JP2545408B2 - Method of manufacturing optical waveguide stop preform - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to the manufacture of optical waveguide preforms. Specifically, the present invention relates to a technique of depositing glass soot to produce a single mode optical waveguide soot foam.

BACKGROUND OF THE TECHNOLOGY A vapor phase axial deposition (VAD) method for multimode optical fiber fabrication is described in US Pat. No. 3,966,446. The VAD process begins with the deposition of germanium phosphorus silicate particles, which are formed on the end of a rotating seed rod by reducing SiCl ₄ , GeCl ₄ and POCl ₃ in an oxyhydrogen flame. The seed rod is slowly separated from the torch, resulting in an essentially cylindrical porous soot foam. The porous soot foam is then dehydrated and solidified at high temperature into a transparent glass ball. The solidified glass ball is then drawn into a long rod that will be used as the core of the preform from which the multimode optical waveguide fiber is drawn. The preform cladding is formed by the so-called "rod-in-tube" method, which involves collapsing a silica tube on a core rod that is inserted therein.

In the VAD method for producing single mode optical waveguide fiber preforms, it is necessary to produce small holes to deposit several cladding layers. Easy-to-handle soot foam size 15: 1 with one deposition
18mm core diameter for optimum clad to core ratio
Should not be any larger than (Suet). Under growth conditions, the porous core material must be of sufficient density to support the heavy cladding layer deposited on it. Also, the cosart must have a uniform radial density in order to avoid the formation of bubbles at the clad-core interface during solidification.

In the manufacture of single mode optical fibers by the VAD method, as described in US Pat. No. 4,345,928 approved by Kawachi, which is hereby incorporated by reference, the glad material is a rod-in-type material as described above. It cannot be manufactured only by the tube method. Small diameter of single mode fiber core (eg 5 to 10 μm)
Therefore, a part of the light wave propagating along it passes through a part of the clad near the core. The rod-in-tube method of making all clad materials has a high damping due to the presence of OH in the clad. Therefore, VAD
In the case of low loss single mode fibers made by the method, some of the cladding material, as well as the core, must be manufactured by a soot deposition step followed by dehydration before solidifying into a transparent glass optical waveguide preform.

Typically, the amount of cladding material produced in this way should result in a soot foam with a deposited cladding-core diameter ratio higher than 6 to 1. Therefore, for jingle mode fiber production, the VAD method requires the use of several oxygen-hydrogen torches to simultaneously form soot. One torch is for axial deposition of the core and one or more torches are for radial deposition of the cladding. After dehydration and solidification, the resulting transparent glass balls are also stretched to obtain a suitable clad-core diameter ratio that gives the desired cutoff wavelength, by the rod-in-tube method described above. Will be added. Typically, a cutoff wavelength of 1.15 μm and Δn =
For a 125 μm outer diameter fiber with a 0.004 index step, the core has a diameter of about 8 μm.

In multi-mode VAD manufacturing, when manufacturing the preform core, the soot foam is made.
A torch with an annular cross-section is used, which consists of individual concentric circularly spaced quartz tubes. Two internal tubes are chemicals, SiC
It carries l ₄ , GeCl ₄ and POCl ₃ , and the other three tubes supply H ₂ , Ar and O ₂ for the acid-hydrogen flame. Typically, soot foams made with this torch configuration have a core diameter of 50 mm aud. If a similar torch configuration is used for single mode fiber fabrication, the overall soot foam surrounding both the core and cladding will be surprisingly large.
(Eg 300mm) Such a large soot foam makes handling and sintering extremely difficult. Therefore, in the case of single mode fiber fabrication, techniques must be developed to produce single mode soot foam cores with diameters that are somewhat larger than 18 mm.

US Pat. No. 4,465,708 is directed to the use of a torch in a VAD process to form a small core of a single mode optical waveguide preform. The torch includes a plurality of concentrically mounted glass tubes that pass the reactants and flammable materials. A sloping shroud is mounted near the end of the torch and shield gas is directed along its interior surface to confine and direct the gas and reactants to the surface of the growing soot foam.

While this technique is effective for producing small diameter cores (eg, 16.5 mm), the refractive index and density distributions are adversely affected by the fact that the shroud can produce an uneven temperature distribution on the core surface. Receive. This process results in a core with regions that gradually cool from the hot (about 900 ° C) portion of the tip as it moves upwards along the surface. Such a thermal pattern causes a dense germanium-free region to be formed in the center of the core. As it travels radially to the edges, the germanium increases significantly, and the density is such that the outer surface of the core is a very loosely bound fluffy layer of germanium-rich soot particles.
Significantly reduced. This fluffy layer causes problems not only during bowl growth but during solidification as well. This loosely bound layer of soot during growth cannot support weight from the cladding volume prior to cracking. Furthermore, during solidification, a loose layer of germanium-rich soot can form bubbles at the cladding-core interface,
Some of the resulting preforms are unsuitable for fiber manufacturing.

