JP2771737B2 - Fabrication method of core expanded optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a core-enlarged optical fiber which is manufactured by using an optical fiber used as an optical transmission line, and which is used for connection between different kinds of optical fibers or connection between an optical component and an optical fiber. It is about.

[0002]

2. Description of the Related Art In a core-expanded optical fiber, a doping agent added to form an optical fiber core is diffused in a radial direction, thereby increasing the mode / field diameter of an optical signal propagating through the expanded core. Optical fiber.

[0003] Fabrication of a core-expanded optical fiber is performed by heating the optical fiber at a temperature at which the optical fiber does not melt and the dopant in the core thermally diffuses. There have been the following three methods for producing a conventional core-enlarged optical fiber. 1) A method in which an optical fiber is vacuum-sealed in a quartz tube and heated in an electric furnace. 2) A method in which heating is performed in the atmosphere using a micro burner. 3) A discharge heating method in the atmosphere.

[0004] The paper "One Constitution Method of Optical Fiber Embedded Device" (Kawakami, Shiraishi, Aizawa) in C-457, Proceedings of the IEICE Spring National Convention, 1988, and the IEICE Spring National Conference, 1989. As described in the paper of the conference paper C-451 "Prototype of spot size conversion fiber for optical fiber embedded device" (Shiraishi, Aizawa, Kawakami)
In the method (1), the coating of the optical fiber having a length of about 30 cm is removed, and the optical fiber is heat-treated after being vacuum-sealed in a quartz tube. Therefore, a pretreatment of vacuum-sealing is necessary. Before the heat treatment, the optical fiber is cut in a short length to remove the coating, so that it is necessary to connect to another coated optical fiber in actual use. Since the heating temperature can be applied only up to about 1400 ° C., it has a disadvantage that a long heat treatment time is required. According to the above-mentioned article, heating at 1300 ° C. for 5 hours was necessary as a typical example of the heat treatment conditions for expanding the core.

The method 2) is based on the electronic letter 27.
Vol. 21 (1991), pages 1968-1969, "THERMALL-DIFFUSED EXPANDED CORE FIBERS FOR LOW-
LOSS AND INEXPENSIVE PHOTONIC COMPONENTS "BY H.HAN
As described in AFUSA, M. HORIGUCHI, AND J. NODA, the heating time is greatly reduced, but it is essentially difficult to stop the fluctuation and the aging of the flame because the heating means is a burner. However, there is a drawback that the reproducibility of the core diameter is poor and the diameter distribution of the enlarged core in the length direction of the optical fiber is not smooth. Further, the core expansion range is limited by the size of the flame, and it is difficult to expand a wide range. Fig. 5 shows Fig. 1 described in the above paper. This figure shows the dependence of the mode field diameter on the heating time. It can be seen that the mode field diameter varies. FIG. 6 is FIG. 2 described in the above paper. This figure shows the distribution of the mode field diameter in the length direction of the optical fiber, but it can be seen that the symmetry and the smoothness are lacking. Further, the range of the core expansion is not greatly expanded even if the burner is swung in the length direction of the optical fiber.

The method 3) is described in Electronics Letters, Vol. 27, No. 17, 1991, pp. 1597-1599.
Paper “SIMPLE FUSION SPLICING TECHNIQUE FOR REDUC
INGSPLICING LOSS BETWEEN STANDARD SINGLEMODE FIBER
As in S AND ERBIUM-DOPED FIBER "BY HYTAM,
Although the reproducibility is superior to the method of 2), the heating range is 1
mm or less and the change in the length direction of the enlarged core diameter is sharper than in the method of 2), so that the upper limit of the core enlargement while suppressing the increase in the optical transmission loss is limited. Further, when connecting the optical component by cutting the enlarged core surface, the positional accuracy of the cut surface needs to be 0.1 mm or less, which is a practical problem. For these reasons, the application example of the method 3) is only connection of heterogeneous optical fibers, and the application range is limited.

