A process for manufacturing an optical fiber having a ceramic coating applied at the preform stage or drawn fiber stage wherein the coating hermetically seals the fiber against penetration of environmental gases and fluids and increases the fiber tensile strength, and a fiber produced by the process. An optical fiber preform, from which a fiber is to be drawn, preferably after a preliminary cleaning and drying, is coated with a ceramic layer by reacting hot gaseous compounds of tin or titanium with chlorine, bromine or iodine and water or hydrogen peroxide, typically with nitrogen or oxygen carrier gases. The reaction produces a thin ceramic preform coating of one or more layers of an oxide of tin, titanium, or tin with titanium. The thus coated preform is drawn to the dimensions of a desired optical fiber. The drawn fiber may itself be coated, typically in an on-line process fed directly from a drawing furnace, and comprising one or more reaction zones which apply a further or initial ceramic layer to the fiber. The thus coated or multiply coated fiber is typically buffer coated. Prior to the application of the buffer coating, the bonding between buffer and ceramic is improved by the application of a coupling enhancement compound.