Compact unambiguous differential path-length metrology with dispersed Fabry-Perot laser diodes for a space interferometer array

Abstract

A space interferometer could reach a sensitivity and angular resolution which is unattainable on Earth due to the distortion and absorption of the atmosphere. It would enable many unique science cases, including the direct imaging and characterisation of temperate terrestrial exoplanets. This ambitious vision relies on the formation flying of individual spacecrafts, and the demonstration of precision metrology measuring positions in to better than 1mm in at least 2 dimensions, and velocities in the range of nm/s. These significant technical challenges are one of the main reasons progress in space interferometry has been seriously hampered in the two last decades. To overcome this obstacle, we propose a novel metrology concept operating in two steps. The coarse positioning of the array elements is achieved through commercially demonstrated components, such as GPS, wide angle cameras and time-of-flight sensors. For the critical fine metrology, multiple longitudinal mode Fabry-Perot lasers in a central spacecraft are split and retro-reflected off each telescope bearing spacecraft. The reflected beams are then coherently combined in the central spacecraft and the resulting fringes are spectrally dispersed. In this manner, the phase difference is measured at the different Fabry-Perot wavelengths, allowing the unambiguous differential position measurements over a couple of mm capture range. We present the concept together with a prototype system in the laboratory.