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, 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.

Potential of commercial SiN MPW platforms for developing mid/high-resolution integrated photonic spectrographs for astronomy.

Integrated photonic spectrographs offer an avenue to extreme miniaturization of astronomical instruments, which would greatly benefit extremely large telescopes and future space missions. These devices first require optimization for astronomical applications, which includes design, fabrication, and field testing. Given the high costs of photonic fabrication, multi-project wafer (MPW) silicon nitride (SiN) offerings, where a user purchases a portion of a wafer, provide a convenient and affordable avenue to develop this technology. In this work, we study the potential of two commonly used SiN waveguide geometries by MPW foundries, i.e., square and rectangular profiles, to determine how they affect the performance of mid/high-resolution arrayed waveguide grating (AWG) spectrometers around 1.5 µm. Specifically, we present results from detailed simulations on the mode sizes, shapes, and polarization properties, and on the impact of phase errors on the throughput and cross talk as well as some laboratory results of coupling and propagation losses. From the MPW run tolerances and our phase-error study, we estimate that an AWG with R ∼10,000 can be developed with the MPW runs, and even greater resolving power is achievable with more reliable, dedicated fabrication runs. Depending on the fabrication and design optimizations, it is possible to achieve throughputs ∼60% using the SiN platform. Thus, we show that SiN MPW offerings are highly promising and will play a key role in integrated photonic spectrograph developments for astronomy.