JPH0314787B2 - - Google Patents

Description

Detailed Description of the Invention Technical Field The present invention relates to a rod-in-tube method, which is characterized by removing the surface layer of the rod with a hydrofluoric acid solution and modifying the part to be fused between the rod and tube. Regarding the manufacturing method of the optical fiber base material,
This provides a base material with a low OH ion content. Background Art When transmitting light in the wavelength band of 1.3 μm in a silica-based optical fiber, the OH ion content becomes a problem.
In particular, the core portion, which serves as a substantial transmission path for light, is strongly required to have a low OH ion content in order to reduce absorption loss. On the other hand, it is also necessary for the optical fiber as a whole to have little radiation loss and scattering loss due to bubbles, foreign matter, etc. Conventionally, a rod-in-tube (RT) method has been known in which an optical fiber base material is produced by loading a rod into a tube and fusing and integrating the rods under heating (Japanese Patent Publication No. 11071-1971). ). Although this method is superior in terms of manufacturing efficiency, yield, dimensional accuracy of the obtained base material, and low knitting of the core, it has the disadvantage that air bubbles and foreign matter tend to remain at the fused interface. There is. On the other hand, vapor deposition (VAD) involves sequentially depositing glass-forming oxide soot in the axial direction of a quartz glass rod, heating it with an oxyhydrogen burner, etc., and sintering it to form a transparent glass rod.
A method (Phase Axial Deposition) is also known (Japanese Patent Publication No. 35217 of 1983). However, the obtained transparent glass rod is not fully satisfactory in terms of OH ion content. Summary of the Invention The present inventors have overcome the drawbacks of the glass rods obtained by the above-mentioned RT method and VAD method, and have no interfacial defects.
In addition, as a result of extensive research in order to obtain an optical fiber base material with a low OH ion content, we have discovered a method that can achieve this objective, and have completed the present invention. That is, the present invention includes a first step in which a silica-based glass rod produced by a vapor phase shafting method is treated with a hydrofluoric acid solution to remove the surface layer with a thickness of 30 μm or more; A dopant-containing quartz glass layer formed by a deposition method is placed in a quartz-based glass tube having an inner wall, and heated while supplying only oxygen gas or inert gas to the air space formed between the rod and the tube. The second step is to modify the parts of the rod and tube to be fused together.
The present invention provides a method for manufacturing an optical fiber base material, which comprises a third step of fusing and integrating the knitted rod and tube under heating. Conventionally, it has been known to treat a silica-based glass rod to be loaded into a tube with a hydrofluoric acid solution before loading it, and to modify the welded part of the rod and tube (Japanese Patent Laid-Open No. 90434/1983). Public bulletin). However, the purpose of the treatment with the hydrofluoric acid solution was to clean the surface of the rod in order to remove dirt adhering to the rod. That is,
The treatment was not intended to remove the surface layer of the rod.
On the contrary, when a rod is treated with hydrogen fluoride gas, which has a strong penetration force into the inside, the transmittance of light in the 1.3 μm wavelength band often decreases. , treatment that extended to the inside was considered undesirable. Further, the modification treatment of rods and tubes proposed in the above-mentioned publication uses a special treatment agent together with oxygen gas. Rather, treatment using oxygen gas alone without the treatment agent in combination is rejected because good results cannot be obtained. The present invention combines and applies these techniques that have been considered undesirable in the past, and as a result, has produced unexpected effects. DISCLOSURE OF THE INVENTION (First Step) The first step of the present invention is a step of treating a silica-based glass rod produced by a vapor phase shafting method with a hydrofluoric acid solution to remove the surface layer of the rod to a thickness of 30 μm or more. As a result, it is possible to obtain a silica-based glass rod for forming a core portion with less absorption loss due to OH ions or less influence caused by OH ions. An aqueous solution is sufficient as the hydrofluoric acid solution to be used, and its concentration is not limited, but should be determined in terms of ease of handling, processing operability, processing efficiency, etc.
10 to 50% by weight, especially 20 to 30% by weight is suitable. As for the treatment operation, a method in which the rod is immersed in a hydrofluoric acid solution is advantageous in terms of workability and controllability of the removed layer. The surface layer thickness of the rod to be removed by this treatment is 30 to 150 μm, especially 50 to 150 μm.
100 μm is sufficient. The silica-based glass rod to be subjected to this treatment is manufactured using the vapor phase axis attachment method, and may be made of pure silica glass or may be made of one or more types of glass to change the refractive index. It may contain a dopant of. Examples of the dopant include P ₂ O ₅ ,
_{GeO2 , TeO2} , _{Sb2O3 , TiO2} , _Al2O3 , _{Ta2O5 ,}
Examples include Nb ₂ O ₅ , B ₂ O ₃ and F.
