For a standard-size fiber with a 125-µm cladding diameter and a 250-µm coating diameter, 75% of the fiber’s three-dimensional volume is the polymer coating. The core and cladding glass account for the remaining 25% of the coated fiber’s total volume. Coatings play a key role in helping the fiber meet environmental and mechanical specifications as well as some optical performance requirements.
If a fiber were to be drawn and not coated, the outer surface of the glass cladding would be exposed to air, moisture, other chemical contaminants, nicks, bumps, abrasions, microscopic bends, and other hazards. These phenomena can cause flaws in the glass surface. Initially, such defects may be small, even microscopic, but with time, applied stress, and exposure to water, they can become larger cracks and eventually lead to failure.
That is, even with state-of-the-art manufacturing processes and top-quality materials, it is not possible to produce a fiber with absolutely no flaws. Fiber manufacturers go to great lengths to process preforms and control draw conditions to minimize the flaw sizes and their distribution. That said, there will always be some microscopic flaws, such as nanometer-scale cracks. The coating’s job is to preserve the “as drawn” glass surface and protect it from extrinsic factors which could damage the glass surface such as handling, abrasion etc.
Hence, all fiber receives a protective coating when it is drawn. Uncoated fiber occurs for only a short span on the draw tower, between the time the fiber exits the bottom of the preform oven and enters the first coating cup on the draw tower. This uncoated interval is just long enough for the fiber to cool so that the coating can be applied.