• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 261
  • 41
  • 31
  • 26
  • 20
  • 17
  • 12
  • 9
  • 6
  • 5
  • 2
  • 2
  • 1
  • 1
  • Tagged with
  • 485
  • 216
  • 169
  • 168
  • 76
  • 66
  • 56
  • 48
  • 47
  • 45
  • 43
  • 42
  • 41
  • 40
  • 38
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
171

Photonic devices and subsystems for future WDM PON and radio over fiber technologies. / CUHK electronic theses & dissertations collection

January 2010 (has links)
Besides colorless ONUs, we investigate potentially low cost, high speed vertical-cavity-surface-emitting lasers (VCSELs) for use in future access networks. VCSELs are attractive because they may meet the stringent size, power dissipation and cost constraints of access network components. We carry out experiments to demonstrate that up to 20 Gb/s direct modulation of long wavelength VCSEL is possible and evaluate their performance as high-speed transceivers. / In order to reduce the complexity and costs of ONU transceiver, we propose a scheme based on a nonreciprocal optical modulator and a linear loop mirror for receiving downstream and sending upstream data We show that the nonreciprocity of traveling wave electrodes can selectively impress signal modulation onto the reflected upstream signals only. Monolithic integrated transceivers may thus remodulate downstream signals for upstream data transmission without needing integrated optical circulators. The proposed ONU is thus compatible with monolithic integration. / In this thesis, we describe our research on photonic devices and subsystems for future access networks. Since optical network units (ONUs) are the most cost-sensitive parts, we first investigate the use of advanced modulation format in colorless ONU structure. We implement a scheme which uses dark return-to-zero (DRZ) for downstream transmission and remodulation of it using a differential-phase-shift-keying (DPSK) for upstream both at 10 Gb/s. We also experimentally demonstrate silicon microring based optical frequency discriminators for use in demodulating DPSK and differential-quadrature-phase-shift-keying (DQPSK) signals. We show that the scheme is robust to variations in bit-rates in contrast with conventional Mach-Zehnder delay interferometer scheme. / Internet traffic has undergone tremendous growth in the past decades and has already penetrated into the daily lives of the general population. Demand for new high bandwidth services is beginning to drive the deployment of optical fiber-based access networks to solve the so-called last mile bottleneck around the world. Passive optical networks (PON) are attractive because there are no active components in the transmission line, thus reducing operational and deployment costs. Time-division-multiplexing (TDM) used in currently deployed PON, in which the bandwidth is shared among the users by time domain multiplexing, does not fully utilize the bandwidth potential of optical fibers and will not be able to satisfy the bandwidth demand in access networks in the near future. Among the advanced multiplexing techniques, wavelength-division-multiplexing (WDM) PON is a good candidate technology for providing sustained bit-rates beyond 10 Gb/s in access networks. However, reduction of costs in WDM PON remains a key challenge for their practical deployment. / Wired and wireless hybrid optical access networks are also investigated. Radio-over-fiber is one low-cost approach to deliver broadband wireless services, in which radio signals at the carrier frequency are delivered over optical networks from a central office to remote antenna base stations. Generation of high frequency carrier and radio frequency fading are the main research challenges. We propose and demonstrate frequency upconversion based on frequency doubling and quadrupling. Novel wired and wireless hybrid subsystems that mitigate millimeter-wave signal distortion are also demonstrated. / Xu, Lin. / Adviser: H. K. Tsang. / Source: Dissertation Abstracts International, Volume: 72-04, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. Ann Arbor, MI : ProQuest Information and Learning Company, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.
172

All-optical devices for wavelength-division multiplexing (WDM) and optical time-division multiplexing (OTDM). / CUHK electronic theses & dissertations collection

January 2002 (has links)
Wong Chi Sang. / "August 2002." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2002. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.
173

Design of optical burst switches based on dual shuffle-exchange network and deflection routing.

