Efficient ultra-broadband low-resolution astrophotonic spectrographs

Broadband low-resolution near-infrared spectrographs in a compact form are crucial for ground- and space-based astronomy and other fields of sensing. Astronomical spectroscopy poses stringent requirements including high efficiency, broad band operation ($>$ 300 nm), and in some cases, polarization insensitivity. We present and compare experimental results from the design, fabrication, and characterization of broadband (1200 - 1650 nm) arrayed waveguide grating (AWG) spectrographs built using the two most promising low-loss platforms - Si₃N₄ (rectangular waveguides) and doped-SiO₂ (square waveguides). These AWGs have a resolving power (λ/Δλ) of ~200, a free spectral range of ~ 200-350 nm, and a small footprint of ~ 50-100 mm². The peak overall (fiber-chip-fiber) efficiency of the doped-SiO₂ AWG was ~ 79% (1 dB), and it exhibited a negligible polarization-dependent shift compared to the channel spacing. For Si₃N₄ AWGs, the peak overall efficiency in TE mode was ~ 50% (3 dB), and the main loss component was found to be fiber-to-chip coupling losses. These broadband AWGs are key to enabling compact integrations such as multi-object spectrographs or dispersion back-ends for other astrophotonic devices such as photonic lanterns or nulling interferometers.