Alternating cascade of spectrally different erbium-doped fiber amplifiers for link-loss-insensitive long-haul WDM transmission

This paper numerically investigates a cascade of erbium-doped fiber amplifiers (EDFAs) with different spectral characteristics in an alternating scheme. Thereby, in a transmission band with appropriate spectral properties (1544-1550 nm), the sensitivity of optically amplified long-haul WDM transmission links on the span loss between amplifiers can be reduced to that of single-channel systems. This may be as much as an order of magnitude better than in a traditional, nonalternating, WDM cascade. While passive bandpass filters are necessary to control the wavelength range, there is no need for any active gain equalization or feed-back control. The gain in the cascade can also to some extent automatically adjust to compensate for a spectral tilt of the loss. We show that the range of this automatic loss tilt compensation can be increased, up to three times its original value, by enhancing the difference of the net gain that signals at different wavelengths experience in EDFAs of different types