  Integrated optic beam combiners offer many advantages over conventional bulk
optic implementations for astronomical imaging. To date, integrated optic beam
combiners have only been demonstrated at operating wavelengths below 4 microns.
Operation in mid-infrared wavelength region, however, is highly desirable. In
this paper, a theoretical design technique based on three coupled waveguides is
developed to achieve fully achromatic, broadband, polarization-insensitive,
lossless beam combining. This design may make it possible to achieve the very
deep broadband nulls needed for exoplanet searching.
