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- Title
- Broad Bandwidth, All-fiber, Thulium-doped Photonic Crystal Fiber Amplifier for Potential Use in Scaling Ultrashort Pulse Peak Powers.
- Creator
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Sincore, Alex, Richardson, Martin, Shah, Lawrence, Amezcua Correa, Rodrigo, University of Central Florida
- Abstract / Description
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Fiber based ultrashort pulse laser sources are desirable for many applications; however generating high peak powers in fiber lasers is primarily limited by the onset of nonlinear effects such as self-phase modulation, stimulated Raman scattering, and self-focusing. Increasing the fiber core diameter mitigates the onset of these nonlinear effects, but also allows unwanted higher-order transverse spatial modes to propagate. Both large core diameters and single-mode propagation can be...
Show moreFiber based ultrashort pulse laser sources are desirable for many applications; however generating high peak powers in fiber lasers is primarily limited by the onset of nonlinear effects such as self-phase modulation, stimulated Raman scattering, and self-focusing. Increasing the fiber core diameter mitigates the onset of these nonlinear effects, but also allows unwanted higher-order transverse spatial modes to propagate. Both large core diameters and single-mode propagation can be simultaneously attained using photonic crystal fibers.Thulium-doped fiber lasers are attractive for high peak power ultrashort pulse systems. They offer a broad gain bandwidth, capable of amplifying sub-100 femtosecond pulses. The longer center wavelength at 2 ?m theoretically enables higher peak powers relative to 1 ?m systems since nonlinear effects inversely scale with wavelength. Also, the 2 ?m emission is desirable to support applications reaching further into the mid-IR.This work evaluates the performance of a novel all-fiber pump combiner that incorporates a thulium-doped photonic crystal fiber. This fully integrated amplifier is characterized and possesses a large gain bandwidth, essentially single-mode propagation, and high degree of polarization. This innovative all-fiber, thulium-doped photonic crystal fiber amplifier has great potential for enabling high peak powers in 2 ?m fiber systems; however the current optical-to-optical efficiency is low relative to similar free-space amplifiers. Further development and device optimization will lead to higher efficiencies and improved performance.
Show less - Date Issued
- 2014
- Identifier
- CFE0005260, ucf:50611
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005260
- Title
- Distribution of Laser Induced Heating in Multi-Component Chalcogenide Glass and its Associated Effects.
- Creator
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Sisken, Laura, Richardson, Kathleen, Richardson, Martin, Shah, Lawrence, University of Central Florida
- Abstract / Description
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Chalcogenide glasses are well known to have good transparency into the infrared spectrum. These glasses though tend to have low thresholds as compared to oxide glasses for photo-induced changes and thermally-induced changes. Material modification such as photo-induced darkening, bleaching, refractive index change, densification or expansion, ablation of crystallization have been demonstrated, and are typically induced by a thermal furnace-based heat treatment, an optical source such as a...
Show moreChalcogenide glasses are well known to have good transparency into the infrared spectrum. These glasses though tend to have low thresholds as compared to oxide glasses for photo-induced changes and thermally-induced changes. Material modification such as photo-induced darkening, bleaching, refractive index change, densification or expansion, ablation of crystallization have been demonstrated, and are typically induced by a thermal furnace-based heat treatment, an optical source such as a laser, or a combination of photo-thermal interactions. Solely employing laser-based heating has an advantage over a furnace, since one has the potential to be able to spatially modify the materials properties with much greater precision by moving either the beam or the sample.The main properties of ChG glasses investigated in this study were the light-induced and thermally-induced modification of the glass through visible microscopy, white light interferometry, and Raman spectroscopy. Additionally computational models were developed in order to aid in determining what temperature rise should be occurring under the conditions used in experiments.It was seen that ablation, photo-expansion, crystallization, and melting could occur for some of the irradiation conditions that were used. The above bandgap energy simulations appeared to overestimate the maximum temperature that should have been reached in the sample, while the below bandgap energy simulations appeared to underestimate the maximum temperature that should have been reached in the sample. Ultimately, this work produces the ground work to be able to predict and control dose, and therefore heating, to induce localized crystallization and phase change.
