Manufacturing of chalcogenide and silver-halide single-mode fibres for modal wavefront filtering for DARWIN

Modal wavefront filtering is mandatory in nulling interferometers dedicated to detect extrasolar planets. Several activities have been initiated by ESA for developing single-mode waveguides for the mid-infrared. We present the development of fibres to be used for modal filtering within the European DARWIN mission and its scientific precursor GENIE: Chalcogenide fibres fit the wavelength range up to about 11 microns, while silver halide fibres can cover the full DARWIN wavelength range from 6.5 to 20 microns. A wide range of different manufacturing methods have been applied for producing step-index fibres. We also present the first results of manufacturing photonic crystal silver halide fibres. We tested the modal wavefront filtering capability of the fibres in a Mach-Zehnder interferometer fed by a CO 2 -laser. In addition we recorded the transverse output beam profile for each fibre. The results of both measurements are strong indicators for single-mode operation. We identified the critical issues experienced in the course of this manufacturing activity. The efficient removing of cladding modes and the required length of the fibres, commonly strongly underestimated, turned out as the keys for successful demonstration of singlemode behaviour. We found dedicated and compatible materials acting as mode stripper for both fibre materials used. We highlight the required steps for further improvement of the manufactured fibres and for a reasonable continuation of the fibre development activities for DARWIN.