January 2003 (has links)
Choy Man Ting. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2003. / Includes bibliographical references (leaves 66-69). / Abstracts in English and Chinese. / Acknowledgments --- p.ii / Abstract --- p.v / Table of Contents --- p.vii / List of figures --- p.viii / Chapter Chapter 1 --- p.12 / Chapter 1.1 --- OBS Network Architecture --- p.3 / Chapter 1.2 --- Offset Time and Reservation Schemes --- p.5 / Chapter 1.3 --- Research Objectives --- p.7 / Chapter 1.4 --- Overview --- p.8 / Chapter Chapter 2 --- p.9 / Chapter 2.1 --- WDM crossbar architectures --- p.9 / Chapter 2.2 --- Switch Based on Optical Crossbars --- p.10 / Chapter 2.3 --- Switch Based on Wavelength Grating Routers --- p.11 / Chapter Chapter 3 --- p.14 / Chapter 3.1 --- Basics of Dual Shuffle Exchange Network --- p.14 / Chapter 3.2 --- Dual Shuffle-exchange Network --- p.16 / Chapter 3.3 --- Proposed Architecture based on DSN --- p.19 / Chapter 3.4 --- Analysis on blocking due to output contention --- p.20 / Chapter 3.5 --- Implementation issues on the 4x4 switching module --- p.23 / Chapter 3.6 --- Analysis: Non-blocking versus banyan --- p.25 / Chapter Chapter 4 --- p.30 / Chapter 4.1 --- First Scheme --- p.30 / Chapter 4.2 --- Simulation on the first scheme --- p.33 / Chapter 4.3 --- Second Scheme: Tunable wavelength converter --- p.37 / Chapter 4.4 --- Third Scheme: Route to specific wavelength port --- p.42 / Chapter 4.5 --- Analysis on blocking due to insufficient stages --- p.46 / Chapter Chapter 5 --- p.49 / Chapter 5.1 --- Delay analysis of DSN --- p.49 / Chapter 5.2 --- Vertical Expansion --- p.51 / Chapter 5.3 --- Simulation results on vertical expansion --- p.52 / Chapter 5.4 --- Building DSN with 8x8 MEMS switches --- p.54 / Chapter 5.5 --- Prove of the proposed Quarter shuffle network --- p.56 / Chapter 5.6 --- Comparison between Quarter shuffle and doubled links approaches --- p.58 / Chapter Chapter 6 --- p.64 / Conclusion --- p.64 / Bibliography --- p.66
174

Técnicas combinadas de autoalimentação e reuso de comprimento de onda em transmissores refletivos para redes WDM-PON / Combined self-seeding and wavelength reuse for reflective transmistters in WDM-PON

