Serpentine Integrated Grating Spectrometer for Extreme Precision Radial Velocimetry

We propose a novel ultra-high-resolution Serpentine Integrated Grating (SIG) spectrometer for use in Precision Radial Velocimetry (PRV) measurements of minute Doppler shifts gravitationally imparted on stellar spectra by orbiting Earth-size exoplanets. Detecting such small spectral shifts is extremely challenging, requiring exquisite instrument and spectral reference stability and spectral resolving powers exceeding 100,000 to maintain few cm/s precision for year(s). To overcome atmospheric limits on ground-based PRV, planned space missions require precision spectrometers with low size, weight and power (SWaP). SIG generalizes photonic gratings to two dimensions, and relies on the exquisite manufacturing fidelity of photonic integrated circuits (PICs), instead of grating ruling machines, to produce PIC gratings with record resolution. These folded gratings form the basis of a new class of miniature spectrometers with comparable resolutions to spectroscopic instruments thousands of times larger and more expensive. A SIG spectrometer requires only a few small optical components and can be readily integrated with emerging astrophotonic photonic lantern and microcomb technologies to implement a low-SWaP instrument suitable for space-based PRV. To date, we have demonstrated a proof-of-concept SIG combining a 5.2 cm (equivalent to 14.8 cm in free space) folded delay line with grating couplers in a footprint of just ~0.4 mm^2 to attain a resolving power of ~100,000 in the 1540-1650nm regime. During Phase 1 we will develop a photon-efficient SIG spectrometer design specifically targeting PRV at NIR wavelengths and investigate extending it to the visible regime. We will design new SIG variants with up to 1M resolving power, study SIG efficiency limits, compare low-SWaP calibration methods, evaluate instrument noise and stability, investigate new architectures, and design a prototype spectrometer which we will build and demonstrate during Phase II to attain TRL4.

The SIG astrophotonics technology could not only facilitate ultra-high-resolution low-SWaP stable spectrometry for ground-based and especially space-based PRV missions, but could also resolve Rossiter-Mclaughlin effects and optical absorption lines of transiting exoplanets, enable many-channel high-resolution spectroscopy using multiple robotically-positioned fibers in wide-field stellar photometry campaigns, and benefit a variery of NASA missions requiring low SWaP, high-resolution spectroscopy, whether aimed at the stars, the Sun, or at Earth. Spectroscopy is relied upon in nearly every branch of science, including recent advances in atomic physics and quantum computing requiring high spectral resolution to resolve atomic transitions. The SIG technology not only outperforms traditional spectrometers, but can do so within an ultra-miniature "sugar cube" form factor that can be incorporated into space-constrained experiments or arrayed to measure multiple spectra.

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