JP2007045643A - Method of manufacturing glass preform for optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a glass preform for a low-loss optical fiber using a porous glass preform with a very little water content prepared by using a high-frequency induction heat plasma torch. <P>SOLUTION: The porous glass preform is manufactured by providing a glass material and oxygen to a high-frequency induction heat plasma torch to synthesize glass microparticles and spraying the formed glass microparticles onto the surface of a glass rod to be deposited. A quartz glass substantially free from OH group is preferred to be used as the glass rod. As the glass material, silicon tetrachloride or the like is used. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a glass preform for optical fiber that is produced by depositing glass fine particles generated using a high frequency induction thermal plasma torch on a target (glass rod) to produce a porous glass preform for optical fiber, and forming a transparent glass. It relates to a manufacturing method.

  Transmission loss is a characteristic of an optical fiber used in an optical communication system. This is an index indicating how much light is attenuated per unit propagation distance, and the lower this value, the more the signal can be transmitted. The most commonly used optical fiber is a germanium-doped core fiber in which quartz glass doped with germanium is used to increase the refractive index and pure silica glass is used for the cladding.

On the other hand, as described in Non-Patent Document 1, pure silica core fiber using pure silica glass for the core and silica glass doped with fluorine to reduce the refractive index is used for the cladding. is there.
In the germanium-doped core fiber, Rayleigh scattering is increased and transmission loss is increased because germanium is doped in the core as compared with a pure silica core fiber.

  As a variation of the pure silica core fiber, there is a core doped with chlorine as described in Patent Document 1. In terms of Rayleigh scattering, pure silica core fiber is superior to germanium doped core fiber, but germanium doped core fiber is generally superior in terms of OH absorption loss near 1.38 μm. .

  In recent years, germanium-doped core fiber has been commercially available as an optical fiber with low OH absorption loss contribution of less than 0.01 dB / km (OH concentration less than 0.15 ppb), that is, Low Water Peak Fiber (LWPF). Yes. This is because when a general VAD method is used as a method for producing germanium-doped fiber, the core and the clad portion in the vicinity of the core can be synthesized at the same time, and the OH group in the region where light bleeds is almost completely removed in the dehydration process By being possible.

On the other hand, as described in Patent Document 1, a pure silica core fiber doped with fluorine in the clad generally has an OH absorption loss of about 0.3 dB / km or more.
Fig. 1 shows examples of transmission loss spectra of germanium-doped LWPF and pure silica core fiber (PSCF). In this example, the PSCF has an OH absorption loss of approximately 0.5 dB / km.
Re-published patent WO 00/42458 SEI Technical Review, Sept., 2003, No.163, pp. 10-13,

  Pure silica core fiber is an effective means to reduce transmission loss in the 1.3 μm and 1.55 μm bands, but commercially available means to reduce OH absorption loss in the vicinity of 1.38 μm have been known so far. It wasn't. If the OH absorption loss near 1.38 μm can be sufficiently reduced in a pure silica core fiber, low loss transmission can be performed in a wide wavelength band of approximately 1200 to 1650 nm, and wavelength division multiplexing transmission using this wavelength band is possible. It is very effective for such as.

  The present invention uses a high frequency induction thermal plasma torch to produce a porous glass preform for an optical fiber having a very low moisture content, and provides a method for producing a low loss optical fiber glass preform using the same. The purpose is to do.

  The method for producing a porous glass preform for an optical fiber according to the present invention comprises synthesizing glass particles by supplying glass raw material and oxygen to a high-frequency induction thermal plasma torch, and spraying and depositing the generated glass particles on the surface of the glass rod. The glass rod is made of pure quartz glass substantially free of OH groups. The glass raw material is silicon tetrachloride.

  The method for producing a glass preform for an optical fiber according to the present invention comprises synthesizing glass fine particles by supplying a glass raw material and oxygen to a high frequency induction thermal plasma torch, and depositing the produced glass fine particles on a glass rod to form a porous glass preform. The porous glass base material is heated in a chlorine-containing atmosphere at 900 to 1200 ° C and dehydrated, and then heated and transparentized in a fluorine-containing atmosphere at 1300 to 1600 ° C. For the glass rod, pure quartz glass substantially free of OH groups is used. The glass raw material is silicon tetrachloride. The glass base material thus obtained is further provided with a fluorine-doped quartz glass layer on the outside thereof, and a pure silica core glass base material for optical fibers is obtained.

  According to the present invention, a porous glass preform for optical fiber with an extremely low water content can be obtained, and after this is made into a transparent glass, a fluorine-doped quartz glass layer is further provided on the outside thereof, so that an extremely low loss can be obtained. A glass preform for a pure silica core optical fiber is obtained.

