The present invention relates to a curved optical waveguide which is free
of any axis-deviation structure in the middle of the waveguide and which
has a sigmoidal core shape, wherein the curvature thereof at one end is
zero and the curvature thereof at the other end is finite (>0) and the
use of such a curved optical waveguide permits the reduction of optical
loss at the connected portions to the lowest possible level even when it
is applied to, for instance, an optical splitter or a directional coupler
and the curved optical waveguide of the present invention can provide an
optical waveguide which never requires the use of any offset.