<p> The Thulium Fiber Laser (TFL) has been studied as a potential alternative to the conventional Holmium:YAG laser (Ho:YAG) for the treatment of kidney stones. The TFL is more ideally suited for laser lithotripsy because of the higher absorption coefficient of the emitted wavelength in water, the superior Gaussian profile of the laser beam, and the ability to operate at arbitrary temporal pulse profiles. The higher absorption of the TFL by water helps translate into higher ablation of urinary stones using less energy. The Gaussian spatial beam profile allows the TFL to couple into fibers much smaller than those currently being used for Ho:YAG lithotripsy. Lastly, the ability of arbitrary pulse operation by the TFL allows energy to be delivered to the stone efficiently so as to avoid negative effects (such as burning or bouncing of the stone) while maximizing ablation. Along with these improvements, the unique properties of the TFL have led to more novel techniques that have currently not been used in the clinic, such as the ability to control the movement of stones based on the manner in which the laser energy is delivered. Lastly, the TFL has led to the development of novel fibers, such as the tapered fiber and removable tip fiber, to be used for lithotripsy which can lead to safer and less expensive treatment of urinary stones. Overall, the TFL has been demonstrated as a viable alternative to the conventional Ho:YAG laser and has the potential to advance methods and tools for treatment of kidney stones. </p>

<p> Virtual aperture culling of the eigenmodes of a resonator (VACER) is a technique to focus light within turbid media at arbitrary locations. A seed pulse of light is directed through a phase-conjugate mirror (PCM) into a turbid medium. Though much of the light may be lost, any light which reaches the second PCM is phase conjugated and thus returned to the first PCM where the light will be phase conjugated again. Amplification by the PCMs can prevent decay of the light cycling between the PCMs. Introducing a mechanism which filters light based on position enables attenuation of the modes not traveling through the center of the virtual aperture resulting in a focusing of light at the center of the virtual aperture. The seed pulse and the positioning of the PCMs on opposite sides of the virtual aperture ensure that modes cannot bypass the virtual aperture. Magnetic fields and ultrasound waves are potential means for implementation of a virtual aperture. Generally, only weak filtration mechanisms like magnetic fields and ultrasound waves are innocuous to turbid media. Fortunately, weak effects can strongly cull modes in VACER because the filtration mechanism affects the modes during each pass between PCMs and the modes compete. A combination of theory and computational modeling prove that sound physical principles underlie VACER. Moreover, computational modeling reveals how mode overlap, the seed pulse, and other variables impact VACER performance. Good experimental performance is predicted.</p>

Thesis (Ph.D.)--University of California, San Francisco, 2007. / Source: Dissertation Abstracts International, Volume: 68-11, Section: B, page: 7476. Adviser: Jean Pouliot.

<p> Hemodialysis vascular access, the interface between a dialysis patient and a dialysis machine, is quite literally the lifeblood of a patient's health. Vascular access dysfunction is the leading cause of hospitalization in hemodialysis patients. The occlusive growth of neointimal hyperplasia (NH) in expanded polytetrafluoroethylene (ePTFE) ringed grafts is the primary cause of failure. To further develop a proposed thermal ultrasound treatment to reduce or prevent NH in arteriovenous vascular grafts, the acoustic properties of ePTFE were studied in water and alcohol solutions. Previous reports of ePTFE acoustic properties are critiqued. It was found that the acoustic transmission and attenuation through ePTFE, and therefore the potential for an ultrasound-based therapy for NH, are heavily dependent on the medium in which the graft is immersed, suggesting that the acoustic properties of implanted grafts will change as grafts mature in vivo. The acoustic impedance and attenuation of water-soaked ePTFE were 0.478 &plusmn; 1.43 &times; 10^-2 MRayl and 1.78 &plusmn; 0.111 Np/cm*MHz, respectively, while the acoustic impedance and attenuation of ePTFE in alcohol were 1.49 &plusmn; 0.149 MRayl and 0.77 &plusmn; 1.1 &times; 10^-2 Np/cm*MHz, respectively. The use of focused ultrasound to heat implanted ringed ePTFE grafts was numerically modeled from 1.35- and 1.443-MHz transducers for in vitro geometries. Power deposition and heating, in turn, differed by an order of magnitude between various graft acoustic properties. Graft rings were predicted to be substantial absorbing and scattering features. In vitro phantom models were constructed: one with and one without thermocouples. At 1 W of acoustic power, the maximum temperature rise was 8&ring; C. The thermocouple model containing a water-soaked graft did not experience heating in the far graft wall. The MRTI model confirmed that the graft rings are an absorbing/scattering feature. Heating was not prevented in the presence of water flow through the graft. Water was not heated significantly. Overall, results suggest ultrasound exposure can be used to generate temperature rises corresponding with the potential prevention or inhibition of NH in ringed ePTFE vascular grafts. A hybrid therapeutic/diagnostic transducer design with a therapeutic semi-annular array surrounding a diagnostic linear array is presented. Compared to a solid transducer of the same dimensions, there were only marginal aberrations in the focal plane. Numerical optimization of the element drive configuration indicated that the least distorted focal plane was produced by uniform phase and magnitude at each element.</p>

