Theses and Dissertations

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Open Access
Study of a high power capillary discharge
(Colorado State University. Libraries, 1999) González, Juan José, author; Rocca, Jorge J., advisor; Wilbur, Paul J., committee member
The direct generation by electrical discharges of hot and dense plasma columns with large length-to-diameter ratio is of interest for the development of efficient soft x-ray lasers and has resulted in the generation of coherent radiation at wavelength as short as 46.9 nm. This work presents the first experimental results of a new high power density capillary discharge designed to explore the generation of axially uniform plasma columns for the development of discharge pumped lasers at shorter wavelengths. A high power pulsed power generator based on a three-stage pulse compression scheme was developed. The final stage consist of a water dielectric Blumlein transmission line designed to generate current pulses of up to 225 kA with a 10-90 % rise-time of = 10 ns through the capillary load. Argon plasmas generated in polyacetal and ceramic capillaries were studied by means of time resolved soft x-ray pinhole camera images and time resolved XUV spectroscopy. The pinhole images show that the current pulse rapidly compresses the plasma to form a column with a soft x-ray emitting region with a diameter of ≈ 250 μm. Spectra in the 18-23 nm region are observed to be dominated by an ArXV line. The experimental data obtain is in agreement with model computations that suggest these discharge conditions should generate plasma columns of ~ 200-300 μm in diameter with electron temperatures > 250 eV and densities of 1-2x10 20 cm-3.
Open Access
Development, characterization and application of a high average power capillary discharge soft x-ray laser
(Colorado State University. Libraries, 2001) Benware, Brady Robert, author; Rocca, Jorge, advisor
A compact high repetition rate, high average power capillary discharge laser operating at a wavelength of 46.9 nm that is of the size of many widely utilized visible and UV lasers has been developed and characterized. Two significant differences from previously developed capillary discharge soft x-rays lasers, the use of ceramic capillaries rather than poly-acetal and significantly longer plasma columns of up to 36 cm, allowed for the generation of greatly increased output pulse energies and average power. Lasing at a repetition rate of 4 Hz and an average laser pulse energy of 0.88 mJ has been obtained, which amounts to an average power of 3.5 mW. Lasing at repetition rates as high as 10 Hz was also achieved, but with lower output pulse energy. In this work the beam energy and divergence were measured as a function of capillary length, and the temporal evolution of the laser pulse was also studied. The combined high pulse energy and high repetition rate of this laser make it unique in the field of soft x-ray lasers to date. This laser has been used to perform angular dependent reflectivity measurements to determine optical constants of materials at 46.9 nm, which are in good agreement with those previously measured, or in some cases represent the first recorded values at this wavelength. This experiment constitutes the first application of a table-top soft x-ray laser to the field of material characterization. In a separate experiment, the output beam was polarized using two multi-layer coated mirrors that were configured for optimum reflectivity at 45 degrees. The resulting 96% polarized beam was then used to characterize the efficiency of a diffraction grating. Finally, in a third experiment, the beam was focused using a spherical multi-layer coated mirror to a spot size where the majority of the energy was confined to a 2 μm diameter. The peak intensity was estimated to be 1x1011 W/cm2. Through ray tracing computations, the focused spot size was determined to be dominated by spherical aberration. This focused beam reached intensities that were sufficient to induce ablation on brass and stainless steel targets realizing the first demonstration of material ablation with a coherent soft x-ray beam.
Open Access
Lasing at 52.9 nm in Ne-like chlorine and steps towards shorter wavelength capillary discharge lasers
(Colorado State University. Libraries, 2001) Frati, Maximo, author; Rocca, Jorge J., advisor; Wilbur, Paul, committee member
Significant advances have been obtained in the past few years in the development of soft x-ray lasers. Both, laser-pumped and discharge-pumped schemes have been successfully demonstrated. In particular, a very compact capillary discharge laser has been demonstrated to deliver an average power of several mW in the 46.9 nm line of Ne-like Ar. The work presented in this thesis, that was motivated by the possibility extending the very practical discharge excitation scheme to other short wavelengths laser transitions, can be divided in two parts. The first resulted in the successful demonstration of amplified spontaneous emission in the 3 p ¹Sₒ - 3s ¹P₁ transition of Ne-like Cl at 52.9 nm. Laser pulses of ~1.5 ns duration with energies up to 10 μJ and a beam divergence 4 mrad were obtained at repetition rates of 0.5 - 1 Hz. This new 23.4 eV table top laser is of particular interest for applications requiring high peak fluxes of photons with energy slightly below the He photoionization threshold. The results discussed in the second part of this thesis represent the first steps necessary for the development of a discharge-pumped Ni-like Cd laser at 13.2 nm. A room temperature source of atomically pure Cd vapor was developed and used to inject Cd into the capillary channel, where it was excited by a fast high current pulse to produce a hot dense plasma. The first spectroscopic data of a capillary discharge plasma containing Ni-like Cd ions (Cdₓₓᵢ) was obtained and analyzed. These results can be of use in future works when trying to develop a collisionally excited discharge-pumped Ni-like Cd laser.
