Propagation of Single-Mode Fibre Laser Beams through an Optical ABCD System with Circular Aperture at the Fibre Output End

Based on the expansion expression of the fundamental mode of a single-mode fibre in terms of Laguerre--Gauss modes, the propagation of a beam of a weakly guiding fibre laser through an optical ABCD system with a circular aperture at the fibre end is studied. The results show that there is much
difference between the propagation of the laser beam described by the
expansion expression and by the Gaussian mode approximation. The depth of focus of the laser beam is longer than that of the Gaussian modes.