Show less - Date Issued
- 2014
- Identifier
- CFE0005261, ucf:50606
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005261
- Title
- Predicting Risk to Reoffend: Establishing the Validity of the Postive Achievement Change Tool.
- Creator
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Martin, Julie, Wan, Thomas, Winton, Mark, Martin, Lawrence, Chen, Hsueh-Fen, University of Central Florida
- Abstract / Description
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In recent years, there has been increased reliance on the use of risk assessment in the juvenile justice system to predict and classify offenders based on their risk to reoffend. Over the years, the predictive validity of risk assessments has improved through the inclusion of actuarial assessment and dynamic risk factors. The predictive validity of certain assessments, such as the Youth Level of Service/Case Management Inventory (YLS/CMI), has been well established through numerous...
Show moreIn recent years, there has been increased reliance on the use of risk assessment in the juvenile justice system to predict and classify offenders based on their risk to reoffend. Over the years, the predictive validity of risk assessments has improved through the inclusion of actuarial assessment and dynamic risk factors. The predictive validity of certain assessments, such as the Youth Level of Service/Case Management Inventory (YLS/CMI), has been well established through numerous replication studies on different subgroups of the population. The validity of other instruments, such as the Positive Achievement Change Tool (PACT), is in its infancy having only been validated on the sample of the population for which it was created. The PACT, a relatively new juvenile risk assessment tool, was adapted from the Washington State Juvenile Court Assessment and validated on the Florida juvenile population. This study sought to demonstrate the predictive validity of the PACT risk assessment, analyze gender differences in juvenile recidivism, and determine the relative importance of individual-level, social-level, and community-level variables in the prediction of recidivism for a sample of juveniles in Tarrant County, Texas. The results of this research confirmed the predictive validity of the PACT for juveniles served by Tarrant County Juvenile Services (TCJS). Despite possessing adequate predictive validity for the entire population, gender-specific analyses revealed differences in the ability of the PACT to accurately classify female delinquents based on risk to reoffend. Not only did gender differences emerge in the predictive validity of the PACT, but males and female recidivism was also predicted by different social-level indicators. The results of this research provided further evidence for social-causation theories of crime and delinquency, with social-level indicators exerting the strongest relationship with recidivism when compared to individual-level and community-level predictors. The inability of community-level predictors to enhance the predictive accuracy of the assessment suggest broad application of the PACT across jurisdictions. TCJS has invested a considerable amount of time, resources, and funding in the implementation and maintenance of the PACT. The results of this study provided support and direction for the continued use of the PACT at TCJS. In addition, establishing the predictive validity of the PACT on the Tarrant County juvenile population satisfied the legislative requirement for a population specific validation of the risk assessment implemented in each county.
Show less - Date Issued
- 2012
- Identifier
- CFE0004221, ucf:48992
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004221
- Title
- Effects of Electronic Media Messages on the Perceived Self-Efficacy of Pedestrian Commuters Living in the Unincorporated Central Florida Community of Conway.
- Creator
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Stephens, Daniel, Yu, Chia-Yuan, Martin, Lawrence, Hawkins, Christopher, Seigler, Daniel, University of Central Florida
- Abstract / Description
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Urban pedestrianism is increasingly perceived as a dangerous form of travel. While roadway design has been historically scaled to cars instead of people, planning professionals are now re-thinking their approach to make roads more inclusive for all travelers. Scholars, however, have learned harbored fear can trump behavior even under ideal travel conditions. Such fear can adversely impact perceived pedestrian self-efficacy, which is the self-generated internal assessment or belief in a...
