Na guide stars are the standard. OTHER METHODS HAVE BEEN PROPOSED AND EVEN A FEW HAVE BEEN USED ooops sorry all caps....

Techniques that dont rely on atomic transitions have also been suggested and tried. However the Na guidestar is preferable for many reasons.

Other layers of different elements also exist in the mesosphere, the sodium layer was chosen due to the strength of the sodium absortion/emission line at 589 nm (yellow/orange). There are iron and nickel layers up there but they have no transitions at reasonable wavelengths with the large cross-section that sodium does.

589 nm has been a difficult wavelength for lasers to reach. Until recently the most practical way to reach this wavelength was with dye lasers, like the one I work on. The VLT is installing a dye laser. Dye lasers are complex and inefficient. The laser I work on consumes ~50 kW and puts out ~15 W of usable laser light at 589 nm- it is essentially 9 laser subsystems that all have to work together for the whole system to work.

Recently, however, due in large part to research funded by the astronomical community and the air force, solid state lasers have been developed at this wavelength. This new generation of "sodium" lasers relies on combining laser beams of two different frequencies in a nonlinear crystal to sum the wavelengths to get 589 nm.

Solid state lasers are far more efficient, the Gemini laser only consumes a few kilowatts and is a small fraction of the size of the dinosaur I work on. Other lasers are being worked on that will be even more efficient and compact.

I had never worked on a dye laser before I came here, and I sincerely hope I never work on one again. They are ugly....

Bringing this back to the topic of the week, the altitude of the Na layer is sensitive to the average temperature of of the lower atmosphere- a trend of falling altitude has been seen over the few decades the layer has been monitored. Higher temperature = lower altitude.