Power EDFAs with optical power?

I'm not exactly the most skilled guy when it comes to submarine cables so I need some help here. I'm looking at the design for a submarine link where they amplify optical noise twice, send it to the WSS and fill up the whole C-band with pure amplified noise. Then when they want to add capacity they use another WSS, and replace the noise WL in the C-band with the one carrying traffic. This made zero sense to me before I was told that all the noise was used to power the EDFAs. How?? Since it's passive (I think?) my best guess is that the erbium doped coil works best when it's getting hit with full power? Sounds a bit weird though so if someone could explain this to me as if I were 5 years old that would be much appreciated.. Thanks in advance!