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.
To this day the standard method of imaging design is optimization. Recently we have introduced tailoring as a radically different paradigm of optical design. Tailoring determines the shape of optical surface, a priory free, by solving one or more differential equations. This method has proved successful in illumination design where a high level of detail needs to be accommodated and indeed a perfect solution is possible often with only one optical surface.
The weakness of tailoring is first that it cannot adequately deal
with weak requirement which need to be optimized, because they can
not be precisely met or at least not met simultaneously. Examples
range from manufacturability, to size, sensitivity to tolerances,
but also includes imaging errors. Wassermann and Wolf showed in a classical paper how tailoring can be used in an imaging system in order to achieve aplanatism with the addition of two aspheres. In our contribution we present a synthesis which combines the virtues of optimization with those of tailoring for imaging design. It encompasses freeform surfaces and thus a huge number of effective parameters, however, only a few of these are subject to optimization. On the other hand our method can adequately use optimality criteria such as conflicting features in a figure of merit, which need to be compromised upon. Finally the result is a mathematically rigorous optimum with respect to whatever figure of merit is specified.