuracy of the thickness of the quartz glass tube for a rod tube is poor, the error is further amplified during the heat drawing process, and the error becomes relatively large. In addition, since the error is further promoted by the multiple coating, the dimensional accuracy needs to be accurate. Mechanical grinding is suitable for processing a quartz glass base material and obtaining a highly accurate quartz glass tube. Mechanical grinding by increasing the size of quartz glass,
In particular, precision grinding by a conventionally known large-sized industrial machine has become possible. However, while high accuracy can be obtained, processing scratches, micro cracks, cracks, processing distortion, etc. occur during processing on the processing surface, which generates bubbles on the internal boundary surface when integrated by the rod-in-tube method. Become. Conventionally, in order to solve such a problem, the internal surface roughness is reduced to, for example, 0.01 μm by performing high precision mechanical polishing of the ground surface over time, performing internal fire polishing, or forming a special glass layer on the internal surface. (See, for example, Japanese Patent Application Laid-Open No. 52-92530). However, these processing methods are cumbersome, and cannot be practically performed when mass-producing large (large-diameter, thick-wall, long) quartz glass tubes. there were. However, it was found that the inner surface finish of the high purity quartz glass raw tube could be solved by using a precision honing device. In this processing method, the original quartz glass tube has an outer diameter of 5 mm.
If it is 0 mm or more, a raw tube with a length of about 3000 mm
The entire length is straight and can be processed into a perfectly round pipe at all positions. Grinding and polishing by changing the grade of whetstone or abrasive grain, crack,
Remove cracks, stress strain, etc., treat them once with hydrofluoric acid aqueous solution to reduce stress concentration, remove surface contamination by ultrasonic cleaning to reduce inner surface roughness to 20 μm or less, and if necessary, remove this When the heat processing is performed, sharp irregularities and cracks in the mechanically grounded portion become gentle surfaces, and processing distortion is released, so that generation of bubbles can be suppressed. According to the experiments by the present inventors, the inner surface roughness due to the grinding was 20%.
If it exceeds μm, it is known that the processing damage is not relaxed or released even by the heating and stretching treatment, and bubbles are generated at the internal boundary surface during integration by the rod-in-tube method.

A cylindrical quartz glass preform is mechanically ground by a core drilling machine or the like to form a hole, or the inner and outer surfaces of a tubular large quartz glass preform made by the OVD method or the like are mechanically roughened. In the case of a ground quartz glass tube, an ultra-precision honing method is preferred for the inner peripheral finish processing. As a result, the clearance with the core glass rod can be reduced. Combining this super-precision honing and outer circumference grinding,
The thickness error of the grinding tube should be 2% or less. If the error is within this range, amplification of the error during the heating and pressurizing stretching hardly occurs, and the eccentricity of the fiber is not adversely affected.

The grinding of the outer peripheral surface is not so severe as the grinding of the inner peripheral surface because the ground surface is directly heated to the high-temperature portion and heated, but the breaking strength after the optical fiber is formed is affected. As a result, sharp stress concentration portions are removed or alleviated by hydrofluoric acid etching, and the surface roughness is reduced to at least 20.
It is necessary to be 0 μm or less, preferably 100 μm or less. Therefore, for the outer peripheral grinding, for example, a standard outer peripheral grinder or a cylindrical grinder, which has been used in semiconductor ingots and various ceramics grinding processes, is used.

After the completion of the drilling, the machined surface is polished to confirm that the thickness error is 2% or less. This is subjected to hydrofluoric acid etching treatment to obtain a quartz glass tube having an inner surface roughness of 20 μm or less. The quartz glass tube is integrated with the core glass rod for optical fiber by the rod-in-tube method, but even if the core glass rod is made under the same conditions, the characteristics will vary, and fiber specifications, user characteristics,
The characteristics change depending on the manufacturing method. Therefore, in order to fit the quartz glass tube to the core glass rod, it is preferable to produce quartz glass tubes of various dimensions by performing a heat stretching process.

