CONTROLLING REFRACTIVE INDEX PROFILE DURING FIBER PREFORM MANUFACTURING

In some implementations, a substrate tube in a modified chemical vapor deposition process may rotate while glass precursors flow into the substrate tube at a fixed rate. Dopants may be delivered into the substrate tube while heat is applied to the substrate tube to deposit, on an inner wall of the substrate tube, a layer of material including the glass precursors and the dopants. A lateral position of an exit of an injection tube used to deliver the dopants may be adjusted while the substrate tube is rotated and heat is applied to the substrate tube such that the material deposited on the inner wall of the substrate tube has an azimuthally non-uniform doping concentration. Alternatively, a rotation of the substrate tube may be adjusted to create opposing temperature gradients within the substrate tube, causing non-uniform layer deposition to occur on different sides of the substrate tube in alternating passes.