JP2003255153A - Single-mode optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a single-mode optical fiber in photonic crystal structure which is easy to manufacture and has the high degree of freedom of design of a dispersion characteristic and has a large core diameter. <P>SOLUTION: The geometric diameter of a core part is made larger than twice as large as the mean interval between columnar holes by arranging two kinds of columnar holes which extend in the extending direction of an optical fiber and have different internal diameters in a clad part discretely at fixed intervals. Further, columnar holes which are arranged at an equal distance from the geometric center of the core part among the columnar holes arranged in the clad part are made equal in diameter. Those columnar holes are filled with a medium which is smaller in refractive index to light of wavelength guided in a vacuum or the fiber than the substance forming the fiber. Furthermore, the core part has a plurality of columnar holes extending in the extending direction of the fiber. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single-mode optical fiber, and more particularly to a single-mode optical fiber having a photonic crystal structure which is easy to manufacture, has a high degree of freedom in designing dispersion characteristics, and has a large core diameter. Regarding fiber.

[0002]

2. Description of the Related Art Optical fibers having a photonic crystal structure are
Characteristic design is possible depending on how the holes provided in the clad are arranged. For example, a structure in which the holes are hexagonally close-packed is the structure of an optical fiber that is the easiest to manufacture. That is, when the cylinders having the same outer diameter are arranged without a gap, the cross section naturally takes a hexagonal close-packed structure. By arranging holes having a desired diameter in the center in the generatrix direction and arranging them, an optical fiber having a structure in which holes having an arbitrary diameter are spatially distributed in the clad portion can be obtained.
Further, by making the diameters of some of the holes different from the diameters of the other holes, it becomes possible to change the local effective refractive index of the optical fiber.

Further, since the characteristics of the optical fiber having the photonic crystal structure depend on the large difference in refractive index between air and glass, the wavelength dependence of the characteristics can be changed by changing the arrangement of the above-mentioned holes. It is also possible to make great changes.

[0004]

However, an optical fiber having a normal photonic crystal structure has an outer diameter equal to that of a glass rod used for forming a core portion for guiding light and a cladding portion as a peripheral portion thereof. Since it is made of a single glass rod, it is necessary to increase the spacing of the holes provided in the cladding when trying to enlarge the core part, which controls the dispersion characteristics of the glass rod used. However, there is a problem that it is difficult to fully utilize the characteristic of the photonic crystal structure that the characteristics can be designed depending on the arrangement of the holes.

Further, when the distance between the holes is widened in this way, the light confining effect in the optical fiber is lowered, and when the fiber is guided in the curved state, the light easily leaks. There was also a problem that (for example, TABr
iks et al., “Endlessly single-mode photonic crystal
fiber ”, Optics Letter, Volume 22, Issue 13, 9
61-963 (1997): hereinafter referred to as "Reference 1."

The present invention has been made in view of the above problems, and an object of the present invention is to facilitate manufacturing, to provide a high degree of freedom in designing dispersion characteristics, and to have a large core diameter. An object of the present invention is to provide a single-mode optical fiber having a photonic crystal structure capable of satisfying both one-mode conditions and bending characteristics.

[0007]

In order to achieve such an object, the present invention according to claim 1 is directed to a plurality of cylindrical columns extending in the fiber extending direction around a core portion. A single-mode optical fiber having a photonic crystal structure including a clad portion in which holes are periodically arranged, wherein the plurality of columnar holes are at least two types of columnar holes having different inner diameters. They are arranged at regular intervals.
The geometric diameter of the core portion is larger than twice the average distance between the cylindrical holes.

The invention described in claim 2 is the same as claim 1.
In the single-mode optical fiber described in the item (1), the diameters of the cylindrical holes arranged at the same distance from the geometric center of the core portion are equal.

The invention described in claim 3 is the same as that of claim 2
In the single-mode optical fiber according to, in the cylindrical holes arranged at equal distances from the geometric center of the core part, the diameter of the cylindrical hole adjacent to the core part is another circle. It is characterized in that it is larger than the diameter of the columnar holes.

The invention according to claim 4 is the same as claim 1
In the single-mode optical fiber described in the paragraph 1, among the cylindrical holes, at least one set of cylindrical holes facing each other with respect to the geometric center of the core portion has the same diameter. .

The invention described in claim 5 is the same as claim 1.
In the single mode optical fiber according to any one of 4 to 4, the inside of the cylindrical hole arranged in the clad is
It is characterized by being filled with a medium which is in a vacuum or has a refractive index lower than that of a substance constituting the fiber for light having a wavelength guided in the fiber.