Duarte, Ulysses Rondina 06 March 2015 (has links)
Transmissores agnósticos em comprimento de onda colorless [1], isto é, transmissores que não estão restritos à operação em um único comprimento de onda, são considerados como um dos pré-requisitos para os projetos de redes ópticas passivas baseadas na multiplexação por divisão em comprimento de onda Wavelength Division Multiplexing Passive Optical Network. Esta configuração reduz consideravelmente os custos operacionais e de instalação da rede WDM-PON, uma vez que um único tipo de transmissor pode ser usado em qualquer terminal de assinante, independentemente dos comprimentos de onda pré-estabelecidos para os usuários em questão. Entretanto, a possibilidade de aumento do número de usuários e da capacidade de transmissão agregada, com poucas modificações na estrutura da rede WDM-PON, garantindo uma melhor relação custo-benefício, são questões também essenciais no projeto de sistemas multi-comprimento de onda.Tendo em vista o panoramo exposto, neste trabalho é proposta uma nova topologia de reuso de portadora óptica para redes WDM-PON, a qual combina sinais autoalimentados e remodulação óptica no domínio da intensidade, visando o atendimento das premissas tecnológicas para cenários de nova geração. Como metodologia, investigações experimentais são conduzidas em termos de caracterizações ópticas em função de vários parâmetros e avaliações sistêmicas em taxas simétricas de 1,25 Gb/s, no âmbito de redes de acesso e acesso estendido. Utilizam-se transmissores colorless do tipo amplificadores ópticos semicondutores refletivos e fontes ópticas banda larga baseadas em emissão espontânea amplificada, de forma a garantir a simplificação dos terminais dos usuários. Adicionalmente, são realizadas análises comparativas entre essa proposta frente aos trabalhos encontrados na literatura técnica, quando possível, em termos de custos operacionais, desempenho sistêmico e viabilidade de expansão para cenários a taxas simétricas de 10 Gb/s. De acordo com os resultados obtidos, acredita-se que essa topologia possa ser expandida para cenários de até 10 Gb/s com o uso de técnicas apropriadas de apagamento de portadora, mantendo a melhor relação custo-beneficio para sistemas de remodulação no domínio da intensidade. / Wavelength agnostic (colorless) transmitters [1], i.e, transmitters which are not fixed to a specific wavelength operation, are regarded as one of the essential keys for the designing of wavelength division multiplexing passive optical networks (WDM-PONs). The use of colorless transceiver leads to a reduction in the mainentence cost of the network, once the same basic optoelectronic hardware can be employed to provide any wavelength required by a specific optical network termination (ONT). However, the possibility of a graceful increase on the number of ONTs and transmission capacity offered by the network, with no disruptive changes in the WDM-PON infra-structure, while keeping a cost-effective topology, are also attractive features in the design of future access networks. In this context, a novel wavelength-reuse topology for WDMPON is proposed in this work. The wavelength-sharing technique is based on combined self-seeded and remodulated signals in the intensity domain, in order to comply with the technology requierements for next generation networks. Experimental analysis are carried out in respect to optical characterization and system evaluation on symmetric rates of 1.25 Gb/s, aiming both short reach and extended reach scenarios. Reflective semicondutor optical amplifiers and broadband light sources based on amplified spontaneous emission are used as colorless transceivers in order to guarantee a costeffective ONT configuration. Also, comparative analyses between this proposal and works found in the technical literatural are established in terms of maintenance costs, system performance and feasibility of expansion to symmetric 10 Gb/s scenario. According to the results, we believe that network could be expanded to symmetric 10 Gb/s scenario with the development of proper erasure technique, while keeping the best cost-effective configuration of intensity-remodulated systems.
175