Show moreUrban pedestrianism is increasingly perceived as a dangerous form of travel. While roadway design has been historically scaled to cars instead of people, planning professionals are now re-thinking their approach to make roads more inclusive for all travelers. Scholars, however, have learned harbored fear can trump behavior even under ideal travel conditions. Such fear can adversely impact perceived pedestrian self-efficacy, which is the self-generated internal assessment or belief in a traveler's agentive abilities to navigate the travel environment. The challenge thus becomes twofold: improve the built environment while bolstering traveler confidence. The following study, therefore, employed a qualitative phenomenological research design to ascertain the concerns and perceptions of vulnerable travelers as it pertained to and was affected by travel-specific media. The study employed denizens selected from the Central Florida community of Conway, who were interviewed using a multi-method approach employing a semi-structured interview technique utilizing individual interviews and small focus group sessions. Using Social Cognitive Theory (SCT) as the theoretical framework, the researcher studied and documented the elements contributing to the perceptions of pedestrian travelers. The rationale for this approach is found in the dynamic relationships that exist between the objective travel environment, the perceived travel environment, and travel behavior (-) all representing the triad of cognition, the external environment, and social stimuli, which encompass Bandura's Triadic Reciprocal Determinism (TRD). The four themes that emerged from the data analysis (-) communication, safety, cost, and happiness (-) characterize the experiences of the participants as they watched positively-themed media images modeling civil travel behavior. This research adds to existing literature on the magnitude such themes have on perception, to include latent perceptions harbored by pedestrian commuters concerning dangers (-) real or imagined (-) of traveling on local roadways.
Show less - Date Issued
- 2018
- Identifier
- CFE0007376, ucf:52100
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007376
- Title
- 2 micron fiber lasers: power scaling concepts and limitations.
- Creator
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Sincore, Alex, Richardson, Martin, Amezcua Correa, Rodrigo, Schulzgen, Axel, Shah, Lawrence, University of Central Florida
- Abstract / Description
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Thulium- and holmium-doped fiber lasers (TDF and HDF) emitting at 2 micron offer unique benefits and applications compared to common ytterbium-doped 1 micron lasers. This dissertation details the concepts, limitations, design, and performance of four 2 micron fiber laser systems. While these lasers were developed for various end-uses, they also provide further insight into two major power scaling limitations. The first limitation is optical nonlinearities: specifically stimulated Brillouin...
Show moreThulium- and holmium-doped fiber lasers (TDF and HDF) emitting at 2 micron offer unique benefits and applications compared to common ytterbium-doped 1 micron lasers. This dissertation details the concepts, limitations, design, and performance of four 2 micron fiber laser systems. While these lasers were developed for various end-uses, they also provide further insight into two major power scaling limitations. The first limitation is optical nonlinearities: specifically stimulated Brillouin scattering (SBS) and modulation instability (MI). The second limitation is thermal failure due to inefficient pump conversion. First, a 21.5 W single-frequency, single-mode laser with adjustable output from continuous-wave to nanosecond pulses is developed. Measuring the SBS threshold versus pulse duration enables the Brillouin gain coefficient and gain bandwidth to be determined at 2 micron. Second, a 23 W spectrally-broadband, nanosecond pulsed laser is constructed for materials processing applications. The temporally incoherent multi-kW peak power pulses can also efficiently produce MI and supercontinuum generation by adjusting the input spectral linewidth. Third, the measured performance of in-band pumped TDF and HDF lasers are compared with simulations. HDF displays low efficiencies, which is explained by including ion clustering in the simulations. The TDF operates with impressive (>)90% slope efficiencies. Based on this result, a system design for (>)1 kW average power TDF amplifier is described. The designed final amplifier will be in-band pumped to enable high efficiency and low thermal load. The amplifier efficiency, operating bandwidth, thermal load, and nonlinear limits are modeled and analyzed to provide a framework for execution. Overall, this dissertation provides further insight and understanding on the various processes that limit power scaling of 2 micron fiber lasers.