SUMMARY OF THE INVENTION The foregoing problem was addressed by directing gas through a torch to form a first frame front having a first temperature.
By directing additional gas through the torch to form a first frame front in the first frame front at a temperature above the temperature of, and directing the reactants through the torch and the second frame front,
This was overcome by forming a stream of glassy soot and depositing the glassy soot axially on the growing optical waveguide soot foam.

Detailed Description Figure 1 (By Suda et al. IOOC-ECOC '85 Proceeding)
FIG. 1 is a schematic diagram of a system for producing a multimode soot preform described in a paper entitled “High-speed production of all-synthesized fiber preform by multiframe VAD method using SiHCl ₃ raw material”. This schematic depicts a soot foam 10 being manufactured using a conventional single frame burner 12 for core synthesis and a dual frame burner 14 for cladding synthesis.

The single flame burner 12 typically includes a plurality of concentric and separated tubes in which the precursor 15 is directed from the inner tube while oxygen and hydrogen are supplied from the outer tube to form a flame. , Heating the precursor because it forms a glass soot. A precursor 15 is fed around a dual frame burner 14, but with oxygen and hydrogen directed to form internal and external oxygen-hydrogen flame fronts 16 and 18, respectively. The inner front must be cooler than the outer front to promote high deposition rates. Undeposited material is removed through exhaust port 22. While such techniques are effective in making multimode soot preforms, they cannot be used for smaller core diameter signal mode preforms for the reasons discussed above.

The torch 30 shown in FIG. 2 overcomes the previous problems. The torch 30 provides two acid-hydrogen flame fronts: one produces an inner front 34 and an outer front 36, thus providing an off-center soot particle stream 32 with suitable gas structure and flow rate. The inner frame front 34 can be adjusted to uniformly heat the lower portion of the soot foam core 38 to 500 ° C to 1000 ° C to maintain a uniform soot density and germanium concentration. The outer front 34 is about 200 degrees lower than the inner front. Soot particles under the inner frame front 34
32 is formed in reaction with oxygen from both interior front 34 and exterior front 36. After the soot particles 32 are deposited on the bottom surface of the core 38, the cooler excess undeposited particles are repelled from the hot core surface by thermal repulsion. By thus limiting the deposition area, small diameter cores 38 are obtained with a uniform density.

A cross section of an embodiment of the torch 30 is shown in FIG.
The torch 30 includes a plurality of cylindrical quartz tubes arranged to form two frame fronts having off-center soot flow. The gases and reactants associated with a particular quartz tube are shown in the table below.

The torch 30 was used to make soot foam with a soot core diameter of 8.0 mm to 13.4 mm with a density of about 0.4 g / cm ³ . Such a density is sufficient to support the clad needed to produce a preform having a clad-core ratio of 15: 1.

FIG. 4 is a sectional view of another example of the torch 30. The gases and reactants associated with a particular quartz tube are shown in the table below.

FIG. 5 shows an embodiment of two frame front torches 30 offset from the center of the rectangle. The gases and reactants associated with specific tubes are shown in Table III.

It should be understood that the embodiments described herein are merely examples of the invention. Various modifications may be made to those skilled in the art within the spirit and scope of the invention to implement the principles of the invention.

[Brief description of drawings]

1 is a schematic diagram of a high speed VAD fabrication system; FIG. 2 is a schematic diagram of a decentered VAD torch implementing the present invention; and FIG. 3 is shown in FIG. 2 taken at plane (3-3). And FIG. 4 and FIG. 5 are sectional views of the torch shown in FIG. 2 depicting another embodiment of the present invention. Explanation of main symbols 10 …… Optical waveguide soot preform 30 …… Glass soot deposition torch 32 …… Glass soot 36 …… Outer frame front 41 …… Passage

 ─────────────────────────────────────────────────── --Continued front page (56) References JP 61-72645 (JP, A) JP 61-21177 (JP, B2)

Claims (6)

(57) [Claims] 1. A method of manufacturing an optical waveguide soot foam (10) by vapor phase axial deposition, the step of flowing gas through a torch to form a first frame front (36) having a first temperature; Outflowing additional gas through the torch to form a second frame front (34) in the first frame front at a temperature higher than the first temperature; Suit (3
A method characterized by the steps of forming a stream of 2) and axially depositing the glass soot on a growing optical waveguide soot foam. 2. A method according to claim 1, wherein the outer surface of the growing optical waveguide soot foam comprises:
A method comprising depositing a glassy soot cladding. 3. The method according to claim 1, wherein a soot foam (10) is solidified and a clad is formed on the soot foam (10) to form an optical waveguide preform, and a part of the preform is re-formed. A method characterized in that the fiber is drawn from the part that has been re-fluidized by heating it to a high temperature. 4. The method according to claim 1, wherein the temperature of the second frame front is higher than that of the first frame front and the second frame front and the first frame. Gas passages (41,
44, 46) and providing a passageway (42) in the first frame front through which the glass soot is directed over the growing soot preform surface. . 5. The method of claim 4 wherein the glass soot passage is offset from the center of the torch. 6. The method according to claim 4, wherein the gas passage is circular.