The SPECIFIC method a fast and accurate method for generating shaped femtosecond laser pulses. The femtosecond pulses are user specified from pulse parameters in the temporal domain. The measured spectral and recovered temporal phase and amplitudes from SEA TADPOLE are compared with the theoretical pulse profile from the user specified input. The SPECIFIC method has been shown to be a technique that can generate a diverse array of spectral/temporal phase and amplitude as well as polarization pulse shapes for numerous scientific applications. The spatio -temporal -spectral properties of focusing femtosecond laser pulses are studied for several pulse shapes that are important for non-linear spectroscopic studies. We have shown with scanning SEA TADPOLE that the spatio-spectral phase of focusing double pulse profile changes across the laterally across the beam profile. The spectral features of the sinusoidal spectral phase shaped pulse has been shown to tilt at with a changing angle away from the focus of the lens. Using spatio-spectral coupling, we have shown that multiple spatio-temporal foci can be generated along and perpendicular to the focusing direction of a femtosecond laser pulse. The spatial position of the spatio-temporal foci is controlled optically. Using sinusoidal spectral phase modulated pulse trains fragment ion production from Benzonitrile parent molecule can be controlled. A spectral transmission window perturbed the temporal pulse amplitudes resulting in fragment ion production dependant on spectral window position. The spectral window ion production was shown to also be dependant on temporal phase sequence. / Chemistry

<p> This thesis reports findings from three series of experiments related to ultrashort laser pulse interactions with materials. The first series investigates the formation of laser induced ripples that have spatial periods much shorter than the irradiation wavelength after laser irradiation. The second series of experiments explores the capabilities of ultrashort pulse laser micromachining on optical fiber modifications for niche applications. Lastly, preliminary work in establishing a double-pulse ablation technique is reported. </p> <p> The first set of experiments reported in this thesis investigates the morphology of surface ripples that are generated when irradiated with multiple ultrashort laser pulses. Two types of surface ripples can form after irradiation. The fust type has spatial periods near the wavelength of the irradiation pulses and the second has spatial periods substantially below the irradiation wavelength (typically 114 to 115 of the free-space irradiation wavelength are observed in our studies). These substantially subwavelength ripples form when the irradiation wavelength corresponds to a photon energy that is below the bandgap of the target material. The Ti:Sapphire laser systems used in this series of experiments provides pulses centered around 800 nm. Gallium phosphide (GaP) was chosen to be the main material for investigation since 800 nm corresponds to a photon energy that is below the bandgap of this material; no frequency conversion needs to be carried out when GaP is the material of choice for subwavelength ripples studies. In this series of experiments substantially different irradiation conditions were investigated: pulse durations varied from 150 fs to 7 ns, laser energies ranges from well above the ablation and modification threshold to well below, both 800 nm and 400 nm wavelengths, and "scrambled" (where polarization was rotated between each successive pulse) polarization as well as circular polarization were used. Microscopy techniques employed to study these ripples include optical microscopy, scanning electron microscopy, atomic force microscopy and transmission electron microscopy. Cross-sectional studies with transmission electron microscopy were also carried out by using focused ion beam milling to prepare thin specimens across irradiated regions. Sapphire was also used as the irradiation target for 800 nm and 400 nm pulses since it has a large bandgap and even 400 nm corresponds to an energy that is below its bandgap. Irradiation conditions where the two types of ripples are observed are determined. Also, microscopy of the ripple features provided insights in to the formation mechanism of the subwavelength ripples. </p> <p> In the second series of experiments, preliminary work was performed to investigate the capabilities of ultrashort laser micromachining in fiber optic applications. This series of experiments can be subdivided in to two categories. </p> <p> The goal of the first fiber investigation was to create a slit in a metallic coating deposited on a fiber facet. Such a feature might eliminate the use of external slits (e.g. for spectrometers), especially ifthe output of the fiber depends on its geometry (e.g. polarization-maintaining fiber). The first experiment carried out was micromachining of a ~ 180 nm layer of gold that was deposited on a glass substrate, in order to determine irradiation conditions where the gold layer can be removed while the glass is not damaged. Once the irradiation condition was established by studying the micromachined gold layer on glass substrate, gold layers were deposited on fiber facets for micromachining experiments. The results showed promising potential, but fme tuning of the irradiation parameters, and processing as well as microscopy techniques are needed before useful applications can be realized. </p> <p> The second set of fiber experiments investigates irradiation conditions that are appropriate to micromachine features into fibers such as v-grooves and beveled ends. Preliminary work was carried out to determine a suitable focusing scheme for this application. Different pulse durations and a pulse train were also employed in hope of minimize chipping and cracking. This investigation did not reach a conclusion on whether micromachining with ultrashort laser pulses are in fact suitable for processing of optical fibers, where high quality facets are required. Future investigation could provide further information on the feasibility of laser micromachining on fabricating features in optical fibers. </p> <p> Lastly, a double-pulse ablation scheme was established and explored. Double-pulse ablation had been reported in the literature to improve material removal rate and the appearance of the fmal morphology. However, this setup can be adapted to investigate the ablation mechanisms and provide insight into the state of the material at different time frames of ablation. While the experimental results are preliminary, this technique showed potential, along with possible extensions of this technique, to further investigate the ablation mechanisms. </p> / Thesis / Master of Applied Science (MASc)

