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Demonstrating improved fibre coupling efficiency by loss-less shaping of top-hat receive beams

The investigations presented below were originally planned on the background of the DARWIN mission. The demand for improved fibre coupling efficiency yet is a general one for applications that cannot cope with the 20% loss of power upon coupling into a fibre. By shaping the transverse amplitude and phase distributions of the receive beam, the single-mode fibre coupling efficiency can potentially be increased to almost 100% (if the beam shaping is lossless), thus allowing for a gain of more than 20% (or 1.07dB) compared to conventional designs. We show that the use of "tailored freeform surfaces" for purpose of beam shaping reduces the 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. As a follow-up to a previously published paper, [1], we present the design and the results of a proof of concept demonstrator (POCD) for the application of tailored surfaces for building a beam shaping optics that shall enhance fibre coupling performance. The demonstrator consists of two main parts, the input beam support equipment and the POCD core. The first part contains the laser source, the top hat beam generation optics and a deformable mirror. The POCD core is set up as a three mirror system focussing into the fibre with 1064 nm being the design wavelength. The experiments performed with the demonstrator aimed at the principle proof of the beam shaping approach and at the test of deviations from the nominal parameters like field angle and aberrations. The results acquired from the experiments with a proof of concept demonstrator in an "as built" configuration show good agreement with the theoretical performance predictions by wave-optical simulations. Limitations of the available manufacturing technologies and of the operating wavelength regime are discussed.