A New Class of Glasses for Double-Crucible Optical Fibers with High Numerical Apertures

A series of boron- and germanium-free silicate glasses was developed for the core glass of high-numerical-aperture double-crucible optical fibers. When combined with a boro-silicate cladding glass from a series also investigated in this study, fibers with a numerical aperture in the range 0.3 to 0.5 can be produced. The relatively high refractive indices in the core glasses are achieved by including large mole fractions of BaO, CaO, and ZnO, accompanied in some cases by smaller additions of ZrO₂ and/or Y₂O₃ in the glass composition. Measurements of the refractive index, density, Rayleigh scattering coefficient, and viscosities and observations of the devitrification behavior and chemical durability of the glasses are presented. Attenuation spectra of suitable core glasses and of double-crucible optical fibers comprised of different core and cladding combinations yielding high numerical apertures are given. The fibers have attenuations near 10 dB/km at 850 nm.