We describe experimental results and a theoretical analysis for propagation in graded-index multimode fiber when diode laser light is launched into the lowest-order propagation modes and the fiber undergoes severe bending perturbations, Experimentally, near-field modal interference images and transmission loss measurements were obtained for different loop diameters. The data indicate that, when the fundamental mode is excited, the light remains in lowest-order modes even for small bend diameters. This is consistent with analysis which predicts that, in a parabolic-index multimode fiber subject to constant diameter bending, the light tends to oscillate between lowest-order modes and remains trapped therein rather than diffusing to high-order modes, implications of these results for an intrusion-resistant communication system with graded-index multimode fiber are discussed. (C) 2000 Optical Society of America.
