The atmosphere has a profound impact on the passage of light, whether this is the starlight imaged by a telescope or the beam from a laser weapon or optical communications link. This is commonly experienced as the shimmering effects of heat haze on optical imagery and the mirage of an apparently wet road on a hot day. These effects are a consequence of atmospheric turbulence; convective eddies and temperature gradients which are manifest as refractive index inhomogeneities that deflect different parts of the beam by different amounts.

This is an extremely complex phenomenon, but it is necessary to model the effects in order to assess the consequences on optical system performance. At the detailed level so-called wave optics models allow a snapshot of the beam to be created, providing information on the spatial structure of the beam and its dynamic behaviour in time (Figure 1). However, such models are computationally intensive and time consuming and systems studies are for the most part based on models of the time averaged behaviour of the beam. Typically, these consider the overall effects on the beam to be a combination of an inherent “broadening”, arising from the small-scale structure of the atmosphere, and “beam wander” a random jitter of the beam which, averaged over time, effectively broadens it further.

LumOptica’s parametric model predicts both the overall spread and the beam wander component, such that the benefits of correcting the latter by tracking the centroid of the beam may be assessed. The model provides the ability to assess the relative importance of systems parameters such as the wavelength, laser quality, optical aperture and pointing accuracy in the presence of varying degrees of atmospheric turbulence (as described by the refractive index structure parameter Cn2). Unlike many such models, which assume that the atmospheric turbulence is uniform along the propagation path (i.e. a horizontal beam), the LumOptica model incorporates a description of the height profile of the turbulence and is therefore able to predict the differing effects of whether the beam is propagating upwards or downwards.

NEW Laser Propagation App
To help visualise the effects of the atmosphere on laser propagation LumOptica has produced an associated “App“, based on a simplified version of our propagation model. This allows the user to “target” a drone with an interactive display of the effects on the beam in response to changes in the relative positions (in height and range) of the laser and target under various levels of turbulence. The App also allows the user to vary key systems parameters, such as the initial beam diameter and laser quality.

This is a simplified model intended to provide an indication of the magnitude of the beam spreading and wander effects. However, LumOptica advises against using these indicated values in any formal design exercise and cannot accept responsibility for any consequent conclusions. For more detailed analyses, please contact us.