A Reconsideration of the Best Wavelengths for Free-Space Optics

The selection of the "optimal" operating wavelength for Free Space Optical (FSO) systems has been a subject of some ongoing controversy over the past several decades. Practical FSO systems have been found to suffer severe performance degradation in adverse atmospheric visibility conditions (high extinction/low-transmission) such as fog, haze, and other atmospheric aerosols (smoke, dust). Claims have been made that certain wavelengths offer generally superior performance and reduced attenuation for FSO system operation. We will revisit the problem of optical propagation through atmospheric particulates, and will show that the specific details of the selected aerosol size distribution function (SDF), which specifies the aerosol number density distribution by radius, and the corresponding wavelength-dependent complex refractive indices can significantly influence the total extinction/transmission behavior of various wavelengths and hence the choice of "optimal" wavelength. We will use a variety of realistic atmospheric SDFs to highlight the sensitivity of the "optimal" wavelength to the SDF composition details. A primary result will be a comparison illustrating extinction performance at selected wavelengths across the spectrum of visible to LWIR for a variety of realistic and clearly-defined atmospheric scenarios: urban, desert, maritime, with fogs, hazes, smoke, and dust.