> Nobody yet knows the theoretical max as you can jack up the power
> until things start melting and get more bandwidth.
Not quite. You can't just jack up the power, unless you also manage to
improve your SNR. The maximum theoretical bandwidth of singlemode fiber is
dependent on SNR and whether the channel is linear or not.
Assuming a SNR of 30dB and a linear channel, the maximum bandwidth of
singlemode fiber is about 100 Tbps.
This is for the usable parts of infrared light (amplifiable C and L bands in
single mode fiber). This is about 10 Thz. According to Shannon's theorem the
theoretical upper limit for symbol rate in a linear channel with 30db SNR is 10
bits/s/Hz, independent of modulation format. Multiplying, we get 100 Tbps.
If you can up the SNR, use non-linear effects or use more spectrum then your
bandwidth goes up.
Current commercial products provide about 10 Tbps per fiber pair. Google does
this on the FASTER subsea cable with 100 DWDM waves at 100 Gbps. The Internet2
uses 88 DWDM waves at 100 Gbps.
However, the optical channel is not the only limiting factor. One of the
reasons why 100 Gbps is where we are at right now is that electronics have
trouble keeping up with faster switching speeds. That's why there is so much
effort to go all optical on all levels of networking.