Abstract¡G Scalar coupled mode theory predicts that the output powers at two nonlaunch fibers are identical for the equilateral 3x3 fiber couplers due to the geometrical symmetry, but several experimental results demonstrate that they are not equal. In this work, a power coupling model with considering the polarization property of input power is proposed for the equilateral 3x3 fiber couplers. Consistent with the measurement results, the model shows that when a polarized light is launched into one of the three fibers the output powers at the other two nonlaunch fibers are unequal.

Several novel designs of printed monopole antennas are proposed and experimentally studied. These antennas have the advantages of low profile, light weight and easy construction and can be printed on dielectric substrates and integrated with associated circuitry on the same substrates, which not only reduces the manufacturing cost but also decreases the required size of the complete system. This dissertation mainly consists of four sections and several novel designs are introduced in each section. First, a novel dual-band antenna with two stacked inverted-F strips and a CPW-fed folded inverted-F strip antenna are proposed. Second, a novel printed dual-band double-T monopole antenna, comprising two stacked T-shaped monopoles, is discussed. Third, three diversity antenna designs obtained by protruding a ground plane with optimal dimensions between two printed monopoles for WLAN communication applications in the 2.4 GHz and 5.2 GHz bands are presented. Good dual-polarized radiation with an enhanced port decoupling (isolation less than ¡V27 dB) for the two feeding ports is obtained. Finally, a novel printed ultra-wideband diversity monopole antenna is shown. The operating bandwidth of the ultra-wideband antenna reaches about 5.4 GHz, covering the WLAN bands at 2.4, 5.2 and 5.8 GHz, and the antenna can provide spatial diversity to combat the multipath fading problem.

In this thesis two different research topics are undertaken, both in the area of compact RF/microwave circuits design. The first topic involves the design of a compact circularly polarized (CP) slot-ring antenna. A study of several compact CP microstrip and slotline antennas reported in the past has been carried out. In this research, a method of reducing the size of a printed slot-ring antenna is proposed. The reduction in size is achieved by introducing meandered-slot sections in the ring. Circular polarization is achieved by introducing an asymmetry, also a meandered-slot section, and feeding the antenna at an angle of 45o from the asymmetry using a microstrip feed line. The minimum axial ratio of 0.4 dB is obtained at 2.46 GHz, which is the operating frequency of the antenna. The size of the proposed antenna is reduced by about 50% compared to a conventional CP slot-ring antenna and it displays a CP bandwidth of about 2.5%. The simulated and measured results are presented, and they are in good agreement. The small size of the antenna makes it very suitable for use in modern RF/microwave wireless systems which require compact, low cost, and high performance circuits. Moreover, its CP behavior makes it more attractive for applications such as satellite communications. The second topic in the thesis involves the design of a compact microstrip bandpass filter using triangular open-loop resonators. A new compact three-pole microstrip bandpass filter using four triangular open-loop resonators is presented. A fourth resonator is placed to provide cross-coupling in the structure which gives a better skirt rejection. The measured pass-band center frequency is 2.85 GHz. The filter demonstrates about 7% bandwidth with insertion loss of less than 1 dB in the passband, a return loss of greater than 15 dB and out-of-band rejection of greater than 30 dB. The simulated and measured results are in good agreement. The proposed filter is very attractive for use in modern wireless systems which require bandpass filters having compact size, low insertion loss, high selectivity, and good out-of-band rejection.

This study proposes a polarization-independent liquid-crystal Fresnel lens using the twisted nematic liquid crystal (TN-LC). At the high voltage state, the TN-LC behaves as an optically anisotropic material which makes the proposed LC Fresnel lens possess a polarization-insensitive characteristic. Two kinds of LC Fresnel lens, photoresist type and patterned electrodes type, are prepared in our experiment. Experimental results reveal that the photoresist type has a high diffraction efficiency and a good polarization-independent characteristic. However, its diffraction efficiency cannot be switched off. Contrarily, the patterned electrode type has the characteristics of the switch-off and good polarization-independent with a moderate diffraction efficiency

This research accomplished a series of investigations on the polarization switching of Vertical-Cavity Surface-Emitting Lasers (VCSEL). The research was preceded experimentally and theoretically to observe the variation of the VCSEL¡¦s polarization switching under various polarization optical feedback and various feedback ratios. A VCSEL with significant polarization-switching hysteresis loop was employed in the experiments to study the interaction between the optical feedback and polarization switching. The experimental results matched the theoretical simulations very well. These results could be interpreted with the graphic analysis of the linear gain model of the VCSEL. In the investigation of the polarization-switching hysteresis loop, experiments found that, inside the loop and at some constant current, the polarization switching would be conducted with a switching delay time. The switching delay time was shortened under stronger optical feedback, revealing an exponential dependence between the switching delay time and optical feedback ratio. In the experiments of small current modulation to continuously switch the laser¡¦s polarization, optical feedback could improve the success of the current-driven polarization switching, significantly enhancing its bandwidth. For a large-range tuning of the laser¡¦s current, as the current modulation frequency increased, the polarization-switching current in the increasing-current process would significantly shift toward the high current end, while the polarization-switching current in the decreasing-current process was affected less. These results greatly contribute to the understanding of the VCSEL¡¦s polarization switching.

