JP4559458B2 - Optical fiber manufacturing method - Google Patents

Description

The present invention relates to a method of manufacturing an optical fiber to be subjected to optical signal transmission, in particular to a method of manufacturing an optical fiber used as a transmission medium in optical communication.

  In general, in optical communication using an optical fiber, an optical signal propagates through the optical fiber in a state of being confined in the core portion due to total reflection due to a difference in refractive index between the core portion and the cladding portion. In recent years, research and development of an optical fiber that propagates an optical signal by total reflection using a difference in refractive index between air and glass by providing a plurality of holes in the cross section of the optical fiber has been advanced. Since the optical fiber can obtain a large difference in refractive index as compared with an optical fiber to which a conventional dopant is added, it is possible to realize characteristics that cannot be realized with a conventional optical fiber.

  For example, in Patent Document 1, by providing a plurality of holes in the cladding of a conventional 1.3 μm band zero-dispersion single mode optical fiber (SMF) in which germanium is added to the core, the mode field diameter is set to the SMF. It is disclosed that the bending loss at a bending radius of 10 mm can be reduced to 1 dB / m or less while maintaining the same 7.9 μm to 10.2 μm.

  Up to now, when the above-described single-mode optical fibers with holes are connected to each other by mechanical splicing using a refractive index matching material, the refractive index matching material is several in the hole, for example, in the axial direction. m has entered, and the propagation characteristics of the optical fiber have been remarkably deteriorated. In addition, when the hole end in the axial direction of the single-mode optical fiber with holes is open, moisture penetrates into the holes and promotes the growth of microcracks, etc. There was a possibility that the mechanical strength would deteriorate.

  For this reason, various methods for sealing the end face of the optical fiber with holes have been developed. For example, Patent Document 2 describes a method in which glass powder is inserted into a hole end, and the glass powder is melted using laser light to seal the hole end. Patent Document 3 describes a method of sealing a hole end portion with quartz fine particles having an index of refraction equal to or smaller than that of the cladding portion and an optical adhesive.

International Publication WO2004 / 092793 Pamphlet JP-A-2005-24842 JP 2005-3252 A

  However, the optical fiber end sealing methods described in Patent Document 2 and Patent Document 3 require that sealing be performed every time connection is performed during wiring, and wiring work becomes complicated. It was.

Therefore, the present invention has been made in view of such problems, and an object of the present invention is to make wiring work complicated in an optical fiber having a plurality of holes in the axial direction of the optical fiber. An object of the present invention is to provide a method of manufacturing an optical fiber that can be reduced.

An optical fiber manufacturing method according to the first invention for solving the above-described problems is as follows.
An optical fiber comprising a core part and a clad part having a refractive index smaller than that of the core part, wherein the clad part is equivalent to the clad part or a plurality of air layers in the optical fiber axial direction. A region where a plurality of glass layers having a smaller refractive index are mixed is continuous in the optical fiber axial direction, and three or more formed in the cladding portion so as to surround the periphery of the core portion. It is a region formed inside each hole, and the hole is closed by the glass layer, an optical fiber manufacturing method,
A first process for producing an optical fiber preform having a core portion and a cladding portion having a refractive index smaller than that of the core portion;
The cladding portion of the optical fiber preform manufactured in the first process is continuous in the optical fiber axial direction, and has three or more holes in the cladding portion so as to surround the core portion. Two processes,
A plurality of glass bodies made of a glass material having a refractive index equal to or smaller than that of the cladding part and having a predetermined three-dimensional shape smaller than the hole diameter are inserted into the holes provided in the second process. A third process;
And a fourth process of melt-stretching the optical fiber preform obtained by the third process.

According to the method for manufacturing an optical fiber according to the first aspect of the present invention, an optical fiber including a core portion and a cladding portion having a refractive index smaller than that of the core portion, the optical fiber being axially disposed in the cladding portion. A region where a plurality of air layers and a plurality of glass layers having a refractive index equal to or smaller than that of the cladding portion are mixed is continuous in the axial direction of the optical fiber, and around the core portion. It is a region formed inside each of three or more holes formed in the cladding part so as to surround, and the holes are closed by the glass layer. A first process for producing an optical fiber preform having a core part and a clad part having a refractive index smaller than that of the core part, and a clad of the optical fiber preform produced by the first process A second process in which three or more vacancies are provided in the cladding portion so as to be continuous in the optical fiber axial direction and surround the periphery of the core portion, and in the vacancies provided in the second process A third process of inserting a plurality of glass bodies made of a glass material having a refractive index equal to or smaller than the cladding part and having a predetermined three-dimensional shape smaller than a hole diameter, and obtained by the third process. And a fourth process for melt-drawing the optical fiber preform thus obtained, the third process is merely added to the conventional method for manufacturing a holey optical fiber, which complicates the manufacturing operation and increases the manufacturing cost. Can be suppressed.

