A Note on the Deterioration of the Coherence Properties of a Laser Beam by Molecular Scattering


  • Eli Brookner


A loss in monochromaticity, or equivalently coherence, occurs when a laser beam is propagated through the atmosphere. One factor causing the loss in coherence is the scattering from the randomly moving atmosphere molecules. In this short note, the effect on a communications system's performance of the loss of coherence arising from molecular scattering is studied. The results are given for the propagation, over a horizontal path of length L, of an ideal-monochromatic, collimated laser beam with a radius R with LR and with the geometric optics condition (λ L)½R satisfied (where λ is the laser wavelength). For the examples studied, it is shown that molecular scattering gives rise to only an inconsequential background noise in the receiver for typical communication systems. In particular, for λ = 1 μm and R = 5 cm and 5 m, the background noise is 74 and 54 db down, respectively. It will be necessary, however, for the communications engineer (and the radar engineer as well) to examine his particular case to ensure that the effects of molecular scattering are negligible or, if they are not negligible, that they are accounted for.