Reduced-confinement GaAlAs tapered waveguide antennas for enhanced far-field beam directionality

Reduced-confinement tapered waveguide antennas are shown to produce reductions of >35% in the transverse far-field beam divergence for radiation emitted from GaAlAs slab waveguides, resulting in far-field beams as narrow as 8.2 degrees full width at half-maximum (FWHM) along the direction perpendicular to the wafer surface. Reduced confinement of the guided mode near the output endface is achieved using a novel molecular-beam-epitaxy growth technique to produce a longitudinal reduction in the refractive index and thickness of the waveguide film. The reduced-confinement geometry has the distinct advantage over horn antennas of being compatible with two-dimensional antenna development. A numerical simulation is used to verify and predict the performance limitations of reduced-confinement antennas. Requirements on both the amplitude and phase profiles of the electric field at the antenna output are shown to impose a stringent lower limit on the acceptable taper length for optimal antenna performance.<>