Briefly, at an altitude of about 90 km there is a layer of sodium which is about 10 km thick. This sodium has a resonant transition at about 590 nm. This wavelength is near the maximum emission of high-power dye lasers using rhodamine 6G dye. When the sodium is illuminated at this wavelength it produces an emission. This emission is what is called a laser guide star. This is used, in conjunction with adaptive optics, to correct for atmospheric turbulence at large terrestial telescopes.
High-power low-beam-divergence narrow-linewidth tunable lasers, are useful in a wide range of fields, from atomic vapor laser isotope separation (AVLIS) to laser guide star
in astronomy. Here, references relevant to tunable laser development for laser guide star applications are given.
D. H. Titterton, in Handbo ok of Laser Technology and Applications (Institute of
Physics, London, 2004).
J. Pique and S. Farinotti, Efficient modeless laser for a mesospheric sodium laser guide star, J. Opt. Soc. Am. B 20, 2093-2101 (2003).
F. J. Duarte, Multiple-return-pass beam divergence and the linewidth equation, Appl. Opt. 40, 3038 - 3041 (2001).
I. L. Bass, R. E. Bonanno, R. P. Hackel, and P. R. Hammond,
High-average-power dye laser at Lawrence Livermore National Laboratory,
Appl. Opt. 33, 6993-7006 (1992).
C. A. Primmerman et al., Compensation of atmospheric optical distortion using a synthetic beacon, Nature 353, 141-143 (1991).