Understanding the limitations of high contrast nulling with photonic components

The Habitable Worlds Observatory (HWO) will be the first observatory where the primary science focus is the detection and characterization of terrestrial planets in the habitable zones of Sun-like stars. To access the light from the exoplanet needed to conduct the spectroscopy, the HWO will utilize an advanced starlight suppression system. Although the front-runner in this race is a coronagraph, they have not reached the necessary contrast levels. In the near-infrared (J or H band), the angular resolution is reduced and coronagraphs able to access the habitable zone of a Sun-like star with the ~6-m aperture are even more difficult to realize. Nulling interferometry is a serious contender for the starlight suppression system in the near infrared channel of the HWO. Photonic technologies, including integrated circuits on chips are a promising approach to realizing a compact and stable nuller for a space mission where Size, Weight and Power (SWaP) are critical. There are currently 5 groups working on different variants of photonic nuller architectures. Key to nearly all designs is a beam-combining chip, where light originating from different parts of the pupil or focal planes is strategically combined to destructively interfere and suppress it. There have recently been demonstrations of simple photonic Mach-Zehnder Interferometers (MZIs) that can suppress light to ~10^-6 in a single stage in monochromatic light, which is encouraging. Achieving this level of suppression requires uniform losses in the arms of the interferometer and even light splitting at the splitter. Despite these demonstrations, the maximum starlight suppression that can be achieved by an MZI is still unknown. The benefit of photonics is that imperfect devices can be compensated via active amplitude and phase modulation, which is trivial to integrate onto a chip. The bandwidth over which the MZI and the active amplitude and phase compensation can achieve the desired level of suppression also needs to be explored. In this project, we aim to design, fabricate, and test the limits of starlight suppression in beam combiners with and without active amplitude and phase compensation. Our goal is to demonstrate <10^-7 starlight suppression in a single stage in monochromatic light, characterize the polychromatic response, and determine the optimum architecture to achieve the requirement over a broader range and to a deeper contrast. We will study the interplay with integrated photonic spectrographs, which can pre-disperse the light and send narrower channels into individual beam combiners optimized for narrower bandwidths. The goal is to exit the project with a clear understanding of the maximum contrast limit of a single MZI in broadband light, the optimal bandwidths for each channel of a photonic nuller, and how a circuit can be designed with minimal components to achieve the daunting contrast requirements set by the HWO over a 20% fractional bandwidth.

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