Optical fiber for amplification, and laser device

An amplification optical fiber comprising:

• a core doped with an active element;
• an inner cladding surrounding the core and comprising a refractive index lower than a refractive index of the core;
• and
• an outer cladding surrounding the inner cladding and comprising a refractive index lower than a refractive index of the inner cladding, wherein
• the inner cladding is twisted about a center axis of the core,
• in a cross section vertical to a longitudinal direction, an outer circumference of the inner cladding comprises a polygon with rounded corners, and
• an angularity c defined by Expression (1) and Expression (2) is 0.15 or more and 0.8 or less,
• A=cos(π/n) (1)
• c={1−(d1/d2)}/(1−A) (2)
• where
• n is a number of vertices of the polygon,
• d1 is a diameter of an inscribed circle of the outer circumference of the inner cladding, and
• d2 is a diameter of a circumcircle of the outer circumference of the inner cladding.