JPH054831A - Production of infrared-transmitting optical fiber preform - Google Patents

Abstract

PURPOSE:To produce an infrared-transmitting optical fiber preform capable of increasing the ratio of clad diameter to core diameter or holding a double- structure core, free from mismatch of structure and having low transmission loss. CONSTITUTION:A molten glass of the 1st composition is poured into a mold 1 having cylindrical hollow part and the mold is rotated to form a pipe-formed 1st clad 3. Continuous to the above 1st process, the rotational speed of the mold is lowered or the rotation is stopped, molten glass materials of the 2nd composition and the 3rd composition are poured into the pipe-formed 1st clad 3 formed in the vertically supported mold to form the 2nd clad and a core 7 in the 2nd clad or to form the 1st core 6 and the 2nd core 7 in the 1st core.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an optical fiber preform, and in particular, a large clad diameter to core diameter ratio (clad / core diameter ratio) or a core having a double structure can be provided and a structure The present invention relates to a method for manufacturing an optical fiber preform for infrared ray transmission that is free from irregularities and has low loss.

[0002]

2. Description of the Related Art An infrared transmitting optical fiber made of an optical material having an infrared absorption end on the wavelength side longer than that of silica glass is expected to have a transmission loss of 10 ^-2 dB / km or less, which is higher than that of silica fiber. Among them, the fluoride optical fiber is the most promising as a transmission medium capable of long-distance non-repeater. The suction casting method, the two-layer melt method, the build-in casting method, and the rotation casting method have been known as the methods for producing a fluoride glass optical fiber preform capable of obtaining a low-loss fiber. It was In the suction casting method (Japanese Patent Laid-Open No. 63-11535), the clad melt is first poured into the cylindrical hollow portion of the mold, the core melt is continuously injected, and the volume shrinkage occurs when the clad melt solidifies. This is a method of producing a base material having a core / clad structure by introducing a core melt into the central portion. Two-layer melt method (Patent No. 143
No. 8419, Japanese Patent Publication No. 62-45181), the clad melt is first poured into the cylindrical hollow part of the mold, the core melt is continuously injected, and the bottom of the mold is pulled out before the clad melt solidifies. In this method, a part of the clad melt is moved downward to introduce the core melt into the central part. Build-in casting method (Patent No. 1345722,
Japanese Patent Publication No. 61-5662) pour a glass melt of the clad composition into a mold having a cylindrical hollow portion, and in the state where the center part does not solidify, the melt in the center part is poured out from the melt injection port of the mold, It is a method of producing a base material by pouring a core melt into a hollow portion formed in a central portion. Rotational casting method (DC Trans
t. al, Electron. Lett. vol. 1
8, P. 59, (1982)) is obtained by pouring a clad glass melt into a cylindrical hollow portion of a mold, rotating the mold to form a pipe-shaped clad glass layer, and pouring a core glass melt into the inside of the clad. This is a method of producing a base material.

[0003]

With these methods, it is difficult to increase the clad / core diameter ratio over the entire length of the base material, so that these methods alone cannot produce a single-mode fiber. Also, E used for optical amplification
In the r-doped silica-based fiber, a core having a double structure in which Er is added is formed only in the central region of the core in order to increase efficiency, but such a double structure cannot be provided by the above method. The rod-in-tube method is known as a method for solving these problems (Japanese Patent Application No. 2-1340).
No. 18). This is a method in which the base material produced by the above method is inserted into a jacket glass pipe produced by applying the rotational casting method, heated and drawn to produce a new base material. In this method, a base material having a core / clad structure is used as the base material to be inserted, and the same cladding glass as the base material is used for the jacket tube, whereby
The core diameter ratio can be increased. Therefore, the single mode can be easily realized. In addition, a base material having a double-structured core can be manufactured by using a core having a double-structure as the base material to be inserted and using a jacket tube as a clad. However, in this method, the jacket tube and the base material to be inserted must be manufactured separately, and the multi-step complicated process such as polishing and surface treatment of the base material to be inserted, and further heating and stretching to integrate them is required. It was necessary, so there was a problem in efficiency,
In addition, there is a drawback that the interface between the base material and the jacket tube is crystallized during heating and stretching, and is contaminated with dust and dust, resulting in increased loss.

