A reflector includes a gain fiber and a periodic refraction structure unit. The gain fiber has a core doped with a rare earth element. The periodic refraction structure unit includes a high-refractive-index region that has a predetermined width, that is formed at a predetermined spacing along an optical axis direction of the gain fiber, that is formed across an entire section of the core that is orthogonal to the optical axis of the core, and that has a first refractive index, and a low-refractive-index region that is formed adjacent to the high-refractive-index region, that has a width equal to the predetermined spacing, and that has a second refractive index lower than the first refractive index. A width di of an ith periodic structure of refractive index in the periodic refraction structure unit is given by the equation di=Hi·(λ/(2·ni))+λ/(4·ni).