JP4487560B2 - Optical fiber preform manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing an optical fiber preform by a collapse method.

  The collapse method is a method for obtaining an optical fiber preform by solidifying a glass pipe. You may solidify in the state which inserted the glass rod in the glass pipe, and a glass pipe and a glass rod are integrated in this case. FIG. 17 is a conceptual diagram showing the collapse method. The glass pipe 1 is attached to the gripping portion 4 of the glass lathe 13 and rotated while the glass rod 2 is inserted, and one end of the glass pipe 1 is heated and sealed with the heat source 3. The glass pipe 1 is heated and solidified while being moved.

Prior to solidification, the glass pipe 1 is heated to at least 1000 ° C. while flowing chlorine gas into the glass pipe 1 to chemically clean the glass surface, and the glass pipe 1 is evacuated and solidified. Is also known (see, for example, Patent Document 1).
For example, Non-Patent Document 1 describes the diffusion of water in silica glass.
For example, Non-Patent Document 2 describes the release and occlusion of water related to silica glass.

  FIG. 18 is a graph showing an increase in transmission loss due to OH groups in the wavelength band of 1.4 μm. The OH group in quartz glass has a large absorption peak (OH group absorption) in the vicinity of a wavelength of 1.4 μm. The OH group absorption increases transmission loss in the 1.4 μm band and makes it difficult to transmit signals in the 1.4 μm band and excitation light for Raman amplification. Therefore, the smaller the OH group absorption, the better. However, when an optical fiber preform is manufactured by the collapse method, it is difficult to reduce OH group absorption.

JP-A-8-225335 Hajimu Wakabayashi, 1 other, “Diffusion of Water into Silica Glass at Low Temperature”, “Journal of American Ceramic Society.” Ceram. Soc.) ", (United States), Vol. 72, No. 10, 1989, p. 1850-1855 Edited by Katsuro Fukamizu, “Amorphous Silica Material Application Handbook”, Realize Inc., May 31, 1999, p. 56-57

  An object of the present invention is to provide a method for manufacturing an optical fiber preform by a collapse method, which can obtain an optical fiber with reduced increase in transmission loss due to OH group absorption.

The present invention is a method of manufacturing an optical fiber preform by solidifying a glass pipe, and the concentration of hydrogen atom-containing substances in the glass pipe is a total while heating the temperature of the glass pipe to 550 ° C. or less. A drying step of blowing an inert gas of 10 volppm or less, a chemical cleaning step of heating the glass pipe while flowing a gas containing a chlorine element-containing substance in the glass pipe after the drying step, An optical fiber preform having a step of sealing one end, and a step of solidifying the glass pipe by exhausting while flowing a gas so that the absolute pressure in the glass pipe is 100 kPa to 0.1 kPa A manufacturing method is provided.
The present invention also relates to a method of manufacturing an optical fiber preform by inserting a glass rod into a glass pipe and integrating the glass rod, a step of vapor-phase etching the inner surface of the glass pipe, and inserting the glass rod into the glass pipe. A step of blowing an inert gas having a total concentration of 10 volppm or less of hydrogen atom-containing materials into the glass pipe while heating the temperature of the glass pipe and the glass rod to 60 to 550 ° C., a chlorine atom-containing material A step of heating while containing a gas, a step of sealing one end of the glass pipe, and exhausting the gas while flowing gas so that the absolute pressure in the glass pipe is 100 kPa to 0.1 kPa, and the glass pipe and the glass rod A method for producing an optical fiber preform is provided.
In this specification, the optical fiber preform may be a glass body that is drawn as it is to become an optical fiber, or may be a glass body (optical fiber preform intermediate) that is drawn after further processing. .

  According to the present invention, by sufficiently drying the inside of the glass pipe and the glass surface before solidification, an optical fiber preform with a very small increase in transmission loss due to absorption derived from OH groups can be produced by the collapse method. it can. The optical fiber preform of the present invention has a very low OH group content, and an optical fiber obtained by drawing this preform has a low transmission loss in the vicinity of a wavelength of 1.38 to 1.45 μm, and is excellent in a range of 1.48 to 1. As a pumping light source used for amplifying signal light in the .6 μm band, it is suitable as a Raman amplification optical fiber using a light source in the above wavelength band.

A hydrogen atom-containing substance such as a hydrogen molecule (H ₂ ), water (H ₂ O), methanol (CH ₃ OH), methane (CH ₄ ), ketone (CH ₃ COCH ₃ ), hydrogen atom (H When silica glass is heated while adsorbing etc., hydrogen atoms contained in the hydrogen atom-containing substance are converted into the glass surface and chemical formula (1), (2):
SiO ₂ + H → Si—OH (1)
GeO ₂ + H → Ge-OH (2)
It reacts like this. The hydroxyl groups (OH groups) thus produced are difficult to remove.

For example, when the hydrogen atom-containing material is H ₂ O, H ₂ O is chemically adsorbed on the glass surface when heat-treated. When heated above 550 ° C., the adsorption activation energy becomes twice as large as that at a low temperature, and it becomes difficult to remove H ₂ O from the glass surface. When heated to 600 ° C. or higher, H ₂ O reacts with the glass surface to generate OH groups. Also it is between H _{2 O} and the glass surface in heating at 200 ° C. or higher in some cases reacts.

In the optical fiber preform manufacturing method of the present invention, before the glass pipe is solidified by the collapse method, it is contained in the hydrogen atom-containing substance and / or the atmosphere in the glass pipe adsorbed on the glass surface in the glass pipe. The hydrogen atom-containing materials (collectively referred to as “hydrogen-containing materials in the glass pipe”) are reduced. In the present specification, a process for reducing a hydrogen atom-containing substance, not limited to H ₂ O, is referred to as “drying”, and a process for performing this process is referred to as a “drying process”. By performing the drying process, no OH group is generated even when the glass pipe is heated to a high temperature in the step of solidification, so that it becomes possible to obtain an optical fiber preform with reduced transmission loss increase due to OH group absorption. .
By carrying out the drying step of the present invention, the concentration of the OH group in the portion derived from the interface when the optical fiber preform is solidified can be made 100 ppb or less. Further, the OH group absorption at an optical fiber wavelength of 1.38 μm can be made 0.5 dB / km or less.

A drying process is performed at the temperature of 550 degrees C or less. If the heating temperature exceeds 550 ° C., it becomes difficult to remove H ₂ O adsorbed on the silica glass, and if it is 600 ° C. or higher, H ₂ O reacts with the glass surface to generate OH groups. It is.

