Fundamental examination of gain, differential gain, and modulation dynamics in bulk and QW lasers

Quantum well (QW) lasers have been predicted to have enhanced differential gain compared to their bulk counterparts—double heterostructure (DH) Quantum well (QW) lasers have been predicted to have enhanced differential gain compared to their bulk counterparts—double heterostructure (DH) lasers.' Thus, higher modulation bandwidth is expected for QW lasers due to the higher differential gain. However, the modulation bandvvidths of QW lasers do not show too much improvement over the DH lasers in experiments, especially for the case of SQW lasers. Enhanced photon density dependent gain compression,^ which leads to anomalously high damp;; ing, and carrier transport mechanism’ have been proposed to explain modulation dynamics of QW lasers. To fully understand the modulation dynamics of QW lasers it is necessary to carefully investigate the differential gain of QW lasers. The separate confinement heterostructure (SCH) QW structure is usually employed in QW lasers to support the quantum confinement of the injected carriers and confinement of the optical field. At finite temperature, due to the Fermi-Djrac statistics, the injected carriers populate not only the energy states of the QW(s) but also the large density of states in the SCH structures. The carriers populating the SCH states contribute little to the peak gain because of their nonresonance with the optical transition at the peak gain. The carrier population of SCH states makes the quasi-Fermi energy levels change slovvly with the change of injected carrier density. These lead to lower differential gain. Figure 1 shows the calculated differential gain for a typical GaAs/ AlGaAs DH laser and typical GaAs/ AlGaAs QW lasers with various number of wells. The differential gain in SQW structure is lower than that of the DH structure. There is a differential gain enhancement as the number of wells CWR5 Fig, 1. Differential gain as a function of modal gain for a typical GaAs/AlGaAs DH laser and typical GaAs/AlGaAs QW lasers with different quantum well number. increases in MQW structures. The differential gain enhancement in MQW structures is attributed to the fact that the effect of carrier population in SCH structure is distributed among the QWs and the quasi-Fetmi level is lower in MQW structures than that in a SQW structure for the same value of modal gain. These results, combined with considerations of damping effects and operating conditions, are consistent with the experimental observations. Figure 2 shows the maximum modulation bandwidth for DH laser and QW lasers with different well number. We find that some of the previous conclusions about QW lasers need to be changed by considering the unavoidable thermal population of injected carriers in the optica] SCH region. CWR5 Fig. 2. Maximum modulation bandwidth as a function of cavity length for a typical CaAs DH laser and typical GaAs QW laser.s with different well number.