Comparison of gain control techniques to stabilize EDFAs for WDM networks

A serious problem for wavelength-division multiplexing (WDM) networks is the wavelength-dependent gain saturation of the erbium-doped fiber amplifiers (EDFAs). Because of this effect, the loss or removal of one or more channels at the input of an EDFA can cause large changes in the output powers of the remaining channels. Gain clamping, and channel equalization have been shown to be effective methods to stabilize the WDM output of the EDFAs. As an alternative solution, we have investigated EDFA gain control under multiwavelength operation, comparing different methods: (1) pump power control; (2) adding a co-propagating control signal, and (3) adding a counter-propagating control signal. The results of these three gain control methods are summarized. Very precise control of the pump power over a small range of 1.7 db was needed to keep the gain constant with the loss of one to four channels. This result reflects the sensitive dependence of the EDFA's gain on the pump power. The EDFA gain was less sensitive to the control signal power, for both co- and counter-propagating configurations. In addition, the low absolute power levels for the control signal, in the range of -15 to -10 dBm, suggests that one laser could be used to provide control signals for several EDFAs within a WDM network element.