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

Description

Photonic technologies offer numerous advantages for astronomical instruments such as spectrographs and interferometers owing totheir small footprints and diverse range of functionalities. Operating at the diffraction-limit, it is notoriously difficult to efficientlycouple such devices directly with large telescopes. We demonstrate that with careful control of both the non-ideal pupil geometry ofa telescope and residual wavefront errors, efficient coupling with single-mode devices can indeed be realised. A fibre injection wasbuilt within the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument. Light was coupled into a single-mode fibreoperating in the near-IR (J − H bands) which was downstream of the extreme adaptive optics system and the pupil apodising optics. Acoupling 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 resultsallow 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/orsimply exploit photonic technologies in future instrument design.

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