The groundwork has been completed for a large new research initiative involving the development of diode lasers for moderate power ultrashort pulse generation. This thesis reports on the status of three core areas of this initiative: InGaAsP/GaAs diode laser design and characterization, split contact device testing, and thin film interference filter deposition and characterization. Two new short wavelength diode laser designs have been realized and tested. A 980 nm laser was designed, using an InGaAsP barrier/waveguide region. This showed improved far field performance and better contact isolation as compared to an existing 980 nm laser using GaAs barriers. A laser emitting at 850 nm was also designed using GaAs quantum wells surrounded by a new quaternary waveguide region. A test arrangement was developed to facilitate the measurement of IV and LI curves for split contact lasers. Numerous lasers were tested, indicating that short absorber sections and narrow gap widths are preferable for use as saturable absorbing regions in a passively mode locked diode laser. Finally, thin film silicon oxynitride interference filters have been designed, deposited, and characterized for several antireflecting and high reflectance coatings on semiconductor laser facets. A comparison ofsingle layer AR coatings accounting for the modal reflectivity was performed. A four layer high reflectance coating with a peak broadband reflectance of over 90% was deposited on a laser facet. / Thesis / Master of Engineering (ME)

This thesis describes the process of developing a robust phase measurement technique with which to analyse semiconductor based devices intended for use in optoelectronic/all optical networks. The devices measured are prospective dispersion compensators, based either on planar photonic crystal waveguides or coupled microcavities connected by ridge waveguide. The technique was validated by measuring the phase transfer function of a Fabry Perot etalon. This demonstrated that even when detecting low optical powers (sub μW), accurate measurement of phase could quickly be carried out over a significant spectral range (~10nm). Comparison of experimental data taken from the prospective dispersion compensators with theory showed excellent agreement, which provided qualitative (cavity spacing and reflectivity) and quantitative (loss) measures of device performance. The phase measurement technique has been designed to be capable of measuring other classes of device also, including active devices such as semiconductor optical amplifiers. This suggests the phase measurement technique may be valuable in analysing the variation of dispersion as a function of applied bias, peak power or temperature for a variety of devices.

A new pulse-retrieval software for Frequency-Resolved Optical Gating (FROG) technique has been developed. The new software extends the capacity of the original FROG algorithm in two major categories. First is a new method to determine the uncertainty of the retrieved pulse field in FROG technique. I proposed a simple, robust, and general technique?tstrap method?ch places error bars on the intensity and phase of the retrieved pulse field. The bootstrap method was also extended to automatically detect ambiguities in the FROG pulse retrieval. The second improvement deals with the spatiotemporal effect of the input laser beam on the measured GRENOUILLE trace. I developed a new algorithm to retrieve the pulse information, which includes both pulse temporal field and the spatiotemporal parameters, from the spatiotemporal distorted GRENOUILLE trace. It is now possible to have a more complete view of an ultrashort pulse. I also proposed a simple method to remove the spatial profile influence of the input laser beam on the GRENOUILLE trace. The new method extends the capacity of GRENOUILLE technique to measure the beams with irregular spatial profiles.

This thesis contains three components of research: a detailed study of the performance of Frequency-Resolved Optical Gating (FROG) for measuring complex ultrashort laser pulses, a new method for measuring the arbitrary polarization state of an ultrashort laser pulse using Tomographic Ultrafast Retrieval of Transverse Light E-fields (TURTLE) technique, and new approach for measuring two complex pulses simultaneously using PG blind FROG. In this thesis, we compare the performance of three versions of FROG to measure complex ultrashort laser pulses: second-harmonic-generation (SHG) FROG, polarization-gate (PG) FROG, and cross-correlation FROG (XFROG). We found that the XFROG algorithm achieves 100% convergence, while PG FROG and SHG FROG GP algorithm achieve 100% convergence after doing the noise deduction and increasing the sampling range. The second part of this thesis describes a method for measuring the intensity, phase and the complete polarization state of a laser pulse having a time-dependent polarization state (i.e. a polarization shaped pulse). This technique is called tomographic ultrafast retrieval of transverse light E-fields (TURTLE). TURTLE typically involves making three FROG measurements: one of the intensity and phase of the pulse's horizontal polarization component, one of its vertical component, and another of the 45o component. Performing a simple minimization using these three FROG measurements, the time-dependent polarization state of the ultrashort pulse can be determined. The third part of this thesis introduces a method for measuring two complex pulses simultaneously using a single FROG device. This technique is based on Polarization-gate (PG) FROG and it is called PG blind FROG. It involves two measurements: One of them is a PG FROG trace using the intensity of pulse 1 to gate pulse 2 and other one is the PG FROG trace using the intensity of pulse 2 to gate pulse 1. An iterative phase retrieval algorithm based on generalized projection (GP) is used to reconstruct the intensity and phase of these two pulses. This approach is an elegant way to measure complex and/or very spectrally broad pulses such as those due to super continuum.

