Optical waveguide device with reduced insertion loss between optical waveguides coupled to a free space region

1. An optical waveguide device comprising;

at least one free space region with a plurality of optical waveguides integrated as inputs at an interface region integrated between said free space region and said waveguide inputs, the waveguides separated from adjacent waveguides by channels having a bottom portion;

said channels monotonically decreasing in cross-sectional channel width and channel depth between adjacent waveguides in a direction toward said free space region until the channels substantially diminish and merge with adjacent waveguides at the interface region to provide an adiabatic monotonic change in waveguide refractive index so that insertion loss of the waveguides at the interface region with the free space region is substantially reduced.

2. The optical waveguide device of claim 1 wherein said monotonic decrease in cross-sectional channel width and depth includes a monotonic increase in shallowness of depth of the bottom portion until its mergence with the integrated interface region.

3. The optical waveguide device of claim 1 wherein said free space region is part of a combiner/recombiner comprising a power coupler, a star coupler, a MMI coupler, an arrayed waveguide grating (AWG) or an Echelle grating.

4. The optical waveguide device of claim 1 wherein the channel bottom portions comprise a V-shaped bottom portion.