Efficient broad band, high resolution (R~30,000) photonic spectrographs realized through cascading

Arrayed waveguide gratings (AWGs) offer a promising photonic architecture to miniaturize astronomical spectrographs by several orders of magnitudes. 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 ~40%, respectively, thus giving a total peak efficiency of 32% for the cascaded AWG. This high-efficiency cascaded AWG demonstration paves the way for a fully integrated photonic spectrograph for ground- and space-based telescopes.