Amplification optical fiber and laser device

1. An amplification optical fiber comprising:

a core doped with an active element, through which multi-mode light propagates;

an inner cladding that surrounds the core and has a refractive index lower than a refractive index of the core; and

an outer cladding that surrounds the inner cladding and has a refractive index lower than the refractive index of the inner cladding, wherein

the inner cladding has a polygonal outline in a cross section perpendicular to a longitudinal direction, and

the inner cladding has a permanent twist, maintained without application of external force on the inner cladding, around a central axis of the core.

2. The amplification optical fiber according to claim 1 , wherein the permanent twist has at least three but less than 30 turns per a length of one meter in a direction parallel to the longitudinal direction.

3. The amplification optical fiber according to claim 1 , wherein a relative refractive index difference between the inner cladding and the core is 0.08% or higher.

4. The amplification optical fiber according to claim 1 , wherein a difference in propagation constant between light in an LP 01 mode and light in an LP 11 mode propagating through the core is 1000/m or larger.

5. The amplification optical fiber according to claim 1 , wherein light as an effective cross section of light propagating through the core is 300 μm 2 or larger.

6. The amplification optical fiber according to claim 1 , wherein a theoretical cutoff wavelength of light in an LP 03 mode is 1.3 μm or smaller.

7. A laser device comprising:

the amplification optical fiber according to claim 1 ; and

at least one light source that emits light that propagates through the amplification optical fiber.