The object of the present invention is to solve the above-mentioned drawbacks, to obtain a large ratio of the clad diameter to the core diameter, or to provide a core having a double structure, and to provide a low loss optical fiber mother board for infrared ray transmission without structural irregularity. It is to provide a manufacturing method of the material.

[0005]

In order to solve the above-mentioned drawbacks, the present invention provides a pipe-shaped first mold by pouring a glass melt of the first composition into a mold having a cylindrical hollow portion and rotating the mold. 1st step of forming 1 clad, and the number of revolutions of the mold is reduced or stopped continuously in succession to the inside of the pipe-shaped 1st clad formed in the upright mold. A second step of pouring a glass melt of the second composition and the third composition to form a core in the second clad and the inside thereof, or a first core and the second core in the inside thereof. .

Further, in the present invention, a suction casting method, a two-layer melt method, or a build-in casting method can be used to form the second clad and the core or the first core and the second core inside thereof. .

[0007]

According to the present invention, the interface state formed between the first cladding and the second cladding or the first core is essentially different from the conventional rod-in-tube method capable of forming a similar structure, and the interface formation is performed. The number of steps required is extremely different.
That is, according to the present invention, since the interface is continuously formed from the melt, the same good interface as the interface obtained by the conventional rotation casting method can be obtained in one step. Interfacial formation requires a multi-step complex process, and since the interface obtained there is formed from a glass surface that is reheated after being exposed to the atmosphere, it is contaminated by dust and dust, and modified by crystallization. There is a clear difference that they are vulnerable.

Next, the present invention will be described in detail with reference to the drawings. FIG. 1 is a sectional view showing a first step of the present invention. As shown in FIG. (A), a mold 1 having a cylindrical hollow portion heated near the glass transition temperature of the first clad glass is erected upright, and a glass melt 2 for the first clad 2 is introduced from its upper injection port.
Pour into the hollow part. Immediately, as shown in Fig. (B), the mold is laid horizontally, the mold is rotated around the axis of the cylindrical hollow part, and the glass melt is formed into a pipe shape by centrifugal force, and the melt has an appropriate viscosity. The first clad 3 is manufactured by maintaining the rotation until it becomes. 2 to 4 are sectional views showing the second step of the present invention. 2 (A), 3
Each of (A) and 4 (A) shows a state in which the mold is immediately rotated after the completion of the first step, or the mold is stopped and the mold is made to stand upright again.

In FIG. 2, the glass melt 4 for the second cladding or the first core is placed inside the pipe-shaped first cladding 3 in the upright mold as shown in FIG. 2 (B).
When the first core glass melt 5 or the second core glass melt 5 is continuously poured, the volume shrinkage accompanying the solidification of the second clad or first core glass melt 4 causes the temperature of the melt to rise. The high-viscosity central portion is sucked downward, and the second clad 6 and the core 7, or the first core 6 and the second core 7 inside thereof are formed, as shown in FIG. 2 (C).

In FIG. 3, the glass melt 4 for the second cladding or the first core is poured into the pipe-shaped first cladding 3 in the upright mold as shown in FIG. 3 (B). The glass melt 5 for the first core or the glass melt for the second core 5 is continuously poured. Immediately when the stopper 8 provided at the lower portion of the mold is pulled out a little, the central portion of the glass melt 6 for the second cladding or the first core, which is in a state of high temperature and low viscosity, moves downward, and the glass melt shown in FIG. 2), the second clad 6 and the core 7, or the first core 6 and the second core 7 inside thereof are formed.

In FIG. 4, the glass melt 4 for the second clad or core is poured into the pipe-shaped first clad 3 in the upright mold as shown in FIG. When inverted, the central portion of the center of the glass melt for the second clad or the first core, which has a low viscosity and is not solidified, flows out, and the second clad or the second clad having a hollow portion as shown in FIG. One core 6 is obtained. Next, when the glass melt 7 for the core or the second core is immediately poured into the hollow portion, as shown in FIG. 4D, the second clad 6 and the core 7, or the first core 6 and the second core inside the second clad 6 and the core 7. The core 7 is formed.

[0012]

EXAMPLES The present invention will now be described in detail with reference to examples, but the present invention is not limited thereto. Although the examples show the production of the fluoride optical fiber preform, the present invention can be easily applied to the production of other infrared transmissive optical fiber preforms made of halide glass, chalcogenide glass, heavy metal oxide glass, or the like. .

