JP2001201648A - Optical fiber - Google Patents

Abstract

(57) [Summary] PROBLEM TO BE SOLVED: To increase the effective core area of an optical fiber,
Structure with effective core area while reducing bending loss
Low sensitivity to parameters. SOLUTION: A plurality of pieces having a predetermined sectional area in a main medium are provided.
Fine structure including sub-media is provided at least in the cladding region
In an optical fiber where each region extends in the axial direction,
The lad region is a first cladding region surrounding the core region 1
2 and a second cladding region surrounding the first cladding region 2
3 and the average refractive index N of the core region 1 _coreAnd the first
Average refractive index N of cladding region 2_clad1And the second cladding
Average refractive index N of region 3_clad2And N_core> N
_clad1> N_clad2The following relationship is established.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical fiber suitable as an optical transmission line.

[0002]

2. Description of the Related Art FIG. 8 is a sectional view of an optical fiber including a conventionally known fine structure. This optical fiber is
As shown in FIG. 8, it has a cross-sectional structure in which a number of voids 82 (voids) are provided in the material of the silica glass 81. The central portion of the cross section without the voids 82 is the core region 83, and the portion surrounding the core region 83 and including many voids 82 is the cladding region 84.

The principle of optical confinement of an optical fiber having such a fine structure is qualitatively described using the concept of an effective refractive index (for example, TA Birks et al., O.M.
optics Letters Vol. 22 p. 96
1 (1997)). Strictly speaking, the refractive index should show a complicated distribution in the core region 83 and the cladding region 84 due to the fine structure. However, it is necessary to replace each region with a uniform medium to approximate the optical waveguide characteristics. Assuming that
This uniform medium refractive index is called the effective refractive index. The effective refractive index n _eff satisfies the following inequality.

[0004]

(Equation 1) Here, n represents a refractive index, and f represents a volume fraction. The subscript 1 represents silica glass, and the subscript 2 represents air. As for the volume fraction, f ₁ + f ₂ = 1 holds. Normally, since it is n _1> n _2, top left and top right side of the equation is reduced with increasing f _2. Accordingly, the effective refractive index of the cladding region 84 including many voids 82 is smaller than the effective refractive index of the core region 83, and light confinement is realized as in a normal optical fiber.

[0005] Such a model of the effective refractive index is considered to be valid when the wavelength of light is longer than that of the fine structure scale. However, as the wavelength of light becomes shorter, the light becomes localized in a place with a higher refractive index, so that the effective refractive index rises. The assumption that can be replaced by
It is thought to lose validity.

On the other hand, an optical fiber having a larger negative dispersion than such an optical fiber is disclosed in, for example, Japanese Patent Laid-Open No. 10-95.
No. 628. This optical fiber has a fine structure as described above, but the cladding region is composed of an inner cladding region and an outer cladding region, and the effective refractive index of the inner cladding region is smaller than the effective refractive index of the outer cladding region. Having.

[0007]

However, in the optical fiber disclosed in the above publication, the negative dispersion is increased as compared with the optical fiber having a uniform cladding structure as shown in FIG. There are problems such as a decrease in core cross-sectional area, an increase in bending loss, and an increase in sensitivity to structural parameter fluctuations in effective core cross-sectional area.

The present invention has been made in view of such circumstances, and has an effect that the effective core area is increased, the bending loss is reduced, and the effective core area is reduced as compared with an optical fiber having a uniform clad structure. An object of the present invention is to provide an optical fiber capable of suppressing sensitivity to structural parameters low.

[0009]

According to a first aspect of the present invention, there is provided an optical fiber having a core region and a cladding region surrounding the core region in a cross section perpendicular to the axial direction.
A microstructure including a plurality of sub-mediums having a predetermined cross-sectional area in the main medium is provided at least in the cladding region, and in an optical fiber in which each region extends in the axial direction, the cladding region is
A first cladding region surrounding the core region, a second cladding region surrounding the first cladding region, an average refractive index N _core of the _core region, an average refractive index N _clad1 of the first cladding region, An average refractive index N _clad2 of the second cladding region;
, A relationship of N _core > N _clad1 > N _{clad 2} is established.

According to this configuration, the change in the effective core area A _eff with respect to the wavelength change is smaller than that of the conventional optical fiber. Therefore, the effective core with respect to the change in the structural parameter such as the pitch which is the distance between the centers of the two voids is reduced. Cross section A
The sensitivity of the _eff characteristic decreases, the degree of light confinement in the core region increases, and the bending loss decreases. Therefore, it is possible to secure a larger effective core area and to reduce bending loss as compared with the conventional optical fiber. In addition, it is possible to reduce the sensitivity of the effective core area to the structural parameter variation.

According to a second aspect of the present invention, in the optical fiber according to the first aspect, the average refractive index of the region including the fine structure is determined by increasing or decreasing the sectional area of the sub-medium per unit sectional area. The configuration is adopted.

As described above, by increasing or decreasing the sectional area of the sub-medium per unit sectional area, the average refractive index of the region including the fine structure can be determined. , The magnitude relationship of the average refractive index with the second cladding region can be easily determined.

