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Adaptive optics (AO) systems of next generation optical ground telescopes will employ laser guide stars (LGS)
to achieve wide sky coverage. In these systems the mesospheric sodium layer at ~ 90 km height is excited
by means of laser-induced fluorescence of the Na I D2 resonance hyperfine transmission. The finite thickness of
sodium layer, and temporal variations in its density structure, result in LGS that are elongated and have internal
structure that varies with time. This degrades the performance of the AO system due to degeneracy between
effects of atmospheric and sodium layer variations. In order to quantify this and assess the impact on future
extremely large telescopes such as the Thirty-Meter Telescope (TMT), measurements are needed of the density
distribution of the sodium layer with high spatial and temporal resolution.
We describe the design of a new lidar experiment to investigate the spatio-temporal power spectra of the
Na-variations at frequencies as high as 50 Hz. This system employs a 5 W pulsed laser and a 6 m liquid mirror
telescope, which provide sufficient sensitivity for high-resolution studies. The transmitter is a YAG-pumped dye
laser, with an optical collimation system that allows the beam divergence to be controlled over a range from
diffraction-limited to several arcmin. This will also allow the investigation of saturation effects, important for the
next generation high power LGS systems. Backscattered photons will be collected at the prime focus using four
high-efficiency photomultiplier detectors and a fast counting system. The resulting system will provide vertical
density profiles with a spatial resolution as small as 2 m.
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