Single-mode waveguides for GRAVITY: II. Single-mode fibers and Fiber Control Unit

• Published in: Astronomy and astrophysics (Berlin) 2024-01, Vol.681, p.A26
• Main Authors: Perrin, G., Jocou, L., Perraut, K., Berger, J.-Ph, Dembet, R., Fédou, P., Lacour, S., Chapron, F., Collin, C., Poulain, S., Cardin, V., Joulain, F., Eisenhauer, F., Haubois, X., Gillessen, S., Haug, M., Hausmann, F., Kervella, P., Léna, P., Lippa, M., Pfuhl, O., Rabien, S., Amorim, A., Brandner, W., Straubmeier, C.
• Format: Article
• Language: English
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202347587

The second generation Very Large Telescope Interferometer (VLTI) instrument GRAVITY is a two-field infrared interferometer operating in the K band between 1.97 and 2.43 µm with either the four 8 m or the four 1.8 m telescopes of the Very Large Telescope (VLT). Beams collected by the telescopes are corrected with adaptive optics systems and the fringes are stabilized with a fringe-tracking system. A metrology system allows the measurement of internal path lengths in order to achieve high-accuracy astrometry. High sensitivity and high interferometric accuracy are achieved thanks to (i) correction of the turbulent phase, (ii) the use of low-noise detectors, and (iii) the optimization of photometric and coherence throughput. Beam combination and most of the beam transport are performed with single-mode waveguides in vacuum and at low temperature. In this paper, we present the functions and performance achieved with weakly birefringent standard single-mode fiber systems in GRAVITY. Fibered differential delay lines (FDDLs) are used to dynamically compensate for up to 6 mm of delay between the science and reference targets. Fibered polarization rotators allow us to align polarizations in the instrument and make the single-mode beam combiner close to polarization neutral. The single-mode fiber system exhibits very low birefringence (less than 23°), very low attenuation (3.6–7 dB km −1 across the K band), and optimized differential dispersion (less than 2.04 µrad cm 2 at zero extension of the FDDLs). As a consequence, the typical fringe contrast losses due to the single-mode fibers are 6% to 10% in the lowest-resolution mode and 5% in the medium- and high-resolution modes of the instrument for a photometric throughput of the fiber chain of the order of 90%. There is no equivalent of this fiber system to route and modally filter beams with delay and polarization control in any other K -band beamcombiner.