Efficient injection from large telescopes into single-mode fibres: Enabling the era of ultra-precision astronomy

¹ Subaru Telescope, National Astronomical Observatory of Japan, National Institutes of Natural Sciences (NINS), 650 North A’Ohoku Place, Hilo, HI 96720, USA
e-mail: jovanovic.nem@gmail.com
² MQ Photonics Research Centre, Department of Physics and Astronomy, Macquarie University, NSW 2109, Australia
³ Australian Astronomical Observatory, 105 Delhi Rd, North Ryde NSW 2113, Australia
⁴ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁵ College of Optical Sciences, University of Arizona, Tucson, AZ 85721, USA
⁶ Astrobiology Center of NINS, 2-21-1, Osawa, Mitaka, 181-8588 Tokyo, Japan
⁷ Sydney Institute for Astronomy (SIfA), Institute for Photonics and Optical Science (IPOS), School of Physics, University of Sydney, NSW 2006, Australia
⁸ Centre for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS), Australia
⁹ Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Parc Valrose, Bât. H. Fizeau, 06108 Nice, France

Received: 23 December 2016
Accepted: 14 June 2017

Abstract

Photonic technologies offer numerous advantages for astronomical instruments such as spectrographs and interferometers owing to their small footprints and diverse range of functionalities. Operating at the diffraction-limit, it is notoriously difficult to efficiently couple such devices directly with large telescopes. We demonstrate that with careful control of both the non-ideal pupil geometry of a telescope and residual wavefront errors, efficient coupling with single-mode devices can indeed be realised. A fibre injection was built within the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument. Light was coupled into a single-mode fibre operating in the near-IR (J − H bands) which was downstream of the extreme adaptive optics system and the pupil apodising optics. A coupling efficiency of 86% of the theoretical maximum limit was achieved at 1550 nm for a diffraction-limited beam in the laboratory, and was linearly correlated with Strehl ratio. The coupling efficiency was constant to within <30% in the range 1250–1600 nm. Preliminary on-sky data with a Strehl ratio of 60% in the H-band produced a coupling efficiency into a single-mode fibre of ~50%, consistent with expectations. The coupling was >40% for 84% of the time and >50% for 41% of the time. The laboratory results allow us to forecast that extreme adaptive optics levels of correction (Strehl ratio >90% in H-band) would allow coupling of >67% (of the order of coupling to multimode fibres currently) while standard levels of wavefront correction (Strehl ratio >20% in H-band) would allow coupling of >18%. For Strehl ratios <20%, few-port photonic lanterns become a superior choice but the signal-to-noise, and pixel availability must be considered. These results illustrate a clear path to efficient on-sky coupling into a single-mode fibre, which could be used to realise modal-noise-free radial velocity machines, very-long-baseline optical/near-IR interferometers and/or simply exploit photonic technologies in future instrument design.