Abstract
Atmospheric effects have a significant impact on the performance of airborne and space laser systems. Traditional models used to predict propagation effects rely heavily on simplified assumptions of the atmospheric properties and their interactions with laser systems. In the engineering domain, these models need to be continually improved in order to develop tools that can predict laser beam propagation with high accuracy and for a wide range of practical applications such as LIDAR (light detection and ranging), free-space optical communications, remote sensing, etc. The underlying causes of laser beam attenuation in the atmosphere are examined in this paper, with a focus on the dominant linear effects: absorption, scattering, turbulence, and non-linear thermal effects such as blooming, kinetic cooling, and bleaching. These phenomena are quantitatively analyzed, highlighting the implications of the various assumptions made in current modeling approaches. Absorption and scattering, as the dominant causes of attenuation, are generally well captured in existing models and tools, but the impacts of non-linear phenomena are typically not well described as they tend to be application specific. Atmospheric radiative transfer codes, such as MODTRAN, ARTS, etc., and the associated spectral databases, such as HITRAN, are the existing tools that implement state-of-the-art models to quantify the total propagative effects on laser systems. These tools are widely used to analyze system performance, both for design and test/evaluation purposes. However, present day atmospheric radiative transfer codes make several assumptions that reduce accuracy in favor of faster processing. In this paper, the atmospheric radiative transfer models are reviewed highlighting the associated methodologies, assumptions, and limitations. Empirical models are found to offer a robust analysis of atmospheric propagation, which is particularly well-suited for design, development, test and evaluation (DDT&E) purposes. As such, empirical, semi-empirical, and ensemble methodologies are recommended to complement and augment the existing atmospheric radiative transfer codes. There is scope to evolve the numerical codes and empirical approaches to better suit aerospace applications, where fast analysis is required over a range of slant paths, incidence angles, altitudes, and atmospheric conditions, which are not exhaustively captured in current performance assessment methods.
Highlights
While absorption and scattering are well-covered as the dominant attenuation causes in most models, there are several outstanding gaps in both linear and non-linear propagation
Further considerations into continuum absorption, the assumptions of absorption line profiles and uncertainty regarding aerosol content and distribution are some of the areas highlighted towards the development of more representative propagation models
Lack of progress on non-linear propagation modelling is observed, with thermodynamic effects not being well-represented in present-day atmospheric transfer codes, as the effects are considered minimal in comparison to their linear counterparts and are an impediment to faster processing
Summary
Empirical models developed in the 1960s to 1970s are still widely used for laser propagation analysis [5,6] and constitute the basis of atmospheric radiative transfer codes, which are widely utilized for performance predictions. The effect of atmospheric properties on laser beam propagation is presented in some detail, covering the empirical methods that define the major sources of propagation This includes absorption and scattering of molecules and aerosols, atmospheric turbulence effects and non-linear behavior caused by thermal effects such as blooming and bleaching. The most popular of these electro-optical systems are passive visible to infrared imagining and active LIDAR for atmospheric sounding These techniques measure the molecular and aerosol components in the atmosphere but it can be difficult to model certain aspects of the propagation effects, which in turn affects accuracy. The key aeronautical and space applications for laser systems and the underlying principles are summarized below
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