Characteristics, Applications, and Properties of Carbon-Dioxide-Laser-Induced Long-Period Fiber Gratings
Bachim, Brent Leland
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Long-period fiber gratings (LPFGs) are typically fabricated by exposing photosensitive optical fiber to ultraviolet light. However, LPFGs can be fabricated by a variety of other techniques, including exposure to carbon-dioxide (CO2) laser light. The physical process by which the refractive-index change is induced in an optical fiber during exposure to CO2 laser light gives CO2-laser-induced LPFGs unique properties when compared to more traditional LPFGs fabricated by exposure to UV light. As such, CO2-laser-induced LPFGs respond differently to external perturbations and useful behavior has been observed, including variable attenuation tuning at a constant wavelength and wavelength tuning at constant amplitude with applied flexure. In order to manipulate, harness, and enhance the unique features of CO2-laser-induced LPFGs, it is necessary to understand their physical properties and optical characteristics. The main objectives of the research presented in this thesis are to quantify experimentally the optical performance of CO2-laser-induced LPFGs with respect to flexure, torsion, and variable incident polarization, to characterize grating cross-sectional refractive-index profiles, and to demonstrate applications of CO2-laser-induced LPFGs that exploit their unique properties. As part of the investigation of the effects of asymmetry, the fabrication and basic transmission characteristics of CO2-laser-induced LPFGs were examined. The polarization-dependent transmission characteristics, specifically polarization-dependent loss and polarization mode dispersion, of CO2-laser-induced LPFGs were investigated. The unique behavior of the gratings in response to applied flexure and applied torsion was also explored. Example variable optical attenuator, optical tunable filter, and fiber-to-waveguide coupler devices illustrate the potential advantages of the asymmetric index profile present in CO2-laser-induced LPFGs for certain applications. A new cross-sectional refractive-index profiling technique was presented that enables measurement of profiles containing small and irregular index variations. The profiling technique was used to measure the cross-sectional refractive-index profiles of optical fiber exposed to CO2 laser light. Future areas of research concerning CO2-laser-induced LPFGs were identified and discussed.