For the next generation of extremely large telescopes with the primary mirrors over 30 m in diameter, focal anisoplanatism renders single laser guide star AO useless. The laser tomography AO (LTAO) technique demonstrates an effective approach to reduce focal anisoplanatism, although it requires multiple LGSs & WFSs, and complex tomographic reconstruction. Here we propose a novel LGS alternative configuration with the corresponding wavefront sensing and reconstruction method, termed Projected Pupil Plane Pattern (PPPP). A key advantage of this method is that a single collimated beam is launched from the telescope primary mirror, and the wavefront sensed on the uplink path, which will not suffer from the effects of focal anisoplanatism. In addition, the power density of the laser beam is significantly reduced compared to a focused LGS, which decreases aircraft and satellite safety hazards. A laboratory experiment for PPPP has been setup to anchor the PPPP concept and compare against a Shack-Hartmann WFS.

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Huizhe Yang, Alastiar Basden, David Buscher, Francisco Javier de Cos Juez, Aglae Kellerer, Tim Morris, Richard Myers, Eddy Younger, and Nazim Bharmal, "LGS alternative wave-front sensing: Projected Pupil Plane Pattern (PPPP)," Proc. SPIE 10703, Adaptive Optics Systems VI, 107030Q (Presented at SPIE Astronomical Telescopes + Instrumentation: June 11, 2018; Published: 10 July 2018); https://doi.org/10.1117/12.2311991.