About the correlation between the centre and the wings of a hard-apertured Gaussian beam

The paper considers the influence of near-field clipping upon the accuracy of angular divergence measurement of a laser beam. We have shown that power fraction that contain the centre and the wings of a diffracted beam are not necessarily correlated. This effect can lead to systematic important errors on laser beam divergence measurement even when the beam under consideration is weakly truncated. In particular, the monitoring of any variable lensing effect can be greatly affected by such effects since one could be mistaken about its sign (converging or diverging).