TIA's Glossary of Telecommunication Terms
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primary coating
The plastic overcoat in intimate contact with the cladding of an optical fiber, applied during the manufacturing process. Note 1: The primary coating typically has an outside diameter of approximately 250 to 750 
m, and serves to protect the fiber from mechanical damage and chemical attack. It also enhances optical fiber properties by stripping off cladding modes, and in the case where multiple fibers are used inside a single buffer tube, it suppresses cross-coupling of optical signals from one fiber to another. Note 2: The primary coating should not be confused with a tight buffer, or the plastic cladding of a plastic-clad-silica (PCS) fiber. [After FAA] Note 3: The primary coating, which typically consists of many layers, may be color-coded to distinguish fibers from one another, e.g., in a buffer tube containing multiple fibers. Synonyms primary polymer coating, primary polymer overcoat .