PROTOTYPE CHALCOGENIDE FIBERS FOR 10-MICRON WAVEFRONT MODAL FILTERING

Abstract Wavefront cleaning by single-mode fibers has proved to be efficient in optical-infrared interferometry to improve calibration quality. For instance, theFLUOR instrument has demonstrated the capability of fluoride glass single-mode fibers in this respect in the K and L bands. New interferometricinstruments developped for the mid-infrared require the same capability forthe 8–12 „ m range. We have initiated a program to develop single-modefibers in the prospect of the VLTI mid-infrared instrument MIDI and ofthe ESA mission Darwin that require excellent wavefront quality. In orderto characterize the performances of chalcogenide fibers we are developping,we have set up an experiment to measure the far-field pattern radiated at10 „ m. We report here the first and promising results obtained with thisnew component. 1 IntroductionWhatever the cause of corrugations on interfering wavefronts, defects on the mir-rors or effect of the turbulent atmosphere, they result in a loss in the coherencefactor measured by an interferometer. Spatial filtering by a pinhole overcomes thisproblem by removing the high spatial frequencies from the wavefronts. However,the pinhole has to be designed for a specific wavelength and does not provide agood correction of low spatial frequency defects. An alternative consists in modal