First, we investigate the Bouguer-Lambert-Beer (BLB) law as applied to the transmission of ultrashort pulses through water in the linear absorption regime. We present a linear theory for propagation of ultrashort laser pulses, and related experimental results are in excellent agreement with this theory. Thus we conclude that recent claims of the BLB law violations are inconsistent with the experimental data obtained by our group. Second, we study the dynamics of ultrashort pulses in a Lorentz medium and in water via the saddle point method. It shows that the saddle point method is a more efficient and faster method than the direct integration method to study one-dimensional pulse propagation over macroscopic distances (that is, distance comparable to the wavelength) in a general dielectric medium. Comments are also made about the exponential attenuation of the generalized Sommerfeld and Brillouin precursors. By applying the saddle point method, we also determined that the pulse duration estimated by the group velocity dispersion (GVD) approximation is within 2% of the value computed with the actual refractive index for a propagation distance of 6 m in water.

Coherent control of laser induced processes is based on the quantum interference among multiple excitation pathways. Progress in the field has been fueled by advances in pulse shaping techniques, allowing modulation of phase and amplitude across the bandwidth of ultra short pulses. This dissertation makes use of coherent control technique for the optimization of two-photon fluorescence (TPF) and its applications in selective excitation for biomedical imaging. Different physical processes, e.g. TPF, second harmonic generation (SHG) and their ratios (e.g. TPF/SHG) were optimized by using feedback control pulse shaping technique with an evolutionary algorithm. Various nonlinear effects, e.g. filamentation, intensity clamping and white light generation were studied using two-photon fluorescence and Z-scan technique with different dyes and biomarkers. Simultaneous measurements of different nonlinear effects were performed. Novel methods were proposed and implemented to obtain two-photon excitation characteristics in intensity-resolved manner. Understanding of these nonlinear effects can give new solution to the issues of spatial resolution and molecular contrast for cellular and tissue imaging.

The energy absorbing behavior of stainless steel 304L during the pulsed Nd:YAG laser welding is investigated in this thesis. The equivalent absorptivity is estimated from the comparison of measured and finite element method (FEM) results simulated melting pool shape parameters, e.g. pool width, pool depth, cross-section area and total volume of the pool. To simulate the actual pulsed laser beam, the energy density of heating source is performed as a Guassian distribution in the transection of a circular laser beam. For evaluating the feasibility and the accuracy of the estimated equivalent absorptivity, the multi-pulsed Nd:YAG laser welding is simulated by using the estimated absorptivities. A good agreement between this simulated and measured melting pool shapes are found in the multi-pulsed laser welding. The equivalent absorptivity can be interpolated from different parameters of the molten pool. However, absorptivity curve fitted from the cross-section area and total volume of the melting pool provide a more stable value. Results also indicate that the absorptivity and the pulse energy are in inverse proportion. The thermal-elastic-plastic FEM model is employed to simulate the fusion and solidification process of the pulsed laser welding. A complicate residual stress distribution introduced from the shrinkage in the solidification process is also calculated and presented. The distribution of post-weld-deformation near the melting pool has also been studied in this thesis. This post-weld-deformation may be a key factor in high precision laser welding, e.g. laser packaging for the optoelectronic components. The absorptivity estimated in this thesis may be helpful to simulate the laser welding process accurately.

This thesis investigates the performance of pulse charging, which is believed to be superior to constant current charging in some respects, such as charging efficiency and charging speed. The investigation is focused upon the extensively used secondary batteries, lead-acid batteries. The consecutive orthogonal arrays method is applied to search for the optimum operating variables of pulse charging, including pulse amplitude, duty ratio and frequency of the charging current. Unfortunately, the experimental results of consecutive orthogonal arrays reveal that charging efficiency is not obviously affected by pulse amplitude, duty ratio or frequency. Instead, charging rate is dominantly influenced by average charging current. These results indicate that pulse charging scheme is not superior to constant current charging. To compare these two charging schemes further, a series of experiments are carried out to discuss the effects of each operating variables. Unfortunately, no evidence from the experimental results can prove the superiority of pulse charging to constant current charging as formerly documented.

Pulse Compression and Dispersion Control in Ultrafast Optics Vikrant K. Chauhan 116 Pages Directed by Dr. Rick P. Trebino In this thesis, we introduced novel pulse compressors that are easy to align and which also compensate for higher order dispersion terms. They use a single dispersive element or a combination of dispersive elements in single-element-geometry. They solve the problem of extra-cavity pulse compression by providing control of the pulse width in almost all of the experiments performed using ultrashort pulses, and they even compensate for higher order dispersion. We performed full spatiotemporal characterization of these compressors and demonstrated their performance. We also developed a theoretical simulation of pulse compressors which is based on a matrix based formalism. It models the full spatiotemporal characteristics of any dispersion control system. We also introduced a simple equation, in its most general form, to relate the total dispersion and magnification introduced by an arbitrary sequence of dispersive devices. Pulse compressor characterization was done using interferometric measurements in the experiments presented in this work, but we also developed a method to measure pulses that uses polarization gating FROG for measuring two unknown pulses. In the last part, we briefly discuss the designing of a high energy chirped pulse amplification system.

