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

In this paper, we analyse the application of an optical readout system to the gravitational reference sensor of LISA. The goal is not the replacement of the capacitive sensor with an optical one, but the integration of the optical sensor in the present design of LISA. The main motivation is the implementation of a back-up solution, so that there is a significant risk reduction for the mission in case the main sensor fails for some reason. Furthermore, an optical system is potentially more sensitive than the capacitive one for some degrees of freedom. In this paper, we report some preliminary experimental results on the sensitivity of the sensor and describe a possible set-up for the implementation according to the present design of the gravitational reference sensor.