Show less - Date Issued
- 2018
- Identifier
- CFE0007374, ucf:52105
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007374
- Title
- Design and Engineering Criteria for Optical Parametric Chirped Pulse Amplifier Systems.
- Creator
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Bodnar, Nathan, Richardson, Martin, Delfyett, Peter, Likamwa, Patrick, Baudelet, Matthieu, Shah, Lawrence, University of Central Florida
- Abstract / Description
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The generation of a quasi-single-cycle laser light pulse is a goal in many laser applications experiments. Some involve in High Harmonic Generation (HHG) and Attosecond Sciences. The demand for ultrafast laser facilities has grown; the techniques and availability of materials have changed; thereby posing new design challenges in building Optical Parametric Chirped Pulse Amplification (OPCPA) laser facilities. The concepts and challenges are discussed in detail in the development of two laser...
Show moreThe generation of a quasi-single-cycle laser light pulse is a goal in many laser applications experiments. Some involve in High Harmonic Generation (HHG) and Attosecond Sciences. The demand for ultrafast laser facilities has grown; the techniques and availability of materials have changed; thereby posing new design challenges in building Optical Parametric Chirped Pulse Amplification (OPCPA) laser facilities. The concepts and challenges are discussed in detail in the development of two laser systems within the Laser Plasma Laboratory, HERACLES and PhaSTHEUS. This dissertation also gives insight to the challenges that are encountered in other cutting edge OPCPA laser facilities. An overview of the design challenges that need to be addressed in any OPCPA laser facility either high energy or high average power that is suitable for high intensity laser physics is discussed in this dissertation.
Show less - Date Issued
- 2018
- Identifier
- CFE0007158, ucf:52300
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007158
- Title
- Use of Performance Information by Local Government Administrators: Evidence from Florida.
- Creator
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Dimitrijevska-Markoski, Tamara, Kapucu, Naim, Martin, Lawrence, Hu, Qian, Wang, Xiaohu, University of Central Florida
- Abstract / Description
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This study examines the factors that facilitate and hinder the use of performance information by public administrators in local governments in Florida. Acknowledging the incompleteness of many theoretical and conceptual models in previous performance management studies and the absence of theory on the use of performance information; this study utilizes a grounded theory approach to develop and test a model analyzing the use of performance information. The research focuses on cities and...
Show moreThis study examines the factors that facilitate and hinder the use of performance information by public administrators in local governments in Florida. Acknowledging the incompleteness of many theoretical and conceptual models in previous performance management studies and the absence of theory on the use of performance information; this study utilizes a grounded theory approach to develop and test a model analyzing the use of performance information. The research focuses on cities and counties, members of the Florida Benchmarking Consortium (FBC), and surveys public administrators whose tasks are related with the collection and/or reporting of performance information. The study examines three research questions: First, to what extent and in what capacity do local government administrators use performance information? Second, what are the predictors of the use of performance information among local government administrators? Finally, to what extent does the design adequacy of a performance measurement system (PMS), institutionalization of performance measurement (IPM), organizational support (OS), individual factors (IF) and external influences (EI) impact the use of performance information among local government administrators? To collect data on the above questions, an online survey was administrated to public administrators involved in the 2015-2016 FBC data collection cycle. The data were analyzed with structural equation modeling (SEM). The results of the study demonstrate that institutionalization of performance measurement has the strongest statistically significant positive association with the use of performance information followed by the influence of the design adequacy of the performance measurement system. Organizational support, through institutionalization of performance measurement, has an indirect influence on the use of performance information. Interestingly, in this study individual factors were not found to be significantly associated with the use of performance information.
Show less - Date Issued
- 2017
- Identifier
- CFE0006586, ucf:51261
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006586
- Title
- Variations in Health Services Utilization by Patients with Prostate Cancer.
