CC BY
7ALT'23
The 30th International Conference on Advanced Laser Technologies
LS-O-12
On the origin of radial and tangential cracks in optical fiber preforms
G. Bufetova1, A. Kosolapov2, M. Yashkov3, A. Umnikov3, V. Velmiskin2, V. Tsvetkov1, I. Bufetov2
1-Prokhorov General Physics Institute of the Russian Academy of Sciences, 38 Vavilov Str., 119991, Moscow, Russia
2- Prokhorov General Physics Institute of the R A S, Dianov Fiber Optics Research Center
3- Institute of Chemistry of High-Purity Substances of the R A S, Nizhny Novgorod, Russia
It is known that the difference in the coefficients of thermal expansion between the cladding glass (ai) and the core material (a2) during sample cooling leads to the formation of tangential cracks if a2 > ai and to the formation of radial cracks if ai > a2 [1]. We propose to pay attention to the well-known phenomenon-the formation of so-called "stars" in the cross sections of fiber preforms based on silica glass with cores doped with relatively high concentrations of germania or alumina. On many preforms the formation of perturbations at the core-cladding boundary, having the form of "needles" or narrow channels filled with the core substance, is observed.
It is possible that mechanism of their formation could be as follows: in the cooling process due to high values of compressive stress in the still liquid core, cracks appear in a cladding (no longer liquid) directed mainly along the radius of the preform, which are immediately filled with the substance of the liquid core. It was shown [2] that refractive index of the core glass of the preform could increase up to ~1.75 in the visible spectral range at temperatures of ~2000°C (~ 1.46 at room temperature). This fact suggests that pressures close to or exceeding the ultimate strength of silica glass (~20 GPa) occur in the perform core region during the manufacturing process. Since the glass transition temperature of the core (silica glass with a high GeO2 or AhO3 doping level) is significantly lower than the glass transition temperature of SiO2, this may explain why the SiO2 cladding cracks and the core material fills resulting cracks. The cross-section view of the preform (Fig. 1) testifies in favor of this assumption, since the star-shaped core (with rays along the radius) was formed during the manufacture of the preform at high temperature and indicates the compressive stress of the core material. At the same time, an annular crack along which the core is separated from the cladding, which appeared when cutting the cold preform, is a consequence of the tensile stress of the core at room temperature.
This work was carried out with the financial support of the Russian Science Foundation (grant № 19-1200361)
[1] Von Kerkhof, F. Bruchentstehung und Bruchausbreitung im Glas. In Book Glastechnische Fabrikationsfehler, 4th ed.; p. 545 (2011).
[2] G. Bufetova, A. Kosolapov, M. Yashkov et al., Extra-High Pressure in the Core of Silica-Based Optical Fiber Preforms during the Manufacturing
Process, Photonics, Vol. 10, No. 3, paper 335 (2023).
Fig. 1. A cross section view with "star" and an annular crack around the core (1)
