  The evolution of classical optical fields propagating through atmospheric
turbulence is investigated under arbitrary conditions. We use the single-phase
screen (SPS) method and the infinitesimal propagation equation (IPE), a
multi-phase screen (MPS) method, to compute the optical power fractions
retained in an input Laguerre-Gauss (LG) mode or transferred to higher order LG
modes. Although they show the same trend while the scintillation is weak, the
IPE and SPS predictions deviate when the strength of scintillation passes a
certain threshold. These predictions are compared with numerical simulations of
optical fields propagating through turbulence. The simulations are performed
using an MPS model, based on the Kolmogorov theory of turbulence, for different
turbulence conditions to allow comparison in both weak and strong
scintillation. The numerical results agree well with the IPE results in all
conditions, but deviate from the SPS results for strong scintillation.
