Reflective semiconductor optical amplifiers (RSOAs) became a key-technology for the implementation of next generation local area passive optical networks (PONs). In this letter, we present a semi-analytic study of their nonlinear dynamics, based on the derivation of a reduced model that gives the input-output RSOA transmission function as the solution of a standard differential equation. The reduced model is computationally more efficient than full-scale computer simulations of time-domain models, and can be used for a fully analytic study of four wave mixing in the context of WDM- and OFDM-based PONs.