Embodiments of the drying process (A) to (E):
(A) A gas (dry gas) having a low hydrogen atom-containing substance concentration is blown from one end of the glass pipe to the other end, and the inside of the glass pipe is purged (blast purge).
(B) The inside of the glass pipe is evacuated (evacuated),
(C) The inside of the glass pipe is blown and evacuated while purging.
(D) The gas in the glass pipe is discharged to reduce the pressure in the glass pipe, and then, at least one cycle of introducing a dry gas into the glass pipe and increasing the pressure in the glass pipe is performed (cycle) purge),
(E) Combinations of (A) to (D), for example, (B) + (A), (C) + (A), (B) + (C) + (D), (B) + (D) , (C) + (D),
Is mentioned. By performing the drying process, the hydrogen atom-containing substances adsorbed on the glass surface (glass pipe inner surface and glass rod surface) in the glass pipe and the hydrogen atom-containing substances contained in the atmosphere in the glass pipe are almost completely removed. Can do. For example, in the case of adsorbed water, it can be reduced to about 10-2 ppm (parts per million by mass) or less.

Embodiment (A) “Blowing purge” will be described. 1 (A) and 1 (B) are conceptual diagrams showing “blow-out purge” which is an embodiment of the drying process of the present invention. FIG. 1 (A) shows the case of only a glass pipe in FIG. B) shows the case where there is a glass rod in the glass pipe.
In the example shown in FIG. 1 (A), the glass pipe 1 is gripped by the gripping part 4 with both ends open, dry gas is introduced from the gas line 5 attached to one end of the glass pipe 1, and attached to the other end. Blowing is performed toward the gas line 6. In the example shown in FIG. 1 (B), a dry gas is introduced from a gas line 5 attached to one end of the glass pipe 1 with the glass rod 2 inserted into the glass pipe 1, and a gas line 6 attached to the other end. A windsock is directed toward
The glass pipe 1 may be heated when performing windsock. Use of a cylindrical mantle heater 7, a tape heater 7 ′ wound around the outer periphery of the glass pipe 1 or an electric furnace as the heating means is preferable because temperature control can be facilitated. An oxyhydrogen burner, a plasma burner, or the like can also be used as the heating means. Blowing and purging may be performed for a long time without heating.

  Since the dry gas is blown from one end to the other in this way, the hydrogen atom-containing substance adsorbed on the inner surface of the glass pipe 1 and the surface of the glass rod 2 is desorbed, and the hydrogen atoms in the atmosphere in the glass pipe 1 The contained material can be reduced. Further, the desorbed hydrogen atom-containing substance is rarely re-adsorbed on the glass surface.

Examples of the dry gas for purging used in the drying step include inert gases such as nitrogen (N ₂ ), helium (He), and argon (Ar), and reactivity such as chlorine (Cl ₂ ) and thionyl chloride (SOCl ₂ ). A gas, and further oxygen (O ₂ ) can be mentioned. These dry gases preferably have a total hydrogen atom-containing substance concentration of 10 volppm (parts per million by volume) or less, and particularly preferably 1 volppm or less.

  The flow rate of the gas blown in the drying step is preferably 10 times or more the pipe volume in the range where the glass pipe is heated in the drying step. By doing in this way, the spreading | diffusion of the water | moisture content from a pipe downstream side can be prevented, and the moisture content inside a pipe can be kept equal to the dry gas supplied.

  In the drying process, it is desirable that the time for blowing and purging be 1 hour or longer. By purging for a long time, the adsorption amount of the hydrogen atom-containing substance can be sufficiently reduced.

When the glass pipe 1 is heated during blow-off purge, energy desorbed from the glass surface can be given to the adsorbed molecules, and the purge time can be shortened. FIG. 11 is a graph showing the amount of moisture desorbed from the heated silica glass. 60 ° C. or more desorption binding force is weak of _H 2 O and the glass surface to be facilitated with (PeakI), strong binding force between the glass surface desorption is promoted at 400 ° C. or more _H 2 O (PeakII) It has been confirmed that there is. It is desirable to heat the temperature of the glass pipe to 60 ° C or higher. Further, since the boiling point of H ₂ O at atmospheric pressure is 100 ° C., it is also desirable to heat to 100 ° C. or higher.
The temperature of the glass pipe is preferably heated to less than 200 ° C. If purge is performed by blowing at a temperature of less than 200 ° C., generation of OH groups can be suppressed. When the temperature is 200 ° C. or higher, H ₂ O may react with the glass surface.
The temperature of the glass pipe is heated to 60 ° C. or higher and lower than 200 ° C., and H ₂ O having a weak binding force with a large amount of the glass surface is desorbed from the glass surface, and then the temperature of the glass pipe is heated to 300 ° C. or higher. It is also desirable to do. Needless to say, the hydrogen atom-containing substance can be reduced even if blow-by purge is performed at room temperature.

It is desirable to heat the glass pipe in the drying step in a wider range than the range in which the glass pipe is heated to 550 ° C. or higher in the sealing step or the solidifying step. In FIG. 1 (B), a range A indicates a portion that is collapsed to become an effective portion of the optical fiber preform (a portion of the optical fiber preform that can be a product), and this portion is increased to 550 ° C. or higher in the process of solidification. Heated. Range B is the range to be heated in the drying process. As shown in the figure, by taking the range B wider than the range A, the inside of the pipe is dehydrated in advance, and the OH group contained on the inner surface of the glass pipe in the range B is desorbed as H ₂ O, Re-adsorption to the inner surface of the glass pipe in range A can be prevented.

Next, the embodiment (B) “evacuation” will be described. In this embodiment, the absolute pressure in the glass pipe is 4 kPa or less, preferably 0.4 kPa or less, particularly preferably 0.04 kPa or less. Since the saturated water vapor pressure of H ₂ O at a temperature of 25 ° C. is 4 kPa, desorption of H ₂ O can be accelerated by lowering the glass pipe internal pressure. In addition, by lowering the internal pressure, the mean free process of H ₂ O can be lengthened and the probability of collision with the glass wall surface can be lowered, so that re-adsorption to the glass surface can be greatly suppressed. .

  Further, the embodiment (D) “cycle purge” will be described. FIG. 2A to FIG. 2D are conceptual diagrams showing “cycle purge” which is an embodiment of the drying process of the present invention. 2A to 2C show the case where the glass pipe penetrates, and FIG. 2D shows the case where the glass pipe is sealed on the way. In this embodiment, the pressure in the step of reducing the pressure is preferably 4 kPa or less. The pressure in the step of increasing the pressure is preferably 50 kPa or more, and more preferably 100 kPa or more.

The cycle purge when the glass pipe is penetrating is performed by repeating the procedure such as i) to iii) once or more.
i) The valve 9 is closed and the valve 10 is opened to exhaust.
← → (Valve 9 is opened, valve 10 is closed, and dry gas is introduced [FIG. 2 (A)].
ii) The valve 9 is opened, the valve 10 is opened, and the exhaust gas is exhausted while introducing the dry gas.
← → (Valve 9 is opened and valve 10 is closed [FIG. 2 (B)].
iii) The valve 9 is closed and the valve 10 is opened to exhaust.
← → (Valve 9 is opened and valve 10 is opened and exhausted while introducing dry gas [FIG. 2 (C)]. Iii) In the case of iii), even if exhausted by increasing the flow rate of dry gas and weakening the exhaust gas The inside of the glass pipe can be pressurized.

