JPH064490B2 - Method of manufacturing constant polarization optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention relates to a method of manufacturing a constant polarization optical fiber having excellent polarization maintaining characteristics. A constant polarization optical fiber that propagates light while maintaining the polarization plane is expected to be used in coherent optical communication, optical application measurement, and other fields.

[Conventional technology]

Conventionally, as a constant polarization fiber, there has been proposed a system in which an anisotropy stress is applied to the core to make the core have birefringence so as to maintain the polarization plane. FIG. 2 shows an example of the structure of a constant polarization fiber of this kind. Reference numeral 1 is a core, 2 is a cladding, 3 is a stress applying portion, and in the figure, two examples are briefly shown. did. The core 1, the cladding 2, and the stress applying portions 31 and 32 are generally made of silica glass, the core 1 has a higher refractive index than the cladding 2, and the stress applying portions 31 and 32 have a thermal expansion coefficient higher than that of the core and the cladding. A large glass material such as SiO ₂ —B ₂ O ₃ is used. When a glass base material with a structure similar to that of Fig. 2 is heated and melted in a drawing furnace and tension is applied to draw it into fibers, the stress due to the difference in the coefficient of thermal expansion in the cooling process during drawing is illustrated. The stress is generated as shown by the arrow in the middle, and this stress gives an anisotropic stress to the core 1 so that the core has birefringence.

As a method of manufacturing the constant polarization fiber shown in FIG. 2, a silica glass rod having a core portion and a cladding portion is prepared, and a hole into which a stress applying member is inserted is formed at a symmetrical position around the core portion. There is a method (Japanese Patent Laid-Open No. 59-92929) in which a stress-applying member is inserted into a sheet to heat it to a high temperature to soften it and draw it to form a fiber.

As an improved method, a hole corresponding to the shape of the core material and the stress applying portion material is opened in the quartz glass rod to be the cladding, and then one end of the quartz glass rod is closed with a lid having the same diameter as the quartz glass rod. The core material and the stress imparting portion material are respectively inserted into the holes, and the upper portion of the hole in which the stress imparting portion material is put is also covered and closed. In this state, a method has been proposed in which the inside of the quartz glass rod is heated while reducing the pressure to a vacuum to fuse and integrate each part, and drawing is performed. According to this method, the deformation of the stress applying part during drawing is suppressed. It is said that a fiber having good symmetry, preventing bubbles from being generated at the interface, and having improved polarization characteristics can be obtained (Japanese Patent Publication No. 62-28098).

[Problems to be solved by the invention]

The method of inserting the core material and the stress applying portion material into the holes of the cladding material and wire drawing in vacuum as proposed in Japanese Patent Publication No. 62-28098 has various advantages. It has been found that there is a serious defect related to transmission loss that water is mixed into the interface and absorption by the OH group increases.

The object of the present invention is to propose a method for manufacturing a constant polarization fiber which solves the above problems of the present invention.

[Means for solving problems]

As a result of various studies on the above-described conventional vacuum drawing method, the present inventors have found that if the airtightness during depressurization is not perfect, trapping in the atmosphere occurs, and moisture in the atmosphere is taken up in the base material, I noticed that the increase of absorption of OH groups and that the impurities sneak into the pores of the base material during storage of the base material are the cause of the above problems,
This point was improved and the present invention was achieved.

That is, the present invention is mainly used in the manufacture of a constant polarization optical fiber consisting of a core that serves as an optical transmission line, a cladding having a lower refractive index than the core, and a stress imparting portion, and an axis is provided on both sides of the core of the glass base material that is composed of the core portion and the cladding portion. After opening a hole penetrating in the direction, after inserting the glass rod for stress imparting portion into the hole,
The temperature in the hole is 1500 ° C while flowing a chlorine-based gas into the hole.
To remove impurities and perform dehydration treatment by heating from the outside within a range that does not exceed the temperature, and then continuously heat one end of the base material while flowing a chlorine-based gas into the hole to perform fusion sealing and complete fusion. After that, the inside of the hole is depressurized while keeping the inside of the hole free of impurities, and then the other end of the base material is heated and fusion-sealed, thereby sealing the inside in a vacuum state. A base material for optical fibers, and then drawing the base material for optical fibers.