Open Access
Desk-top size high repetition rate 46.9 NM capillary discharge laser as photoionization source for photochemistry applications
(Colorado State University. Libraries, 2006) Heinbuch, Scott, author; Rocca, Jorge J., advisor
A portable high repetition rate desktop-size capillary discharge laser emitting at a wavelength of 46.9 nm (26.5 eV photon energy) was demonstrated and used as a photoionization source in nanocluster mass spectroscopy. The high photon energy allows the single-photon ionization of nanoclusters and other molecules, which, due to their high ionization potential, would otherwise require undesirable multi-photon ionization. This Ne-like Ar capillary discharge laser occupies a table area of approximately 0.4 x 0.4 m², smaller than that occupied by many widely used ultraviolet gas lasers. The laser's power supplies and gas handling system are designed to fit into small racks that can be placed underneath a standard optical table. The main spark-gap is electrically triggered to allow synchronization of the laser pulses with those of other lasers in photochemistry applications. Experiments were performed to characterize the laser output energy, average power and timing jitter. Tests were conducted to determine the capillary lifetime. Laser pulses with energy ~ 13 μJ were generated at 12 Hz repetition rate by single pass amplification in a 21 cm long Ne-like Ar capillary discharge plasma column. The standard deviation of the jitter was found to be 5 ns. Capillary lifetime tests at 12 Hz repetition rate determined that the laser output energy decays by a factor of two after about 2 10⁴ - 3 10⁴ shots. The laser was installed in a photochemistry laboratory where it is operated for many hours on a daily basis. The laser was successfully used as a single photon photoionization source for the study of hydrogen bonded nanoclusters and other small molecules using time of flight mass spectroscopy. The first mass spectra of water nanoclusters and other small molecules using this source have been obtained.
Open Access
Application-aware transport services for sensor-actuator networks
(Colorado State University. Libraries, 2007) Banka, Tarun, author; Jayasumana, Anura P., advisor; Chandrasekar, V., advisor
Many emerging mission-critical sensor actuator network applications rely on the best-effort service provided by the Internet for data dissemination. This dissertation investigates the paradigm of application-aware networking to meet the QoS requirements of the mission-critical applications over best-effort networks that do not provide end-to-end QoS support. An architecture framework is proposed for application-aware data dissemination using overlay networks. The application-aware architecture framework enables application-aware processing at overlay nodes in the best-effort network to meet the QoS requirements of the heterogeneous end users of mission-critical sensor-actuator network applications. An application-aware congestion control protocol performs data selection and real-time scheduling of data for transmission while considering different bandwidth and data quality requirements of heterogeneous end users. A packet-marking scheme is proposed that enables application-aware selective drop and forwarding of packets at intermediate overlay nodes during network congestion to further enhance the QoS received by the end users under dynamic network conditions. Effectiveness of the transport services based on application-aware architecture framework is demonstrated by one-to-many high-bandwidth time-series radar data dissemination protocol for CASA (Collaborative Adaptive Sensing of the Atmosphere) application. Experiment results demonstrate that under similar network conditions and available bandwidth, application-aware processing at overlay nodes significantly improves the quality of the time-series radar data delivered to the end users compared to case when no such application-aware processing is performed. Moreover, it is shown that application-aware congestion control protocol is friendly to the already existing TCP cross-traffic on the network as long as bandwidth requirements of the mission-critical applications are met. Scalability analysis of application-aware congestion control protocol shows that it is able to schedule data at cumulative rates of more than 700M bps without degrading the QoS received by multiple end users.
