Antiresonant guiding hollow-core fibers and their applications in nonlinear optics
Creators: name: Hasan, Md. Imran
Publisher: name: The Australian National University
Year: 2019

description: Hollow-core antiresonant fibers operating in the ultraviolet to mid-infrared regions are currently of great interest to many researchers. Among the large variety of antiresonant guiding hollow-core fibers, negative curvature hollow-core antiresonant fibers have recently emerged, featuring exceptional transmission properties, thanks to an assorted range of innovative cladding structure designs that effectively reduces the light leakage in the core. Currently, there is a big effort in the community to study the guiding properties of negative curvature hollow-core antiresonant fibers, both experimentally and theoretically. Numerical approaches such as finite-element method are effective and accurate for finding the guiding properties of the hollow-core antiresonant fibers. In this thesis, an antiresonant guiding hollow-core fiber is proposed and its guiding properties are investigated numerically. The proposed design has an elliptical nested element in the antiresonant tubes which effectively adds two curvatures, namely a positive and a negative curvature. The numerical study shows that it enhances the confinement of the light in the core. At the same time, the nested elements provide an extra degree of freedom that can be exploited for suppressing higher-order modes through the change of the ellipticity. Depending on the geometrical complexity the numerical approaches often require large computational resources. An alternative approach is to use a capillary approximation. By going through a sequence of thorough analyses on the antiresonant guiding hollow-core fiber's structural parameters, empirical formulae for the dispersion and effective area of the fundamental mode are proposed. The formulae allow one to obtain the guiding properties over a wide spectral bandwidth, without the need for time consuming numerical simulations. The empirical formulae are validated by comparing their results with those obtained using the finite-element method. The effects of changing the number of antiresonant tubes, as well as the presence of the nested elements in the antiresonant tubes on the guiding properties are also analyzed. In an antiresonant guiding hollow-core fiber, light is guided in the central region with only a very small overlap between the light and the waveguide material. This not only allows the propagation of high-intensity pulses without causing damages to the fiber, but also opens up light transmission in the spectral regions where the waveguide material is highly absorbent. This permits the use of silica as the base material for antiresonant guiding hollow-core fibers in the mid-infrared spectral region. Moreover, the fiber can be filled with gas, offering an excellent platform for investigating light-mater interactions. It enables the study of ultrafast nonlinear optics in waveguide geometries, including the supercontinuum generation in the mid-infrared spectral regions where the filling material provides the necessary nonlinearity. Detailed numerical studies on supercontinuum generations and nonlinear light-wave propagation are presented for the case of pumping in the anomalous dispersion and normal dispersion regimes. The pump pulse goes through a series of nonlinear effects leading to a supercontinuum output that spans across a wide bandwidth in mid-infrared. Such sources have great potential for applications in remote sensing of various molecules, medical imaging diagnosis, and surgery.