JPH0431333A - Production of fluoride optical fiber - Google Patents

Abstract

PURPOSE:To reduce transmission loss by forming a second preform having a specific core diameter with drawing a fluoride glass preform in taper-like and making a fluoride glass preform for optical fiber having a uniform outer diameter with polishing outside of the second preform. CONSTITUTION:A fluoride glass preform composed of a taper-like core 1 and a clad 2 having a specific outer diameter is formed by a suction method, etc. Then, the preform having core diameter variation in longitudinal direction expressed by formula I [x (mm) is distance from core top end and y (mm) is core diameter] is drawn in taper-like state at a take-up speed expressed by formula II [X (mm/min) is feeding speed of preform, Y (mm/min) is take-up speed of preform and t (min) is time after attaining of core top end at central part in furnace] to obtain a preform composed of a core 1 having longitudinally uniform diameter and a clad having a taper-like outer diameter. Thus, periphery of resultant preform is polished to make a preform having a longitudinally uniform outer diameter and drawn to afford the aimed fluoride optical fiber having low transmission loss and a long length.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field 1] The present invention relates to a method for manufacturing a fluoride optical fiber with low loss and a long length having a constant core diameter.

(Left below) [Conventional technology J Fluoride optical fiber has a 10-” dB/km performance that exceeds that of quartz fiber.
The following transmission loss is expected, and it is seen as a promising transmission medium capable of long-distance non-relaying. Until now, methods for producing fluoride glass preforms that yield low-loss fibers include build-in casting, suction, and double-layer melting. The build-in casting method (Patent No. 1345722) involves casting a glass melt with a cladding composition into a cylindrical mold, pouring out the melt in the center without solidifying the center, and then pouring the core melt into the center. This is a method of producing a base material by casting. Suction method (JP-A-63-11535
In this method, the cladding melt is first cast into a cylindrical mold, then the core melt is continuously cast, and the core melt is introduced into the center using the volumetric shrinkage when the cladding melt solidifies. This is a method for producing. Two-layer melt method (
Patent No. 1438419) is a cylindrical mold in which the core melt is first cast, then the core melt is continuously cast, and before the clad melt solidifies, the bottom is pulled out and the clad glass flows out, and at the same time the core glass is cast. This is a method of introducing it into the center.

[Problems to be Solved by the Invention] These methods can produce a base material with a small diameter core and low scattering loss, but because a temperature distribution occurs in the longitudinal direction of the melt, it is difficult to create a matrix with a uniform core diameter in the longitudinal direction. It was difficult to make the material. Fluoride optical fibers obtained by drawing the base material produced in this way are difficult to connect because the core diameter is not constant, and the transmission loss characteristics change depending on the direction of light incidence. There were drawbacks such as: Moreover, there was also the disadvantage that the core/cladding diameter ratio could not be controlled and a single mode fiber could not be produced.

On the other hand, as a method for producing a base material with a uniform core/cladding diameter ratio in the longitudinal direction, the rotary silanal casting method (D, C, Tranet al.

Electron, [, ett, vol18. P
, 59. (1982)) have been proposed. In this method, a clad glass melt is poured into the mold while rotating a cylindrical mold, a hollow cylindrical clad glass pipe is created by centrifugal force, and then a core glass melt is poured into the center of the base material. This is a method of manufacturing.

The base material obtained by this method has a constant core/cladding diameter ratio, but due to crystallization due to the cladding surface being reheated by the core melt during the manufacturing process and the slow cooling rate of the core glass melt. However, due to the crystallization that occurs, it is difficult to reduce scattering loss. Furthermore,
Since a core with a small diameter cannot be manufactured, a single mode optical fiber cannot be manufactured either.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a method for manufacturing a long fluoride optical fiber with low loss by solving the above-mentioned drawbacks.

[Means for Solving the Problems 1] In order to solve the above-mentioned drawbacks, the present invention provides a fluoride glass matrix produced by a build-in casting method, a suction method, or a two-layer melt method that can obtain a matrix with low scattering loss. The material is stretched in a tapered shape without changing the speed to produce a second base material with a constant core diameter in the longitudinal direction, and the outer periphery of the second base material is polished to form a low-loss base material with a constant outer diameter of the cladding in the longitudinal direction. Fabricate a base material for a compound optical fiber.

