Axial refractive index depression in preforms and fibers

The refractive index depression that exists along the axis in modified chemical vapor deposited (MCVD) preforms and fibers doped mainly with GeO₂ or P₂O₅ is studied by interference microscopy. The index profile of a preform, determined as a function of its transition from the uncollapsed to the fully collapsed state, shows that this central region is fully depleted of dopant and has an index value corresponding to that of the pure fused silica cladding. The same results are found in samples taken from six preform tips and from a relatively large fiber, indicating that this effect is a general property of an uncompensated fabrication process. Implications for its influence on the fiber's transmission properties are discussed.