Since the above-mentioned heat-treated quartz glass tube is heat-treated at a high temperature, various processing damages such as roughness of a ground surface and processing distortion due to mechanical grinding are alleviated or released.
In particular, the greater the degree of deformation in hot stretching, the more the surface scratches, cracks,
The pits are greatly deformed and enlarged, the grooves become shallow, and the sharp corners disappear, and the high-precision mechanical polishing, internal fire polishing, or special glass layers conventionally required for quartz glass tubes for rod-in tubes Process that is not suitable for mass production, such as a process for forming the inner surface, can be omitted. Therefore, it is advantageous to produce a large quartz glass tube with high precision and to obtain a target large quartz glass tube by utilizing thermal deformation.

In the method for producing natural quartz glass of the present invention, a high-quality portion is selected from crystallites produced naturally, the outer shell of each crystallite is removed, the center is taken out, and this is crushed. After removing the foreign substances, impurities are removed by a chemical treatment. Using this as a raw material, it is manufactured by a method that has been known for a long time, such as a crucible melting drawing method and a mold forming method, and is still generally used. However, a method of preparing a large cylindrical quartz glass preform by the Bernoulli method using oxyhydrogen can be recommended as a method for producing a quartz glass material for optical fibers because it has the least amount of impurities.

As a method for producing a synthetic quartz glass ingot or a tubular body used for producing the quartz glass tube used in the present invention, a conventionally known “Applied Technology of High Purity Silica”, pp. 100-104 (stock) Company CMC,
Various production methods described in, for example, March 10, 1991) can be considered. The high-temperature vapor-phase Bernoulli method is a method for obtaining a glass ingot directly by a gaseous silicon compound, for example, SiCl ₄ and an oxyhydrogen flame, Synthetic quartz glass contains more than 800 ppm of OH groups and is therefore unsuitable as a material for optical fibers (for low OH) and is mainly used for photomask substrates for semiconductors and optical members of exposure equipment. In addition, the improved plasma method, which has a low OH group but requires a large amount of power and requires a high cost, is only used for producing special products such as high-purity core glass for optical fibers. On the other hand, the flame temperature was lowered as compared with the direct vitrification method, the raw material gas was wiped on a rotating base material (target), and a porous soot material was formed. This method is suitable as the method for producing the base material of the present invention without the above-mentioned disadvantages. The VAD method is
The main production is a solid cylindrical quartz glass preform, and the OVD method directly produces a tubular quartz glass preform.

The refractive index of the large quartz glass tube is determined according to the design value. The refractive index of the quartz glass tube and the core glass rod for an optical fiber inserted therein differs depending on the manufacturing conditions, purpose and specifications. ) The refractive index of high-purity synthetic quartz glass slightly changes depending on the contained OH group (low refractive index) and the amount of chlorine (high refractive index). In general, dehydration treatment is performed with high-purity chlorine in an optical fiber core glass, but the degree of the dehydration varies depending on each company and varies depending on the purpose of use. (2) 200 ppm of OH groups in natural quartz glass used for MCVD
The refractive index is low because almost no chlorine is contained. (3) In the depressed type optical fiber, the refractive index required for a large quartz glass tube is different because a step around the core has a refractive index of about 0.05 to 0.2%. The advantage is that the refractive index of the large quartz glass tube should be set to ± 0.02% of the target standard value at the request of each company, rather than determining the refractive index of the large quartz glass tube. Therefore, in the present invention, the target standard value is referred to as a design value.

To slightly modify the above design values, it is possible to control the OH group or the amount of chlorine. However, in order to determine each condition independently, Ge, P, Ti, Al, F, The objective cannot be achieved unless the doping of B or the like is actively carried out. This correction can be easily made in the production of synthetic quartz glass by the soot method.

When the quartz glass clad tube is made of natural quartz, the O.D.
Since the H group and the refractive index are different, it is preferable to provide a composite cladding layer between a natural quartz glass tube and a synthetic quartz glass tube.

With the large quartz glass preform of the present invention, 7
For large diameter preforms of 5 mm or less, make a preform using a quartz glass tube of about 75 mm, redraw a large preform, or combine and stretch a quartz glass tube and a core glass rod in a rod-in-tube process. In the same step at the same time to directly obtain a preform having a target outer diameter.