In another form of the present invention, the fluoride glass base material is further inserted into a fluoride glass pipe, and while maintaining a reduced pressure state between the inner surface of the vibrator and the fluoride glass base material, the tapered material is tapered while changing the drawing speed. By drawing the fiber into a shape, a fluoride optical fiber with a controlled core/cladding diameter ratio is produced.

[Function 1] In the present invention, a fluoride glass base material whose core diameter is not uniform in the longitudinal direction is tapered drawn while changing the speed, thereby forming a base material whose core diameter is constant in the longitudinal direction. By manufacturing the cladding, polishing the outer periphery of the cladding to make the cladding outer diameter constant, and drawing the base material obtained in the above manner, the core diameter is constant and the core/cladding diameter ratio is controlled. By using this method, a long fluoride optical fiber with low structural asymmetric scattering and low loss can be manufactured. Furthermore, by inserting the obtained base material into a fluoride glass tube and tapering it, a long single mode optical fiber with low loss can be produced.

[Example 1] Hereinafter, the present invention will be explained in detail with reference to examples, but the present invention is not limited to the same extent by these examples.

(Example 1) Base material produced by suction method (core composition: 49 mol% Z
rF, -25 mol% BaFi -3,5 mol% LaF,
-2 mol% YF, -2,5 mol% AβF, -18 mol%
LiF, cladding composition: 47.5 mol% ZrF4-2
3.5 mol% BaF, -2,5 mol% LaFs-2 mol% YF, -4,5 mol% F, -20 mol% NaF
) is shown in Figure 1.

As is clear from FIG. 1, the core l of the base material produced by the suction method has a tapered shape. The outer diameter of the cladding 2 is constant. In FIG. 1, the area where the change in core diameter can be approximated by a straight line is in the range of 12 to 51 f+m from the tip of the core. The distance from the core tip is x (am), and the core diameter is y (
am), the change in core diameter in the longitudinal direction can be expressed by the following equation.

! = 0. []41x+1.1...
・(1) When producing a base material by tapering this base material, the conditions for the core diameter to be constant in the longitudinal direction are as follows: the feed speed of the base material is X (mm/a+in>), and the take-up speed of the base material is Y
(Iffm/mfn) and the time t (min) after the tip of the core reaches the center of the electric furnace, it can be expressed by the following equation.

Y=(0,025Xt+0.69) ”X −(
2) The base material shown in FIG. 1 was stretched at a bowing speed of formula (2), with a feed rate of 5 Omm/min and a heating temperature of the base material of 270°C. FIG. 2 shows changes in the outer diameter and core diameter of the obtained base material in the longitudinal direction. As is clear from FIG. 2, a base material was obtained in which the diameter of the core 1 was uniform in the longitudinal direction and the outer diameter of the cladding 2 was tapered. The outer periphery of the cladding 2 of this base material was polished to obtain a base material having a constant outer diameter of 3 mm and a length of 83 mm.

As a result of injecting a He-Ne laser beam into the base material obtained in this way and measuring the scattered light intensity in the vertical direction in the longitudinal direction, there was no change in the scattered light intensity before and after stretching, and scattering due to reheating during stretching. No somatic growth occurred.

FIG. 3 shows the transmission loss characteristics of the fiber obtained by drawing this base material. During the measurement, light was incident on this fiber from both the forward and reverse directions to measure the transmission loss, but no difference was observed in the measurement results. Also, the feed rate was set to 0.5
A base material having a similar shape could be produced even if the change was made within the range of ~10 mm/win.

(Example 2) A base material having the same shape as Example 1 had an outer diameter of 20++v+,
Insert it into a fluoride glass vibrator with an inner diameter of 7 mm and closed at one end, and use a rotary pump to pump it to 10 torr from the other end.
Stretching was carried out under the same conditions as in Example 1 while reducing the pressure.

FIG. 4 shows changes in the outer diameter and core diameter of the base material obtained after stretching in the longitudinal direction. As is clear from FIG. 4, a base material having a uniform core diameter in the longitudinal direction was obtained.