  The cycle purge when the glass pipe is sealed in the middle is performed as follows [FIG. 2 (D)]. In this case, a gas line 8 that can be evacuated is connected to the gas line 5 attached to the gas introduction side end. First, the valve 9 is closed, the valve 13 is opened, and the gas is exhausted through the gas line 5 and the gas line 8 so that the absolute pressure in the glass pipe 11 is, for example, 4 kPa or less, and adsorbed water and the like are evaporated. Next, the valve 9 is opened to introduce the dry gas into the glass pipe 11 and the valve 10 is closed to bring the absolute pressure in the glass pipe 11 to a state of 50 kPa or more, for example. By alternately performing exhaust and gas introduction one or more times, the hydrogen atom-containing substance adsorbed on the surfaces of the glass pipe 11 and the glass rod 2 can be greatly reduced.

  You may have the process of performing the collapse of this invention in the state which connected the handling pipe to the one end or both ends of a glass pipe, ie, connecting a handling pipe to at least one end of a glass pipe before a drying process. FIG. 3 is a conceptual diagram showing a state in which a glass pipe and a handling pipe are connected. Handling pipes 12 are connected to both ends of a glass pipe 11 which is an effective part of the optical fiber preform. This has the advantage that an expensive glass pipe need not be used more than necessary.

When carrying out the collapse of the present invention by connecting a handling pipe, it is preferable that the hydrogen atom-containing substance adsorbed on the inner surface of the handling pipe is small and the OH group concentration in the handling pipe is low. Specifically, the OH group concentration in the handling pipe is desirably 10 ppm (parts per million by mass) or less. This is because the handling pipe portion is also heated in the process of sealing or solidifying, etc., so that the OH group contained therein is desorbed as H ₂ O, and becomes effective on the pipe inner surface of the effective part. This is because there is a risk of re-adsorption.

  In addition, it is preferable that the handling pipe has a part that radiates infrared rays transmitted through the pipe thickness to the outside of the pipe. FIG. 12 is a conceptual diagram showing a state in which one embodiment of the handling pipe of the present invention is connected to the glass pipe 11. The handling pipe 16 is provided with a radiating portion 15 having a pipe-like shape. When the handling pipe 16 is heated by the heat source 3, the temperature on the side opposite to the heat source 3 with respect to the radiating portion 15 of the handling pipe 16 is lower than that in the case where the radiating portion 15 is not provided. When one end of the handling pipe 16 is heated to 1400 ° C. for 10 minutes, the temperature of the handling pipe 16 at a position 1000 mm away from the heat source 3 is 50 ° C. when the radiating portion 15 is in the middle, and 100 ° C. when there is no radiating portion 15. It was.

  When the handling pipe 16 is used, the drying process of the present invention is performed on the glass pipe 11 side from the heat radiation part 15 to remove the hydrogen-containing substance. Since the side opposite to the glass pipe 11 across the heat radiating part 15 of the handling pipe 16 is not heated to a high temperature, the hydrogen-containing substance adsorbed on this part of the effective part of the glass pipe 11 is not produced during the manufacturing process. It will not spread toward you. The heat radiating portion 15 may be formed in a form other than forming the pipe in a nodal shape, for example, an opaque quartz glass pipe that scatters infrared rays, sandwiched between the handling pipes 16. Alternatively, two pipes may be connected to form a handling pipe, and the fusion spliced portion may be used as the heat radiating unit 15.

4 (A) to 4 (C) are conceptual diagrams showing an embodiment of the connecting step of the present invention. The glass pipe 11 and the handling pipe 12 are connected by heating and melting them using the heat source 3. When a heat source that does not generate H ₂ O, such as a plasma burner, an induction furnace, or a resistance furnace, is used, it is possible to greatly suppress the mixing of H ₂ O when the glass pipe and the handling pipe are connected. On the other hand, when a heat source that generates H ₂ O, such as an oxyhydrogen burner, is used, a hydrogen atom-containing substance may be mixed into the glass pipe and the handling pipe when the glass pipe and the handling pipe are connected.
In order to prevent mixing, for example, as shown in FIG. 4A, dry gas is introduced into both pipes from the end surface opposite to the connected side. As shown in FIG. 4B, the sealing material 14 is attached to the opposite end face of one pipe and the dry gas is introduced from the opposite end face of the other pipe. Or as shown in FIG.4 (C), it connects in the state which attached the sealing material 14 to the end surface on the opposite side to the side to which both pipes are connected. By doing so, the amount of outside air mixed into the glass pipe is reduced, and contamination of the inner surfaces of both pipes is reduced. As the drying gas, the same type of gas as the drying gas in the drying step can be used. The total concentration of hydrogen atom-containing substances in the gas is preferably 10 volppm or less, and more preferably 1 volppm or less. It is also a desirable embodiment to perform a drying step after connection.

  The collaps of the present invention preferably has a step of vapor-phase etching the inner surface of a glass pipe (a glass pipe having a handling pipe connected to at least one end is also referred to as a glass pipe) before or after the drying step. By vapor-phase etching, the inner surface of the glass pipe is smoothed, and the chemically adsorbed hydrogen atom-containing substance that could not be removed in the drying process is scraped off, or a certain depth (from several μm from the surface) Impurities that intrude into the order of several millimeters can be removed.

FIG. 5 is a conceptual diagram showing an embodiment of the etching process of the present invention. Heating is performed by the heat source 3 while introducing an etching gas from one end of the glass pipe 1. The range A is a range that is collapsed and becomes an effective part of the optical fiber preform, and is heated to 550 ° C. or higher in the subsequent process, for example, the process of solidification. A range C is a range where gas phase etching is performed. The range C is preferably set so as to cover the range A. This is because when H ₂ O chemically adsorbed on the surface in range A remains, this H ₂ O evaporates when heated to 550 ° C. or higher in the solidification step, This is to prevent contamination.

Examples of gas for gas phase etching include sulfur hexafluoride (SF ₆ ), fluorocarbon (C ₂ F ₆ ), and silicon tetrafluoride (SiF ₄ ). The gas concentration, etching time, and heating temperature may be appropriately selected according to the target degree of etching. Etching gas and Cl ₂ may be mixed and used.

If a preliminary drying step of drying the inside of the glass pipe is performed in advance before the etching step, formation of a strongly acidic liquid due to the reaction between the gas for gas phase etching and H ₂ O can be prevented. In particular, when SF ₆ is used during etching, concentrated sulfuric acid is generated if H ₂ O remains in the pipe. Since it is difficult to remove concentrated sulfuric acid having a low vapor pressure, impurities such as OH groups are mixed into the manufactured optical fiber. The production of concentrated sulfuric acid is very dangerous for workers.

