Photonic integrated circuits are attracting increasing interest in astronomic applications, where device miniaturization is a key issue for reducing cost and improving the stability of the optical measurement systems against mechanical vibrations and thermal drifts. Compact spectrometers that enable wavelength-domain analysis of the light radiated by a galaxy can be implemented on a photonic chip by using arrayed waveguide gratings (AWGs). A limitation of traditional AWG designs is that the optical beams are focused on output ports that are positioned along a circular surface, thus not allowing the coupling with linear detector arrays without defocusing aberrations for side channels.

In this work, J. Zhan and coauthors propose a new design for an integrated AWG, which allows for minimizing such defocusing aberrations on a flat output plane. To this aim, they identify three “stigmatic" points (TPS), which are associated with three different wavelengths, in such a way that edge wavelengths are focused onto that surface with no reduction of the resolving power. An experimental validation of the TPS AWG concept is achieved on a low-loss high-index-contrast Si₃N₄ platform, providing a spectral resolving power as high as 17,000 in a device of less than 1 cm² footprint. The proposed AWG design can be scaled up to several input channels, for instance to interface the spectrometer with an input photonic lantern, and can be implemented in a polarization diversity scheme to fully observe the unpolarized light collected from the universe.

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