Compact laser interferometer for dynamic nanometer-level ranging

Hexapod positioning systems are employed in a variety of industrial and research applications, such as satellite sensor testing, optical metrology, and motion simulation. In the context of a testbed simulating spacecraft-to-spacecraft laser ranging interferometry, a hexapod is used to reproduce orbit dynamics, including spacecraft jitter. Due to the insufficient resolution of its internal sensors, what we believe to be a novel interferometer architecture with the capability for nm-level ranging is presented, to act as an external reference system. The simple optical layout is achieved through the direct bonding of the optical components to a titanium baseplate; an active alignment, based on a closed-loop controller, is performed to maximize the superimposition of the beams at the photodetector. The resulting device meets the performance requirements both in air and in vacuum, where the noise floor was evaluated as under 1nm/Hz in the 1 mHz-1 Hz range. Operating two interferometers in parallel enables a further reduction of the noise levels, identifying thermo-elastic variations as the main contributor to signal noise. The device effectively tracks the motion of the hexapod across its full range and at a maximum speed ≥5mm/s, and is able to identify inaccuracies of the internal sensors of the platform.