Abstract
Refractive index profile of the core is a key design parameter in fiber lasers and amplifiers. During manufacturing, the initial information of the index profile is obtained from the preform, while ultimately the performance is defined by the index profile of the fiber. Depending on stresses and diffusion, the two profiles may be different. It is possible to predict more accurately the laser fiber refractive index when we apply a stress-induced index change model to the measured preform index profile data. The improved capability to predict the fiber index from preform increases the confidence in achieving the designed index profile in fiber, which enables faster process feedback and higher fiber yields.
Highlights
The refractive index difference values of active fibers and preforms are determined by the core composition and influenced by the stress-induced index changes in the preforms and fibers
When the modelled stress-induced index change values were combined with the measured preform index values, the correlation R2 was improved from 82.7% to 97.5%
The preform index profile is measured at the beginning, and the fiber index profile at the end of the production process
Summary
The refractive index difference values of active fibers and preforms are determined by the core composition and influenced by the stress-induced index changes in the preforms and fibers. A refractive index value, measured from a preform, can change during the process steps that follow the preform manufacturing caused by changes in the residual stresses in the glass [1]. To estimate stresses in glass materials, we need to know the transition temperatures, material composition, thermal expansion coefficients, preform and fiber dimensions, elastic modulus and viscosity of the doped and undoped glass. We will present a simplified model to estimate the stresses and refractive index changes in the laser fibers and preforms. We calculate the material parameter values by linearly mixing the parameter values of pure dopant materials and silica These doped material parameter values are applied to calculate the stresses. The stress-induced index change model allows estimating the refractive index changes when the glass composition and dimensions are varied. The model gives us a physical insight into the index change phenomenon and removes the limits set by the measurement data in statistical models [5]
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