Free-space optics system operation in Asian cities under heavy dust loading conditions

Until recently atmospheric dust was not a major concern for free-space optics communication systems, however the heavy pollution and the recent increase in dust-storm events in Asia and particularly in China, has motivated the studies we present in this paper. In this paper we will discuss the impact of dust on free-space optics system operations for transmission wavelengths up to 10μm. We focus our studies on China as dust is normally present in its lower atmospheric regions, and since dust originating in China is transported over most of Asia during heavy dust storms. In general, heavy dust loadings consists of larger dust particles that are lifted from the ground and whose atmospheric concentrations can reach values that are high enough to interrupt the operation of free-space optics communication links. Similar to the characterization of fog conditions, dust storm studies are usually based on visibility data. However, in the case of mineral dust, visibility data are not sufficient to accurately predict the impact of dust on atmospheric transmissions. Transmission coefficients during dusty days cannot be simply calculated from visibility data for the entire range of useful laser wavelengths due to complex wavelength dependencies on mineral dust refractive indices. In order to test laser operating conditions in China, we ran extensive MODTRAN calculations of atmospheric transmissions for different dust conditions in the visible-IR range. Wavelength-dependent optical properties of atmospheric dust, which are required as input to the MODTRAN dust models, were calculated via Mie theory (spherical dust) using representative dust morphological and mineralogical properties such as refractive indices, size distributions and concentrations. We also tested an effect of particle nonsphericity at several wavelengths. We found that the effect of non-sphericity is small for direct beam transmissions, thought, it may become an important factor for diffused forward-scattered radiation. We found that changes in dust particle concentrations and dust particle size distributions during dust-storms storngly affect the transmission in the entire visible and IR wavelength range.