Furthermore, the rod may consist only of a portion that will become the core of the final target optical fiber, or may have a portion that will become a cladding layer around its outer periphery. Further, it may be a step-type optical fiber or a graded-type optical fiber.
In addition, if the rod has a large diameter, it may be one that is produced by a vapor phase axis-setting method and stretched to a diameter of 0.25 to 0.5 times while heating, prior to treatment with a hydrofluoric acid solution. Stretch it to a diameter of 8 to 10
A fiber as thin as mm facilitates the work in the first to third steps, and has the advantage of efficiently producing a good optical fiber base material. The treated rods obtained by treatment with hydrofluoric acid solution are washed with pure water,
After being cleaned by a known method such as ultrasonic cleaning, it is subjected to the next second step. (Second Step) In the second step, the treated rod obtained in the first step is loaded into a quartz-based glass tube whose inner wall has a dopant-containing quartz glass layer formed by an internal chemical vapor deposition method. This is a process in which the welded portion of the rod and tube is modified by heating while supplying only oxygen gas or inert gas to the space formed between the rod and tube. This modification treatment leaves almost no defects such as bubbles that cause radiation loss or scattering loss at the interface.
It becomes possible to fuse and integrate the rod and tube. The quartz-based glass tube used to load the treatment rod in this process has a dopant-containing quartz glass layer formed by internal chemical vapor deposition on its inner wall.
As the dopant, the same ones as in the case of the above-mentioned silica-based glass rod can be mentioned (Japanese Patent Publication No. 51-23185, Japanese Patent Application Laid-Open No. 1983-1983).
120352, JP-A-52-35654, etc.). The tube may essentially serve as a cladding layer, a support layer, or both a cladding layer and a support layer in the final optical fiber. Therefore, it is also possible to appropriately combine the rods and tubes described above to form a base material that provides a modified step type optical fiber having a partially refractive index gradient. In addition, the tube contains a hydrofluoric acid solution,
It is preferable to treat the surface with an appropriate cleaning means such as pure water or ultrasonic waves before use. In addition, examples of the inert gas used in this step include rare gases such as helium gas, neon gas, and alcon gas. In the modification treatment, if the temperature is too low, the modification will not be sufficient, and if the temperature is too high, the tube or rod may be softened and deformed, the dopant contained therein may volatilize, or the distribution state of the dopant contained may change, which is undesirable. . A suitable modification temperature is 1000 to 1900°C, especially 1200 to 1400°C, based on the outer peripheral surface of the tube. There are no particular limitations on the heating method; for example, the heating source is moved slowly back and forth or repeatedly in the axial direction of the tube using an oxyhydrogen burner while rotating the rod and tube. I can list methods etc. In this case, the moving speed of the heating source is 10 to 500 mm/min, especially 50 to 300 mm/min.
mm/min is suitable, and the rotational speed of the rod and tube is suitably 10 to 100 rpm. Within these conditions, in most cases it is possible to heat the rod and tube to a uniform temperature over the circumferential direction. In the present invention, modification treatment refers to supplying a gas between the rod and tube, heating the rod and tube from the outside while supplying the gas, and heating the surface or surface layer of the rod and the tube as the welded parts. This refers to changing the inner wall of a material so that a good fusion interface is formed when these are fused and integrated. However, the content of this change has not been fully elucidated. The present inventors consider the change as follows. In other words, the temperature, gas, gas pressure, and dopant in the rod and/or tube, especially in the surface layer, interact to soften the part to be welded, reduce the melting age, or cause the part of the rod and tube to be welded to soften. As homogenization occurs (denaturation) and the physical or chemical affinity of each welded part increases, a good interface is formed during fusion and integration, and defects that cause radiation loss and scattering loss occur. It is thought that there is no such thing. The modification treatment is sufficiently effective by modifying only the very surface layer of the rods and tubes. The time required for processing is determined by the pressure, flow rate, etc. of the inert gas, and tends to be shorter as the pressure is higher and the flow rate is larger. When the heat source movement method described above is applied at a pressure of 500 to 1000 mmHg and a flow rate of 50 to 2000 ml/min, the denaturation treatment is normally performed by moving the heat source back and forth within the above-mentioned movement speed range about 1 to 20 times. It can be performed. When the denaturation treatment is completed, the next third step is carried out. (Third Step) The third step is a step in which the modified rod and tube are heated and fused together to form an optical fiber base material. In the present invention, this step can be carried out by a known method.
For example, place the rod and tube so that their central axes coincide, and use an oxyhydrogen burner while rotating them synchronously to heat the tube to 1900~
This can be done by heating the tube to about 2300℃, crushing the tube with heat and flame pressure, and fusing and adhering the two to form a single body. An optical fiber can be produced from the obtained optical fiber base material according to a conventional method. That is, for example, the base material is heated using an electric furnace or the like.
