Issue |
A&A
Volume 681, January 2024
|
|
---|---|---|
Article Number | A26 | |
Number of page(s) | 13 | |
Section | Astronomical instrumentation | |
DOI | https://doi.org/10.1051/0004-6361/202347587 | |
Published online | 03 January 2024 |
Single-mode waveguides for GRAVITY
II. Single-mode fibers and Fiber Control Unit
1
LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris,
5 place Jules Janssen,
92195
Meudon,
France
e-mail: guy.perrin@obspm.fr
2
Univ. Grenoble Alpes, CNRS, IPAG,
38000
Grenoble,
France
3
Le Verre Fluoré,
rue Gabriel Voisin,
35170
Bruz,
France
4
Max Planck Institute for Extraterrestrial Physics,
Giessenbachstrasse,
85741
Garching bei München,
Germany
5
European Southern Observatory,
Karl-Schwarzschild-Str. 2,
85748
Garching,
Germany
6
CENTRA, Instituto Superior Tecnico,
Av. Rovisco Pais,
1049-001
Lisboa,
Portugal
7
Max Planck Institute for Astronomy,
Königstuhl 17,
69117
Heidelberg,
Germany
8
I. Physikalisches Institut, Universität zu Köln,
Zülpicher Str. 77,
50937
Köln,
Germany
Received:
27
July
2023
Accepted:
29
September
2023
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 cm2 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.
Key words: instrumentation: high angular resolution / instrumentation: interferometers
© The Authors 2024
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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