Performance issues in optically amplified systems and networks

The growth of wavelength-division multiplexed (WDM) transmission and the prospect of multiwavelength optical networking are perhaps the most significant consequences of the advent of erbium-doped fiber amplifiers (EDFAs). Erbium-doped fiber amplifiers are almost ideal in their characteristics, but they are not quite perfect, and they give rise to effects such as amplifier noise, amplifier spectral gain nonuniformity, and channel cross saturation which do limit the performance of optically amplified transmission systems and networks. Transmission impairments, such as those from optical nonlinearities, dispersion and polarization effects accumulate in amplified systems and have greater impact. The optical signal-to-noise ratio of the system can be improved by decreasing the noise figure of the EDFAs, increasing the channel powers, lowering repeater spacing and/or lowering system reach. Low noise figure and high output power can be achieved simultaneously with two-stage EDFA designs