JP5660673B2 - Optical fiber - Google Patents

Description

  The present invention relates to an optical fiber. More specifically, the present invention relates to an optical fiber used for an optical transmission medium that transmits signal light in an optical transmission system.

  In optical transmission systems, chromatic dispersion and nonlinear effects of optical fibers limit transmission characteristics, and optical fibers with various structures have been developed and widely used to reduce chromatic dispersion and nonlinear effects of such optical fibers. .

  2. Description of the Related Art In recent years, a hole-structured optical fiber having holes inside an optical fiber has various characteristics that cannot be realized by a conventional solid-state optical fiber, and thus has attracted high interest as a new optical transmission medium. In an optical transmission system, when input power increases, transmission quality deteriorates due to nonlinear effects. Therefore, input optical power is limited, which hinders an increase in transmission capacity and an increase in repeater distance.

Here, since the occurrence of the nonlinear effect is inversely proportional to the density of the input power in the optical fiber, the effective area (A _eff ) of the optical fiber, which is the area where the light wave propagates, is increased, and the power density in the optical fiber is increased. It is effective to suppress the non-linear effect to make it not so high.

  On the other hand, the above-described hole-structured optical fiber can be expected to increase the effective cross-sectional area in order to realize a low nonlinear optical transmission line. For this reason, various hole-structured optical fibers that have been studied for increasing the effective cross-sectional area have been reported (see, for example, Non-Patent Document 1 and Non-Patent Document 2).

T. T. et al. Matsui, et al. , “Applicability of Photonic Crystal Fiber With Uniform Air-Hole Structure to High-Speed and Wide-Transmission OverTencon. Lightwave Technol. 27, 5410-5416, 2009. K. Mukasa, et al. , "Comparisons of merits on wide-band transmission systems between using extremely improved solid SMFs with Aeff of 160μm2 and loss of 0.175dB / km and using large-Aeff holey fibers enabling transmission over 600nm bandwidth," the Proceedings of OFC2008, OthR1, Feb. 2008.

However, in the conventional solid optical fiber and the photonic crystal fiber having a uniform structure, the actual cross-sectional area has been limited to 160 μm ² or less.

  The object of the present invention has been made in view of the above-described problems, and provides an optical fiber capable of realizing a larger effective area than a conventional optical fiber.

  In order to achieve the above object, the optical fiber of the present invention has a core part and at least two hole part layers formed around the core part in the cladding part.

Specifically, the optical fiber of the present invention includes a core part, a cladding part surrounding the core part, having a refractive index lower than the refractive index of the core part, and an optical fiber axial direction inside the cladding part. A plurality of hole portions provided continuously and discretely in a cross section orthogonal to the optical fiber axis direction, and having two or more hole portion layers formed in a circular shape with respect to the core portion, The relative refractive index difference between the core part and the clad part satisfies 0.06% or more and 0.1% or less, and the radius of the core part satisfies 6.0 μm or more and 6.5 μm or less. However, at a wavelength of 1550 nm, the effective cross-sectional area is 160 μm ² or more and 210 μm ² or less, the cutoff wavelength is 1450 nm or less, and the bending loss at a bending radius of 30 mm is not more than 1450 nm and 1625 nm or less. , 0.5 dB / 100 turn or less.

  The optical fiber of the present invention can achieve a larger effective area than the conventional optical fiber.

  In the optical fiber of the present invention, it is preferable that the hole portion layer closest to the core portion has a smaller effective refractive index distribution than the hole portion layer outside the hole portion layer closest to the core portion.

  In the optical fiber of the present invention, the hole layer closest to the core part has a smaller effective refractive index distribution than the hole layer outside the hole part layer closest to the core part. A larger effective area than that of the fiber can be realized.

  In the optical fiber of the present invention, it is preferable that the hole portion layer closest to the core portion has a larger hole portion occupation ratio than the hole portion layer on the outside.

  The optical fiber of the present invention realizes a larger effective area than the conventional optical fiber because the hole layer closest to the core part has a larger hole area occupation ratio than the hole layer on the outside. can do.

