Abstract

The ESA/NASA joint space mission LISA (Laser Interferometer Space Antenna) aims at detecting gravitational waves in the frequency band 3 10^-5 Hz to 1 Hz. It consists of 3 satellites which form an equilateral triangle in space with a baseline of ~ 5 million kilometres where changes in the distance between each two satellites are measured interferometrically with pm-sensitivity. Free flying proof masses inside the satellites act as end mirrors of the interferometer anus. In the current baseline design, the position of the proof masses with respect to the satellite housing must be measured with sensitivities of ~ 5 pn/√Hz for the translation measurement (for frequencies above 2.8 10^-3 Hz with an f^-2 requirement relaxation down to 3 10^-5 Hz) and < 20 nrad/√Hz for the tilt measurement (for frequencies above 10^-4 Hz with an f^-1 requirement relaxation down to 3 10^-5 Hz). For this purpose, EADS-Astrium GmbH - in collaboration with the Humboldt-University to Berlin and the University of Applied Sciences, Konstanz - develops a heterodyne interferometer utilizing differential wavefront sensing for the tilt measurement. The design is based on maximum symmetry where reference and measurement arm have the same optical pathlength, frequency and polarization. The interferometer design is shown in Fig. 1. Frequency fl is split at a beam separator cube into measurement and reference beam which are both reflected by the same fixed mirror and superimposed by frequency f2 at a (non-polarizing) beamsplitter in order to generate a heterodyne beat signal at, in our case, 10 kHz. In our lab experiment, the two frequencies are generated by use of two acousto-optic modulators (AOMs) and fibre coupled to the interferometer board which is placed in a vacuum chamber in order to minimize variations in optical pathlength.

© 2007 IEEE