Molten Core Fabrication of Intrinsically Low Nonlinearity Glass Optical Fibers

Optical nonlinearities limit scaling to higher output powers in modern fiber-based laser systems. Paramount amongst these parasitic phenomena are stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and nonlinear refractive index (n2)-related wave-mixing phenomena (e.g., self-phase modulation, SPM, four-wave mixing, FWM). In order to mitigate these effects, the fiber community has largely focused on the development of micro-structured large mode area (LMA) fibers whereby the fiber geometry is engineered to spread the optical power over a larger effective area. In addition to increasing the resultant complexity and cost of these fibers, such LMA designs introduce new parasitic phenomena, such as transverse mode instability (TMI), which presently serves as the dominant limitation in power-scaling. This dissertation explores a different approach for mitigating these nonlinearities in optical fiber lasers; namely attacking the aforementioned effects at their fundamental origin, i.e., the material through which the light propagates. Indeed, the Brillouin gain coefficient (BGC), the Raman gain coefficient (RGC), the thermo-optic coefficient (TOC) and the nonlinear refractive index (n2) are all intrinsic material properties that respectively drive SBS, SRS, TMI and wave-mixing phenomena. Though less well studied within the fiber laser community, such a materials approach offers a powerful yet simpler way to address nonlinearities. Chapter I investigates the thermodynamic origins of light scattering and provides insight into the prime material properties that drive optical nonlinearities. Chapters II and III offer an overview of how these (and other) properties can be measured and modeled in multicomponent glass systems, considering both bulk or fiber geometries. In Chapter IV, a materials road map for binary and ternary glass material systems is provided to identify which compositions should be of specific focus for the development of intrinsically low optical nonlinearity optical fibers. These four Chapters have been adapted from a series of published journal articles1 entitled “A unified materials approach to mitigating optical nonlinearities in optical fiber” [1]–[4]. In Chapter V, the fabrication of oxyfluoride-core silica-cladding optical fibers using the molten core method is described and their core glass compositions and structures investigated. The thermodynamics and kinetics of fluoride-oxide reactions are also studied, and insights on the dominant mechanisms that drive the fluoride-oxide reactions during fiber processing are discussed. In Chapter VI, optical properties that drive optical nonlinearities are studied, and their relationships with glass compositions investigated. Oxyfluoride fibers exhibiting concomitant reductions of 6-9 dB in BGC, 0.5-1.5 dB in RGC, and 1.2-3.2 dB in TOC, relative to conventional silica fibers, as well as reduced linear and nonlinear refractive indices, are reported. Spectroscopic properties of active Yb-doped fibers are also considered, and suggest enhanced laser performance and higher lasing efficiencies for these systems compared to conventional silica fibers. Finally, Chapter VII portrays the current challenges and the future perspectives of these oxyfluoride-core silica-cladding glass optical fibers, along with approaches to overcome them