Theoretical and experimental investigation of the effect of state filling on high speed modulation dynamics of quantum well lasers

Several mechanisms have been proposed to explain the large variation in high speed modulation of semiconductor quantum well (QW) lasers, including carrier transport effects [1] and well barrier hole burning enhanced gain suppression [2]. We believe, however, that there exists another important effect, state filling effect [3,4], which has significant impact on the modulation dynamics in the QW lasers. The state filling effect accounts for the unavoidable permanent thermal population of injected carriers in the upper subbands of the QW structure, especially in the states of the separate confinement heterostructure region. The differential gain and transparency current, both, depend on the rate of increase of the quasi Fermi energies with increasing injected carrier density. The presence of upper subbands with large density of states tends to clamp the Feimi energies thus leading to low differential gain and high transparency current.