Hereinafter, an optical fiber and a manufacturing method thereof according to the best mode for carrying out the present invention will be described in detail with reference to the drawings.
FIG. 1 is a diagram schematically showing an optical fiber according to the best embodiment of the present invention, and FIG. 2 is a sectional view thereof. FIG. 3 is a diagram schematically showing a glass material used for manufacturing an optical fiber according to the best embodiment of the present invention.

  As shown in FIGS. 1 and 2, an optical fiber 10 according to the best embodiment of the present invention includes a core portion 11 having an arbitrary refractive index distribution that is located at the axial center and doped with a dopant, A clad part 12 having a uniform refractive index disposed around, a concentric circle centering on the core part 11 in the clad part 12 and extending in the longitudinal direction of the optical fiber 10 and having a circular cross-sectional shape And a plurality of holes (six in the illustrated example). The air holes 15 are provided with a glass layer 13 having an index of refraction that is equal to or smaller than that of the cladding portion 12 and an air layer 14. That is, the air holes 15 are regions in which a plurality of air layers 14 and a glass layer 13 having a refractive index smaller than or equal to that of the cladding portion 12 are mixed. The clad part 12 has a refractive index smaller than that of the core part 11. In the optical fiber 10, the core part 11 and the clad part 12 form a main light waveguide structure.

  The arrangement of the air layer 14 may be random in the holes 15 or may be regular. For example, the air layers 14 may be arranged at equal intervals in the longitudinal direction of the air holes 15, or may be arranged with gradually increasing intervals from one end side toward the other end side. As described above, when the air layer 14 is gradually widened in the longitudinal direction of the hole 15, the mode field diameter is increased on one end side of the optical fiber and on the other end side. Can be small.

  In the optical fiber 10, in addition to the six regions (holes) 15 in which the air layer 14 and the glass layer 13 coexist as described above and the concentric circles, the four regions 15 are arranged in a square shape. Alternatively, three regions 15 may be arranged in a triangular shape. Further, the shape of the region 15 (shape in a cross section orthogonal to the longitudinal direction of the optical fiber 10) may be other than a circle.

Then, the manufacturing method of the optical fiber 10 mentioned above is demonstrated below.
First, an optical fiber preform having a core portion and a cladding portion having a refractive index smaller than that of the core portion is manufactured (first process). Subsequently, a plurality of holes extending in the optical fiber axial direction are provided in the clad portion of the optical fiber preform manufactured by the first process using a drill or the like (second process).

  Subsequently, glass substances having a refractive index that is equal to or smaller than that of the cladding part are arranged in the axial direction in the holes provided in the second process (third process). However, one end in the longitudinal direction of the optical fiber preform is closed with a member made of the same material as the core or cladding. This prevents the glass material inserted into the holes from being discharged out of the holes.

  Subsequently, the optical fiber preform obtained in the third process is melt-drawn, for example, by drawing, to manufacture the optical fiber 10 described above (fourth process). When the optical fiber preform is heated to a high temperature, the glass material inserted into the pores of the optical fiber preform is melted (softened) in the same manner as the optical fiber preform. Therefore, when the optical fiber preform is stretched, the glass material is also stretched in the same manner as the optical fiber preform. Therefore, an optical fiber in which a glass layer and an air layer are mixed in the air holes is manufactured.

  However, the glass material used in the third process described above may be a cubic glass material 21 as shown in FIG. 3A, a spherical glass material 22 as shown in FIG. As shown in c), there is a conical glass material 23. It is desirable that the size of the glass material is smaller than the diameter of the hole and can be mixed with an appropriate amount of air when inserted into the hole.

  Therefore, according to the method for manufacturing an optical fiber according to the best embodiment of the present invention, the third process is merely added to the conventional method for manufacturing a holey optical fiber. Between the second process of providing holes in the reform and the fourth process of producing an optical fiber by melt-drawing an optical fiber preform that has been subjected to predetermined processing, a glass material is formed in the holes of the optical fiber preform. Only the third process of inserting the process is provided, and the complexity of the manufacturing operation and the increase in the manufacturing cost can be suppressed.

  The best mode of the optical fiber according to the present invention will be specifically described below based on examples.

  In this embodiment, in the optical fiber, the air layer and the glass layer having a refractive index which is equal to or smaller than that of the cladding portion are mixed into six circles from the center of the core portion. The shape of the glass material placed in the holes and inserted into the holes was a sphere.