  In the optical fiber of the present invention, the hole portion has a circular cross-sectional shape perpendicular to the optical fiber axial direction, and the diameter of the hole portion constituting the hole layer closest to the core portion is the outer side. It is preferable that it is larger than the diameter of the hole part constituting the hole part layer in

  In the optical fiber of the present invention, the diameter of the hole portion constituting the hole portion layer closest to the core portion is larger than the diameter of the hole portion constituting the hole portion layer on the outside, A larger effective area than a conventional optical fiber can be realized.

  In the optical fiber of the present invention, it is preferable that the hole portion layer closest to the core portion has a larger hole portion distribution density than the hole portion layer on the outside.

  The optical fiber of the present invention realizes a larger effective area than the conventional optical fiber, because the hole layer closest to the core part has a higher hole distribution density than the hole layer on the outside. can do.

  In the optical fiber of the present invention, it is preferable that the hole portion is formed in an annular shape or a regular polygonal shape with the core portion as a center.

  The optical fiber of the present invention can be easily designed by forming the hole portion in an annular shape or a regular polygonal shape with the core portion as the center.

In the optical fiber of the present invention, the effective cross-sectional area of the optical fiber, which is the area through which light propagates, is preferably 160 μm ² or more and 210 μm ² or less.

  The optical fiber of the present invention preferably has a cutoff wavelength of 1450 nm or less.

  The optical fiber of the present invention preferably has a bending loss of 0.5 dB / 100 turn or less at a bending radius of 30 mm for light having a wavelength of 1450 nm or more and 1625 nm or less.

  The optical fiber according to the present invention can realize a larger effective area than the conventional optical fiber.

It is the schematic which shows the cross-section of the optical fiber which concerns on 1st Embodiment of this invention. It is the schematic which shows the cross-section of the optical fiber which concerns on 1st Embodiment of this invention. FIG. 3 is a diagram showing the maximum effective cross-sectional area obtained with respect to the radius a of the core portion in the optical fiber having the structure of FIGS. 1 and 2. FIG. 2 is a diagram showing the wavelength dependence of the bending loss of the fundamental mode in the optimum structure in the optical fiber having the structure of FIG. 1. FIG. 3 is a diagram showing the wavelength dependence of the bending loss of the fundamental mode at the time of optimal design in the optical fiber having the structure of FIG. 2. FIG. 3 is a diagram showing a change in the maximum effective cross-sectional area obtained with respect to the radius a of the core portion in the optical fiber having the structure of FIGS. FIG. 3 is a diagram showing a maximum effective cross-sectional area obtained with respect to a relative refractive index difference Δ in the optical fiber having the structure of FIGS. 1 and 2. It is the figure which showed the structure which maximizes an effective area in the optical fiber of the structure of FIG. FIG. 3 is a diagram showing a structure for maximizing an effective area in the optical fiber having the structure of FIG. FIG. 2 is a diagram showing a relationship between a bending radius and a bending loss in the optical fiber having the structure of FIG. 1. FIG. 3 is a diagram showing a relationship between a bending radius and a bending loss in the optical fiber having the structure of FIG. 2. It is the schematic which shows the cross-section of the optical fiber which concerns on 2nd Embodiment of this invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. Embodiment described below is an example of implementation of this invention, and this invention is not restrict | limited to the following embodiment. In the present specification and drawings, the same reference numerals denote the same components.

(First embodiment)
(1) Structure of optical fiber 1 FIG.1 and FIG.2 is schematic which shows the cross-section of the optical fiber 1 which concerns on 1st Embodiment of this invention. Here, in FIGS. 1 and 2, 1 is an optical fiber, 2 is a core part, 3 is a cladding part, 4 is a hole part, and 5 is a hole part layer (first layer 51, second layer 52). It is.

The optical fiber 1 having the cross-sectional structure shown in FIG. 1 is continuous with the optical fiber axial direction (the direction perpendicular to the paper surface of FIG. 1 and agrees with the so-called longitudinal direction. The same applies to FIGS. 2 and 12 below). A core portion 2 having a circular outer shape with a uniform outer diameter, and a clad portion 3 having a circular shape in a sectional view surrounding the periphery of the core portion 2. Further, the refractive index n ₁ of the core portion 2 is higher than the refractive index n ₂ of the cladding portion 3 (n ₁ > n ₂ ).

