An optical readout system for the drag free control of the LISA spacecraft

摘要

LISA is an ESA-NASA joint project for the realization of a space interferometric gravitational wave (GW) antenna. LISA is designed for the measurement of GWs in a very low frequency band (0.1-100 mHz). The antenna is composed by three spacecraft (SC) in suitable heliocentric orbits placed at the corners of a huge equilateral triangle, each side being 5 million km long. The SCs are linked by lasers, forming a sort of optical transponder. By means of phase locking techniques, any round-trip phase delay change gives a measurement of a change in the SC distance (measured as light transit time), due to incoming GWs. An essential requirement is that the SCs are set as close as possible to pure geodetic motion, in the measurement frequency band. This is hardly fulfilled because the SCs are disturbed by several external forces, like solar radiation pressure, cosmic rays etc. In each SC there are two free falling proof masses (PM) that are as much isolated as possible by all external force but gravity. The relative position between each PM and the SC is measured, in six degrees of freedom, by the so-called inertial sensor (IS). The IS signal is then used for drag-free servo-loops that force the SC to follow the geodetic motion of the PMs. The current solution for the IS is the adoption of capacitive sensing. This gives a reliable device but poses several limitations due to back action and cross couplings. In this work, we present an optical lever sensor as an alternative solution. In particular we analyze the potential sensitivity and discuss the advantages in terms of relaxed specifications for the drag free control loops. We also report on bench-top measurements that confirm the performance in the required frequency band.