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Surface engineering provides solutions to wear and corrosion degradation of engineering components. Laser cladding is a surfacing process used to produce wear and corrosion resistant surfaces by covering a particular part of the substrate with another material that has superior properties, producing a fusion bond between the two materials with minimal dilution of the clad layer by the substrate. The advantages of laser cladding compared to conventional techniques include low and controllable heat input into the workpiece, a high cooling rate, great processing flexibility, low distortion due to the low heat input to the workpiece and minimal post-treatment. The main processing parameters of laser cladding include laser power, laser spot size, processing speed, and powder feed rate. Within an optimized operational window, all these variables have some effect on the temperature of the clad interaction zone. The laser cladding technique is very complicated because it involves metallurgical and physical phenomena, such as laser beam-materials interaction, heat transfer between the clad and the substrate, and the interdiffusion of the clad and the substrate materials. Laser cladding is currently an open-loop process, relying on the skills of the operator and requiring dedication to specialty to make it successful. Unless the required expertise is provided, attempts to make the process successful will be futile. The objective in conducting the project was to investigate and develop prototype sensors to monitor and control Nd:YAG laser cladding process. Through a LabVIEW software based monitoring program, real-time process monitoring of optical emissions in the form of light and heat radiation was carried out, and correlated with the properties of the produced clad layers. During various experiments, single- and multiple-track laser cladding trials were performed. The responses of such sensors to the selected conditions were examined and an in depth analysis of detected heat and optical radiation signals was carried out. The results of these experiments showed the ability of such sensors to recognize changes in process parameters, and detected defects on layer surfaces along with the presence of oxides. A multi-function closed-loop laser power and CNC motion table feed rate control interface based on a LabVIEW platform has been designed and built, which is capable of accepting and interpreting sensors� data and adjusting accordingly the laser power and CNC motion table feed rate to produce sound clad layers. The developed dual control strategy utilized in this study forms a relatively inexpensive and less-complicated system that allows end-users to achieve lower failure rates during laser cladding (within its own limitations) and, therefore, through successful concurrent control of melt pool temperature and motion table feed rate provide better productivity and quality in the experimentally produced clad layers.

