JPH048381B2 - - 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.

<Prior art> As an example of a conventional manufacturing method for an optical fiber preform with a high numerical aperture, a quartz-based glass core rod doped with germanium dioxide at a high concentration is inserted into a clad pipe, and the two are heated and melted into one body. The so-called rod-in-tube method is known. In this case, pure silica glass is most commonly used as the clad pipe, but fluorine-doped silica glass may be used to produce an optical fiber preform with a higher numerical aperture. 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 and preform rupture. Something happened. Furthermore, by combining a core material doped with a large amount of germanium dioxide and a cladding material doped with fluorine, the transmission loss in the ultraviolet region of the resulting optical fiber tends to increase.

The reason for the former is that 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 it remains as bubbles. The latter cause is presumed to be due to defects induced by fluorine in the cladding diffusing into the core in a high-temperature atmosphere and reacting 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, since germanium dioxide is practically doped near the surface of the core rod, its volatilization is difficult to occur, and when the core rod and the clad pipe are integrated, glass doped with germanium dioxide and fluorine are Since the doped glass does not come into direct contact with each other, it is possible to prevent an increase in transmission loss in the ultraviolet region and to stably produce a high numerical aperture optical fiber 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 counter this, in anticipation of volatilization, a thin layer of pure silica glass is applied in advance, or the corrod is first stretched to a predetermined outer diameter.
We tried a method of applying a thin layer of pure silica glass, but since synthesis of pure silica glass by direct vitrification requires very high temperatures, it would be difficult to process the core rod for a long time or to process a small diameter core rod. There was a problem in that the deformation became large and the subsequent integration of the core rod and clad pipe could not be easily performed.

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 the Problems> The present invention provides 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. In the above, the core rod is 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 silica glass layer containing at least one of P ₂ O ₅ , B ₂ O ₃ , and TiO ₂ is applied so that the thickness is 1/125, and the above-mentioned clad pipe has a relative refractive index with respect to pure silica glass. Made of quartz glass doped with fluorine in an amount equivalent to a difference of -0.3% or less,
This is a method for manufacturing an optical fiber preform characterized by the following. 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 relative refractive index difference of the clad pipe used. The above-mentioned methods are mentioned in the range of values corresponding to .

The present inventors focused on the fact that the quartz vitrification temperature was lowered by adding three types of dopants, P ₂ O ₅ , B ₂ O ₃ , and TiO ₂ , and as a result of intensive study,
By forming a thin layer of silica glass doped with at least one of the above three types of dopants on the outermost periphery of the starting material rod, it can be formed at a lower temperature than when forming _two layers of pure SiO. By minimizing deformation of the core rod even during long-term direct vitrification processing, and forming the coating to a thickness that takes into account the amount that will be volatilized during subsequent high-temperature processing such as stretching, It has been found that since a sufficient thickness of the quartz glass layer can be ensured and deformation can be prevented, a thicker coating can be formed than before, and this also allows for free stretching, increasing the degree of freedom in processing.

In addition, by using a core rod coated with a coated quartz glass layer, an optical fiber can be obtained with less increase in loss in the ultraviolet region than an optical fiber obtained from an optical fiber preform using a rod without this coating as the 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 clad pipe used, and the thickness is 1/30 to 1/125 of the outer diameter of the core rod. It is preferable from the viewpoint of transmission characteristics and processability.

<Example> 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 the figure, 1 indicates a quartz glass rod, and 2 indicates a quartz-based glass pipe. In this embodiment, the glass rod 1 is a glass rod with a relative refractive index containing germanium dioxide.
A silica glass layer doped with P ₂ O ₅ and having a relative refractive index difference of approximately 0% is applied around the outer periphery of a starting material rod made of 1.8% silica glass. As the glass pipe 2, a quartz glass pipe with a relative refractive index difference of −0.3% by adding fluorine was used. Furthermore, the glass rod 1 used in this example was made by synthesizing P ₂ O ₅ doped silica glass to a thickness of 0.7 mm (1/45) on the outer periphery of the starting material rod with an outer diameter of 32 mm using a high-temperature plasma method. Using a rod obtained by stretching it to an outer diameter of 16 mm while heating it with a hydrogen flame, it was found that 30% by weight of the coated quartz glass on the outermost part of the rod was lost by comparing its weight before and after stretching. I expected that the thickness would be 1/64. Further, numeral 3 in the figure is a burner for heating the glass rod 1 and the glass pipe 2. As shown in Fig. 1, the glass rod 1 is inserted into the glass pipe 2, heated by the burner 3, and rotated in one direction around the concentric axis of the glass rod 1 and the glass pipe 2 as shown by the arrow to increase the temperature in the outer circumferential direction. Equalize the distribution. In addition, the gap between the glass rod 1 and the glass pipe 2 is reduced in thickness to facilitate integration, and 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 and spinning the preform had a transmission loss of 12 dB/Km at a wavelength of 0.6 μm.

Comparative Example According to the structure shown in Fig. 1, in this Comparative Example, a glass rod 1 having a diameter equal to the outer diameter of the starting material rod is attached to 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%. A thin layer of pure silica glass having a thickness of /125 was applied, and as the glass pipe 2, a silica glass pipe with a relative refractive index difference of −0.3% by adding fluorine was used. The outer diameter of the starting material rod, the synthesis conditions for the silica glass thin layer, and the stretching conditions were the same as in Example 1, and a glass rod 1 having an outer diameter of 16 mm after stretching was obtained.

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 example, and the preform burst during natural cooling.

<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. It becomes possible to select an effective processing method. Specifically, when synthesizing quartz glass around the outer periphery of a glass rod, for example, the larger the outer diameter of the target, the faster the synthesis speed, which is very advantageous in terms of productivity improvement.

[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: 1/30 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 silica 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.
The clad pipe is coated with a silica glass layer containing at least one of P ₂ O ₅ , B ₂ O ₃ , and TiO ₂ so as to have a thickness of 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 0.3% or less. 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. A method for manufacturing an optical fiber preform.