  A plurality of hole portions 4 having a cross section that is continuous in the optical fiber axis direction and orthogonal to the optical fiber axis direction are disposed around the core portion 2 inside the cladding portion 3. The portion 4 has a circular shape in cross section and forms a hole portion layer 5 in a circular shape around the core portion 2 serving as the optical fiber axis. In this embodiment, the portion 4 is closest to the core portion 2. Adjacent to the outer side of the hole part layer 51 (first layer 51) in the direction away from the hole part layer 51 (first layer 51) (the outer peripheral direction of the optical fiber 1; the same applies hereinafter). And the two layers called the hole part layer 52 (2nd layer 52) in the outer side of this hole part layer 51 have shown the aspect by which all are formed in the regular hexagon shape. Each of the plurality of hole portions 4 having a circular shape in a cross-sectional view disposed inside the cladding portion 3 is constant with a diameter d having a uniform size in the optical fiber axial direction of the optical fiber 1. Further, the distance between the centers of the adjacent hole portions 4 in the same layer (the first layer 51 and the second layer 52) is also constant.

Further, the optical fiber 1 having the cross-sectional structure shown in FIG. 2 also surrounds the core portion 2 having a circular shape in cross-section and the periphery of the core portion 2, similarly to the optical fiber 1 having the cross-sectional structure shown in FIG. A clad part 3 having a circular shape in section, and the refractive index n ₁ of the core part 2 is higher than the refractive index n ₂ of the clad part 3. In addition, a plurality of hole portions 4 are disposed around the core portion 2 inside the clad portion 3 as in the case of the optical fiber 1 in FIG. The hole portion layer 5 is formed in a circular shape around the core portion 2 in the shape.

  The hole layer 5 is formed on the outer side of the hole layer 51 (first layer 51) in the direction away from the hole layer 51 (first layer 51) closest to the core 2. Adjacent to each other, two layers called a hole layer 52 (second layer 52) outside the hole layer 51 are formed in a regular hexagonal shape. The plurality of hole portions 4 arranged in the clad portion 3 have a constant distance between the centers of the adjacent hole portions 4 in the same layer (the first layer 51 and the second layer 52). Yes.

The optical fiber 1 having the structure shown in FIG. 2 has a diameter of the hole portion 4 constituting the first layer 51 and the second layer 52 on the side closer to the core portion 2 (first layer 51). d ₁ is formed to be larger than the diameter d ₂ of the hole 4 constituting the second layer 52. The first layer 51, which is a region where the hole portion 4 having a relatively large diameter exists, is more relative to the second layer 52, which is a region where the hole portion 4 having a relatively small diameter exists. In particular, the effective refractive index distribution (effective refractive index distribution) of the first layer 51 having a large hole portion occupation ratio and a large hole portion occupation ratio is the second. It becomes smaller than the effective refractive index distribution of the layer 52.

  Since the refractive index of the core part 2 is higher than the refractive index of the cladding part 3, the effective refractive index distribution of the core part 2, the effective refractive index distribution of the first layer 51, and the effective refractive index distribution of the second layer 51 are compared. Then, the effective refractive index distribution of the core part 2 is the highest, and the refractive index distribution of the first layer 51 sandwiched between the core part 2 and the second layer 52 is the smallest, so-called W-type distribution ( Effective refractive index distribution of core part 2> effective refractive index distribution of second layer 52> effective refractive index distribution of first layer 51). The optical fiber 1 in which the refractive index distribution is a W-type distribution can be expected to have a large effective area, and the optical fiber 1 of the present embodiment is formed so that the refractive index distribution has a W-type distribution. Therefore, an effective area larger than that of the conventional optical fiber 1 can be realized.

  In addition, although the number of the hole part layers 5 formed by the hole part 4 is represented as two layers in FIGS. 1 and 2, it is not limited to this and may be three or more layers. When the number of the hole part layers 5 is 3 or more, the diameter of the hole part 4 of the x layer is the hole part of the y layer (where x and y are positive integers and x + y = N) surrounding it. By making the diameter larger than 4, the same refractive index distribution relationship can be obtained effectively.

  1 and 2 show an arrangement in which the hole layer 5 is arranged around the core part 2 so that the hole part 4 is formed in a regular hexagonal shape. It may be formed so as to have a regular polygonal shape other than the regular hexagon, or an annular shape centering on the core portion 2.