- Creator
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McKee, Roberta, Wan, Thomas, Martin, Lawrence, Zhang, Ning, Sivo, Stephen, University of Central Florida
- Abstract / Description
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Among men living in the United States, prostate cancer is the second leading cause of cancer death, and, excluding skin cancers, it is the cancer diagnosed most frequently. While incidence and mortality rates have been declining, the American Cancer Society estimated that there were 220,800 men diagnosed with prostate cancer and more than 27,500 prostate cancer deaths in 2015. Various patient-level and community-level factors have been shown to influence the differential patterns of diagnosis...
Show moreAmong men living in the United States, prostate cancer is the second leading cause of cancer death, and, excluding skin cancers, it is the cancer diagnosed most frequently. While incidence and mortality rates have been declining, the American Cancer Society estimated that there were 220,800 men diagnosed with prostate cancer and more than 27,500 prostate cancer deaths in 2015. Various patient-level and community-level factors have been shown to influence the differential patterns of diagnosis, care, and outcomes for men with prostate cancer. Detailed information regarding the utilization of health services by prostate cancer patients, particularly those with higher propensity for health services use, could be used to inform efforts intended to improve the coordination and delivery of care to work towards the elimination of disparities. The purpose of the study is to facilitate a better understanding of the determinants of health services utilization by older males with prostate cancer in the United States by examining the relative influence and interaction effects of factors characterizing individual patients and their county of residence. Andersen's behavioral model of health services utilization is used as a framework to guide this study. A cross-sectional design is used to analyze administrative claims data from the 2008 Medicare Provider Analysis Review (MEDPAR) file (n=5,754). County-level data from Area Health Resources File (ARHF) are merged to include the community and contextual characteristics. American Hospital Association (AHA) annual survey data are also used to examine the importance of hospital attributes in a subset analysis (n=555). A two-stage approach is used for analyzing the data. First, several social and demographic variables are included in automatic interaction detector (AID) analysis to identify relatively homogenous subgroups of patients with similar service utilization patterns for emergency room visits and hospital length of stay. Second, regression analysis is performed in the full dataset including all patients, and in each subgroup to determine the amount of variance explained by predictor variables categorized as predisposing, enabling, and need-for-care factors. Hierarchical logistic regression is performed to analyze the variability in emergency room use, and hierarchical multiple regression is performed to analyze the variability in hospital length of stay. The results show that the need-for-care factors are dominant predictors of service use. However, the relative importance of the predictor variables varies by subgroups of prostate cancer patients identified in the initial AID analysis. The findings lend some support of the use of an integrated approach to examine the personal and social determinants of health services utilization by prostate cancer patients enrolled in the U.S. Medicare program. The theoretical framework and analytic approach employed in this study make it possible to obtain an in-depth understanding of the influential factors associated with emergency room use and length of stay for all-cause hospitalizations, which can be used to inform future research and efforts aimed at developing targeted interventions to improve the coordinated care and to reduce health disparities among Medicare beneficiaries with prostate cancer.
Show less - Date Issued
- 2016
- Identifier
- CFE0006352, ucf:51523
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006352
- Title
- Design and Engineering of Ultrafast Amplifier Systems.
- Creator
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Webb, Benjamin, Richardson, Martin, Chang, Zenghu, Delfyett, Peter, Gaume, Romain, Shah, Lawrence, Klemm, Richard, University of Central Florida
- Abstract / Description
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Recently, the design and engineering of ultrafast laser systems have led to an extraordinary increase in laser power and performance which have brought about advances in many fields such as medicine, material processing, communications, remote sensing, spectroscopy, nonlinear optics, and atomic physics. In this work, several ultrafast amplification techniques -- including chirped-pulse amplification (CPA), optical parametric chirped-pulse amplification (OPCPA), and divided-pulse amplification...
