<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/m²) 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/m² the irradiance at the peak of luminescence for the samples varied between 15.03-29.35 mW/m². 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>

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.

Paper that is subjected to moisture undergoes dimensional changes. It expands during moistening and shrinks during drying. When the paper is under tension between rollers, the effects are complex since shrinkage and expansion are restricted in the width direction. Waves can then appear on the paper web. This can be a problem in heatset web offset printing. The problem is known as waviness or fluting. The printed papers exhibit a wavy shape, which is visually disturbing due to light reflections which create glossy streaks. The aim of the work described in this thesis was to develop a method suitable for studying the moisture- and tension-induced waviness. Experiments were carried out on a laboratory scale to study how such waves develop during moistening and drying. The experimental setup was based on a modified tensile tester. A CCD camera and image analysis based on the STFI-OptiTopo technique was used to characterise the waviness. Moistening and drying were achieved by changing the surrounding air humidity. The method was used to study the effect of moisture uptake by the paper, and to evaluate the effect of tension on the waviness. It was found that increasing moisture resulted in a higher waviness amplitude, but that the web tension controlled the wavelength of the waviness. A high tension gave rise to a shorter wavelength. The measured wavelength was compared with a previously suggested model and the predicted wavelength was about twice as high as the measured wavelength. / When paper becomes damp it can develop waves which can be a problem with heatset web offset printing. Paper with waves may have glossy streaks after printing. Laboratory experiments studied the effect of moisture uptake by the paper. Readings were taken at a range of air humidities. The effect of tension on the waviness was also measured. The waviness was characterised using a charge coupled device (CCD) camera and the STFI-Packforsk OptiTopo technique to analyse the images. When the humidity was increased the amplitude of the waves increased. A high tension resulted in shorter wavelengths. Results from a previous model were used as a comparison.

Vertical-cavity surface-emitting lasers (VCSELs) areattractive as potential inexpensive high-performance emittersfor fibre-optical communication systems. Their surface-normalemission together with the small dimensions are beneficial forlow-cost fabrication since it allows on-wafer testing,simplified packaging and effective fibre-coupling. Forhigh-speed data transmission up to hundreds of metres, 850-nmVCSELs are today the technology of choice. For higher bandwidthand longer distance networks, emission at long-wavelength(1.3-1.55 µm) is required. Long-wavelength VCSELs are,however, not available since no materials system offershigh-index-contrast distributed Bragg reflectors (DBRs) as wellas high-gain active regions at such wavelengths.High-performance DBRs may be built up from AlGaAs/GaAsmultilayers, but long wavelength quantum wells (QWs) are onlywell established in the InP system. Therefore, the bestperforming devices have relied on wafer-fusion betweenInP-based QWs and AlGaAs-DBRs. More recently, however, the mainefforts have been shifted towards all-epitaxial GaAs-baseddevices, employing 1.3-µm GaInNAs QWs. In this thesis, different processing technologies forlong-wavelength VCSELs are described. This includes a thoroughinvestigation of wafer-fusion between InP and GaAs regardingelectro-optical as well as metallurgical properties, and thedevelopment of a stable low-pressure process for the selectiveoxidation of AlAs. Optimised AlGaAs/GaAs DBRs were designed andfabricated. An important and striking observation from thatstudy is that n-type doping potentially is much moredetrimental to device performance than previously anticipated.These investigations were exploited in the realisation of twonew VCSEL designs. Near-room-temperature continuous-waveoperation of a single-fused 1.55-µm VCSEL was obtained.This demonstrated the potential of InGaAsP/InP DBRs inhigh-performance VCSELs, but also revealed a high sensitivityto self-heating. Further efforts were therefore directedtowards all-epitaxial GaAs-based structures. This resulted in ahigh-performance 1215-nm VCSEL with a highly strained InGaAssingle QW. This can be viewed as a basis for longer-wavelengthVCSELs, i.e., with an emission wavelength approaching 1300 nm,either by an extensive device detuning or with GaInNAs QWs. <b>Keywords</b>: VCSEL, vertical cavity laser, semiconductorlaser, long-wavelength, DBR, oxidation, wafer fusion, InGaAs,semiconductor processing

