Small-signal analysis of two cascaded semiconductor optical amplifiers in a counterpropagating configuration

In this work, we present a small-signal analysis which reveals its usefulness to explain the evolution of the optical frequency responses obtained at the outputs of two-cascaded semiconductor optical amplifiers (SOAs) used in a counterpropagating configuration (TC-SOA-CC). To obtained these responses, the TC-SOA-CC configuration has been implemented on an electric simulator and we have used for each SOA a multisection rate equation based model. This model permits us to study both the static and dynamic behavior of the TC-SOA-CC optical outputs in the context of bidirectional transmission. Thanks to this model, we can also obtain the evolution of the carrier lifetimes in each SOA in order to determinate their influence on the optical frequency responses. The small-signal responses derived from our small-signal analysis are given as a function of the intrinsic parameters and for each specific situation for the value of the average injected optical power. This small-signal analysis is used to comment the behavior of the frequency responses obtained by simulation at the TC-SOA-CC optical outputs. Thanks to our small-signal analysis, we explain why for such a configuration, the features of the frequency responses obtained at the SOAs optical outputs can change from a low-pass, to a high-pass, and/or to a band-pass form by varying the average injected optical powers in the SOAs.