Means to find nonlinear modal fields using the finite difference beam propagation method

A method to construct modal fields for an arbitrary one-or two-dimensional intensity dependent refractive index structure is described. An arbitrary starting field is propagated along a complex axis using the slowly varying envelope approximation for the Finite Difference Beam Propagation Method. By suitably choosing the complex value of the propagation step, one mode is maximally increased in amplitude. The applicability of the method is discussed and illustrated by a one-dimensional cross section test example with Kerr-type nonlinearity