First results from a laboratory hypertelescope using single-mode fibers

Context. In the future, giant optical interferometric arrays will be developed with kilometric baselines and a large number of telescopes. Such arrays could have direct imaging capabilities if optimized beam combiners are used. Aims. This paper aims at studying the performance of an interferometric beam combiner using single mode fibers and in the frame of a hypertelescope. Methods. A laboratory testbed called SIRIUS was developed. We describe the general concept, the technical specifications and the results obtained. These results are analyzed with the help of a numerical simulator. Results. Direct images were obtained at the densified focus of SIRIUS. We show that the fibers greatly ease the pupil rearrangement. They also greatly improve the quality and the stability of the direct image. The computed images allow us to reproduce the effects of differential photometry and the influence of optical path difference variations. Optical path difference errors less than $\lambda/10$ and differential photometries less than 60% are required to keep the quality of the direct image. Conclusions. These results demonstrate the great potential of direct imaging interferometric beam combiners for future optical large arrays. The excellent comparison between experience and simulation clearly shows the simplicity of the fibered pupil densifier. It also gives us a great confidence in the extrapolation of these results and specifications for future arrays with a very large number of apertures.