[xlevus]

I'd wager it's even simpler than that.

At some point the signal must

a) be converted from electrical to optical

b) be boosted down the wire

c) be converted from optical to electrical

Easiest solution? Tap in at A, B or C.

Who's going to notice that your repeater at the bottom of the ocean actually has a NSA listener plugged into its debug socket?

[marshray]

No, actually in modern systems the light signal is boosted without an intermediate conversion to an electrical signal: http://en.wikipedia.org/wiki/Optical_amplifier

[beloch]

Some types of optical amplifiers are vulnerable to side channel attacks, so they might be a weak point. However, they're also probably under more scrutiny from both maintenance and security people.