HOLE ASSISTED FIBER AND DESIGN METHOD

1 . A hole-assisted optical fiber comprising:

a core region having a diameter 2a and a uniform refractive index distribution;

a uniform cladding region that surrounds the core region and has a relative refractive index difference Δ from the core region; and

holes arranged in a single layer or a plurality of layers in a circumferential direction within the cladding region excluding the core region to be a hexagonal close-packed manner,

wherein Condition A is satisfied where Condition A is that a combination of a core radius a and a radius Rin of an inscribed circle inscribed in an innermost layer of the layers of the holes is observed in a left-hand region of ΔαR=−0.01 dB/km, when a ratio ΔαR is plotted on a graph having the core radius a and the radius Rin as axes, the ratio ΔαR is a ratio of loss αR to loss αRs, the loss αR is a Rayleigh scattering loss of the hole-assisted optical fiber calculated by varying the core radius a and the radius Rin, the loss αRs is a Rayleigh scattering loss calculated by varying the core radius a for a normal optical fiber whose structure is the same as the hole-assisted optical fiber except for the absence of holes.

2 . The hole-assisted optical fiber according to claim 1 ,

wherein the holes are six in number and are arranged in a single layer,

wherein the relative refractive index difference Δ is 0.25% or more and 0.4% or less, and an occupancy rate S defined by Expression (C1) is 0.42 or more and 0.54 or less,

wherein the combination is observed within a polygon in the graph, the polygon having vertexes of

A1 (3.23, 20.00),

A2 (2.25, 15.23),

A3 (2.14, 14.26),

A4 (2.11, 12.00),

A5 (2.14, 10.45),

A6 (2.36, 8.00),

A7 (5.32, 8.00),

A8 (5.05, 9.16),

A9 (4.70, 12.65),

A10 (4.55, 15.16),

A11 (4.57, 18.26),

A12 (4.50, 20.00), and

[

Math

.

C

⁢

1

]

S

=

6

×

(

d

2

)

2

R

⁢

o

⁢

u

⁢

t

2

-

R

⁢

i

⁢

n

2

(

C

⁢

1

)

wherein, d represents a diameter of the hole, and Rout represents a radius of a circumscribing circle circumscribing the layer of the holes.

3 . The hole-assisted optical fiber according to claim 1 ,

wherein the holes are eighteen in number and are arranged in two layers,

wherein the relative refractive index difference Δ is 0.20% or more and 0.35% or less, and an occupancy rate S defined by expression (C2) is 0.15 or more and 0.25 or less,

wherein the combination is observed within a polygon in the graph, the polygon having vertexes of

B1 (2.79, 17.00),

B2 (2.70, 16.00),

B3 (2.27, 14.00),

B4 (2.18, 12.70),

B5 (2.18, 12.10),

B6 (2.32, 10.80),

B7 (2.29, 9.60),

B8 (2.54, 8.00),

B9 (5.21, 8.00),

B10 (4.96, 9.10),

B11 (4.79, 10.80),

B12 (4.77, 12.00),

B13 (4.66, 12.90),

B14 (4.66, 14.50),

B15 (4.32, 17.00), and

[

Math

.

C

⁢

2

]

S

=

1

⁢

8

×

(

d

2

)

2

R

⁢

o

⁢

u

⁢

t

2

-

R

⁢

i

⁢

n

2

(

C

⁢

2

)

wherein, d represents a diameter of the hole, and Rout represents a radius of a circumscribing circle circumscribing the layers of the holes.

4 . The hole-assisted optical fiber according to claim 1 ,

wherein the holes are thirty-six in number and are arranged in three layers,

wherein the relative refractive index difference Δ is 0.15% or more and 0.40% or less, and an occupancy rate S defined by Expression (C3) is 0.04 or more and 0.18 or less,

wherein the combination is observed within a polygon in the graph, the polygon having vertexes of

C1 (2.45, 14.00),

C2 (1.96, 11.80),

C3 (1.86, 11.57),

C4 (2.16, 8.00),

C5 (5.52, 8.00),

C6 (5.45, 8.07),

C7 (5.23, 9.14),

C8 (5.18, 10.86),

C9 (4.50, 12.64),

C10 (4.21, 14.00), and

[

Math

.

C

⁢

3

]

S

=

3

⁢

6

×

(

d

2

)

2

R

⁢

o

⁢

u

⁢

t

2

-

R

⁢

i

⁢

n

2

(

C

⁢

3

)

wherein, d represents a diameter of the hole, and Rout represents a radius of a circumscribing circle circumscribing the layers of the holes.

5 . A method for designing a hole-assisted optical fiber,

wherein the hole-assisted optical fiber has a structure including a core region having a diameter 2a and a uniform refractive index distribution, a uniform cladding region that surrounds the core region and has a relative refractive index difference Δ from the core region, and holes arranged in a single layer or a plurality of layers in a circumferential direction within the cladding region excluding the core region to be a hexagonal close-packed manner, and

wherein the method comprises:

calculating a desired bending loss condition, a desired confinement loss condition, and a desired cutoff condition when a core radius a and a radius Rin of an inscribed circle inscribed in an innermost layer of the layers of the holes are changed in any relative refractive index difference Δ and any hole occupancy rate S;

plotting the bending loss condition, the confinement loss condition, and the cutoff condition on a graph having the core radius a and the radius Rin as axes;

calculating a Rayleigh scattering loss of the hole-assisted optical fiber when the core radius a and the radius Rin are changed;

calculating a Rayleigh scattering loss when the core radius a is changed for a normal optical fiber whose structure is the same as the hole-assisted optical fiber except for the absence of holes;

plotting a ratio ΔαR on the graph, which is a ratio of the Rayleigh scattering loss of the hole-assisted optical fiber to the Rayleigh scattering loss of the normal optical fiber;

detecting, on the graph, an overlapping region where a right-hand region of the bending loss condition or the confinement loss condition, a left-hand region of the cutoff condition, and a left-hand region of any ratio ΔαR overlap each other; and

defining the core radius a and the radius Rin included in the overlapping region as design values of the hole-assisted optical fiber.