Ph.D Thesis

Ph.D StudentGready David
SubjectCarrier Dynamics in Quantum Dot Lasers
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROFESSOR EMERITUS Gad Eisenstein


This thesis deals with a study of dynamic and static properties of high speed quantum dot lasers. Quantum dot lasers have unique properties such as: Low threshold currents, high temperature stability and near zero linewidth enhancement factor. For many years, such lasers could not be modulated at high speeds (compared to quantum well lasers). The reason is difficulties in achieving high differential gain in low dimensionality media. This has changed in recent times due to important advances in epitaxial technology for InP quantum dots.

For the theoretical part of the thesis, an extensive spatially resolved model was developed, taking into account the interaction between carriers (electrons, holes) from different energy levels. The model also includes gain broadening effects (homogeneous and inhomogeneous broadenings). The model was used to study the dynamical behavior of InP based quantum dot lasers operating at the important wavelength of 1550nm. InP based quantum dot lasers were rare until recently and the ones that existed had inferior behavior compared to their GaAs counterparts which operate near 1300nm.  Recent advances brought to the scene high quality lasers with very high modal gain and high speed modulation capabilities so that the proper modeling of these lasers became crucial.

A similar model was also used for studying the dynamical properties of GaAs based quantum dot lasers exploiting tunneling injection scheme. Quantum dot tunneling injection lasers have shown a remarkable dynamic behavior, which can be explained using gain broadening effects.

The first generation of the new InP based QD lasers showed record modulation rate of 15 Gbit/s, measured at Technion.  Using the spatially resolved model we designed new structures which enabled our collaborators to produced optimal lasers which showed large signal record rate of 22 Gbit/s.

In addition to the high speed measurements for both small and large signal, the relation between the small and large signal direct modulation were investigated. Recent results for quantum dot lasers suggest that the small signal modulation bandwidth is not a good predictor for large signal capabilities. We analyzed this relation using a numerical model in addition to an analytic investigation and proved that in QD lasers, a narrow small signal modulation bandwidth may be accompanied by large signal modulation at very high rates.

Finally the numerical model was expanded to include the gain and refractive index coupling so that the index dynamics of the laser is also accounted for.