Also, FIG. 2 is a cross section for an optical amplification medium fiber which is used for the structure for the incidence of the excited light shown in FIG. 1. [0074] As shown in FIG. 2, an 10 which is used for a structure for the incidence of the excited light according to the present embodiment comprises a core 1 to which a rare-earth element is doped, an inner cladding 2 which is disposed on