TABLE OF CONTENTS The angular spread, ?, (half angle of a cone) of a beam projected along an atmospheric path is [1], [2]
| where |
 | |
| a | = beam radius when projected |
The parameter ? is the transverse coherence distance and is related to the effect that the turbulence of the atmosphere has on propagation of light. For situations in which the atmosphere is uniform in properties over the path length, we find that the coherence distance is [2]
| where | L | = path length |
| C n 2 | = refractive index structure constant, as further defined in the " C n 2 Estimates rule" (p. 52) |
For situations in which the atmosphere is not uniform, additional computations, shown in the references, must be performed.
Here, we have made some assumptions to avoid introducing a complicated calculation. The first term is the beam spread caused by diffraction. The second term is the additional spreading associated with turbulence effects.
This rule comes from a combination of the analytic description of laser beam propagation in a vacuum (the diffraction component) along with a simplified assumption about the way that other beam spreading effects, such as the atmosphere, add to the theoretical beam spread.
These estimates of beam size are as good as the quality of the estimates of C n 2, as the description of beam spreading derived from the theory of laser resonators is a mature science.
It should be noted that various authors use two different...
