Gas discharge in hollow-core fibers and its application in waveguide gas lasers

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Gas discharge in hollow-core fibers and its application in waveguide gas lasers

 

Author: Shi, Xin
Title: Gas discharge in hollow-core fibers and its application in waveguide gas lasers
Degree: Ph.D.
Year: 2009
Subject: Hong Kong Polytechnic University -- Dissertations.
Glow discharges.
Gas lasers.
Lasers -- Resonators.
Vacuum technology.
Wave guides.
Department: Dept. of Electrical Engineering
Pages: vi, 141 p. : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2307182
URI: http://theses.lib.polyu.edu.hk/handle/200/5144
Abstract: Compact and flexible solid-state lasers are challenging the dominant market of gas lasers (especially the COi lasers) in the recent years. Solid-state lasers seem to be the direction of the future. The features of compactness and portability are the dream of researchers in the field of gas lasers. When the gas lasers, especially the vessel containing the gain medium, reduce to small size, they form a new branch of gas lasers-waveguide gas lasers. The waveguide CO2 gas lasers were developed rapidly since 1970s and are widely used in commercial products nowadays. However the sizes of previous waveguide gas lasers are still huge compared with their rival-solid-state lasers. Conventional hollow waveguide theory indicates that the waveguide loss of a simple hollow waveguide is considerable if the aperture of the waveguide is small. The advances in hollow-core photonic bandgap (PEG) fiber gave us the inspiration to develop novel fiber gas lasers based on such new hollow-core fibers. These novel waveguide structures break the limitation predicted by the simple hollow waveguide theory and provide us with the opportunity for building compact, flexible, and miniature fiber gas lasers which are expected to find applications in many fields. However there are still many unknowns in the construction of hollow-core fiber gas lasers. For example: whether the population inversion (prerequisite for lasing) will happen in such a small size tube, and which type of excitation will be the most efficient to produce the population inversion? Many other details need be addressed before a successful experimental demonstration can be made. Although the research on waveguide gas lasers has made significant progress over the past three decades, the research on discharge in micro-size (i.e., size of inner diameter down to 200 um or less) capillaries and lasers based on such capillaries are still not yet reported to our knowledge. The low waveguide losses in hollow-core fibers indicate that they are good waveguides as well as discharge tubes. The calculations which follow the theory of laser resonators suggest that the feedback configuration of flat mirrors placed at the ends of hollow-core fiber is the best choice for hollow-core fibers. Hollow-core PEG fibers can be coiled down to very small size with negligible loss and are good candidates for making long cavity length but still compact fiber gas lasers. Calculations of population inversion indicate that there exists gain for gas-filled hollow-core fibers with inner diameter bigger than 11 um. Our calculations also suggest that considerable gas pressure is needed for the very small tubes to achieve population inversion. These provide the theoretical support for constructing a novel fiber gas laser from the viewpoint of gas discharge physics. The vacuum system for constructing fiber gas lasers is studied in detail. Gas flow in hollow-core fibers is different from that in conventional laser tube because of their very small bore sizes. When considering the outgassing rate of inner surface of the hollow-core fibers, the calculations indicate that outgassing might pose a problem in achieving high degree of vacuum and hinder the lasing of the fiber gas laser. Based on the theoretical analysis and experimental results, we also discuss the technical problems encountered in the construction of fiber gas lasers, such as vacuum requirement, optical alignment problem, and fiber cleaving. We carried out a series of experiments to explore the gas discharge in small size gas-filled hollow-core fibers and succeeded in obtaining gas discharge in 250, 150, and 50 um inner-diameter (i.d.) hollow-core fibers by using longitudinal direct current excitation. Stable glow discharges of at least several minutes were observed for these hollow-core fibers. A flash glow was also observed for a hollow-core fiber with an i.d. of -20 um. Breakdown of helium and argon gases in a 26.2cm-length 250 um-i.d. hollow-core fiber was achieved with a voltage of less than 30kV. Considering the facts that a waveguide gas laser with 430 um-i.d and 20cm-length has been constructed eariler and that a smaller bore-size and longer length of gain tube would provide larger gain, it should then be possible to construct hollow-core fiber gas lasers with an inner diameter of 250 um or less. The successful demonstration of miniature fiber gas lasers would open doors for many new applications such as rotation sensing and flow measurement.

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