In the present invention, before the drying step, preferably between the etching step and the drying step, the inner surface of the glass pipe is subjected to a modified chemical vapor deposition method (MCVD method), a plasma-activated chemical vapor deposition method (PCVD method), or the like. A step of depositing a layer may be performed. From one end of the glass pipe, silicon tetrachloride (SiCl ₄ ) is used as a glass source gas, and germanium tetrachloride (GeCl ₄ ), phosphorus oxychloride (POCl ₃ ), SiF ₄ or boron trichloride (BCl ₃ ) is used as a refractive index adjusting dopant source gas. ), O ₂ and He are introduced into the glass pipe as a reactive gas and a carrier gas. Then, the glass pipe is heated by a heat source provided so as to be movable relative to the glass pipe outside, and a glass layer is formed on the inner surface of the glass pipe. As the heat source, for example, an oxyhydrogen burner, a plasma flame, an electric furnace such as an induction furnace or a resistance furnace, or the like can be used.

  The glass pipe after carrying out the etching step is in a state in which almost no hydrogen atom-containing substance gas is present in the pipe and is not adsorbed on the glass surface, so that the starting glass pipe of the MCVD method or PCVD method is used. Is very suitable. There is no need to synthesize a so-called optical cladding layer, and even if the core layer is deposited directly on the inner surface of the glass pipe, a high-quality optical fiber can be obtained.

  Similarly, a step of inserting a glass rod into the glass pipe may be performed before the drying step, preferably between the etching step and the drying step. Moreover, you may carry out in order of the process of depositing after the process of etching, the process of inserting, the process of drying, the process of sealing, and the process of solidification.

In addition, when there are metal impurities that cannot be removed only by the drying process, a chemical cleaning process may be performed after the drying process by heating the glass pipe with a gas containing a chlorine element-containing substance. Good. FIG. 8 is a conceptual diagram showing an embodiment of the chemical cleaning process of the present invention. While introducing a reactive gas containing a chlorine atom-containing substance such as Cl ₂ , SOCl ₂ , SiCl ₄ , GeCl ₄ , or carbon tetrachloride (CCl ₄ ) from the gas line 5 into the glass pipe 1, the glass is formed by the heat source 3. The pipe 1 is heated to about 1000 ° C. A metal or metal oxide having a low vapor pressure is reacted with a chlorine atom-containing substance and removed as a metal chloride having a high vapor pressure (for example, nickel chloride is 993 ° C. and iron chloride is about 1020 ° C. and has a vapor pressure of 1 atm). It becomes possible.
A post-drying step of drying the inside of the glass pipe may be performed after the chemical cleaning step. This is because Cl ₂ used in the chemical cleaning step often contains about 1 volppm of H ₂ O, and it is beneficial to remove this H ₂ O. It can be replaced with an adsorbent that reduces the moisture in the Cl ₂ gas in advance (for example, a purifilter (trade name) manufactured by Nippon Oxygen Co., Ltd.). The same applies to other supply gases.

  After the drying process or after the drying process and the chemical cleaning process, the exhaust pipe side of the glass pipe (or handling pipe) is sealed as a sealing process. FIG. 9A and FIG. 9B are conceptual diagrams showing an embodiment of the sealing process of the present invention. As shown in FIG. 9A, the glass pipe 1 is heated by an external heat source 3 to fuse the ends. Instead of fusing the end of the glass pipe, the valve 10 of one gas line 6 may be closed and sealed (not exhausted) as shown in FIG. 9B. Immediately before the solidification step after the sealing step, the gas in the glass pipe is discharged to reduce the pressure in the glass pipe, and then the dry gas is introduced into the glass pipe to introduce the glass pipe. Performing at least one cycle purge for increasing the internal pressure is very effective for reducing the hydrogen atom-containing material.

FIG. 10A and FIG. 10B are conceptual diagrams showing an embodiment of the process of solidifying the present invention. After completion of the sealing step or after completion of the above cycle, the glass pipe 1 is collapsed and solidified by heating with the heat source 3 as shown in FIG. At this time, Cl ₂ , O ₂ , a mixed gas of Cl ₂ and O _2, a mixed gas of Cl ₂ , O _2, and He may flow through the glass pipe, and these gases contain hydrogen atoms contained therein. It is desirable that the total concentration of the substances is 10 volppm or less. Further, when the inside of the glass pipe is exhausted from 100 kPa to 0.1 kPa while flowing gas, it is possible to carry out well without generating bubbles at the solidification interface. Furthermore, the inside of the glass pipe may be exhausted without introducing gas. Furthermore, the pressure in the glass pipe may be adjusted while supplying the gas so that the inside of the glass pipe is slightly positive pressure (+0.01 to +4 kPa, more preferably +0.01 to +1 kPa with respect to the external pressure). .

In the example shown in FIG. 10 (A), the glass pipe 1 and the glass rod 2 are fused and integrated by collaps, but Cl ₂ , N ₂ , O _2, etc. are introduced into the glass pipe 1 during solidification. Since the exhaust gas is exhausted, the concentration of the air mixed in the glass pipe 1 can be reduced. As shown in FIG. 10 (B), it is more preferable that a gas inlet and an outlet are provided independently at the end of the glass pipe 11.

  As an example of a preferred embodiment of the present invention, the step of depositing and / or inserting is performed following the step of connecting, drying, and etching, and then the drying step is performed again to perform chemical cleaning. It is possible to obtain a glass rod by performing a process, a sealing process, a cycle purge, and a solidification process.

As the glass rod and glass pipe used in the present invention, those produced by Vapor-phase Axial Deposition method (VAD method), MCVD method, Outside Vapor Deposition method (OVD method) and other known means can be used. In addition, it is also preferable that the glass rod obtained by carrying out the collapse of the present invention is used as it is as a starting glass rod or as a starting glass pipe in the form of a pipe by drilling or the like, and then the present invention is performed again for collapsing. It is an embodiment.
For example, in the case of processing a solid glass body obtained by the VAD method into a glass pipe, it is desirable to make a hole after stretching rather than stretching after making a hole. If the hole is made first, the moisture adsorbed on the inner surface of the glass pipe becomes OH groups when the glass pipe is heated during stretching and diffuses into the glass, making it difficult to remove the OH groups later. It is. If the hole is made after stretching, it is preferable that the inner surface of the glass pipe is vapor-phase or liquid-phase etched to a thickness of about 1 to 2 mm in the subsequent process.

EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated in detail, this invention is not limited only to these Examples. In Examples and Reference Examples, the "dry _{N 2", H 2} O content of 0.5volppm less, and other hydrogen atom-containing substance content of the gaseous _{N 2} or less 0.1volppm Use.

