Propagation of nonparaxial fields by parabasal field decomposition

The propagation of harmonic fields through homogeneous media is an essential simulation technique in optical modeling and design by field tracing, which combines geometrical and physical optics. For paraxial fields the combination of Fresnel integral and the Spectrum of Plane Waves (SPW) integral solves the problem. For non- paraxial fields the Fresnel integral cannot be applied and SPW often suffers from a too high numerical effort. In some situations the far field integral can be used instead, but a general solution of the problem is not known. It is useful to distinguish between two basic cases of non-paraxial fields: 1) The field can be sampled without problems in the space domain but it is very divergent because of small features. A Gaussian beam with large divergence is an example. 2) The field possesses a smooth but strong phase function, which does not allow its sampling in space domain. Spherical or cylindrical waves with small radius of curvature are examples. We refer to such fields as fields with a smooth phase term. The complete phase is the sum of the smooth phase term and the residual. For both cases we present a parabasal field decomposition, in order to propagate the field. In the first case we perform the decomposition in the Fourier domain and in the second case in the space domain. For each of the resulting parabasal fields we separate a linear phase factor which has not to be sampled. In order to propagate the parabasal fields we present a rigorous semi-analytical SPW operator for parabasal fields, which can handle the linear phase factors without sampling it at any time. We show that the combination of the decomposition and this modified SPW operator enables an ecient propagation of non-paraxial fields. All simulations were done with the optics software VirtualLab™.