Thesis (Ph. D.)--New York University, School of Education. / Also on film. eContent provider-neutral record in process. Description based on print version record.

published_or_final_version / Electrical Engineering / Master / Master of Philosophy

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Remediation of DNAPL contaminated sites in fractured rock has proven to be very difficult. No current technology can be used to remediate such sites in a timely and economic manner due to the inherent heterogeneity of fractured rock and back diffusion of contaminants stored in the rock matrix. This study was conducted in order to evaluate the viability of biostimulation of native biofilm as a means to control flow and back diffusion at fractured rock sites. A field trial was conducted at an uncontaminated site in southern Ontario. The site is underlain by dolomites of the Lockport formation. Three major fracture zones have been identified in the study area. Two closely spaced (5.04 m) boreholes were used to isolate a fracture zone at a depth of 17 m with straddle packers. These boreholes were used to create an injection-withdrawal system with recirculation, which was used for tracer injection in order to load the rock matrix with a conservative dye tracer and to inject nutrients for 21 days in order to stimulate the growth of biofilm in the fracture. Evaluation of the ability of the biofilm to control flow through the fracture was conducted through pulse interference tests. Pulse interference tests were conducted before and after the injection of nutrients. The results from the pulse interference tests showed a maximum 65% reduction in transmissivity, which is equivalent to a 28% reduction in fracture aperture shortly after the cessation of biostimulation. In order to investigate the effect of the biofilm stimulation on matrix diffusion the rock matrix was loaded with Lissamine, a conservative fluorescent dye tracer prior to biostimulation and its concentration was monitored at injection and withdrawal wells. The effect that biostimulation had on matrix diffusion was determined by comparing field concentration measurements with a model that simulates a system unaffected by biofilm stimulation. The biostimulation lowered the concentration of tracer attributable to back diffusion at the withdrawal well by about 20% for approximately 30 days following the cessation of biostimulation. It is also thought that large amounts of tracer might have been trapped in the biofilm as it formed and was then released back into the fracture as the biofilm deteriorated. / Thesis (Master, Civil Engineering) -- Queen's University, 2009-08-11 19:27:44.232

This project explores optimization of two-photon excited fluorescence (TPEF) enhancement between tunable narrowband and un-tuned broadband femtosecond (fs) laser pulse excitations for two-photon microscopy (TPM). The research is conducted preliminarily in time domain and comprehensively in frequency domain to understand the physics behind TPEF enhancement by un-tuned sub-10 fs nearly transform-limited pulse (TLP) versus tunable 140 fs pulse. The preliminary study on inverse proportionality of TPEF yield to fs-pulse duration delimits a general lower-bound to narrowband fs-pulse regime (pulse duration > 40 fs) with assumption of dye-molecule frequency invariant response. Deviations from this inverse proportionality in broadband fs-pulse regime (pulse duration < 40 fs) highlights dye-molecule frequency variant response, necessity of group delay dispersion (GDD) compensation, and broadband TLP for TPEF enhancement. The follow-up comparative study is made on un-tuned sub-10 fs TLP versus tunable 140 fs pulse excitations using three dye-phantoms (Indo-1, FITC, and TRITC) representative of fluorescent probes with similar TPEF characteristics. The integrated experimental system, with custom-designed GDD compensation, dispersion-less laser-beam expanding and focusing, and compound-lens for efficient fluorescence collection with good spectral resolution, ensures accurate TPEF measurements. Differentiated TPEF enhancements of Indo-1 (1.6), FITC (6.7), and TRITC (5.2) proportionally agree with calculated ones due to the overlap of fs-pulse second harmonic (SH) power spectrum with dye-molecule two-photon excitation (TPE) spectrum. Physically speaking, with broadband sub-10 fs TLP readily involved in both degenerate (v1 = v2) and non-degenerate (v1 ≠ v2) two-photon absorption (TPA), this un-tuned ultrashort fs-pulse excitation simultaneously allows for more accessibility to TPA-associated final states and diversely promotes population of thus excited dye-molecules with the three dye-phantoms. Under environmental influences (mutual quenching through one-photon absorption(s) and solvent effect), multicolor TPEF enhancement observed from a mixture of the three dyes shows promise of sub-10 fs TLP as simultaneous excitation for multiple-dye labeled samples in contrast to compromised excitation with narrowband fs-pulse tuning. Both single- and multicolor TPEF enhancements clarify tradeoff between tunability of narrowband fs-pulse and un-tuned broadband fs-pulse excitations, being instructive to further considerations on optimization of TPEF enhancement by strategic utilization of broadband fs-pulse for better performance of TPM.