Open Access
Ultrafast quantum coherent control apparatus
(Colorado State University. Libraries, 2007) Wilson, Jesse, author; Bartels, Randy, advisor; Levinger, Nancy, committee member; Rocca, Jorge J. G., committee member
In recent years, the availability of ultrafast laser sources has opened up a number of opportunities for exploring molecular dynamics that take place on femtosecond time scales. Coherent control experiments involve creating, manipulating, and measuring these ultrafast phenomena. Such controllable processes include second harmonic generation (SHG), creation of vibrational wavepackets, high-harmonic generation, photodissociation, and more.The foundation to all these experiments is an ultrafast pulse shaper and a high-dimensional search algorithm. Here we present the design and construction of a spectral phase-only pulse shaper, including details on alignment and calibration. We also demonstrate the functionality of the device by producing several pulse profiles that could be potentially useful in coherent control experiments. A covariance matrix analysis evolutionary strategy (CMAES) is also implemented, and demonstrated to optimize SHG in a nonlinear crystal. Finally, recognizing that phase-only shapers cannot produce the full range of temporal shapes available to a given input pulse, we show the design and construction of a pulse shaper which uses only a single linear phase mask to gain control over both spectral phase and amplitude by use of phase gratings.
Open Access
The study and real-time implementation of attenuation correction for X-band dual-polarization weather radars
(Colorado State University. Libraries, 2008) Liu, Yuxiang, author; Bringi, V. N., advisor; Chandrasekar, V., advisor
Attenuation of electromagnetic radiation due to rain or other wet hydrometeors along the propagation path has been studied extensively in the radar meteorology community. Recently, use of short range dual-polarization X-band radar systems has gained momentum due to lower system cost compared with the much more expensive S-band systems. Advances in dual-polarization radar research have shown that the specific attenuation and differential attenuation between horizontal and vertical polarized waves caused by oblate, highly oriented raindrops can be estimated using the specific differential phase. This advance leads to correction of the measured reflectivity (Zh) and the differential reflectivity (Zdr) due to path attenuation. This thesis addresses via theory, simulations and data analyses the accuracy and optimal estimation of attenuation-correction procedures at X-band frequency. Real-time implementation of the correction algorithm was developed for the first generation of X-band dual-polarized Doppler radar network (Integration Project 1, IP1) operated by the NSF Center for Collaborate Adaptive Sensing of the Atmosphere (CASA). We evaluate the algorithm for correcting the Zh, and the Zdr for rain attenuation using simulations and X-band radar data under ideal and noisy situations. Our algorithm is able to adjust the parameters according to the changes in temperature, drop shapes, and a certain class of drop size distributions (DSD) with very fast convergence. The X-band radar data were obtained from the National Institute of Earth Science and Disaster Prevention (NIED), Japan, and from CASA IP1. The algorithm accurately corrects NIED's data when compared with ground truth calculated from in situ disdrometer-based DSD measurements for a Typhoon event. We have implemented, in real-time, the algorithm in all the CASA IP1 radar nodes. We also evaluate our preliminary method that separately estimates rain and wet ice attenuation using microphysical outputs from a previous supercell simulation using the CSU-RAMS (Regional Atmospheric Modeling System). The retrieved rain and wet ice specific attenuation fields were found to be in close correspondence to the 'true' fields calculated from the simulation. The concept to correct rain and wet ice attenuation separately can be also applied to the CASA IP1 network with additional constraint information possibly provided by the WSR-88D network.
Open Access
Top-down clustering based self-organization of collaborative wireless sensor networks
(Colorado State University. Libraries, 2008) Bandara, H. M. N. Dilum, author; Jayasumana, Anura P., advisor; Massey, Daniel F., committee member; Chandrasekar, V., committee member
The proposed cluster tree based routing strategy facilitates both node-to-sink and node-to-node communication. Hierarchical addresses that reflect the parent-child relationship of cluster heads is used to route data along the cluster tree. Utilization of cross-links among neighboring cluster heads and a circular path within the network approximately doubles the capacity of the network. Under ideal conditions, this approach guarantees delivery of events/queries and has a lower overhead compared to routing strategies such as rumor routing and ant routing. The cluster tree formed by our algorithm is used to identify and form Virtual Sensor networks (VSNs), an emerging concept that supports resource efficient collaborative WSNs. Our implementation of VSN is able to deliver unicast, multicast, and broadcast traffic among nodes observing similar events, efficiently. Efficacy of the VSN based approach is evaluated by simulating a subsurface chemical plume monitoring system. The algorithm is further extended to support the formation of a secure backbone that can enable secure upper layer functions and dynamic distribution of cryptographic keys, among nodes and users of collaborative sensor networks.
