Challenges for Multiband Optical Amplification and Solutions Therefore

Increasing transmission capacity of installed fiber optical communication systems by multiband (MB) transmission offers large advantages compared with other options if physical access to all intermediate amplification sites is granted. This is typically the case in terrestrial networks or unrepeatered submarine links without remote optically pumped amplifier (ROPA). Challenges for introducing this technology and solutions for overcoming them are detailed with a focus on energy efficiency, transmission performance, system installation, amplifier manufacturing, and device control. However, only few systems will be prepared for such a capacity upgrade such that traffic interruption is unavoidable. Splitting an incoming wavelength–division multiplexing (WDM) signal into different bands for amplification, which is mandatory when using rare–earth doped fiber amplifiers, leads to performance degradation and reduced power conversion efficiency. Based on a hypothetical energy level diagram, it is explained why using lanthanides other than erbium results in reduced power conversion efficiency. Due to the intrinsic wavelength characteristics of the cross sections of thulium, efficiency is further significantly reduced when designing gain–flattened thulium–doped fiber amplifiers for operation beyond 1510 nm. Simulation results show that gain and tilt adjustment in reaction to power transients will take at least 200$\,\mu \text{s}$.