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Comment by staplung | Hacker News Reader

I'm no expert either but a complicating factor for undersea fiber-optic cables is that they need amplifiers every N kilometers. So even if the cable itself could theoretically support more bandwidth, the amplifiers might not. The amplifiers contain lasers of their own that fire at very specific wavelengths.

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> undersea fiber-optic cables is that they need amplifiers every N kilometers

Huh, I had no idea it needs to be done so frequently.

> These days optical cable repeaters are photon amplifiers that operate at full gain at the bottom of the ocean for an anticipated service life of 25 years.

> Repeaters are a significant cost component of the total cable cost, and there is a compromise between a ‘close’ spacing of repeaters, every 60km or so, or stretching the inter-repeater distance to 100km and making significant savings in the number of repeaters in the system. On balance it is the case that the more you are prepared to spend on the cable system the higher the cable carrying capacity.

https://blog.apnic.net/2020/02/12/at-the-bottom-of-the-sea-a...

https://www.rp-photonics.com/propagation_losses.html

https://www.rp-photonics.com/fiber_amplifiers.html

I never thought about it, but I’m assuming that means the cables also carry enough electricity to power the amplifiers?

I've read sharks see the cables as potential (ha!) prey due to slight EMF leakage. They chew on them damaging them. Not the fiber optic ones of course.

All trans-oceanic cables carry an electric current in addition to light in fiber(s). Electricity is what powers the optical amplifiers of which there needs to be one every hundred or so kilometers.

For short-ish cables (a couple hundred kilometers), driving the amplifiers via light (interestingly this can be done from both the data-sending and receiving end!) is possible, but not for trans-oceanic distances.

Yeah, the amplifiers need 10,000 volts. The way the cable is constructed is to have the fiber pairs inside a copper tube (there's some other stuff in between too). The voltage is carried by the tube. The ocean itself serves as the ground.

In general the power to the amplifiers is series connected and the ocean does not come into the question at all. If it did you would have huge issues with corrosion even ignoring the effects on the ocean itself. So, along the cable there is a wire that carries small DC current that goes between the amplifiers, each amplifier places zener diode along this wire and gets its power from it. At each landing station there is a current source (that is capable of developing significant open-circuit voltage) that powers this (this scheme is the reason why the cables are laid not only in loops, but in "loops-of-loops").

Correct

Okay, I did a little more digging and it seems you're right eszed! Even undersea cables can increase their bandwidth, at least in some cases; seems to be dependent on some modulation factors that I don't fully understand.

If I'm reading this right, BPSK - used on the longest transpacific cables - can't really be upgraded to have higher bandwidth but QPSK and other modulation schemes used for shorter distances can be (?).

BPSK and QPSK are modulation schemes, but the optical amplifiers used don't demodulate and re-modulate at all, so the modulation used is irrelevant.

Correct. Modulation is encoding the bits in different aspects of the light wave. The better the modulation scheme, the more expensive is the division multiplexing equipment.

Not really. I know of no cases where optical amplifiers posed a barrier to upgrading. Ciena and Infinera have been able to upgrade cable capacity by several X.

Optical amplifiers simply boost the signal. A photo is absorbed and then re-emitted with more power, but the same frequency and phase.

There is no optical/electrical/optical conversion.

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