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The continually increasing sensitivity required for the advancement of far-infrared astronomy dictates that the next generation of space-based observatories must employ cryogenically cooled telescopes and instrumentation. Operating cryogenic instrumentation in orbit poses several challenges, including the need for extremely low power dissipation and precise position measurement and control. In prior work, we reported on the development of a homodyne three-phase range-resolved laser interferometer, which demonstrated a displacement measurement uncertainty of 2:3nmrms at <4K.1 Effects of low frequency noise (1=f) in the electrical signals at low velocities were performance limiting due to the interpretation of noise as interference fringes. To avoid 1=f noise, a frequency-modulated continuous-wave (FMCW) heterodyne approach was adopted. An FMCW prototype was developed, and the preliminary results yielded an uncertainty of 29nmrms at <4K.2 In this paper we present the design of an integrated cryogenic FMCW range-resolved laser interferometer which features real time data processing for simultaneous displacement measurements of up to 8 axes. The performance of fibers and their coupling under ultra-high vacuum at cryogenic temperatures is largely unexplored, and we present the cryogenic characterization results of several key variables, including fiber type, termination, and mating, along with alignment effects due to the thermal contraction of fiber components. These results have been incorporated into the design of our cryogenic FMCW interferometer. Applications of cryogenic range resolved interferometry are discussed, with a focus on the integration of the FMCW interferometer with a custom 3-axis cryogenic accelerometer. |
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