Surface crystallization behavior during thermal processing of low-photoelastic ZnO–SnO–P₂O₅ glasses

Pb-containing glasses such as the PbO–SiO₂ series are useful in optical devices because of their low-photoelastic constants (PECs); however, the toxic Pb component in these glasses is problematic. Transparent ternary ZnO–SnO–P₂O₅ glasses are potential substitutes for low-PEC Pb-containing glasses, but their thermal behavior is not well understood, which inhibits their ability to be drawn into fibers. Herein, we report the surface crystallization behavior of transparent ternary ZnO–SnO–P₂O₅ glasses, including those with low-PEC compositions, heated under isothermal conditions to temperatures above their glass-transition temperatures. The crystallization conditions for low-PEC phosphate glasses during their thermal processing into shapes of optical modules were deduced. The crystalline compounds precipitated on the surfaces were investigated via X-ray diffraction analysis, micro-Raman spectroscopy, and microscopic observations. A time–temperature–transformation diagram was constructed on the basis of the thermal stability of the glasses. We expect that this enhanced understanding of the crystallization behavior induced by the reshaping process will enable fabrication of precise polarization control devices with low PECs, such as optical-fiber current sensors, optical lenses, and filters.