Abstract: An optical device 201, having one or more input waveguides 25 coupled to one side of a slab waveguide 20 and an array of output waveguides 26 coupled to an other side of the slab waveguide, is improved by including a transition region 22 immediately adjacent to the slab that operates to reduce insertion loss between the waveguide array 26 and the slab waveguide. The transition region includes a number of silica paths (a.sub.I . . . a.sub.n) that intersect the waveguide array. The silica paths have widths W(a.sub.n) that progressively decrease as they become further away from the slab waveguide. The silica paths are parallel to each other and perpendicular to the waveguide array. Disclosed embodiments include an M.times.N star coupler, a Dense Waveguide Division Multiplexer, and a 1.times.N power splitter. In each of these embodiments, insertion loss is substantially reduced over similar devices not employing such a transition region.