Abstract: The specification describes a method for addressing defects in the center of the core of an optical fiber that are formed during high temperature steps associated with collapsing a hollow core fabricated by the MCVD, PCVD, or OVD methods. These defects form absorption centers and impair the optical transmission properties of the optical fiber. The defects are reduced or eliminated according to the invention by forming a buffer layer as the last deposited layer before collapse. The buffer layer is undoped, or lightly doped, and provides a diffusion barrier to prevent or slow a change in the oxide glass stoichiometry. The result is that fewer dopant and oxygen atoms exit from the core layers through the free surface during collapse, resulting in fewer defects and lower fiber attenuation.