Arrayed Waveguide Grating Spectrometers for Astronomical Applications: New Results

TL;DR

This paper reports on the development of arrayed waveguide grating spectrometers for astronomy, achieving notable throughput, spectral resolution, and broadband coverage, with practical improvements for future astronomical instrumentation.

Contribution

The paper introduces new fabrication techniques and device designs for AWG spectrometers, enhancing throughput, polarization insensitivity, and spectral coverage for astronomical applications.

Findings

Peak throughput of ~23% achieved

Spectral resolving power of ~1300 demonstrated

Coverage of the entire H band (1450-1650 nm)

Abstract

One promising application of photonics to astronomical instrumentation is the miniaturization of near-infrared (NIR) spectrometers for large ground- and space-based astronomical telescopes. Here we present new results from our effort to fabricate arrayed waveguide grating (AWG) spectrometers for astronomical applications entirely in-house. Our latest devices have a peak overall throughput of ~23%, a spectral resolving power (${\lambda}/{\delta}{\lambda}$) of ~1300, and cover the entire H band (1450-1650 nm) for Transverse Electric (TE) polarization. These AWGs use a silica-on-silicon platform with a very thin layer of Si3N4 as the core of the waveguides. They have a free spectral range of ~10 nm at a wavelength of ~1600 nm and a contrast ratio or crosstalk of about 2% (-17 dB). Various practical aspects of implementing AWGs as astronomical spectrographs are discussed, including the coupling of the light between the fibers and AWGs, high-temperature annealing to improve the throughput of the devices at ~1500 nm, cleaving at the output focal plane of the AWG to provide continuous wavelength coverage, and a novel algorithm to make the devices polarization insensitive over a broad band. These milestones will guide the development of the next generation of AWGs with wider free spectral range and higher resolving power and throughput.