Optical Tomography is a medical imaging modality that can be used to non- invasively image functional changes within the body. As near-infrared light is highly scattered by biological tissue, the process of image reconstruction is ill-posed and, in general is also under-determined. As such, model based iterative image reconstruction methods are used. These methods require an accurate model of light propagation through tissue, also known as the forward model. The diffusion approximation (DA) to the radiative transport equation is one of the most widely used forward models. It is based on the assumption that scattering events dominate over absorption events resulting in a diffuse light distribution. This is valid in cases with low absorption coefficients or large geometries (greater than a few scattering lengths). In many cases, however, such as in small animal imaging where the source-detector separation is small, this assumption is not valid and so a higher-ordered approximation is required. In this thesis, a three-dimensional frequency domain forward model based on the simplified spherical harmonics (SPN) approximation to the radiative transport equation is introduced. By comparison with a Monte- Carlo model, the SPN approximation is shown to be more accurate than the DA, especially in regions near to the sources and detectors and the increase in accuracy is greater in cases with stronger absorption. This is particularly important for bioluminescent imaging of small animals which involve both small geometries and strong absorption. Due to the asymptotic nature of the 3 SPN approximation, the highest ordered model was not necessarily the most accurate, but all models with N&gt;1 were more accurate than the DA. The SPN based forward model has also been implemented into an image reconstruction algorithm. Despite the fact that the SPN approximation does not combine the scattering coefficient and anisotropy factor into a single variable, as is the case in the DA, it was found that it is not possible to reconstruct them uniquely. The SPN based models were shown to be able to reconstruct optical maps with greater accuracy than the DA. However, due to the increased number of unknowns to be recovered, the SP7 based reconstructed images contained significant artefact and cross-talk. Finally, a SPN-Diffusion hybrid model was developed in which the SPN model was used in the regions near to the source and the DA elsewhere. This model provides the increase of accuracy of the SPN models in the regions where the DA is insufficient, whilst retaining the computational efficiency of the DA. It was shown that the hybrid model leads to increased accuracy not only in the regions solved using the SPN model, but also in the DA based regions where as in a pure DA model, the errors near the source were propagated throughout the domain. It is also shown that the hybrid model can be solved in half the time of the full SPN model.