This dissertation describes a new electrochemical synthesis of novel composite materials based on montmorillonite (MMT) clay and intrinsically conducting polyaniline (PANI). PANI was successfully incorporated into MMT galleries to form PANI−MMT nanocomposites. Electropolymerization of anilinium ions which are intercalated inside the clay layers have been carried out at a constant applied potential. The synthetic conditions have been optimized taking into account the effect of concentration of aniline, magnetic stirring and potential cycling. The resulting organic-inorganic hybrid material, PANI-MMT has been characterized by various physicochemical techniques. Results of elemental analysis show that nanocomposite contains only 10 % of conducting PANI. Formation of PANI inside the clay tactoid has been confirmed by the expansion of inter layer distance of MMT as revealed by X-ray diffraction studies. Relatively lower interlayer expansion for PANI-MMT than that of anilinium-MMT indicates the higher stereoregularity in PANI-MMT which has strong influence on electrical properties of nanocomposites. Infrared spectroscopy studies reveal the presence of physicochemical interaction, probably hydrogen bonding, between clay and polyaniline. Cyclic voltammetry studies indicate that presence of electroinactive clay does not influence the electrochemical activity of PANI. Electrochromic behaviour of PANI-MMT nanocomposites have been studied using in situ UV-Vis spectroscopy which reveals that electrochromism of PANI in the composite material has been retained. One of the main technological applications of conducting polymers, particularly PANI, is in the area of corrosion protection of active metals. PANI-MMT nanocomposites synthesized using the present method and a chemically synthesized PANI which is soluble in organic solvents have been used to protect C45 steel surface against corrosion. Corrosion studies have been performed using electrochemical impedance measurements(EIM)and anodic polarization studies. Electrochemical impedance data has been analyzed using a suitable equivalent circuit. Corrosion protection of steel offered by both PANI-MMT and organically soluble PANI is evident form the increase in the value of charge transfer resistance of the coated steel surfaces. Time dependent EIM measurements reveal that charge transfer resistance gradually decreases with time, however, the values are much higher than that of uncoated surfaces. Two capacitive loops, one at higher and another at lower frequencies, observed in the Nyquist plots have been assigned to the electrical properties of coating material (in the present case, PANI-MMT or soluble PANI) and electrochemical process at the interface, respectively. An anodic shift in the corrosion potential, a decrease in the corrosion rate and a significant increase in the polarization resistance indicate a significant anti-corrosion performance of both PANI-MMT nanocomposite and organically soluble PANI deposited on the protected steel surface. / Diese Dissertation beschreibt eine neue elektrochemische Synthese neuartiger Compositmaterialien basierend auf dem Tonmineral Montmorillonite (MMT) und intrinsisch leitfähigem Polyanilin (PANI). Die Elektropolymerisation von Aniliniumionen, welche in die Tonmineralschichten eingebaut sind, wurde bei einem konstanten Potenzial durchgeführt. Das resultierende organisch-anorganische Hybridmaterial PANI-MMT wurde mit verschiedenen physikochemischen Methoden charakterisiert. Die Ergebnisse der Elementaranalyse zeigen, dass nur 10 % des Nanocompositmaterials aus leitfähigem PANI bestehen. Die Vergrößerung des Zwischenschichtabstandes von MMT, die bei Röntgendiffraktometrieuntersuchungen beobachtet wurde, lässt auf die Bildung von PANI innerhalb der Tonmineral-Taktoide schließen. IR-spektroskopische Untersuchungen deuten auf das Vorhandensein von Wechselwirkungen physikochemischer Art, wahrscheinlich Wasserstoffbindungen zwischen dem Tonmineral und Polyanilin, hin. Untersuchungen mit zyklischer Voltammetrie zeigten, dass die Anwesenheit von elektroinaktivem Tonmineral die elektrochemische Aktivität von PANI nicht beeinflusst. Das elektrochrome Verhalten von PANI-MMT Nanocompositen wurde mit UV-Vis-Spektroskopie untersucht, wobei sich herausstellte, dass das elektrochrome Verhalten vom PANI im Compositmaterial erhalten bleibt. Eines der technologischen Hauptanwendungsgebiete von leitfähigen Polymeren, insbesondere von PANI, ist der Korrosionsschutz von aktiven Metallen. PANI-MMT Nanocomposite die mit der angegebenen Methode (elektrochemisch) synthetisiert wurden und chemisch synthetisiertes in organischen Medien lösliches PANI wurden zum Korrosionsschutz von C45 Stahl eingesetzt. Die Korrosionsuntersuchungen wurden mit Hilfe von elektrochemischen Impedanzmessungen (EIM) und anodischen Polarisationsuntersuchungen durchgeführt. Der von PANI-MMT und von in organischen Medien löslichem PANI gebotene Korrosionsschutz ist wahrscheinlich auf die Zunahme des Ladungsdurchtritts widerstandes der beschichteten Stahloberfläche zurückzuführen. Die anodische Verschiebung des Korrosionspotenzials, eine Verringerung der Korrosions-geschwindigkeit und eine deutliche Zunahme des Polarisationswiderstandes sind eindeutige Hinweise für das Antikorrosionsvermögen von PANI-MMT und auch von in organischen Medien löslichem PANI, welche auf der zu schützenden Stahloberfläche abgeschieden wurden.

Thesis (Ph. D.)--Rutgers University, 2009. / "Graduate Program in Physics and Astronomy." Includes bibliographical references (p. 86-91).

Thesis (M. S.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2007. / Committee Chair: A. Rahman Zaghloul ; Committee Members: W. Russell Callen and Doug Yoder. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (M.Ch.E.)--University of Delaware, 2009. / Principal faculty advisor: Jochen A. Lauterbach, Dept. of Chemical Engineering. Includes bibliographical references.

Thesis (Ph. D.)--Florida State University, 2003. / Advisor: Dr. Kirby W. Kemper, Florida State University, College of Arts and Sciences, Dept. of Physics. Title and description from dissertation home page (viewed Nov. 20, 2003). An arrow pointed to the right over the Li in title. Includes bibliographical references.