Further, the invention according to claim 6 is the same as claim 1.
The single-mode optical fiber according to any one of items 1 to 5, wherein the core portion has a plurality of cylindrical holes extending in the extension direction of the fiber.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

1 to 6 are cross-sectional views of a fiber for explaining an example of the structure of a single mode optical fiber of the present invention.
3 to 3 are cross-sectional views of a fiber having a structure in which holes are arranged in the clad portion, and FIGS. 4 to 6 show structures in which holes are arranged in the clad portion and holes are also arranged in the core part. It is sectional drawing of a fiber.

The holes arranged in the clad portion of the fiber having the structure shown in FIGS. 1 to 6 are periodically arranged with a distance of Λ (Λ _{1 to 6} ) between them. The shaded core portion in the figure means a core region in which no holes are arranged. The cores shown in these figures have a generally hexagonal shape, but these are merely examples and may have other shapes.

[0016] The cladding of the fiber of the structure shown in FIGS. 1-3, is provided with two types of holes having different diameters, in FIG. 1 hole diameter d ₁ and the diameter d _{1 '(d 1>} d
₁ '), in FIG. 2 pore diameter _{d 2} and the diameter _{d 2' (d} 2 _{<d 2}
'), Hole diameter _{d 3} and the diameter _d 3 in FIG. 3' _(d 3 _<d
3 ') holes are periodically arranged.

Further, holes having hole diameters d ₄ (FIG. 4) and d ₅ (FIG. 5) are periodically arranged in the cladding portion of the fiber having the structure shown in FIGS. , Different diameters (δ ₄ (FIG. 4), δ
₅ (FIG. 5)) are provided. Further, the optical fiber having the structure shown in FIG. 6 has substantially the same structure as the structure shown in FIG. 4, but the structure shown in FIG. 4 has three sets facing each other with respect to the geometric center of the core portion. While the holes have the same diameter, the hole diameter of one set is different from the diameters of the other two holes.

It should be noted that, directly focusing on the spatial arrangement relationship between holes having different diameters, the structure shown in FIG. 1 is random, and the structures shown in FIGS. 2 to 5 are d ₂ ′ and d ₃ ′. ,
Voids having a diameter of δ ₄ and δ ₅ are arranged concentrically (that is, equidistant from the center of the core) with the center of the core as a reference.

The ratio (d / Λ) of the hole spacing Λ and the hole diameter d is γ
And the area of the core part is defined as the area (S) of the convex polygon inscribed in the hole closest to the core part of the holes arranged in the cladding part, this area is S ₀ : Approximately 4 (√3−) with respect to the area of the parallelogram obtained by connecting the centers of the holes adjacent to the core part)
γ) ² times (FIG. 1), about [1 + 3√3 (3-γ) (2-
γ)] / 2 times (Fig. 2), about [2 + √3 (10-3γ)
The area ratio r (S / S ₀ ) is (2-γ)] / 2 times (FIG. 3).

On the other hand, the area ratio r of the conventional structure shown in the above-mentioned Document 1 is (2-γ) ² times, and γ = 0.3.
In this case, by adopting the structure shown in FIGS. 1 to 3, the area ratio r can be increased to 2.8 times, 4.3 times, and 5.0 times, respectively, thereby increasing the core area. That is, the core diameter can be increased.

It should be noted that FIGS. 1 to 6 only show holes in the clad portion only in the vicinity of the core portion, and the holes are periodically formed over the entire clad portion in the arrangement relationship shown here. It is arranged.

The arrangement and number of holes in the core portion are not limited to those shown in these figures, and the arrangement of holes having different diameters in the clad portion is also shown in these figures. It is not limited to the arrangement shown.

In the single mode optical fiber having such a structure, the outer diameters of the cylindrical glass rod (the cylindrical glass tube if desired) for forming the core portion and the cylindrical glass tube for forming the cladding portion are It can be produced by making them equal and bundling them.

FIG. 7 is a view for explaining a state in which the above-mentioned cylindrical glass tubes and cylindrical glass rods are bundled in order to manufacture the single mode optical fiber having the structure shown in FIG.
These cylindrical glass rods 72 and cylindrical glass tubes 71, 7
3 has the same outer diameter of about 100 μm to 1 mm, while the cylindrical glass tubes 71 and 73 have different inner diameters, and the inner diameter of the cylindrical glass tube 71 for forming the clad portion is d.
₄ , the inner diameter of the cylindrical glass tube 73 for forming the holes in the core portion is δ ₄ . In the example shown in this figure, d ₄ <δ ₄ .

The cylindrical glass rod 72 and the cylindrical glass tubes 71 and 73 are bundled as shown in FIG. 7 to form a base material, which is heated and stretched, whereby the cylindrical glass rod 72 and the cylindrical glass tube 71 The gap between the cylindrical glass tubes 73 disappears during the drawing process, and as a result, a single mode optical fiber having the cross-sectional structure shown in FIG. 4 is obtained.

