Electrical and Computer Engineering ETDs

Nader Naderi

Publication Date

2-7-2008

Abstract

Semiconductor quantum dot (QD) lasers are competitive candidates for many applications such as high-speed long-haul optical communication systems. This is due to their superior lasing characteristics (compared to conventional quantum well (QW) lasers) and also their potential for high differential gain and direct modulation with negligible chirp. Recently, substantial efforts have been made to improve the modulation characteristics of QD semiconductor lasers such as enhancing the modulation efficiency and improving the overall modulation bandwidth. The gain lever effect is a method used to enhance the efficiency of amplitude modulation and optical frequency modulation at microwave frequencies by taking advantage of the sub-linear nature of the gain versus carrier density. Previously, two-section quantum well lasers have been investigated theoretically and experimentally to explore the gain lever effect. As for QD devices, which are extremely promising because of the strong gain saturation effect in dots, have not been investigated until recently. In this thesis, first the characteristics and applications of conventional two-section gain lever semiconductor lasers are presented. In related previous studies, gain levered single and multiple QW lasers have been used to enhance the modulation efficiencies in both intensity (IM) and frequency (FM) modulation. In this work, the modulation characteristics of a gain lever QD laser diode is demonstrated for the first time. In this work we report an amplitude modulation enhancement of 8-dB for a p-doped two-section quantum dot laser and discuss the relation between the normalized 3-dB bandwidth and the modulation section gain for different power levels. Also based on rate equations and small signal analysis, a novel modulation response equation is derived to describe the device dynamics. Using the new modulation response function the actual gain lever ratio can be measured for various power levels. For future work, the gain lever laser structure can be optimized to reduce the effect of non-linear gain suppression which directly limits the efficiency enhancement through the damping factors and relaxation oscillation frequency.

Keywords

Quantum dots

Sponsors

US. Army and Air Force Research Laboratories

Document Type

Thesis

Language

English

Degree Name

Electrical Engineering

Level of Degree

Masters

Department Name

Electrical and Computer Engineering

First Committee Member (Chair)

Christodoulou, Christos

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