Abstract The reflective semiconductor optical amplifier (RSOA) capability to compress the relative intensity noise (RIN) is well-known and widely exploited in wavelength division multiplexing passive optical networks (WDM PON). While this feature has been previously analysed using SOA theory, in this paper we show that RSOAs present specific gain saturation properties. According to the injected power, three nonlinear operation regimes can be highlighted: the first one, where RIN is moderately compressed, the second one, where RIN maximum compression takes place, and the final one, where RIN grows again, this last regime being RSOA specific. We focus on a spectrum-sliced WDM PON classical topology, evaluating the impact of the injection process and filtering. RSOA injection and optical filtering have opposite effects on the RIN: by injection into the RSOA the RIN is reduced, after filtering RIN increases. We experimentally evaluate RIN both in the time and in the spectral domain. A simple numerical model mainly based on the correct reproduction of the component gain behaviour is exploited to verify experimental results. Through our experimental analysis and by simulations, we identify the correct injected power range to best take advantage of the RSOA features.