JPS62176935A - Method for manufacturing optical fiber preform - Google Patents

Abstract

PURPOSE:To easily and stably produce the titled preform for optical fiber having a high numerical aperture by forming the thin layer of quartz glass doped with a specified dopant on the outer periphery of a starting rod. CONSTITUTION:A quartz glass layer having a thickness 1/30-1/125 times the outer diameter of the starting rod and contg. >=1 kind among P2O5, B2O3, and TiO2 is furnished on the outer periphery of the starting rod contg. G2O3 in the amt. necessary to control the specific refractive index difference from pure quartz glass to >=1.5% to form a core rod 1. A clad pipe 2 is formed with quartz glass whose specific refractive index difference is adjusted to <=-0.3% by the addition of fluorine. Since the thin film of quartz glass doped with the dopant is formed, the thin film can be formed at a lower temp. as compared with the case where a pure SiO2 layer is formed, the deformation of the core rod can be controlled to the minimum even in the direct vitrification for many hours, the coat can be formed in the amt. including the amt. to be volatilized in the succeeding high-temp. processing, the drawing can be freely performed, and the degree of freedom in the processing can be expanded.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a novel method for easily and stably manufacturing an optical fiber preform having a high numerical aperture.

<Conventional technique> As an example of a conventional method for manufacturing an optical fiber preform with a high numerical aperture, a quartz-based glass core rod doped with germanium dioxide to a high degree of flexibility is inserted into a clad pipe, and both The so-called rod-in-tube method is known, in which the rods are integrated by heating and melting. in this case,
Pure silica glass is most commonly used as the clad pipe, but fluorine-doped silica glass is sometimes used to produce optical fiber preforms with higher spacing.

Since the refractive index of quartz glass is lowered by adding fluorine, a high numerical aperture can be easily obtained by using it as a cladding material.

However, a problem when using quartz glass doped with germanium dioxide at a high concentration as a core rod is that air bubbles tend to remain in the preform, and these air bubbles often cause cracks to occur and the preform to burst. There was something that happened. Furthermore, by combining a core material doped with a large amount of germanium dioxide and a clad material doped with fluorine, the transmission loss in the ultraviolet region of the resulting optical fiber tends to increase.

The former cause is because the vapor pressure of germanium dioxide is high, so germanium dioxide volatilized from near the core rod surface in high-temperature ambient air enters the inner surface of the pipe or irregularities on the rod surface, preventing sufficient integration of the pipe and rod. It is thought that it remains as air bubbles. The latter cause is due to fluorine in the cladding diffusing into the core in a high m atmosphere.
It is speculated that this is due to defects induced by reaction with germanium.

As a solution to these problems, a method has been proposed in which a thin layer of pure silica glass having a thickness of 1/60 to 1/125 of the outer diameter of the core rod is applied directly to the outer periphery of the core rod by vitrification. According to this method, germanium dioxide is not doped near the surface of the core rod, so its volatilization is less likely to occur. Since the doped glass does not come into direct contact, it is possible to prevent an increase in transmission loss in the ultraviolet region and to stably produce high numerical aperture optical fibers with good characteristics.

<Problems to be Solved by the Invention> The method described above is a very excellent method for manufacturing high numerical aperture optical fibers, but after applying a thin layer of pure silica glass around the outer periphery of the core rod, When processing at high temperatures, such as when stretching to a predetermined outer diameter, the pure silica glass thin layer evaporates due to the high temperature, making it impossible to obtain the required thickness.

To deal with this, we tried applying a thin layer of pure silica glass in advance in anticipation of volatilization, or first stretching the core rod to a predetermined outer diameter and then applying a thin layer of pure silica glass. However, when synthesizing pure silica glass using the direct vitrification method, very high temperatures are required, so processing is required for a long time, and when processing a small diameter core rod, the deformation of the core rod becomes large, and the subsequent core rod and clad pipe There was a problem that it was not easy to integrate the two.

The present invention is intended to solve these problems and provide a new method that can stably manufacture optical fiber preforms having a high numerical aperture.

Means for Solving Problems> The present invention inserts a core rod made of high refractive index glass into a clad pipe made of low refractive index glass, melts the core rod and the clad pipe simultaneously, and integrates them to form an optical fiber preform. In the manufacturing method, the core rod is made of a starting material rod made of quartz glass containing germanium dioxide in an amount corresponding to a relative refractive index difference of 1.5% or more with respect to pure silica glass. 11 in diameter
P, Os, B, so that the thickness is 50 to 1/125.
The clad pipe is coated with a silica glass layer containing at least 1R of O, Tie.
A method for manufacturing an optical fiber preform characterized in that the preform is made of quartz glass doped with fluorine in an amount corresponding to a relative refractive index difference of -0.3% or less with respect to pure silica glass.

In a particularly preferred embodiment of the present invention, the relative refractive index difference of the silica glass thin layer containing the dopant on the outermost periphery of the starting material rod is a value corresponding to the relative refractive index difference of the core rod used or the cladding layer used. The above methods include the range of values corresponding to the relative refractive index difference of the pipe.

We,P,Os,B,03. Tie, no 3
We focused on the fact that adding dopants to glazes lowers the quartz vitrification temperature, and as a result of intensive study, we found that the above 3.
By forming a thin layer of silica glass doped with at least one of the various dopants on the outermost periphery of the starting material rod, it can be performed at a lower temperature than forming a pure 3101 layer, so it can be directly applied for a longer period of time. By minimizing the deformation of the core rod through the vitrification process, and by forming the coating to a thickness that takes into account the amount that will be volatilized during subsequent high-temperature processing such as stretching, a sufficient thickness of the silica glass layer can be achieved. It has been found that since it is possible to secure the film and prevent deformation, it is possible to process a thicker coating than before, and that this also allows for free stretching processing, increasing the degree of freedom in processing.