According to a third aspect of the present invention, in the optical fiber according to the first or second aspect, the average refractive index of each of the core region, the first cladding region and the second cladding region is an effective core breakage at an optical wavelength of 1550 nm. A configuration selected so that the area is 10 μm ² or more is adopted.

According to this configuration, since a large effective core area is provided, the occurrence of nonlinear optical phenomena can be suppressed, and the transmission quality can be improved.

[0015]

Embodiments of the present invention will be described below with reference to the accompanying drawings. In each of the drawings, the same elements are denoted by the same reference numerals, and redundant description will be omitted. Also, the dimensional ratios in the drawings do not always match those described.

FIG. 1 is a view showing a region division in a cross section of an optical fiber according to an embodiment of the present invention.
FIG. 3 is a cross-sectional view illustrating a fine structure of the optical fiber according to the embodiment. As shown in FIG. 1, in the present embodiment, the cross section is the core region 1 and the first region surrounding the core region 1.
A second cladding region surrounding the first cladding region;
And a cladding region 3. As shown in FIG. 2, each of these regions is composed of a silica glass 4 as a main medium and a number of voids 5 as a sub-medium.

In the present embodiment, the concept of average refractive index is used to distinguish the refractive index of each region. The effective refractive index is not used because the definition is ambiguous because it is defined using approximation and is not suitable for describing the structure.
FIG. 3 is a diagram for explaining how to determine the average refractive index of the optical fiber according to the present embodiment. For one void 5 focused on a certain area, a perpendicular bisector is drawn between all voids. With these vertical bisectors, void 5
And defines a polygon that does not include a vertical bisector inside. This is called cell 6. In this cell 6, the average refractive index _navg is calculated by the following equation.

[0018]

(Equation 2) Here, n is the refractive index of the fiber material, A _cell is the cell area, and A _hole is the void area. The set {( _navg , r)} is represented as a scatter diagram, where the center P of the void 5 is the position of the cell 6 and the distance from the origin O (fiber axis) to P is r.

In the present embodiment, the average refractive index of each region determined by the above method is determined with the following magnitude relation. That is, the average refractive index N of the core region 1
_{a core} , an average refractive index N _clad1 of the first cladding region 2,
Between the second cladding region 3 and the average refractive index N _clad2 ,

[0020]

(Equation 3) The relationship is given.

FIG. 4 is a scatter diagram of the average refractive index of each region in the present embodiment. As can be seen from FIG. 4, the relation of the average refractive index in the mathematical formula is given.

The average refractive index of each region is determined by adjusting the size of the void 5. That is, by increasing or decreasing the cross-sectional area of the void 5 per unit cross-sectional area, the ratio between the silica glass 4 as the main medium and the void 5 as the sub-medium increases or decreases, so that the average refractive index of the region is determined. be able to. In the present embodiment, the size of each void is determined as shown in the following table. In the table, for comparison, an optical fiber including a conventional microstructure is also described. Here, in FIG. 3, the pitch L = 2.8 μm, the number of layers m = 7, and the refractive index n of silica glass = 1.444. D ₀ is the diameter of the void 5 in the core region 1 and d ₁ is the first
The diameter of the void 5 in the cladding region 2, and d ₂ is the diameter of the void 5 in the second cladding region 3. Further, # 1 shows data of an optical fiber having a conventional uniform cladding structure, # 2 shows data of an optical fiber according to the present invention, and # 3 shows a structure disclosed in Japanese Patent Application Laid-Open No. 10-95628. Is shown.

[0023]

[Table 1] When the sectional structure of the optical fiber according to the present invention is determined based on the data in Table 1, the result is as shown in FIG. FIG. 5 is a diagram showing a cross-sectional structure of the optical fiber according to the present embodiment. The voids 5 are arranged on the hexagonal lattice, and the core region 1 has no voids 5. In the first cladding region 2, d ₁ / L = 0.
34, the void 5a is formed in the second cladding region 3 by d ₂ /
A void 5b with L = 0.4 is provided.

As described above, by increasing or decreasing the cross-sectional area of the sub-medium per unit cross-sectional area, the average refractive index of the region including the fine structure can be determined, so that the core region 1 and the first cladding region 2 and the second cladding region 3 can easily determine the magnitude relationship of the average refractive index.

FIG. 6 is a diagram showing a calculation result of the effective core area of the optical fiber according to the present embodiment. here,
The calculation results when the pitch is set to 1.53 μm in all of Structure # 1, Structure # 2, and Structure # 3 are shown. In FIG. 6, focusing on the effective core area A _eff at the wavelength λ = 1550 nm,
_eff is larger in structure # 2 than in structure # 1 and structure # 3. Focusing on the increase rate d (A _eff ) / dλ of the effective core area at the wavelength λ = 1550 nm,
The structure # 2 is smaller than the structures # 1 and # 3. The fact that the pace of increase in the effective core area is small means that the effective core area A with respect to fluctuations in structural parameters such as pitch.
_This means that the sensitivity of the _eff characteristic is small, the light confinement in the core region is good, and the bending loss is small. In general, as the effective core area increases, the bending loss increases. Therefore, a small bending loss means that the effective core area can be increased when the bending loss is constant.

