Insights and Aspects to the Modeling of the Molten Core Method for Optical Fiber Fabrication

Abstract

The molten core method (MCM) is a versatile technique to fabricate a wide variety of optical fiber core compositions ranging from novel glasses to crystalline semiconductors. One common feature of the MCM is an interaction between the molten core and softened glass cladding during the draw process, which often leads to compositional modification between the original preform and the drawn fiber. This causes the final fiber core diameter, core composition, and associated refractive index profile to vary over time and longitudinally along the fiber. Though not always detrimental to performance, these variations must, nonetheless, be anticipated and controlled as they directly impact fiber properties (e.g., numerical aperture, effective area). As an exemplar to better understand the underlying mechanisms, a silica-cladding, YAG-derived yttrium aluminosilicate glass optical fiber was fabricated and its properties (core diameter, silica concentration profile) were monitored as a function of draw time/length. It was found that diffusion-controlled dissolution of silica into the molten core agreed well with the observations. Following this, a set of first order kinetics equations and diffusion equation using Fick’s second law was employed as an initial effort to model the evolution of fiber core diameter and compositional profile with time. From these trends, further insights into other compositional systems and control schemes are provided.

Powder-in-Tube Reactive Molten-Core Fabrication of Glass-Clad BaO-TiO2-SiO2 Glass–Ceramic Fibers

“…Since the Si presented in the core comes purely from the reaction between the core material and the fused silica tubing during the drawing process, the average Si content inside the core with different core sizes are summarized in Table 1. The results indicate that larger core diameters have less Si inside the core, which can be explained by the longer diffusion length of the Si in the core [24,36]. For all the fabricated fibers with different core sizes, the molar ratio of Si is within 65-75 mol%.…”

“…On the other hand, reactive molten-core fabrication has proven to be an effective method to fabricate fibers with core materials which cannot be drawn into fibers on their own [22][23][24]. With the housing material reacting with the core material via various mechanisms, such as reactive chemistry, dissolution, and diffusion, fibers with various core materials, such as semiconductors, special glass, and crystalline oxides, have been reported [22][23][24][25][26][27][28][29][30]. Therefore, in this paper we report on the fabrication of glass-clad BTS glass-ceramic fibers using the powder-in-tube reactive molten-core approach with a successive isothermal heat treatment.…”

Dissolution and Diffusion-Based Reactions within YBa2Cu3O7−x Glass Fibers

“…In this way, we are able to confirm that the considerable dimensional variations in the core diameters of the as-drawn fibers in vacuum and oxygen atmosphere are caused by the different induced pressures. Moreover, given the larger core diameter of the oxygen drawn fiber, a lower diffusion-induced silicon content inside the core region should have been expected due to the longer diffusion pathway as discussed in [8,12,13]. As this was not the case (i.e., the silicon content in the oxygen drawn fiber core is higher than the silicon content of the vacuum fiber core, as shown in Table 2), this indicates that the higher silicon content measured in the oxygen drawn fiber core actually originates from the oxygen-rich drawing environment.…”

“…Although the studies in [5,6,8,9] reported the dissolution and diffusion based processes of the fused silica cladding into the molten core, in-depth analyses of these processes and of the potentially occurring phase separations were not performed. The first attempts to analyze the dissolution and diffusion based processes within glass-clad fibers in more depth were performed recently for YAG-derived yttrium aluminosilicate glass optical fibers in [12,13]. However, the significance of the oxygen diffusion from the cladding into the molten core, which is indispensable for analyzing the dissolution and diffusion based process in glass-clad fiber systems drawn using the molten-core approach, was not investigated in these works [12,13].…”

“…The first attempts to analyze the dissolution and diffusion based processes within glass-clad fibers in more depth were performed recently for YAG-derived yttrium aluminosilicate glass optical fibers in [12,13]. However, the significance of the oxygen diffusion from the cladding into the molten core, which is indispensable for analyzing the dissolution and diffusion based process in glass-clad fiber systems drawn using the molten-core approach, was not investigated in these works [12,13]. Hence, this highlights the need for further in-depth analyses of these dissolution and diffusion based reactions within glass-clad fibers.…”