Gain leveling in a two-level system for EDFA using Quantum-interference effects

Much work has been focused on gain leveling for EDFA for its importance in long distant transmission in dense wavelength division multiplexing (WDM) system. For 1480 nm pumping, energy from the pump field excites the erbium ions in the low lying levels of the
⁴
I
_15/2
manifold to the high lying ones in the
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I
_13/2
manifold, and light amplification occurs from 1520 nm to 1560 nm because of the stokes shift existing between the emission and the absorption bands of this transition. However, the gain profile of EDFA is uneven and therefore leads to error performance in different WDM channels in 1530-1560 nm. Here we propose a scheme of solving gain leveling for EDFA by one of quantum interference effects. We apply an additional strong coherent field to drive the transitions from
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I
_15/2
manifold to
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I
_13/2
manifold. The susceptibility chi induced by the strong field is reverse with the original gain spectrum as a filter does. This scheme can reach a flat gain operating around 1.53 mum