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The performance of adaptive optics systems employing sodium laser guide starscan be improved by continuously monitoring the vertical density structure ofmesospheric sodium along the line of sight. We demonstrate that sodium densityprofiles can be retrieved by amplitude modulation of continuous wave (CW)lasers. In an experiment conducted at the Large Zenith Telescope (LZT), ESO'sWendelstein Raman-fibre laser was amplitude-modulated with a pseudo-randombinary sequence and profiles were obtained by cross-correlation of themodulation pattern with the observed return signal from the laser guide star.For comparison, high-resolution profiles were obtained simultaneously using thelidar system of the LZT. The profiles obtained by the two techniques show noisecontamination, but were found to agree to within the measurement error. As afurther check, a comparison was also made between several lidar profiles andthose obtained by simultaneous observations using a remote telescope to imagethe laser plume from the side. The modulated CW lidar technique could beimplemented by diverting a small fraction of the returned laser light to aphoton counting detector. Theoretical analysis and numerical simulationsindicate that, for 50 per cent modulation strength, the sodium centroidaltitude could be retrieved every 5 s from a single laser guide star, with anaccuracy which would induce a corresponding wavefront error of 50 nm for theELT and less than 30 nm for the TMT and GMT. If multiple laser guide stars areemployed, the required modulation amplitude will be smaller.

Measuring line-of-sight sodium density structure using laser guide stars