“…In long-haul MDM transmission systems, the recirculating loop schemes can also be applied by means of mode multiplexers and demultiplexers, and the length of the few-mode fibers (FMFs) used in the loops is usually less than 65 km [12][13][14][15][16] . In order to get rid of the complicated and expensive loop configuration, in the paper, we use the equivalent spectrum method [17] to experimentally simulate the DMG performance of the cascade FM-EDFA system, in which the process of multi-stage amplification is replaced by multiple measurements on the FM-EDFA. First, we fabricate a kind of new device, namely, optical isolation and wavelength division multiplexers (IWDMs), to build up all-fiber FM-EDFAs capable of flexibly supporting forward, backward, and bi-directional pumping.…”