In a single mode core glass rod, it is important to select characteristics such as a mode field diameter, a cutoff wavelength, and a dispersion. In recent years, the characteristics have become much higher, so if the produced core glass rod is used as it is, the characteristics often vary slightly. Therefore, after the clad thickness of the core glass rod is adjusted once with a quartz glass tube, the characteristics are checked, and then the outer diameter is adjusted by re-jacking the large quartz glass tube or by combining etching and the like. The large quartz glass preform has a feature that a higher accuracy can be obtained because the adjustment range is wide.

[0038]

Example 1 Using a shafting method (VAD method), SiCl ₄ was vaporized, flame-hydrolyzed in an oxyhydrogen flame, and sprayed on a rotating quartz glass rod to produce a large-sized quartz porous soot material. This soot material was placed in an electric furnace, and dehydrated by heating with a mixed gas of He and Cl _{2 in} consideration of the conditions of the core glass rod.
A transparent glass was formed at 1550 ° C. by a zone melt method to obtain a large cylindrical quartz glass base material. This quartz glass base material was first cut at both ends, and the center portion was alternately opened from both ends with a core drilling machine of # 80 grindstone.
The dimensions are adjusted by rough grinding with an outer peripheral grinding machine of No. 0 whetstone, outer diameter 94 mmφ, inner diameter 30 mmφ, outer diameter / inner diameter ratio = 3.1
3. Thickness of about 32mm, length of 730mm, weight of about 10kg
Was obtained.

The inner surface of the synthetic quartz glass tube was machined by a long automatic honing machine for ultra-precision finishing to obtain a synthetic quartz glass tube having a straight length and a perfect circular hole. The outer peripheral surface is ground using an NC outer peripheral grinder so that the center of the inner diameter and the center of the outer diameter match, and the thickness error is 2%.
Internal and external grinding and polishing were repeated until the following conditions were reached, and the inner periphery was finally finished with # 800, and the outer periphery was finished with # 140. Next, in order to remove surface contamination and to reduce surface processing distortion, the surface of the quartz glass raw tube is etched and finished with hydrofluoric acid having a concentration of 30% to 5%, and ultrasonically cleaned with pure water to synthesize. A quartz glass tube was used. This synthetic quartz glass tube has an outer diameter (D _o ) of 91.5 mm,
Inner diameter (D _i ) 32.4 mm, outer diameter (D _o ) / inner diameter (D _i )
Ratio = 2.82, wall thickness 29.55 mm, wall thickness error (t _max.
-T _min. ) 0.48 mm (1.62%), length 730 m
m, weight 9.2 kg. Further, the surface was moved 8 mm in the vertical direction by a stylus-type simple roughness meter, and the inner surface roughness (Rmax _. ) Was 4.8 μm and the outer surface roughness (Rmax _. ) 53 μm.

On the other hand, a 1.3 μm single mode core glass rod having a core and cladding refractive index difference (Δn) of 0.343%, an outer diameter of 54.5 mm and a length of 455 mm by a VAD method was prepared. It is heated and drawn to an outer diameter of 30.1 mm by a precision automatic drawing machine with outer diameter control, and the core glass rod and the quartz glass tube are designed to have a cutoff wavelength (λ _c ) of 1.25 μm. Was slightly melted and then melted at a length of 730 mm. After carefully inserting the core glass rod into the synthetic quartz glass tube, fix the core glass rod and the synthetic quartz glass tube so that their centers are aligned with each other. Bending and twisting due to processing were corrected. This was inserted into the vertical electric furnace from the top, and
After melting the tip at 80 ° C., the pressure was reduced by a vacuum pump. Temperature (2000-2800 ° C.) and degree of vacuum (20
The moving speed was changed while adjusting the water column (Aq) from 0 to 1000 mm, and the bubble condition at the interface was examined. Under the condition without foaming, the entire length was slowly moved at 2 mm / min to prepare a preform. The preform portion obtained under the stable condition had an outer diameter of 90.2 mmφ, a length of 595 mm, and a weight of 8.3 kg, which corresponded to a fiber length of about 300 km. A part of the preform was heated and stretched to an outer diameter of about 50 mmφ, and as a result of examination with a preform analyzer, the difference in refractive index at the boundary with the cladding layer was almost 0.01% or less, and the eccentricity was 0.153 mm. (0.34%).

