Five degrees of freedom test mass readout via optical levers

Abstract

High precision measurement of all six degrees of freedom of freely floating test masses is necessary for future gravitational space missions as the sensing noise is frequently a limiting factor in the overall performance of the instrument. Femto-meter sensitivity has been demonstrated with LISA Pathfinder which used a complex laser interferometric setup. However, these measurements where restricted to the length changes in one degree of freedom only. When aiming for sensing multiple degrees of freedom, typically capacitive sensing is used, which facilitates a compact setup but does not provide competitive precision. An alternative approach to improve the sensitivity beyond capacitance readout systems and to reduce the complexity of the setup, is to use optical levers. Here, we report on the realization of a test mass sensing system by means of a modulation/demodulation technique in combination with four optical levers detected by quadrant photodiodes. The results of our table-top experiment show that this configuration allows us to extract information on five degrees of freedom of a cubic test mass. With basic off-the-shelf laser diodes we demonstrate an angular resolution of below 600 nrad Hz-1/2 at frequencies between 10 mHz and 1 Hz (which is better than a conventional autocollimator) while simultaneously measuring the linear motion of the test mass with a precision of better than 300 nm Hz-1/2 in the same frequency band. Extension of the geometry will enable optical sensing of all six degrees of freedom of the test mass. © 2019 IOP Publishing Ltd.