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

TLDR

Designers often assume 1550 nm light experiences less atmospheric attenuation than 785 nm or 850 nm, a belief supported by a common attenuation equation that holds in hazy conditions but fails in fog, where all wavelengths are attenuated similarly, underscoring the need for short lasercom links to meet high‑availability telecom demands. The study proposes a revised atmospheric‑attenuation equation that more accurately captures the wavelength‑independent loss observed in fog. The authors derived the modification through an extensive literature review and full Mie‑scattering calculations of laser propagation in fog. The results confirm that fog, heavy snow, and extreme rain cause wavelength‑independent attenuation, which can disrupt short (< 500 m) lasercom links.

Abstract

12 There is currently a misconception among designers and users of free space laser communication (lasercom) equipment that 1550 nm light suffers from less atmospheric attenuation than 785 or 850 nm 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. In hazy weather (visibility > 2 km), the prediction of less atmospheric attenuation at 1550 nm 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.