[Reference Example 1]
FIG. 6 shows a flowchart of Reference Example 1 .
1) A silica glass body to which 27 mol% germanium oxide (GeO ₂ ) was added in the vicinity of the center was manufactured by the VAD method, and processed into a glass rod serving as a core portion having a diameter of 7.5 mm and a length of 600 mm.
2) A silica glass body to which 1.5% (mass percentage) of fluorine was added was manufactured by the VAD method, and processed into a glass pipe that became a depressed portion having an outer diameter of 40 mm, an inner diameter of 8.5 mm, and a length of about 500 mm. . The OH group concentration in the glass pipe was below the detection limit (0.01 ppm) by an infrared spectrometer. The glass pipe was heated to about 1500 ° C. while flowing SF ₆ as an etching gas, and subjected to vapor phase etching.
3) The glass rod prepared in 1) was inserted into the glass pipe prepared in 2).

4) As shown in FIG. 1 (B), evacuated from the gas line 6 while flowing dry N ₂ from the gas line 5 into the glass pipe 1 at 2000 sccm standard cubic center per minute (cubic centimeter per hour in the standard state). And the internal pressure of the glass pipe 1 was 2.5 kPa. At this time, followed by chemical cleaning step of the glass pipe 1 and the glass rod 2, the step of sealing a long range from both ends of the part to be heated to 550 ° C. or higher in each step of solid step by 200 mm, Heated to 200 ° C. with a tape heater 7 ′. The flow rate of the dried N ₂ is about 40 times the volume of the heated portion of the glass pipe 1 of about 50 cm ³ . This state was maintained for 4 hours (first drying step).

5) As shown in FIG. 8, Cl ₂ was introduced into the glass pipe 1 at a flow rate of 500 sccm and heated to 1150 ° C. by the heat source 3 to remove metal impurities (chemical cleaning step).
6) As shown in FIG. 9A, the exhaust end side in the drying process of the glass pipe 1 was heat-sealed with a heat source 3 and sealed.
7) After reducing the pressure inside the glass pipe 1 to 0.01 kPa or less from the gas line 8 by a vacuum pump, dried N ₂ was introduced into the glass pipe 1 and the vacuum pump was stopped to increase the pressure to 105 kPa. This was repeated three times to desorb the adsorption gas (mainly H ₂ O) on the surface of the glass rod 2 and the inner surface of the glass pipe 1.

8) As shown in FIG. 10A, the glass pipe 1 and the glass rod 2 were integrated (solidified) to obtain a glass body having a diameter of about 39 mm and a length of 400 mm. At this time, 500 sccm of Cl ₂ and 500 sccm of O ₂ were introduced into the glass pipe 1. The pressure in the glass pipe 1 was −1 kPa with respect to the atmospheric pressure, and the outer surface temperature (value measured with a radiation thermometer) of the glass pipe 1 at the time of collapse was 1600 ° C.

9) After extending the glass body to an outer diameter of 11.7 mm, the outer periphery was ground by mechanical grinding to obtain a glass rod having an outer diameter of 6.4 mm and a length of 600 mm.
10) A silica glass body to which about 0.9% of fluorine (F) was added by the VAD method was manufactured and processed into a glass pipe that became a clad portion having an outer diameter of 43 mm, an inner diameter of 9 mm, and a length of about 500 mm. Prior to collapsing, this pipe was heated to about 1500 ° C. while flowing SF ₆ and subjected to vapor phase etching.
11) The glass rod prepared in 9) was inserted into the glass pipe prepared in 10).

12) While flowing 2000 sccm of dry N ₂ into the glass pipe from one end, the other end was evacuated so that the internal pressure of the glass pipe was not lower than the outside air, and the internal pressure of the glass pipe was set to 103 kPa. At this time, a 200 mm long range from both ends of the portion heated to 550 ° C. or higher in each step of the subsequent chemical cleaning step, sealing step, and solidification step is heated to 170 ° C. with a tape heater. The state was maintained for 1 hour and then purged by blowing off (second drying step). The flow rate of N ₂ is about 35 times the volume of the heated portion of the glass pipe of about 60 cm ³ .

13) Cl ₂ was introduced into the glass pipe at a flow rate of 1000 sccm and heated to 1120 ° C. to remove metal impurities.
14) The exhaust end side of the glass pipe was sealed.
15) A cycle purge was performed under the same conditions as in 7).
16) The glass pipe and glass rod of 15) were integrated (solidified) as shown in FIG. 10A to obtain a glass body having a diameter of 42 mm and a length of 400 mm. At this time 100sccm a Cl ₂ to the glass pipe 1, and the _{O 2} was introduced 900 sccm. The pressure in the glass pipe 1 was −4 kPa with respect to the atmospheric pressure, and the outer surface temperature of the glass pipe 1 at the time of collapse was 1500 ° C.

17) Silica glass in which about 0.9% of fluorine is added to the outer periphery of the glass body by the OVD method so that the ratio D / 2a of the outer diameter D of the cladding portion to the diameter 2a of the core portion is 43.1 times. The optical fiber preform was obtained by synthesis. A part of this optical fiber preform was cut into pieces, and the OH group concentration of the part integrated in 8) was measured by an infrared absorption method, and found to be 15 ppb.
18) The optical fiber preform was drawn to obtain an optical fiber having a glass portion with an outer diameter of 100 μm. The refractive index profile of the optical fiber is shown in FIG.

The transmission characteristics of the optical fiber at a wavelength of 1550 nm are as follows: transmission loss: 0.50 dB / km, dispersion: -41 ps / km / nm, dispersion slope: -0.01 ps / km / nm ² , Aeff: 8.5 μm ² , cut-off Wavelength: 1050 nm, PMD: 0.1 ps / km ^1/2 , and it was confirmed that a highly nonlinear fiber was obtained. This optical fiber has good characteristics as an optical fiber for Raman amplification. Further, the wavelength characteristic of the transmission loss of this optical fiber is as shown by the solid line in FIG. 14, and the excessive transmission loss due to the OH group in the 1.4 μm band can be suppressed to 0.1 dB / km or less at the wavelength of 1.38 μm. ing.

[Comparative Example 1]
An optical fiber was obtained in the same manner as in Reference Example 1 except that the first drying step and the chemical cleaning step with Cl _{2 in} 5) were omitted. The wavelength characteristic of the transmission loss of this optical fiber is as shown by the broken line in FIG. 14, and the absorption by the OH group was 1.3 dB / km.

FIG. 7 shows a flowchart of the first embodiment .
1) A silica glass body to which 22 mol% of GeO ₂ was added in the vicinity of the center was manufactured by the VAD method, and processed into a glass rod serving as a core portion having a diameter of 8.5 mm and a length of 600 mm.

2) By the same method as in Reference Example 1 , a glass pipe to be a depressed part having an outer diameter of 40 mm, an inner diameter of 8.5 mm, and a length of about 500 mm to which 1.5% by mass of fluorine was added was prepared.
3) Using a horizontal glass lathe, using an oxyhydrogen flame burner as a heat source, a handling pipe having an outer diameter of 40 mm, an inner diameter of 10 mm, and a length of about 400 mm was connected to both ends of the glass pipe. When measured with an infrared spectrometer, the concentration of OH groups contained in the handling pipe was 8 ppm. When connecting the glass pipe and the handling pipe, dried N ₂ was allowed to flow through each pipe from the end opposite to the end to be connected as shown in FIG. 4 (A).

