Active element added-optical fiber, preform for active element added-optical fiber, resonator, and fiber laser device

An active element added-optical fiber comprising:

• a core, having a radius d, including a first region and a second region, and configured to propagate light in a few modes, and
• a cladding that surrounds an outer peripheral surface of the core without a gap, has a refractive index lower than a refractive index of the second region of the core, and propagates light in a few modes, wherein
• the first region is a region from a central axis of the core to a radius ra and contains ytterbium as an active element that can be pumped by light,
• the second region is a region to the radius d that surrounds the first region without a gap and contains a plurality of dopants, one of which is germanium,
• the active element is not added to a region within the second region from a radius rc to the radius d,
• the germanium is not added to a region within the first region from the central axis to a radius rb,
• a concentration of the germanium is highest among the plurality of dopants,
• the active element and the germanium are added to a first diffusion region from the radius rb to the radius rc,
• 0.1d<ra<d,
• when a distance from the central axis of the core in a radial direction is r, in a region of 0.2d<r≤0.9d, there is at least one local maximum value position, whose refractive index is locally maximum and is higher than an average refractive index in a region of 0≤r≤0.9d,
• an average concentration of the active element in a region of 0<r≤0.1d is higher than an average concentration of the active element in a region of 0.1d<r≤ra, and
• an average refractive index in the region of 0≤r≤0.1d is higher than a refractive index at r=0.2d.

An active element added-optical fiber comprising:

• a core, having a radius d, including a first region and a second region, and configured to propagate light in a few modes, and
• a cladding that surrounds an outer peripheral surface of the core without a gap, has a refractive index lower than a refractive index of the second region of the core, and propagates light in a few modes, wherein
• the first region is a region from a central axis of the core to a radius ra and contains ytterbium as an active element that can be pumped by light,
• the second region is a region to the radius d that surrounds the first region without a gap and contains a plurality of dopants, one of which is germanium,
• the active element is not added to a region within the second region from a radius rc to the radius d,
• the germanium is not added to a region within the first region from the central axis to a radius rb,
• a concentration of the germanium is highest among the plurality of dopants,
• the active element and the germanium are added to a first diffusion region from the radius rb to the radius rc,
• 0.1d<ra<d,
• when a distance from the central axis of the core in a radial direction is r, in a region of 0.2d<r≤0.9d, there is at least one local maximum value position, whose refractive index is locally maximum and is higher than an average refractive index in a region of 0≤r≤0.9d, and
• an average refractive index in a region of 0.1d≤r≤0.8ra is higher than an average refractive index in a region of 1.1ra≤r≤0.9d.

An active element added-optical fiber comprising:

• a core, having a radius d and including a first region and a second region, and
• a cladding that surrounds an outer peripheral surface of the core without a gap and propagates light in a few modes, wherein
• the first region is a region from a central axis of the core to a radius ra and contains ytterbium as an active element that can be pumped by light,
• the second region is a region to the radius d that surrounds the first region without a gap and contains a plurality of dopants, one of which is germanium,
• the active element is not added to a region within the second region from a radius rc to the radius d,
• the germanium is not added to a region within the first region from the central axis to a radius rb,
• a concentration of the germanium is highest among the plurality of dopants,
• the active element and the germanium are added to a first diffusion region from the radius rb to the radius rc,
• 0.1d<ra<d,
• when a distance from the central axis of the core in a radial direction is r, in a region of 0.2d<r≤0.9d, there is at least one local maximum value position, whose refractive index is locally maximum and is higher than an average refractive index in a region of 0≤r≤0.9d, and
• an average refractive index in a region of 0.1d≤r≤0.8ra is equivalent to an average refractive index in a region of 1.1ra≤r≤0.9d.