Improving the fiber coupling efficiency for DARWIN by loss-less shaping of the receive beams

For the DARWIN mission the extremely low planet signal levels require an optical instrument design with utmost efficiency to guarantee the required science performance. By shaping the transverse amplitude and phase distributions of the receive beams, the singlemode fibre coupling efficiency can be increased to almost 100%, thus allowing for a gain of more than 20% compared to conventional designs. We show that the use of "tailored freeform surfaces" for purpose of beam shaping dramatically reduces the coupling degradations, which otherwise result from mode mismatch between the Airy pattern of the image and the fibre mode, and therefore allows for achieving a performance close to the physical limitations. We present an application of tailored surfaces for building a beam shaping optics that shall enhance fibre coupling performance as core part of a space based interferometer in the future DARWIN mission and present performance predictions by wave-optical simulations. We assess the feasibility of manufacturing the corresponding tailored surfaces and describe the proof of concept demonstrator we use for experimental performance verification.