Bending loss in multimode fibers with graded and ungraded core index

Parabolic grading of the core index in a multimode fiber (Selfoc) diminishes mode dispersion and interface loss. This paper shows that this grading affects the mode volume and the loss in bends very little, if the index difference of the graded core (between the core axis and the cladding) is twice that of the homogeneous core. Curvature radii of several centimeters are tolerable. Mode coupling (or ray deflection) in random bends is slightly decreased by grading. Both the graded and the homogeneous multimode fiber are particularly sensitive to certain critical deviations of the guide axis from straightness. These deviations must be less than avoided. a fraction of a micrometer in order that catastrophic mode loss be avoided.