Open Access
Simulation of space-based radar observations of precipitations
(Colorado State University. Libraries, 2008) Khajonrat, Direk, author; Chandra, Chandrasekar V., advisor
The Tropical Rainfall Measurement Mission (TRMM) will soon be followed on by the Global Precipitation Measurement (GPM). The GPM satellite will be the next generation observation of precipitation from space. The GPM will carry a dual-frequency precipitation radar (DPR) operating at 13.6 GHz (Ku-band) and 35.6 GHz (Ka-band), as opposed to a single-frequency 13.8 GHz (Ku-band) precipitation radar (PR) in TRMM. A greater degree of accuracy of precipitation measurements can be achieved by a dual-frequency radar using measurements from the two channels. The DPR on the GPM will be the first space-based dual-frequency precipitation radar. Since spaceborne precipitation observations have never been done in Ka-band before, extensive research on dual-frequency radar, including electromagnetic wave propagation characteristics from space and retrieval algorithms are essential for system development and system evaluations. Because the DPR is the first of its kind, a simulation-based study can provide significant assessment of the GPM system which is presented here. The research reported here focuses on developing methodologies for simulating the precipitation characteristics that would be observed from space by DPR using current space-based radar observations and earth-based radar measurements. The underlying microphysics of precipitation structures are important for developing a simulation model and a realistic model of precipitation is desired for representative simulation results. In this research; a microphysical model of precipitation is developed based on airborne radar measurements. The simulation of precipitation observations in Ku- and Ka-band are performed using both TRMM-PR observations and ground-based radar measurements. The simulation of a wide variety of precipitation regimes reveals the characteristics of the precipitation observed in Ku- and Ka-band, and allows testing of different retrieval algorithms-either the single-frequency (TRMM-like algorithm) or dual-frequency techniques. A significant degradation of signal in the Ka-band channel in intense precipitation such as an intense convective storm and tropical storms directly affect the retrieval algorithms that can be used. Vertical reflectivity profiles classification and drop size distribution parameters estimation of tropical storms are studied and results are presented here.
Open Access
Propagation and frequency conversion of ultrashort pulses in the presence of coherent nuclear motion
(Colorado State University. Libraries, 2008) Hartinger, Klaus Karl, author; Bartels, R. A., advisor
We have investigated linear as well as nonlinear propagation effects on a relatively weak ultrashort pulse arising from coherent nuclear motion. To this end, we have developed analytical and numerical models used to calculate the molecular response to a strong, ultrashort pump pulse, and propagate a weak probe pulse in the presence of the nuclear wave packet. The molecular response is described in terms of an "effective" susceptibility, which can be split into linear and nonlinear contributions. While a lot of what is discussed in terms of propagation effects is applicable to both rotational and vibrational wave packets, molecular alignment, i.e., coherent rotational motion of linear molecules, is where the focus lies. We have applied spectral interferometry to detect molecular alignment, both in scanning and single-shot configurations, to observe propagation effects due to the effective linear susceptibility, as well as carried out calculations and measurements showing the dependence of the effective third-order susceptibility on coherent nuclear motion. Lastly, a strong enhancement in the conversion efficiency to the third harmonic of a relatively weak probe pulse is observed in a variety of molecular and atomic gases.
Open Access
Robust resource allocation in heterogeneous parallel and distributed computing systems
(Colorado State University. Libraries, 2008) Smith, James T., II, author; Siegel, H. J., advisor; Maciejewski, A. A., advisor
In a heterogeneous distributed computing environment, it is often advantageous to allocate system resources in a manner that optimizes a given system performance measure. However, this optimization is often dependent on system parameters whose values are subject to uncertainty. Thus, an important research problem arises when system resources must be allocated given uncertainty in system parameters. Robustness can be defined as the degree to which a system can function correctly in the presence of parameter values different from those assumed. In this research, we define mathematical models of robustness in both static and dynamic stochastic environments. In addition, we model dynamic environments where estimates of system parameter values are provided as point estimates where these estimates are known to deviate substantially from their actual values. The main contributions of this research are (1) mathematical models of robustness suitable for dynamic environments based on single estimates of system parameters (2) a mathematical model of robustness applicable to environments where the uncertainty in system parameters can be modeled stochastically, (3) a demonstration of the use of this metric to design resource allocation heuristics in a static environment, (4) a mathematical model of robustness in a stochastic dynamic environment, (5) we demonstrate the utility of this dynamic robustness metric through the design of resource allocation heuristics, (6) the derivation of a robustness metric for evaluating resource allocation decisions in an overlay network along with a near optimal resource allocation technique suitable to this environment.
