High-Speed Semiconductor Lasers based on Low-Dimensional Active Materials for Optical Telecommunication

The scope of this work is the fundamental growth, tailoring and characterization of self-organized indium arsenide quantum dots (QDs) and their exploitation as active region for diode lasers emitting in the 1.55 µm range. This wavelength regime is especially interesting for long-haul telecommunications as optical fibers made from silica glass have the lowest optical absorption. Molecular Beam Epitaxy is utilized as fabrication technique for the quantum dots and laser structures. The results presented in this thesis depict the first experimental work for which this reactor was used at the University of Kassel. Most research in the field of self-organized quantum dots has been conducted in the InAs/GaAs material system. It can be seen as the model system of self-organized quantum dots, but is not suitable for the targeted emission wavelength. Light emission from this system at 1.55 µm is hard to accomplish. To stay as close as possible to existing processing technology, the In(AlGa)As/InP (100) material system is deployed. Depending on the epitaxial growth technique and growth parameters this system has the drawback of producing a wide range of nano species besides quantum dots. Best known are the elongated quantum dashes (QDash). Such structures are preferentially formed, if InAs is deposited on InP. This is related to the low lattice-mismatch of 3.2 %, which is less than half of the value in the InAs/GaAs system. The task of creating round-shaped and uniform QDs is rendered more complex considering exchange effects of arsenic and phosphorus as well as anisotropic effects on the surface that do not need to be dealt with in the InAs/GaAs case. While QDash structures haven been studied fundamentally as well as in laser structures, they do not represent the theoretical ideal case of a zero-dimensional material. Creating round-shaped quantum dots on the InP(100) substrate remains a challenging task. Details of the self-organization process are still unknown and the formation of the QDs is not fully understood yet. In the course of the experimental work a novel growth concept was discovered and analyzed that eases the fabrication of QDs. It is based on different crystal growth and ad-atom diffusion processes under supply of different modifications of the arsenic atmosphere in the MBE reactor. The reactor is equipped with special valved cracking effusion cells for arsenic and phosphorus. It represents an all-solid source configuration that does not rely on toxic gas supply. The cracking effusion cell are able to create different species of arsenic and phosphorus. This constitutes the basis of the growth concept. With this method round-shaped QD ensembles with superior optical properties and record-low photoluminescence linewidth were achieved. By systematically varying the growth parameters and working out a detailed analysis of the experimental data a range of parameter values, for which the formation of QDs is favored, was found. A qualitative explanation of the formation characteristics based on the surface migration of In ad-atoms is developed. Such tailored QDs are finally implemented as active region in a self-designed diode laser structure. A basic characterization of the static and temperature-dependent properties was carried out. The QD lasers exceed a reference quantum well laser in terms of inversion conditions and temperature-dependent characteristics. Pulsed output powers of several hundred milli watt were measured at room temperature. In particular, the lasers feature a high modal gain that even allowed cw-emission at room temperature of a processed ridge wave guide device as short as 340 µm with output powers of 17 mW. Modulation experiments performed at the Israel Institute of Technology (Technion) showed a complex behavior of the QDs in the laser cavity. Despite the fact that the laser structure is not fully optimized for a high-speed device, data transmission capabilities of 15 Gb/s combined with low noise were achieved. To the best of the author`s knowledge, this renders the lasers the fastest QD devices operating at 1.55 µm. The thesis starts with an introductory chapter that pronounces the advantages of optical fiber communication in general. Chapter 2 will introduce the fundamental knowledge that is necessary to understand the importance of the active region`s dimensions for the performance of a diode laser. The novel growth concept and its experimental analysis are presented in chapter 3. Chapter 4 finally contains the work on diode lasers.

