Investigation On Carrier Dynamics In Semiconductor Optical Amplifiers

Semiconductor optical amplifier (SOA) is an important nonlinear device which has been widely used in wavelength conversion, all-optical logic and3R regeneration for all-optical signal processing techniques. In order to investigate the dynamics of SOA in communication system, it is required to develop the theoretical model of SOA. With the increasing of the communication speed, it is necessary to pay attention to the ultrafast physical processes in the SOA.The characteristics of material and structure of the active region and the carrier recovery dynamics of the SOA are analyzed. We focus on the effect of the differential gain, the step quantum well structure, the interaction between the distributed feedback grating structure and polarization dependence on the carrier recovery dynamics in the SOA.Research achievements and contributions are summarized as following:(1) The optical model and the material model about the physical theory in the SOA are investigated. The governing equations which describe different physics processes are all derived from basic principles. The light wave equation is derived from the Maxwell equations, material gain equation is derived from the Schrodinger equation, the carrier rate equation is derived from the Heisenberg equation. The models builted in this thesis are developed by modifying these fundamental equations.(2) It is demonstrated theoretically that the quantum well SOAs with high differential gain will show fast gain recovery dynamic characteristics. The methods to enhance the differential gain are also discussed. Based on the calculation of energy band structure, the effects of compressive strain and p-type modulation doping on the gain, the differential gain and linewidth enhancement factor (α-factor) are investigated. The peak of the differential gain spectrum shifts to longer wavelength, and the peak value is significantly enhanced by increasing compressive strain. Meanwhile the gain is obviously increased by increasing the p-type modulation doping concentration. By comparing the gain and phase recovery dynamics in three different types of SOA samples, it is shown that the quantum well SOA with highest differential gain has the shortest gain recovery time among the three samples.(3) The gain recovery dynamic characteristics in the step quantum well SOA are theoretically investigated via a detailed model. We numerically solve the coupled rate equations including microscopically calculated carrier-phonon scattering rates between the carrier reservoir and the ground state. It is demonstrated that the electron scattering rate in the step quantum well SOA depends on the potential parameters of the carrier reservoir region. Finally, it is shown that the SOA with larger transition rate has shorter carrier recovery time than the other SOAs via analyzing and comparing the gain and phase recovery dynamics in different types of SOA samples.(4) The gain recovery dynamic characteristics of the SOA with distributed feedback (DFB) grating are theoretically investigated. The effects of assist light on the gain recovery dynamics of the SOA are mainly analyzed. The influences of assist light on the steady-state carrier density and field intensity distributions of the DFB-SOA are studied respectively. Results show that the recovery time in the DFB-SOA is successfully reduced by injecting relatively high power assist light, whose wavelength is set at the gain region. Finally, it is shown that the gain recovery in the SOA with DFB grating is faster than that in the SOA without DFB grating under assist light injection. In addition, the coupling factor in the DFB grating structure can be optimized to shorten the gain recovery time.(5) Polarization dependence of patterning effects in Quantum Well (QW) SOA-based wavelength conversion is theoretically and experimentally investigated. By comparing experimental and computational results, it is demonstrated that the polarization of the injection signal has significantly influence on the gain recovery time of QW-SOA. Under the different polarized signals injection, the output signals suffer various degrees of patterning effects both for unstrained and tensile strained QW-SOAs.