REFLECTING MIRROR, FIBER RESONATOR, AND FIBER LASER

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To provide a reflecting mirror which enables the reflection wavelength property thereof to be precisely set, has high reliability and is adaptable even to wavelength shortening, and to provide a fiber resonator, and a fiber laser.SOLUTION : The reflecting mirror is provided that includes : a gain fiber including a core part to which rare earth elements are added; and a periodical structure of refractive index including a high refractive index region which has a predetermined width, is formed at a predetermined interval in an optical axis direction of the gain fiber and formed over an entire cross section of the core part orthogonal to an optical axis and has a first refractive index, and a low refractive index region which is formed adjacently to the high refractive index region, and which has a width of a predetermined interval and which has a second refractive index that is lower than the first refractive index. The reflecting mirror has dgiven by the following formula : d=H(λ/(2 n)+λ/(4 n) (In the formula, Hindicates a dimension and (i) is a natural number) when an operation wavelength of the reflecting mirror is defined as λ, a width of a (i)th periodical structure of refractive index in the periodical structure of refractive index is defined as dand a refractive index is defined as n.SELECTED DRAWING : Figure 1