  The performance of adaptive optics systems employing sodium laser guide stars
can be improved by continuously monitoring the vertical density structure of
mesospheric sodium along the line of sight. We demonstrate that sodium density
profiles can be retrieved by amplitude modulation of continuous wave (CW)
lasers. In an experiment conducted at the Large Zenith Telescope (LZT), ESO's
Wendelstein Raman-fibre laser was amplitude-modulated with a pseudo-random
binary sequence and profiles were obtained by cross-correlation of the
modulation pattern with the observed return signal from the laser guide star.
For comparison, high-resolution profiles were obtained simultaneously using the
lidar system of the LZT. The profiles obtained by the two techniques show noise
contamination, but were found to agree to within the measurement error. As a
further check, a comparison was also made between several lidar profiles and
those obtained by simultaneous observations using a remote telescope to image
the laser plume from the side. The modulated CW lidar technique could be
implemented by diverting a small fraction of the returned laser light to a
photon counting detector. Theoretical analysis and numerical simulations
indicate that, for 50 per cent modulation strength, the sodium centroid
altitude could be retrieved every 5 s from a single laser guide star, with an
accuracy which would induce a corresponding wavefront error of 50 nm for the
ELT and less than 30 nm for the TMT and GMT. If multiple laser guide stars are
employed, the required modulation amplitude will be smaller.
