First stellar photons for an integrated optics discrete beam combiner at the William Herschel Telescope

Abstract

We present the first on-sky results of a four-telescope integrated optics discrete beam combiner (DBC) tested at the 4.2 m William Herschel Telescope. The device consists of a four-input pupil remapper followed by a DBC and a 23-output reformatter. The whole device was written monolithically in a single alumino-borosilicate substrate using ultrafast laser inscription. The device was operated at astronomical H-band (1.6 µm), and a deformable mirror along with a microlens array was used to inject stellar photons into the device. We report the measured visibility amplitudes and closure phases obtained on Vega and Altair that are retrieved using the calibrated transfer matrix of the device. While the coherence function can be reconstructed, the on-sky results show significant dispersion from the expected values. Based on the analysis of comparable simulations, we find that such dispersion is largely caused by the limited signal-to-noise ratio of our observations. This constitutes a first step toward an improved validation of the DBC as a possible beam combination scheme for long-baseline interferometry.

A report on the status of astrophotonics for optical/IR interferometry and beyond

“…Instrumental visibilities of less than unity were retrieved on the six baselines, whereas the closure phases appeared paradoxically highly noisy for being self-calibrated quantities. It was concluded that, while the experiment could demonstrate the proof-of-principle of the on-sky DBC, the low flux conditions coupled to unavoidable long integration times had limited the yield of the experiment 37 . In a next step, the consideration of a 3D-printed lenslet array could significantly improve the stability of the transfer function.…”

Ultrafast laser inscription of integrated optics 2-telescope beam combiners for K-band interferometry at the CHARA array

“…One manufacturing technology that has the potential to enable the fabrication of integrated optics in a diverse range of optical materials is ultrafast laser inscription (ULI), and ULI has already been used to manufacture devices for a range of astrophotonic applications [7][8][9][10][11]. Recently, ULI has been used to inscribe K-band waveguide components in commercial Infrasil glass (IG) which exhibits improved transparency in the K-band compared to standard fused silica.…”

On-sky experiment of the discrete beam combiner: lessons learned and strategies for improved calibration of the transfer function

“…We retrieved the visibility amplitude and closure phase (CP), but with significant dispersion that was recently published in Ref. 21. Though most of the analysis was presented in Ref.…”

“…Though most of the analysis was presented in Ref. 21, in this proceeding, we attempted to look further at our on-sky experimental layout by studying the input power imbalance and phase errors at the DBC and at different photon levels. This kind of study will better prepare us for further on-sky tests of the DBC, in particular when combining a large number of apertures.…”