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  • 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.

Microwave Switching and Attenuation with Superconductors

Poulin, Darcy G. 07 1900 (has links)
<p>The discovery of high temperature superconducting (HTS) materials having a critical temperature above the boiling point of liquid nitrogen has generated a large amount of interest in both the basic and applied scientific communities. Considerable research effort has been expended in developing HTS microwave devices, since thin film, passive, microwave components will likely be the first area to be successfully commercialized.</p> <p>This thesis describes a new thin film HTS microwave device that can be operated as a switch or as a continuously variable attenuator. It is well suited for low power analog signal control applications and can easily be integrated with other HTS devices. Due to its small size and mass, the device is expected to find application as a receiver protection switch or as an automatic gain control element, both used in satellite communications receivers. The device has a very low insertion loss, and the isolation in the OFF state is continuously variable to 25 dB. With minor modifications, an isolation exceeding 50 dB is readily achievable. A patent application for the device has been filed, with the patent rights assigned to COM DEV.</p> <p>The device is based on an unusual non-linear response in HTS materials. Under a non-zero DC voltage bias, the current through a superconducting bridge is essentially voltage independent. We have proposed a thermal instability to account for this behaviour. Thermal modelling in conjunction with direct temperature measurements were used to confirm the validity of the model.</p> <p>We have developed a detailed model explaining the microwave response of the device. The model accurately predicts the microwave attenuation as a function of the applied DC control voltage and fully explains the device operation. A key feature is that the device acts as a pure resistive element at microwave frequencies, with no reactance. The resistance is continuously variable, controlled by the DC bias voltage. This distinguishes it from a PIN diode, since PIN diodes have a capacitive reactance that limits their frequency range.</p> <p>Measurements made to confirm the microwave model validity resulted in the development of a new cryogenic de-embedding technique. The technique allows accurate microwave measurements to be made on devices at cryogenic temperatures using only room temperature calibration standards. We have also investigated the effect of kinetic inductance on coplanar waveguide transmission lines, and indicate under what conditions kinetic inductance must be considered in transmission line design.</p> / Doctor of Philosophy (PhD)

Determination of Two-Phase Flow Parameters for Nuclear Fuel Channels using a Real-Time Neutron Radiography Method

Harvel, David Glenn 04 1900 (has links)
<p>Multi-dimensional modelling of two-phase flow requires accurate constitutive relationships for interfacial parameters such as interfacial heat transfer, void fraction distribution, interfacial area, etc. However, existing diagnostic systems for measurement of two-phase flow parameters have difficulty measuring two or three-dimensional void distributions required for determination of interfacial parameters.</p> <p>In this work, a Real-Time Neutron Radiography (RTNR) system is developed for non-intrusive measurement of two-phase flow parameters in nuclear fuel channels at low thermal neutron fluxes (on the order of 10⁶n/cm²-s). This advanced radiation technique has the advantage of measuring two-phase flow in 3½ dimensions(x,∫dy,z,t) where the ½ dimension refers to an integrated or averaged space dimension. Pipe flow channels, annulus flow channels, MAPLE-type nuclear fuel flow channels, and CANDU-type nuclear fuel flow channels are investigated. Measurements of flow regime, void fraction, void fraction distribution, bubble diameter, bubble velocity, and interfacial area are conducted.</p> <p>The RTNR system is compared to ultrasonic and optical video measurement systems in pipe flow channels. Good agreement is obtained for flow regime, void fraction, bubble diameter, and interfacial area measurements.</p> <p>The RTNR system is compared to High-Speed X-ray Computed Tomography (X-CT) and optical video measurement systems in the annulus flow channel. Good agreement is obtained for the determination of flow regime, void fraction, void fraction distribution, and bubble velocity measurements.</p> <p>Application of the RTNR system to the vertical MAPLE-type 37 rod hexagonal finned fuel bundle and the horizontal CANDU-type 37 rod cylindrical fuel bundle is conducted. Measurements of flow regime, void fraction, and void fraction distribution are obtained and interfacial wave motion is observed. Flow regime observations in the MAPLE-type nuclear fuel channel show good agreement with previous work whereas flow regime observations in the CANDU-type nuclear fuel channel show significant discrepancies with previous work. A new large amplitude stratified wavy (LASW) flow regime is observed by the RTNR images in the CANDU-type nuclear fuel channel. The MAPLE-type nuclear fuel channel is shown to inhibit void migration and bubble coalescence at the entrance to the bundle and significantly affect the flow. The CANDU-type nuclear fuel channel is also observed to influence the cross flow in the core of the rod bundle.</p> <p>Based on this work, an RTNR diagnostic technique is shown to be successful in measuring two-phase flow parameters in nuclear fuel channels.</p> / Doctor of Philosophy (PhD)

