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Characterisation of the ultra-short optical pulses produced by infrared ffee-electron lasers (FELs) is an important task, not only for the further development of free electron lasers and their theory, but also for their operation as a research tool. The setting up and optimisation of the FEL requires effective and reliable diagnostics tools. This thesis presents techniques for the measurement of sub-picosecond optical and electron pulses. A range of techniques is developed that allows measurements of the electric field of both optical pulses and electron bunches to be made with an accuracy of better than 100 fs. These techniques have been used to obtain the first complete electricfield characterisation of ultra-short pulses from a far-infrared FEL; to study the formation of singlesided exponential optical pulses in two FELs; and to obtain the longitudinal profile of electron bunches, both by probing the near-field transition radiation and by directly sensing the Coulomb field of the electron bunches. Although the techniques described are not truly single-shot - requiring measurements averaged over a period of a few microseconds - ways in which they could be extended to provide single-shot capability are discussed.

Due to the large penetration depth in tissues, ultra-broadband supercontinuum (SC) in the 1m wavelength range, which is regarded as a diagnostic window in bio-photonics, represents a versatile light source for a wide range of bio-imaging and spectroscopy applications. In particular, dispersive Fourier transform (DFT) has recently emerged as anultrafast optical technique forimaging and spectroscopy. Thus, In order to employ the SC source for DFT, it has to exhibit ultra-broad bandwidth as well as good temporal stability– the two important metrics for practical high-speed bio-imaging and spectroscopy applications. In this thesis, we first demonstrate stabilized and enhanced SC generation (in the anomalous dispersion regime) at 1m by a minute continuous-wave (CW) seeding scheme. By introducing a weak CW which is around 200,000 times weaker than the pump, a significant broadening in the SC bandwidth and an improvement in the temporal stability is obtained. This seeding scheme allows, for the first time,1m DFT at a spectral acquisition rate of 20MHz with good temporal stability-paving the way toward realizing practical real-time, ultrafast biomedical spectroscopy and imaging. For the DFT part, instead of using the regular specialty 1m single mode fiber (SMF) as the dispersive elements, we here explore and demonstrate the feasibility of using the standard telecommunication single-mode fibers (e.g. SMF28 and dispersion compensating fiber (DCF)) as few-mode fibers (FMFs) for optical time-stretch confocal microscopy in the 1m region. By evaluating group velocity dispersion (GVD) of different FMF modes and thus the corresponding time-stretch performances, we show that the fundamental modes (LP01) of SMF28 and DCF, having sufficiently high dispersion-to-loss ratios, are particularly useful for practical time-stretch spectroscopy and microscopy in the 1m region, without the need for the specialty 1m(single mode fiber) SMF. More intriguingly, the ability of selective modal excitation in FMFs also enables us to utilize the higher-order FMF modes (e.g. LP11) for time-stretch imaging. Such additional degrees of freedom create a new avenue for optimizing and designing the time-stretch operations, such as by tailored engineering of the modal-dispersion as well as the GVD of the individual FMF modes. In the search for stable and efficient SC generation for practical bio-imaging and spectroscopy, we also numerically investigate the active enhancement of the seeded-SC generation pumped in the normal dispersion regime. Similarly, we introduced a minute CW seeding scheme, more specifically seeding spectrally coincides with the Raman gain peak of the pump. With this design, the SC bandwidth can be enhanced to more than one octave, even when the pump is far away from zero dispersion wavelengths (ZDW) (~100nm) in the normal dispersion regime. This new seeding mechanism opens opportunities to expand the scope of active seeding mechanism for enhancing SC generation to the normal dispersion regime. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Thesis (MSc)--University of Stellenbosch, 2008. / Bibliography. Also available via the Intenet.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1981. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.)--Physics, Georgia Institute of Technology, 2008. / First, Phillip, Committee Member ; Adibi, Ali, Committee Member ; Raman, Chandra, Committee Member ; Buck, John, Committee Member ; Trebino, Rick, Committee Chair.

Thesis (Ph.D.)--University of Nebraska-Lincoln, 2006. / Title from title screen (site viewed April 26, 2007). PDF text: xii, 99 p. : ill. (some col.) UMI publication number: AAT 3229554. Includes bibliographical references. Also available in microfilm and microfiche formats.

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A dissertation submitted to the Faculty Of Science in partial fulfilment of the requirements for the degree of Master of Science by research only in the School of Physics, University of the Witwatersrand, Johannesburg. July 11, 2017. / A discrete Q-switched laser that gives a side-lobed single pulse as a laser output was implemented; followed by studying energy extraction e ciencies and pulse characterisation. The aim was to help design a passively Q-switched laser that gives a smooth single pulse of optimum energy as a laser output. The smoothness feature in a single pulse is important in some applications such as range nding. The concepts are demonstrated both experimentally and numerically; the latter using Fox-Li approach to modeling resonator modes with the Fresnel's integral for the system under study. In the rst two chapters, fundamentals of how a laser works and the spatial mode development are studied. In chapter 1, the principles of a laser are discussed: absorption, spontaneous and stimulated emission. Also, di erent types of pumping sources and resonator con gurations that can be used are discussed. In chapter 2, the focus is on developing spatial modes of a laser. The fundamental and high order modes are discussed together with their propagation laws. Then a numerical method is used to nd the eigenmodes of an arbitrary resonator con guration. This numerical method is used to simulate propagation of a fundamental mode and the simulation results are compared to analytical propagation laws. Then, this numerical method is used to simulate a laser resonator. The eigenmode of the lowest loss in the resonator was found. In chapters 3 and 4, experimental work is done on a Q-switched laser where the focus is on the overall laser performance. In chapter 3, Q-switched laser output energies are studied for di erent combinations of Q-switch transmission values and output coupler re ectivities. In addition, the in uence of spatial modes on a Q-switched pulse shape and pulse width are studied, taking into account beam divergence. In chapter 4, conclusions and future work are presented. In future work, the knowledge of spatial mode in uence on pulse shape, pulse width and beam divergence from chapter 3 is exploited. Then particular resonator con guration that gives optimised output results (Q-switched laser output energy, beam divergence, pulse shape and pulse width) is chosen. On that particular resonator, di erent Q-switch transmission values are studied, but now looking only at beam divergence and pulse width. Also, some suggestions on further improving laser performance are given. / LG2018

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