“…By its symmetry, we keep the second-order term, and letting u(r→,Z,t)=ψ(r→,z,t)exp(iρR0P0z), Eq. (1) is transformed as follows: 26 , 28 i∂ψ∂Z+12(∂2ψ∂X2+∂2ψ∂Y2+∂2ψ∂T2)−P02Pcr2ψ=0.Equation (2) is the normalized dimensionless Snyder–Mitchell model, 2 where ψ(X,Y,Z,T) is the envelope of the optical field. (X,Y,Z,T) are the dimensionless coordinates.…”