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.
Highlights
The molten core method (MCM) has been employed for ~25 years to produce “nonconventional” optical fibers with intriguing properties [1], including crystalline semiconductors (Si, Ge) core–silica (SiO2 ) cladding [2], and low optical nonlinearity multicomponent fluorosilicate core–silica cladding [3] glass, to name just a few
One intrinsic feature of such fibers fabricated using the MCM is the inherent incorporation of SiO2 into the core as the fiber is being drawn. This SiO2 incorporation typically is accompanied with a proportionate decrease of the fiber core diameter and the establishment of a fiber core graded index profile [4,5]
This paper provides a first investigation into the evolution of properties— core diameter, silica concentration and its associated graded index profile—of fibers drawn using the molten core method (MCM)
Summary
The molten core method (MCM) has been employed for ~25 years to produce “nonconventional” optical fibers with intriguing properties [1], including crystalline semiconductors (Si, Ge) core–silica (SiO2 ) cladding [2], and low optical nonlinearity multicomponent fluorosilicate core–silica cladding [3] glass, to name just a few. Most optical fibers fabricated using the MCM are silica-clad, that is, the preform is a fused silica capillary tube and will serve as the fiber cladding material post draw. One intrinsic feature of such fibers fabricated using the MCM is the inherent incorporation of SiO2 into the (molten) core as the fiber is being drawn. This SiO2 incorporation typically is accompanied with a proportionate decrease of the fiber core diameter and the establishment of a fiber core graded index profile [4,5]. Some mixing between SiO2 from the preform and the molten precursor is desired as it facilitates the formation of a glassy core during fiber drawing.
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