An optical element includes an optical waveguide layer. The optical waveguide includes a periodic structure of grooves. The optical waveguide layer has a layer-thickness equal to or greater than 1.5 μm and is made of material selected from a group consisting of Ta2O5, Al2O3, LiNbO3, LiTaO3, AlN, GaN, SiC, and Yttrium aluminum garnet (YAG). (D/0.5Λ)≥2.5 is satisfied where D indicates the depth of groove; and Λ indicates the pitch of the arranged grooves in the periodic structure. The unit of Λ is identical to the unit of D.