University of Macau / Faculty of Science and Technology. / Department of Electrical and Computer Engineering

This thesis presents a continuous time bandpass delta sigma modulator with frequency translation inside the delta sigma loop. The input IF signal is down converted to baseband after amplification by a low Q, wideband bandpass resonator. The down converted IF signal is digitized by a continuous time, second order lowpass delta sigma modulator. The output of the lowpass delta sigma modulator is upconverted and fedback in to the low Q wideband bandpass resonator. Unlike the conventional delta sigma modulators, sinusoidal pulses are used for feedback. The system level design of the frequency translating delta sigma modulator is discussed. A prototype frequency translating delta sigma modulator to digitize IF signals at 100 MHz was designed in CMOS 0.35 μm process. Transistor level simulation shows that 80 dB SNR is achievable at a power dissipation of 100 mW. The frequency translating delta sigma modulator is less sensitive to time delay jitter in the DAC feedback pulse. If we use edge triggered sinusoid pulses for feedback, the DAC jitter performance of frequency translating delta sigma modulator will be better than that of conventional bandpass delta sigma modulator. / Graduation date: 2003

The effects of modulation of CICR on spontaneous and systolic Ca2+ release were investigated in isolated rat ventricular myocytes. Spontaneous waves of Ca2 + release were initially abolished and then resumed at a lower frequency during exposure to 100 J.LMtetracaine. Both the duration of the initial quiescent period and oscillation frequency in tetracaine were dependent on the control oscillation frequency and the concentration of tetracaine applied. Electrophysiological quantification of the SR Ca2+ content of myocytes revealed a significant increase during exposure to tetracaine. The amplitude of spontaneous Ca2 + release was also increased such that despite decreased frequency, efflux per unit time activated by Ca2+ waves was not changed significantly. Using confocal microscopy, the spatial and temporal properties of Ca2+ waves were studied revealing that tetracaine inhibits the propagation of Ca2 + release. The increased SR Ca2+ content and the increased amplitude of Ca2+ release can reverse this effect. Application of 100 J.LMtetracaine to electrically stimulated cells transiently depressed systolic Ca2+ release and contraction but had no effect in the steady state. Removal of tetracaine was associated with potentiation of systolic Ca2+ release followed by gradual recove~. Quantification of the SR Ca2+ content revealed that in tetracaine the SR Ca + content was significantly increased in the steady state. This increase was accounted for by inhibition of systolic Ca2+ release activating less Ca2+ efflux in the presence of the same or increased Ca2+ influx on the L-type Ca2+ current. As the SR Ca2+ content increases, more efflux is activated until eventually efflux and influx balance in the steady state. The transient potentiation of contraction on removal of tetracaine is due to the increased SR Ca2 + content, which increases the gain of CICR in the absence of inhibition of Ca2 + release. The mechanism of post rest potentiation in rat cardiac tissue has not been conclusively elucidated by previous studies. This investigation provides evidence that changes in SR Ca2 + content and recovery of channels from inactivation could contribute to the potentiation of contraction observed in rat ventricle after a period of rest. Tetracaine enhances the degree of potentiation of contraction, which can only partially be attributed to its ability to enhance SR Ca2+ accumulation. During myocardial ischaemia dramatic changes in the substrate and metabolite levels in cells occur and a number of these changes are known to affect the RyR. However, the overall effects of metabolic blockade on the sensitivity CICR in intact cells have been overlooked. Experiments were carried out to investigate the effect of metabolic inhibition on spontaneous Ca2+ release and SR Ca2+ content in isolated rat ventricular myocytes. The results show that CICR is inhibited during metabolic inhibition. This could contribute to the degree of damaging and potentially fatal Ca2+ overload experienced on reperfusion of ischaemic tissue.

No description available.

