Optical fiber for light amplification

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PROBLEM TO BE SOLVED : To provide an optical fiber for optical amplification which amplifies signal light for outputting ultrahigh output with reliability for extended period. SOLUTION : Around a core 1 containing rare earth metal, a first clad layer 2, a second clad layer 3, and a third clad layer 4, there are provided in order with their refractive indexes as core 1>; first clad layer 2>; second clad layer 3>; third clad layer 4. The second clad layer is formed of a quartz glass while the third clad layer 4 formed of silicon resin. Though excitation light mainly comes into the first clad layer 2, a part of it comes in the second clad layer 3 to suppress heating due to reflection of the excitation light on the interface between the first and second clad layers 2 and 3. The profiles, in XY section, of the first and second clad layers 2 and 3 are not truly round so that the excitation light is efficiently reflected, with the signal light propagating through the efficiently amplified core 1.