Demonstration of an efficient, photonic-based astronomical spectrograph on an 8-m telescope

TL;DR

This paper demonstrates a highly efficient, photonic-based astronomical spectrograph on an 8-m telescope, combining adaptive optics and waveguide technology to achieve significant throughput improvements for future large telescopes.

Contribution

It introduces a novel integration of adaptive optics with photonic spectrograph technology on a large telescope, achieving high efficiency and setting a new precedent for future astronomical instruments.

Findings

Achieved 5% sky-to-detector throughput, with potential to reach 13%.

Coupling efficiency into single-mode fiber reached 47% with 60% Strehl ratio.

Future improvements could enable 90% Strehl ratio and 67% coupling efficiency.

Abstract

We demonstrate for the first time an efficient, photonic-based astronomical spectrograph on the 8-m Subaru Telescope. An extreme adaptive optics system is combined with pupil apodiziation optics to efficiently inject light directly into a single-mode fiber, which feeds a compact cross-dispersed spectrograph based on array waveguide grating technology. The instrument currently offers a throughput of 5% from sky-to-detector which we outline could easily be upgraded to ~13% (assuming a coupling efficiency of 50%). The isolated spectrograph throughput from the single-mode fiber to detector was 42% at 1550 nm. The coupling efficiency into the single-mode fiber was limited by the achievable Strehl ratio on a given night. A coupling efficiency of 47% has been achieved with ~60% Strehl ratio on-sky to date. Improvements to the adaptive optics system will enable 90% Strehl ratio and a coupling of up to 67% eventually. This work demonstrates that the unique combination of advanced technologies enables the realization of a compact and highly efficient spectrograph, setting a precedent for future instrument design on very-large and extremely-large telescopes.