Further, about 5 km of a fiber having an outer diameter of 125 μm was prepared by a drawing machine, and the fiber characteristics of the element wire for each 1 km were examined. As a result, the eccentricity was 0.22 μm as an average value.
m, cut-off wavelength (λ _c ) 1.285 μm, 1.3 μm
The transmission loss at 0.3 m was 0.355 dB / km, and the OH group loss at 1.38 μm was 0.86 dB / km, which was excellent for a single mode optical fiber.

[0042]

Example 2 A large porous soot base material was prepared by an external method (OVD method), dehydrated, and subjected to refractive index adjustment to vitrify to form a cylindrical synthetic quartz glass base material. Both ends were cut in parallel, and the outer circumference was roughly ground with an outer peripheral grinder having a # 80 grindstone. After the outer diameter was almost set to a target outer diameter, this was set in a honing machine and the inner surface was polished using a # 80 grindstone. Next, the inner surface was polished by changing the grindstone to # 140, # 400, and # 800. Next, apply this to an ultrasonic thickness gauge and measure the length to 50 mm.
Each rotation, the thickness was measured at eight points on one circumference, and the thickness was moved sequentially, the thickness variation over the entire length was examined, and the thickness error was plotted by a computer. Further, based on this, the outer periphery was polished and corrected by an NC outer peripheral grinder, and the thickness accuracy was confirmed. This large synthetic quartz glass tube has an outer diameter of 164 mm and an inner diameter of 58 mm.
9mm, outer / inner diameter ratio = 2.78, wall thickness 52.55m
m, thickness error 440 μm (0.84%), length 1870
mm, weight about 75kg, inner surface roughness (Rmax _. ) 3.5μ
m, and a large quartz glass tube having an outer surface roughness (Rmax _. ) of 77 μm.

On the other hand, assuming this synthetic quartz glass tube, a large single mode quartz core glass rod for a single mode was prepared by the VAD method, and three rods having substantially the same characteristics were selected. From the calculation of the cutoff wavelength (λ _c ), the core diameter required for the synthetic quartz glass tube was calculated, and a part of the clad portion of the core glass rod was adjusted by etching. Next, these three core glass rods were welded, and the outer diameters were substantially the same (55 m).
m), and after etching, the entire surface was fire polished.

The core glass rod was placed in the large synthetic quartz glass tube, and the lower end was fixed. This was placed in a vertical electric furnace, heated at a temperature of 2000 to 2800 ° C., and melted and softened from the lower end. Temperature and vacuum (200-1000m
The rod was moved while adjusting the mAq) to perform a rod-in tube. If the temperature is not appropriate or the speed is high, foaming will occur at the internal interface and air bubbles will remain, so the outer diameter will be about 50 mm at first.
Confirm that the interface is sufficiently wet and welded while stretching
Thicknesses were increased to 75 mm, 100 mm, 125 mm, and 150 mm, and preforms having five different thicknesses were prepared. The obtained preform had a maximum diameter of 152 mm in outer diameter, and the total weight of the five types of preforms was about 71 kg, which was equivalent to 2600 km in terms of optical fiber.

For the purpose of examining the characteristics of the preform in detail, a preform having a length of 50 mm was selected, and the characteristics of the core were examined with a preform analyzer. There is no refractive index step, and the center shift between the core and clad is 0.28
%.

The above 50 mm preform was drawn by an optical fiber drawing device to have an outer diameter of 125 μm ± 0.5 μm. The transmission characteristics of this wire were examined.
Cutoff wavelength (λ _c ) 1.270 μm, transmission loss at 1.3 μm 0.361 dB / km, O at 1.38 μm
The H group loss was 0.65 dB / km.

[0047]

Example 3 A large porous soot base material was prepared by a shafting method (VAD method), heated and dehydrated and turned into a transparent glass according to Example 1, and this was roughly ground to obtain an outer diameter of 96 mm and a length of about 820 mm.
Was obtained.