Graduation date: 1995

Electroabsorption modulators (EAM) have attract a lot of interests in high-speed optical communication due to low chirp, high-efficiency operation and the capability to be integrated with other semiconductor devices. Enhancing the operation by overcoming the trade-off between RC-limitation and high-speed performance, traveling-wave types of EAMs (TWEAM) have been documented to be a good candidate. In this thesis, a novel type of TWEAMs, namely undercut-etching-the-active-region type (UEAR), have been characterized and analyzed. There are two topics in this thesis, namely (1) one is to compare the performance of the UEAR and conventional ridge-waveguide (RW) types of TWEAM. The regime of D.C. to 40GHz small-signal modulation and 10 Gbits/sec large-signal data transmission are used to characterize and compare the performance of TWEAMs. In comparison with conventional ridge-waveguide (RW) TWEAM, 3dB lower optical-insertion-loss, at least 6dB higher in RF-link (D.C. to 40GHz) and faster electro-optical response (3dB bandwidth of 25GHz at 50£[-termination for UEAW and 15GHz for RW) are obtained in UEAW-TWEAM. Error-free 10Gbits/sec operation with high sensitivity of ¡V36.2 dBm and low swing voltage of 0.6V have been achieved in UEAW-TWEAM, a 3.2dB enhancement over RW-TWEAM, indicating the trade-off in designing electroabsorption modulators can be greatly released by novel type structure (UEAW). (2) The other topic is the application of TWEAM to all-optical wavelength converters. The generating photocurrent by optical absorption is the effect accompanying with the electrical-to-optical modulation in the EAM. Using the properties of cross-absorption and generating photocurrent, high-speed all-optical modulation can be potentially implemented in the application of EAMs. Long -waveguide high-speed TWEAMs can thus have high-extinction ratio performance in all-optical conversion. The conversion efficiency of -26dB and high extinction ration of >20dB are obtained in this experiment, showing the potential in the application of all-optical conversion.

Abtract Three different microstrip transmission-line layouts between the pads and an optical modulator waveguide have been designed and compared for their circuit response. In the first design, two 50£[ input and output microstrip transmission lines are connected to the center of the 24.58£[ semiconductor waveguide. In the second design, two 78£[ input and output microstrip transmission lines are connected to the center of waveguide. The third design represents a traveling-wave EA modulator. After optimizing the lengths of the transmission lines through our circuit simulation, we find that the traveling-wave optical modulator (the third layout) have the best circuit response of bandwidth ~22.4GHz and voltage reflection coefficient ~40%. The traveling-wave optical modulator not only overcomes the RC constant limit but also makes use of long waveguide to satisfied the high-speed requirement. At the same time, this circuit increases the extinction ratio and optical saturation power. In the fabrication process, we use polyimide to planarize both sides of the ridge waveguide and evaporate the top electrode on the polyimide. In addition, we use a special wet etching to form sloped edges for rasing ground pads up to the same level of the signal pad. This will allow us to use coplanar microwave probes to measure the devices.

The purpose of this thesis is to fabricate the quantum dot waveguide and measure electro-optic (EO) modulation characteristics. We focus on the refractive index change (£Gn), phase retardation, and electro-optic coefficients for quantum dot samples. The £fg=1.3£gm InGaAs quantum dot structures were grown on n+ GaAs substrates by MBE. We design a series of 2.2£gm straight and slope 7¢X single mode waveguide, and use the cleavage surface method to produce Fabry-Perot cavity. In fabrication process, we first defined the device pattern by using photo-lithography technique. Second, we etched ridge waveguide by using dry etching or multi-step wet etching method. Finally, we used the etching solution HBr:HCl:H2O2:H2O=5:4:1:70 to smooth the sidewall and reduce the scattering loss. We analyzed the quantum dot EO properties by measuring phase retardation of Fabry-Perot resonance at different bias. From measuring the Fabry-Perot resonance at£f=1515nm TE-polarization, the linear electro-optic (LEO) and quadratic electro-optic (QEO) coefficients of C311 sample are 7.26¡Ñ10-12m/v and 1.14¡Ñ10-18m2/v2 , respectively. And the LEO and QEO coefficients of C251 sample are 2.99¡Ñ10-11m/v and 4.10¡Ñ10-17m2/v2, respectively. By coupling £f=1515nm TM-polarized light, we found the main influence of C251 sample is Kerr effect, and the QEO coefficient is 3.52¡Ñ10-17m2/v2. Both electro-optic coefficients are significantly larger than those measured in quantum wells and bulk GaAs. These results are applicable to QD-based low drive voltage and small size modulators.

Thesis (Ph. D.)--School of Electrical and Computer Engineering, Georgia Institute of Technology, 2004. Directed by David V. Anderson. / Includes bibliographical references (leaves 58-60).

Thesis (M.S.)--Brigham Young University. Dept. of Electrical and Computer Engineering, 2003. / Includes bibliographical references (p. 89-91).