Although, the technological breakthroughs such as WDM had allowed the capacity per fiber to be increased around tenfold every four years in the past decade, however, the capacity of the optical communication systems based on these transmission technologies is slowly becoming saturated. To satisfy the exponential growth of the Internet traffic, for the next generation short reach systems, including data center transmission and optical interconnect (OI) applications, the space-division multiplexing (SDM) can be a way forward. The SDM technology based on the multicore fiber (MCF) has recently attracted much attention as a potential approach. In this paper, design strategy of computer-compatible 8-core trench-assisted MCF (TA-MCF) is presented to reduce the intercore crosstalk. Moreover, the influence of butt-coupled TA-MCF OI on coupling loss is also discussed. On the other hand, another alternative approach, the mode division multiplexing (MDM) is also showing promise and mode (de)multiplexer is one of the key devices in such a MDM system. Designs of mode splitters using asymmetric directional couplers for the fundamental quasi-TE (TM) mode with the higher order quasi-TE (TM) modes (de)multiplexer including the Hy21 (Hx21), Hy31(Hx31, Hy41(Hx41, and Hy51(Hx51) modes are optimized by using a full-vectorial H-field finite element method.