This paper estimates the loss budget of wavelength division multiplexing (WDM) single-fiber loopback access networks that employ a broadband amplified spontaneous emission (ASE) light source at the central office (CO) and an optical modulator with optical amplification function in each optical network unit (ONU). This study considers the backreflection lights from two sources, a broadband ASE light from the CO (Reflection-I) and a modulated signal from the ONU (Reflection-II). A gain-saturated semiconductor optical amplifier (GS-SOA) is placed at the ONU to reduce the excess intensity noise of sliced ASE and the spontaneous-spontaneous beat noise caused by Reflection-II; this technique increases the loss budget of the transmission line and removes an optical pre-amplifier from the optical receiver. Experiments clarify that with the gain compression effect of the GS-SOA the loss budget is 22.7dB.