Optical trapping techniques have become an important and irreplaceable tool in many research disciplines for reaching non-invasively into the microscopic world and to manipulate, cut, assemble and transform micro-objects with nanometer precision and sub-micrometer resolution. Further advances in optical trapping techniques promise to bridge the gap and bring together the macroscopic world and experimental techniques and applications of Microsystems in areas of physics, chemistry and biology. In order to understand the optical trapping process and to improve and tailor experimental techniques and applications in a variety of scientific disciplines, an accurate knowledge of trapping forces exerted on particles and their dependency on environmental and morphological factors is of crucial importance. Furthermore, the recent trend in novel laser trapping experiments sees the use of complex laser beams in trapping arrangements for achieving more controllable laser trapping techniques. Focusing of such beams with a high numerical aperture (NA) objective required for efficient trapping leads to a complicated amplitude, phase and polarisation distributions of an electromagnetic field in the focal region. Current optical trapping models based on ray optics theory and the Gaussian beam approximation are inadequate to deal with such a focal complexity. Novel applications of the laser trapping such as the particle-trapped scanning near field optical microscopy (SNOM) and optical-trap nanometry techniques are currently investigated largely in the experimental sense or with approximated theoretical models. These applications are implemented using the efficient laser trapping with high NA and evanescent wave illumination of the sample for high resolution sensing. The proper study of these novel laser trapping applications and the potential benefits of implementation of these applications with complex laser beams requires an exact physical model for the laser trapping process and a nanometric sensing model for detection of evanescent wave scattering. This thesis is concerned with comprehensive and rigorous modelling and characterisation of the trapping process of spherical dielectric particles implemented using far-field and near-field optical trapping modalities. Two types of incident illuminations are considered, the plane wave illumination and the doughnut beam illumination of various topological charges. The doughnut beams represent one class of complex laser beams. However, our optical trapping model presented in this thesis is in no way restricted to this type of incident illumination, but is equally applicable to other types of complex laser beam illuminations. Furthermore, the thesis is concerned with development of a physical model for nanometric sensing, which is of great importance for optical trapping systems that utilise evanescent field illumination for achieving high resolution position monitoring and imaging. The nanometric sensing model, describing the conversion of evanescent photons into propagating photons, is realised using an analytical approach to evanescent wave scattering by a microscopic particle. The effects of an interface at which the evanescent wave is generated are included by considering the scattered field reflection from the interface. Collection and imaging of the resultant scattered field by a high numerical aperture objective is described using vectorial diffraction theory. Using our sensing model, we have investigated the dependence of the scattering on the particle size and refractive index, the effects of the interface on the scattering cross-section, morphology dependent resonance effects associated with the scattering process, and the effects of the incident angle of a laser beam undergoing total internal reflection to generate an evanescent field. Furthermore, we have studied the detectability of the scattered signal using a wide area detector and a pinhole detector. A good agreement between our experimental measurements of the focal intensity distribution in the back focal region of the collecting objective and the theoretical predictions confirm the validity of our approach. The optical trapping model is implemented using a rigorous vectorial diffraction theory for characterisation of the electromagnetic field distribution in the focal region of a high NA objective. It is an exact model capable of considering arbitrary amplitude, phase and polarisation of the incident laser beam as well as apodisation functions of the focusing objective. The interaction of a particle with the complex focused field is described by an extension of the classical plane wave Lorentz-Mie theory with the expansion of the incident field requiring numerical integration of finite surface integrals only. The net force exerted on the particle is then determined using the Maxwell stress tensor approach. Using the optical trapping model one can consider the laser trapping process in the far-field of the focusing objective, also known as the far-field trapping, and the laser trapping achieved by focused evanescent field, i.e. near-field optical trapping. Investigations of far-field laser trapping show that spherical aberration plays a significant role in the trapping process if a refractive index mismatch exists between the objective immersion and particle suspension media. An optical trap efficiency is severely degraded under the presence of spherical aberration. However, our study shows that the spherical aberration effect can be successfully dealt with using our optical trapping model. Theoretical investigations of the trapping process achieved using an obstructed laser beam indicate that the transverse trapping efficiency decreases rapidly with increasing size of the obstruction, unlike the trend predicted using a ray optics model. These theoretical investigations are in a good agreement with our experimentally observed results. Far-field optical trapping with complex doughnut laser beams leads to reduced lifting force for small dielectric particles, compared with plane wave illumination, while for large particles it is relatively unchanged. A slight advantage of using a doughnut laser beam over plane wave illumination for far-field trapping of large dielectric particles manifests in a higher forward axial trapping efficiency, which increases for increasing doughnut beam topological charge. It is indicated that the maximal transverse trapping efficiency decreases for reducing particle size and that the rate of decrease is higher for doughnut beam illumination, compared with plane wave illumination, which has been confirmed by experimental measurements. A near-field trapping modality is investigated by considering a central obstruction placed before the focusing objective so that the obstruction size corresponds to the minimum convergence angle larger than the critical angle. This implies that the portion of the incident wave that is passed through the high numerical aperture objective satisfies the total internal reflection condition at the surface of the coverslip, so that only a focused evanescent field is present in the particle suspension medium. Interaction of this focused near-field with a dielectric micro-particle is described and investigated using our optical trapping model with a central obstruction. Our investigation shows that the maximal backward axial trapping efficiency or the lifting force is comparable to that achieved by the far-field trapping under similar conditions for either plane wave illumination or complex doughnut beam illumination. The dependence of the maximal axial trapping efficiency on the particle size is nearly linear for near-field trapping with focused evanescent wave illumination in the Mie size regime, unlike that achieved using the far-field trapping technique.

