EFFICIENT EXCITATION OF A MESOSPHERIC SODIUM LASER GUIDE STAR BY INTERMEDIATE-DURATION PULSES

Calculations of backscatter emission of mesospheric sodium atoms in a laser guide star that is excited by pulses ranging from 30-ns to 0.9-mu s duration are described. The efficient use of such pulses at saturating irradiance values is shown to require similar to 3 GHz of spectral broadening to provide access to the full absorption spectrum of the D-2 line. The broadening is provided by frequency modulation. A set of density matrices was used to account for all 24 hyperfine states and inhomogeneous Doppler broadening. At the broadband (3-GHz) saturation irradiance of 4 W/cm(2), both linearly and circularly polarized laser beams are shown to produce emission rates exceeding 60% of the maximum possible rate-equation rate for the 0.9-mu s pulses. As expected, circular polarization produced more backscatter than did linear polarization, but the enhancement never exceeded 1/3 in the calculations that are reported. A brief estimate of state precession in the Earth's magnetic field suggests that achieving even this enhancement will require that the time scale for optical pumping be held to less than 1 mu s, which will require the use of irradiances greater than 0.7 W/cm(2) and spectral coverage of the full 3-GHz hyperfine-plus-Doppler absorption profile, at least until most of the population is pumped out of the F = 1 ground states.