JPH048380B2 - - Google Patents

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 technology]

As an example of a conventional manufacturing method for an optical fiber preform with a high numerical aperture, a so-called rod-in method is used in which a silica-based glass core rod doped with germanium dioxide at a high concentration is inserted into a clad pipe and then heated and melted to integrate the two. The Tube method is known. In this case, pure silica glass is most commonly used as the clad pipe, but fluorine-doped quartz glass may be used to produce optical fiber preforms with higher numerical apertures. 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, when using quartz glass doped with germanium dioxide at a high concentration as a core rod, air bubbles tend to remain in the preform, and these air bubbles often cause cracks and preform rupture. It was hot. Furthermore, the transmission loss in the ultraviolet region of optical fibers obtained by combining a core material heavily doped with germanium dioxide and a cladding material doped with fluorine tends to increase.

The former cause is because germanium dioxide has a high vapor pressure, so germanium dioxide volatilizes from near the core rod surface in a high-temperature atmosphere and enters the inner surface of the pipe or irregularities on the rod surface, inhibiting sufficient integration of the pipe and rod. It is thought that the gas remains as bubbles. The latter cause is presumed to be due to defects induced by fluorine in the cladding diffusing into the core under high temperature conditions and reacting with germanium.

As a solution to these problems, the outer circumference of the core rod is 1/60 to 1/125 of the outer diameter of the core rod.
A method has been proposed in which a thin layer of pure silica glass with a thickness of . According to this method, germanium dioxide is not doped near the surface of the core rod, so its volatilization is difficult to occur, and when the core rod and the clad pipe are integrated, germanium dioxide-doped glass and fluorocarbon 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 manufacture high numerical aperture optical fibers with good characteristics.

[Problem to be solved by the invention]

The method described above is an excellent method for manufacturing optical fibers with high numerical apertures, but it is difficult to do so by applying a thin layer of pure silica glass around the outer periphery of the core rod and then drawing the core rod to a predetermined outer diameter at high temperatures. During processing, the thin layer of pure silica glass evaporates due to the high temperature, making it impossible to obtain the required thickness.

To deal with this, attempts have been made to apply a thin layer of pure silica glass in advance in anticipation of volatilization, or to stretch it to a predetermined outer diameter and then apply a thin layer of pure silica glass, but the direct vitrification method has not been successful. Extremely high temperatures are required when synthesizing pure silica glass, so when processing for a long time or processing a small-diameter core rod, the deformation of the core rod becomes large, making it easier to integrate the core rod and clad pipe afterwards. There was a problem that I couldn't do it.

The present invention aims at a novel method that can solve these problems and stably manufacture an optical fiber preform having a high numerical aperture.

[Means for solving problems]

The present invention provides a method for 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. 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, 1/60 to 1/60 of the outer diameter of the starting material rod is placed.
A layer of pure silica glass is applied to a thickness of 1/125,
Further, a quartz glass layer doped with at least fluorine is applied around the outer periphery of the pure silica glass layer, and the clad pipe is doped with fluorine in an amount equivalent to a relative refractive index difference of -0.3% or less with respect to the pure silica glass. The present invention relates to a method for manufacturing an optical fiber preform, characterized in that the preform is made of quartz glass.

In a particularly preferred embodiment of the present invention, the fluorine content concentration of the quartz glass layer doped with at least fluorine is such that the relative refractive index difference with respect to pure silica glass is -
Examples include the above method in which the amount is equivalent to 0.3% or less, and the thickness of the layer is 1/30 to 1/30 of the outer diameter of the starting material rod.
The above method is 1/200.

The present inventors focused on the fact that the transparent vitrification temperature of quartz glass is lowered by adding fluorine, and as a result of detailed study, we added fluorine to the outer periphery of the pure silica glass layer provided on the outer periphery of the rod. By forming an added quartz glass layer on the outermost periphery of the core rod, deformation of the core rod can be minimized even during direct vitrification processing for a longer period of time, and the deformation of the core rod can be minimized even during subsequent high-temperature processing such as drawing. It has also been found that the degree of freedom in processing can be expanded without compromising the thickness of the coated pure silica glass layer.

