Laser-pulse fluctuations due to thermal self-action in a turbulent atmosphere

Conclusions

Refractive-index fluctuations caused by heating alter the intensity and phase fluctuation spectra arising from the turbulence. The large-scale part of the spectra is weakened, whereas the small-scale part is strengthened, and the spatial-frequency range in which the fluctuations are attenuated becomes narrower as the beam energy increases. Correspondingly, the fluctuation structure functions alter. For values of the arguments ρ in those functions less than the radius of the first Fresnel zone$\sqrt {L/2k}$, the thermal nonlinearity causes increases in the structural functions D_{ϕ, χ} with the energy. For ρ ≫$\sqrt {L/2k}$, the dependence of D_{ϕ, χ} on energy is nonmonotone, with the fluctuations at first weakened but then strengthened. The reduction in fluctuation variance and the improvement in coherence is less pronounced over long lines than on short ones. The range in which geometrical optics can be used to derive the fluctuations is independent of the nonlinearity and is defined by k₂z/2k ≤ π/4. The trend to the diffraction asymptote becomes slower as the nonlinearity increases.