Optical Science and Engineering ETDs

Publication Date

6-9-2016

Abstract

Intensive investigations during the past two decades have focused on potential applications of near-infrared (NIR) femtosecond filament for remote spectroscopy. The short length (less than 1 m) and low energy (only several millijoule) of a single NIR filament limit these applications. Long-pulse UV filaments have therefore been proposed at UNM to overcome such limitations. This dissertation describes our investigation and optimization of the high-power UV source, focusing on details of the generation and characterization of the generated pulses, as well as the applications of UV filaments for remote sensing and high-voltage discharge. On the topic of pulse generation, a 266 nm UV system delivering laser pulses below 200 ps with up to 0.4 J per pulse at 1.25 Hz repetition rate is developed. Two aspects of the laser source are closely investigated. On the one hand, the spatio-temporal profile of the laser pulses that has been overlooked for decades is systematically studied. It is shown that a curved energy front, i.e., pulses away from the beam center delayed from the center pulse, originates from the Q-switched unstable cavity Nd:YAG oscillator and evolves during the processes of laser amplification and pulse compression. To eliminate the energy front curvature, a possible solution is proposed based on the numerical simulation. On the other hand, a long existing debate on the minimum pulse duration that can be achieved through stimulated Brillouin scattering (SBS) pulse compression is resolved by this work. It is demonstrated that the lower limit of the compressed pulse duration is not set by the phonon lifetime of the SBS medium. The energy exchange between the pump and compressed Stokes pulses is responsible for the pulse compression below phonon lifetime. Next, using the newly developed powerful and stable UV source, the generation of UV filaments in air is studied. It is shown that, when focused by a 3 m lens, a single filament is generated inside the laboratory, while multiple filaments are observed in an open environment with a 9 m focusing lens. Detailed characterization of the filament spatial profile and the conductivity of the plasma channel created by the filament are performed. Two applications of the UV filaments are investigated. The single UV filament is applied to spectroscopy studies, including both Raman and Laser Induced Breakdown Spectroscopy (LIBS). A UV filament is shown to be very efficient in exciting forward stimulated Raman scattering (SRS) in gases. Backward emission of SRS signal, which could be utilized for remote sensing, has not been observed. However, a side experiment carried out in water discloses a new mechanism of driving efficient backward SRS generation, which can possibly be employed in the case of gas medium. A second study with the single filament is carried out for LIBS. The dynamics of self-absorption dip in LIBS spectrum is investigated, which can be further applied for the high resolution spectroscopy. The last application is filament-induced high-voltage discharge. A fully guided 40 cm long discharge is demonstrated with the UV filament alone, at 1/2 the self-breakdown voltage in air. Two additional lasers are tested to improve the discharge triggering by photo-detaching oxygen negative ions and heating the plasma. The anticipated improvement in reducing the discharge delay or enhancing the discharge probability has not been observed.

Degree Name

Optical Science and Engineering

Level of Degree

Doctoral

Department Name

Optical Science and Engineering

First Committee Member (Chair)

Diels, Jean-Claude

Second Committee Member

Arissian, Ladan

Third Committee Member

Krishna, Sanjay

Fourth Committee Member

Lenzner, Matthias

Document Type

Dissertation

Language

English

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