Mode coupling coefficients were measured in a multimode step-index fiber at different steps of the process of manufacturing a multistrand optical fiber cable. It was established that the mode coupling coefficients of the unsheathed fiber were relatively small and nearly the same for all the guided modes. By sheathing with nylon and by multistranding, the mode coupling coefficients between lower-order modes increased rapidly due to microbends, while in the vicinity of the highest-order mode, they remained unchanged. From impulse response waveforms and baseband frequency responses, it was observed that mode mixing effects became more noticeable as the mode coupling coefficients increased. Also, it was found that the excess loss caused by microbends was relatively small.
Kenichi Kitayama and Masahiro Ikeda, "Mode mixing effects in optical fibers caused by sheathing and multistranding: measurements," Appl. Opt. 17, 3660-3664 (1978)