> 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. 


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