Fiber Technology For Subsea Networks
Today vs. Tomorrow
By Sergei Makovejs and John Hedgpeth
March 26, 2021
Today’s Submarine Fiber Outlook:
The subsea fiber and cable market continues to show strong demand, and multiple tens of new subsea projects are expected over the next few years, with a substantial portion of them representing long-haul routes. For these routes, space division multiplexing (SDM) with ?12 fiber pairs per cable is rapidly becoming a de-facto technology to achieve lower cost/bit compared to traditional, non-linear systems.
The main premise of SDM design has not changed much since its inception and it continues to use lower fiber launch powers compared to traditional “non-linear optimum” to achieve linear or semi-linear transmission regime (Fig. 1). In this figure, generalized signal-to-noise ratio (GSNR) is an industry metric frequently used to determine the performance of the wet plant for open subsea cables. In the presence of inherent electrical power constraints, the reduction of launch power leads to lower capacity per fiber. However, this is offset by the ability to design cables with higher fiber counts, ultimately achieving higher overall cable capacity. As a rule of thumb, cost-optimized SDM design favors semi-linear transmission regime, whereas power-optimized SDM design pushes the fiber launch power into a more linear regime.
These design features have important implications for submarine fiber itself. To meet the needs of today’s SDM designs, two optical fiber characteristics are often in the center of discussion: attenuation and effective area. Ultra-low attenuation continues to play a key role, and every 0.001 dB/km improvement is essential in the pursuit to further reduce overall cost per bit. Conversely, for SDM designs, effective area (Aeff) moves lower in the decision-making process for fiber choice. While non-SDM systems favored both ultra-low loss and ultra-large Aeff (e.g. 150 µm2) fibers, SDM systems typically gravitate towards 80 – 115 µm2 Aeff. This is because in SDM systems the value of fiber Aeff is somewhat diminished, while value of bend performance is significantly augmented. The latter is particularly important given that bend performance tends to deteriorate with Aeff. In addition, a transition from 80 to 115 µm2 Aeff provides a meaningful decrease in attenuation (approximately 0.01 dB/km), while beyond ~115 µm2 Aeff there is little or no further improvement in attenuation. For all those reasons, 80 – 115 µm2 seems to be a sweet spot for submarine SDM systems, with the specific choice of 80 vs. 115 µm2 Aeff being frequently driven by several inter-dependent, techno-economic factors.
Near Future View of Submarine Fiber:
As the industry continues to progress on the SDM trajectory, a drive towards fiber count densification in a cable to achieve further reduction in system cost per bit will continue. An expectation for the future is that both 80 and 115 µm2 Aeff fibers will continue to play an important role, but now in thinner 200 µm coating variants (Fig. 2) as well as the regular fiber outer diameter. The fiber choice will likely be decided on a project-by-project basis, using similar techno-economic arguments as the ones used today. As further advancements are made in SDM design, attenuation will remain the key attribute, and every 0.001 dB/km of attenuation reduction will continue to matter.