[0007]

As is apparent from the above situation, it has been difficult to perform a heat treatment with an electric furnace in the air in the conventional core-enlarged optical fiber manufacturing technique.

[0008] On the other hand, the present inventors have proposed an optical fiber drawing and
A small electric furnace for processing an optical fiber for heating such as fusion was first proposed (see Japanese Patent Application Laid-Open No. 3-187937). As shown in FIG. 7, such a small electric furnace has both ends open, a split 2 is provided in the axial direction, and a platinum foil 3 is connected in a comb shape to an alumina insulating tube (furnace tube) 1 for housing an optical fiber 4. The optical fiber 4 is stretched and fused by applying a current to the platinum foil 3 from both ends and heating the alumina insulating tube 1 using the platinum foil 3 as a heating element.

Therefore, the core-expanded optical fiber is manufactured in the atmosphere using this small electric furnace. However, the temperature of the optical fiber exposed outside the furnace rises and the optical fiber 4
Is easy to bend, which is not practical from the viewpoint of reproducibility,
There was a problem.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method of manufacturing a core-enlarged optical fiber having excellent characteristics, a wide application range, and high productivity and reproducibility. .

[0011]

According to the present invention, there is provided a method of manufacturing a core-expanded optical fiber, comprising the steps of: In the method of the above, using a small electric furnace as a heating means, in the atmosphere, the maximum temperature near the furnace center of the optical fiber is 1500 ℃ ~ 1700 ℃,
The heat treatment is performed at a temperature of 900 ° C. or less at both ends of the furnace.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments showing the effects of the present invention will be described below.

Embodiment 1 As a first embodiment, an example of the dependence of the mode / field diameter change on heating conditions by heating a small electric furnace for core enlargement processing will be described. First, a manufacturing procedure of the core-enlarged optical fiber will be described. The optical fiber used is a silica-based single mode optical fiber having a core formed by doping GeO ₂ and having a core diameter of 7.3 μm, a relative refractive index difference of 0.32%, a fiber outer diameter of 125 μm, and a UV coating outer diameter of 250 μm. A standard single mode optical fiber was used.
Before the heat treatment, the coating was removed to a length of 55 mm, and the surface of the bare optical fiber was washed with acetone. The bare optical fiber portion was fixed between two fixing jigs, and the central portion thereof was inserted into a small electric furnace for processing an optical fiber to perform a heat treatment.

This heat treatment is performed so that the maximum temperature of the bare optical fiber near the center of the furnace is 1500 to 1700 ° C. and the temperature near both ends of the furnace is 900 ° C. or less. Here, the maximum temperature is set to 1500 to 1700 ° C.
This is because, when heating is performed at a temperature exceeding 0 ° C., the surface of the optical fiber evaporates due to its melting point or higher, which is undesirable.
If the temperature is lower than ° C, heating takes a long time, which is not preferable. The reason why the temperature near both ends of the furnace is 900 ° C. or lower is that if the temperature exceeds 900 ° C., the optical fiber is likely to bend, which is not preferable. Therefore, in this embodiment, even if the furnace core tube is lengthened and the center temperature of the furnace is set to the temperature for core expansion processing (1500 to 1700 ° C.),
The temperature of the optical fiber at both ends of the furnace did not exceed 900 ° C.

FIG. 1 is a view showing the structure of a small electric furnace for core enlargement treatment used, in which 10 is a small electric furnace for core enlargement treatment, and 11 is an alumina insulation having an inner diameter of 2 mm and a length of 50 mm. A tube, 12 is an alloy thin film resistor heat source, 13 is a current terminal, the distance between current terminals on both sides is 35 mm, 14 is a heat insulating material of foamed alumina, 15 is a refractory cement for holding the alumina insulating tube 11, and 16 is heat treatment. Each optical fiber is shown. In addition, the alumina insulating tube 11 and the insulating material 14 of foamed alumina are provided on the upper surface with a width of 0.4 mm in the length direction.
7, 18 are respectively inserted. Since the optical fiber 16 is inserted into the small electric furnace 10 for core enlargement processing from the lateral direction through the splits 17 and 18, there is no need to cut the optical fiber 16 in particular.
The core enlargement process can be performed at an arbitrary point on an optical fiber having an arbitrary length.

