Optical amplifying fiber, optical fiber amplifier, and optical communication system

An optical amplifying fiber comprising:

• at least one single core portion doped with a rare-earth element;
• an inner cladding portion configured to enclose the at least one core portion, the inner cladding portion having a lower refractive index than maximum refractive index of each core portion;
• and
• an outer cladding portion configured to enclose the inner cladding portion, the outer cladding portion having a lower refractive index than refractive index of the inner cladding portion, wherein:
• the inner cladding portion includes a plurality of air bubbles, and
• in the inner cladding portion, in cross-sectional surface orthogonal to axial direction of the optical amplifying fiber, two or more layers having low density of air bubbles are present or two or more layers having high density of air bubbles are present.

An optical amplifying fiber comprising:

• at least one single core portion doped with a rare-earth element;
• an inner cladding portion configured to enclose the at least one core portion, the inner cladding portion having a lower refractive index than maximum refractive index of each core portion;
• and
• an outer cladding portion configured to enclose the inner cladding portion, the outer cladding portion having a lower refractive index than refractive index of the inner cladding portion, wherein:
• the inner cladding portion includes a plurality of air bubbles, and
• when a hexagonal close-packed lattice is defined in cross-sectional surface perpendicular to axial direction of the optical amplifying fiber, regions of the air bubbles are positioned at lattice points.