As described above, since the present embodiment has a large effective core area, the occurrence of nonlinear optical phenomena can be suppressed, and the transmission quality can be improved.

FIG. 7 shows the calculation result of the effective core area of this embodiment having the above structure. Here, at the light wavelength λ = 1550 nm, the effective core area A _eff = 1
The pitch L was determined to be 2 μm ² . In FIG. 7, focusing on the change in the effective core area A _eff at λ = 1550 nm, the effective core area A _eff with respect to the wavelength change is shown.
_Regarding the magnitude of the change in _{e ff} , the structure # 1 is smaller than the structure # 3 and the structure # 2 is smaller than the structure # 1. Since the change of the effective core area A _eff with respect to the wavelength change is small, the sensitivity of the effective core area A _eff characteristic to the variation of the structural parameters such as the pitch is small, the degree of light confinement to the core region 1 is high, and the bending loss Become smaller. The pitches giving the same effective core area A _eff at the same wavelength are 1.46 μm for structure # 3, 1.31 μm for structure # 1, and 1.11 μm for structure # 2. Since the effective refractive index of the cladding region becomes smaller as the pitch becomes smaller than the wavelength, the degree of light confinement in the core region 1 becomes higher.
Therefore, when compared at the same bending loss, the structure # 1 can increase the effective core area A _eff than the structure # 3, and the structure # 2 can increase the effective core area A _eff than the structure # 1.

As described above, since it has a large effective core area, the occurrence of nonlinear optical phenomena can be suppressed.
Transmission quality can be improved.

As described above, according to the optical fiber according to the embodiment of the present invention, a larger effective core area can be secured and bending loss can be reduced as compared with the conventional optical fiber. Further, the sensitivity of the effective core area to the structural parameter can be reduced.

[0030]

As is apparent from the above description, according to the present invention, a microstructure including a plurality of sub-mediums having a predetermined sectional area in a main medium is provided at least in a cladding region, and each region is formed in an axial direction. In the extending optical fiber, the cladding region has a first cladding region surrounding the core region, and a second cladding region surrounding the first cladding region, and has an average refractive index N _core of the core region, and a first cladding region. the mean refractive index N _clad1 region, the average refractive index of the second cladding region N _clad2
, A relationship of N _core > N _clad1 > N _clad2 is established, so that the change of the effective core area A _eff with respect to the wavelength change is smaller than that of the conventional optical fiber. For this reason, the effective core area A with respect to the variation of the structural parameter such as the pitch which is the distance between the centers of the two voids
The sensitivity of the _eff characteristic decreases, the degree of light confinement in the core region increases, and the bending loss decreases. Therefore, a larger effective core area can be secured as compared with the conventional optical fiber, and the bending loss can be further reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing region divisions in a cross section of an optical fiber according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view illustrating a fine structure of the optical fiber according to the embodiment.

FIG. 3 is a diagram illustrating how to determine an average refractive index of the optical fiber according to the embodiment.

FIG. 4 is a diagram showing an average refractive index of the optical fiber according to the embodiment.

FIG. 5 is a diagram showing a cross-sectional structure of the optical fiber according to the embodiment.

FIG. 6 is a diagram showing a calculation result of an effective core area of the optical fiber according to the present embodiment.

FIG. 7 is a diagram showing a calculation result of an effective core area of the optical fiber according to the embodiment.

FIG. 8 is a cross-sectional view of an optical fiber including a conventionally known fine structure.

[Explanation of symbols]

Reference numeral 1 denotes a core region, 2 denotes a first cladding region, 3 denotes a second cladding region, 4 denotes silica glass, 5 denotes a void, and 6 denotes a cell.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Eiji Sasaoka 1-chome, Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa F-term in Sumitomo Electric Industries, Ltd. Yokohama Works 2H050 AB03Z AC36

Claims (3)

[Claims] A core region and a cladding region surrounding the core region in a cross section perpendicular to the axial direction;
In an optical fiber in which a microstructure including a plurality of sub-mediums having a predetermined cross-sectional area in a main medium is provided at least in the cladding region, and each of the regions extends in an axial direction, the cladding region surrounds the core region. a first cladding region, and a second cladding region surrounding the first cladding region, the mean refractive index N _core of the core region, the mean refractive index N _{cl ad1} of the first cladding region, the second An optical fiber, _wherein a relationship of N _core > N _clad1 > N _clad2 is established between the average refractive index N _clad2 of the two cladding regions. 2. The optical fiber according to claim 1, wherein an average refractive index of the region including the microstructure is determined by increasing or decreasing a cross-sectional area of the sub-medium per unit cross-sectional area. . 3. The core region, the first cladding region and
And the average refractive index of the second cladding region is 15
10 μm effective core area at 50 nm ^TwoOver
Claim 1 or Claim characterized in that it has been selected to
Item 3. The optical fiber according to Item 2.