(Embodiment 1) The first cladding and the second cladding are 47.5ZrF ₄ -23.5BaF, respectively.
₂ -2.5LaF ₃ -2YF ₃ -4.5AlF ₃ -20
NaF (mol%), core 48.5 ZrF ₄ -23.5
BaF ₂ -3.5LaF ₃ -2YF ₃ -2.5AlF ₃
Fluoride raw materials were weighed and mixed so as to have a composition of -7LiF-13NaF (mol%), placed in a metal crucible, and held at 850 ° C for 1.5 hours in an Ar + HF gas atmosphere to melt, and then 700 ° C. The temperature dropped to. The glass melt for the first clad is poured into a brass mold having a cylindrical hollow portion with a diameter of 20 mm and a length of 140 mm which is heated to 170 ° C., and immediately the mold is laid horizontally and the mold is laid on the shaft of the cylindrical hollow portion. Was rotated at 3000 rpm for 5 minutes. Immediately thereafter, the rotation of the mold was reduced to 10 rpm to erect the mold, and the second clad glass melt was poured into the formed pipe-shaped first clad having an outer diameter of 20 mm and an inner diameter of 7 mm, and the mold was continuously used for the core. The glass melt was poured. afterwards,
The mold was gradually cooled to room temperature.

The core of the obtained base material was tapered, and the range of 12 to 58 mm from the tip of the core could be approximated by the following equation.

[0015]

## EQU1 ## y = 0.041x + 1.1 (1) Here, x (mm) is the distance from the tip of the core, and y (mm) is the core diameter. That is, the core / clad diameter ratio of this taper portion was a large value of 5.7 to 13.

He--Ne was added to the base material thus prepared.
As a result of measuring the scattering state at the interface between the formed second clad and the first clad by injecting a laser, the same good scattered light intensity characteristics as at the interface between the first clad and the core were obtained.

(Embodiment 2) The clad is 47.5ZrF ₄
-23.5BaF ₂ -2.5LaF ₃ -2YF ₃ -4.
5AlF ₃ -20NaF (mol%), the first core 49Z
_{rF 4 -25BaF 2 -3.5LaF 3 -2YF 3} -
2.5AlF ₃ -13LiF-5NaF (mol%), the second core 49ZrF ₄ -25BaF ₂ -3.45LaF
₃ -2YF ₃ -2.5AlF ₃ -13LiF-5NaF
Weigh and mix the fluoride raw materials so that the composition is -0.05 PrF ₃ (mol%), and put each in a gold crucible.
After holding at 850 ° C. for 1.5 hours in an Ar + HF gas atmosphere to melt, the temperature was lowered to 700 ° C. A cylindrical hollow portion having a diameter of 20 mm and a length of 140 mm, which is heated to 170 ° C.,
The glass melt for clad was poured into a brass mold having a stopper at the bottom, the mold was immediately laid horizontally, and the mold was rotated at 3000 rpm for 5 minutes around the axis of the cylindrical hollow part. Then immediately stop the rotation of the mold and erect the mold,
The glass melt for the first core was poured into the formed pipe-shaped clad having the outer diameter of 20 mm and the inner diameter of 5 mm, and the glass melt for the second core was continuously injected. Immediately after, the stopper at the bottom of the mold was slightly pulled out to move a part of the first core glass melt downward. Then, the mold was gradually cooled to room temperature. The obtained base material has an outer diameter of 20 mm and a core diameter of 5 mm,
In the core, a Pr-doped region having a diameter of about 2.5 mm is formed in the center, that is, two regions of the first core and the second core inside thereof.
A heavy structure was formed over 80 mm.

He--Ne was added to the base material thus prepared.
As a result of measuring the scattering state at the interface between the clad formed and the first core by injecting a laser, good scattered light intensity characteristics comparable to those at the interface between the first core and the second core inside the clad were obtained. Was given.

