Bichromatic two-photon coherent excitation of sodium to provide a dual-wavelength guidestar.

A bichromatic sodium guide star based on two-photon coherent excitation followed by cascade emission is considered. Results of density matrix computations, including Doppler averaging, are presented for both a three-level model and the full hyperfine structure of the sodium $3S$, $3P$, and $4S$ levels. These results indicate that the proposed scheme for a bichromatic guide star is feasible.