Integrated distributed feedback lasers and electroabsorption modulators fabricated using helium-plasma-assisted indium phosphide defect induced quantum well intermixing

Letal, Gregory 27 April 2000 (has links)
<p>An integrated DFB laser and electroabsorption modulator was fabricated by using a novel quantum-well-intermixing technique that uses the defects from a layer of InP grown by He-plasma-assisted gas source molecular beam epitaxy (He*-InP). This is the first investigation into the He*-InP defect induced intermixing, and the first time a device has been fabricated by using this technique. The first part of this thesis outlines the first investigation into the effects of defects within a layer of He*-InP on annealing-induced quantum-well intermixing. Such a layer can be used to either decrease or increase the total amount of intermixing depending upon the placement of the He*-InP layer relative to the QW and the thickness of the layer. When the layer is far from the QW (∼0.5μm ), the total amount of intermixing is decreased. When the layer is close to the QW, the total amount of intermixing can be increased as well as being accompanied by a reduction of the photoluminescence intensity. The main benefit of this intermixing technique is that, unlike dielectric-enhanced intermixing, it is possible to regrow over material that has been intermixed by the He*-InP defect. The He*-InP defect-induced-intermixing technique has been applied to the fabrication of an integrated electroabsorption modulator and a distributed feedback laser. The techniques developed to fabricated the integrated device are discussed in detail in the second part of the thesis, followed by the device results. The device characteristics vary with device geometry. For example, the extinction ratios range from 9-16dB for voltage of 3V applied to a 400μm modulator due to the variation of the lasing wavelength with ridge width (for a 3QW structure). The threshold currents of discrete, 600μm long DFB lasers ranged from 18 to 35mA depending on its ridge width and where on wafer it was taken from.</p> / Doctor of Philosophy (PhD)

Study of III-V Semiconductors by Spatially-resolved and Polarization-resolved Photoluminescence

Yang, Jian (Jim) 02 1900 (has links)
<p>Spatially-resolved and polarization resolved photoluminescence has been used to study the photoelastic effect and the correlation between strain fields on the facet and along the active region in semiconductor diode lasers. The spectral mapping ability of this technique has been developed and has been demonstrated to be a useful tool for the study of quantum well intermixing processes in InGaAsP systems.</p> / Doctor of Philosophy (PhD)

A wavelength monitor based on electroabsorption in quantum well waveguide photodiodes

