Developing high-efficiency high-resolution photonic spectrographs for astronomy

Abstract

Astrophotonic spectrographs offer a unique capability to miniature astronomical spectrographs by several orders of magnitude and eliminate any moving parts by leveraging single-mode (i.e. diffraction-limited) input. This is a critical technology for attaining high thermo-mechanical stability, which is a requirement for high resolving power spectrographs (R = λ/Δλ > 10,000), as driven by science cases such as exoplanet detection using radial velocity measurements. Thanks to advances in adaptive optics and photonic lanterns, efficient injection of light from the telescope into single-mode fibers has become feasible. Arrayed waveguide gratings are one of the most promising architectures for building high-resolving-power spectrographs. However, for astronomical science cases, achieving a high end-to-end efficiency (fiber to chip to free space) is critical. To identify the most efficient material platforms, we constructed high-resolution spectrographs in two leading low-loss photonic material platforms: Silicon nitride (Si3N4), and doped silica. In particular, we built spectrograph chips with R ∼ 30,000 using both material platforms spanning astronomical J and H bands (1200-1700 nm). These spectrographs have a free spectral range of ∼13 nm. Here, we present a comparison of the two material platforms at these high resolving powers. This study paves the foundation for selecting an appropriate material platform for astrophotonic devices (in particular, spectrographs), to achieve the highest efficiency and thereby enable high-stability acquisition of spectra at a resolving power R > 10,000 for enabling precision radial velocity measurements, measuring chemical composition in the high-redshift universe, and unveiling exoplanet atmospheres.