Compact Delay Lines for Space Interferometry

Introduction: This work will demonstrate long but compact delay lines suitable for future formation-flying space interferometers. NASA's Astrophysics Strategic Technology Gaps state that "Interferometric baselines in the tens of meters, up to ~100 m, are required to provide the spatial resolution needed to follow up on discoveries made with the Spitzer and Herschel space telescopes, and to provide information complementary to that attainable with ALMA and JWST" (see Far-IR Spatio-Spectral Interferometry, tier 2 gap). They also express the need to "Develop most efficient orbits for formation flying, maintaining the baseline and the uv-coverage required to science [sic] in the Astro2020 decadal report" (see Far-IR imaging interferometer for high-resolution spectroscopy, tier 2 gap). Large formation-flying Far-IR interferometers, such as the Terrestrial Planet Finder, have reached mid-TRLs in the past before being postponed or canceled. The Keck Institute for Space Studies (KISS) report on Exploring Exoplanets with Interferometry outlines multiple high-risk technologies necessary for a successful large mission. These challenging technologies include precision formation flying, starlight transfer between spacecraft, and fringe tracking. The KISS report recommendations (soon to come out) include one or several small-scale technology demonstration missions as stepping stones for flagship missions in the 2040s. In particular, recent advances in CubeSats, formation flying, and free space optical communication make such small-scale missions a very appealing demonstration platform. Approach: We will implement a delay line that would a) significantly relax formation-flying requirements, b) reduce the total number of spacecraft in a formation-flying interferometer to a minimum of two, and c) increase the number of potential science targets via dual-tracking a target and a reference. The proposed free-space delay line will incorporate four high-reflectance mirrors on mounts and linear stages with piezo-adjusters. A variable number of reflections (from 7 to 35) will enable a large, continuously adjustable delay range, and a fifth mirror will be used to (time-)switch between two delay channels for dual tracking. We will develop an algorithm for precisely controlling the number of reflections and the delay length. We will implement metrology to measure the delay precisely while switching between the two delay channels. We will validate our approach using a tunable visible laser and a standard delay line, and characterize optical losses and polarization effects. We will also analyze the requirements from the delay line in the context of the space environment with potential small and large missions in mind. Expected results: 1) A delay line that fits within a 10 cm x 20 cm footprint, offers one optical delay channel between 1 and 5 meters controlled with 20 nm precision, and can instantly switch to a second channel with a delay different by up to 0.5 meters for dual-tracking. 2) Optical simulations and in-lab validation of delay control and switching algorithms. 3) Assessment of delay-line performance, requirements, and benefits in the context of a CubeSat tech demo mission and a large FIR mission. Significance of results: This work will decrease the technological risks associated with formation flying, both large and small space interferometers. In particular, it will make a CubeSat/SmallSat technology demonstration much more feasible by reducing the number of spacecraft to a total of just two. At the same time, this work will increase the science yield potential of all missions through dual-tracking.

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