  Understanding exoplanet formation and finding potentially habitable
exoplanets is vital to an enhanced understanding of the universe. The use of
nulling interferometry to strongly attenuate the central starlight provides the
opportunity to see objects closer to the star than ever before. Given that
exoplanets are usually warm, the 4 microns Mid-Infrared region is advantageous
for such observations. The key performance parameters for a nulling
interferometer are the extinction ratio it can attain and how well that is
maintained across the operational bandwidth. Both parameters depend on the
design and fabrication accuracy of the subcomponents and their wavelength
dependence. Via detailed simulation it is shown in this paper that a planar
chalcogenide photonic chip, consisting of three highly fabrication tolerant
multimode interference couplers, can exceed an extinction ratio of 60 dB in
double nulling operation and up to 40 dB for a single nulling operation across
a wavelength window of 3.9 to 4.2 microns. This provides a beam combiner with
sufficient performance, in theory, to image exoplanets.
