<p>An optical element has a substrate (11) having or not having a channel (14) for optical waveguide; and a material which has a refractive index higher than that of the substrate and is filled in the channel for optical waveguide or is disposed on the substrate; wherein the refractive index in a part of the material varies substantially periodically or is substantially continuously monotone increasing or decreasing in the direction of light propagation. <IMAGE></p>
