Design and construction of an optical test bed for eLISA imaging systems and tilt-to-length coupling

The evolved Laser Interferometer Space Antenna (eLISA) is a future space-based interferometric gravitational-wave detector consisting of three spacecraft in a triangular configuration. The interferometric measurements of path length changes between satellites will be performed on optical benches in the satellites. Angular misalignments of the interfering beams will couple into the length measurement and represent a significant noise source (tilt-to-length (TTL) coupling). Imaging systems are foreseen to reduce this TTL coupling. We have designed and built two different imaging systems to suppress this coupling. To investigate tilt-to-length coupling experimentally we designed and constructed an optical test bed. It consists of a minimal optical bench and a telescope simulator. The minimal optical bench comprises the science interferometer where the local laser is interfered with light from a remote spacecraft. In our experiment, a simulated version of this received beam is generated on the telescope simulator. The telescope simulator provides a tilting flat-top beam, a reference interferometer and an additional static beam as a phase reference. We avoid TTL coupling in the reference interferometer by using a small photo diode placed in a copy of the beam rotation point. We show that the test bed is operational with an initial measurement of tilt-to-length coupling without imaging systems.