Line of sight mesospheric sodium profiles obtained from the LGS signal for optimal ELT LGS-AO

Next-generation extremely large telescopes will be equipped with adaptive optics systems that use laser guide stars produced by resonant excitation of mesospheric sodium. The wavefront sensing of these systems could be enhanced by measurements of the vertical density profile of the mesospheric sodium along the line of sight, i.e. the sodium profile producing the LGS itself. Normally in AO system a few percent ( 3%) of the light from the LGS can be retrieved as leftover from the dichroic or the optics feeding the LGS WFS. This light, filtered and sent to a photon counter could be used for retrieving sodium density profiles. One approach is to partially modulate the intensity of the continuous wave lasers with a pseudo-random binary sequence of pulses. The sodium density profile can then be retrieved by cross-correlation of the measured LGS flux with the modulation sequence. With sufficient return flux, this technique can provide profiles every few seconds, useful for matching filters applied on elongated LGS, and can provide the centroid of the sodium layer profile, needed for the adaptive optics proper focus offset corrections. We have tested the continuous wave lidar concept by making simultaneous measurements using the modulation technique and a conventional lidar system, which provides the 'truth' in the comparison of the profiles. The experiments were conducted using the ESO Wendelstein laser guide star unit and the 6 m Large Zenith Telescope's sodium lidar system, located near Vancouver, Canada. This paper describes the experiment and presents some preliminary results. The possible implementation and potential performance of this technique for extremely large telescopes will be discussed.