  When the production method described in Patent Document 1 is examined in detail, the production of a pure silica core fiber cannot simultaneously synthesize the core and the cladding portion adjacent to the core. In order to synthesize the clad portion adjacent to the core, it is necessary to deposit glass fine particles generated by flame hydrolysis with an oxyhydrogen flame or the like on the outer periphery of the glass rod that becomes the core. It was found that the final OH absorption loss was increased by generating OH groups on the surface of the glass rod and diffusing it to the inside of the glass rod.

The porous glass base material manufactured in this way is dehydrated and sintered to become a transparent glass base material, but the OH group in the porous glass layer portion deposited on the glass rod in this dehydration process is removed. However, the OH group diffused to the inside of the core glass rod is not removed.
As a result of intensive studies, the present inventor has found that this problem can be solved by using a plasma flame without using an oxyhydrogen flame usually used in a cladding deposition process.
Hereinafter, the present invention will be described in detail with reference to the drawings.

  FIG. 2 is a partial longitudinal sectional view showing an outline of the cladding deposition process of the present invention. On the target rod 1 that rotates and reciprocates, glass fine particles generated by plasma 3 serving as a clad are deposited, and a porous glass base material 2 is formed. The plasma 3 is generated by a high frequency induction thermal plasma torch (hereinafter simply referred to as a plasma torch), and the plasma torch is a quadruple tube comprising a first tube 4, a second tube 5, a third tube 6 and a fourth tube 7. The high frequency coil 8 is disposed outside the fourth tube 7.

Silicon tetrachloride and argon are supplied to the flow path 9 of the plasma torch, argon is supplied to the flow path 10, and oxygen and argon are supplied to the flow path 11. Note that cooling water is supplied to the flow path 12. The gas flowing in the flow paths 9 to 11 is converted into plasma by induction of the high frequency coil 8, becomes high temperature plasma of several thousand degrees C. or more, reacts inside the plasma to generate quartz glass fine particles, and deposits on the target rod 1. Thus, the porous glass base material 2 is formed.
Since no hydrogen content is supplied into the plasma, OH groups do not essentially penetrate and diffuse into the target rod 1.

The porous glass preform 2 thus obtained is subjected to dehydration and transparent vitrification in the dehydration and sintering apparatus shown in FIG. The porous glass base material 2 is connected to a rotating / lifting device 14 via a shaft 13 and is suspended in the furnace core tube 15.
Dehydrated gas such as chlorine diluted with helium gas is supplied from the gas inlet 16 into the furnace core tube 15 and discharged from the exhaust port 17. In this state, the heating furnace 18 is heated to 900 to 1200 ° C., and the porous glass base material is slowly pulled down by the rotating / lifting device 14 to perform dehydration.

  Once the entire porous glass base material has been dehydrated, the porous glass base material is once pulled up, and this time it contains fluorine such as silicon tetrafluoride and sulfur hexafluoride diluted with helium as needed from the gas inlet 16 In this state, the heating furnace 18 is heated to 1300 to 1600 ° C., and the porous glass base material is slowly pulled down while rotating by the rotating / lifting device 14, and the porous glass layer is doped with fluorine and transparent glass. Process.

  The necessary core / cladding ratio may be obtained by depositing the porous glass layer once, but the same process may be repeated again to obtain the necessary core / cladding ratio. In addition, when the amount of clad is deposited beyond the area where light oozes out in the first step, the second step is usually performed by flame hydrolysis such as the well-known OVD method or shafting method. A necessary core / cladding ratio may be obtained by using the porous glass preform manufacturing process.

(Example 1)
A synthetic silica glass rod with an OH group content of 0.15 ppb or less, synthesized using the VAD method, is heated and stretched in an electric furnace in a nitrogen atmosphere, and the surface is ground and polished to an outer diameter of 10 mm and a length of 1000 mm. A core rod made of pure quartz was prepared. A natural quartz glass dummy rod was connected to both ends of the core rod and attached to a deposition apparatus, and glass particles were deposited around the core rod using the plasma apparatus shown in FIG. The core rod was rotated at a rotational speed of 60 rpm and reciprocated in the vertical direction at a moving speed of 75 mm / min.

  The high frequency coil 8 is supplied with high frequency power of 3.5 MHz and 9 kW, the flow channel 9 is silicon tetrachloride 4 L / min and argon 4 L / min, the flow channel 10 is argon 20 L / min, the flow channel 11 was supplied with argon 30 L / min and oxygen 40 L / min. With a deposition time of 745 minutes, a porous glass base material having an adhesion amount of 4147 g and an outer diameter of 103 mm was obtained.