4) The inside of the glass pipe was evacuated by an oil rotary pump, and the pressure was reduced to an absolute pressure of 40 Pa. Thereafter, dry N ₂ was blown into the pipe. The pipe internal pressure at this time was about 102 kPa in absolute pressure. The depressurization and blowing were repeated three times to discharge the hydrogen element-containing material in the atmosphere in the pipe and the hydrogen element-containing material adsorbed on the pipe inner surface. (Preliminary drying process)
5) Gas phase etching was performed by heating with a plasma burner while flowing SF ₆ at 100 sccm, Cl ₂ at 200 sccm, and He at 100 sccm. The pipe was reciprocated several times until the inner diameter of the pipe of the effective part became 11 mm.

6) The glass rod prepared in 1) was inserted into the pipe of 5). After the pipe was evacuated to 0.1 kPa or less by a vacuum pump, dried N ₂ was introduced into the glass pipe, the vacuum pump was stopped, and the pressure was increased to 103 kPa. This was repeated three times to desorb the adsorbed gas on the glass rod surface and the inner surface of the glass pipe. At this time, a 300 mm long range was heated to 450 ° C. with a mantle heater from both ends of the portion that became 550 ° C. or higher in the subsequent step. Thereafter, the glass pipe was heated to 1200 ° C. while introducing Cl ₂ into the pipe at a flow rate of 500 sccm to remove metal impurities.

7) A plasma flame was used as a heat source to seal between the glass pipe and the glass rod. Subsequently, a dry O ₂ gas (concentration of H ₂ O of 0.5 volppm, the concentration of other hydrogen atom-containing substances is 0.1 volppm or less) flows through 400 sccm and has a performance capable of reducing the pressure to 0.03 kPa. The glass pipe and the glass rod were integrated (solidified) while evacuating with a vacuum pump to obtain a glass body having an outer diameter of 39 mm and a length of about 400 mm. During the integration, the pressure inside the pipe was about 0.1 kPa and the outer surface temperature of the glass pipe was 1200 ° C. Further, when the amount of air entrained in the glass pipe was separately investigated, it was about 0.01 sccm.

  8) After stretching the glass body to an outer diameter of 13.7 mm, the outer periphery was ground by mechanical grinding or etching with a hydrogen fluoride solution to obtain a glass rod having an outer diameter of 9.6 mm and a length of 600 mm.

9) A silica glass body to which 3% (mass percentage) of chlorine (Cl) is added is manufactured by the VAD method, and the outer diameter is 45 mm and the length is processed by stretching to a uniform diameter and opening in the center. It was set as the glass pipe used as a clad part of about 650 mm. The concentration of OH groups in this pipe was below the detection limit.
10) Using the same apparatus as in 3), a handling pipe having an outer diameter of 42 mm, an inner diameter of 15 mm, and a length of 400 mm was connected to both ends of the glass pipe prepared in 9). When the concentration of OH groups contained in the handling pipe was measured with an infrared spectrometer, it was about 8 ppm. At the time of connection, dry N ₂ was blown from the end of each pipe opposite to the end to be connected.

11) The same pre-drying process as 4) was performed on the glass pipe of 10).
12) Gas phase etching was performed by heating with a plasma burner while flowing SF ₆ at 150 sccm, Cl ₂ at 200 sccm, and He at 100 sccm in a glass pipe. The burner was reciprocated several times until the inner diameter of the glass pipe of the effective part became 14.5 mm.
13) A silica glass layer having a thickness of 1.7 mm was synthesized by an MCVD method in which a glass pipe was heated with a plasma flame to form a ring portion to which 4 mol% of GeO ₂ was added on average. The inner diameter of the glass pipe was 11.2 mm.

14) The glass rod prepared in 8) was inserted into the glass pipe, and N ₂ dried while heating with a mantle heater at about 200 ° C. was introduced into the glass pipe at 5000 sccm and blown away. The range of heating was set to cover a range of 250 mm from both ends of the portion heated to 550 ° C. or higher in the subsequent step. The flow rate of N ₂ is about 44 times the volume of the heated portion of the glass pipe, which is about 113 cm ³ . This state was maintained for 5 hours to desorb the adsorbed gas on the glass rod surface and the pipe inner surface. Thereafter, Cl ₂ was introduced into the pipe by 1000 sccm and heated to 1200 ° C. to remove metal impurities by reacting with Cl ₂ .

15) The glass pipe and glass rod of 14) were sealed and integrated (solidified) in the same manner as in 7) to obtain a glass body having an outer diameter of 43 mm and a length of 500 mm.
16) An optical fiber mother having a ratio of the outer diameter D of the clad to the outer diameter 2a of the core D / 2a of 44.64 times synthesized by synthesizing 3% of silica glass by the VAD method on the outer periphery of the glass body. The material was obtained.

17) The optical fiber preform was drawn to obtain an optical fiber having an outer diameter of the glass portion of 125 μm. FIG. 15 shows the refractive index profile of this optical fiber.
FIG. 7 is a flowchart of the first embodiment .
The transmission characteristics of this optical fiber at a wavelength of 1550 nm are as follows: transmission loss: 0.49 dB / km, dispersion: -159.5 ps / km / nm, dispersion slope: -0.65 ps / km / nm ² , Aeff: 17 μm ² , cut Off wavelength: 1380 nm, PMD: about 0.05 ps / km ^1/2 .

The transmission loss-wavelength characteristic of this optical fiber is as shown in FIG. 16, and the excessive transmission loss due to the OH group in the 1.4 μm band is suppressed to 0.05 dB / km or less at the wavelength of 1.38 μm. Recognize. This dispersion compensating fiber is a broadband dispersion compensating fiber that compensates the chromatic dispersion of an optical fiber having a zero dispersion wavelength in the 1.3 μm band over 1.45 to 1.62 μm, but excessive transmission loss due to the OH group. Therefore, the transmission loss near 1.45 μm is low and good. Further, Raman amplification using this optical fiber can be suitably performed.
When this optical fiber was immersed in an H ₂ atmosphere at a temperature of 80 ° C. and a concentration of 100% (volume percentage) for 24 hours and the loss at a wavelength of 1.38 μm was measured again, the loss increase was less than the measurement limit (0.05 dB / km). It was extremely small.

