Various dopants are added in commercially available optical glass fibers. The specific atomic species and charge state of lanthanide dopants are known to significantly influence the fiber's optical properties. For understanding the role of dopants on the optical properties, atomic-scale identification of the lanthanide dopants in the optical fiber is crucial. Aberration-corrected scanning transmission electron microscopy (STEM) is especially powerful for visualizing individual atoms of heavy elements buried in a matrix composed of light elements. Here, we apply aberration-corrected high-angle annular dark field (HAADF)-STEM to directly visualize individual erbium (Er) dopants buried in the optical glass fiber. Molecular dynamics and image simulations are used to interpret the experimental images and draw quantitative conclusions. The visibility of the buried Er atoms in the amorphous glass is strongly dependent on the defocus and specimen thickness, and only Er atoms in very thin regions can be reliably identified.