Multi-core fiber
A multicore fiber includes six or more of core elements having a core, a first clad surrounding the outer circumferential surface of the core and a second clad surrounding the outer circumferential surface of the first clad, and includes a clad surrounding the core elements. All of expressions are satisfied: n 1 >n 2 >n 3 , n 1 >n 4 , n 3 <n 4 , wherein the refractive index of the core is n 1 , the refractive index of the first clad is n 2 , the refractive index of the second clad is n 3 , and the refractive index of the clad is n 4 . The core elements are disposed so that the inter-center pitch between the cores adjacent to each other is disposed at regular spacing and the centers of the cores are annularly disposed.
1. A multi-core fiber comprising:
six or more of core elements, each of the core elements including:
a core;
a first clad surrounding an outer circumferential surface of the core; and
a second clad surrounding an outer circumferential surface of the first clad; and
a clad surrounding the core elements, wherein:
all of expressions are satisfied:
n 1 >n 2 >n 3
n 1 >n 4
n 3 <n 4
wherein a refractive index of the core is n 1 , a refractive index of the first clad is n 2 , a refractive index of the second clad is n 3 , and a refractive index of the clad is n 4 ;
the core elements are disposed so that an inter-center pitch between the cores adjacent to each other is disposed at regular spacings and centers of the cores are annularly disposed, and
wherein an expression is satisfied:
A
eff
1
2
Λ
2
≧
0.0001
XT
+
0.01
wherein crosstalk when light propagates through the cores for 100 km is XT (dB), effective areas of the cores are A eff (μm 2 ), and an inter-center pitch between the cores adjacent to each other is Λ.
2. The multi-core fiber according to claim 1 , wherein when a number of the core elements is n, the cores are disposed in an n-sided regular polygon.
3. The multi-core fiber according to claim 1 , wherein when a number of the core elements is n, the cores are disposed in a regular polygon in which n is a number of six or more that is divided by an integer of two or more and the cores are disposed at vertices whose number is a number that n is divided by the integer.
4. The multi-core fiber according to claim 3 , wherein:
a number of the core elements is a multiple of six, which is 12 or more; and
the cores are disposed in a regular hexagon.
5. The multi-core fiber according to claim 1 , wherein a difference of an effective area between of the cores is within 20 (μm 2 ).
6. The multi-core fiber according to claim 1 , wherein none of the core elements element is disposed at the center of the clad.
7. The multi-core fiber according to claim 1 , wherein the refractive index of the first clad n 2 and the refractive index of the clad n 4 are different from each other.