The host galaxies of luminous reddened quasars at z~2

Wethers, Clare January 2018 (has links)
The work in this thesis concerns the host galaxies of a class of luminous, yet heavily-obscured, quasars at z$\sim$2 - a peak epoch of both star formation and black hole accretion. Here, we seek to characterise the star-forming properties of these obscured quasars to improve our understanding of galaxy-quasar coevolution. A key issue facing host galaxy studies among populations of the most luminous quasars is being able to disentangle the galaxy emission from that of the quasar. With combined observations from the Dark Energy Survey (DES), the VISTA Hemisphere Survey (VHS) and the UKIDSS Large Area Survey (ULAS), we exploit the quasar dust extinction in our sample to demonstrate that the quasar and galaxy emission can be separated via SED-fitting in these systems. By isolating the galaxy emission in this way, we estimate instantaneous SFRs for the galaxies in our sample, based on the restframe UV emission. In general, we find obscured quasars to reside in prodigiously star forming hosts with 25 $\lesssim$ SFR$_{\rm{UV}}$ $\lesssim$ 365 M$_{\rm{\odot}}$yr$^{-1}$. Furthermore, we show that the most luminous quasars reside in the most actively star-forming galaxies, potentially indicating the same gas supply is fuelling both star formation and accretion on to the black hole. Having isolated the galaxy emission via SED-fitting, we test our ability to model the restframe-UV emission of obscured z$\sim$2 quasar hosts in 2D. Until now, morphological studies of luminous quasar hosts have typically been limited to low redshifts or relied on space-based imaging. By making use of a multi-band modelling code however, we demonstrate that it is possible to accurately infer several galaxy properties (i.e. the position of the galaxy in the image, (X,Y), its radius, R$_{\rm{eff}}$, axis ratio, q$_{\rm{GAL}}$, angle of orientation, $\theta$, and S\'rsic index, $n$), based on current ground-based imaging, accross the full range of galaxy and quasar luminosities considered in our sample. This potentially opens the door to future ground-based morphological studies of obscured quasars at high redshift. At sub-mm wavelengths, thermal emission from cold dust peaks, meaning these wavelengths can be used to probe the dust heating by star formation, effectively giving a measure of the obscured star formation in the galaxy. Using targeted observations from SCUBA-2, we trace the 850$\mu$m emission in a sample of obscured quasars, finding evidence for prodigious star formation $ > $ 2400 M$_$yr$^$ in three of the 19 quasars in our sample. The detection rate of our obscured quasar sample is found to be consistent with that of both more heavily-obscured Hot-DOGs and UV-luminous quasars, once the samples have been matched in luminosity and redshift. Furthermore, we find evidence that several of the obscured quasars lie in overdense regions of the sky ($\sim$ 3 times denser than sub-mm blank fields).
176

Investigation of rectangular, uni-directional, horizontally polarised waveguide antennas with longitudinal slotted arrays operating at 2.45 GHz

Perovic, Una 14 November 2006 (has links)
Student Number : 0002066T - MSc dissertation - School of Electrical and Information Engineering - Faculty of Engineering and the Built Environment / Investigations of uni-directional, horizontally polarized waveguide antennas with longitudinal slotted arrays operating at 2.45GHz and their applications to wireless local area networks (WLAN) are presented in this paper. Requirements, considerations, and limitations associated with the design process of this particular waveguide are discussed and presented. Various antenna parameters were simulated using MATLAB® and SuperNEC® software simulation programs, and were applied to a sensitivity analysis of antenna design. End-fed and center-fed antennas were designed, built, and measured at WLAN frequencies. Measured antennas had high gain above 15dBi, broad beam around the azimuth, and high efficiency, but were limited by their impedance dependency and narrow bandwidth. The center-fed antenna had 3dBi higher gain than the 18dBi gain of the end-fed antenna. The VSWR ratio of both antennas was less than 1:1.5 at the operating frequency. The center-fed antenna had broader azimuth and elevation patterns by 40° and 10°, respectively. The end- fed antenna had more stable gain and VSWR, 50% wider VSWR bandwidth of 100MHz, and more directional elevation pattern. The design criteria generated using waveguide theory and simulated analysis was validated by the physical design and performance of the measured antennas.
177

A Study of Denitrosation of N-Nitroso Compounds by Irradiation with Long-Wavelength UV Light

Wang, Zhenyu J. 01 May 1989 (has links)
N-Nitrosamines and N-nitrosamides have been reported to be mutagenic. N-Nitrosamides are direct mutagens that need no activation to be mutagenic, whereas N-nitrosamines need to be enzymatically activated to exert their mutagenicity. Oxidative demethylation of nitrosamines is a commonly accepted activation mechanism. Another pathway of nitroso compounds, denitrosation, has recently been proposed. The mechanism of denitrosation, however, is still unknown. The purpose of this study was to use a photo -reaction model to explore the possible denitrosation mechanism of N-nitroso compounds. An N-nitrosamine, N-nitrosomorpholi ne (NMOR), and an N-nitrosamide, N-methyl-N'-nitrosoguanidine (MMNG), were irradiated with long-wavelength UV light in the presence of phosphate. Denitrosation of these nitroso compounds occurred, and free radicals were generated during the photolysis of N-nitroso compounds. The free radical from NMOR is carbon centered, and the free radical from MNNG is nitrogen centered. They have different responses to phosphate. A stable mutagen is formed during the photolysis of NMOR in the presence of phosphate. The formation of this stable mutagen is through a free-radical mechanism. MNNG appears to be inactivated in terms of mutagenicity by the photodenitrosation. Free radicals produced during the photolysis of NMOR and MNNG are mutagenic through a direct radical interaction mechanism. The results of this study indicate that the newly found pathway of N-nitroso compounds, denitrosation, may be related to a free-radical mechanism.
178

