SPATIAL DIVISION MULTIPLEXING IN POWER-CONSTRAINED SUBSEA CABLES By Jose Chesnoy & Jean-Christophe Antona
SDM is a system‑level optimization: constrained by cable limits on electrical power, and by nonlinear physics and Shannon’s capacity law per fibre, the most effective way to scale transoceanic capacity is to add spatial and frequency channels (more fibre pairs) and operate each channel at low optical power and OSNR. It enables to yield a higher total
74
capacity per cable and a lower cost per bit for long‑haul links, with already a doubling of capacities across the Atlantic ocean.
WHY THE INDUSTRY MOVED FROM “MORE POWER PER FIBRE” TO “MORE FIBRES PER CABLE” Submarine cables carry the vast majority of global Internet traffic. Over the last decade the dominant strategy for increasing cable capacity shifted from squeezing more bits into each fibre to multiplying the number of fibres inside a single cable with less bits per fibre, while consuming the same
SUBBTEL FORUM | Issue 148
flow of electrons and minimizing the cost per bit. That architectural change — Space Division Multiplexing (SDM) — is not an optical fad but the direct consequence of three hard physical limits: the electrical power a cable can safely carry, the nonlinear physics inside optical amplifiers and fibres, and the information‑theory ceiling described by Shannon.
WHAT SDM ACTUALLY IS (IN PRACTICAL TERMS) The SDM paradigms consists in optimizing the capacity and the cost of a multi-fibre cable under electrical powering constraints to meet the customer demand, rather than trying to extract every last bit from a single or a few fibres. To do so, new degrees of freedom are unfrozen such as fibre count and fibre type, along with a smart management of powering (with sharing of amplifier pump across multiple fibres, aka pump “farming” or “sharing”). This leads to