Gain anisotropy in a semiconductor optical amplifier: confinement factors or material gain

We show that if gain anisotropy in a bulk semiconductor optical amplifier (SOA) is attributed to different confinement factors for the TE and TM modes, the anisotropy as a function of pump current is quite different from the case when another mechanism, material gain anisotropy due to weak strain in the active layer, is the cause of anisotropy. With a simple model for the latter mechanism we obtain a good description of trends seen in a recent experiment and give a prescription for the phenomenological hole reservoir anisotropy factor f that has been introduced before in model simulations.