As a constituent material of the optical fiber 1 according to the present invention, it is formed of a uniform material in the optical fiber axial direction, and quartz can be used as a main component. In the optical fiber 1 according to the present invention, in order to to the refractive index n ₁ of the core portion 2 is higher than the refractive index n ₂ of the cladding portion 3 (n _{1> n 2)} is, for example, the core portion 2 constituting material Can be realized by using quartz to which an impurity such as germanium (Ge) or aluminum (Al) is added to increase the refractive index, and using pure quartz without impurities as the material of the cladding part 3. As another means, the constituent material of the core part 2 is made of pure quartz having no impurities, and the constituent material of the cladding part 3 is made of quartz to which impurities such as fluorine (F) and boron (B) are added to reduce the refractive index. This can be realized by using Furthermore, the constituent material of the core portion 2 is made of quartz to which impurities such as germanium (Ge) and aluminum (Al) having an increased refractive index are added, and the constituent material of the cladding portion 3 is fluorine. You may make it use the quartz which added the impurity which reduces refractive indexes, such as (F) and boron (B).

There is no restriction | limiting in particular in manufacturing the optical fiber 1 which concerns on this invention. Around the core portion 2 of the quartz rod in which the core portion 2 and the clad portion 3 in which the refractive index is adjusted to satisfy n ₁ > n ₂ , such as the above-described constituent materials, are evacuated by a drill or an ultrasonic processing method. An appropriate number of capillary tubes formed of a quartz tube made of a constituent material of the cladding portion 3 around a solid quartz rod made of the constituent material of the core portion 2 or a grinding method for manufacturing the hole portion 4 mechanically. An optical fiber preform having a hole structure is obtained using a capillary method or the like that is manufactured by inserting the bundled quartz rod and capillary tube into a quartz jacket tube. The optical fiber 1 can be easily obtained by heating the optical fiber preform on which the hole structure is formed and drawing in the optical fiber axial direction.

(2) Various characteristics of the optical fiber 1 according to the first embodiment:
Next, examples of various characteristics of the optical fiber 1 according to the present invention shown in FIGS. 1 and 2 will be described.

FIG. 3 shows the maximum effective obtained with respect to the radius a of the core portion 2 when the relative refractive index difference Δ of the core region is 0.1% in the optical fiber 1 having the structure shown in FIGS. It is the figure which showed the change of cross-sectional area ( _Aeff ). Here, as shown in FIGS. 1 and 2, R represents the shortest distance from the center of the core portion 2 to the hole portion 4.

Further, the relative refractive index difference Δ (%) of the core region is calculated by the following formula (I). In the formula (I), n ₁ represents the refractive index of the core portion 2, and n ₂ represents the refractive index of the cladding portion 3.
Δ (%) = (n ₁ ² −n ₂ ² ) / (2 × n ₁ ² ) (I)
In the calculation in formula (I), the international standard ITU-T G.I. Designed to meet the bending loss and cutoff wavelength requirements recommended in 656. That is, such ITU-T G.I. In accordance with 656, the cutoff wavelength is 1450 nm or less, and the bending loss of the fundamental mode is 0.5 dB / 100 turns (turns: number of turns) or less at a bending radius of 30 mm. Note that the cutoff wavelength is a loss of the first higher-order mode with respect to a bending radius of 140 mm, which is considered to be an effective bending radius when the optical fiber 1 is laid, which is the condition described in Non-Patent Document 1 described above. Is set to a wavelength of 1 dB / m.

As shown in FIG. 3, the optical fiber 1 having the structure shown in FIGS. 1 and 2 can realize an effective cross-sectional area of 160 μm ² or more when the radius a of the core portion 2 is in the range of 5 to 7 μm. Further, from FIG. 3, in the structure of FIG. 1, the radius a of the core portion 2 is about 6.0 μm and the effective sectional area becomes the maximum, and in the structure of FIG. 2, the radius a of the core portion 2 is It can be confirmed that the effective area is maximized at about 6.5 μm.

  FIG. 4 shows the bending loss of the fundamental mode in the optimum structure when the relative refractive index difference in the core region is 0.1% and the radius a of the core part 2 is 6.0 μm in the optical fiber 1 having the structure of FIG. FIG. 5 is a diagram showing the wavelength dependence (relationship between bending radius and bending loss). FIG. 5 shows the optical fiber 1 having the structure shown in FIG. It is the figure which showed the wavelength dependence (relationship between a bending radius and a bending loss) of the bending loss of the fundamental mode at the time of the optimal design in case the radius a of 6.5 [mu] m.