It is heated to about 2000°C and drawn at a speed of about 10 to 100 m/min to form an optical fiber with an outer diameter of about 50 to 200 μm. Advantages of the invention According to the present invention, the hydrofluoric acid solution obtained by treating
A quartz-based glass rod with extremely low absorption loss due to OH ions can be fused and coated with a quartz-based glass tube with almost no interface defects. From the obtained base material, it is possible to obtain an optical fiber that has extremely low absorption loss due to OH ions, radiation loss due to bubbles, foreign objects, etc., and scattering loss, and therefore transmits light with a wavelength of 1.3 μm and its vicinity. This method is suitable as a method for obtaining a fiber with Examples, Comparative Examples Reference Example 1 (Production of graded rod by CAD method) Using a concentric five-layer burner, from the first layer in the center
SiCl ₄ : 150ml/min, GeCl ₄ : 20ml/min, POCl ₃ :
5ml/min, Ar: 500ml/min, SiCl ₄ from the second layer:
50ml/min, Ar: 200ml/min, from the third layer in the middle
The raw material gas was supplied under the conditions of H ₂ : 4/min, Ar from the 4th layer: 1/min, and O ₂ : 6/min from the outermost 5th layer, and the soot produced by the flame hydrolysis reaction method was A porous matrix with a diameter of 60 mm and a length of 200 mm was prepared by depositing and growing on the tip of a seed rod. Then, using an electric furnace, the porous base material is
1600℃ while supplying helium gas containing 2 mol% oxygen gas as a dehydrating agent at a rate of approximately 10/min.
and sintered at a rate of 200 mm/hour to obtain a graded quartz-based glass rod containing Ge as a dopant and having a diameter of 20 mm, a length of 100 mm, and a refractive index (n ²⁰ _D max) of 1.465. . Reference example 2 (Preparation of pseudo step rod using VAD method) From the first layer, SiCl ₄ : 200ml/min, GeCl ₄ : 20ml/min
min, POCl ₃ : 5 ml/min, Ar: 500 ml/min, Ar from the second layer: 200 ml/min, H ₂ from the third layer: 4/min,
Ar from the 4th layer: 1/min, O ₂ from the 5th layer: 6
The diameter containing Ge (dopant) was prepared in the same manner as in Reference Example 1 except that the raw material gas was supplied under the conditions of /min.
A pseudo-step type silica-based glass rod of 18 mm in length, 100 mm in length, and a refractive index (n ²⁰ _D max) of 1.465 was obtained. Reference example 3 (Preparation of tube with internal CVD layer) O ₂ (800 ml/min), CCl ₂ F ₂ were placed inside a quartz glass tube with an outer diameter of 20 mm, a thickness of 1.5 mm, and a refractive index (n ²⁰ _D ) of 1.452.
(200ml/min) and oxyhydrogen burner to 150mm/min.
The tube was heated to 1200°C while being moved at a rate of 30 minutes, and the inner surface was smoothed, and then SiCl ₄ : 240 ml/minute, SiF ₄ : 30 ml/minute were added to the tube.
GeCl ₄ : 30ml/min, POCl ₃ : 10ml/min and O ₂ :
Each raw material gas was supplied under the conditions of 1000ml/min, and the oxyhydrogen burner was moved at a rate of 150mm/min.
Ge, F, by chemical vapor deposition method by heating to °C.
Contains P as a dopant and has a refractive index (n ²⁰ _D )
A quartz-based glass tube with an internal CVD layer of 1.450 mm and a thickness of 1.6 mm was obtained. Reference example 4 SiCl ₄ (240 ml/min), BF ₃ (150 ml/min) as raw material gas
ml/min), the heating temperature was 1500°C, and the same procedure was used as in Reference Example 3, but the inner layer contained B and F as dopants, had a refractive index (n ²⁰ _D ) of 1.450, and a thickness of 1.6 mm.
A quartz-based glass tube with a CVD layer was obtained. Example 1 A graded rod obtained in the same manner as in Reference Example 1 was heated to about 2000°C with an oxyhydrogen burner, and stretched to make a rod with a diameter of 10 mm (0.5 times the diameter). 2% in aqueous hydrofluoric acid solution (room temperature)
The sample was immersed for a period of time, and about 50 μm of the surface layer was removed. Next, this treated rod was placed concentrically in a tube obtained in the same manner as in Reference Example 3, and while the rod and tube were rotated synchronously at 80 rpm, oxygen gas (760 mmHg at room temperature) was supplied at 800 ml/hour. The tube and rod were heated from the outside by continuously supplying the tube and rod at a flow rate of 150 mm/min to the air space formed between the rod and the tube, and by moving an oxyhydrogen burner repeatedly in parallel to the tube 5 times at a speed of 150 mm/min. , denaturation treatment was performed. The denaturation treatment temperature was 1600°C (infrared thermometer) on the outer surface of the tube. Immediately after the denaturation treatment, the tube, which continues to rotate synchronously with the rod, is heated for about 2200 m using an oxyhydrogen burner.
