The invention relates to waveguiding structures in planar lightwave circuit devices that include a transition region between a slab waveguide and channel waveguides to reduce optical coupling loss. In particular star couplers and arrayed waveguide gratings incorporating the transition region of the present invention demonstrate reduced insertion loss. By creating a transition region composed of transverse rows intersecting the output waveguide array, where the rows have equal dimensions and the effective refractive index is controlled by increasing the spacing width gradually from row to row, an adiabatic transition is created from slab waveguide to channel waveguide array. This structure provides low insertion loss within practical manufacturing tolerances. In addition, the present invention has found that by incorporating the transition region of the present invention into an AWG, the reduced insertion loss can be controlled as uniform insertion loss across the channels.