It should be noted that the relationship between d (the diameters of all the hole portions 4 in the structure of FIG. 1; hereinafter the same applies to FIG. 8) and the diameter 2a of the core portion 2 (d / 2a) and the relationship between R and a are as shown in FIG. , D ₁ (the diameter of the hole portion 4 constituting the first layer 51 in the structure of FIG. 2; hereinafter, the same applies to FIGS. 8 and 9) and the diameter 2a of the core portion 2 (d ₁ / 2a ), The relationship between R and a, and the relationship (d ₂ / d ₁ ) between d ₁ and d ₂ (the diameter of the hole 4 constituting the second layer 52) is as shown in FIG. is there.

  Assuming that the effective bending radius when the optical fiber 1 is installed is 140 mm from the condition of the cutoff wavelength described in Non-Patent Document 1, the optical fiber 1 having the structure of FIG. Losses due to the waveguide structure excluding material-specific losses are suppressed to 0.2 dB / km or less at 1450 to 1625 nm. As shown in FIG. 5, the optical fiber 1 having the structure of FIG. 2 is suppressed to 0.05 dB / km or less in the wavelength band of 1450 nm to 1625 nm as compared with the structure of FIG. Compared to, it is possible to realize a low bending loss.

6 shows the maximum effective cross-sectional area obtained with respect to the radius a of the core portion 2 when the relative refractive index difference Δ of the core region is 0.08% in the optical fiber 1 having the structure of FIGS. It is the figure which showed the change of ( _Aeff ). In addition, in FIG. 6, similarly to the results shown in FIG. It is designed to satisfy the bending loss and cutoff wavelength conditions recommended in 656. As shown in FIG. 6, it can be confirmed that the optical fiber 1 having the structure shown in FIGS. 1 and 2 can realize an effective cross-sectional area of 160 μm ² or more when the radius a of the core portion 2 is in the range of 5 to 7 μm. The upper limit of the effective cross-sectional area is about 210 μm ² .

FIG. 7 is a diagram showing the maximum effective cross-sectional area (A _eff ) obtained for the relative refractive index difference Δ calculated by the formula (I) in the optical fiber 1 having the structure of FIGS. 1 and 2. As shown in FIG. 7, it can be seen that the effective cross-sectional area is the maximum when the relative refractive index Δ is 0.08% in both the structures shown in FIGS. From FIG. 7, it can be confirmed that the optical fiber having the structure shown in FIG. 2 can realize a larger effective area compared to the optical fiber 1 having the structure shown in FIG.

FIG. 8 is a diagram showing a structure that maximizes the effective area when the relative refractive index Δ is set to 0.08, 0.1, and 0.12% in the optical fiber 1 having the structure of FIG. FIG. 9 is a diagram showing a structure for maximizing the effective area when the relative refractive index Δ is the same in the optical fiber 1 having the structure of FIG. 8 and 9, the photonic crystal fiber structure (Δ = 0%) is also assumed, and Λ indicates the distance between the hole portions 4. As shown in FIG. 8, in the optical fiber 1 having the structure shown in FIGS. 1 and 2, an effective area of 160 μm ² or more is obtained at a relative refractive index Δ in the range of 0.08 to 0.12%. Can be confirmed.

  FIG. 10 shows the relationship between the bending radius and the bending loss when the relative refractive index Δ is 0.06%, 0.08%, 0.1% and 0.12% in the optical fiber 1 having the structure shown in FIG. FIG. FIG. 11 is a diagram showing the relationship between the bending radius and the bending loss when the relative refractive index Δ is the same in the optical fiber 1 having the structure of FIG.

  As shown in FIGS. 10 and 11, in the case of the relative refractive index Δ = 0% (photonic crystal fiber structure), the bending loss is not significantly reduced even when the bending radius is increased, and 1 dB / km. Since it converges to the above values, the bending loss becomes 1 dB / km or more when the optical fiber 1 is laid. On the other hand, in the structure shown in FIG. 1, the bending loss radius can be greatly improved by increasing the relative refractive index Δ, and the effective bending when the optical fiber 1 is laid by setting Δ to 0.1% or more. The bending radius at 140 mm considered to be the radius can be suppressed to 0.1 dB / km or less. In the structure shown in FIG. 2, the bending loss can be further reduced as compared with the structure shown in FIG.

