Hybrid Si/III-V is a promising platform for semiconductor narrow-linewidth lasers, since light can be efficiently stored in low loss silicon and amplified in III-V materials. The introduction of a high-Q cavity in silicon as an integral part of the laser's resonator leads to major reduction of the laser linewidth. However, the large intra-cavity field intensity resulting from the high-Q operation gives rise to non-linear effects. We present a theoretical model based on non-linear rate equations to study the effect of two-photon absorption and induced free-carrier absorption in silicon on the laser's performance. The predictions from this model are compared to the experimental results obtained from narrow-linewidth lasers fabricated by us. It is shown to have an effect on the linearity of the L-I curve, and to reduce the achievable Schawlow- Townes linewidth.