an optical fiber protected from degradation caused by corrosion from environmental fluids . the optical fiber includes a core - cladding assembly surrounded with a thin quasi metal oxide film . over time , particularly under the influence of the elevated temperatures , the initially deposited reagents of the thin film oxidize or hydrolyze , removing the organic components , and bonding a quasi metal oxide film on the surface of the cladding . the resulting film significantly retards corrosion by blocking the environmental fluids from reacting with flaws in the cladding surface of the fiber , thus improving the mechanical performance and reliability of the optical fiber . in addition , the thin film protects the fiber against incidental abrasions . alternatively , the quasi metal oxide reagents are deposited into a protective polymer layer of the fiber or a filling compound of a fiber cable , rather than directly onto the cladding surface , whereby the same protective effect is achieved when the reagents diffuse through the polymer layer or filling compound to the core - cladding surface .