  Here we outline a description of paraxial light propagation from a modal
perspective. By decomposing the initial transverse field into a spatial basis
whose elements have known and analytical propagation characteristics, we are
able to analytically propagate any desired field, making the calculation fast
and easy. By selecting a basis other than that of planes waves, we overcome the
problem of numerical artefacts in the angular spectrum approach and at the same
time are able to offer an intuitive understanding for why certain classes of
fields propagate as they do. We outline the concept theoretically, compare it
to the numerical angular spectrum approach, and confirm its veracity
experimentally using a range of instructive examples. We believe that this
modal approach to propagating light will be a useful addition to toolbox for
propagating optical fields.
