Direct visualization of bimodal-propagation-induced spatial self-imaging

Spatial self-imaging (SSI) consists of the periodic replication of the transverse profile of an optical beam along its propagation. It can be achieved in guided wave systems when all excited modes interfere in phase [1] . Remarkable is the case of graded-index multimode fibers, where the modes propagation constants are equally spaced, and the interference condition becomes trivial, giving rise to a periodic breathing of the field effective area [2] . In the singlemode propagation regime, SSI is by definition impossible. However, bimodal propagation can be attained in a nominal singlemode fiber (SMF-28), when operating in proximity of its cut-off wavelength. The latter imposes the condition for which only the fundamental mode is guided, so that higher-order modes (HOM) vanish over a certain damping distance (typically of about a few centimeters, depending on the wavelength). Therefore, effects associated to the presence of HOM can be present, over propagation lengths shorter than the damping distance.