Bidirectional modeling of reflection-induced gain and noise dynamics in semiconductor optical amplifiers

Reflections in semiconductor optical amplifiers (SOAs), especially in heterogeneously integrated silicon photonics platforms where isolators are absent, can severely degrade performance through nonlinear gain dynamics and excess noise. Even weak reflections can induce backward-propagating fields that can coherently interfere with the forward-propagating signal, causing forward gain suppression resulting from reflection-induced carrier depletion. Existing models, which primarily focus on unidirectional propagation, do not adequately address the dynamic noise generated by this bidirectional coupling. In this work, we present a self-consistent bidirectional model that includes nonlinear gain saturation, phase-sensitive interference, and the resulting excess intensity noise. We show that modest chip-level reflections, when combined with mechanical vibrations, thermal drift, current noise, or laser phase jitter, can elevate excess intensity noise to levels approaching the fundamental amplified spontaneous emission (ASE) noise floor, making it necessary to account for its effect in system design and performance evaluation. This framework enables predictive modeling of reflection-sensitive behavior, providing a valuable tool for optimizing active photonic devices, particularly in heterogeneously integrated III-V/Si platforms.