Methods for forming optical fiber preforms with selective diffusion layers

A method for forming an optical fiber preform, the method comprising:

• constructing a soot preform by:
• forming a preform core portion;
• forming an inner cladding layer around the preform core portion, wherein the inner cladding layer has an inner cladding density;
• forming an outer selective diffusion layer around the inner cladding layer, wherein the outer selective diffusion layer has an outer as-formed density greater than the inner cladding density;
• and
• forming an outer cladding layer around the outer selective diffusion layer, wherein the outer cladding layer has an outer cladding density which is less than the outer as-formed density;
• diffusing at least one diffusing species through the outer selective diffusion layer into the inner cladding layer;
• and
• sintering the soot preform such that the outer selective diffusion layer has an outer barrier density greater than the outer as-formed density and the outer selective diffusion layer prevents diffusion of the at least one diffusing species through the outer selective diffusion layer.