Hereinafter, the present invention will be described in detail with reference to the drawings. Fig. 1
(a) ~ (h) is an explanatory view showing an embodiment of the present invention in the order of steps, in which stress is applied to symmetrical positions on both sides of the core material 1'of the glass base material comprising the core material 1'and the cladding material 2 '. The holes 41 and 42 into which the material 3'for the applying part is inserted are opened to form a glass preform pipe [Fig. 1 (a)], and as the opening means, for example, an ultrasonic perforator is used. The inner surfaces of the holes 41 and 42 are preferably smoothed by optical polishing or by heat treatment while flowing a fluorine-based gas such as SF ₆ . The materials 31 'and 32' for stress applying portions are inserted into the holes 41 and 42. [Fig. 1 (b)]. Up to this step, it is the same as the conventional method. Next, the dummy tubes 5 made of quartz glass are attached to both ends of the glass base material pipe. Although it is not essential to mount the dummy pipe 5, it is advantageous to use the dummy pipe 5 because the effective use length of the base material can be increased. [Fig. 1 (c)]. Next, one end of the base material pipe or the dummy pipe 5 is fused with the lid 7 connected to the gas passage 6, and is attached to the glass lathe 8 in this state. The gas passage 6 of the lid 7 communicates with an atmosphere gas supply source (not shown) and the vacuum pump 9, and a valve 10 is provided between the passage 6 and the vacuum pump 9. First valve 1
Chlorine gas is closed inside the base metal pipe, that is, hole 4
Introduced into the hollow portions of 1, 42, the base material pipe is heated to about 1000 ° C. by a moving external heating source such as an oxyhydrogen burner 11 to remove impurities from the inner surfaces of the holes 41, 42 and perform dehydration treatment. [Fig. 1 (d)]. Next, while the chlorine-based gas is still flowing, the end portion A of the dummy tube 5 is connected to the oxyhydrogen burner 11
And heat seal to completely seal the pipe [Fig. 1
(e)]. At the same time as the one end A is completely sealed, the valve 10 is opened and the pressure of the pump 9 is reduced to keep the chlorine-based gas flowing [Fig. 1 (f)]. When the inside of the pipe can be depressurized to 0.1 to 20 Torr, the portion B of the other dummy pipe 5 is fused and sealed [Fig. 1 (g)]. As a result, a preform can be obtained in which the hollow portion inside is almost completely evacuated. The preform is heated in a drawing furnace 12 and drawn to obtain a fiber 13 having the structure shown in FIG. 2 (FIG. 1 (h)).

The glass base material comprising the core and the cladding in the present invention is a combination of glass materials in which the refractive index of the core is higher than that of the cladding, for example, the core is a GeO ₂ -doped quartz glass and the cladding is pure quartz, the core Examples thereof include a combination of pure quartz and a cladding of F-doped quartz glass, but other combinations may of course be used. The method for producing the glass base material composed of these cores and claddings is not particularly limited, and there are conventionally known methods such as VAD method (vapor phase axis method), MCVD method (internal CVD method), OVPO method (external method). ), The rod-inch-Yeub method or the like.

As the stress applying member used in the present invention, glass having a larger thermal expansion coefficient than the core cladding material is preferable, and for example, B ₂
Examples thereof include O ₃ -doped quartz glass.

In addition, examples of the chlorine-based gas used for removing impurities and dehydration in the present invention include Cl ₂ and CCl ₄ , and the heating temperature is about 1100 to 1500 ° C. If it is less than 1100 ° C, no sufficient effect is obtained, and if it exceeds 1500 ° C, vitrification occurs, which is not preferable.

In the present invention, the conditions for drawing the preform are not particularly limited, but for example, the temperature is 1950 to 210.
Heated to 0 ° C, linear tension force 10-30g, linear tension degree 50-
Generally, it is performed at about 200 m / min.

[Action]

In the method of the present invention, by flowing the chlorine-based gas into the hollow portion while heating while inserting the glass rod for the stress-applying part, in addition to removing impurities and hydroxyl groups, since decompression is performed while directly flowing the chlorine-based gas, Hydroxyl groups do not remain in the preform and can be sealed in vacuum. As a result, it is possible to obtain a constant polarization fiber free from air bubbles and residual hydroxyl groups, since atmospheric air injection from the leak portion can no longer occur during drawing. Further, in the conventional method, bubbles are likely to be formed at the interface with the stress-applying part due to the heat integration during drawing, which is a cause of increased loss due to structural irregularity and deterioration of crosstalk due to structural nonuniformity. . On the other hand, in the present invention, since the surface of the pores and the surface of the stress applying member are cleaned by the cleaning effect of burning off the dust, foreign matters, etc. by air-burning while flowing the chlorine-based gas, the loss due to the hydroxyl group is conventionally. In addition to the above, the loss due to structural imperfections and crosstalk degradation can be suppressed, and a constant polarization fiber with good characteristics can be obtained.

