[crispyambulance]

The article says this cable uses SDM (space division multiplexing). Which, for fiberoptics, means that you have multiple fibers. Of course they HAVE TO put many wavelengths on each fiber, each wavelength carrying a signal.

The "state-of-the-art" AFAIK is to use many wavelengths per fiber, each one carrying ~192 wavelengths each wavelength transporting at up to 100Gbps (this is known as DWDM).

So so with SDM, you just have more fibers? So what? It seems like I am missing something here? Why is "SDM" the key concept rather than "DWDM"? Why not just say DWDM with 12 fiber pairs?

[aappleby]

I thought the same thing, but they really are sending N completely separate signals spatially separated at the transmitters, then deconvolving them (sort of) at the other end. Relies on very complicated structure inside the glass of the fiber.

[DoomHotel]

You can send spatially-separated signals down a single multi-mode fiber.

https://www.nature.com/articles/s41598-019-53530-6

[jisco]

It's not the case here. On their website, google states: ... Dunant is the first long-haul subsea cable to feature a 12 fiber pair space-division multiplexing (SDM) design ...

Multi-mode fibers are not feasible for long distance transmission. For long distance communications, using suggested approach, may be better to use multi-core fibers.

[crispyambulance]

That's interesting! But multimode fiber isn't feasible for thousands of kilometers? This is transatlantic. Wouldn't that have to be singlemode just for the distances involved?

[sp332]

Even single-mode needs repeaters along the length of the cable to get across an ocean. I guess you could use multimode and a lot more repeaters, but that seems more expensive and more failure-prone.

[aidenn0]

When I was first learning about fiber, graded-index multimode was the "hot new thing" with corning promising the modal-dispersion of single-mode fiber with the light-carrying capacity of multimode, which should reduce repeaters compared to either. Since these are single-mode fibers, I assume those promises were overstated?

[crispyambulance]

Yes, and the SDM as described in the nature article in parent^2, it would require a something far more complex than a repeater (which in most cases is actually just a purely optical amplifier).

Current practice is to use erbium doped fiber amplifier or raman amp for boosting optical signal at long intervals for transoceanic runs. Given the complexity of spatial signal, I don't think a regular optical amplifier will work? I could be wrong, this tech is changing but submarine fiber-optics tech is necessarily conservative and slow moving.