Open Access
Applications of extreme ultraviolet compact lasers to nanopatterning and high resolution holographic imaging
(Colorado State University. Libraries, 2008) Wachulak, Przemyslaw Wojciech, author; Marconi, Mario C., advisor
This dissertation describes two applications of extreme ultraviolet light in nanotechnology. Using radiation with a wavelength in the extreme ultraviolet (EUV) range allows to reach scales much smaller than with a conventional visible illumination. The first part of this dissertation describes a series of experiments that allowed the patterning at nanometer scales with sub-100nm resolution. Two types of photoresists (positive tone - PMMA and negative tone - HSQ) were patterned over the areas up to a few mm2 with features as small as 45nm using the interferometric lithography approach, reaching resolution equivalent to the wavelength of the illumination - 46.9nm. For the nanopatterning experiments two types of interferometers were studied in detail: Lloyd's mirror configuration and an amplitude division interferometer. Both approaches are presented and their advantages and drawbacks are discussed. The second part of the dissertation focuses on holographic imaging with ultimate resolution approaching the wavelength of the illumination. Different experiments were performed using Gabor's in-line holographic configuration and its capabilities in the EUV region were discussed. Holographic imaging was performed with different objects: AFM probes, spherical markers and carbon nanotubes. The holograms were stored in a high resolution recording medium - photoresist, digitized with an atomic force microscope and numerically reconstructed using a code based on the Fresnel propagator algorithm achieving in the reconstructed images the ultimate wavelength resolution. The resolution for the carbon nano-tubes images was assessed by two independent measurements: the knife-edge test resulting 45.5nm and an algorithm based on the correlation between the reconstructed image and a set of templates with variable resolution obtained by successive Gaussian filtering. This analysis yielded a resolution ~46nm. A similar algorithm that allowed for the simultaneous assessment of the resolution and the size of the features was used in EUV microscopy images confirming the validity and robustness of the code. A very fast, non-recursive reconstruction algorithm based on fast Fourier transform allowed for three dimensional surface reconstruction of the hologram performed by optical numerical sectioning, with a lateral resolution ~200nm and depth resolution ~2μm.
Open Access
Robust resource-allocation methods for QOS-constrained parallel and distributed computing systems
(Colorado State University. Libraries, 2008) Shestak, Valdimir, author; Maciejewski, A. A., advisor; Siegel, Howard Jay, advisor
This research investigates the problem of robust resource allocation for distributed computing systems operating under imposed Quality of Service (QoS) constraints. Often, such systems are expected to function in a physical environment replete with uncertainty, which causes the amount of processing required over time to fluctuate substantially. In the first two studies, we show how an effective resource allocation can be achieved in the heterogeneous shipboard distributed computing system and IBM cluster based imaging system. The general form for a stochastic robustness metric is then presented based on a mathematical model where the relationship between uncertainty in system parameters and its impact on system performance are described stochastically. The utility of the established metric is exploited in the design of optimization techniques based on greedy and iterative approaches that address the problem of resource allocation in a large class of distributed systems operating on periodically updated data sets. One of the major reasons for possible QoS violations in distributed systems is a loss of resources, frequently caused by abnormal operating conditions. One aspect that makes a resource allocation problem extremely challenging in such systems is a random nature of resource failures and recoveries. The last study presented in this work describes a solution method that was developed for this case based on the concepts of the Derman-Lieberman-Ross theorem. The experimental results indicate a significant potential of this approach to generate robust resource allocations in unstable distributed systems.
Open Access
Differential gene expression in Escherichia coli following exposure to non-thermal atmospheric-pressure plasma
(Colorado State University. Libraries, 2008) Sharma, Ashish, author; Collins, George, advisor; Pruden, Amy, advisor
Plasma decontamination provides a low temperature and non-toxic means of treating objects where heating and exposure to poisonous compounds is not acceptable especially in applications relating to medical devices and food packaging. The effects of various plasma constituents (UV photons, reactive species, charged particles etc.) acting independently and/or synergistically on bacteria at the biomolecular level is not well understood. High-density oligonucleotide microarrays were used to explore the differential gene expression of the entire genome of E. coli following plasma treatment. The results indicate a significant induction of genes involved in DNA repair and recombination suggesting that plasma exposure caused substantial DNA damage in the cell. There was also evidence of oxidative stress and suppression of genes involved in housekeeping functions of energy metabolism and ion transport. Experiments were also carried out to optimize plasma operating parameters to achieve a higher rate of inactivation of microbes. Overall, the results of this study will help to further optimize non-thermal plasma applications for bacterial inactivation.
