Thesis (M. S.)--Oklahoma State University, 2004. / Vita. Includes bibliographical references (p. 49-52).

Thesis (Ph. D.)--Oklahoma State University, 2004. / Vita. Includes bibliographical references (p. 109-124).

Thesis (Ph. D.)--Oklahoma State University, 2005. / Vita. Includes bibliographical references (p. 58-62).

Thesis (Ph. D.)--Oklahoma State University, 2005. / Vita. Includes bibliographical references (p.102-109).

The application of Sample Grated Distributed Bragg Reflector (SGDBR) wavelength tunable lasers for swept-wavelength Optical Coherence Tomography (OCT) is explored. OCT is a method of measuring reflectivity versus distance into samples under test with a focused infrared light source. Swept wavelength OCT requires a laser light source that is capable of sweeping its wavelength quickly over the entire wavelength range of the tunable laser. Fast sweeping of the laser's wavelength enables real-time imaging of a wide surface area of the surface under test. This thesis will show that SGDBR lasers can be designed to meet the fast wavelength ramp speeds of swept wavelength OCT and to even exceed the capability of present swept-wavelength OCT source solutions. SGDBR lasers were originally developed for telecommunications applications using Wavelength Division Multiplexing. In the telecommunications application, the wide wavelength tuning range of the device (1520-1575 nm) is utilized but the devices are only required to change wavelength over 50 ms time intervals. Research on SGDBR lasers has shown that wavelength switching speeds of 5 ns have been obtained using pre-distortion of the current drive waveforms. This thesis explores the inherent modulation speed of chip-level and packaged SGDBR lasers and the associated capability to make high speed continuous wavelength ramps for swept wavelength OCT. It will be shown that frequency modulation speeds of over 100 MHz can be accomplished with the laser drive and packaging techniques presented in this work. The result of the work shows that SGDBR lasers are very promising sources for swept wavelength OCT applications. In order to understand the present generation OCT application in more detail, work is first presented demonstrating the capability of white-light interferometry OCT in a meat tenderness measurement application. White light interferometer measurement OCT has been the standard solution for OCT measurements for at least 15 years. Measurement of a range of beef samples was done in conjunction with the college of Agriculture. Results show that the OCT setup has a penetration depth of up to 1.5 mm. The work did not show strong correlation between OCT measurement signatures and meat tenderness. The work helped to understand the OCT measurement and clearly pointed out the value of increased measurement speed using swept wavelength OCT and the potential use of SGDBR lasers as the swept wavelength source. One of the conclusions drawn from this application of OCT measurements is that the process can be improved using a faster measurement technique. The thesis then studies the characteristics of the SGDBR laser and how they map into the characteristics needed for swept-wavelength OCT applications. A major part of the work was design of both chip-level and package-level sources that were used to evaluate the laser characteristics. Specific properties of this SGDBR laser are measured: wavelength tuning characteristics, optical laser linewidth, amplitude modulation speed, frequency modulation speed and wavelength switching speed on each of the control inputs to the SGDBR device In the end, it is shown that SGDBR lasers can improve the wavelength ramp speeds in OCT. Device concerns include the laser linewidth and its limitations for swept wavelength OCT. This work provided the basis for other graduate students to build up a more complete implementation of an OCT measurement system.

Detection within the deep ultraviolet (DUV) region (˜200-280 nm) offers unique fundamental advantages to probe certain optical traces. Therefore, many applications have emerged including flame and missile detection, and non-line of sight and space-to-space communication. Ga2O3 has become a natural choice for DUV detection owing to its intrinsic ultra-wide optical bandgap (˜4.85 eV), extrinsic n-type dopability, and excellent chemical and physical stability. However, Ga2O3 has no viable p-type doping to date, and fabricated photodetectors show only partial coverage of the entire solar-blind region (˜200-245nm). Furthermore, there is a limited understanding of how various growth parameters for ß-Ga2O3 and its alloys impact the material properties (i.e. defects), and how these ultimately play a role in functional device characteristics. This dissertation aims to address the aforementioned challenges with systematic studies spanning across epitaxial growth by molecular beam epitaxy (MBE), device fabrication, and characterization, leading to a comprehensive understanding of how these impact the optical, structural, compositional, and device properties. The experiments start with homoepitaxial and heteroepitaxial growth of Ga2O3 on bulk n-Ga2O3, sapphire, and advance to the growth on Si by MBE for monolithic integration. A novel nucleation technique of Ga2O3 on n-Si substrate allowed achieving one of the fastest functional DUV photodetectors with high responsivities. Furthermore, bandgap engineering via alloying Ga2O3 with In and Sn improved the DUV coverage, extending the cut-off wavelength beyond ˜245 nm, while benefitting higher responsivities. A record-setting photoresponsivity (˜35 kA/W) among planar devices was achieved with Sn alloying. The mechanisms leading to the unusually high photoconductive gains in these devices were investigated to determine the root cause. Point defects, particularly gallium vacancy-related complexes, are identified as the most likely source of ultra-high gains by hole-trapping at space-charge-region of Schottky barrier photodetectors. Moreover, a direct trade-off between bandwidth and responsivity was observed due to these complexes.