Coherent multiwavelength sources for tropospheric aerosol lidar

Rawle, Christopher B., n/a January 2005 (has links)
The monitoring and study of the earth�s atmosphere is becoming an increasingly important task given the current uncertainties in climate prediction. Areas where lidar has been used to further understanding of the atmosphere include monitoring of greenhouse gases, global warming, stratospheric ozone depletion, photochemical smog and aerosol photochemistry. However, the potentially severe long term effects of anthropogenic aerosols on earth�s biosphere are poorly understood. This project seeks to apply state of the art laser technology to develop an innovative multiwavelength lidar system capable of providing new information and new insights into the field of tropospheric aerosol lidar. Several novel tunable laser and laser-like sources have been investigated and developed for the purpose of tropospheric aerosol lidar at The National Institute of Water and Atmospheric Research (Niwa), Central Otago. Multiwavelength operation in the visible and near infrared portion of the spectrum has been emphasised with the sources developed collectively spanning the wavelength interval of 400-1369 nm. The laser sources investigated were the LiF:F2+ colour centre, Titanium Sapphire (Ti:sapphire) and barium nitrate Raman lasers. In addition to the laser sources, the β-barium borate optical parametric oscillator (BBO OPO) was characterised. For each of the sources, lidar relevant aspects were studied. The results recorded include conversion efficiency with respect to the pump source, linewidth and tuning characteristics, beam quality, temporal behaviour, and device reliability and ruggedness. It was found that the LiF:F2+ laser offered significantly lower threshold, broader tuning and higher output pulse energies than the Ti:sapphire laser in the 900-1000 nm region. The high optical gain of the LiF:F2+ medium facilitated cavity optical alignment and operation of the system. The high gain also resulted in temporal behaviour well suited to the existing Niwa lidar detection scheme. When using a 5 ns pump source, amplified spontaneous emission (ASE) was found to limit the laser tuning range and efficiency. The barium nitrate Raman laser was based on a simple linear cavity arrangement which resulted in a compact and robust device with no moving components. The stimulated Raman scattering process offers relatively narrow linewidth laser operation at the first and second Stokes wavelengths of 1197 nm and 1369 nm respectively. This laser offered efficient operation once the high operation threshold was reached. Second harmonic generation was used to extend the number of potential lidar transmitter lines produced. The barium nitrate Raman laser possessed high beam divergence and a maximum of three discrete transmitter wavelengths. The BBO OPO used a type I collinear signal resonant configuration. A plane-plane cavity configuration with pump reflection was found to provide simplicity of design, low threshold, highly efficient operation and output pointing stability. The BBO OPO signal wavelength could be tuned over the wavelength interval of 400-700 nm. The disadvantage of the plane cavity was high output beam divergence. However, this was successfully brought within the required limits through the use of a 40 mm long cavity in conjunction with an expanding and collimating telescope. As a result of the study, a Tunable lidar Transmitter (TLT) system based on the BBO OPO was designed and constructed at the Physics Department. The TLT was computer-controlled using custom written software and constructed in a self contained modular manner with all required mechanical, electrical and optical components. A user manual was also written to accompany the TLT. The TLT was installed at Niwa and was successfully used to gather preliminary multiwavelength lidar data. The TLT BBO OPO threshold occurred for a pump energy of 5.2 mJ (10.6 MW/cm2) and had a maximum slope efficiency of 53%. Signal efficiency varied from 24-41-35% over the intervals of 410-500-600 nm. A maximum signal energy of 21 mJ was obtained for a signal wavelength of 492 nm when using the maximum available pump energy of 42 mJ. OPO signal linewidth varied from 0.1-1-8 nm over the signal wavelength intervals of 400-600-700 nm. The associated OPO finesse varied between 370 and 100 as the signal wavelength was tuned over the wavelength interval of 400-600 nm. The temporal behaviour of the BBO OPO was a slowly varying function of pump energy and closely followed the temporal behaviour of the pump laser, making it well suited to the existing Italian lidar detection and timing scheme.
179

