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.

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. Beyond its applications for PRV, SIGS could be well suited for applications in earth science, heliophysics, and planetary science. Compared to traditional high-resolution spectrographs, a core advantage of the SIGS technology is its low SWaP (Size, Weight and Power), which is far more critical to space flight mission instruments than their ground-based counterparts. For space-based missions enabling continuous observations without telluric contamination or aberrations from atmospheric turbulence, high resolving power of order R~100,000 opens a number of science avenues such as detecting biosignature gas absorptions like H2O, O2 and CH4 in transiting exoplanet atmospheres or precision radial velocity studies to characterize exoplanetary systems. High spectral resolution is also critical for near-infrared studies of strong H3+ aurorae at all outer planets, and necessary to isolate faint emissions in the thin lunar atmosphere from the brightly scattered moonlight. On Venus, Doppler shifts map wind patterns and thermal linewidth broadening informs the altitude where plasmas precipitate, and on Mars the narrowly resolved D/H isotopic ratio tells us how water escapes with season and sporadic global dust storms. One of our long-term goals is to develop a modular SIG spectrograph that could easily fit on a CubeSat and make high-resolution spectroscopy from space far more attainable. 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. State-of-the-art high-resolution spectrometers are typically large, heavy, and expensive, rely on slow scanning of a grating or a reference mirror, and often rely on narrow slits that waste most of the photons. On the other hand, SIGS is capable of attaining equally high resolutions (e.g. 5pm) at the full frame rate of the image sensor (potentially > 1 KHz) without any mechanical scanning and with high photon efficiency, making it possible to monitor rapid changes in laser spectra in real time, capture spectra of pulses, as well as to resolve sub-GHz laser line spacings, drifts, and mode hops. Such measurements are important for laser characterization, studying nonlinear laser physics, laser pulse shaping, experiments involving electro-optical laser modulation, and initial tuning of lasers to atomic transitions in multi-laser atomic physics experiments, for example. Furthermore, we envision a SIGS research instrument that can easily fit in a bag (potentially even a pocket) and is powered from a battery, making it possible to perform laser characterization and diagnostics in the field, or to place multiple fiber-coupled SIGS instruments within a physics lab to monitor spectra at different points in a complex experiment in real time, for example.

This is a mature technology (TRL 7+) — the realistic path in is usually NASA's Technology Transfer Program: licensing an existing NASA patent, or a Space Act Agreement to use NASA facilities/expertise directly. NASA also runs a startup licensing program with no upfront fee for companies formed to commercialize a specific NASA technology.

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