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Abstract

The next generation of far-infrared space observatories must be fully cryogenically cooled to fully exploit state-of-the art detectors. The need for high-resolution broadband spectroscopy dictates that a Fourier transform spectrometer (FTS) used in such a mission must be cooled to <4 K. Robust and precise cryogenic displacement metrology is required as the quality of the recovered spectrum is dependent on precise knowledge of the position of the moving FTS components. Past missions have used capacitive, inductive, and encoder displacement metrology systems, however, to date, no mission has flown an interferometer for such a purpose. Interferometers confer advantages of high precision and low thermal power dissipation that can meet the stringent requirements of future missions. We have developed a cryogenic multiaxis range-resolved laser interferometer based on sinusoidal frequency modulation (SFM) that requires only a single laser and a single photodetector due to its inherent frequency-division multiplexing nature. Here we present the theory, key design considerations, calibration methods, and application of the SFM to simultaneous multiaxis measurements under ambient and cryogenic conditions. To the best of our knowledge, this is the first reporting of multiaxis cryogenically launched displacement measurements using a range-resolved interferometer.