Over 99% international data are transmitted over optical submarine cables. However, the main challenges of optical submarine communications are the feeding power and mechanical size limitation. Coupled Core-Multi Core Fibers (CC-MCF) are well-suited for submarine cable systems with physical space limitations, as they offer higher spatial multiplexing density and serve as an effective key transmission medium for expanding capacity. One of the critical impairments limiting the enhancement of transmission performance in CC-MCF is Mode Dependent Loss (MDL). This paper focuses on the mechanism of MDL in transoceanic transmission using coupled MCF, aiming to investigate the impact of Spatial Mode Dispersion (SMD) in heterogeneous CC-MCF on the estimation and mitigation of MDL.

Specifically, by establishing a coupled 4-core transoceanic transmission model and conducting self-consistent validation, this study explores the evolution of the accuracy of MDL estimation based on Multiple In Multiple Out (MIMO) taps under different SMD conditions. Additionally, it also investigates, for the first time, the MDL mitigation performance of a space-time coding technique based on the Hadamard transform under the influence of SMD.

The results indicate a low level of SMD coefficient below 10 ps/km is helpful to the efficient MDL estimation and mitigation. Moreover, the use of Space Time Coding (STC) has provided a 5.5 dB Q factor gain to our 4-core CC-MCF transmission system.

This paper provides a comprehensive analysis of the actual transmission characteristics of CC-MCF. Its findings offer valuable insights into the potential development of next-generation CC-MCF submarine cable communication technologies, particularly those aiming for high spatial multiplexing density.