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I'm curious as to where you came up with your symbol rate numbers. The theoretical limit[1] is 2 bits/Hz which for infrared light (~400THz) would put the limit at ~800Tbps.

[1] https://en.wikipedia.org/wiki/Nyquist_rate

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The Nyquist signaling rate only applies to binary/two-level signals. Most optical communications systems use many levels to improve the data rate. StackExchange discussion: http://electronics.stackexchange.com/questions/21854/why-is-...

Ahhh, that makes sense! I wasn't aware that the Nyquist rate only applied to binary signal. Thanks for the SE link.

The usable parts of infrared light (amplifiable C and L bands in single mode fiber) 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. This is how I derived my rough estimate of the capacity of single mode fiber.

The Nyquist rate is modulation dependent. Optical fiber can certainly achieve spectral efficiency over 2 bits/s/Hz.

Ok, Throndor provided a link which has a better explanation about this. I have some reading to do!

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