Measurement of intermodal coupling in weakly multimode fibre optics

Using selective mode excitation via the cladding of an etched step-index weakly multimode fibre, and spatial mode separation at the output using a similarly etched section, intermodal coupling has been measured, mode-by-mode in the first seven guided modes of the fibre. Experimentally the observation has been made that most coupling terms remain less than -20 dB after 500 m of fibre.