In order to observe laser phase noise, this noise must be converted to amplitude noise, which can be achieved using either an interferometer or an absorption resonance in an atomic/molecular vapor or gas. When phase noise is converted to amplitude noise, it is manifested as a heterodyne signal in the output of an optical square-law detector. Thus, phase noise is measured by optical heterodyne spectroscopy, or, equivalently, laser phase noise spectroscopy. In recent work on diode laser noise spectroscopy of rubidium and oxygen, the observed spectroscopic lineshapes were not in total agreement with theoretical predictions. We have repeated the previous work on the oxygen A-band transitions, and we now find qualitative agreement with theory. In addition, we have measured the diode laser noise spectrum of a Lummer- Gehrcke interferometer (LGI), comparing the heterodyne lineshape of a LGI transmission spectrum with a qualitative theory that we develop in this thesis. A theory, from other workers, predicts the intensity fluctuations from a Doppler-broadened, two-level atomic/molecular system driven with a phase-diffusing laser field. We show that a simplified version of this theory, which ignores Doppler effects of the system, is a useful approximation to the complete theory, by comparing computer-generated heterodyne lineshapes of each, for a rubidium transition. We apply this approximate theory to an oxygen A-band transition, and compare these results with our experimental measurements. For the experimental arrangement used in the present work, diode laser noise spectroscopy may also include effects of selective reflection, which is dealt with experimentally and theoretically. Diode laser phase noise has practical importance in optical communications and atomic clocks. / Graduation date: 1999

We have developed simple and inexpensive laser systems using grating-feedback diode lasers with electronic feedback to the injection current. These grating-feedback lasers can be continuously scanned up to 10 GHz and have a linewidth of 150 kHz. The three electronic frequency-stabilization systems we developed use polarization spectroscopy, etalon transmission and modified heterodyne signals as the frequency discriminators to drive an integrating servo control circuit. These laser systems are used for laser cooling and trapping of rubidium and atomic beam diagnostics. The rubidium D��� line at 780 nm is a strong, cycling transition that can be used for laser cooling and trapping. We use chirped cooling and Zeeman-tuned cooling to slow atoms from a thermal atomic beam. These atoms are loaded into a two-dimensional magneto-optic trap, or funnel. Using a frequency offset of the trapping lasers, the atoms are ejected from the funnel at a controllable velocity. The diode laser systems we have developed are a central component of this rubidium atomic funnel. We will use the funnel's bright, cold atomic beam as a source for matter-wave interferometry. We also developed an ionization detector to measure the flux and the spatial profile of the atomic beam when the background of scattered light makes fluorescent detection difficult. / Graduation date: 1997

Thesis (M.S.)--Oregon Graduate Center, 1974.

Thesis (Ph. D.)--Oregon Graduate Center, 1974.

Thesis (Ph. D.)--Oregon Graduate Center, 1982.

M.S. / Applied Physics / Not available.

Ph.D. / Applied Physics / Using the extended Huygens Fresnel principle, the effect of the atmospheric turbulence on the statistical properties of a polychromatic speckle field, generated by a diffuse target, is studied in detail. The results, substantiated by experimental data, indicate that the atmospheric perturbation increases the variance of the received intensity substantially and is sensitive to the wavelength, beam size and beam geometry. The results for the covariance of the received intensity, normalized to the variance, indicate that, at low turbulence levels, reduction in vacuum speckle contrast ratio (VSCR) also reduces the normalized covariance but, with further increase in the turbulence level, reduction in the vacuum speckle contrast ratio increases the normalized covariance. Also it is found that for small detector spacings, the normalized covariance remains approximately constant even with substantial increase in the turbulence level. By resolving the time delayed covariance of the received intensity (TDC), into coherent and incoherent terms, it is shown that for large time delays, the time delayed covariance is determined by the incoherent fluctuations and for poor vacuum speckle contrast ratio, the time delayed covariance is not very sensitive to the wind velocity. Finally it is shown that due to the atmospheric perturbation the probability density function of the received intensity changes from an M-distribution or a sum of exponential distributions in vacuum to a K-distribution or a weighted sum of K-distributions in the presence of the turbulent atmosphere.