In addition, by using a core rod coated with a coated quartz glass layer, an optical fiber with less increase in loss in the ultraviolet region can be obtained than an optical fiber obtained from an optical fiber preform using a rod without this layer as a core rod. It also became clear that

The relative refractive index difference of the covering quartz glass layer is within a value range corresponding to the relative refractive index difference between the core rod used and the cladding/cub used, and the thickness is 1/1 of the outer diameter of the core rod.
30 to 1/125 is preferable from the viewpoint of transmission characteristics and workability.

<Examples> The present invention will be described in detail below using examples of the present invention with reference to the drawings.

Embodiment Figure 1 is a diagram explaining an embodiment of the present invention.
2 represents a quartz-based glass rod, and 2 represents a quartz-based glass pipe. In this example, as glass rod 1, on the outer periphery of a starting material rod made of silica-based glass containing germanium dioxide and having a relative refractive index difference of 1.8%, 1/45 of the starting material rod
0% relative refractive index doped with P, o, with a thickness of
A glass layer plate 2 is used.
A quartz glass pipe with a relative refractive index difference of -α3tX by adding fluorine was used. In addition, the glass rod 1 used in this example was made by synthesizing PIO doped silica glass to a thickness of 17 wm (1/45) on the outer periphery of the above-mentioned starting material rod with an outer diameter of 32 mm using a high-temperature plasma method. Using the material obtained by stretching it to an outer diameter of 16 pieces while heating it with a hydrogen flame, we compared its weight before and after the stretching process (50% by weight of the coated quartz glass on the outermost part of the rod was lost). Since it was predicted that the thickness would be 1/64, the thickness would be 1/64. Also, 3 in the figure is a burner that heats the glass rod 1 and the glass pipe 2.As shown in Figure 1, the glass rod 1 is connected to the glass pipe. The glass rod 1 and the glass pipe 2 are inserted into the tube 2 and rotated in one direction around the concentric axes of the glass rod 1 and the glass pipe 2 as shown by the arrows while being heated by the burner 3 to make the temperature distribution uniform in the outer circumferential direction. After reducing the pressure in the gap between the two to make it easier to integrate them, the burner 3 is moved parallel to the longitudinal direction of the glass rod 1 and the glass pipe 2 to be integrated, thereby heating and melting the glass rod 1 and the glass pipe 2. While moving the glass rod 1 and the glass pipe 2, the entire length of the glass rod 1 is integrated.The preform obtained in this way has no residual air bubbles and is suitable for transmission of optical fibers obtained by heating and spinning the preform. The loss was 12 aB/km at a wavelength cL6pm.

Comparative Example According to the structure shown in FIG. 1, the glass rod 1 in this comparative example is a glass rod 1 containing germanium dioxide with a relative refractive index difference of 1.8%.
A thin layer of pure silica glass having a thickness of 1/125 of the outer diameter of the starting material rod is applied to the outer periphery of a starting material rod made of silica-based glass, and the glass pipe 2 is made by adding fluorine. A quartz glass pipe with a relative refractive index difference of -0.3% was used. The outer diameter of the starting material rod,
The synthesis conditions and stretching conditions for the quartz glass thin layer were the same as in Example 1, and a glass rod 1 having an outer diameter of 16 mm was obtained after stretching.

Comparing the weight before and after stretching, it was estimated that 20% of the pure silica glass thin layer was lost due to stretching. Hereinafter, the optical fiber preform obtained by integrating the glass rod 1 and the glass pipe 2 by substantially the same operation as in the example contained residual air bubbles and burst during natural cooling.

<Advantageous Effects of the Invention> As described above, the method of the present invention can expand the degree of freedom in processing when stably manufacturing an optical fiber preform with a high numerical aperture and good transmission characteristics. Therefore, it becomes possible to select a more efficient processing method. Specifically, for example, when synthesizing quartz glass around the outer periphery of a glass rod, the larger the outer diameter of the target, the faster the synthesis speed becomes, which is very advantageous in terms of improving productivity.

[Brief explanation of drawings]

FIG. 1 is a schematic explanatory diagram of an embodiment of the present invention. Figure 1

Claims (2)

[Claims] (1) In a method of manufacturing an optical fiber preform by inserting a core rod made of high refractive index glass into a clad pipe made of low refractive index glass and simultaneously melting and integrating the core rod and the clad pipe, the core rod is pure. On the outer periphery of a starting material rod made of quartz glass containing germanium dioxide in an amount corresponding to a relative refractive index difference of 1.5% or more with respect to quartz glass, 1/3 of the outer diameter of the starting material rod is added.
P_2O_5, B_ so that the thickness is 0 to 1/125.
A silica glass layer containing at least one of 2O_3 and TiO_2 is applied, and the above-mentioned clad pipe has a relative refractive index difference of -0.3% with respect to pure silica glass.
A method for manufacturing an optical fiber preform, characterized in that it is made of quartz glass doped with fluorine in an amount equivalent to: (2) The silica glass layer has a relative refractive index difference within the range of a value corresponding to the relative refractive index difference of the starting material rod to a value corresponding to the relative refractive index difference of the clad pipe. 1) A method for manufacturing an optical fiber preform as described in section 1).