Fundamental information-density limits in optically amplified transmission: an entropy analysis

Fundamental information-density limits are evaluated for amplified systems, based on entropy analysis. It is shown that in-line amplification introduces information loss, which is more important in distributed than in lumped amplification. In the absence of any other degradation (such as that which comes from dispersion and nonlinearity), spectral efficiencies of 6¿14 (bits/s)/Hz represent theoretical limits for medium- and long-haul amplified systems.