Proof-of-concept Demonstrator for the Off-Axis Retroreflector Interferometer Configuration for NGGM

This paper presents the design and breadboarding of the proof of concept demonstrator for the so called retro-reflector interferometer scheme in off-axis configuration for the ‘Next Generation Gravity Mission’ (NGGM) studied at the European Space Agency (ESA). This configuration can offer benefits in terms of overall satellite configuration compared to the transponder scheme, which is currently flying on board of GRACE-FO. However, it relies on very low received laser signal levels due to the fact that the laser light is travelling about 100 km from the master satellite to the remote satellite and is reflected back to the master satellite by a retro-reflector. In comparison to the transponder scheme, where the signal is amplified on the remote satellite using a laser, which is optically phase locked to the laser signal of the master spacecraft, this reflection does not amplify the signal. Thus, even with higher emitted laser power, instead of some nanowatt, only a few picowatt are available on the according science detector. Therefore, less than a femtowatt of straylight within the detectable heterodyne frequency and angular range is allowed on the detector to fulfil the ranging noise requirement. The paper gives insights into the main opto-mechanical design topics of the Optical Bench Assembly (OBA). It includes the optical analysis results as well as mechanical design to suppress straylight below the required limit. The optomechanical design of the OBA is complemented by the opto-mechanical design of the test setup and by the electro-optical design of the phase read-out chain. Finally, preliminary results from the test campaign are presented.