Wu, xiucheng January 1999 (has links)
<p>A wavelength monitor, based on p-i-n quantum well waveguide photodiodes, and its application to fiber Bragg grating strain sensor have been demonstrated. Ridge waveguide photodiodes with five different quantum well structures have been de signed, fabricated and characterized. In a wavelength monitoring system that employed a single-segment photodiode, the waveguide photodiode was used as either a tunable edge filter or a wavelength selective photodetector. A wavelength resolution of 1 pm with an operating range of 18 nm was achieved with a GaAs/AlGaAs multiple quantum well device. In a wavelength monitoring system that employed a pair of in-line waveguide photodiodes, the photocurrent ratio of the two photodiodes varied linearly with wavelength in a region near the absorption band edge. A wave-length resolution of 0.4 pm was achieved with InGaAs/GaAs quantum well devices for TE-polarized light with intensities of about 30 μW and a signal averaging time of 0.2 seconds. An operating range of 40 nm was obtained by voltage tuning of the quantum wells. The photocurrent ratios in the in-line waveguide photodiodes were also used to determine the absorption coefficients of the photodiodes. The shifts of the exciton peak positions with the applied bias voltage showed fairly good agreements with the predictions of a variational theory. The photocurrent ratio of the in-line quantum well waveguide photodiodes showed strong polarization dependence. However, the separation between the absorption edges for the TE and TM polarizations was so large that this device could be used in the wavelength range near the absorption edge of the TE component even in the presence of a large TM component in the input. An optical fiber Bragg strain sensing system, that consists of a superluminescent diode, a 2 x 2 fiber coupler, a fiber Bragg grating and the waveguide detector, was assembled to demonstrate the application of the developed wavelength monitor. A strain resolution of 5 microstrain has been obtained with the fiber Bragg grating strain sensor. The grating strain sensor shows a linear response over a range of ±2500 microstrain.</p> / Doctor of Philosophy (PhD)

Linewidth enhancement factors of short external cavity semiconductor lasers

Nguyen, An H. 03 1900 (has links)
<p>This thesis describes the development of a method for measuring the tuning of the emission frequency of semiconductor lasers. This method employs a short external cavity (SXC) mirror, as an external optical feedback element, to alter the resonant condition of the lasers. The experimental system, which includes an SXC laser module constructed out of a flexure mount concept, was derived from this method. The system was used to make measurements of the frequency tuning of InGaAsP multi-quantum well lasers. Linewidth enhancement factors of these lasers were calculated from the experimental data. It was found that the linewidth enhancement factors of these lasers depend on the wavelengths of emission and are fairly independent of the output powers. It was also found that the linewidth enhancement factors of these lasers decrease as the band gap of the barriers of the quantum wells increases.</p> / Doctor of Philosophy (PhD)

Analysis and monitoring of a CANDU nuclear power plant using multivariate statistical process control methods

Leger, Robert P. 08 1900 (has links)
<p>In the nuclear power industry, the ability to efficiently analyse historical data, and to detect and diagnose process faults in a timely manner are critical tasks in operating and maintaining a nuclear power plant. The objectives of this research were to prove that established Statistical Process Control (SPC) techniques could be used to analyse nuclear power plant data and to develop a hierarchical process monitoring methodology which could deliver relevant information to different functional groups within a plant. The use of established Multivariate SPC techniques to analyse nuclear power plant data was successfully proven in several areas and is considered a significant contribution to the development of data analysis tools for the nuclear energy industry. By analysing actual data from an operational nuclear power plant, the techniques were able to provide key insights into the process. Process tests and different plant configurations were easily identified. The multivariate techniques could relate the different plant configurations to sensor calibrations and process changes. Also, the techniques were able to identify two anomalies in the data which were not previously detected using the existing analysis tools. In order to produce a hierarchical process monitoring methodology, a multi-block, multi-level Principal Component Analysis algorithm, and associated prediction code, was developed and tested. This algorithm is an extension of existing multi-block, two-level algorithms and represents a contribution to the current state of Multivariate SPC techniques. It was found that the algorithm was very useful for analysis but marginal for delivering relevant information in a process monitoring capacity. This finding resulted in the third major contribution of this research work. The Multivariate SPC techniques are very useful for analysing nuclear power plant data but not as feasible for monitoring the process in an on-line manner. This was attributed to the goals defined for the monitoring methodology, the scaling method used for the data, and the numerous normal plant operating states.</p> / Doctor of Philosophy (PhD)

Measurement of Stress in III-V Semiconductors using the Degree of Polarization of Luminescence