  As described above, in the optical fiber having the structure of FIGS. 1 and 2, the refractive index of the core portion 2 is made larger than the refractive index of the cladding portion 3, and in the structure of FIG. Is formed larger than the diameter of the hole 4 constituting the second layer 52, the effective refractive index distribution becomes larger toward the outer layer, and the bending radius is 140 mm. In this case, the bending loss can be 0.1 dB / km or less.

(3) Effects of First Embodiment As described above, in the optical fiber 1 according to this embodiment, the refractive index of the core portion 2 is higher than the refractive index of the cladding portion 3 in the optical fiber 1 having a hole structure. Two or more hole part layers 5 formed in the cladding part 3 are formed. Therefore, the optical fiber of this embodiment can realize a larger effective area than the conventional optical fiber.

Further, the diameter of the hole portion 4 of the hole portion layer 51 (first layer 51) closest to the core portion 2 is equal to the hole portion layer 52 (second layer 52) outside the hole portion layer 51. ) Is larger than the diameter of the hole 4. Therefore, the effective refractive index distribution of the first layer 51 having a large hole portion occupancy is smaller than the refractive index distribution of the second layer 52 having a relatively small hole portion occupancy. With this configuration, the optical fiber 1 according to the present embodiment has a so-called W-type refractive index distribution, and maintains various characteristics such as bending loss equivalent to those of the single mode optical fiber, while maintaining the conventional optical fiber 1. A larger effective cross-sectional area, for example, an effective cross-sectional area exceeding 160 μm ² can be realized, the restriction of the input power in the optical transmission system can be relaxed, and the nonlinear effect of the optical fiber 1 can be suppressed.

(Second Embodiment)
(1) Structure of Optical Fiber 1 FIG. 12 is a schematic view showing a cross-sectional structure of the optical fiber 1 according to the second embodiment.
In the following description, parts that are the same as or substantially the same as those already described in the first embodiment are denoted by common reference numerals and description thereof is omitted. The range of the outer diameter of the hole 4, the arrangement of the hole layer 5, the constituent material of the optical fiber 1, the manufacturing method, and the like are also the same as those in the first embodiment, and will not be described.

  The optical fiber 1 having the cross-sectional structure shown in FIG. 12 is a core having a circular outer shape having a uniform outer diameter continuously in the axial direction of the optical fiber, like the optical fiber 1 having the structure shown in FIGS. And a clad part 3 having a circular shape in cross section surrounding the periphery of the core part 2, and the refractive index of the core part 2 is higher than the refractive index of the clad part 3.

  A plurality of hole portions 4 having a cross section that is continuous in the optical fiber axial direction and orthogonal to the optical fiber axial direction are disposed around the core portion 2 inside the cladding portion 3. The hole portion 4 has a circular shape in cross section, and forms a hole portion layer 5 in a circular shape around the core portion 2. In this embodiment, the hole portion closest to the core portion 2 is formed. A layer 51 (first layer 51) and a hole layer 52 (on the outer side of the hole layer 51 (first layer 51) adjacent to the first layer 51 in a direction away from the core 2 The second layer 52) and three layers of the hole layer 53 (third layer 53) are formed on the outer side adjacent to the hole layer 52 (second layer 52), and the cladding portion A plurality of hole portions 4 having a circular shape in cross section disposed in the inside of the optical fiber 1 are all constant in a diameter d having a uniform size in the optical fiber axial direction of the optical fiber 1, The distance between the centers of the adjacent hole portions 4 in the same layer (the first layer 51, the second layer 52, and the third layer 53) is also constant.

  On the other hand, in the present embodiment, the second layer 52 and the third layer 53 are both formed in a regular hexagonal shape, but the second layer 52 and the third layer 53 are formed for the first layer 51. 3 is formed in a regular octagon shape in which the distribution density of the holes 4 is larger than that of the third layer 53. Therefore, the first layer 51 has a higher hole portion distribution density than the second layer 52 and the third layer 53, and the occupation ratio of the hole portions 4 per unit area also increases. Is configured to be larger than the hole area occupation ratio in the second layer 52 and the third layer 53.