A He-Ne laser was applied to this base material to measure the scattered light intensity in the vertical direction in the longitudinal direction, which was comparable to the scattered light intensity of the base material before stretching, and no growth of scatterers had occurred. . Furthermore, no change in the intensity of scattered light was observed at the interface between the base material and the glass vibrator, indicating that the interface was completely fused.

When the atmospheric pressure was maintained between the glass vibrator and the base material during stretching, some air bubbles remained at the interface, but even a small amount (differential pressure of 1
torr) By reducing the pressure, the generation of bubbles could be suppressed.

The outer periphery of the cladding 2 of this base material was polished to an outer diameter of 19.5 mm, and the resulting base material was drawn to obtain an optical fiber of about 800 m. The refractive index difference of this base material is 0.61%,
Since the core diameter was 10.3 μm, the obtained fiber was a single mode fiber with a cutoff wavelength of 2.2 μm. The transmission loss characteristics of this fiber are shown in FIG. The minimum loss of this fiber is 1.5dB/km in 2.55μs
Using this method, we were able to fabricate a long single-mode optical fiber with low loss.

(Example 3) Base material produced by build-in casting method (core composition: 53 mol% ZrF4-20 mol% BaFi 4 mol% LaFs-3 mol% iF, -20 mol% NaF
, cladding composition: 27 mol% ZrF4-26 mol% Hf
The core diameters of F4-20 mol% BaFz-4 mol% LaF, -3 mol% iF, -20 mol% NaF) are shown in FIG.

As is clear from FIG. 6, the core 1 of the base material produced by the build-in casting method has a tapered shape.

In Figure 6, the portion where the core diameter change can be approximated by a straight line is
The range is 16 to 40 mm from the tip of the core. If the distance from the tip of the core is x (mm) and the core diameter is y (mm), then the change in the core diameter in the longitudinal direction is expressed by the following formula (%). The conditions for the diameter to be constant in the longitudinal direction are the feeding speed of the base material, X (mm/m1n), and the take-up speed of the base material, Y (
mm /win) and the time t (win) after the core tip reaches the center of the electric furnace, it can be expressed by the following equation.

Y = (0,055Xt+0.13)"X ・
...(4) The feed rate was 1.5 mm/win, the heating temperature of the base material was 270°C, and the base material shown in Fig. 6 was heated to (4).
) was drawn at the bowing speed of the formula.

FIG. 7 shows changes in the outer diameter and core diameter of the obtained base material in the longitudinal direction. As is clear from FIG. 7, a base material was obtained in which the core diameter was uniform in the longitudinal direction and the outer diameter of the cladding 2 was tapered. The outer periphery of the cladding 2 of this base material was polished to obtain a base material having a constant outer diameter of 3 mm and a length of 45 m.

A He-Ne laser is incident on the base material obtained in this way,
As a result of measuring the scattered light intensity in the vertical direction in the longitudinal direction, there was no change in the scattered light intensity before and after stretching, and no growth of scatterers occurred due to reheating during stretching.

FIG. 8 shows the transmission loss characteristics of the fiber obtained by drawing this base material. For this fiber, when measuring:
Light was incident from both the forward and reverse directions and the transmission loss was measured, but no difference was observed in the measurement results.

(Example 4) Base material produced by two-layer melt method (core composition: 48.5 mol%
ZrF4 - 23.5 mol% BaF, -3.5 mol% La
Fs 2 mol% YFs - 2.5 mol% iF, -5 mol% LiF - 15 mol% NaF, cladding composition: 47
．． 5 mol% ZrF < -23.5 mol% BaFz 2
．． 5 mol% LaFi-2 mol% YF.

-4.5 mol% AI! , FS-20 mol % NaF) was stretched at the following speed using a glass pipe with an outer diameter of 12 mm and an inner diameter of 6 mm, with the degree of vacuum between the pipe and the base material being 700 torr.

Y = (0,014Xt+0.81)”X
-(5) The outer shape of the cladding of the obtained base material was tapered, and the cladding of this base material was polished to obtain a base material in which the outer diameter of the cladding 2 was constant in the longitudinal direction (outer diameter 12 mm).
) * As a result of drawing this base material, a 700 mm fluoride optical fiber was obtained. As a result of measuring its transmission loss characteristics,
The minimum loss for this fiber is 2.0 dB at 2.55 μm.
/km. Also, this fiber has a refractive index difference of 0
．． 24% and a core diameter of 17.1 μm, it was a single mode fiber with a cutoff wavelength of 2.3 μm.

