Feasibility analysis of a 4-QAM colorless transmitter based on two RSOAs

Multilevel modulations are becoming attractive, not only for metro-core optical communications, but also for the access sector. Designing cost-effective, easy-to-integrate multilevel modulators is mandatory to support high-bandwidth access interconnections, which may also require colorless operation. In this work, we propose a novel colorless multilevel transmitter architecture, based on reflective semiconductor optical amplifiers (RSOAs) which meets the above criteria. In our architecture, Fig. 1(a), the in-phase (I) and quadrature (Q) components of the optical QAM signal are obtained by direct modulation of two RSOAs, using the currents I
I
and I
q
. The RSOAs are coupled together and fed by the same seed source so that a π/4-phase shift in the in-phase path allows the output field to comprise two quadrature components.