Chalcogenide-glass negative curvature fibers.

Hollow-core negative curvature fibers have drawn lots of attention due to their attractive properties, including a low transmission loss, a wide transmission bandwidth, low power ratio in the glass, and a simple structure. In this dissertation, we describe the history, guiding mechanism, advances, and future prospects for negative curvature fibers. We focus our studies on negative curvature fibers using chalcogenide glasses, which have low material loss in the mid-infrared region. Optical fibers used for lasers in mid-infrared region have important applications for chemical sensing, environmental monitoring, homeland security, and medical diagnostics. We study one-dimensional slab waveguides, two-dimensional annular core fibers, and negative curvature tube-lattice fibers to illustrate the inhibited coupling guiding mechanism. Antiresonance in the glass at the core boundary and a wavenumber mismatch between the core and cladding modes inhibit coupling between the modes and have led to remarkably low loss in negative curvature fibers. We show computational studies to design negative curvature fibers that improve the performance of the hollow core fibers. First, we compare loss in silica and chalcogenide negative curvature fibers and consider both simple and nested geometries as the transmission wavelength varies. At wavelengths shorter than 4.5 µm, silica negative curvature fibers have a loss that is around or below 10⁻¹ dB/m and are preferable to chalcogenide fibers. At wavelengths longer than 4.5 µm, it is preferable to use As₂S₃ chalcogenide or As₂Se₃ chalcogenide negative curvature fibers since their loss is one or more orders of magnitude lower than the loss of silica negative curvature fibers. Second, we find the impact of cladding tubes in chalcogenide negative curvature fibers. The leakage loss is decreased by a factor of 19 and the operating bandwidth is almost doubled when the optimal gap between cladding tubes is used in negative curvature fibers with 6 tubes. The optimal gap in a fiber with 6 cladding tubes is 3 times as large as the optimal gap in fibers with 8 or 10 cladding tubes. A larger gap is needed in a fiber with 6 cladding tubes to remove the weak coupling between the central core mode and the tube modes. Third, we study conditions for suppression of higher-order core modes in chalcogenide negative curvature fibers with an air core. An avoided crossing between the higher-order core modes and the fundamental modes in the tubes surrounding the core can be used to resonantly couple these modes, so that the higher-order core modes become lossy. Fourth, we study bend loss in chalcogenide negative curvature fibers with different polarizations, different tube wall thicknesses, and different bend directions relative to the mode polarization. The coupling between the core mode and tube modes induces bend loss peaks in the two non-degenerate modes at the same bend radius. There is as much as a factor of 28 difference between the losses of the two polarization modes. Last, we propose a polarization-filtering and polarization-maintaining negative curvature fiber in which two nested resonant tubes are added to a standard negative curvature fiber with one ring of tubes. The coupling between the glass modes in the nested resonant tubes and the fundamental core modes is used to increase the birefringence and differential loss for the fundamental core modes in the two polarizations. At the end, we discuss the future prospects for negative curvature fibers and give a summary.