Colbourne, Dwight Paul 10 1900 (has links)
<p>The techniques of polarization-resolved electroluminescence and photoluminescence have been demonstrated to be accurate methods of measuring mechanical stress in luminescent semiconductors. These techniques have been applied to measure the stresses in AlGaAs/GaAs and InGaAsP/InP superluminescent diodes and diode lasers and bulk GaAs and InP crystals. Stresses due to various diode laser manufacturing processes have been measured. Individual dislocations in bulk crystals and strained epitaxial layers have been detected and characterized by their stress patterns.</p> / Doctor of Philosophy (PhD)

Safety Characteristics of a Suspended-Pellet Fission Reactor System

Kingdon, Ross David 09 1900 (has links)
<p>A new fission reactor system with passive safety characteristics to eliminate the occurrence of loss-of-coolant accidents, reduce reactivity excursion effects, and which also provides for closure of the nuclear fuel cycle through on-site spent fuel management is examined. The concept uses multi-coated fuel pellets which are suspended by an upward moving coolant in vertical columns of the reactor core and electro-refining elemental separation to remove selected fission products prior to actinide recycling. The possibility of fuel melt following a loss-of-coolant is avoided as a decrease in coolant flow results in the removal of fuel from the core through the action of gravity alone. Average fluid velocities in the columns which are necessary to suspend the pellets are calculated and found to be consistent with the necessary heat extraction to yield ~1-10MWth per column. The total output power of such suspended pellet-type reactors is compared to the power necessary to provide the suspending fluid flow, yielding favourable ratios of ~10² - 10³ The reduction of reactivity excursion tendencies is envisaged through an ablative layer of material in the pellets which sublimates at temperatures above normal operating conditions. In the event of a power or temperature increase the particles fragment and thereby change their hydrodynamic drag characteristics, thus leading to fuel removal from the core by elutriation. Comparison of nuclear-to-thermal response times and elutriation rates for limiting power transients indicate that the present design assets in reactivity excursion mitigation. Closure of the nuclear fuel cycle is attained through a spent fuel management strategy which requires only on-site storage of a fraction of the fission products produced during reactor operation. Electro-refining separation of selected fission products combined with complete actinide recycling yields no isolation of plutonium or highly enriched uranium during the procedure. The out-of-core waste stream has a significantly reduced radioactivity, volume and lifetime compared to the once-through waste management strategy and thus provides an alternative to long-term geological disposal of fission reactor wastes. The Pellet Suspension Reactor concept possesses some unique operating characteristics and, additionally, is shown to be similar to conventional fission reactors in terms of common performance features.</p> / Doctor of Philosophy (PhD)

Integrating InP Waveguides: Measuring Processing Stress and Modelling Intersecting Waveguides With Low Crosstalk

Daly, George Michael 02 1900 (has links)
<p>Two major problems in InP waveguide design were studied. The first of these is the effect of processing induced stress on the mode of an InP/lnGaAsP waveguide. The stress in such a waveguide was measured by the technique of the Degree of Polarization of Photoluminescence. The measured stress map was modeled to quantify the spatial distribution of index change within the waveguide itself. The mode of the single-mode waveguide with the stress effects was calculated using the beam propagation technique. The resultant mode exhibited a double peaking which agreed well with the observed mode of the actual waveguide.</p> <p>The second problem studied was that of crossed waveguides. Crossed waveguides are necessary in optoelectronic integrated circuits due to the multiple light paths and planar nature of the fabrication process. Each arm of these crosses requires good throughput with very little power coupled into the other waveguide. Although this is easy to achieve at large crossing angles, at angles below ≈ 10° a significant fraction of the light may couple into the crossing waveguide.</p> <p>This thesis uses the beam propagation method to characterize the loss and crosstalk. Two modified versions of the X-crossed waveguide structure were developed and have shown crosstalk improvements, at angles below 10°, of greater than 20 dB. These improvements are easy to implement and are important due to the extra design freedom they allow the designers of optoelectronic circuits.</p> / Doctor of Philosophy (PhD)

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