Projected Pupil Plane Pattern:an alternative LGS wavefront sensing technique

Abstract

For the next generation of extremely large telescopes, Focal Anisoplanatism (FA) renders single Laser Guide Star AO useless. Here we analyze a novel LGS alternative configuration with corresponding wavefront sensing and reconstruction method, termed Projected Pupil Plane Pattern, to solve the problem of Focal Anisoplanatism. With PPPP, turbulence is sensed during uplink by a laser beam projected as a collimated beam from the whole telescope primary mirror. Phase changes due to the turbulence introduce intensity variations that then increase in amplitude with propagation distance. By observing the distribution of intensity at two distant planes, the Transport-of-
Intensity equation can be used to determine the phase aberration encountered during the uplink path. A simple imaging camera can then be used to measure the wavefront by imaging the backscattered light patterns.
We have successfully demonstrated PPPP works both by simulation and laboratory experiment, where we find that PPPP can achieve equivalent performance to a SH WFS associated with a NGS. However it is shown that the main problem of PPPP is its low Signal-to-Noise Ratio if a 20W laser is used. To reduce the requirement for high laser power, an alternative reconstructor based upon nonlinear Artificial Neural Networks has been developed, and provides a wavefront with measurement error around 160nm RMS with a single 200W laser on a 4-m diameter telescope. PPPP is therefore ready for a practical on sky test, which we are currently undertaking at Electro Optical Systems (EOS) Debris Laser Ranging (DLR) system, Australia.