FIG. 2 shows the results measured with a 1.31 μm LD light source.
The heating time dependence of the mode field diameter at the center of heating at 1600 ° C. is shown. The heating temperature of the optical fiber at both ends of the furnace during this heating was about 800 ° C. mode·
It can be seen that the field diameter is small and increases monotonically on almost one curve. The heating time for doubling the mode / field diameter is about 55 minutes, which means that the processing time is reduced to about 1/5 as compared with the conventional vacuum-filled electric furnace heating method.

FIG. 3 shows the distribution of the mode field diameter in the longitudinal direction of the optical fiber. It can be seen that the mode and field diameters are distributed symmetrically and smoothly. Although a small electric furnace having a length of 50 mm was used here, the core expansion range was expanded by changing the furnace length, and a more gentle core diameter distribution could be obtained.

As can be seen from this procedure and the results, there is no need to cut the optical fiber into a short length as compared with the conventional vacuum-filled electric furnace heating method. There are advantages that there is no need to remove the coating to an unnecessarily long length, there is no need to vacuum-enclose the optical fiber in the quartz tube, and the heat treatment time is significantly shortened.

Compared with the micro burner heating method, the obtained enlarged core diameter is excellent in reproducibility, the distribution of the enlarged core diameter in the longitudinal direction of the optical fiber is smooth and excellent in symmetry, and the range of the core expansion is changed. It has the advantage that it can be performed.

Further, since the core expansion range can be made wider and gentler than the discharge heating method, there is an advantage that the core can be expanded more greatly and the application range to optical component connection and the like is not limited.

Also, as compared with the case of using only a small electric furnace for processing an optical fiber disclosed in Japanese Patent Application No. 1-326087, there is an advantage that the optical fiber is not bent during the core enlarging process. That is, the small electric furnace for processing optical fibers according to the publication employs a terminal structure capable of shortening the entire length in order to suppress the length of coating removal of the optical fiber to be processed. The optical fiber was softened due to the rise in the fiber temperature, and the temperature distribution in that portion was not stable due to the influence of disturbance. This caused bending, but this method has overcome the drawback.

Embodiment 2 Next, an embodiment of manufacturing a core-expanded optical fiber using the aforementioned small electric furnace 10 for core-expansion processing will be described.

FIG. 4 is a view for explaining the present embodiment, and shows an apparatus for producing a core-expanded optical fiber using a small electric furnace for core expansion processing. In the figure, 10 is a small electric furnace for core enlargement processing, 21 is a lifting apparatus for a small electric furnace for core enlargement processing, 22
a and 22b are fiber fixing bases, 23 is an optical fiber, 24
Denotes an enlarged portion of the core of the optical fiber from which the coating has been removed, 25 denotes a light source, 26 denotes a light receiver, 27 denotes a power source of a small electric furnace for core expansion processing, and 28 denotes a control device.

In the above configuration, first, the optical fiber 23
After removing the coating on the portion where the core is desired to be enlarged and washing it, the core is fixed to the fiber fixing tables 22a and 22b with a constant tension. Next, the small electric furnace 10 for core enlargement treatment
1 and insert the core enlarged portion 24 of the optical fiber into the center of the alumina insulating tube. The controller 28 raises the temperature of the small electric furnace 10 for core enlargement processing to a set temperature, heats the core enlargement section 24 at that temperature for a set time, ends the heating, and ends the small electric furnace 20 for core enlargement processing.
And the process ends.

During heating, the light source 25 connected to the optical fiber
The change in the transmission loss can be monitored by the optical receiver 26 and the optical receiver 26. However, if the processing conditions are established, the monitoring of these optical transmissions is not always necessary.

The maintenance of this apparatus only requires the replacement of a new electric furnace and the corresponding adjustment when the small electric furnace for core enlargement processing is exhausted, and does not require any special skilled person including the operation.