Higher Education Governance in Vietnam: University Action, the State and Changing Relationships

Diese Dissertation stellt eine Studie da, welche sich mit den Änderungen in der Governance der Hochschulbildung in Vietnam beschäftigt. Das zentrale Ziel dieser Forschungsarbeit ist die Untersuchung der Herkunft und Änderung in der Beziehung der Mächte zwischen dem vietnamesischen Staat und den Hochschulbildungsinstituten (HI), welche hauptsächlich aus der Interaktion dieser beiden Akteure resultiert. Die Macht dieser beiden Akteure wurde im sozialen Bereich konstruiert und ist hauptsächlich durch ihre Nützlichkeit und Beiträge für die Hochschulbildung bestimmt. Diese Arbeit beschäftigt sich dabei besonders mit dem Aspekt der Lehrqualität. Diese Studie nimmt dabei die Perspektive einer allgemeinen Governance ein, um die Beziehung zwischen Staat und HI zu erforschen. Zudem verwendet sie die „Resource Dependence Theory“ (RDT), um das Verhalten der HI in Bezug auf die sich verändernde Umgebung zu untersuchen, welche durch die Politik und eine abnehmende Finanzierung charakterisiert ist. Durch eine empirische Untersuchung der Regierungspolitik sowie der internen Steuerung und den Praktiken der vier führenden Universitäten kommt die Studie zu dem Schluss, dass unter Berücksichtigung des Drucks der Schaffung von Einkommen die vietnamesischen Universitäten sowohl Strategien als auch Taktiken entwickelt haben, um Ressourcenflüsse und Legitimität zu kontrollieren. Die Entscheidungs- und Zielfindung der Komitees, die aus einer Mehrheit von Akademikern bestehen, sind dabei mächtiger als die der Manager. Daher werden bei initiativen Handlungen der Universitäten größtenteils Akademiker mit einbezogen. Gestützt auf die sich entwickelnden Muster der Ressourcenbeiträge von Akademikern und Studierenden für die Hochschulbildung prognostiziert die Studie eine aufstrebende Governance Konfiguration, bei der die Dimensionen der akademischen Selbstverwaltung und des Wettbewerbsmarktes stärker werden und die Regulation des Staates rational zunimmt. Das derzeitige institutionelle Design und administrative System des Landes, die spezifische Gewichtung und die Koordinationsmechanismen, auch als sogenanntes effektives Aufsichtssystem zwischen den drei Schlüsselakteuren - der Staat, die HI/Akademiker und die Studierenden – bezeichnet, brauchen eine lange Zeit zur Detektion und Etablierung. In der aktuellen Phase der Suche nach einem solchen System sollte die Regierung Management-Tools stärken, wie zum Beispiel die Akkreditierung, belohnende und marktbasierte Instrumente und das Treffen informations-basierter Entscheidungen. Darüber hinaus ist es notwendig die Transparenz der Politik zu erhöhen und mehr Informationen offenzulegen.

Sensor-based Control of Chewing and Rumination Behavior of Dairy Cows

The measurement of feed intake, feeding time and rumination time, summarized by the term feeding behavior, are helpful indicators for early recognition of animals which show deviations in their behavior. The overall objective of this work was the development of an early warning system for inadequate feeding rations and digestive and metabolic disorders, which prevention constitutes the basis for health, performance, and reproduction. In a literature review, the current state of the art and the suitability of different measurement tools to determine feeding behavior of ruminants was discussed. Five measurement methods based on different methodological approaches (visual observance, pressure transducer, electrical switches, electrical deformation sensors and acoustic biotelemetry), and three selected measurement techniques (the IGER Behavior Recorder, the Hi-Tag rumination monitoring system and RumiWatchSystem) were described, assessed and compared to each other within this review. In the second study, the new system for measuring feeding behavior of dairy cows was evaluated. The measurement of feeding behavior ensues through electromyography (EMG). For validation, the feeding behavior of 14 cows was determined by both the EMG system and by visual observation. The high correlation coefficients indicate that the current system is a reliable and suitable tool for monitoring the feeding behavior of dairy cows. The aim of a further study was to compare the DairyCheck (DC) system and two additional measurement systems for measuring rumination behavior in relation to efficiency, reliability and reproducibility, with respect to each other. The two additional systems were labeled as the Lely Qwes HR (HR) sensor, and the RumiWatchSystem (RW). Results of accordance of RW and DC to each other were high. The last study examined whether rumination time (RT) is affected by the onset of calving and if it might be a useful indicator for the prediction of imminent birth. Data analysis referred to the final 72h before the onset of calving, which were divided into twelve 6h-blocks. The results showed that RT was significantly reduced in the final 6h before imminent birth.