Figure 5: Si-core forming mechanism in fibre draw.

(a) The chemical reaction at the core-cladding interface. Al atoms (blue) break Si-O bond, reduce Si (black), release O²⁻ ions (red) and change to Al³⁺ ions (green). All the products of ions and atoms dissolve in the Al melt and diffuse into the core. (b) The Si atoms (black), because of their miscibility, diffuse and remain in the Al melt (blue), while ions of O and Al form Al₂O₃ molecules (yellow) and precipitate out at the bottom. (c–e) Sketches of the evolution of the core in the heating zone throughout the drawing process. As the draw continues, more and more Si atoms accumulate in the Al-Si melt and eventually one obtains a highly pure Si melt on top of the Al₂O₃ with all the Al consumed in the redox reaction (Stage II). Further drawing then results in a highly pure Si-core fibre (Stage III). The diagrams corresponding to each figure demonstrate the ratio of different materials along the core axially. Blue, black and yellow denote Al, Si and Al₂O₃, respectively. The green dashed line that gets lower and lower during the draw marks the position of the core. The cyan shadows in b and c illustrate the location of the transition region.