The conventional electric furnace heating apparatus for vacuum-sealing an optical fiber requires a separate apparatus for vacuum-sealing the fiber in addition to the main body. This operation not only requires labor and time, but also requires a specialized expert. Although needed, this method solved this problem.

The conventional micro-burner heating device requires precise control of the flow rate of the combustion gas, and furthermore, it is necessary to swing the micro-burner in the length direction of the optical fiber. Although this method has been expensive and expensive, the present method simplifies the apparatus and can be manufactured at low cost.

The application range of the core-expanded optical fiber obtained by the conventional discharge heating apparatus was limited because the expansion range of the core was narrow, but the application range was expanded by the present method.

Further, an apparatus only using a small electric furnace for processing an optical fiber disclosed in JP-A-3-187937 is
Although the optical fiber is likely to bend during the core enlargement processing and is not practical from the viewpoint of reproducibility, the present method has enabled the core enlargement processing with good reproducibility.

[0031]

As described above, the method of the present invention makes it possible to expand the core by an electric furnace in the atmosphere, so that it is not necessary to cut the optical fiber into a short length, and the length of the coating can be made longer than necessary. No need to remove, no need to vacuum-enclose optical fiber in quartz tube, remarkably shortened heat treatment time, excellent reproducibility of obtained enlarged core diameter, lengthwise direction of optical fiber with enlarged core diameter Distribution is smooth and excellent in symmetry, the range of core enlargement can be changed,
There is an advantage that the optical fiber is not bent during the heat treatment. The device has advantages such as a simple device configuration, low cost, simple operation, simple maintenance, and unlimited application range.

[Brief description of the drawings]

FIG. 1 is a schematic view of a small electric furnace for core enlargement processing used in a first embodiment.

FIG. 2 shows 160 measured with the 1.3 μmL D light source of the same example.
It is a graph which shows the heating time dependence of the mode field diameter of the center of heating at the time of 0 degreeC heating.

FIG. 3 is a graph showing the distribution of the mode field diameter in the length direction of the optical fiber of the embodiment.

FIG. 4 is a schematic view of an apparatus for manufacturing a core-enlarged optical fiber using a small electric furnace for core-enlargement processing according to an embodiment.

FIG. 5 Electronics Letter, Vol. 27, No. 21 (199)
1 year) on pages 1968-1969 "THERMALL-DIFFUS
ED EXPANDED CORE FIBERS FOR LOW-LOSS AND INEXPENSI
VE PHOTONIC COMPONENTS "BY H.HANAFUSA, M.HORIGUCH
It is a figure which shows Fig.1 of I, AND J.NODA.

FIG. 6 is a diagram showing FIG. 2 of the same paper.

FIG. 7 is a schematic view of a small electric furnace for processing an optical fiber according to the prior art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Small electric furnace for core expansion processing 11 Alumina insulating tube 12 Heat source of alloy thin film resistor 13 Current terminal 14 Insulation material of foamed alumina 15 Refractory cement 16 Optical fiber 17 Splitting of alumina insulating tube 18 Splitting of heat insulating material 21 Small size for core expansion processing Electric furnace elevating device 22a, 22b Fiber fixing stand 23 Optical fiber 24 Optical fiber core enlargement part 25 Light source 26 Photodetector 27 Power supply of small electric furnace for core enlargement processing 28 Controller

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-98202 (JP, A) JP-A-4-65326 (JP, A) JP-A-3-187937 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) G02B 6/10 G02B 6/14

Claims (1)

(57) [Claims] In a method for manufacturing a core-enlarged optical fiber, wherein a doping agent added to form an optical fiber core is heated and diffused, a small electric furnace is used as a heating means, and the optical fiber furnace is heated in the air. The maximum temperature near the center is 1
A method for producing a core-expanded optical fiber, wherein the heat treatment is performed at a temperature of 500 ° C. to 1700 ° C. and 900 ° C. or less at both ends of a furnace.