Review of soft glass step-index preform fabrication methods

Step-index core/clad optical fibers made from fluoride, tellurite, and chalcogenide soft glasses, that have minimum propagation loss in the near- and mid-infrared (IR) regions, are gaining remarkable attention in various applications. The propagation loss originates primarily from the absorption and scattering of light due to several reasons, such as the existence of impurities and/or defects within the fiber. This review explores the preform fabrication techniques on selected examples of soft glasses to seek an understanding of the impact of fabrication methods on the dimension and quality of the core/clad interface and propagation loss of the corresponding fibers. First, a brief background about the main characteristics of soft glasses considered here and the origin of propagation loss in optical fibers is explored. Later, the fabrication methods of soft glass-based optical fibers and preforms are highlighted with emphasis on the preform fabrication methods. More specifically, three main categories of the preform fabrication are explored: direct casting (i.e., built-in casting, rotational casting, and suction casting), direct extrusion (i.e., axially stacked core/clad billet extrusion and radially stacked core/clad billet extrusion), and cane-in-tube assembly. The rationale for choosing these methods is that the interface is generated at various viscosities and temperatures, which would give an overview of the influence of fabrication conditions on the core/clad interface dimension and quality, core tapering, and propagation loss of the resulting fiber. The review concludes with a summary and outlook for developing soft glass fibers with a low core/clad ratio, high-quality interface (i.e., interface that does not show voids, crystals, and other scattering centers when inspected with microscopy), and reduced propagation loss.