  We present a method of Gaussian-beam-mode decomposition to calculate
classical diffraction of optical beams by apertures. This method offers a
entirely different approach to examine the classic problem. Although our method
is based on a very straightforward setup, it is surprisingly effective. We
validate our method by comparing its results with those of Kirchhoff's full
diffraction formula. Not only does our method have a simple and organized
analytical framework, it also offers significant computational advantage.