Vertical cavity lasers (VCLs) are of great interest aslow-cost, high-performance light sources for fiber-opticcommunication systems. They have a number of advantages overconventional edge-emitting lasers, including low powerconsumption, efficient fiber coupling and wafer scalemanufacturing/testing. For high-speed data transmission overdistances up to a few hundred meters, VCLs (or arrays of VCLs)operating at 850 nm wavelength is today the technology ofchoice. While multimode fibers are successfully used in theseapplications, higher transmission bandwidth and longerdistances require single-mode fibres and longer wavelengths(1.3-1.55 µm). However, long-wavelength VCLs are as yetnot commercially available since no traditional materialssystem offers the required combination of bothhigh-index-contrast distributed Bragg reflectors (DBRs) andhigh-gain active regions. Earlier work on long-wavelength VCLshas therefore focused on hybrid techniques, such as waferfusion between InP-based QWs and AlGaAs DBRs, but more recentlythe main interest in this field has shifted towardsall-epitaxial GaAs-based devices employing novel 1.3-µmactive materials. Among these, strained GaInNAs/GaAs QWs aregenerally considered one of the most promising approaches andhave received a great deal of interest. The aim of this thesis is to investigate monolithicGaAs-based long-wavelength (&gt;1.2 µm) VCLs with InGaAsor GaInNAs QW active regions. Laser structures - or partsthereof - have been grown by metal-organic vapor phase epitaxy(MOVPE) and characterized by various techniques, such ashigh-resolution x-ray diffraction (XRD), photoluminescence(PL), atomic force microscopy, and secondary ion massspectroscopy (SIMS). High accuracy reflectance measurementsrevealed that n-type doping is much more detrimental to theperformance of AlGaAs DBRs than previously anticipated. Asystematic investigation was also made of the deleteriouseffects of buried Al-containing layers, such as AlGaAs DBRs, onthe optical and structural properties of subsequently grownGaInNAs QWs. Both these problems, with their potential bearingon VCL fabrication, are reduced by lowering the DBR growthtemperature. Record-long emission wavelength InGaAs VCLs were fabricatedusing an extensive gain-cavity detuning. The cavity resonancecondition just below 1270 nm wavelength occurs at the farlong-wavelength side of the gain curve. Still, the gain is highenough to yield threshold currents in the low mA-regime and amaximum output power exceeding 1 mW, depending on devicediameter. Direct modulation experiments were performed on1260-nm devices at 10 Gb/s in a back-to-back configuration withopen, symmetric eye diagrams, indicating their potential foruse in high-speed transmission applications. These devices arein compliance with the wavelength requirements of emerging10-Gb/s Ethernet and SONET OC-192 standards and may turn out tobe a viable alternative to GaInNAs VCLs. <b>Keywords:</b>GaInNAs, InGaAs, quantum wells, MOVPE, MOCVD,vertical cavity laser, VCSEL, long-wavelength, epitaxy, XRD,DBR

Theoretical and experimental studies of real-time interrogation of fiber Bragg grating (FBG) sensors based on chirped pulse compression with increased interrogation resolution and signal-to-noise ratio are presented. Two interrogation systems are proposed in this thesis. In the first interrogation system, a linearly chirped FBG (LCFBG) is employed as the sensing element. By incorporating the LCFBG in an optical interferometer as the sensor encoding system, employing wavelength-to-time mapping and chirped pulse compression technique, the correlation of output microwave waveform with a chirped reference waveform would provide an interrogation result with high speed and high resolution. The proposed system can provide an interrogation resolution as high as 0.25 μ at a speed of 48.6 MHz. The second interrogation system is designed to achieve simultaneous measurement of strain and temperature. In this system, a high-birefringence LCFBG (Hi-Bi LCFBG) is employed as a sensing element.

Theoretical and experimental studies of real-time interrogation of fiber Bragg grating (FBG) sensors based on chirped pulse compression with increased interrogation resolution and signal-to-noise ratio are presented. Two interrogation systems are proposed in this thesis. In the first interrogation system, a linearly chirped FBG (LCFBG) is employed as the sensing element. By incorporating the LCFBG in an optical interferometer as the sensor encoding system, employing wavelength-to-time mapping and chirped pulse compression technique, the correlation of output microwave waveform with a chirped reference waveform would provide an interrogation result with high speed and high resolution. The proposed system can provide an interrogation resolution as high as 0.25 μ at a speed of 48.6 MHz. The second interrogation system is designed to achieve simultaneous measurement of strain and temperature. In this system, a high-birefringence LCFBG (Hi-Bi LCFBG) is employed as a sensing element.

No description available.

No description available.

No description available.