Abstract:
We use two-dimensional simulations to study the design of tapers to provide efficient, low reflection coupling between a waveguide in a two-dimensional photonic crystal (PC) and free space.
We find that, largely independent of the PC parameters, or of the length and width of the tapered
region, the same type of concave, horn-shaped tapering profile is optimal for coupling from the
waveguide into free space, and significantly out-performs the widely-used linear taper. We also
find that optimal tapers can radiate nearly Gaussian beams, and therefore they can also provide
efficient coupling of Gaussian beams from free space into the PC waveguide. These properties are
better exhibited by rod-type PCs with Ez polarization than by hole-type PCs with Hz polarization.
This study of taper couplers exemplifies a design strategy for photonic circuits which optimizes
positioning of the cylinders immediately surrounding the light path, and then builds the rest of
the crystal structure around these cylinders .
