Darwin Nulling Interferometer Breadboard Iii – Symmetry Requirements and Modal Filtering

In a nulling interferometer, the requirements on uniformity of phase, amplitude, and state of polarisation among the interferometer arms are stringent and hardly to be met in practice. The application of modal filters makes deep nulling possible as such filters may reduce the mismatch of interfering fields. We review the principle of modal filtering and discuss the achievable filter bandwidth and the effect of coupling phase. We give an overview over waveguides suitable for modal filtering, including also lossy multi-mode waveguides.