Studies of Light Emission from N-B doped 6H-SiC

Reimers, Petra January 2010 (has links)
<p>The purpose of this thesis work was to find a way to measure basic light emission properties of nitrogen-and-boron-doped 6H-SiC, which are fabricated with a growth method developed at Linköping University. The research is in its initial phase and the light properties as well as optical measurement techniques are important. The aim is that the results of the measurements will provide feedback to the growth process what quality and doping levels that are required to get the maximum amount of light. The measurements were performed at the Laboratory of Lighting Technology, Technical University of Darmstadt, Germany.</p><p>Two measuring methods with different excitation sources were tested: a double monochromator and a setup using near UV-filters. While the double monochromator was able to project wavelengths in steps down to 0.5 nm with a high accuracy, the filters were only available in steps of 10 nm where the accuracy of the wavelength values varied. The double monochromator was chosen for the continuing measurements.</p><p>When using excitation light between 375-390 nm the emitted light was in the visible wavelength region. The light properties measured were the irradiance (measured in W/m<sup>2</sup>) and the peak wavelength were maximum luminescence occurred.</p><p>The result showed that sample 2-4 had a peak wavelength at approximately 580‑582 nm for the excitation wavelength 375 nm. For sample 5 the peak wavelength occurred at 582 nm at the excitation wavelength 390 nm. Sample 1, the unintentionally doped, did not show any measurable results as expected.</p><p>When irradiance of the excitation light was approx. 8 W/m<sup>2</sup> the irradiance at the peak of luminescence for the samples varied between 15.03-29.35 mW/m<sup>2</sup>. The low values are believed to be the result of the emitted light scattering in all directions whereas the measurements are done in one direction and only from a small area of the sample.</p><p>The measurements has shown that it is possible to measure the light properties of the grown material even though the samples were not finalized (capsulated) LED’s. The results from the measurements are of interest for the continuing development of the material.</p>
180

Oberflächengitter in azobenzenhaltigen Schichten für organische DFB-Laser / Surface relief gratings in azobenzene containing layers for organic DFB lasers

