Large-bandwidth, low-loss, efficient mode mixing using long-period mechanical gratings.

We propose a new architecture for using long-period fiber gratings (LPGs) to induce strong mode mixing with low loss for space-division multiplexing. In this architecture, LPGs are installed in step-index (SI) few-mode fibers that support more modes than the transmission fiber. Such a design could significantly reduce losses due to coupling from the highest-order mode group to cladding modes. In our experiment, efficient mixing of three spatial modes over a broad bandwidth was achieved by a mechanical long-period grating on a SI fiber that supports eight spatial modes. The insertion loss, including two splice losses, is less than 0.5 dB, and the coupling matrix and mode-dependent loss (MDL) are characterized experimentally for the first time, to the best of our knowledge. Strong mixing between LP01 and LP11 for a whole C band is demonstrated, and MDL introduced to the system is negligible.