[Reference Example 2]
1) A silica glass body to which 0.4% of chlorine was added by the VAD method was manufactured, and processed into a glass rod serving as a core portion having a diameter of 4 mm and a length of 600 mm. The relative refractive index difference of the glass rod was 0.06% with respect to the silica glass.
2) A silica glass body to which 1.0% of fluorine was added was manufactured by the VAD method, and processed into a glass pipe serving as a clad portion having an outer diameter of 25 mm, an inner diameter of 4 mm, and a length of about 500 mm. While flowing SF ₆ through the glass pipe, the glass pipe was heated to about 1500 ° C. to vapor-phase etch the inner surface of the glass pipe, so that the inner diameter of the glass pipe was 6 mm. The relative refractive index difference of the glass pipe was -0.33% with respect to silica glass.
3) The glass rod prepared in 1) was inserted into the glass pipe prepared in 2).
4) it is evacuated from the other end while passing 2000sccm the _{N 2} dried from one end to the glass pipe was a glass pipe internal pressure 2.5 kPa. At this time, followed by chemical cleaning step of the glass pipe and the glass rod, the step of sealing a long range from both ends of the part to be heated to 550 ° C. or higher in each step of solid step by 200 mm, the tape heater At 200 ° C. This state was maintained for 4 hours (drying process).
5) Cl ₂ was introduced into the glass pipe at a flow rate of 500 sccm, and the metal impurities were removed by heating to 1150 ° C. with a heat source.
6) The exhaust end side in the glass pipe drying step was heat-sealed with a heat source and sealed.
7) After reducing the pressure inside the glass pipe to 0.01 kPa or less with a vacuum pump, dried N ₂ was introduced into the glass pipe, and the vacuum pump was stopped and pressurized to 105 kPa. This was repeated three times to desorb the adsorbed gas (mainly H ₂ O) on the glass rod surface and the inner surface of the glass pipe.
8) The glass pipe and the glass rod were integrated (solidified) to obtain a glass body having a diameter of about 24.3 mm and a length of 400 mm. At this time, 500 sccm of Cl ₂ and 500 sccm of O ₂ were introduced into the glass pipe. The pressure inside the glass pipe was −1 kPa with respect to atmospheric pressure, and the outer surface temperature (value measured with a radiation thermometer) of the glass pipe 1 at the time of collapse was 1600 ° C.
9) An optical fiber in which silica glass with about 1.0% fluorine added to the outer periphery of the glass body is synthesized by the OVD method, and the ratio D / 2a of the outer diameter D of the cladding portion to the diameter 2a of the core portion is 15 times. I got the base material.
10) The optical fiber preform was drawn to obtain an optical fiber having a glass portion having an outer diameter of 125 μm.
The increase in transmission loss due to the OH group of the optical fiber was 0.03 dB / km at a wavelength of 1.38 μm.

[Comparative Example 2]
In Reference Example 2 , an optical fiber was manufactured by omitting step 4) (drying step). The increase in transmission loss due to the OH group of the optical fiber was 2.0 dB / km at a wavelength of 1.38 μm.

1) In the same manner as 1) to 8) of Reference Example 2 , it has a core part made of silica glass added with 0.35% chlorine and a clad part made of silica glass added with 1.1% fluorine. A glass body having a diameter of about 44 mm and a length of 400 mm was obtained. The relative refractive index difference of the core part with respect to the clad part was 0.39%.
2) After extending the glass body to an outer diameter of 11.7 mm, the outer periphery was ground by mechanical grinding to obtain a glass rod having an outer diameter of 6.4 mm and a length of 600 mm.
3) A silica glass body to which about 1.1% of fluorine was added by the VAD method was manufactured and processed into a glass pipe to be an outer cladding portion having an outer diameter of 43 mm, an inner diameter of 9 mm, and a length of about 500 mm. While flowing SF ₆ through the pipe, it was heated to about 1500 ° C. to perform a gas phase etching process.
4) The glass rod prepared in 2) was inserted into the glass pipe prepared in 3).

5) while passing 2000sccm the _{N 2} dried into the glass pipe from one end, the glass pipe pressure is evacuated to a degree that does not become lower than the outside air, and the glass pipe internal pressure 103kPa at the other end. At this time, a 200 mm long range from both ends of the portion heated to 550 ° C. or higher in each step of the subsequent chemical cleaning step, sealing step, and solidification step is heated to 170 ° C. with a tape heater. The state was maintained for 1 hour and then purged by blowing off (second drying step). The flow rate of N ₂ is about 35 times the volume of the heated portion of the glass pipe of about 60 cm ³ .
6) Cl ₂ was introduced into the glass pipe at a flow rate of 1000 sccm and heated to 1120 ° C. to remove metal impurities.
7) The exhaust end side of the glass pipe was sealed.
8) A cycle purge was performed.

9) Light in which the glass pipe and glass rod of 8) are integrated (solidified), and the ratio D / 2a of the outer diameter D of the clad part to the diameter 2a of the core part 42 mm, the length 400 mm, and the core part 15 times. A fiber preform was obtained. At this time 100sccm a Cl ₂ to the glass pipe, and the _{O 2} was introduced 900 sccm. The pressure inside the glass pipe was −4 kPa with respect to atmospheric pressure, and the outer surface temperature of the glass pipe at the time of collapse was 1500 ° C.
10) The optical fiber preform was drawn to obtain an optical fiber having a glass portion having an outer diameter of 125 μm.

The increase in transmission loss due to the OH group of the optical fiber was 0.02 dB / km at a wavelength of 1.38 μm. Since glass body corresponding to the glass body 1 of Example 2) 8) of Reference Example 2 is not heated directly by the oxyhydrogen flame, the increase in transmission loss of the optical fiber of Example 2 as compared to the optical fiber of Reference Example 2 It is getting smaller.

[Comparative Example 3]
In Example 2 , an optical fiber was manufactured by omitting the first and second drying steps. The increase in transmission loss due to the OH group of the optical fiber was 1.5 dB / km at a wavelength of 1.38 μm.

In Reference Example 1 and Example 1 , a tape heater or a mantle heater was used as a heat source used in the drying process, but other heat sources such as an induction furnace, a resistance furnace, a laser, and the like can also be used. Although an oxyhydrogen flame burner and a plasma flame burner are used as heat sources for the solidification process, other heat sources such as an induction furnace and a resistance furnace can also be used. A vertical apparatus may be used in place of the horizontal glass lathe. When a heat source having an axisymmetric heat distribution is used in a vertical apparatus, it is not necessary to rotate the glass pipe and the glass rod. The glass pipe and the glass rod can be produced by any means known in this technical field, for example, any of the VAD method, OVD method, MCVD method, and collapse method. Mechanical grinding for surface cleaning and size adjustment of glass pipes and glass rods may be performed by chemical etching (gas phase, liquid phase). Needless to say, the present invention can also be applied to the case where only the glass pipe is made solid.

