Efficient, broadband, and compact high-resolution photonic spectrographs at R ~ 30,000 realized through cascading

Arrayed waveguide gratings (AWGs) offer a promising photonic architecture to miniaturize astronomical spectrographs by several orders of magnitude. However, the overlapping spectral orders in an AWG need to be separated using bulk-optics cross-dispersion to retrieve the full spectrum, which reduces the photonic advantage. In this paper, we present experimental results from a proof-of-concept cascaded AWG setup, which eliminates the need for bulk-optics cross-dispersion. A low-resolution, high-FSR AWG (channel spacing ∼ 8.75 nm , FSR ∼ 200 nm) splits the input spectrum into coarse channels which are then dispersed at high-resolution (R ∼ 30,000, FSR ∼ 12 nm) using the fine AWG. Thus, the entire spectrum across 200 nm can be imaged using one coarse AWG and a stack of fine AWGs, one corresponding to each coarse channel. Both coarse and fine AWGs are built using doped-SiO₂ platform. The peak efficiencies of the low- and high-resolution chips are ∼80% and ∼22%, respectively, thus giving a total peak efficiency of 18% for the cascaded AWG. This high-efficiency cascaded AWG demonstration paves the way for a fully integrated photonic spectrograph for future space-based missions.