Döring, Sebastian January 2011 (has links)
Ein neuentwickeltes azobenzenhaltiges Material, das auf einem supramolekularen Konzept basiert, wird bezüglich seiner Strukturbildung während einer holografischen Belichtung bei 488 nm untersucht. Im Mittelpunkt stehen dabei eindimensionale, sinusförmige Reliefs mit Periodizitäten kleiner 500 nm. Es wird gezeigt, wie der Grad der Vernetzung der photosensitiven Schicht die Strukturbildung in diesem Größenbereich beeinflusst. Zur Maximierung der Strukturtiefe werden gezielt Prozessparameter der Belichtung sowie Materialparameter variiert. Unter Standardbedingungen und moderaten Belichtungsintensitäten von ca. 200 mW/cm² bilden sich innerhalb weniger Minuten bei einer Periode von 400 nm Strukturtiefen von bis zu 80nm aus. Durch die Beeinflussung von Materialparametern, wie Oberflächenspannung und Viskosität, wird die maximale Strukturtiefe auf 160nm verdoppelt. Durch Mehrfachbelichtungen wird auch die Bildung von zweidimensionalen Gittern untersucht. Die Originalstrukturen werden in einem Abformverfahren kopiert und in Schichten von unter UV-Licht aushärtenden Polymeren übertragen. Durch das Abformen kommt es zu einer geringfügigen Verschlechterung der Oberflächenqualität sowie Abnahme der Strukturtiefe. Dieser Verlust wird durch eine Verringerung der Prozesstemperatur verringert. Mithilfe kopierter Oberflächengitter werden organische Distributed Feedback-(DFB)-Laser zweiter Ordnung hergestellt, um den Einfluss von Gitterparametern auf die Emissionseigenschaften dieser Laser zu untersuchen. Dazu erfolgt zunächst die Charakterisierung der optischen Verstärkungseigenschaften ausgewählter organischer Emittermaterialien mittels der Variablen Strichlängenmethode. Das mit dem Laserfarbstoff Pyrromthen567 (PM567) dotierte Polystyrol (PS) zeigt dabei trotz konzentrationsbedingter geringer Absorption eine vergleichsweise geringe Gewinnschwelle von 50µJ/cm² bei ca. 575 nm. Das aktive Gast-Wirt-System der konjugierten Polymere MEH-PPV und F8BT* weist eine hohe Absorption und eine kleine Gewinnschwelle von 2,5 µJ/cm² bei 630 nm auf. Dieses Verhalten spiegelt sich auch in den Emissionseigenschaften der damit hergestellten DFB-Laser wieder. Die Dicke der aktiven Schichten liegen im Bereich hunderter Nanometer und wird so eingestellt, dass sich nur die transversalen Grundmoden im Wellenleiter ausbreiten können. Die Gitterperiode sind so gewählt, dass ein Lichtmode im Verstärkungsbereich des Emittermaterials liegt. Die Emissionslinien der Laser sind mit FWHM-Werten von bis zu 0,3 nm spektral sehr schmalbandig und weisen auf eine sehr gute Gitterqualität hin. Die Untersuchungen liefern minimale Laserschwellen und maximale differentielle Effizienzen von 4,0µJ/cm² und 8,4% für MEH-PPV in F8BT* (bei ca. 640nm) sowie 80 µJ/cm² und 0,9% für PM567 in PS (bei ca. 575 nm). Die Vergrößerung der Strukturtiefe von 40nm auf 80nm in mit MEH-PPV dotierten F8BT*-Lasern zu einem deutlichen Anstieg der ausgekoppelten Energie sowie der differentiellen Effizienz und einem geringen Absinken der Laserschwelle. Dies ist ein Resultat der erhöhten Kopplung von Lasermode und Gitter. Die Emission von DFB-Lasern mit zweidimensionalen Oberflächengittern zeigen eine Verringerung der Divergenz aber kein Einfluss auf die Laserschwelle. Abschließend erfolgt eine Vermessung der Photostabilität von DFB-Lasern unter verschiedenen Bedingungen. Das Einbringen eines konjugierten Polymers in eine aktive Matrix sowie der Betrieb in einer Stickstoffatmosphäre führen dabei zu einer Erhöhung der Lebensdauer auf über eine Million Pulse. Durch die Kombination von Oberflächengittern in PDMS-Filmen mit elektroaktiven Substraten wird eine elektrisch steuerbare Deformation des Beugungsgitters erreicht und auf einen DFB-Laser übertragen. Die spannungsinduzierte Verformung wird zunächst in Beugungsexperimenten charakterisiert und ein optimaler Arbeitspunkt bestimmt. Mit den beiden Elastomeren SEBS12 und VHB4910 werden in den Gittern maximale Periodenänderungen von 1,3% bzw. 3,4% bei einer Steuerspannung von 2 kV erreicht. Der Unterschied resultiert aus den verschiedenen Elastizitätsmoduln der Materialien. Übertragen auf DFB-Laser resultiert eine Variation der Gitterperiode senkrecht zu den Gitterlinien in einer kontinuierlichen Verschiebung der Emissionswellenlänge. Mit einem