Hi,
EDFAs could be used in a single or double stage setup in metro, long haul, and ULH networks; while Raman amplifiers for LH & ULH systems. The advantages of using the EDFA double stage setup are:
ยท Providing high gain/low noise at the first stage with980nm pump laser (low NF) and high power output on the 1480nm pump laser second stage
ยทAbility to place a loss element (DCU, OADM, gain flattening amplifier) between the stages without degrading the overall performance effectively
ยท Redundancy
The critical consideration in using RAMAN is the requirement to provide amplification in the S & U bands. However, the associated challenges are: requiring high & constant pumping power level (producing nonlinearities) and crosstalk among wavelengths.