1,720,968 research outputs found
Modekopplung, dynamische und nichtlineare Eigenschaften der MOVPE-gewachsenen Quantenpunkt- und Quantenfaden-Halbleiterlasern
No full textDie eingereichte Arbeit untersucht die Eigenschaften von auf InP basierenden optischen Quantenpunkt- (quantum dot, QDot) und Quantenfaden (QDash)- Halbleiterlasern. Der erste Teil der Arbeit widmet sich den Grundlagen und Vorteilen der QDot/QDash Laser. Im zweiten Teil der Arbeit wird die Herstellung und Entwicklung der QDot/QDash Laser gezeigt.
Der dritte Teil der Arbeit widmet sich den systematischen Experimenten der QDot/QDash Einsektions- und Zweisektions- (modegekoppelter-) Lasern. Erstmalig wird demonstriert, dass die MOVPE-gewachsenen passiv modegekoppelter QDot Laser Ex-Facette optische Pulse mit Halbertsbreiten im ps Bereich und Pulsewiederholfrequenz von ~33 GHz emitieren. Die Modenkopplungsfrequenz der passiven modegekoppelten Laser kann in 300-400 MHz Bereich angepasst werden, wobei in Einsektions modegekoppelten Lasern die Modenkopplungsfrequenz 10-fach weniger abstimmbar ist. Die Einsektions modegekoppelten QDot/Qdash Laser zeigen ein erheblich geringeres Zeit-Bandbreiten-Produkt und ein auf Autokorrelations-Messung basierendes Lorentzprofil des Pulses. Die Einsektions modegekoppelten Laser zeigen keine Ex-Facette Pulseemission und somit ist eine externe Komprimierung des Pulses erforderlich. Die Messung der Pulsform und Pulsphase der Einsektions modegekoppelten QDot/Qdash Laser mittels eines FROG Aufbaus (Frequenzaufgelösten Autokorrelation) zeigt einen nonlinearen Chirp und Intensiäts-Oszillationen. Zusammengefast, Qdot aund Qdash basierte passiv Zweisektionsslaser weisen gleiche Modekopplungs- und dynamische Eigenschaften auf. Im Gegensatz dazu, zeigen Einsektionsmodegekoppelte QDot/Qdash Laser zeigen unterschiedliche dynamische Eigenschaften; Grund dafür ist die Konkurenz zwischen sog. Supermode.
Im vierten Teil der Arbeit wird Vierwellenmischung basierte Wellenlängenkonversion in der Kavität der Qdot und Qdash Laser untersucht. 1490-µm lange Qdash Laser demonstrieren eine normalisierte Konversionseffizienz von -18.6 dB mit einem Signal-zu-Rausch-Verhältnis von 37 dB und 3 THz Frequenzverstimmung, wobei 1250-µm lange Qdot laser 12 dB und 14.7 dB niedrigere Konversionseffizienz und Signal-zu-Rausch-Verhältnis aufweisen. Der Grund dafür ist, dass Vierwellenmischung in Qdash Laser von Zwei-Photonen-Absorption, einer schnellen Sättigung des Gewinns und erhöhter Ladungsträgermodulation profitieren kann. Im Vergleich mit Qdot Laser, eignen Qdash basierte Laser sich daher besonders für effiziente Vierwellenmischung.