(Embodiment 3) The first cladding and the second cladding are each 47.5ZrF ₄ -23.5BaF.
₂ -2.5LaF ₃ -2YF ₃ -4.5AlF ₃ -20
NaF (mol%), core is 50ZrF ₄ -24BaF ₂
-3.5LaF ₃ -2YF ₃ -2.5AlF ₃ -14L
Fluoride raw materials were weighed and mixed so as to have a composition of iF-4NaF (mol%), put into a gold crucible, and Ar
After being held in a + HF gas atmosphere at 850 ° C. for 1.5 hours to melt, the temperature was lowered to 700 ° C. The glass melt for the first clad is poured into a brass mold having a cylindrical hollow portion with a diameter of 15 mm and a length of 140 mm heated to 200 ° C., and the mold is immediately laid horizontally and the shaft of the cylindrical hollow portion is placed on the mold. Centered at 3000 rpm for 3 minutes. Next, the rotation of the mold was immediately stopped, the mold was made to stand upright, and the glass melt for second cladding was poured into the formed pipe-shaped first cladding having an outer diameter of 15 mm and an inner diameter of 7 mm. Immediately, the mold was turned upside down, the central portion of the glass melt for the second cladding, which had a low viscosity and was not solidified, was poured out from the melt injection port of the mold, and was made upright again. Immediately, the glass melt for core was poured into the hollow portion. Then, the mold was gradually cooled to room temperature.

The core of the obtained base material was tapered, and the range of 16 to 40 mm from the tip of the core could be approximated by the following equation.

[0021]

## EQU00002 ## y = 0.088x + 0.2 (2) where x (mm) is the distance from the tip of the core, and y (mm) is the core diameter. That is, the core / clad diameter ratio of this taper portion was as large as 4.1 to 9.3.

He--Ne was added to the base material thus prepared.
As a result of measuring the scattering state at the interface between the formed second clad and the first clad by injecting a laser, the same good scattered light intensity characteristics as at the interface between the first clad and the core were obtained.

(Application Example 1) The base material obtained in Example 1 was taper stretched by the method described in Japanese Patent Application No. 2-134018. That is, the base material having the tapered core portion is 5 mm / m
inward, and the base material is Y (mm / mi)
It was collected at the speed of n).

[0024]

## EQU3 ## Y = 5 (0.186t + 1) ² (3) where t (min) is the time after the core tip reaches the center of the electric furnace held at 275 ° C. The stretched base material has a constant core diameter, and the outer diameter of the base material decreases from the tip to the end. The outer circumference of this stretched base material was polished to have an outer diameter of 9 mm and a core diameter of 1.1 mm, that is, 8.2.
With a large clad / core diameter ratio of 85 mm
Got the base material. Next, this base material was zone-heated and drawn into fibers to obtain 440 m of fibers having an outer diameter of 125 μm and a core diameter of 15.2 μm. The obtained fiber was a single mode fiber having a cutoff wavelength of 2.3 μm, and the transmission loss at a wavelength of 2.55 μm was 0.5 dB / km. As described above, a low loss single mode optical fiber could be manufactured by the base material obtained by the present invention.

(Application Example 2) The base material obtained in Example 2 was drawn to have an outer diameter of 5.8 mm, and a fluoride glass pipe having the same composition as the clad with one end closed (outer diameter 20 mm,
It was inserted into an inner diameter of 6 mm) and stretched to an outer diameter of 5.8 mm while reducing and integrating the pressure. Next, insert the obtained base material again into another fluoride glass pipe (outer diameter 20 mm, inner diameter 6 mm) of the same composition as the clad with one end closed and draw it while decompressing and integrating it, outer diameter 125 μm, core diameter 4.6μ
2 km of m fiber was obtained. The obtained fiber has a cutoff wavelength of 0.99 μm and a diameter of 2.
The transmission loss at a wavelength of 1.3 μm was 30 dB / km with a 3 μm single-mode fiber having a Pr-doped region of about 500 ppm. When the excitation light and the signal light having a wavelength of 1.017 μm were made to enter using the obtained fiber 30 m, the signal light was amplified in the wavelength range of 1.29 μm to 1.33 μm, and when the excitation light was 800 mW, 1. 31μ
A gain of 35 dB was obtained at m.

(Application Example 3) The base material obtained in Example 3 was taper-stretched by the method described in Japanese Patent Application No. 2-134018. That is, the base material having the tapered core portion is 3 mm / m
inward, and the base material is Y (mm / mi)
It was collected at the speed of n).

