Rare earth doped double clad fiber

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PROBLEM TO BE SOLVED : To provide a rare earth-added double clad fiber with a high conversion efficiency and high reliability.SOLUTION : The rare earth-added double clad fiber 1 comprises : an Yb-added core 2; a first clad 3 surrounding the core 2; and a second clad 4 surrounding the first clad 3. The core 2 has a numerical aperture NA between 0.06 and 0.15 inclusive, and a core diameter D0 between 20 μm and 100 μm inclusive. The ytterbium(Yb) is added to only a part of the region of the core 2 ranging from the center of the core 2 to the first clad 3. The Yb-added region expands to 0.11-2.1 times as large as the LP01 mode diameter of the core 2 from the center of the core toward the first clad 3.