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 - Si_3N_4 (rectangular waveguides) and doped-SiO_2 (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 mm^2. The peak overall (fiber-chip-fiber) efficiency of the doped-SiO_2 AWG was   79% (1 dB), and it exhibited a negligible polarization-dependent shift compared to the channel spacing. For Si_3N_4 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.