A hole was formed in the center of this ingot by using a hot carbon drill press-fitting method.
The outer periphery was ground and subjected to hydrofluoric acid treatment for cleaning. At this time, the synthetic quartz glass raw tube had an outer diameter of 101 mm, an inner diameter of 40 mm,
Outer diameter / inner diameter ratio = 2.525, length 775 mm, weight about 1
It was 1.5 kg. Because the inner surface of this original tube is perforated by heating and melting, mechanical shock, cutting failure, cracks,
No cracks, processing distortions, etc. were observed.

Therefore, for the purpose of examining the roughness of the inner surface of the synthetic quartz glass raw tube, polishing was performed by changing the polishing roughness on a honing machine every 150 mm in length, and the sample was etched as shown in Table 2. Approximately 38m in this synthetic quartz glass tube
The core glass rod having a length of m was inserted and preformed in the same manner as in Example 1. The interface state is as shown in Table 2.

[0050]

[Table 2]

[0051]

Example 4 In the same manner as in Example 2, four large tubular soot base materials were prepared by an external method (OVD method), and after dehydration and vitrification, the inner and outer surfaces were roughly polished. After ultra-precision honing of the inner diameter, the center line of the inner diameter and the outer diameter was artificially decentered, eccentric, the outer periphery was polished, and hydrofluoric acid treatment was performed, followed by cleaning. This synthetic quartz glass tube had an outer diameter (D _o ) of 100 mmφ, an inner diameter (D _i ) of 32 mm, and D _o / D _i = 3.125. It was placed in a vertical electric furnace, heated to 2200 ° C. and stretched under pressure. The stretching conditions are as shown in Table 3 when the outer diameter of the quartz glass tube after stretching is (d _o ) and the inner diameter is (d _i ).

[0052]

[Table 3]

Table 4 shows errors of the synthetic quartz glass tube subjected to the stretching and errors based on the pressure stretching.

[0054]

[Table 4]

As shown in Table 4 above, in the quartz tube having a large error, the error after stretching increases as the pressure ratio and the stretching ratio increase. In particular, the pressure ratio directly changed the thickness error of the quartz glass tube greatly, and the tube of 3.9% or more was sufficiently heated at the start and deformed asymmetrically, then expanded rapidly and burst in the furnace.

[0056]

Example 5 A special high-quality portion was selected and washed from a crystal mass calculated naturally. Further, the outer shell of each bulk crystal was removed, the center was taken out, and this was crushed to make the particle diameter uniform. After removing surface impurities by chemical treatment, a large cylindrical natural quartz glass base material was prepared by the Bernoulli method using an oxyhydrogen flame. This quartz glass base material was subjected to hot carbon drill press-fitting in accordance with Example 3 to form a hole at the center thereof, and the outer periphery thereof was machined to finish, washed with hydrofluoric acid, washed with water and dried. The quartz glass tube has an outer diameter of 175 mm, an inner diameter of 60 mm, and an outer diameter / inner diameter ratio =
2.916, length 3m, weight 150kg, total thickness is checked at 50mm intervals, average thickness error is 0.3m
m, which was within the target range. Further, when the surface roughness at the end of the quartz glass tube was examined with a stylus type simple roughness meter, the inner surface roughness ( _Rmax. ) Was 0.8 μm and the outer surface roughness (Rmax _. ) Was 95 μm _. there were.

On the other hand, a 1.3 μm single-mode optical fiber core rod with a partially clad, prepared by the VAD method, was prepared, inserted into the quartz glass tube, and set in a vertical electric furnace. After elevating the temperature to 2250 ° C. in the furnace to seal the tip, vacuum was drawn from above. The conditions of the rod-in-tube method are as follows: the degree of vacuum is set to 200 to 1000 mmAq, the outer diameter of the start is set to 50 mm, and the temperature, the moving speed and the degree of vacuum are determined while observing the fusion state of the interface between the core rod and the quartz glass tube. Was changed to obtain a preform having a maximum outer diameter of 160 mm.