This porous glass base material is transferred to the apparatus shown in FIG. 3 and dehydrated by heating to 1100 ° C. in a helium atmosphere containing 3% chlorine, followed by a helium atmosphere containing 5% silicon tetrafluoride. A transparent vitrification treatment was performed by heating to 1500 ° C. below.
In addition, a porous glass base material is produced by depositing glass fine particles on the outside of the glass rod thus obtained by using a normal OVD process, and this is similarly dehydrated and sintered to obtain a pure silica core light. A glass base material for fiber was obtained.

  FIG. 4 shows the transmission loss spectrum of the optical fiber obtained by heating and drawing this at about 2000 ° C. The transmission loss at 1385 nm was 0.24 dB / km, and the increase in loss due to OH absorption was approximately 0.01 dB / km.

(Comparative Example 1)
A synthetic silica glass rod with an OH group content of 0.15 ppb or less, synthesized using the VAD method, is heated and stretched in an electric furnace in a nitrogen atmosphere, and the surface is ground and polished to an outer diameter of 10 mm and a length of 1000 mm. The core rod was manufactured. Natural quartz glass dummy rods were connected to both ends of the core rod and attached to a deposition apparatus, and glass particles were deposited around the core rod using a multi-tube oxyhydrogen flame burner. The core rod was rotated at a rotational speed of 60 rpm and reciprocated in the vertical direction at a moving speed of 75 mm / min.

  The multi-tube oxyhydrogen flame burner was supplied with silicon tetrachloride 4 L / min, argon 4 L / min, nitrogen 3 L / min, air 4 L / min, hydrogen 80 L / min and oxygen 40 L / min. With a deposition time of 745 minutes, a porous glass base material having an adhesion amount of 4410 g and an outer diameter of 112 mm was obtained.

This porous glass base material is transferred to the apparatus shown in FIG. 3 and dehydrated by heating to 1100 ° C. in a helium atmosphere containing 3% chlorine, followed by a helium atmosphere containing 5% silicon tetrafluoride. A transparent vitrification treatment was performed by heating to 1500 ° C. below.
On the outside of the glass rod thus obtained, glass fine particles are deposited using a normal OVD process to produce a porous glass preform, which is similarly dehydrated and sintered for a pure silica core optical fiber. A glass base material was obtained.

  FIG. 4 shows the transmission loss spectrum of the optical fiber obtained by heating this to about 2000 ° C. and drawing. The transmission loss at 1385 nm was 0.73 dB / km, and the increase in loss due to OH absorption was approximately 0.50 dB / km.

  The transmission loss is extremely small in a wide band and is effective for long-distance wavelength multiplex transmission.

It is a graph which shows the example of the transmission loss spectrum of germanium dope LWPF and pure silica core fiber (PSCF). It is a fragmentary longitudinal cross-section which shows the outline of the clad deposition process of this invention. It is a figure which shows the outline of a sintering apparatus. 6 is a graph showing transmission loss spectra of optical fibers obtained in Example 1 and Comparative Example 1.

Explanation of symbols

1 ... Target stick,
2 ... Porous glass base material,
3 ... Plasma,
4 ... 1st pipe,
5 ... second pipe,
6 ... Third pipe,
7 ... 4th tube,
8 ... High frequency coil,
9, 10, 11, 12 ... flow path,
13 ... shaft,
14: Rotating / elevating device,
15 ... Furnace core tube,
16 ... gas inlet,
17 ... Exhaust port,
18 ... heating furnace.

Claims (7)

  A method for producing a porous glass preform for an optical fiber, comprising synthesizing glass fine particles by supplying glass raw material and oxygen to a high-frequency induction thermal plasma torch, and spraying and depositing the generated glass fine particles on the surface of a glass rod .   The method for producing a porous glass preform for an optical fiber according to claim 1, wherein the glass rod is pure quartz glass substantially free of OH groups.   The method for producing a porous glass preform for an optical fiber according to claim 1, wherein the glass raw material is silicon tetrachloride.   Glass raw material and oxygen are supplied to a high frequency induction thermal plasma torch to synthesize glass particles, and the generated glass particles are deposited on a glass rod to form a porous glass base material. A method for producing a glass preform for an optical fiber, comprising heating and transparentizing in a fluorine-containing atmosphere at 1300 to 1600 ° C after dehydration by heating in a chlorine-containing atmosphere at 1200 ° C.   The method for producing a glass preform for an optical fiber according to claim 4, wherein the glass rod is pure silica glass substantially free of OH groups.   The method for producing a glass preform for an optical fiber according to claim 4, wherein the glass raw material is silicon tetrachloride. The manufacturing method of the glass preform for optical fibers which provides a fluorine dope quartz glass layer on the further outer side of the glass preform for optical fibers manufactured by the method in any one of Claims 4 thru | or 6.

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