The rod and tube were heated to ℃ and crushed, and the rod and tube were fused and integrated to obtain a primary base material with an outer diameter of 13 mm (core part 10 mm), which was then covered with a quartz pipe to form an optical fiber base material with an outer diameter of 25 mm. Obtained. While heating this base material to 2100℃ in an electric furnace,
It was drawn at a speed of 40 m/min to obtain an optical fiber with a core diameter of 50 μm, a cladding of 10 μm, and an outer diameter of 125 μm. The loss characteristics of the obtained optical fiber were as shown in the table. Note that the scattering loss value is obtained by extrapolating the linear region between wavelengths 0.7 to 1.1 μm in the loss characteristic curve using the λ ^-4 display method to wavelength infinity, and is a loss value that does not depend on wavelength (radiation loss, scattering loss). Furthermore, the loss value (dB/Km) of light at a wavelength of 0.95 μm can be evaluated as the OH group content (ppm) in the core portion. The results show that absorption loss due to OH ions, radiation loss due to bubbles, foreign matter, etc., and scattering loss are significantly small. Comparative Example 1 A core diameter of 50 μm, a cladding insertion of 10 μm, an outer diameter of
A 125 μm optical fiber was obtained. Its loss characteristics were as shown in the table. This result shows that there is a large amount of absorption loss due to OH ions. Comparative example 2 Surface cleaning of rod with hydrofluoric acid solution (soaking time 10
The core diameter was 50 μm and the clad layer was 10 μm in the same manner as in Example 1, except that the thickness of the removed layer was 3 μm or less).
An optical fiber with an outer diameter of 125 μm was obtained. Its loss characteristics were as shown in the table. Comparative Example 3 Same as Example 1 except that no modification treatment was performed, core diameter 50 μm, cladding layer 10 μm, outer diameter 125 μm.
obtained optical fiber. Its loss characteristics were as shown in the table. This result shows that there are large scattering losses and radiation losses. Example 2 An optical fiber having a core diameter of 50 μm, a cladding layer of 10 μm, and an outer diameter of 125 μm was obtained in the same manner as in Example 1, except that the layer removed by the hydrofluoric acid solution was 150 μm. Its loss characteristics were as shown in the table. Example 3 The rod was not stretched, had an outer diameter of 28 mm, and a wall thickness of 1.5 mm.
The core diameter was 100 μm and the cladding layer was made under the same conditions as Example 1 except that a tube with an internal CVD layer of 1.5 mm was used.
An optical fiber with an outer diameter of 8 μm and an outer diameter of 150 μm was obtained. Its loss characteristics were as shown in the table. Comparative Example 4 A tube with a core diameter of 50 μm and an outer diameter of 50 μm was used, except that a tube without a CVD layer on the inner wall was used.
A 125 μm optical fiber was obtained. Its loss characteristics were as shown in the table. Example 4 An optical fiber with a core diameter of 50 μm, a cladding layer of 10 μm, and an outer diameter of 125 μm was obtained in the same manner as in Example 1, except that the pseudo step type rod of Reference Example 2 was stretched to 0.56 times the diameter (10 mm in diameter). Ta. The loss characteristics are shown in the table. Example 5 Denaturation treatment was performed using helium gas (1000ml/min) at 1800 ml/min.
An optical fiber having the same dimensions was obtained in the same manner as in Example 1 except that the temperature was 0.degree. The loss characteristics were as shown in the table. Example 6 Using the tube obtained in Reference Example 4, an optical fiber of the same size was obtained under the same conditions as Example 1, except that the modification treatment was carried out using argon gas (800 ml/min) at 1200°C. The loss characteristics were as shown in the table.

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Claims (1)

[Claims] 1. The first step is to treat a silica-based glass rod produced by a vapor phase shafting method with a hydrofluoric acid solution to remove a surface layer of 30 μm or more. A dopant-containing quartz glass layer formed by a phase deposition method is placed in a quartz glass tube having an inner wall, and heated while supplying only oxygen gas or inert gas to the gap formed between the rod and tube. A method for producing an optical fiber base material, comprising: a second step of modifying the welded portions of the rod and tube; and a third step of integrating the modified rod and tube under heating. 2. The manufacturing method according to claim 1, wherein the quartz-based glass rod is produced by a vapor-phase axial method and then stretched to a diameter 0.25 to 0.5 times.