Exact Paraxial Quantization
  A non-perturbative quantization of a paraxial electromagnetic field is achieved via a generalized dispersion relation imposed on the longitudinal and the transverse components of the photon wave vector. The theoretical formalism yields a seamless transition between the paraxial- and the Maxwell-equation solutions. This obviates the need to introduce either "ad hoc" or perturbatively-defined field operators. In the limit of narrow beam-like fields, the theory is in agreement with approximated quantization schemes provided by other authors. 
A. Aiello and J. P. Woerdman