Development of high numerical aperture fiber bundles (FBs) requires use of thermally matched pair of glasses with a high difference of refractive indices. We have developed a pair of glasses with high refractive index contrast ΔnD>0.2, suitable for fabrication of optical fiber bundles with numerical aperture NA > 0.85. Core glass was synthetized in the lanthanum oxide system Nb2O5-Ta2O5-SiO2-ZrO2-B2O3-Al2O3-La2O3-BaO-SrO. Borosilicate glass synthetized in oxide system SiO2-Al2O3-B2O3-MgO-CaO-Na2O-K2O, thermally matched to the core glass, is used for the fiber cladding. The glasses also have high transmission from 350 to over 600 nm, which makes them ideal for fluorescence imaging applications. These thermally stable, crystallization-free lanthanum and borosilicate glasses have been successfully applied to development of proof-of-concept large diameter optical fiber.