Show moreRecently, the design and engineering of ultrafast laser systems have led to an extraordinary increase in laser power and performance which have brought about advances in many fields such as medicine, material processing, communications, remote sensing, spectroscopy, nonlinear optics, and atomic physics. In this work, several ultrafast amplification techniques -- including chirped-pulse amplification (CPA), optical parametric chirped-pulse amplification (OPCPA), and divided-pulse amplification (DPA) -- are described and demonstrated in the design and construction of two ultrafast laser facilities. An existing Ti:Sapphire laser system was completely redesigned with an increased power of 10 TW for experiments capable of generating hundreds of laser filaments in ordered arrays. The performance of DPA above the Joule-level was investigated in a series of experiments utilizing various DPA schemes with gain-saturated amplifiers at high pulse energy. A new high energy OPCPA facility has been designed and its pump laser system constructed, utilizing the technique of DPA for the first time in a flashlamp-pumped amplifier chain and with a record combined energy of 5 Joules in a 230 ps pulse duration. The demonstrated OPCPA pump performance will allow for the generation of 50 TW quasi-single cycle 5 fs pulses at 2.5 Hz from a table-top OPCPA system.
Show less - Date Issued
- 2016
- Identifier
- CFE0006547, ucf:51349
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006547
- Title
- Fiber Optimization for Operation Beyond Transverse Mode Instability Limitations.
- Creator
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Bradford, Joshua, Richardson, Martin, Gaume, Romain, Amezcua Correa, Rodrigo, Shah, Lawrence, University of Central Florida
- Abstract / Description
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Transverse Mode Instabilities (TMIs) stand as a fundamental limitation to power and brightness scaling in laser systems based upon optical fiber technologies. This work comprises experimental and theoretical investigations into fiber laser design that should minimize the effects of Stimulated Thermal Rayleigh Scattering. Theoretical discussions and simulations focus on how fiber parameters affect transverse mode coupling. These include core geometry optimization, pump geometry optimization,...
Show moreTransverse Mode Instabilities (TMIs) stand as a fundamental limitation to power and brightness scaling in laser systems based upon optical fiber technologies. This work comprises experimental and theoretical investigations into fiber laser design that should minimize the effects of Stimulated Thermal Rayleigh Scattering. Theoretical discussions and simulations focus on how fiber parameters affect transverse mode coupling. These include core geometry optimization, pump geometry optimization, in addition to the effects of HOM content and losses on the TMI threshold. Experimentally, a high-power laser facility is commissioned with beam quality diagnostics to quantify the thresholds of the onset of modal interferences and their impacts on beam quality. These diagnostics include high-resolution Fourier Transform Interferometry (FTI) and in-situ power-in-the-bucket measurements. The design and characterization capabilities developed here are crucial to the development of next-generation high-power fiber laser capabilities.
Show less - Date Issued
- 2018
- Identifier
- CFE0006980, ucf:51646
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006980
- Title
- Cities of Service: A Grounded Theory Exploration of Volunteer Service.
- Creator
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Hill, Brandy, Martin, Lawrence, Bryer, Thomas, Kapucu, Naim, Wan, Thomas, University of Central Florida
- Abstract / Description
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This research explores how volunteer service can be both included and impactful in strategies to address local challenges. Prior research would suggest that volunteer service has largely been overlooked when it comes to governmental efforts to solve society's biggest problems, particularly at the local level. Ignoring the potential for volunteer service in problem-solving approaches may hinder the development of effective problem-solving strategies. This research describes the ways cities...
Show moreThis research explores how volunteer service can be both included and impactful in strategies to address local challenges. Prior research would suggest that volunteer service has largely been overlooked when it comes to governmental efforts to solve society's biggest problems, particularly at the local level. Ignoring the potential for volunteer service in problem-solving approaches may hinder the development of effective problem-solving strategies. This research describes the ways cities drive demand for volunteer service. In particular, volunteer service demand can be explained through certain motivational bases(-)economic, aspirational, and need-based(-)together with various feasibility considerations. A grounded theory model for volunteer service demand from the perspective of the city and a generalized logic model for service as a strategy to address local challenges are presented.
Show less - Date Issued
- 2013
- Identifier
- CFE0004690, ucf:49849
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004690
