Single-mode excitation of multimode fibers with ultrashort pulses

Single-mode excitation of step-index multimode fibers with light sources with short temporal coherence lengths is demonstrated. Multimode fiber designs with reduced microbending-induced mode coupling are described that allow the propagation of the fundamental mode over long lengths with negligible mode coupling even in the presence of tight fiber bends. At a wavelength of 1.56 mu m a fiber with a core diameter of 45 mu m can preserve the fundamental mode for a propagation length of similar to 20 m, Such fibers allow coiling with a coil diameter as small as 7 cm. (C) 1998 Optical Society of America.