Open Access
Towards emulation of large-scale IP networks using end-to-end packet delay characteristics
(Colorado State University. Libraries, 2008) Vivanco, Daniel A., author; Jayasumana, Anura, advisor
Network emulation combines concepts from network simulation and measurements and provides a emulated network testbed over which application and protocols can be tested. Existing network emulators are not scalable due to the limitations of available computer hardware infrastructure and the reliance on one-to-one packet mapping and modeling scheme. This research proposes a measurement-based modeling methodology for the design of a network-in-a-box emulator. Methodology aims at overcoming the limitation of computational overhead and end-to-end network system characterization. A framework for large scale IP network emulation, named Overall Trend Replicating Network Emulator Tool (OTRENET), is presented. OTRENET intercepts data packet streams and modify them, based on network system models, in real-time. The complexity and overhead of packet-by-packet mapping and modeling, while producing results consistent with measurements is achieved by a traffic sampling algorithm. Such algorithm monitors traffic metrics in a per-packet level, to dynamically separate it into frames. A comprehensive study of end-to-end packet delay dynamics, in the context of network system modeling, is presented. Theoretical basis, techniques and measurements for network packet delay dynamics characterization and modeling for various sending rate conditions and network stages have been developed. Goodness-of-fit results demonstrate the modeling accuracy for both packet delay and IPG processes for cases where sending bit rate is relatively small compared to the link capacity. However, as the sending bit rate increases, as a fraction of the bandwidth, IPG becomes a better alternative for network system modeling. A novel approach for online modeling end-to-end packet delay dynamics is proposed to address non-stationarity of network systems. Proposed methodology models and captures the network system characteristics taking into account the non-stationarity of the packet delay samples. In general, results presented show that analyzing packet delay processes by modeling the segmented traces yield a better understanding of the network system dynamics.
Open Access
Characterization of integrated optical waveguide devices
(Colorado State University. Libraries, 2008) Yuan, Guangwei, author
At the Optoelectronics Research Lab in ECE at CSU, we explore the issues of design, modeling and measurement of integrated optical waveguide devices of interest, such as optical waveguide biosensors and on-chip optical interconnects. A local evanescent-field array coupled (LEAC) sensor was designed to meet the needs for low-trace biological detection without florescent chemical agent aids. The measurement of LEACs sensor requires the aid of either a commercial near-field scanning optical microscope (NSOM) or new proposed buried detector arrays. LEAC sensors were first used to detect pseudo-adlayers on the waveguide top surface. These adlayers include SiNx and photoresist. The field modulation that was obtained based on NSOM measurement was approximately 80% for a 17 nm SiNx adlayer that was patterned on the waveguide using plasma reactive ion etching. Later, single and multiple regions of immunoassay complex adlayers were analyzed using NSOM. The most recent results demonstrated the capability of using this sensor to differentiate immunoassay complex regions with different surface coverage ratio. The study on buried detectors revealed a higher sensitivity of the sensor to a thin organic film on the waveguide. By detecting the optical intensity decay rate, the sensor was able to detect several nanometer thick film with 1.7 dB/mm/nm sensitivity. In bulk material analysis, this sensor demonstrated more than 15 dB/mm absorption coefficient difference between organic oil and air upper claddings. In on-chip optical interconnect research, optical waveguide test structures and leaky-mode waveguide coupled photodetectors were designed, modeled and measured. A 16-node H-tree waveguide was used to deliver light into photodetectors and characterized. Photodetectors at each end node of the H-tree were measured using near-field scanning microscopy. The 0.5 micrometer wide photodetector demonstrated up to 80% absorption ratio over just a 10 micrometer length. This absorption efficiency is the highest among reported leaky-mode waveguide coupled photodetectors. The responsivity and quantum efficiency of this photodetector are 0.35 A/W and 65%, respectively.
