Date of Award
Doctor of Philosophy in Electrical Engineering
A theoretical analysis of the performance of InGaAsP multi-quantum well electro-absorption modulators is presented. A comprehensive model of the quantum confined Stark effect is described to determine the absorption and index change spectra versus applied field. This model is based on previously developed models for GaAs/AIGaAs structures, but includes improvements in the handling of exciton line broadening and the variation of exciton oscillator strength with field.
The analysis of line broadening due to composition fluctuations is presented, revealing a previously neglected factor. Two numerical methods for calculating the line broadening, based on the resonant tunneling method, are presented and compared. A theoretical analysis of barrier composition fluctuation broadening is presented, which separately calculates the contribution from the electron and hole and their different penetration into opposite barriers when field is applied. The total linewidth model is compared with published linewidth measurements.
Theoretical results of absorption spectra versus applied field were compared with two sets of experimental measurements. With appropriate choice of several unknown factors related to the quantum well fabrication quality, the theory and experimental data were well matched in shape of the absorption edge, shift of the edge with field, and decrease in the exciton oscillator strength with field.
The theoretical model was used to optimize modulator device design, through the calculation of thousands of design combinations of device length, well number, well width, barrier width, well composition, and applied voltage. For each design, bandwidth, contrast ratio, loss, detuning, and several chirp parameters were calculated. It is shown that long distance transmission performance may be optimized with negative values of a specific chirp parameter called the 3dB Henry factor. Modulator design can be optimized for such values by operating close to the exciton and accepting high optical loss. Loss may be reduced by optimum choice of device length, well number, and barrier width, while it can be compensated by an optical amplifier.
The optimum design changes considerably when the requirement for negative chirp is eliminated. Such designs use more quantum wells and tune the device further from the exciton. Finally, the model provides a means to choose the optimum well width.
Ames, Gregory H., "Design Optimization of Indium-Gallium-Arsenide-Phosphide Multi-Quantum Well Electroabsorbtion Modulators" (1996). Open Access Dissertations. Paper 683.