Thermo-optic coefficient of B₂O₃ and GeO₂ co-doped silica fibers

New measurements of the thermo-optic coefficients (TOCs) in ternary GeO₂ and B₂O₃ co-doped silica core optical fibers are presented. Material additivity models are applied to the measurements to shed more light on the relative contributions by each constituent. Three of the ternary fibers studied are drawn at different temperatures, but from the same preform, providing insight into the influence of fabrication conditions. It is found that the TOC of the B₂O₃ constituent is somewhat less negative than previously reported and trends towards more negative values with increasing draw temperature. Two approaches are taken in determining the TOC of one of the fibers, including measuring the LP₁₁ cutoff wavelength as a function of temperature. The latter suggests an attractive application for fiber cores with a TOC lower than the cladding: fibers whose V-number intrinsically decreases with increasing temperature. Such thermally-mode-reducing optical fibers could be of great consequence for high-energy laser applications.