Objective: This activity aims at investigating different wavefront sensor technologies in order to identify the best suited one foroperation under strong atmospheric turbulence conditions in optical communication applications. Targeted Improvements: 100% increased wavefront sensor performance under strong atmospheric turbulence conditions. Description: Optical beams propagating through the turbulent atmosphere undergo phase and amplitude fluctuations, causing random fading and surges in the signal which leads to irradiance fluctuations when coupled into a single mode fibre. Adaptive Optics aims at counteracting these wavefront impairments. Wavefront sensors are one of the key elements of an Adaptive Optics loop, together with the deformable mirror and the controller. Shack-Hartmann wavefront sensors (SH WFS) are among the most popular sensors used to infer the wavefront error in astronomy due to their simplicity, speed, and efficient use of light. However, SH WFS were shown to not perform well under strong atmospheric turbulence (occurring, for instance, at low elevations during LEO/Ground optical links or during daytime). This is due to the fact that standard least squares reconstruction algorithms assume that SH WFS measurements are equal to the average phase gradient within each sub-aperture. However, this assumption does not hold anymore in case of strong scintillations. Furthermore, the re-constructor cannot form an estimate of the branch points appearing in the phase front in the presence of strong scintillations. Other effects occurring during strong turbulence conditions contribute to make the reconstruction more complex (e.g. the phase is not uniform over a sub-aperture, the spots formed by the micro-lenses can be imaged out of the nominal area on the detector). The present activity aims at exploring and testing alternative wavefront sensing methods to identify the one showing the best performance under strong atmospheric turbulence conditions (e.g. WFS based on interferometers, holographic elements, pyramid, curvature, or combined methods etc.). Different wavefront sensor technologies shall be investigated theoretically and an experimental test bench shall be developed to compare the two best suited ones. Strong turbulence conditions shall be defined by a Rytov variance >0.3. Procurement Policy: C(1) = Activity restricted to non-prime contractors (incl. SMEs). For additional information please go to: http://www.esa.int/About_Us/Business_with_ESA/Small_and_Medium_Sized_En…

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