Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisor: Dennis G. Deppe. Vita. Includes bibliographical references. Also available from UMI.

Thesis (Ph. D.)--Oregon Graduate Center, 1985.

This research investigates the characteristics of Parity Time symmetry breaking in two optically coupled, time delayed semiconductor lasers. A theoretical model is used to describe the controllable parameters in the experiment and intensity output of the coupled lasers. The PT parameters we control are the spatial separation between the two lasers, the frequency detuning, and the coupling strength. We find that the experimental data agrees with the predictions from the theoretical model confirming the intensity behaviors of the lasers, and the monotonic change in PT-threshold as a function of coupling scaled by the time delay. <br>

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The first part of this dissertation presents a comprehensive study of quantum dot (QD) lasers threshold characteristics. The threshold temperature dependence of a QD laser diode is studied in different limits of p-doping, hole level spacing and inhomogeneous broadening. Theoretical analysis shows that the threshold current of a QD laser in the limit of uniform QDs is not temperature independent and actually more temperature sensitive than the quantum well laser. The results also explain the experimental trends of negative characteristic temperature observed in QD lasers and clarify how the carrier distribution mechanisms inside and among the QDs affect the threshold temperature dependence of a QD laser diode. The second part is on the experimental demonstration of lithographic lasers. Today’s vertical-cavity surface-emitting lasers (VCSELs) based on oxide-aperture suffer from serious problems such as heat dissipation, internal strain, reliability, uniformity and size scaling. The lithographic laser provides solutions to all these problems. The transverse mode and cavity are defined using only lithography and epitaxial crystal growth providing simultaneous mode- and current-confinement. Eliminating the oxide aperture is shown to reduce the thermal resistance of the device and leading to increased power density in smaller lasers. When it is combined with better mode matching to gain for smaller devices, high output power density of 58 kW/cm2 is possible for a 3 μm VCSEL with threshold current of 260 μA. These VCSELs also have gratingfree single-mode single-polarization emission. The demonstration of lithographic laser diodes with good scaling properties is therefore an important step toward producing ultra-small size laser diodes with high output power density, high speed, high manufacturability and high iv reliability. Lithographic VCSELs ability to control size lithographically in a strain-free, high efficiency device is a major milestone in VCSEL technology.

<p> The semiconductor distributed feedback (DFB) laser is mainly characterised by the single-longitudinal-mode operation with a narrow spectral linewidth, which leads to its wide application in fiber-optic communication systems. Several numerical models ranging from physics-based to phenomenological ones have been developed with different level of complexities and for different applications. However, with the continuous improvement in designs of DFB lasers, more efficient simulation methods with sufficient accuracy are highly desirable. In this thesis, I mainly focus on developing new one-dimensional (ID) simulation methods of DFB lasers with improved computational efficiency and physical insight without compromise on accuracy. Further, a new design idea for DFB lasers are explored and investigated by using the simulation techniques developed.</p> <p> Starting with the well-known ID time-dependent coupled-wave equations, we have examined two different solution schemes, i.e. the traveling wave model (TWM) and the standing wave model (SWM). The TWM has the merits of straightforward implementation, and being able to simulate a large variety of the structure even if the laser cavity has a small quality factor (Q-factor). Firstly, the existing time-domain solution schemes are reviewed and compared under a unified framework. A high-order split-step traveling wave method is then developed. Its validity and efficiency are examined through the comparison made with the conventional split-step scheme.</p> <p> For laser structures with large variations of the carrier/photon density, however, the TWM is not computationally economical. The SWM on the other hand has its advantages in dealing with the laser cavity with a relatively large Q-factor. Two different standing wave models are proposed to simulate the index-coupled and gain-coupled DFB lasers, respectively. The complexities of these two numerical models are further reduced through an approximation made on the time-dependent carrier distribution. Finally, the proposed SWMs are reduced to a similar form to the rate equation formulations for establishing the linkage between the 1D model and the rate equation model. More physical insights into the conventional and powerful rate equations will be gained through this linkage.</p> <p> The final part of the thesis focus on the analysis of a novel design of single-mode operation DFB laser employing the dispersive grating. The design idea is verified by the proposed SWM.</p> / Thesis / Doctor of Philosophy (PhD)

<p>New types of laser using the basic structure of FP cavity are designed and modeled, to achieve high SMSR single-mode lasing that can be immune to high level of optical feedbacks for optical network communication applications.</p> <p>The work includes design of asymmetric Bragg reflection waveguide laser that employs wavelength selective Bragg reflectors as the claddings to confine and filter desired FP longitudinal modes for amplification and lasing. Si-rich SiO₂ single-mode laser based on this structure is also proposed and analyzed.</p> <p>To optimize a recent design of discrete mode laser that is re-growth free and feedback-perturbation insensitive, a comprehensive implementation of the time domain transfer matrix method, including temperature and feedback effects, is carried out. The model helps to obtain a optimized DM structure that is balanced between high SMSR and low feedback sensitivity.</p> / Doctor of Science (PhD)

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The research described in this thesis relates to the design, fabrication and testing of novel semiconductor-based light sources that have been designed for the generation of infra-red light. The thesis is formatted to account for two distinct components of my work, where the first part concerns sources producing coherent light by direct laser emission, notably, ultrashort-pulse quantum-dot lasers. These types of lasers continue to show considerable promise as efficient, compact sources of ultrashort pulses with durations of hundreds of femtoseconds, while giving rise to unique and interesting electronic properties such as low lasing thresholds through the quantum nature of their density of states. At the outset a study of the most relevant aspects of the lasing dynamics of an optically pumped quantum-dot laser is outlined. Pumping of the device with intense discrete optical pulses leads to output from multiple electronic states, each having a characteristic wavelength and temporal properties. I show that pulses produced by excited-state emission have shorter durations (24 ps) and arrive earlier in time than those due to transitions from the ground state, which themselves have durations of around 180 ps. Investigations are then made on two different mode-locked quantum-dot laser systems. One is an all-quantum-dot external-cavity laser that is mode locked using a quantum-dot SESAM device at a repetition frequency of 860 MHz with output power approaching 20 mW. This is followed by a study of a monolithic two-section quantum-dot laser that is mode locked stably in a wide temperature range of 20°C to 70°C. The excellent performance characteristics presented serve to demonstrate both the versatility of quantum-dot material as components in mode-locked laser systems and the temperature stability of such laser devices. The second part of the thesis relates to structures that are designed to take advantage of nonlinear frequency conversion in GaAs-based semiconductors. This material system possesses a nonlinear coefficient of ~170 pm/V and is transparent from around 0.9 μm through to 17 μm, making it attractive for the realisation of a new class of efficient, integrable, quasi-phase-matched, optical parametric oscillator devices. Initially, ion implantation is utilised as a vector to create a periodically-switched nonlinear ridge waveguided device. The observation is made that in the course of implantation the transmissive properties of the device are severely degraded. Unfortunately, the high losses incurred, which reached 250 dB/cm, could not be removed without also destroying the modulation in nonlinearity. During the course of this investigation, significant technological advances were made in the production of orientation-patterned GaAs structures. By recognising the elegance and potential of this new orientation-patterned (OP) methodology, a study of its implications and applicability in the context of my project is initiated.