Surface acoustic wave (SAW) devices have provided solutions to many sensor applications. With the increasing use of the electromagnetic spectrum, the adoption of higher frequencies for new developments is becoming a necessity. SAW devices represent a solution for advancing many emerging sensor's needs given their inherent advantages such as: wireless operation, low cost, and ease of fabrication. However, the SAW technology has been typically limited to frequencies under 3 GHz due to size limitations, increased SAW losses, and the need of an interrogation system suitable for reading SAW sensors at higher frequencies. This dissertation presents the work done to push up the frequency limitations of SAW sensors by using the harmonic response of a SAW device to sense temperature and strain at 4.74 GHz. Transducer configurations for harmonic response are studied using the theory on SAW transduction. The development of 4.3 GHz harmonically operated SAW devices is presented as initial work and part of a NASA SBIR project to investigate the feasibility of the harmonic operation concept applied to the Wireless Avionics Intra-Communications (WAIC) band. Measurements of the devices at fundamental and harmonic frequencies were validated using the coupling of modes (COM) model. 4.74 GHz SAW devices were fabricated using a SAW mask provided by Pegasense, LLC. This mask, designed for 4.3 GHz devices using LN YZ cut, was used to fabricate higher frequency devices using LN YX 128 cut. Temperature and strain SAW sensors harmonically operated at 4.74 GHz were fabricated. Finally, a 4.74 GHz SAW interrogation system is developed, the UCF SAW SDR software is improved, a transceiver is fabricated, and the interrogation of 4.74 GHz harmonically operated SAW temperature and strain sensors is demonstrated.

Multiphoton polymerization (MPP) as one of the direct laser writing techniques is capable of manufacturing three-dimensional (3D) micro-structures with complex shapes and novel functionalities. However, current MPP methods rely on point-by-point or layer-by-layer scanning and therefore are time-consuming. The low fabrication throughput of conventional MPP is the key factor that limits its wider adoption for industrial manufacturing over large surface area. One way to increase the fabrication speed is to turn layer-by-layer process into a volumetric process. An ideal volumetric printing method can fabricate structures with complex 3D geometry by single exposure and should be easy to implement. As a step towards this goal, this dissertation discusses a volumetric fabrication method based on 3D structured light fields. I will discuss the application of zero-order Bessel beams for rapid fabrication of various high-aspect-ratio structures such as polymer fibers and scaffolds. Next, I demonstrate the generation of superposed high-order Bessel beams using a spatial light modulator (SLM). Such beams have multiple foci with long depth of focus and are suitable for parallel fabrication of high-aspect-ratio structures. Furthermore, I investigate optical aberration observed when Bessel beams propagate through tilted interfaces. A method is introduced in which a single phase map on an SLM is used to generate an "elliptical axicon" that compensates for such aberration. Finally, I develop the theory of "helical beams", whose transverse intensity distribution rotates while propagating along the optical axis. Both transverse and longitudinal shape of such beams are tunable. These beams are generated as a superposition of high-order Bessel modes and have a closed-form expression relating the design of the phase mask to the rotation rate of the beam. I demonstrate rapid fabrication of helical microstructures with tunable shape in polymer by single exposure using these beams. This volumetric fabrication technique increases the throughput by orders of magnitude compared to conventional MPP, paving the way for adopting MPP in many industrial applications. The presented technique should have potential applications in other fields such as laser processing of bulk semiconductor/dielectric with increased throughput.

This dissertation focuses on the structural transformations in photo-thermo-refractive (PTR) glass that enable recording phase holograms for efficient transformation of optical beams and high resolution spectroscopy. PTR glass is a multicomponent silicate matrix doped with Ce, Ag, Sn and Sb. It is a holographic phase medium with the ability of permanent refractive index change after UV exposure and thermal development above 500°C due to the precipitation of a NaF crystalline phase. Electronic processes are studied by analyzing the structure of absorption and luminescence bands of PTR glass matrix and its dopants. We analyze the structural transformations in PTR glass from the perspective of induced optical scattering. A high-sensitivity experimental setup for measuring scattering at 90° with respect to a 1-mm diameter probe beam at 405 nm was constructed. A specific algorithm of UV exposures and thermal regimes to reveal the lowest temperature at which inhomogeneities arise was proposed. It was shown that no scattering increase observed in UV exposed PTR glass after thermal processing at temperatures below 450°C. This means that all structural transformations produced by this treatment occurred at atomic scale. Finally, we discuss the low temperature ion exchange method. We describe the theoretical basis of this method and present an experimental layout capable of performing low-temperature ion exchange in the surface of PTR samples. Results of a system that provides measurements of surface refractive index called 'the waveguide method' is presented. It is shown how this system can be used for the characterization of planar optical waveguides created on the surface of PTR glass samples and how their refractive index profile can be calculated by the inverse Wentzel-Kramers-Brillouin (WKB) method.

Lithium niobate has numerous extraordinary features that make it useful for a wide range of applications, particularly in optics. The material's strong electro-optic effect and second-order nonlinearities are two prime examples with applications in optical modulation and wavelength conversion, respectively. The thin-film lithium niobate platform has revitalized the conventional applications of lithium niobate during the last decade. The platform is now one of the most actively investigated subdisciplines in integrated photonics. The waveguides on this innovative platform are high index contrast, resulting in a size reduction of more than 20 times and a bending radius decrease of about two orders of magnitude when compared to traditional counterparts. These ultracompact waveguides facilitate the realization of highly efficient photonic devices, some of which are presented in this work. First, tunable dual-channel integrated Bragg filters with ultra-narrow linewidths are demonstrated. These filters have potential applications in optical communication, sensing, and quantum optics. Next, high-speed Mach-Zehnder electro-optic modulators with an extrapolated 3-dB bandwidth of 170 GHz and low half-wave voltage-length product of 3.3 V.cm are presented. Furthermore, microwave-to-optical converters with integrated antennas and optical waveguides are demonstrated with improved efficiency compared to the currently existing devices for integrated microwave photonics applications. Afterward, fabricated periodically-poled lithium niobate devices are utilized to illustrate nonlinear wavelength translators through cascaded sum- and difference-frequency generations. Finally, further works on these research topics, which are appropriate for future research, are discussed.