A high precision heterodyne interferometer for relative and absolute displacement measurement

The gravitational wave detector LISA utilizes as current baseline a high sensitivity Optical Readout (ORO) for measuring the relative position and tilt of a free flying proof mass with respect to the satellite housing. The required sensitivities in the frequency band from 30 µHz to 1 Hz are in the picometer range for the translation and in the nanoradiant range for the tilt measurement. EADS Astrium, in collaboration with the Humboldt University Berlin and the University of Applied Sciences Konstanz, has realized a prototype ORO over the past years. The interferometer is based on a highly symmetric design where both, measurement and reference beam have a similar optical pathlength, and the same frequency and polarization. The technique of differential wavefront sensing (DWS) for tilt measurement is implemented. With our setup noise levels below 5pm/√HZ for translation and below 10nrad/√Hz for tilt measurements-both for frequencies above 10mHz-were demonstrated. We give an overview over the experimental setup, its current performance and the planned improvements.