In the above description, the example in which the stress applying section is two is given.
It goes without saying that the case of having a larger number of stress applying portions is also included in the scope of the present invention.

〔Example〕

Example Pure quartz core and F-doped quartz glass designed with a cutoff wavelength of 1.22 μm (F-added amount: 0.9% by weight, relative refractive index difference)
0.3%) Two holes with a diameter of 12 mmφ were drilled with an ultrasonic perforator on both sides of a core of a base material having a diameter of 40 mmφ for a single mode fiber made of cladding. After connecting the dummy quartz tubes to both ends of this base material, rods of B ₂ O ₃ -doped quartz glass (B ₂ O ₃ addition amount of 26% by weight) with a diameter of 11 mmφ, which become stress-applying portions, are respectively attached to the two holes. Inserted.

In this state, as shown in Fig. 1 (d), it was attached to a glass lathe, gas was introduced into one side of the dummy tube, a lid with an exhaust port was attached, the valve was closed, and Cl ₂ 500cc was placed inside the hollow part of the base material.
Approximately 1 with an oxyhydrogen burner from the outside while flowing at a flow rate of / min.
By heating to 000 ° C., impurities in the hollow portion were removed and dehydration treatment was performed. Subsequently, while flowing Cl ₂ gas, the dummy tube on the side opposite to the lid was heated to be fused and integrated, but at the same time as completely melting, the valve was opened and Cl ₂ was depressurized while flowing. After decompressing the inside of the hollow part to about 10 torr, the dummy tube on the lid side was integrated by heating to obtain a base material for constant polarization fiber (invention product) in which the inside of the hollow part was vacuum-sealed [Fig. e) to (h)].

This base material was heated to 2050 ° C. in a drawing furnace and drawn with a tension of 15 g to give an outer diameter of 1125 μm and a cutoff wavelength of 1.
A single mode constant polarization fiber having a diameter of 22 μm and a stress applying portion diameter of 34 μm was obtained. The transmission loss of this fiber at a wavelength of 1.30 μm was 0.40 dB / km, and the absorption loss due to the OH group at a wavelength of 1.38 μm was 1.0 dB / km, and a constant polarization fiber with a very constant loss was obtained. The crosstalk at 1.30 μm was -30 dB (fiber length 1000 m), which was good.

Comparative Example Using a base material consisting of a core and a cladding of the same composition and size as the example and a rod for stress applying part, without performing treatment with Cl ₂ gas on the inner surface of the holes formed on both sides of the core, vacuum drawing was performed. The inside of the hole was depressurized to 10 torr or less and the rod was vacuum-sealed. The obtained constant polarization fiber base material (comparative product) was drawn under the same conditions as in the example, and the characteristic value of the obtained fiber was measured. The transmission loss at a wavelength of 1.30 μm was 0.76 dB / km. , The absorption loss due to the OH group at the wavelength of 1.38 μm is 3.7 dB / km, and the crosstalk is -2.
5 dB (fiber length 1000 m), which was inferior to the fiber of the present invention.

〔The invention's effect〕

As described above, the method for manufacturing a polarization-maintaining optical fiber of the present invention can prevent contamination of the OH group at the interface of the stress-applying portion, so that a polarization-polarizing optical fiber with low transmission loss and good crosstalk can be easily formed. This is an excellent method that enables stable production.

[Brief description of drawings]

1 (a) to 1 (h) are schematic views illustrating an embodiment of the present invention in the order of steps, and FIG. 2 is a cross-sectional view showing the structure of an example of a polarization maintaining optical fiber according to the present invention.

Claims (1)

[Claims] 1. In the manufacture of a constant polarization optical fiber mainly composed of a core to be an optical transmission line, a cladding having a lower refractive index than the core, and a stress applying portion, both sides of the core of the glass base material composed of the core portion and the cladding portion are provided. After opening a hole penetrating in the axial direction and inserting the glass rod for stress imparting portion into the hole, while heating the inside of the hole from the outside while flowing a chlorine-based gas into the hole, the temperature in the hole does not exceed 1500 ° C. Impurities are removed and dehydrated, and then one end of the base material is heated and fusion-sealed while flowing a chlorine-based gas into the holes, and after completely fusion-bonding, the atmosphere in the holes is free of impurities. The inside of the hole is depressurized while keeping it, and then the other end of the base material is heated and fusion-sealed to obtain a base material for an optical fiber whose inside is sealed in a vacuum state. Characterized by drawing a base material for optical fiber Method of manufacturing that polarization-maintaining optical fiber.