GRAVITY is a new instrument to coherently combine the light of the European Southern Observatory Very Large Telescope Interferometer to form a telescope with an equivalent 130 m diameter angular resolution and a collecting area of 200 m 2 . The instrument comprises fiber fed integrated optics beam combination, high resolution spectroscopy, built-in beam analysis and control, near-infrared wavefront sensing, phasetracking, dual-beam operation, and laser metrology. GRAVITY opens up to optical/infrared interferometry the techniques of phase referenced imaging and narrow angle astrometry, in many aspects following the concepts of radio interferometry. This article gives an overview of GRAVITY and reports on the performance and the first astronomical observations during commissioning in 2015/16. We demonstrate phase-tracking on stars as faint as m K ≈ 10 mag, phase-referenced interferometry of objects fainter than m K ≈ 15 mag with a limiting magnitude of m K ≈ 17 mag, minute long coherent integrations, a visibility accuracy of better than 0.25%, and spectro-differential phase and closure phase accuracy better than 0.5• , corresponding to a differential astrometric precision of better than ten microarcseconds (µas). The dual-beam astrometry, measuring the phase difference of two objects with laser metrology, is still under commissioning. First observations show residuals as low as 50 µas when following objects over several months. We illustrate the instrument performance with the observations of archetypical objects for the different instrument modes. Examples include the Galactic center supermassive black hole and its fast orbiting star S2 for phase referenced dual-beam observations and infrared wavefront sensing, the high mass X-ray binary BP Cru and the active galactic nucleus of PDS 456 for a few µas spectro-differential astrometry, the T Tauri star S CrA for a spectro-differential visibility analysis, ξ Tel and 24 Cap for high accuracy visibility observations, and η Car for interferometric imaging with GRAVITY.

We present the installed and fully operational beam stabilization and fiber injection subsystem feeding the 2nd generation VLTI instrument GRAVITY. The interferometer GRAVITY requires an unprecedented stability of the VLTI optical train to achieve micro-arcsecond astrometry. For this purpose, GRAVITY contains four fiber coupler units, one per telescope. Each unit is equipped with actuators to stabilize the telescope beam in terms of tilt and lateral pupil displacement, to rotate the field, to adjust the polarization and to compensate atmospheric piston. A special roof-prism offers the possibility of on-axis as well as off-axis fringe tracking without changing the optical train. We describe the assembly, integration and alignment and the resulting optical quality and performance of the individual units. Finally, we present the closed-loop performance of the tip-tilt and pupil tracking achieved with the final systems in the lab..

We present in this paper the design and characterisation of a new sub-system of the VLTI 2 nd generation instrument GRAVITY: the Calibration Unit. The Calibration Unit provides all functions to test and calibrate the beam combiner instrument: it creates two artificial stars on four beams, and dispose of four delay lines with an internal metrology. It also includes artificial stars for the tip-tilt and pupil guiding systems, as well as four metrology pick-up diodes, for tests and calibration of the corresponding sub-systems. The calibration unit also hosts the reference targets to align GRAVITY to the VLTI, and the safety shutters to avoid the metrology light to propagate in the VLTI-lab. We present the results of the characterisation and validtion of these differrent sub-units.

Three-dimensional integration techniques offer not only a method for increasing the packing density, but also most promising opportunities for the realization of multifunctional circuits: mixed circuit technologies, (e.g. digital I analog), mixed process technologies, (e.g. CMOS I bipolar), and the combination of different semiconductor materials. Fig. 1. As one of the steps towards this aim we developed a 2 pm 3D-CMOS process which allows the fabrication of MOS devices in two layers. We realized NMOS devices in the silicon substrate and CMOS devices, as inverters and ring oscillators, in a 0.5 lun-thick recrystallized polysilicon layer. A schematic cross-section of a fabricated 3D-device is shown inThe as-deposited polysilicon upper device layer is recrystallized by means of an argon laser system. Care has to be exercized to prevent substrate damage. We investigated the influence of different recrystallization parameters on substrate damage, detected by Wright etching of bevelled samples and x-ray topograms. On the basis of these investigations we obtain a high quality SO1 layer without generating any crystal damage in the underlying silicon. This is also confirmed by the electrical characteristics of fabricated MOS devices, which do not show any degradation as compared with customary bulk devices. Typical results are shown in Figs. 2 and 3. An undesired effect is the occurrence of mass transport upon recrystallization of the silicon film. With the standard film thickness of 0.5 pm this is no problem, but it is known that thin film SO1 MOS transistors (thickness about 0.1 pm) exhibit remarkably improved properties. Therefore we are working on measures to minimize mass transport in order to allow the thinning of recrystallized silicon layers and to fabricate thin film SO1 devices. 72

The VLTI instrument GRAVITY combines the beams from four telescopes and provides phase-referenced imaging as well as precision-astrometry of order 10 µas by observing two celestial objects in dual-field mode. Their angular separation can be determined from their differential OPD (dOPD) when the internal dOPDs in the interferometer are known. Here, we present the general overview of the novel metrology system which performs these measurements. The metrology consists of a three-beam laser system and a homodyne detection scheme for three-beam interference using phase-shifting interferometry in combination with lock-in amplifiers. Via this approach the metrology system measures dOPDs on a nanometer-level.