We propose a new approach to multiple-wavelength interferometry, targeted to high bandwidth absolute distance measurement, with nanometer accuracy over long distances. Two cw lasers are stabilized over a wide range of frequency intervals defined by an optical frequency comb, thus offering an unprecedented large choice of synthetic wavelengths. By applying a superheterodyne detection technique, we demonstrated experimentally an accuracy of 8 nm over 800 mm for target velocities up to 50 mm/s.

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 propose a new tunable laser source concept for multiple-wavelength interferometry, offering an unprecedented large choice of synthetic wavelengths with a relative uncertainty better than 10 −11 in vacuum. Two lasers are frequency stabilized over a wide range of frequency intervals defined by the frequency comb generated by a mode-locked fiber laser. In addition, we present experimental results demonstrating the generation of a 90 m synthetic wavelength calibrated with an accuracy better than 0.2 parts in 10 6 . With this synthetic wavelength we can resolve one optical wavelength, which opens the way to absolute distance measurement with nanometer accuracy. Two-wavelength interferometry allows absolute distance measurements 1,2 over a range defined by the synthetic wavelength 1 ⌳ = 1 2 / ͑ 1 − 2 ͒ = c / ⌬, where 1 and 2 are the individual wavelengths. Starting from a long synthetic wavelength (e.g., 1 m), a nanometer-level accuracy can be ultimately achieved by gradually decreasing ⌳ until single-wavelength interferometry is performed. This requires the capability to resolve an optical wavelength; thus the accuracy obtained with the smallest synthetic wavelength must be better than / 4. The measurement accuracy at large distances may be limited by the instability and uncertainty of the synthetic and optical wavelengths. We propose a new concept of a tunable twowavelength laser source stabilized with high accuracy and over large frequency ranges that replaces the previously used frequency comb of a Fabry-Perot resonator 3 with the highly accurate frequency comb of a femtosecond (fs) laser. The use of an optical frequency comb has been recently proposed for gauge block length calibration. 4 However, the reported detection technique is based on consecutive measurements at different wavelengths and therefore requires high mechanical stability during the measurement time. We present in this Letter a tunable two-wavelength source with which dynamic distance measurements with nanometer accuracy can be achieved by means of superheterodyne interferometry. 4 An active frequency comb is based on a fs modelocked laser, 5 whose repetition rate, f rep , defines exactly the frequency separation between two adjacent modes of its frequency spectrum. Several cw lasers can be locked to different modes of the comb by beat frequency measurements and electronic phase locked loops. The stability of the laser frequency separation is determined entirely by the relative stability of the frequency reference used to control the repetition rate of the fs laser. To cancel the frequency drift of the comb, either the comb can be self-referenced 6 or one of the lasers can be locked onto a molecular transition and the comb locked to that laser through control of the comb offset.The tunable two-wavelength source (Fig. 1) consists of a Nd:YAG laser ( 1 = 1.319 m, Lightwave Model 125), an external cavity laser diode (ECLD, 2 ϳ 1.3 m, Thorlabs INTUN 1300), and finally a mode-locked fiber laser (Menlo Systems TC-1500). A 10 MHz frequency reference ...

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Aims. To date, infrared interferometry at best achieved contrast ratios of a few times 10−4 on bright targets. GRAVITY, with its dual-field mode, is now capable of high contrast observations, enabling the direct observation of exoplanets. We demonstrate the technique on HR 8799, a young planetary system composed of four known giant exoplanets. Methods. We used the GRAVITY fringe tracker to lock the fringes on the central star, and integrated off-axis on the HR 8799 e planet situated at 390 mas from the star. Data reduction included post-processing to remove the flux leaking from the central star and to extract the coherent flux of the planet. The inferred K band spectrum of the planet has a spectral resolution of 500. We also derive the astrometric position of the planet relative to the star with a precision on the order of 100 μas. Results. The GRAVITY astrometric measurement disfavors perfectly coplanar stable orbital solutions. A small adjustment of a few degrees to the orbital inclination of HR 8799 e can resolve the tension, implying that the orbits are close to, but not strictly coplanar. The spectrum, with a signal-to-noise ratio of ≈5 per spectral channel, is compatible with a late-type L brown dwarf. Using Exo-REM synthetic spectra, we derive a temperature of 1150 ± 50 K and a surface gravity of 104.3 ± 0.3 cm s2. This corresponds to a radius of 1.17−0.11+0.13 RJup and a mass of 10−4+7 MJup, which is an independent confirmation of mass estimates from evolutionary models. Our results demonstrate the power of interferometry for the direct detection and spectroscopic study of exoplanets at close angular separations from their stars.