Efficient ultra-broadband low-resolution astrophotonic spectrographs

Pradip Gatkine; Greg Sercel; Nemanja Jovanovic; Ronald Broeke; Katarzyna Lawniczuk; Marco Passoni; Ashok Balakrishnan; Serge Bidnyk; Jielong Yin; Jeffrey Jewell; J. Kent Wallace; Dimitri Mawet

arXiv · 2404.02376

Efficient ultra-broadband low-resolution astrophotonic spectrographs

Pradip Gatkine·Greg Sercel·Nemanja Jovanovic·Ronald Broeke·Katarzyna Lawniczuk·Marco Passoni·Ashok Balakrishnan·Serge Bidnyk·Jielong Yin·Jeffrey Jewell·J. Kent Wallace·Dimitri Mawet

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Abstract

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$_3$N$_4$ (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$_3$N$_4$ 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.

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Pradip Gatkine, Greg Sercel, Nemanja Jovanovic, Ronald Broeke, Katarzyna Lawniczuk, Marco Passoni, Ashok Balakrishnan, Serge Bidnyk, Jielong Yin, Jeffrey Jewell, J. Kent Wallace, Dimitri Mawet. 2024-04-03. Efficient ultra-broadband low-resolution astrophotonic spectrographs. https://arxiv.org/abs/2404.02376

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