Provided is an optical device, which includes a substrate, a first cladding disposed on the substrate, a first optical waveguide extended in a first direction on the first cladding, and having a first refractive index, a side grating formed in at least one side of the first optical waveguide, a second optical waveguide filling a space of the side grating, extended in a second direction across the first direction on the first cladding, and having a second refractive index, and a second cladding disposed on the second optical waveguide, and having a third refractive index, wherein the first refractive index is greater than the second refractive index, and the second refractive index is greater than the third refractive index.