Open Access
Rainfall estimation from spaceborne and ground based radars using neural networks
(Colorado State University. Libraries, 2009) Alqudah, Amin, author; Chandra, Chandrasekar V., advisor
Rainfall observed on the ground is dependent on the four dimensional radar observations. However it is difficult to express this in a simple form. A simple Z-R relation is not sufficient and has large uncertainty and it needs to be adaptively adjusted. Prior research has shown that neural networks can be used to estimate ground rainfall from radar measurements. Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) is the first space borne observation platform for mapping precipitation over the tropics. TRMM measured rainfall is important in order to study the precipitation distribution all over the globe in the tropics. TRMM ground validation is a critical important component to ensure the measurement accuracy. However, this ground validation has quite different characteristics from TRMM in terms of resolution, scale, viewing aspect, and uncertainties. This makes the use of ground radar rainfall information to correct TRMM rainfall estimates a very challenging task. In this dissertation, rainfall estimation using neural networks is investigated in order to improve rainfall estimation based on measurements taken by ground radars and TRMM-PR. Ground Radar measurements will be used to estimate rainfall using adaptive neural networks. Improvements are also suggested and performed including the use of Principal Components Analysis, ensemble average neural network, and the use of Bayesian Neural Networks. For TRMM-PR purposes a single neural network is not efficient to extract the relation between TRMM-PR measurements and the rain gauges; this is because of the resolution differences between TRMM-PR profile and the rain gauges and the low number of TRMM overpasses over these gauges which will make the training data set to have less number of profiles and not be able to generalize. Therefore, a novel hybrid Neural Network model is presented to train ground radars for rainfall estimate using rain gauge data and subsequently the trained ground radar rain estimates to train TRMM-PR based Neural Networks for rainfall estimation. This hybrid neural network model will derive the relation between rain gauges and ground radar measurements, and transfer this relation to adaptive rainfall estimation for TRMM-PR in order to estimate rainfall and generate global rainfall maps.
Open Access
Three-dimensional water vapor retrieval using a network of scanning compact microwave radiometers
(Colorado State University. Libraries, 2009) Padmanabhan, Sharmila, author; Reising, S. C., advisor
Quantitative precipitation forecasting is currently limited by the paucity of observations on sufficiently fine temporal and spatial scales. In particular, convective storms have been observed to develop in regions of strong and rapidly evolving moisture gradients that vary spatially on sub-meso γ scales (2-5 km). Therefore, measurements of water vapor aloft with high time resolution and sufficient spatial resolution have the potential to improve forecast skill for the initiation of convective storms. Such measurements may be used for assimilation into and validation of numerical weather prediction (NWP) models. Currently, water vapor density profiles are obtained using in-situ sensors on radiosondes and remotely using lidars, GPS ground-based networks, CPS radio occultation from satellites and a relatively small number of space-borne microwave and infrared radiometers. In-situ radiosonde measurements have excellent vertical resolution but are severely limited in temporal and spatial coverage. In addition, each radiosonde takes 45-60 minutes to rise from ground level to the tropopause, and is typically advected by upper-level winds up to tens of km horizontal displacement from its launch site. Tomographic inversion applied to ground-based measurements of GPS wet delay is expected to yield data with 0.5-1 km vertical resolution at 30-minute intervals. COSMIC and CHAMP satellites in low earth orbit (LEO) provide measurements with 0.1-0.5 km vertical resolution at 30-minute intervals but only 200-600 km horizontal resolution, depending on the magnitude of the path-integrated refractivity. Microwave radiometers in low-earth orbit provide reasonable vertical resolution (2 km) and mesoscale horizontal resolution (20 km) with long repeat times. Both the prediction of convective initiation and quantitative precipitation require knowledge of water vapor variations on sub-meso γ scales (2-5 km) with update times on the order of a few tens of minutes. Due to the relatively high cost of both commercially-available microwave radiometers for network deployment and rapid radiosonde launches with close horizontal spacing, such measurements have not been available. Measurements using a network of multi-frequency microwave radiometers can provide information to retrieve the 3-D distribution of water vapor in the troposphere. An Observation System Simulation Experiment (OSSE) was performed in which synthetic examples of retrievals using a network of radiometers were compared with results from the Weather Research Forecasting (WRF) model at a grid scale of 500 m. These comparisons show that the 3-D water vapor field can be retrieved with an accuracy varying from 15-40% depending on the number of sensors in the network and the location and time of the a priori. To deploy a network of low cost radiometers, the Compact Microwave Radiometer for Humidity profiling (CMR-H) was developed by the Microwave Systems Laboratory at Colorado State University. Using monolithic microwave integrated circuit technology and unique packaging yields a radiometer that is small (24 x 18 x 16 cm), light weight (6 kg), relatively inexpensive and low-power consumption (25-50 W, depending on weather conditions). Recently, field measurements at the DOE Atmospheric Radiation Measurement (ARM) Southern Great Plains site in Oklahoma have demonstrated the potential for coordinated, scanning microwave radiometers to provide 0.5-1 km resolution both vertically and horizontally with sampling times of 15 minutes or less. This work describes and demonstrates the use of algebraic reconstruction tomography to retrieve the 3-D water vapor field from simultaneous brightness temperatures using radiative transfer theory, optimal estimation and Kalman filtering.
