A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Dynamics and applications of long-distance laser filamentation in air(2024) Goffin, Andrew; Milchberg, Howard; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Femtosecond laser pulses with sufficient power will form long, narrow high-intensity light channels in a propagation medium. These structures, called “filaments”, form due to nonlinear self-focusing collapse in a runaway process that is arrested by a mechanism that limits the peak intensity. For near-infrared pulses in air, the arrest mechanism is photoionization of air molecules and the resulting plasma-induced defocusing. The interplay between plasma-induced defocusing and nonlinear self-focusing enables high-intensity filament propagation over long distances in air, much longer than the Rayleigh range (~4 cm) corresponding to the ~200 µm diameter filament core. In this thesis, the physics of atmospheric filaments is studied in detail along with several applications. Among the topics of this thesis: (1) Using experiments and simulations, we studied the pulse duration dependence of filament length and energy deposition in the atmosphere, revealing characteristic axial oscillations intimately connected to the delayed rotational response of air molecules. This measurement used a microphone array to record long segments of the filament propagation path in a single shot. These results have immediate application to the efficient generation of long air waveguides. (2) We investigated the long-advertised ability of filaments to clear fog by measuring the dynamics of single water droplets in controlled locations near a filament. We found that despite claims in the literature that droplets are cleared by filament-induced acoustic waves, they are primarily cleared through optical shattering. (3) We demonstrated optical guiding in the longest-filament induced air waveguides to date (~50 m, a length increase of ~60×) using multi-filamentation of Laguerre-Gaussian LG01 modes with pulse durations informed by experiment (1). (4) We demonstrated the first continuously operating air waveguide, using a high-repetition-rate laser to replenish the waveguide faster than it could thermally dissipate. For each of the air waveguide experiments, extension to much longer ranges and steady state operation is discussed.Item SEMICONDUCTOR WAVEGUIDES FOR NONLINEAR OPTICAL SIGNAL PROCESSING(2009) Apiratikul, Paveen; Murphy, Thomas E.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis investigates nonlinear effects in semiconductor waveguides for optical signal processing. Two semiconductor waveguides are studied : nanoporous silicon waveguide and GaAs/AlGaAs waveguide. First, nonlinear optical properties of nanoporous silicon waveguides including two-photon absorption, self-phase modulation and free-carrier effects are characterized and compared with similar measurements conducted on conventional silicon-on-insulator waveguides. Then, we experimentally demonstrate 10-Gb/s wavelength conversion using cross-amplitude modulation, cross-phase modulation and four-wave-mixing in GaAs/AlGaAs waveguides. Finally, we propose an ultrafast optical sampling system based on non-degenerate two-photon absorption in a GaAs photodiode. Using this technique, we successfully demonstrate a background-suppressed measurement of quasi 4-Tb/s eye diagrams.