  It is discussed the limitations of the widely used markovian approximation
applied to model the turbulent refractive index in lightwave propagation.
  It is well-known the index is a passive scalar field. Thus, the actual
knowledge about these quantities is used to propose an alternative stochastic
process to the markovian approximation: the fractional Brownian motion. This
generalizes the former introducing memory; that is, there is correlation along
the propagation path.
