Amplification optical fiber and fiber laser device using the same

1. An amplification optical fiber comprising: a core; and a clad surrounding the core, the amplification optical fiber allowing light having a predetermined wavelength to propagate in at least an LP01 mode and an LP11 mode, wherein

the core includes a first core portion covering a center area of the core and a second core portion surrounding the first core portion,

a refractive index of the first core portion is higher than that of the clad,

a refractive index of the second core portion is higher than that of the first core portion,

when light of the LP01 mode and light of the LP11 mode are standardized by power, in the core, an active element that stimulates to emit light of the predetermined wavelength is doped at a higher concentration in at least a part of an area where power of light of the LP01 mode is larger than that of light of the LP11 mode than at least a part of an area where the power of light of the LP11 mode is larger than that of light of the LP01 mode to satisfy the formula:

∫ 0 ∞ n ( r )×{ I 01 ( r )− I 11 ( r )} rdr> 0

wherein, r represents a distance from the center in the radial direction of the core, I oi (r) represents the power of light of the LP01 mode at the distance r from the center in the radial direction of the core, I 11 (r) represents the power of light of the LP11 mode at the distance r from the center in the radial direction of the core, and n(r) represents an additive concentration of the active element at the distance r from the center in the radial direction of the core.

2. The amplification optical fiber according to claim 1 , wherein the second core portion is formed on an outer peripheral side of a position where the power of light of the LP01 mode and the power of light of the LP11 mode are identical when light of the LP01 mode and light of the LP11 mode are standardized by power.

3. The amplification optical fiber according to claim 2 , wherein a peak of the power of light of the LP11 mode is positioned in the second core portion when light of the LP11 mode is standardized by power.

4. The amplification optical fiber according to claim 2 , wherein an average concentration of the active element doped in the first core portion is higher than an average concentration of the active element in the second core portion.

5. The amplification optical fiber according to claim 4 , wherein the active element is not doped in the second core portion.

6. The amplification optical fiber according to claim 5 , wherein the active element is doped in an area from the center of the core to a position where the power of light of the LP01 mode and the power of light of the LP11 mode are identical when light of the LP01 mode and light of the LP11 mode are standardized by power.

7. The amplification optical fiber according to claim 6 , wherein the active element is doped at a uniform concentration.

8. The amplification optical fiber according to claim 1 , wherein the active element is doped in an area from the center of the core to a predetermined position at a uniform concentration.

9. The amplification optical fiber according to claim 8 , wherein the predetermined position is set to be position where the power of light of the LP01 mode and the power of light of the LP11 mode are identical when light of the LP01 mode and light of the LP11 mode are standardized by power.

10. The amplification optical fiber according to claim 9 , wherein the active element is not doped in an area where the power of light of the LP11 mode is larger than that of light of the LP01 mode when light of the LP01 mode and light of the LP11 mode are standardized by power.

11. A fiber laser device comprising:

the amplification optical fiber according to claim 1 ;

a seed light source which inputs seed light to the amplification optical fiber; and

a pumping light source which outputs pumping light pumping the active element of the amplification optical fiber.

12. A fiber laser device comprising:

the amplification optical fiber according to claim 1 ;

a pumping light source which outputs pumping light pumping the active element of the amplification optical fiber;

a first FBG which is provided on one side of the amplification optical fiber and reflects light with at least a part of a wavelength of light emitted by the active element pumped by the pumping light; and

a second FBG which is provided on the other side of the amplification optical fiber and reflects light with the same wavelength as the light, reflected by the first FBG, at a lower reflectance than the first FBG.

13. The amplification optical fiber according to claim 3 , wherein an average concentration of the active element doped in the first core portion is higher than an average concentration of the active element in the second core portion.