Abstract: Methods for modifying preform core ovality during and subsequent to the formation of an optical fiber preform. After MCVD deposition forms the core rod, but prior to overcladding of the core rod, the code rod may be etched to change its ovality. In order to etch the core rod, the core rod may be mounted to lathe, rotated by at least two rotors, and subjected to a heat source. Additionally, one of the at least two rotors may be phase-shifted from another one of the at least two rotors after the core rod is mounted on the lathe.

Abstract: The specification describes methods for the manufacture of very large optical fiber preforms wherein the core material is produced by MCVD. Previous limitations on preform size inherent in having the MCVD starting tube as part of the preform process are eliminated by removing the MCVD starting tube material from the collapsed MCVD rod by etching or mechanical grinding. Doped overcladding tubes are used to provide the outer segments of the refractive index profile thus making most effective use of the MCVD produced glass and allowing the production of significantly larger MCVD preforms than previously possible.

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.