Spannungssignal von 3,25 kV wird die schmalbandige Emission eines elastischen DFB-Lasers kontinuierlich um fast 50nm von 604 nm zu 557 nm hin verschoben. Aus dem Deformationsverhalten sowohl der reinen Beugungsgitter als auch der Laser werden Rückschlüsse auf die Elastizität der verwendeten Materialien gezogen und erlauben Verbesserungen der Bauteile. / The photoinduced surface relief formation in a newly developed azobenzene containing material is investigated. A photosensitive film based on a supramolecular material concept is illuminated in a holographic setup with light at 488 nm leading to sinusoidal surface structures. It can be shown how the degree of network formation within the material influences structure formation at periods below 500nm. Different material and process parameters are varied to determine maximum possible structure amplitude. At moderate recording conditions and illumination intensities of 200 mW/cm² maximum structure amplitudes of 80 nm are formed within several minutes at periods of approximately 400 nm. Changing material parameters like surface tension and viscosity leads to an increase of maximum amplitude to 160 nm. Additionally, twodimensional structures are fabricated by multiple recording steps. Original surface structures are copied by soft lithography methods into films of UV cured polymers. This replication leads to a decrease of structure contrast and increase of surface roughness. It is shown that structure contrast is preserved by reduction of process temperature. Organic second order distributed feedback (DFB) lasers are prepared with the help of replicated surface gratings covered with organic active layers with thicknesses of several hundred nanometers. The lasers are investigated regarding the influence of grating amplitude on laser threshold and slope efficiency. For this, the optical gain of two green/red emitter materials, polystyrene (PS) doped with laser dye pyrromethene567 (PM567) and active guest host system of conjugated polymers MEH-PPV and F8BT*, is measured. PS doped with PM567 shows relatively low gain threshold of 50 µJ/cm² at 575 nm caused by the low dye concentration while F8BT* doped with MEH-PPV shows threshold gain of 2.5 µJ/cm² at 630 nm. Similar behavior is observed during laser action. Following Bragg-condition grating periods at approximately 400nm lead to laser emission within the gain spectra of the materials. DFB lasers show single mode narrow line emission with FWHMs of down to 0.3nm. This is a result of high grating quality. Lasers based on MEH-PPV-F8BT* exhibit laser thresholds and slope efficiencies of 4.0µJ/cm² and 8.4%, respectively. PM567-PS lasers exhibit 80 µJ/cm² and 0.9%. An increase of grating amplitude of a MEH-PPV-F8BT*-DFB laser from 40nm to 80nm leads to a considerable increase of energy output and a slight decrease of laser threshold. This is a result of an increase of coupling between light mode and grating which relates to an increase of reflectivity. The use of a two dimensional grating leads to a decrease of divergence angle of laser emission but shows no influence on laser threshold. Finally, laser lifetime is observed under different conditions. Doping the conjugated polymer MEH-PPV in an active matrix in combination with the allocation of nitrogen atmosphere leads to an prolongation of half life time to several million pulses. The combination of elastic surface relief gratings with electro active substrates enables electrical controlled deformation of the surface structure. This deformation is characterized by diffraction measurements. At optimal working conditions electro active substrates based on elastomers SEBS12 and VHB4910 show maximum grating deformation of 1.3% und 3.4% at actuation voltages of 2kV, respectively. The different results are caused by the different elastic modulus of the elastomer materials. The transfer of deformation principle to elastic DFB lasers leads to a continuously shift of emission wavelength. At an actuation voltage of 3.25kV an elastic DFB laser based on polyvinylacetate doped with PM567 shows a shift of nearly 50 nm, from 604 nm to 557 nm. Deformation behavior of grating and laser allow to draw conclusions on elasticity of the material and with this is tool for the improvement of the device.

Page generated in 0.021 seconds