Im fünften Teil der Arbeit wurden dynamische Eigenschaften von Qdot/Qdash Lasern mit kurzen Kavitätslängen unter Klein-/Grosssignal Modulation untersucht. Qdot Laser stellen höheren D-faktor bei niedrigerem Strom dar, was bedeutet, dass Qdot Laser eine höhere Resonanzfrequenz und Sätigung der -3 dB Modulationsbandbreite bei niedrigem Strom bieten. Das Merkmal kann in Konzepte für energieeffiziente Laserquellen nützlich sein. Im Vergleich mit Qdash Laser, demonstrieren Qdot Laser stärkere Dämpfung der Resonanzoszillationen aufgrund starker Verstärkungskompression. Qdot Laser besitzen eine -3 dB Bandbreite von 8.1 GHz und ermöglichen eine fehlerfreie Übertragung und Detektion von intensitätsmodulierten Signalen (OOK) mit einer Rate bis zu 22.5 Gbit/s. Qdash Laser bieten gleiche -3 dB Bandbreite aber bei mind. zweimal grösserem Strom, und OOK fehlerfreie Übertragung mit einer Rate bis zu 17.5 Gbit/s mit Potenzial bis zu 20 Gbit/s. Wärmeableitung-zu-Bitraten-Verhältnis ist 8 bzw. 11 pJ/bit für Qdot bzw. Qdash Laser. Ein optimierter Laser Chip Aufbau reduziert Parasitäreffekte und Aufwärmung des Lasers, dadurch können Energieeffizienz bzw. dynamische Eigenschaften verbessert werden.This thesis focuses on the study of novel InAs quantum dot (QDot) and quantum dash (QDash) structures based lasers grown on InP substrates. It firstly covers device fabrication and processing, as well as the static characterization. Systematic investigation of passively mode-locked two-section lasers and single-section mode-locked lasers based on QDot and QDash active material is presented. For the first time to our knowledge, we demonstrate ps range ex-facet pulse emission from MOVPE-grown InP/InAs QDot passively mode-locked two-section laser at ~33 GHz repetition frequency. The mode-locking frequency of QDot and QDash passively mode-locked two-section lasers can be tuned in a range of 300–400 MHz, whereas single-section mode-locked lasers allow a factor of 10 lower frequency tunability. Regarding pulse characteristics, we observe significantly lower time-bandwidth product for single-section mode-locked lasers and Lorentzian shaped pulse emission, nevertheless single-section mode-locked lasers do not exhibit ex-facet pulse emission and thus are subject to external compression. FROG-measurements reveal the presence of the non-linear chirp component and intensity oscillations on the leading edge of the optical pulses both in QDot and in QDash single-section mode-locked lasers. To summarize, QDot and QDash passively mode-locked two-section lasers demonstrate close mode-locking properties and dynamics, while some difference between mode-locking dynamics is observed between their single-section mode-locked counterparts mainly related to the artifacts of supermodes competition. Discussing intracavity based four-wave mixing frequency conversion, we report on the large normalized conversion efficiency of -18.6 dB, OSNR of 37 dB, and a frequency detunings up to 3 THz 1490 µm-long InP/InAs quantum-dash lasers operating at 1550 nm under dual-mode optical injection. We observe a larger conversion efficiency for 1490 µm-long QDash devices in comparison to 1250 µm-long QDot lasers, fabricated by the same growth and processing techniques. QDot devices with delta-function DOS show a four times lower third-order optical susceptibility, a 12 dB-lower normalized conversion efficiency and 14.7 dB-lower optical signal-to-noise ratio. The analysis confirms that such a difference is not only due to the cavity length but also to additive contributions to the gain such as TPA, faster gain saturation, and enhanced modulation of carrier populations. Our results unveil which structural approach must be followed in realization of lasers for efficient nondegenerate four-wave mixing based wavelength conversion. Finally, we have compared dynamic properties of short-cavity quantum-dot and quantum-dash lasers under small-and large-signal operation. We observed a larger D–factor for quantum-dot lasers, which implies a larger resonance frequency at lower bias current, and a saturation of the modulation bandwidth at lower bias levels. This feature can be useful in concepts for energy efficient laser sources. Larger gain compression is the reason for the overdamped resonance frequency peak in quantum-dot lasers, whereas moderate damping is observed in quantum-dash devices. We have shown 8.1 GHz modulation and error-free 22.5 Gbit/s large signal modulation for InP/InAs quantum-dot lasers. Quantum-dash show similar small signal modulation bandwidths, though at significantly higher injection currents and a bit rate of 17.5 Gbit/s (error-free) with potential for 20 Gbit/s. Dissipated heat-to-bit rate ratios are as low as 8 and 11 pJ/bit for quantum-dot and quantum-dash lasers, respectively. It is important to note that parasitics and heating effects can be strongly reduced for the present devices in the future by means of optimized laser chip and mounting leading to further improved dynamic properties of the lasers.EC/FP7/264687/EU/Postgraduate Research on Photonics as an Enabling Technology/PROPHE
Comparison of dynamic properties of InP/InAs quantum-dot and quantum-dash lasers
Get PDFThis article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 109, 161104 (2016) and may be found at https://doi.org/10.1063/1.4965846.The dynamic properties of MOVPE grown InP/InAs quantum-dot and quantum-dash lasers, showing identical structural design, emitting in the C-band are investigated and compared to each other. Based on the small-signal measurements, we show the impact of the density of states function on the cut-off frequency, being larger for quantum dots at low currents, and reaching similar values for quantum dashes only at higher currents. The large-signal measurements show error-free data transmission at 22.5 and 17.5 Gbit/s for the quantum-dot and quantum-dash lasers.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeEC/FP7/EU/264687/Postgraduate Research on Photonics as an Enabling Technology/PROPHE
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
- …