FIG. 4 shows structural parameters of the optical fiber according to the first embodiment of the present invention. As shown in FIG. 4, the diameter of the core part 31 is defined as 2a ₁ , the distance from the center 31a of the core part to the hole 35 is defined as a _2, and the radius of the hole 35 is defined as R. In this embodiment, the core portion 31 has a step type refractive index distribution having a relative refractive index difference of 0.35% with respect to the cladding portion. Further, the refractive index of the glass material inserted into the cladding part and the holes 35 was made the same as that of pure quartz glass.

Here, the ratio P of the glass substance per unit volume of a void | hole is demonstrated using FIG. FIG. 5 is a diagram schematically showing the holes of the optical fiber according to the first embodiment of the present invention and the glass material inserted therein. FIG. 5A shows the radius r _{1 of the} glass material. vacancy ratio R / r ₁ radius R represents a case 1, in FIG. 5 (b), the ratio R / r ₂ of the pore radius R to the radius r ₂ of the glass material indicates the case 2 against. When the ratio R / r ₁ , R / r _{2 of the} hole radius R to the radius r ₁ , r ₂ of the glass substance is ₁ , ₂ , the ratio P of the glass substance per unit volume of the hole is respectively 50% and 25%.

Here, an optical fiber having only a core portion and a clad portion based on the conventional material addition, that is, a normalized electric field strength and a core portion in the case where there is no hole in the optical fiber according to the first embodiment of the present invention The relationship of the normalized radius with respect to the radius will be described with reference to FIG. However, the characteristic of FIG. 6 shows the electric field distribution at a wavelength of 1550 nm. FIG. 6 shows that, for example, at the point where the standardized radius is 1.5, the standardized electric field strength attenuates to 20% or less. That is, when the distance a ₂ from the center of the core part to the hole is 1.5 times the radius a ₁ of the core part, a normalized electric field strength of about 20% exists.

The relationship of the mode field diameter at the wavelength of 1550 nm with respect to the ratio a ₂ / a ₁ of the distance from the center of the core to the hole to the radius of the core will be described with reference to FIG. From FIG. 7, it can be seen that in the region where the ratio a ₂ / a ₁ of the distance from the center of the core portion to the hole with respect to the radius of the core portion is 2 or less, the mode field diameter rapidly decreases. This is because the confinement in the cladding part from the center of the core part of the electric field distribution to the hole is improved by the holes. Further, from FIG. 6, the normalized electric field strength in the conventional optical fiber in the region where the ratio a ₂ / a ₁ of the distance from the center of the core portion to the hole with respect to the radius of the core portion is 2 is about 8%.

Therefore, in the optical fiber according to the present embodiment, the distance a ₂ from the center of the core portion to the hole is a region where the intensity of the electric field distribution formed in the region from the center of the core portion to the hole is 8% or less. By setting to, the mode field diameter characteristic equivalent to that of the conventional optical fiber can be maintained. Conversely, the distance a ₂ from the center of the core part to the hole is set to a region where the intensity of the electric field distribution formed in the region from the center of the core part to the hole is 8% or more. It is also possible to improve the confinement of the distribution near the core and reduce the mode field diameter.

Here, the relationship of the chromatic dispersion at the wavelength of 1550 nm to the ratio a ₂ / a ₁ of the distance from the center of the core part to the hole with respect to the radius of the core part will be described with reference to FIG. FIG. 8 shows that the chromatic dispersion increases rapidly in the region where the ratio a ₂ / a ₁ of the distance from the center of the core part to the hole to the radius of the core part is 2.5 or less. This is because the confinement of the electric field distribution from the center of the core part to the clad part from the hole to the clad part occurs and the waveguide dispersion characteristic is affected. Further, as shown in FIG. 6, the normalized electric field strength in the conventional optical fiber in the region where the ratio a ₂ / a ₁ of the distance from the center of the core to the hole to the radius of the core is 2.5 is about 3%.

Therefore, in the optical fiber according to the present embodiment, the distance a ₂ from the center of the core portion to the hole is a region where the intensity of the electric field distribution formed in the region from the center of the core portion to the hole is 3% or less. By setting to, it becomes possible to maintain the same wavelength dispersion characteristic as that of the conventional optical fiber. On the other hand, by setting the distance a ₂ from the center of the core portion to the hole to a region where the intensity of the electric field distribution formed in the region from the center of the core portion to the hole is 3% or less, It is also possible to make the dispersion characteristic variable. In particular, the chromatic dispersion characteristic of 30 ps / nm · km or more at a wavelength of 1550 nm is a characteristic that cannot be realized with a conventional optical fiber due to restrictions on the amount of change in refractive index. Considered as one.