  With such a configuration, the effective refractive index distribution of the first layer 51 having a relatively large hole portion occupancy ratio of the second layer 52 and the third layer 53 having a relatively small hole portion occupancy ratio is obtained. Since the refractive index distribution is smaller than the effective refractive index distribution, the refractive index distribution of the optical fiber 1 is also a so-called W-type distribution, and a larger effective area than the conventional optical fiber can be realized.

  In the optical fiber 1 according to the present embodiment, the hole density is determined by the distribution density of the hole portions in the hole portion layer 5 (first layer 51 in the present embodiment) closest to the core portion 2. In order to make it larger than the hole portion distribution density of the hole portion layer 5 (the second layer 52 and the second layer 53 in this embodiment) outside the layer, the hole portion 4 has the same size ( In the case of the same diameter), the first layer 51 has a regular octagonal shape, and the second hole layer 52 and the third layer 53 outside the layer are formed as shown in FIG. The hole layer 5 (first layer 51) closest to the core 2, such as a hexagonal shape, is formed on the hole layer (the second layer 52 and the second layer 53) outside the hole layer. ) The hole portion layer 5 may be in a state where the distribution density of the hole portions 4 is high, such as a regular polygon having more corners.

(2) Effects of the second embodiment:
As described above, in the optical fiber 1 according to the present embodiment, the refractive index of the core portion 2 is higher than the refractive index of the cladding portion 3 in the optical fiber 1 having a hole structure, and is formed in the cladding portion 3. Two or more hole portion layers 5 are formed. Therefore, the optical fiber of this embodiment can realize a larger effective area than the conventional optical fiber.

  In addition, the pore density distribution of the pore layer 51 (first layer 51) closest to the core portion 2 indicates that the void portion layers 52 and 53 (second layer 52 and third layer 53) located outside the layer. The layer 53) is formed so as to be larger than the hole density distribution of the layer 53). Therefore, as in the optical fiber 1 having the structure shown in FIG. 2, the occupancy ratio of the first layer 51 is relatively large, and the effective refraction of the first layer 51 having a large occupancy ratio is large. Since the refractive index distribution is smaller than the refractive index distribution of the second layer 52 and the third layer 53 having a relatively small hole portion occupation ratio, and the refractive index distribution is a so-called W-type distribution, the first embodiment described above. You can enjoy the effects of

  The present invention can be used as a transmission medium in an optical transmission system.

1: Optical fiber 2: Core part 3: Clad part 4: Hole part 5: Hole part layer 51: Hole part layer (first layer)
52: Hole part layer (second layer)
53: Hole part layer (third layer)

Claims (6)

The core,
Surrounding the core portion, and a cladding portion having a refractive index lower than the refractive index of the core portion;
A two-layer structure in which a plurality of hole portions provided continuously in the optical fiber axial direction and discretely in a cross section orthogonal to the optical fiber axial direction are formed in a circular shape with respect to the core portion inside the cladding portion. Including the above-described hole portion layer,
The relative refractive index difference between the core part and the clad part satisfies 0.06% or more and 0.1% or less, and the radius of the core part satisfies 6.0 or more and 6.5 μm or less. fiber.
However,
At a wavelength of 1550 nm, the effective area is 160 μm ² or more and 210 μm ² or less,
The cutoff wavelength is 1450 nm or less,
For light with a wavelength of 1450 nm or more and 1625 nm or less, the bending loss at a bending radius of 30 mm is 0.5 dB / 100 turn or less.   2. The optical fiber according to claim 1, wherein the hole portion layer closest to the core portion has a smaller effective refractive index distribution than the hole portion layer outside the hole portion layer closest to the core portion. .   3. The optical fiber according to claim 1, wherein the hole portion layer closest to the core portion has a larger hole portion occupation ratio than the hole portion layer on the outer side. The hole has a circular cross-sectional shape perpendicular to the optical fiber axis direction,
4. The diameter of the hole part constituting the hole part layer closest to the core part is larger than the diameter of the hole part constituting the hole part layer on the outside. 5. The optical fiber in any one of.   5. The optical fiber according to claim 1, wherein the hole portion layer closest to the core portion has a hole portion distribution density higher than that of the hole portion layer on the outer side.   The optical fiber according to any one of claims 1 to 5, wherein the hole portion is formed in an annular shape or a regular polygon shape with the core portion as a center.

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