Weakly-coupled mode division multiplexing (MDM) technique is considered a promising candidate to enhance the capacity of an optical transmission system, in which mode multiplexers/demultiplexers (MMUX/MDEMUX) with low insertion loss and modal crosstalk are the key components. In this paper, a low-modal-crosstalk 4-mode MMUX/MDEMUX for the weakly-coupled triple-ring-core few-mode fiber (TRC-FMF) is designed and fabricated with side-polishing processing. The measurement results show that a pair of MMUX/MDEMUX and 25 km weakly-coupled TRC-FMF MDM link achieve low modal crosstalk of lower than -17.5 dB and insertion loss of lower than 11.56 dB for all the four modes. Based on the TRC-FMF and all-fiber MMUX/MDEMUX, an experiment for 25 km real-time 4-mode 3-λ wavelength division multiplexing (WDM)-MDM transmission is conducted using commercial 400G optical transport network (OTN) transceivers. The experimental results prove weakly-coupled MDM techniques facilitate a smooth upgrade of the optical transmission system.

Low-cost, flexible and intelligent optical performance monitoring and management is a key enabling technology for network quality guarantee, especially in the era of explosive growth of communication capacity and network scale. However, to the best of our knowledge, it is extremely challenging to implement real-time performance monitoring and operations, administration and maintenance (OAM) in a highly complex dynamic network. In this paper, we propose an innovative optical identification (OID) scheme that can realize both performance monitoring and some advanced OAM sub-functions. The basic concepts, applications, challenges and evolution directions of this OID tool are also discussed.