It is the concept which shows "blow-out purge" which is one Embodiment of the drying process of this invention, and shows the case of only a glass pipe. It is a conceptual diagram which shows "blow-out purge" which is one Embodiment of the drying process of this invention, and shows the case where a glass rod exists in a glass pipe. It is a conceptual diagram which shows "cycle purge" which is one Embodiment of the drying process of this invention, and shows the case where the glass pipe has penetrated. It is a conceptual diagram which shows "cycle purge" which is one Embodiment of the drying process of this invention, and shows the case where the glass pipe has penetrated. It is a conceptual diagram which shows "cycle purge" which is one Embodiment of the drying process of this invention, and shows the case where the glass pipe has penetrated. It is a conceptual diagram which shows "cycle purge" which is one Embodiment of the drying process of this invention, and shows the case where the glass pipe is sealed on the way. It is a conceptual diagram which shows the state which connected the glass pipe and the handling pipe. It is a conceptual diagram which shows one Embodiment of the process of connecting of this invention. It is a conceptual diagram which shows one Embodiment of the process of connecting of this invention. It is a conceptual diagram which shows one Embodiment of the process of connecting of this invention. It is a conceptual diagram which shows one Embodiment of the process of etching of this invention. 10 is a flowchart illustrating Reference Example 1 . 2 is a flowchart for explaining Example 1 ; It is a conceptual diagram which shows one Embodiment of the chemical cleaning process of this invention. It is a conceptual diagram which shows one Embodiment of the process to seal of this invention. It is a conceptual diagram which shows one Embodiment of the process to seal of this invention. It is a conceptual diagram which shows one Embodiment of the process of solidifying this invention. It is a conceptual diagram which shows one Embodiment of the process of solidifying this invention. It is a graph which shows the moisture content which remove | desorbs from the heated silica glass. It is a conceptual diagram which shows the state which connected one Embodiment of the handling pipe of this invention to the glass pipe. It is a figure which shows the refractive index profile of the optical fiber obtained by the reference example 1. FIG. It is a figure which shows the wavelength characteristic of the transmission loss of the optical fiber obtained by the reference example 1 and the comparative example 1. FIG. It is a figure which shows the refractive index profile of the optical fiber obtained in Example 1. FIG. It is a figure which shows the wavelength characteristic of the transmission loss of the optical fiber obtained in Example 1. FIG. It is a conceptual diagram which shows the collapse method. It is a figure which shows the wavelength transmission loss increase by OH group in a wavelength 1.4micrometer band.

1 glass pipe, 2 glass rod, 3 heat source,
4 gripping part, 5 gas line, 7 mantle heater,
7 'tape heater, 9 valve, 11 glass pipe,
12 handling pipe, 13 glass lathe, 14 sealing material,
15 Radiation part, 16 Handling pipe.

Claims (21)

A method of manufacturing an optical fiber preform by solidifying a glass pipe,
A drying step in which an inert gas having a total concentration of hydrogen atom-containing substances of 10 volppm or less is blown into the glass pipe while heating the glass pipe to 550 ° C. or less;
A chemical cleaning step of heating the glass pipe while flowing a gas containing a chlorine element-containing substance in the glass pipe after the drying step;
And a step of sealing one end of the glass pipe, and a step of solidifying the glass pipe by exhausting gas while flowing it so that the absolute pressure in the glass pipe is 100 kPa to 0.1 kPa. Manufacturing method of fiber preform.   The method of manufacturing an optical fiber preform according to claim 1, wherein in the drying step, the temperature of the glass pipe is heated to 60 ° C or higher.   The method of manufacturing an optical fiber preform according to claim 1, wherein in the drying step, the temperature of the glass pipe is heated to 300 ° C or higher.   The method of manufacturing an optical fiber preform according to claim 1, wherein in the drying step, the temperature of the glass pipe is heated to 60 ° C or higher and lower than 200 ° C and then heated to 300 ° C or higher.   The said drying process WHEREIN: The said glass pipe is heated in the range wider than the range in which the said glass pipe is heated to 550 degreeC or more by the said solidification process, The Claim 1 thru | or 4 characterized by the above-mentioned. Manufacturing method of optical fiber preform.   The flow rate per minute of the gas having a total concentration of the hydrogen atom-containing substances of 10 volppm or less is 10 times or more the volume of the range in which the glass pipe is heated in the drying step. 2. A method for producing an optical fiber preform according to 1.   The method for manufacturing an optical fiber preform according to any one of claims 1 to 6, further comprising a step of connecting a handling pipe to at least one end of the glass pipe before the drying step.   In the drying step, at least one cycle in which the gas in the glass pipe is discharged to reduce the pressure in the glass pipe, and then the drying gas is introduced into the glass pipe to increase the pressure in the glass pipe. The method for manufacturing an optical fiber preform according to any one of claims 1 to 7, wherein the method is performed more than once.   8. The method of manufacturing an optical fiber preform according to claim 7, wherein an OH group concentration in the handling pipe is 10 ppm or less.   The method of manufacturing an optical fiber preform according to claim 7 or 9, wherein the handling pipe has a part that radiates infrared rays transmitted through the pipe thickness to the outside of the pipe.   The method for manufacturing an optical fiber preform according to any one of claims 1 to 10, wherein an absolute pressure in the glass pipe is set to 4 kPa or less in at least a part of the drying step.   The method for manufacturing an optical fiber preform according to any one of claims 1 to 11, wherein the drying step is performed for 1 hour or more.   The method of manufacturing an optical fiber preform according to any one of claims 1 to 12, further comprising a step of depositing a glass layer on the inner surface of the glass pipe before the drying step.   The method for producing an optical fiber preform according to any one of claims 1 to 13, further comprising a step of inserting a glass rod into the glass pipe before the drying step.   The method for producing an optical fiber preform according to any one of claims 1 to 13, further comprising a step of performing vapor phase etching of an inner surface of the glass pipe before the drying step.   The method for manufacturing an optical fiber preform according to claim 15, further comprising a preliminary drying step of drying the inside of the glass pipe before the etching step.   The optical fiber preform according to claim 15 or 16, wherein, in the etching step, the glass pipe is etched in a range wider than a range in which the glass pipe is heated to 550 ° C or higher in the subsequent step. Production method.   The method for producing an optical fiber preform according to claim 1, further comprising a post-drying step of drying the inside of the glass pipe after the chemical cleaning step. .   After the sealing step, the gas in the glass pipe is discharged to reduce the pressure in the glass pipe, and then the dry gas is introduced into the glass pipe to increase the pressure in the glass pipe. The method for manufacturing an optical fiber preform according to any one of claims 1 to 18, wherein the cycle is performed at least once.   The method for manufacturing an optical fiber preform according to any one of claims 1 to 19, wherein in the solidifying step, the absolute pressure in the pipe is 4 kPa or less. A method of manufacturing an optical fiber preform by inserting a glass rod into a glass pipe and integrating the glass rod,
Vapor phase etching of the inner surface of the glass pipe,
Inserting a glass rod into the glass pipe;
A step of blowing an inert gas in which the concentration of the hydrogen atom-containing substance is 10 volppm or less in total in the glass pipe while heating the glass pipe and the glass rod to 60 to 550 ° C.,
Heating while flowing a gas containing a chlorine atom-containing substance,
Sealing one end of the glass pipe, and exhausting the gas while flowing gas so that the absolute pressure in the glass pipe is 100 kPa to 0.1 kPa, and the glass pipe and the glass rod are heated and integrated. An optical fiber preform manufacturing method characterized by the above.

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