The single mode optical fiber having the cross-sectional structure shown in FIGS. 1 to 6 is also a cylindrical glass depending on the geometrical diameter of the core portion and the hole arrangement of the cladding portion (core portion if desired). By appropriately selecting the number of rods and cylindrical glass tubes and the outer and inner diameters, it is possible to manufacture them by the same method as described above.

For example, when manufacturing a single mode optical fiber having the structure shown in FIGS. 1 to 4, a cylindrical glass tube is selected according to the arrangement of holes in the cladding portion, and at the same time, for forming the core portion, 7 (Fig. 1), 12 (Fig. 2),
For example, 14 (FIG. 3) and 13 (FIG. 4) cylindrical glass rods are used, but of course, other numbers may be used. Furthermore, when forming the core part shown in these figures, a part of the cylindrical glass rod was replaced with a cylindrical glass tube to prepare a single mode optical fiber in which holes were also arranged in the core part. Good.

(Example 1) FIGS. 8A and 8B show the electric field distribution obtained by the FDTD method when light is guided in the single mode optical fiber having the structure shown in FIG. FIG. 8A shows the electric field distribution of the fundamental mode, and FIG. 8B shows the electric field distribution of the secondary mode. Note that this optical fiber has
Is 3 μm, the hole diameter d is 0.6 μm, and the hole diameter δ of the core part is 2.4 μm (d / Λ = 0.2, δ / Λ = 0.
8), the geometric diameter of the core portion is about 15 μm, and calculation is performed assuming that light with a wavelength λ = 850 nm is guided through this optical fiber.

From these figures, the mode field diameter (MFD) obtained from the electric field distribution of the fundamental mode is about 9 μm, and the secondary mode shown in FIG. It can be understood that it is a fiber. The effective refractive indexes of the fundamental mode and the secondary mode are 1.4480 and 1.4452, respectively, and the relative refractive index difference is about 0.2%. Under this condition, δ /
Λ may be 0.7 or more, and may be 1 or more.

Λ = 4 μm, d = 0.8 μm, δ =
When 3.2 μm (d / Λ = 0.2, δ / Λ = 0.8), the MFD obtained from the electric field distribution of the fundamental mode is about 12 μm, and the relative refractive index difference is about 0.13. %.

(Embodiment 2) FIGS. 9A and 9B show the electric field distribution obtained by the FDTD method when light is guided in the single mode optical fiber having the structure shown in FIG. 9A is a diagram for explaining the electric field distribution, FIG. 9A shows an electric field distribution of a fundamental mode, and FIG. 9B shows an electric field distribution of a secondary mode. Note that this optical fiber has
Is 10 μm, the hole diameter d is 2 μm, and the core hole diameter δ is 4
μm (d / Λ = 0.2, δ / Λ = 0.4),
The geometric diameter of the core part is about 32 μm, and calculation is performed assuming that light with a wavelength λ = 1500 nm is guided through this optical fiber.

From these figures, the mode field diameter (MFD) obtained from the electric field distribution of the fundamental mode is about 25 μm, and the relative refractive index difference between the fundamental mode and the secondary mode is about 0.
1%. Under this condition, δ / Λ may be 0.3 or more, and may be 1 or more.

(Embodiment 3) In the optical fiber having the structure shown in FIG. 5, the void interval Λ in the cladding portion is 5 μm, the void diameter d in the cladding portion is 1.25 μm, and the void diameter δ in the core portion is 1.25 μm. If the pore diameters of the clad portion and the core portion are the same (d / Λ = 0.25, δ / Λ = 0.2)
In 5), when a simulation by the FDTD method was performed with light of wavelength λ = 1500 nm being guided through this optical fiber, it was found that light of this wavelength acts as a multimode optical fiber.

Here, if the hole diameter of the core portion is enlarged so that δ / Λ is 0.3, it will act as a single mode optical fiber. In order to make the optical fiber of this structure act as a single mode optical fiber, δ / Λ is 0.
It may be 3 or more and may be 1 or more.

(Embodiment 4) The optical fiber having the structure shown in FIG. 6 is opposed to each other with respect to the geometric center of the core portion.
There is a structure in which the diameters of the groups of holes are equal, but the diameter of one group of holes is different from the diameters of the other two groups of holes. With such a structure, it is possible to eliminate the need for high rotational symmetry of three times or more, and to provide polarization dependency.

In the optical fibers having the structures shown in FIGS. 1 to 6, if the arrangement of Λ, d, δ and the holes in the core and the clad is variously changed, various core diameters are obtained. It becomes possible to manufacture an optical fiber having an MFD.

Also, by arranging the holes inside the core portion, the effective refractive index of the core portion can be freely changed, and the degree of freedom in design can be improved.