The fluorine content concentration of the fluorine-doped silica glass thin layer is preferably close to or the same as the fluorine concentration of the silica glass pipe used for the cladding, from the viewpoint of consistency when integrated, and particularly preferably. -0.3% relative refractive index difference compared to pure silica glass
The following fluorine concentrations can be mentioned: In addition, the thickness should be 1/30 to 1/20 of the outer diameter of the starting material rod.
It is preferable that the value is 0 from the viewpoint of transmission characteristics, workability, etc. In addition to fluorine, Clad also contains P, B,
A dopant such as Ge may be included.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS The effects of the present invention will be described below with reference to the drawings.

Embodiment FIG. 1 is a diagram illustrating an embodiment of the present invention, in which 1 indicates a quartz-based glass rod and 2 indicates a quartz-based glass pipe. A thin layer of pure silica glass with a thickness of 1/125 of the outer diameter of the starting material rod is provided on the outer periphery of the starting material rod made of silica glass with a difference in ratio of 1.8%, and a A thin layer of silica glass with a thickness of 1/45 of the diameter and a relative refractive index difference of -0.3% is used as the glass pipe 2. A quartz glass pipe with -0.3% was used. The glass rod 1 used here was made of pure silica glass 0.24 mm by high-temperature plasma method on the above starting material rod with an outer diameter of 32 mm.
(1/125) thickness and fluoridated quartz glass 0.7mm (1/45)
After adjusting the thickness, the material obtained by stretching to an outer diameter of 16 mm while heating with an oxyhydrogen flame was used. 30% of the fluorinated quartz glass in the outermost part of the rod by comparing its weight before and after drawing.
was expected to be lost. Further, numeral 3 in the figure is a burner for heating the glass rod 1 and the glass pipe 2. As shown in Figure 1, with the glass rod 1 inserted into the glass pipe 2, the burner 3
The glass rod 1 and the glass pipe 2 are rotated in one direction around their concentric axes as shown by the arrows while being heated by the arrows to make the temperature distribution in the outer circumferential direction uniform.
In addition, after reducing the pressure in the gap between the glass rod 1 and the glass pipe 2 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. The glass rod 1 and the glass pipe 2 are integrated over the entire length while moving the heated and melted part of the pipe 2. The preform thus obtained had no residual bubbles, and the optical fiber obtained by heating the preform had a transmission loss of 12 dB/μm at a wavelength of 0.6 μm.

Comparative Example In accordance with the configuration shown in Figure 1, in this Comparative Example, a glass rod 1 is made of silica-based glass containing germanium dioxide and having a relative refractive index difference of 1.8%. As the glass pipe 2, a silica glass pipe with a relative refractive index difference of -0.3% was used by adding fluorine.
Glass rod 1 used here is a core rod with an outer diameter of 32 mm, which is coated with a layer of pure silica glass to a thickness of 0.24 mm (1/125) using a high-temperature plasma method, and then stretched to an outer diameter of 16 mm while heating with an oxyhydrogen flame. This is what I got by doing this. Comparing the weight before and after the stretching process, it was estimated that 20% of the pure silica glass thin layer was lost due to the stretching process. Thereafter, residual air bubbles were observed in the optical fiber preform obtained by integrating the glass rod 1 and the glass pipe 2 by operations roughly similar to those in the examples, and the preform burst during natural cooling.

〔Effect of the invention〕

As described above, according to the method of the present invention, it is possible to stably manufacture an optical fiber preform with a high numerical aperture and good transmission characteristics, and it is possible to expand the degree of freedom in processing, thereby making it more efficient. This makes it possible to select a suitable 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, which is very favorable in terms of improving productivity.

Furthermore, the present invention has the advantage that the outermost layer of the core rod and the cladding can have the same or similar compositions, so that the consistency between the core and the cladding is extremely good during integration.

[Brief explanation of drawings]

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

Claims (1)

[Scope of Claims] 1. A method for manufacturing an optical fiber preform by inserting a core rod of high refractive index glass into a clad pipe of low refractive index glass, and simultaneously melting and integrating the core rod and the clad pipe, the core rod comprising: 1/60 to 1/125 of the outer diameter of the starting material rod is placed 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 pure silica glass.
A pure quartz glass layer is applied to the layer to a thickness of
A method for producing an optical fiber preform, characterized in that the clad pipe 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. 2 The fluorine concentration of the fluorine-doped silica glass layer has a relative refractive index difference of -0.3 with respect to pure silica glass.
Claim 1, which is an amount corresponding to % or less
A method for manufacturing an optical fiber preform described in Section 1. 3. The method for manufacturing an optical fiber preform according to claim 1, wherein the fluorine-doped quartz glass layer has a thickness of 1/30 to 1/200 of the outer diameter of the starting material rod.