The large sizes of conventional astronomical spectrographs pose a challenge with their thermo-mechanical stability, which in turn adversely affects the spectral precision of high-resolution spectrographs, particularly for the upcoming extremely large telescopes. Astrophotonic spectrographs can resolve this issue by enabling compact, single-mode fiber-fed diffraction-limited spectroscopy on a chip.
Here we demonstrate a continuous-output, curved focal-plane photonic Arrayed Waveguide Grating (AWG) chip which allows a continuous sampling of the spectrum, thus alleviating the issues of discrete-output AWGs. This chip was fabricated using a commercial Silicon-nitride platform with a square waveguide cross-section of 800 × 800 nm. The AWG has a resolving power of 12,000, a free-spectral range of 18 nm, and a wide operational band of 1425-1650 nm. The chip is highly compact with a size of only 8.75 × 3 mm. Notably, the AWG chip is cleaved along its curved focal plane (i.e. Rowland curvature) to avoid defocusing of the spectrum due to planar cleaving. The curved focal plane can be imaged onto a planar detector using focusing optics. However, in this paper, the focal plane was sampled by translating a single-mode fiber across it. While this device highlights the capability of the commercial high-contrast SiN platform for building compact, high-resolution, and continuoussampling spectrographs for astronomy, it also underlines the key challenges in imaging / characterizing the high numerical aperture chip output at low loss. We discuss these challenges and the way forward.