Coupled multi-core optical fiber
The present embodiment relates to a CMCF including a structure to achieve more efficient reduction in transmission loss by suppressing decrease in concentration of alkali metal due to diffusion of alkali metal. In the CMCF including a plurality of cores, a power coupling coefficient h between adjacent cores is set to 1×10 −3 /m or more, to maintain an optical coupling state between the adjacent cores. In addition, alkali metal contributing to reduction in transmission loss is added to each of the cores such that a stress maximum value σ_ max between adjacent cores has a negative value.
1. A coupled multi-core optical fiber comprising:
a plurality of cores extending from one end to the other end; and
a single cladding covering each of the cores, wherein
each of the plurality of cores includes alkali metal contributing to reduction in transmission loss,
a power coupling coefficient h between adjacent cores in the plurality of cores is 1×10 −3 /m or more, and
stress on a line segment connecting centers of the adjacent cores is compressive stress.
2. The coupled multi-core optical fiber according to claim 1 , wherein
a maximum value σ_ max of the stress on the line segment is −20 MPa or less.
3. The coupled multi-core optical fiber according to claim 1 , wherein
a distance Λ core between the adjacent cores is 35 μm or less.
4. The coupled multi-core optical fiber according to claim 1 , wherein
the cladding includes a diffusion center position in which concentration of the alkali metal has a local maximum value, and a distance Λ core-clad between the diffusion center position and a center position of a core adjacent to the diffusion center position among the plurality of cores is 45 μm or less.
5. The coupled multi-core optical fiber according to claim 1 , wherein
each of the plurality of cores is comprised of SiO 2 glass in which a concentration of GeO 2 molecules is set to be 0 wt % or more to 1 wt % or less,
fluorine is doped to the cladding, and
transmission loss of each of the plurality of cores at a wavelength of 1550 nm is 0.16 dB/km or less.
6. The coupled multi-core optical fiber according to claim 1 , wherein
germanium is doped to at least one of the plurality of cores, and
transmission loss of the core with added germanium is 0.18 dB/km or less at the wavelength of 1550 nm.
7. The coupled multi-core optical fiber according to claim 1 , wherein
average concentration of the alkali metal in each of the plurality of cores is 0.2 atom ppm or more and 50 atom ppm or less.
8. The coupled multi-core optical fiber according to claim 1 , wherein
average concentration of halogen elements in each of regions corresponding to the plurality of cores in a preform of the coupled multi-core optical fiber before drawing is 1000 atom ppm or more and 30000 atom ppm or less.
9. The coupled multi-core optical fiber according to claim 1 , wherein
concentration of the alkali metal in a surface of the cladding is 1 atom ppm or less.