Here, the relationship of the bending loss at the wavelength of 1550 nm to the ratio R / a ₁ of the hole radius to the core radius will be described with reference to FIG. In FIG. 9, the vertical axis represents the loss increase amount when bending with a radius of 5 mm is applied 10 turns. The thick line and the solid line in the figure represent the bending loss characteristics when the ratio P of the glass material per unit volume of the pores is 25% and 50%, respectively. In FIG. 9, in any ratio P of the glass material per unit volume of the holes, the bending loss is a ratio R / a ₁ of the hole radius to the core radius, that is, as the hole radius increases. It can be seen that it is reduced. Moreover, it turns out that a favorable bending loss characteristic is acquired, so that the ratio P of the glass substance per unit volume of a void | hole decreases.

This is because, in the optical fiber according to the present embodiment, the effective refractive index of the cladding portion is reduced by expanding the holes and increasing the air ratio, and from the center of the core portion of the electric field distribution to the holes. This is because the confinement in the clad portion is improved, and the function and effect do not depend on the holes and the shape and number of air in the holes. Further, the effect does not depend on the local air layer in the longitudinal direction of the optical fiber or in the cross section, and the nonuniformity of the holes. As shown in FIG. 9, when the ratio P of the glass material per unit volume of the holes is 50%, the ratio R / a ₁ of the hole radius to the core radius is set to 1.3 or more. Compared to the above, the bending loss can be improved to 1/10.

  Therefore, according to the optical fiber according to the first embodiment of the present invention, the glass having a plurality of air layers in the optical fiber axial direction in the clad portion and a refractive index that is equal to or smaller than that of the clad portion. Due to the existence of the mixed region, the air layer and the glass layer having a refractive index equal to or smaller than that of the cladding portion are mixed in the axial direction of the cladding portion of the optical fiber. It has better bending loss characteristics than fiber, has the effect of preventing the intrusion of refractive index matching material when connecting with mechanical splice and the infiltration of moisture, and suppressing the deterioration of optical characteristics and mechanical strength, and Since it is not necessary to seal each time connection is performed, wiring work can be performed efficiently.

It is the figure which showed typically the optical fiber which concerns on the best embodiment of this invention. 1 is a cross-sectional view of an optical fiber according to the best embodiment of the present invention. It is a figure which shows typically the glass substance used for manufacture of the optical fiber which concerns on the best embodiment of this invention. It is a figure which shows the structural parameter of the optical fiber which concerns on 1st Example of this invention. It is the figure which showed typically the hole of the optical fiber which concerns on 1st Example of this invention, and the glass substance inserted in the inside. It is a figure which shows the relationship of the normalized radius with respect to the normalized electric field strength and the radius of a core part. Is a view from the center of the core portion to the radius of the core portion shows the relationship between mode field diameter at the wavelength 1550nm for the ratio a ₂ / a ₁ of the distance to the holes. Is a diagram showing the relationship between the wavelength dispersion at the wavelength of 1550nm from the center of the core portion with respect to the ratio a ₂ / a ₁ of the distance to the holes to the radius of the core portion. Is a diagram showing the relationship between the bending loss at a wavelength of 1550nm to the radius of the core portion to the radius of the ratio R / a ₁ of the holes.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical fiber 11 Core part 12 Cladding part 13 Glass layer 14 Air layer 15 Air holes 21, 22, 23 Glass material 34, 44 Glass material a ₁ Core part radius a ₂ Distance from center of core part to air hole R Hole radius r ₁ and r ₂ Glass material radius

Claims (1)

An optical fiber comprising a core part and a clad part having a refractive index smaller than that of the core part, wherein the clad part is equivalent to the clad part or a plurality of air layers in the optical fiber axial direction. A region where a plurality of glass layers having a smaller refractive index are mixed is continuous in the optical fiber axial direction, and three or more formed in the cladding portion so as to surround the periphery of the core portion. It is a region formed inside each hole, and the hole is closed by the glass layer, an optical fiber manufacturing method,
A first process for producing an optical fiber preform having a core portion and a cladding portion having a refractive index smaller than that of the core portion;
The cladding portion of the optical fiber preform manufactured in the first process is continuous in the optical fiber axial direction, and has three or more holes in the cladding portion so as to surround the core portion. Two processes,
A plurality of glass bodies made of a glass material having a refractive index equal to or smaller than that of the cladding part and having a predetermined three-dimensional shape smaller than the hole diameter are inserted into the holes provided in the second process. A third process;
And a fourth process of melt-drawing the optical fiber preform obtained in the third process.

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