Integrated optics for astronomical interferometry - V. Extension to the K band

We report laboratory and on sky characterizations of planar integrated optics beam combiners in the K ({[}2.0 mum; 2.4 mum]) and K' ({[}2.02 mum; 2.30 mum]) bands. Because of the strong scientific interests of the K band, we have extended the integrated optics technologies available in the telecom range (i.e. at 0.8 mum, 1.3 mum and 1.5 mum) to 2.0-2.5 mum. Ion exchange components optimized for these atmospheric bands provide stable contrasts higher than 95% with a laboratory white-light source and global throughputs of 35% in this spectral range. These results are completed with first stellar interferograms obtained with a silica-on-silicon two-way beam combiner on the IOTA interferometer. We characterized in the H and K bands the throughput of this beam combiner optimized for the H band ({[}1.47 mum; 1.78 mum]). On-sky fringes obtained on iotaAur in the H and K' bands clearly demonstrate a high instrumental contrast (larger than 50%) in both bands. This shows that integrated optics works with high performance outside its usual wavelength domain and provides good solutions for astronomical interferometry in a large wavelength range. We have measured single-mode ranges over 1 mum on our components which would allow to observe in two spectral bands simultaneously or to integrate both metrology reference and science signals in a single chip for astrometric applications.