Fig. 1. Installed optical fiber worldwide (data: R. Mack, CRU International^[16]).

Fig. 2. Shannon limit and record experimental SEs on available optical fibers.

Fig. 3. Low-loss window of commercial optical transmission fiber across telecommunication bands. Also shown are Raman pumps amplifying the L-band, which share spectra with S-band signals.

Fig. 4. Maximum possible transmission distance vs. allowed crosstalk per km of MCF for various modulation formats. Owing to the inherent reach reduction with increasing QAM order, a universal MCF crosstalk specification of around −55  dB/km emerges.