META's 1 Petabit Cable And The Evolution Of Spatial Division Multiplexing

In a recent interview META subsea engineers suggested their next project would be a 1 Petabit Trans-Atlantic cable. They pointed to three possible ways of accomplishing it: multicore fibre, use of the C and L spectrum bands, and 50 fibre pair SDM.

Pros and Cons

1. Multicore is subject to cross talk. Right now the only multicore cable is the Taiwan-Phillipines-US cable scheduled to go live this year. A Google project with NEC as the vendor, it has two cores per fibre strand and 26 Tbps total per fibre strand or 13 Tbps per core. The challenge is cross talk. The light spills from one core to the other and vice versa. Obviously this distorts the signals and at high transmission rates the distortions become greater and greater and the signal-to-noise ratio disintegrates. The advantage of multicore fibre is that no redesigning of the optical amplifiers is required. Since everyone acknowledges that SDM alone is unlikely to surpass the half terabit mark, Google is obviously interested in multicore as a way to expand SDM capacity as fibre counts eventually reach the limit of what is possible. Right now two core works. But multicore defined as 3 cores or more is still in development. Two core works because there is sufficient space to limit cross talk. But more cores mean less space between them and more interference. Perhaps a special material is needed to shield each core and prevent bleeding.

2. The other way to take SDM to the next level is to use the C and L bands. Engineers have told me that the challenge is that the optical amplifiers must be redesigned, are probably significantly more expensive, and will be bigger. It is necessary to split the light into the C and L band frequencies and send each band to a separate laser pump or laser pumps for amplification. So a lot more laser pumps would be required per amplifier as well as more power. An open question is whether there is enough space in standard sized repeaters to accommodate that many pump lasers. The PLCN cable uses the C and L bands to transfer data from Taiwan and the Philippines to the US. It uses 6 fibre pairs operating at 24 Tbps per pair or 12 Tbps for each band.

3. A third possibility would be sheer brute force by using 48 fibre pairs at 20 Tbps per pair. This would probably increase cable diameter beyond the acceptable range. It also might break the power budget. 

4. Both two core and multi-band technologies work. But there is little public information about costs or performance (bit error rates). Ironically, both TPU and PLCN connect Taiwan and the Philippines to the US. A comparison of the two cables that details construction costs, operating expense and network performance would be fascinating. The most likely outcome is that a cable would use a combination of all 3 approaches to achieve economically 1 terabit throughput.

Reading Material:

1. https://cloud.google.com/blog/products/infrastructure/delivering-multi-core-fiber-technology-in-subsea-cables