(Example 5) Using the same base material as in Example 1, it was stretched according to the following formula.

Y=(1/(1-0,021Xt))"X ・(6
) The shape of the obtained base material is shown in FIG. In this case, both the core 1 and the cladding 2 are tapered in the longitudinal direction. As is clear from FIG. 9, by using the method of the present invention, it is possible to produce a fluoride optical fiber whose core shape is changed as desired in the longitudinal direction.

As in Example 2, a fluoride glass base material was inserted into a fluoride glass pipe, and while maintaining a reduced pressure state between the inner surface of the pipe and the base material, it was stretched according to equation (6). It is also possible to form a second base material.

[Effects of the Invention 1] As is clear from the above, by using the method of the present invention, a fluoride glass base material whose core diameter is not uniform in the longitudinal direction is subjected to taper drawing while changing the speed. By creating a base material with a constant core diameter in the longitudinal direction, polishing the outer periphery of the cladding to make the cladding outer diameter constant, and drawing the resulting base material, it is possible to maintain a constant core diameter. In addition, the core/cladding diameter ratio is controlled, and a long fluoride optical fiber with low structural asymmetric scattering and low loss can be manufactured. Furthermore, by inserting the obtained base material into a fluoride glass tube and tapering it, there is an advantage that a long single mode optical fiber with low loss can be produced. Alternatively, according to the present invention, it is also possible to produce a fiber in which the core diameter is changed as desired in the longitudinal direction.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of the shape of the base material produced by the suction method in Example 1, Figure 2 is a sectional view showing the shape of the base material after stretching the base material shown in Figure 1, and Figure 3 is Figure 4 shows the transmission loss characteristic diagram of the optical fiber obtained in Example 1. Figure 4 shows the shape of the base material obtained by inserting the base material obtained by the suction method in Example 2 into a glass pipe and then drawing it. 5 is a transmission loss characteristic diagram of the optical fiber obtained in Example 2, FIG. 6 is an explanatory diagram of the shape of the base material produced by the built-in casting method in Example 3, and FIG. is a sectional view showing the shape of the base material after stretching the base material shown in Figure 6, Figure 8 is a transmission loss characteristic diagram of the optical fiber obtained in Example 3, and Figure 9 is a diagram showing the shape of the base material after stretching in Example 5. FIG. 3 is a cross-sectional view showing the shape of the obtained base material. 1...Core. 2...Clad.

Claims (1)

[Claims] 1) A method for producing a fluoride optical fiber having a uniform core diameter in the longitudinal direction by drawing a fluoride glass preform having a non-uniform core diameter in the longitudinal direction, comprising: a first step of forming a second base material having a constant core diameter in the longitudinal direction by stretching it in a tapered shape while changing the speed; A method for producing a fluoride optical fiber, comprising a second step of producing a uniform glass preform for a fluoride optical fiber. 2) In the first step, the fluoride glass base material is inserted into a fluoride glass pipe, and while maintaining a reduced pressure state between the inner surface of the pipe and the fluoride glass base material,
2. The method for producing a fluoride optical fiber according to claim 1, wherein the fluoride optical fiber is drawn in a tapered shape while changing speed. 3) In a method of producing a fluoride optical fiber by drawing a fluoride glass preform whose core diameter is not uniform in the longitudinal direction, the fluoride glass preform is drawn into a tapered shape while changing the speed to form a core in the longitudinal direction. a first step of forming a second base material whose diameter changes with a predetermined distribution; and a glass base material for a fluoride optical fiber having a uniform outer diameter in the longitudinal direction by polishing the outer periphery of the second base material. A method for producing a fluoride optical fiber, comprising: a second step of producing a fluoride optical fiber. 4) In the first step, inserting the fluoride glass base material into a fluoride glass pipe, and maintaining a reduced pressure state between the inner surface of the pipe and the fluoride glass base material,
4. The method for producing a fluoride optical fiber according to claim 3, wherein the fluoride optical fiber is drawn in a tapered shape while changing speed.