Implementation of automatic gain controlled bidirectional EDFA in WDM networks

Wavelength-division-multiplexing (WDM) techniques combined with n erbium-doped fiber amplifier (EDFA) is essential for realizing high capacity lightwave transmission and flexible optical networks. Compared with unidirectional transmission, bidirectional transmission over a single fiber has the advantage of reducing not only the number of fiber link, but also the number of passive components such as splitters and WDM multiplexers. Recently, lots of problems in bidirectional EDFAs were investigated, and various structure schemes of the EDFA were reported to overcome the problems, such as back reflections. An automatic gain control (AGC) function for bidirectional EDFAs, however, has been rarely reported. In a bidirectional EDFA, optical gains of survival channels are subject to change with the input variations of both directions, such as the addition or dropping of channels. To preserve the system performance, these gain changes should be suppressed by using thee AGC function. For the unidirectional EDFA, several AGC functions demonstrated such as optical clamping methods, link control laser methods and pump control methods. Nowadays, the AGC method by using the pump control is attractive because the implementation is simple and less costs. It may be modified for the bidirectional EDFA, which shares the same port for input and output. In the paper, we propose and demonstrate an AGC scheme for the bidirectional EDFA by means of pump control.