When the preform pulled out at 50 mm was examined by a preform analyzer, the eccentricity was 0.52.
%, A negative step of about -0.005% was found in the boundary layer between the clad of the core glass rod and the quartz glass tube, and the quartz glass tube was slightly lowered.

The preform was made into a 125 μm fiber by a drawing machine, and the optical fiber characteristics were examined.
Transmission loss at 41 μm and 1.3 μm wavelength is 0.346 dB
/ Km, which is an excellent characteristic for a single mode quartz glass fiber.

[0060]

Embodiment 6 A synthetic quartz glass tube was manufactured in accordance with Embodiment 1.
Done. This tube has an outer diameter (D_o) 93.5 mm, inner diameter
(D_i) 31.6 mm, D_o/ D_i= 2.96, average thickness
(T_AV.) 30.95 mm, thickness error (t_max.−
t_min.) 0.42 mm (1.36%), inner surface roughness (R
_max.) 8.5 μm, outer surface roughness (R _max.) 68 μm
Was.

Next, the synthetic quartz glass tube was placed in a vertical electric furnace, heated to 2200 ° C., the bottom was sealed, and then the quartz glass tube was pulled out while the inside of the tube was pressurized with argon gas. Was prepared. As shown in Table 5, the inner surface roughness ( _Rmax. ) After the heat treatment was low in all the tubes.

[0062]

[Table 5]

No. A core rod for an optical fiber was inserted using a 3 (62 mm) tube, and an optical fiber preform was prepared by a rod intubing method. However, no air bubbles were generated at the interface.

[0064]

The large-sized preform of the present invention has high precision,
A large-sized coated quartz glass with a small inner surface roughness and a core glass rod are welded and integrated to form a preform with no bubbles at the boundary and a small refractive index step at the boundary. By drawing, a high-quality optical fiber having a small eccentricity can be manufactured with good mass productivity.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a large quartz glass tube.

FIG. 2 is a cross-sectional view of a large quartz glass preform.

FIG. 3 is a schematic cross-sectional view of a single-mode fiber made from a large quartz glass preform.

FIG. 4 is a conceptual diagram of a refractive index distribution and a light power distribution of a single mode fiber.

FIG. 5 is a partial longitudinal sectional view of a method for manufacturing a large quartz glass tube using a core drilling machine.

[Explanation of symbols]

1 core glass rod second cladding layer 3 covering a large quartz glass tube 4 core 5 optical cladding 6 O - Ba - cladding 7 outer diameter D _i large quartz glass tube of cylindrical quartz glass mother material 8 a core drill 9 grindstone D _o large quartz glass tube Inner diameter a core diameter b optical cladding diameter c outer diameter of optical fiber

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kiyoshi Yokokawa 88 Kawakubo, Kanaya, Tamura-cho, Koriyama-shi, Fukushima Shin-Etsu Quartz Co., Ltd. Inside the Koriyama Plant (72) Inventor Masanori Suzuki 88 Kawakubo, Kanaya, Tamura-cho, Koriyama City, Fukushima Prefecture Inside the Koriyama Plant (72) Inventor Toshiyuki Kato 88, Kawakubo, Kanaya, Tamura-cho, Koriyama, Fukushima Prefecture Inside the Koriyama Plant (72) Inventor Yutaka Watanabe 88, Kawakubo, Kanaya, Tamura-cho, Koriyama City, Fukushima Prefecture Inside the Koriyama Plant, Shin-Etsu Quartz Co., Ltd.

Claims (1)

[Claims] 1. High-purity natural quartz having an outer diameter of 50 to 300 mm, an outer diameter to inner diameter ratio of 1.1 to 7, a wall thickness of 10 mm or more, a wall thickness error of 2% or less, and an inner surface roughness of 20 μm or less. The glass tube or the synthetic quartz glass tube and the optical glass core glass rod are welded and integrated, and the refractive index of the high-purity natural quartz glass tube or the synthetic quartz glass tube is 0.
And the refractive index step at the boundary between the high-purity natural quartz glass tube or the synthetic quartz glass tube and the cladding layer of the core glass rod for an optical fiber is ± 2%.
A large quartz glass preform characterized by being 0.01 or less.