Comparison of laser beam propagation at 785 nm and 1550 nm in fog and haze for optical wireless communications

There is currently a misconception among designers and users of free space laser communication (lasercom) equipment that 1550 nn light suffers from less atmospheric attenuation than 785 or 850 nn light in all weather conditions. This misconception is based upon a published equation for atmospheric attenuation as a function of wavelength, which is used frequently in the free-space lasercom literature.(1,2) In hazy weather (visibility > 2 lan), the prediction of less atmospheric attenuation at 1550 nn is most likely true. However, in foggy weather (visibility < 500 m), it appears that the attenuation of laser light is independent of wavelength, ie. 785 nm, 850 nm, and 1550 nm are all attenuated equally by fog. This same wavelength independence is also observed in snow and rain. This observation is based on an extensive literature search, and from full Mie scattering calculations. A modification to the published equation describing the atmospheric attenuation of laser power, which more accurately describes the effects of fog, is offered. This observation of wavelength-independent attenuation in fog is important, because fog, heavy snow, and extreme rain are the only types of weather that are likely to disrupt short (< 500 m) lasercom links. Short: lasercom links will be necessary to meet the high availability requirements of the telecommunications industry.