Further, as shown in FIGS. 8 and 9, when the electric field distribution in the single mode optical fiber having these structures is obtained, it is found that the electric field slightly exudes in the holes. I understand. Because of the "exudation of the electric field" into this hole,
In order to reduce the absorption of light in the optical fiber,
It is important that the holes are filled with a material that is transparent to vacuum or light having a wavelength to be guided. Specifically, for example, for light having a wavelength of 1.5 μm, it is necessary that the inside of the holes be filled with a gas such as dry nitrogen or dry air. It is also important that its refractive index is lower than that of the material of the optical fiber.

[0039]

As described above, according to the present invention,
Since the core part is formed by using multiple glass rods, it is possible to increase the core diameter, and further, the outer shape of the glass rod used for forming the core part and the glass part used for forming the clad part. By making the values equal to each other, it is possible to obtain an optical fiber with less disorder of arrangement and uniform characteristics.

Further, by making a part of the holes provided in the clad portion have a diameter different from the diameters of the other holes, it becomes possible to improve the characteristics while maintaining a large MFD.

Furthermore, by enlarging or reducing the diameter of one or more sets of holes facing each other, it becomes possible to provide a single-mode optical fiber having good polarization-maintaining characteristics.

[Brief description of drawings]

FIG. 1 is a sectional view of a fiber for explaining a first structural example of a single mode optical fiber of the present invention.

FIG. 2 is a cross-sectional view of a fiber for explaining a second structural example of the single mode optical fiber of the present invention.

FIG. 3 is a cross-sectional view of a fiber for explaining a third structural example of the single mode optical fiber of the present invention.

FIG. 4 is a sectional view of a fiber for explaining a fourth structural example of the single mode optical fiber of the present invention.

FIG. 5 is a cross-sectional view of a fiber for explaining a fifth structural example of the single mode optical fiber of the present invention.

FIG. 6 is a cross-sectional view of a fiber for explaining a sixth structural example of the single mode optical fiber of the present invention.

FIG. 7 is a diagram for explaining how a cylindrical glass tube and a cylindrical glass rod are bundled in order to manufacture the single mode optical fiber having the structure shown in FIG.

FIG. 8 is a diagram for explaining a result obtained by an FDTD method of an electric field distribution when light is guided in a single mode optical fiber having the structure shown in FIG. 4, and FIG. 2B is a diagram showing the electric field distribution of FIG.

9A and 9B are views for explaining a result obtained by an FDTD method of an electric field distribution when light is guided in a single mode optical fiber having the structure shown in FIG. 5, and FIG. 2B is a diagram showing the electric field distribution of FIG.

[Explanation of symbols]

71, 73 Cylindrical glass tube 72 cylindrical glass rod

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Satoru Kawanishi             2-3-1, Otemachi, Chiyoda-ku, Tokyo             Inside Telegraph and Telephone Corporation (72) Inventor Kazunori Suzuki             2-3-1, Otemachi, Chiyoda-ku, Tokyo             Inside Telegraph and Telephone Corporation (72) Inventor Shigeki Koyanagi             4-3 Ikejiri, Itami City, Hyogo Prefecture Mitsubishi Electric Cable             Industrial Co., Ltd. Itami Works (72) Inventor Masatoshi Tanaka             4-3 Ikejiri, Itami City, Hyogo Prefecture Mitsubishi Electric Cable             Industrial Co., Ltd. Itami Works (72) Inventor Moriyuki Fujita             4-3 Ikejiri, Itami City, Hyogo Prefecture Mitsubishi Electric Cable             Industrial Co., Ltd. Itami Works F-term (reference) 2H050 AC09 AC62 AC71

Claims (6)

[Claims] 1. A single-mode optical fiber having a photonic crystal structure, comprising a clad part around a core part, wherein a plurality of cylindrical holes extending in the fiber extension direction are periodically arranged, The plurality of cylindrical holes are arranged such that at least two types of cylindrical holes having different inner diameters are spaced from each other at a substantially constant interval, and the geometric diameter of the core portion is the cylindrical holes. A single-mode optical fiber characterized by being greater than twice the average spacing between. 2. The single mode optical fiber according to claim 1, wherein the cylindrical holes arranged at equal distances from the geometric center of the core portion have the same diameter. 3. Among the cylindrical holes arranged at equal distances from the geometric center of the core part, the diameter of the cylindrical hole adjacent to the core part is the same as the diameter of other cylindrical holes. The single-mode optical fiber according to claim 2, wherein the single-mode optical fiber is larger than. 4. The diameter of at least one set of cylindrical holes facing each other with respect to the geometric center of the core portion among the cylindrical holes has the same diameter. Single-mode optical fiber. 5. The inside of the cylindrical hole arranged in the clad portion is a vacuum, or a medium having a refractive index lower than that of the substance forming the fiber for light of a wavelength guided in the fiber. It is filled, It is characterized by the above-mentioned.
5. The single mode optical fiber according to any one of 1 to 4. 6. The single mode optical fiber according to claim 1, wherein the core portion has a plurality of cylindrical holes extending in a fiber extending direction.