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

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.