Demonstrated pre-compensation of a focused laser beam with up to 0.27 mrad point-ahead-angle over a 1-km horizontal communication path
In the development of laser communication networks, atmospheric turbulence remains an obstacle in the efficiency optimisation of bidirectional optical links. Post-compensation of the downlink signal through adaptive optics (AO) has been verified as a method of increasing the efficiency of satellite-to-ground optical links, however, similar efficiencies have yet to be achieved in the opposite direction (uplink) largely due to the shower curtain effect and the consequent impossibility of conducting post-compensation over the smaller receiver aperture at the satellite terminal. Pre-compensation is anticipated to be a promising method of increasing the efficiency in uplink communications for Earth-to-GEO optical feederlinks, by pre-distorting the uplink wavefront prior to propagation through the atmosphere using measurements of the downlink. Here, we present the recent modifications to our current experimental setup and initial results of a pre-compensation experiment conducted over a 1 km horizontal path. The experiment aimed to investigate pre-compensation under a point-ahead-angle for the special case of a focussed uplink beam. Two optical terminals were developed, a "ground terminal breadboard" (GTB) consisting of a commercially available 30 cm reflective telescope in combination with an AO-box  capable of simultaneous post-and pre-compensation and a "satellite terminal breadboard" (STB) with separate transmission and receiving apertures for measurement of the uplink beam under a point-ahead-angle (PAA). We present a first look at and discussion of the experimental results which show that AO pre-compensation increased the measured intensity of the uplink beam over the receiving aperture of the STB for a PAA of up to 0.27 mrad.