Microstructured fibers for broadband wavefront filtering in the mid-IR

The European Space Agency's space-based Darwin mission aims to directly detect extrasolar Earth-like planets using nulling interferometry. However, in order to accomplish this using current optical technology, the interferometer input beams must be filtered to remove local wavefront errors. Although short lengths of single-mode fiber are ideal wavefront filters, Darwin's operating wavelength range of 4 - 20 microm presents real challenges for optical fiber technology. In addition to the fact that step-index fibers only offer acceptable coupling efficiency over about one octave of optical bandwidth, very few suitable materials are transparent within this wavelength range. Microstructured optical fibers offer two unique properties that hold great promise for this application; they can be made from a single-material and offer endlessly single-mode guidance. Here we explore the advantages of using a microstructured fiber as a broadband wavefront filter for 4 - 20 microm.