An alternate way to obtain the aberration expansion in Helmholtz Optics
  Exploiting the similarities between the Helmholtz wave equation and the Klein-Gordon equation, the former is linearized using the Feschbach-Villars procedure used for linearizing the Klein-Gordon equation. Then the Foldy-Wouthuysen iterative diagonalization technique is applied to obtain a Hamiltonian description for a system with varying refractive index. Besides reproducing all the traditional quasiparaxial terms, this method leads to additional terms, which are dependent on the wavelength, in the optical Hamiltonian. This alternate prescription to obtain the aberration expansion is applied to the axially symmetric graded index fiber. This results in the wavelength-dependent modification of the paraxial behaviour and the aberration coefficients. Explicit expression for the modified coefficients of the aberration to third-order are presented. Sixth and eighth order Hamiltonians are derived for this system. 
Sameen Ahmed Khan