Quantum-dot based microdisk lasers and semiconductor optical amplifiers operating at 1.55 μm

Abstract

Optical data transmission allows for high-speed and low-loss transmission over longer distances than the electronic counterpart. Yet, the advantage of using fiber-optic communications has been restrained by power hungry opto-electronic conversions at the nodes. These are required for switching and/or signal processing purposes. Hence, recent efforts have focused on implementing signal processing functions in the optical domain. Key requirements for the constituent devices are low power consumption, ease to integrate, ability to distribute high bit rate optical streams and to perform signal processing functions. The optical interconnection between boards and chips and even within chips is also envisaged in the optical domain. Therefore, research for novel structures and their capability to improve the performance of semiconductor devices employing them is a must. This thesis focuses on the investigation of two devices whose characteristics make them highly promising as components of optical communications systems: microdisk lasers and semiconductor optical amplifiers. The investigated devices are based upon III-V semiconductors. In particular, they are based upon the InGaAsP/InP material system, for which a well-established opto-electronic integration technology exists. Furthermore, the investigated devices employ quantum dots in their active regions and operate in the 1.55 µm wave-length region. Quantum-dot based devices confine carriers in three dimensions and therefore operate in a fundamentally different manner compared to commercially available bulk and quantum-well based devices. This thesis focuses on the modeling and simulation of both quantum-dot microdisk lasers (QD-MDLs) and quantum-dot semiconductor optical amplifiers (QD-SOAs). In addition, an experimental characterization of QD-SOAs is performed. These results are essential for the understanding, design and exploitation in network systems of the investigated devices. Chapter 2 presents a comprehensive quasi-three-dimensional frequency-domain model to compute the lasing modes and the Q-factors of an In-GaAsP/InP quantum-dot microdisk laser. The model contributes to the understanding of the mode behavior and size limitations of the QD-MDLs. The developed model is highly computationally efficient and can be used to study MDLs based on a different active medium. Hence, the model also provides a support tool to design microdisk lasers. Chapter 3 deals with the experimental characterization of QD-SOAs both in the static and dynamic regimes. In the static regime, the inhomogeneously broadened amplified spontaneous emission (ASE) spectra of these devices are measured. In addition, the continuous wave (CW) gain saturation characteristics are characterized, exposing the high saturation output power of these devices. The dynamical behavior of the QD-SOAs was assessed using a pump-probe technique, using either CW or pulsed probe. Using this technique, the gain and phase dynamics are measured, showing that the gain recovery is dominated by an ultrafast process while the phase is dominated by a slow process. Cross-gain modulation (XGM) and cross-phase modulation (XPM) effects in QD-SOAs are also investigated. The manifestation of XGM and XPM on the probe spectrum at the QD-SOA output shows strong-blue chirped components. This property has already been exploited in system experiments and therefore its understanding is of interest. In Chapter 4 the gain and phase dynamics experimental results are analyzed using an impulse response formalism. Using this method, the timescales and strengths of each of the recovery processes are extracted, and their dependencies on different operating conditions (bias current and average pump power) are investigated. The overall experimental characterization and analysis of experimental results presented in Chapters 3 and 4 contribute to the understanding of InAs/InP QD-SOAs performance, whose behavior has been little investigated. Chapter 5 presents a simple two level rate equation model of QD-SOAs. Rather than a quantum mechanical description of the quantum-dot semiconductor optical amplifier, a phenomenological model is preferred which can be based on parameters extracted experimentally. Therefore, based on the experimental findings from Chapters 3 and 4 the model is developed and used to gain insight into some of the basic characteristics of QD-SOAs: large saturation output power, fast gain dynamics and enhanced blue chirp. A simple model to describe the saturation characteristics is derived and shows that the saturation photon density of QD-SOAs is enhanced by a dimensionless factor that accounts for the QD-SOA timescales and distribution of carriers among the considered populations. An analytical model for the recovery dynamics is proposed, and anticipates an enhanced blue chirp in QD-SOAs compared to their counterparts bulk or quantum-well amplifiers, due to the QD-SOA fast recovery timescales. Overall, the simple model results are well in agreement with the static and dynamic characteristics of QD-SOAs observed experimentally, providing a fairly accessible tool for the understanding and simulation of these devices. Finally in Chapter 6 the main results obtained in this thesis are outlined and an outlook on future research directions is given.