The throughput capacity of a single-mode fiber optic cable (1,550 nm) is 20 Tb/s (20,000 Gb/s).
Carrying Capacity (aka Shannon's Law) = W x log2(1+s/n)
W = bandwidth (Hz) (assumed to be 12.5 THz in this case)
s/n is signal-to-noise ratio (s/n value is typically 20 dB for optical fiber, making the ratio equal to 2)
Carrying Capacity = 12.5 THz x log2(1+2)
= 12.5 x 1.6
= 20 Tb/s
Sources: About Fiber Optics (http://www.aboutfiberoptics.com), NetClear on Shannon's Law (http://www.netclear-channel.com/shannonslaw/Default.htm)