[0027]

## EQU4 ## Y = 3 (0.165t + 1) ² (4) where t (min) is the time from when the core portion 16 mm from the core tip reaches the center of the electric furnace held at 270 ° C. Is. The stretched base material has a constant core diameter, and the outer diameter of the base material decreases from the tip to the end. The outer periphery of this stretched base material was polished to have an outer diameter of 7 mm and a core diameter of 1.
A base material having a length of 51 mm in a uniform portion having a large clad / core diameter ratio of 6 mm, ie, 4.4 was obtained. This base material was inserted into a fluoride glass pipe (outer diameter 20 mm, inner diameter 7.2 mm) having the same composition as the first clad, one end of which was closed, and wire drawing was performed while decompressing and integrating, outer diameter 125 μm, core diameter 10. 1.2 km of 1 μm fiber was obtained. The obtained fiber was a single mode fiber having a cutoff wavelength of 2.2 μm, and the transmission loss at a wavelength of 2.5 μm was 1.0 dB / km. As described above, a long loss single-mode optical fiber can be manufactured with the base material obtained by the present invention.

[0028]

As described above, by using the method of the present invention, an infrared transmitting optical fiber preform having a large ratio of the clad diameter to the core diameter and no structural irregularity can be efficiently produced. be able to. Therefore, as shown in the application example, the base material is stretched and drawn, or after being stretched, it is inserted into an infrared ray transmitting glass tube and subjected to a drawing process, whereby a low-loss infrared ray with small structural irregular scattering is obtained. A single mode optical fiber for transmission can be easily manufactured. Further, as shown in the examples, an optical fiber preform for infrared ray transmission, which has a core having a double structure doped with an active element in the center and has no structural irregularity, can be easily manufactured. Therefore, as shown in the application example, by inserting this into an infrared transmitting glass tube and performing a drawing and drawing process, a low loss single mode in which the active element doping is doped in the core center Since an optical fiber can be produced, it has an advantage that it can be applied as a highly efficient functional optical fiber element such as optical amplification and optical nonlinear application.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view for explaining a first step in the present invention.

FIG. 2 is a schematic cross-sectional view for explaining an example of a second step in the present invention.

FIG. 3 is a schematic cross-sectional view for explaining another embodiment of the second step in the present invention.

FIG. 4 is a schematic cross-sectional view for explaining still another embodiment of the second step in the present invention.

[Explanation of symbols]

1 mold 2 Glass melt for 1st clad 3 First clad 4 Glass melt for second cladding or first core Glass melt for 5 cores or 2nd core 6 Second clad or first core 7 1st core or 2nd core 8 stoppers

Claims (4)

[Claims] 1. A first step of forming a pipe-shaped first clad by pouring a glass melt of a first composition into a mold having a cylindrical hollow portion and rotating the mold, and continuously to this step. The number of revolutions of the mold is decreased or the rotation is stopped, and the glass melts of the second composition and the third composition are poured into the pipe-shaped first clad formed in the mold upright, A method of manufacturing an optical fiber preform for infrared transmission, comprising: a clad and a core inside thereof, or a second step of forming a first core and a second core inside thereof. 2. In the second step, a glass melt for the second clad or for the first core is poured into the pipe-shaped first clad, and is continuously melted for the core or for the second core. And a second core is formed inside the second clad and the core or the first core by volumetric contraction when the glass melt for the second clad or the first core is solidified by pouring the respective liquids. The method for producing an optical fiber preform for infrared transmission according to claim 1. 3. In the second step, a glass melt for a second clad or a core is poured into the inside of the pipe-shaped first clad, and a glass melt for a core or a second core is continuously supplied. By pouring, removing the stopper provided at the bottom of the mold, and moving a part of the glass melt for the second cladding or the first core downward.
The method for manufacturing an infrared transmitting optical fiber preform according to claim 1, wherein a clad and a core or a first core and a second core are formed inside the clad and the core. 4. In the second step, a glass melt for the second clad or for the first core is poured into the inside of the pipe-shaped first clad, and the center of the glass melt is solidified before the center is solidified. Is poured out from the melt injection port of the mold, and the core or second core glass melt is poured into the hollow portion formed in the central portion of the mold to form a second
The method for manufacturing an infrared transmitting optical fiber preform according to claim 1, wherein a clad and a core or a first core and a second core are formed inside the clad and the core.