In recent years

Mode Division Multiplexing（MDM）techniques have been widely considered as an effective approach to break out the capacity bottleneck of conventional single-mode optical-fiber communication systems and networks.The typical schemes include strongly-coupled and weakly-coupled ones.In a weakly-coupled MDM scheme

the modal crosstalk should be suppressed as much as possible.Therefore

a signal in each mode can be transmitted and received independently.In this paper

we first introduce the technical principles of strongly-coupled and weakly-coupled schemes.Then a weakly-coupled Few-Mode Fiber（FMF）with double-ring core structure

together with the mode Multiplexer（MUX）and Demultiplexer（DEMUX）based on mode-selective coupler

are designed and fabricated.Next

transmission experimental demonstration based on 6-mode10-km weakly-coupled MDM is achieved with intensity modulation and Digital-Signal-Processing-free（DSP-free）direct detection.A degenerate mode demultiplexing scheme is designed

which is then fabricated based on the heating and tapering method.Finally

the first real-time video MDM transmission is realized

which indicates that a big step has been made for the MDM technique towards practical implementation.