How many times is a laser beam diffraction-limited?

Abstract

We present an approach to the definition and estimation of the “times diffraction limit” (TDL) of highly diffracted laser beams that takes into account the near- and far-field energy distribution of the real beam instead of an ideal Gaussian beam. We have used it to find the TDL factor of two hard-edge-unstable-resonator laser beams having different quality and spatial energy distributions. The results are compared with the widely used beam quality parameter M 2. The discrepancy between our TDL and the M 2 coefficient becomes significant when the near-field energy distribution of a spatially coherent beam deviates from the pure Gaussian shape.