THE BEST QUALITY OPTICAL BEAM BEYOND THE PARAXIAL APPROXIMATION

The propagation of tightly focused light beams is analyzed from the non-paraxial scalar theory for light beam propagation. In particular, we show how to minimize the width of the focused beam, measured with the second order moment width of the irradiance distribution, by appropriately selecting the beam profile and the beam divergence angle. The optimum profile and divergence, and the minimum waist width obtained are found. A quality factor for non-paraxial beams is then proposed and its lowest value found. These results are, on one hand, compared with the paraxial ones, on the other, are shown to remain valid to a good degree of accuracy when polarization effects are taken into account.