Integrated optics for astronomical interferometry - VI. Coupling the light of the VLTI in K band

Aims. Our objective is to prove that integrated optics (IO) is not o nly a good concept for astronomical interferometry but also a working technique with high performance. Methods. We used the commissioning data obtained with the dedicated K-band integrated optics two-telescope beam combiner which now replaces the fiber coupler MONA in the VLTI/VINCI instrument. We characterize the behaviour of this IO device and compare its properties to other single mode beam combiner like the previously used MONA fiber coupler. Results. The IO combiner provides a high optical throughput, a contrast of 89% with a night-to-night stability of a few percent. Even if a dispersive phase is present, we show that it does not bias the measured Fourier visibility estimate. An upper limit of 5× 10 −3 for the cross-talk between linear polarization states has been measured. We take advantage of the intrinsic contrast stability to test a ne w astronomical prodecure for calibrating diameters of simple stars by simultaneousl y fitting the instrumental contrast and the apparent stellar diameters. This method reaches an accuracy with diameter errors of the order of previous ones but without the need of an already known calibrator. Conclusions. These results are an important step of integrated optics, si nce they prove its maturity in an astronomical band where the technology has been specially developed for astronomical conveniences. It paves the road to incoming imaging interferometer projects.