The present invention is a method and system for transmitting data between two objects via a free space optical data link. In an exemplary aspect of the present invention, the method may start with a step in which a laser beam modulated with data is transmitted to the vicinity of an optical collector. The laser beam may be transmitted by a modulated laser transmitter. The laser beam may cause atmospheric nitrogen in the vicinity of the optical collector to fluoresce and emit a set of characteristic line spectra. The set of characteristic line spectra may be received by the optical collector, and at least one characteristic spectral line may be obtained with an optical filter. Preferably, a wavelength of the at least one characteristic spectral line is selected from a group consisting of 3995.00 angstroms, 4630.54 angstroms, 5005.15 angstroms, and 5679.56 angstroms, which have high relative amplitude. The at least one characteristic spectral line may be detected and converted to electrical signal by a photodetector. The electrical signal may be amplified from a small signal to a large signal, and the amplified electrical signal may be processed with a communications receiver to obtain the data.