Open Access
Networked radar systems: waveforms, signal processing and retrievals for volume targets
(Colorado State University. Libraries, 2009) Bharadwaj, Nitin, author; Venkatachalam, Chandrasekar, advisor
Networked radar systems consisting of a dense set of agile short-range high frequency radars operating as Distributed Collaborative Adaptive System (DCAS) is an emerging innovative concept for atmospheric remote sensing that offer great potential to address several challenging problems in atmospheric remote sensing. This research addresses some of the unique challenges that must be overcome to successfully deploy a networked radar system. This research also provides a novel waveform and methodology for a networked radar environment and wideband waveforms for next generation precipitation radars.
Open Access
Pose estimation of spherically correlated images using eigenspace decomposition in conjunction with spectral theory
(Colorado State University. Libraries, 2009) Hoover, Randy C., author; Maciejewski, Anthony A., advisor; Peterson, Christopher Scott, 1963-, committee member; Roberts, Rodney G., committee member; Chong, Edwin Kah Pin, committee member; Young, Peter M., committee member
Eigenspace decomposition represents one computationally efficient approach for dealing with object recognition and pose estimation, as well as other vision-based problems, and has been applied to sets of correlated images for this purpose. The general idea behind eigenspace decomposition is that a large set of highly correlated images can be approximately represented by a much smaller subspace. Unfortunately, determining the dimension of the subspace, as well as computing the subspace itself is computationally prohibitive. To make matters worse, this off-line expense increases drastically as the number of correlated images becomes large (which is the case when doing fully general three-dimensional pose estimation or illumination invariant pose estimation). However, previous work has shown that for data correlated in one-dimension, Fourier analysis can help reduce the computational burden of this off-line expense. The first part of this dissertation extends some of the ideas developed for one-dimensionally correlated image data to data correlated in two- and three-dimensions making fully general three-dimensional pose estimation possible (assuming the object is illuminated from a single distant light source). The first step in this extension is to determine how to capture training images of the object by sampling the two-sphere (S2), and the rotation group (SO(3)) appropriately. Therefore, a thorough analysis of spherical tessellations is performed as applied to the problem of pose estimation. An algorithm is then developed for reducing the off-line computational burden associated with computing the eigenspace by exploiting the spectral information of this spherical data set. The algorithm is based on the fact that, similar to Fourier analysis on the line or circle, spherically correlated functions can be expanded into a finite series using spherical harmonics. It is then shown that the algorithm can be extended to higher dimensions by applying a proper rotation to each of the samples defined on the surface of the sphere. Using this sampling technique, a parameterization of SO(3) is obtained. It is shown that SO(3) correlated functions can be expanded into a finite series by applying a rotation to the set of spherical harmonics and expanding the function using Wigner-D matrices. Experimental results are presented to compare the proposed algorithm to the true eigenspace decomposition, as well as assess the computational savings. The second part of this dissertation deals with the problem of pose estimation when variations in illumination conditions exist. It is shown that the dimensionality of a set of images of an object under a wide range of illumination conditions and fixed pose can be significantly reduced by expanding the image data in a series of spherical harmonics. This expansion results in a reduced dimensional set of \harmonic images". It is shown that the set of harmonic images are capable of recovering a significant amount of information from a set of images captured when both single and multiple illumination sources are present. An algorithm is then developed to estimate the eigenspace of a set of images that contain variation in both illumination and pose. The algorithm is based on projecting the set of harmonic images onto a set of Fourier harmonics by applying Chang's eigenspace decomposition algorithm. Finally, an analysis of eigenspace manifolds is presented when variations in both illumination and pose exist. A technique for illumination invariant pose estimation is developed based on eigenspace partitioning. Experimental results are presented to validate the proposed algorithm in terms of accuracy in estimating the eigenspace, computational savings, and the accuracy of determining the pose of three-dimensional objects under a range of illumination conditions.