No description available.

This dissertation demonstrates the usefulness of exactly solvable quantum models in the investigation of light-matter interaction phenomena associated with the propagation of ultrashort laser pulses through gaseous media. This work fits into the larger research effort towards remedying the weaker portions of the standard set of medium modeling equations commonly used in simulations. The ultimate goal is to provide a self-consistent quantum mechanical description that can integrate Maxwell and Schrödinger systems and provide a means to realistically simulate nonlinear optical experiments on relevant scales. The study of exactly solvable models begins with one of the simplest quantum systems available, one with a 1D Dirac-delta function potential plus interaction with the light field. This model contains, in the simplest form, the most important "ingredients" that control optical filamentation, i.e. discrete and continuum electronic states. The importance of both states is emphasized in the optical intensity regime in which filaments form, where both kinds of electronic states simultaneously play a role and may not even be distinguishable. For this model atom, an analytical solution for the time-dependent light-induced atomic response from an arbitrary excitation waveform is obtained. Although this system is well-known and has been studied for decades, this result is probably the most practically useful and general one obtained thus far. Numerical implementation details of the result are also given as the task is far from trivial. Given an efficient implementation, the model is used in light-matter interaction simulations and from these it is apparent that even this toy model can qualitatively reproduce many of the nonlinear phenomena seen in experiments. Not only does this model capture the basic physics of optical filamentation, but it is also well-suited for high harmonic generation simulations. Next, a theoretical framework for using Stark resonant states (or metastable states) to represent the medium's polarization response is presented. Researchers have recognized long ago the utility of Gamow resonant states as a description of various decay processes. Even though a bound electron experiences a similar decay-like process as it transitions into the continuum upon ionization, it was unclear whether field-induced Stark resonant states carry physically relevant information. It is found that they do, and in particular it is possible to use them to capture a medium's polarization response. To this end, two quantum systems with potentials represented by a 1D Dirac-delta function and a 1D square well are solved, and all the necessary quantities for their use as medium models are presented. From these results it is possible to conjecture some general properties that hold for all resonance systems, including systems that reside in higher than one dimensional space. Finally, as a practical application of this theory, the Metastable Electronic State Approach (MESA) is presented as a quantum-based replacement for the standard medium modeling equations.

This thesis presents the results of a study of the mechanisms responsible for limiting the laser pulse duration obtainable in xenon chloride lasers which are excited by UV-preionized, self-sustained gas discharges. The xenon chloride laser system, the principal emission band of which is centred around 308 nm, belongs to the class of high pressure gas lasers known as 'rare-gas halides'(RGH). RGH lasers are now well known for their high peak power output at a number of wavelengths from 193 nm to 353 nm in the ultraviolet region of the spectrum. To date, however, they have only been operated in the pulsed mode with laser pulse durations of ^~1000 ns for devices employing electron beam excitation and ^~30 ns for devices employing transverse discharge excitation. There is no a priori kinetic limitation which prevents RGH lasers from operating in the CW mode, and an attempt to extend the duration of the laser pulse would enable the quality of laser output to be improved. The laser pulse duration of a discharge excited XeCl^* laser was extended by about one order of magnitude - to 270 ns FWHM - by the use of a distributed resistance electrode to stabilize the discharge. The typical gas mixture used in the laser was ~2 atm of Ne (buffer gas), ~25 mbar of Xe, and 2.5 mbar of HC1. However, the laser pulse duration obtained was considerably shorter than the 500 ns duration, 2000 A peak current, discharge excitation pulse. The cause of this difference between the duration of the laser output pulse and the discharge current pulse was found by carrying out a comprehensive parametric study of the laser, combined with a detailed spectroscopic analysis and the results of a semi-empirical computer model. Two interrelated factors were identified as being responsible for the short duration of the laser output: namely, a temporal collapse of the discharge volume and a spatially non-uniform depletion of the HCl within this volume. The experimental results presented here contradict an earlier theory which ascribed the onset of discharge instabilities in RGH lasers to step-wise ionization of the minority rare gas atoms, and which attributed stability enhancement properties to the electronegative halogen gases used in RGH lasers.

Thesis (MSc (Physics))--University of Stellenbosch, 2009. / The exciting field of ultrashort laser optics has experienced tremendous growth since it's inception. One of it's branches that has been of continuous interest is the characterization of ultrashort laser pulses ...