<p> <b>Overview:</b> Optical concepts as they relate to the ophthalmologic correction of vision in corneal laser vision correction and intraocular lens design was examined. </p><p> <b>Purpose:</b> The interaction between the excimer laser and residual corneal tissue in laser vision correction produces unwanted side effects. Understanding the origin of these artifacts can lead to better procedures. Furthermore, accommodating intraocular lenses offer a potential for eliminating presbyopia. Understanding the properties of a new accommodating intraocular lens incorporating a deformable interface may lead to advances in cataract surgery. </p><p> <b>Introduction:</b> Corneal surface irregularities following laser refractive procedures are commonly seen. They regularly result in a patient&rsquo;s decreased best corrected visual acuity and decreased contrast sensitivity. These changes are only seen in biologic tissue and the etiology has been elusive. A thermal response has been theorized and was investigated in this research. In addition, intraocular lenses using a mechanically deforming interface to change their power in order to duplicate natural accommodation have been developed. The deforming interface(s) induce optical aberrations due to irregular deformations. Design efforts have centered on minimizing these deformations. Both of the ophthalmic applications have been analyzed using finite element analysis (FEA) to understand their inherent optical properties. </p><p> <b>Methods:</b> FEA modeling of thermal theory has been applied to verify that excimer laser induced collagen contraction creates corneal surface irregularities and central islands. A mathematical model which indicates the viability of the theory was developed. The modeling results were compared to post ablation changes in eyes utilizing an excimer (ArF 193 nm), as well as non-ablative thermal heating in eyes with a CO₂ laser. </p><p> Addition modeling was performed on an Intraocular lens prototype measuring of actuation force, lens power, interface contour, optical transfer function, and visual Strehl ratio. Prototype verified mathematical models were utilized to optimize optical and mechanical design parameters to maximize the image quality and minimize the required force. </p><p> Results: The predictive model shows significant irregular central buckling formation and irregular folding. The amount of collagen contraction necessary to cause significant surface changes is very small (0.3%). Uniform scanning excimer laser ablation to corneal stroma produces a significant central steepening and peripheral flattening in the central 3mm diameter. Isolated thermal load from uniform CO₂ laser irradiation without ablation also produces central corneal steepening and paracentral flattening in the central 3mm diameter. </p><p> The iterative mathematical modeling based upon the intraocular lens prototype yielded maximized optical and mechanical performance through varied input mechanical and optical parameters to produce a maximized visual Strehl ratio and a minimized force requirement. </p><p> <b>Conclusions:</b> The thermal load created by laser irradiation creates a characteristic spectrum of morphologic changes on the porcine corneal stromal surface which correlates to the temperature rise and is not seen inorganic, isotropic material. The highly similar surface changes seen with both lasers are likely indicative of temperature induced transverse collagen fibril contraction and stress re-distribution. Refractive procedures which produce significant thermal load should be cognizant of these morphological changes. </p><p> The optimized intraocular lens operates within the physiologic constraints of the human eye including the force available for full accommodative amplitude using the eye&rsquo;s natural focusing feedback, while maintaining image quality in the space available. Optimized optical and mechanical performance parameters were delineated as those which minimize both asphericity and actuation pressure. The methodology combines a multidisciplinary basic science approach from biomechanics, optical science, and ophthalmology to optimize an intraocular lens design suitable for preliminary trials.</p>

<p> Optical microscopy is one of the oldest and most important imaging techniques; however, its far-field resolution is diffraction-limited. In this dissertation, we proposed and developed a novel method of optical microscopy with super-resolution by using high-index dielectric microspheres immersed in liquid and placed on the surface of the structures under study. We used barium titanate glass microspheres with diameters of D~2-220 &mu;m and refractive indices <i> n</i>&sim;1.9-2.1 to discern minimal feature sizes &sim;&lambda;/4 (down to &sim;&lambda;/7) of various photonic and plasmonic nanostructures, where &lambda; is the illumination wavelength. We studied the magnification, field of view, and resolving power, in detail, as a function of sphere sizes.</p><p> We studied optical coupling, transport, focusing, and polarization properties of linear arrays of dielectric spheres. We showed that in arrays of spheres with refractive index n=<rad><rcd>3</rcd></rad>, a special type of rays with transverse magnetic (TM) polarization incident on the spheres under the Brewster&#129;'s angle form periodically focused modes with radial polarization and <i> 2D</i> period, where <i> D</i> is the diameter of the spheres. We showed that the formation of periodically focused modes in arrays of dielectric spheres gives a physical explanation for beam focusing and extraordinarily small attenuation of light in such chains. We showed that the light propagation in such arrays is strongly polarization-dependent, indicating that such arrays can be used as filters of beams with radial polarization. The effect of forming progressively smaller focused beams was experimentally observed in chains of sapphire spheres in agreement with the theory.</p><p> We studied optical coupling,transport, focusing, and polarization properties of linear arrays of dielectric spheres. We showed that in arrays of spheres with refractive index n=&#129;&atilde;3, a special type of rays with transverse magnetic (TM) polarization incident on the spheres under the Brewster&#129;'s angle form periodically focused modes with radial polarization and 2D period, where D is the diameter of the spheres. We showed that the formation of periodically focused modes in arrays of dielectric spheres gives a physical explanation for beam focusing and extraordinarily small attenuation of light in such chains. We showed that the light propagation in such arrays is strongly polarization-dependent, indicating that such arrays can be used as filters of beams with radial polarization. The effect of forming progressively smaller focused beams was experimentally observed in chains of sapphire spheres in agreement with the theory.</p>

Thesis (M.S.)--University of California, San Francisco, 2009. / Source: Masters Abstracts International, Volume: 48-03, page: 1526. Adviser: Orion Weiner.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006. / Source: Dissertation Abstracts International, Volume: 67-11, Section: B, page: 6252. Adviser: Enrico Gratton. Includes bibliographical references (leaves 80-87) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--Brown University, 2008. / Vita. Advisor : Jimmy Xu. Includes bibliographical references (leaves 101-124).