Improving the performances of current optical interferometers & future designs Proceedings of Haute Provence Observatory Colloquium (23-27 September 2013) Edited by L. Arnold, H. Le Coroller & J. Surdej Discrete beam combiners: 3D photonics for future interferometers S. Minardi, A. Saviauk, F. Dreisow, S. Nolte, & T. Pertsch Institute of Applied Physics, Abbe Center of Photonics, Friedrich-Schiller-Universit¨ at Jena, Max-Wien-Platz 1,07743 Jena, Germany Abstract. We present the results of an experimental research aiming at investigating the potential of three-dimensional (3D) photonics for astro- nomical interferometry. We found that a simple two dimensional array of evanescently coupled waveguides (the so called Discrete Beam Combiner - DBC) can be used to retrieve the mutual coherence properties of light col- lected by three telescopes with a precision comparable to state-of-the-art interferometric beam combiners. On the basis of these results, we envisage the future use of DBCs in optical/IR interferometry, with particular atten- tion to large arrays of telescopes. 1. Introduction Photonics and, in particular, micro-optical components such as optical fibers and waveguides have started to play a more and more important role in optical/IR as- tronomical interferometry. Nearly two decades ago it was recognized how optical fibers could be used to perform effective spatial filtering and beam combination of telescopes (Coud´ e Du Foresto et al. 1996). The instrument FLUOR demon- strated unprecedented accuracy in the measurement of stellar visibilities (Coud´ e Du Foresto et al. 1998). It was soon afterwards these pioneering experiments that Kern et al. (1996) proposed the use of planar integrated optical components (2D photonics) to miniaturize even further the optical setup for interferometric beam combination and enhance its thermo-mechanical stability. In the last decade, this proposal lead to the development and on-sky test of several prototypes designed to com- bine 2 (Berger et al. 2001), 3 (Berger et al. 2003) and 4 (Le Bouquin et al. 2011) telescopes simultaneously. The integrated devices allowed a considerable reduc- tion of the size and maintenance tasks (such as alignment) of the interferometric instruments, while delivering first class scientific results. The main limitation of the planar integrated optics approach is that it is difficult to scale up the devices to allow the combination of a higher number of telescopes and baselines, as would be desirable for interferometric imaging applications. Integrated 8 telescope com- biners were proposed but not yet realized (Berger et al. 2000). The difficulty in scaling up planar photonic beam combiners raises from the management of the 121