Optical and electrical properties of novel quantum dot lasers and amplifiers

Self-assembled quantum dot (QD) lasers and amplifiers have drawn much attention due to their potentially better performance over quantum well (QW) or bulk lasers and amplifiers. Two new structures of QD active media have been proposed to improve the performance of conventional QD lasers and amplifiers. One is a p-doped QD structure and the other is a tunneling injection QD structure. Although the improved performance of the new structures of QD lasers over conventional QD lasers has been demonstrated, there are still many underlying physical mechanisms to be further investigated for better design of QD lasers. In this dissertation, the optical and electrical properties of the novel QD active structures are investigated both theoretically and experimentally.; In the case of a p-doped InAs QD Fabry-Perot (FP) laser, the linewidth enhancement factor (LEF) is measured based on cross-gain modulation (XGM) technique. The measured LEF agrees with the calculated LEF when the thermal effect is isolated. In addition, it is theoretically demonstrated that the LEF of the p-doped QD laser is smaller than that of the undoped QD laser due to the reduced transparency carrier density. The high-speed small-signal XGM responses are measured from a p-doped QD semiconductor optical amplifier, depending on the device length and pump-probe detuning. The measured XGM responses indicate that the total carrier density change is the dominant gain saturation mechanism of the p-doped SOA.; In the case of the tunneling injection InP QD FP laser, a particular transverse-electric (TE) gain narrowing and transverse-magnetic (TM) gain pinning is measured below threshold. A polarization-dependent photon-mediated carrier redistribution with the coupled rate equation model is proposed to explain this unique optical gain evolution behavior. To experimentally demonstrate the proposed polarization-dependent photon-mediated carrier redistribution, the TE modal optical gain spectra of the tunneling injection QD laser with the grating-based external cavity are measured. The measured TE gain narrowing spectra are changed by tuning the external feedback wavelength. On the other hand, a typical band-filling behavior without gain narrowing is observed from the measured modal optical gain spectra of a QW laser both with and without the external optical feedback.