Comment by pbmonster | Hacker News Reader

Comment by pbmonster | Hacker News Reader

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You're right about submarine fibers, but you seem to suggest that the pump light for the laser amplifiers is transmitted through the fiber from the cable landing point - like the technology discussed in the OP.

That is certainly not the case, the pump light is generated from electricity right where the laser amplifier sits in the fiber. No real amounts of energy are sent optically down the fiber. To power the amplifier, a high voltage DC line is designed right into the submarine fiber cable. And those things carry a lot of power, a long fiber cable will draw tens of kilowatts of DC for all the optical repeaters.

The reason is, of course, that thousands of miles of cable has a pretty insane optical attenuation, no matter what you do, because optical attenuation rises exponentially with length. The electrical resistance of a high voltage DC power line only rises linearily, on the other hand.

You're right about submarine cables running DC along the shielding/armor to power the optical amplifiers. However it's worth pointing out that there are so-called "repeater-less" systems that do use optical delivery of pump power to the amplifiers (typically they combine this with Raman amplification). Those systems can deliver high capacity communication (not sure where the record stands at but 100s of Gb/s to low Tb/s) over >500 km without any electrical connection (you still need power at the receiver though).

These are typically used for short submarine connections to e.g. connect an island. As it is much cheaper than running a full repeatered system.

Just to prove I never took physics, where are the photons actually going in a long distance undersea cable that makes it impossible to just flash a signal across an ocean sized length of fiber? (As I had assumed was the case.) Is it more a loss of clarity/resolution in terms of wavelength rather than the photons going astray?

Think of it as the same as the internal resistance of a conductor. Some of the photons are effectively absorbed by the fiber and turned into heat.

Thanks! I am embarrassingly so much a 'software' kind of person that this didn't occur to me, but I get the idea a bit better now.

> because optical attenuation rises exponentially with length

I believe the attenuation is stated in dB/km, therefore rises linearly (or even logarithmically if you look at it from the uncommon energy-wise point of view) with distance. Why should exponential be the case?

Imagine a length of fibre that transmits 90% of the light put in. Take the output, and pass it through another equal length. At the output of the second fibre length, we have 90% of 90%.

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