Serpentine Integrated Grating Spectrometer for Extreme Precision Radial Velocimetry

We propose a novel ultra-high-resolution Serpentine Integrated Grating Spectrograph (SIGS) 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 spectrographs with low size, weight and power (SWaP). SIGS 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 spectrographs with comparable resolutions to spectroscopic instruments thousands of times larger and more expensive. SIGS 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. We have previosuly demonstrated a proof-of-concept SIGS combining a 5.2 cm (equivalent to 14.8 cm in free space) folded delay line with grating couplers in a footptint of just ~0.4 mm^2 to attain a resolving power of ~100,000 in the 1540-1650nm regime. During Phase I we for the first time obtained a spectrum using a more recent test chip, clearly resolving spectral lines in an acetylene gas cell, while attaining a total photon loss below 10dB. During this effort we will continue improving SIGS efficiency and extend the design to NIR and VIS wavelengths through 4 fabrication iterations. We will also calibrate SIGS using an EO comb, build several SIGS prototypes, and measure solar and stellar spectra to attain TRL4.