Coating of optical fiber is done to protect the bare silica fiber from outside environment. This is what generally comes in to our mind when we go through terms such as ‘protective layer’, ‘Outer protection’ etc. It is true that primary coating of bare silica glass fiber with a soft and then hard acrylate is done to protect it from environmental effects.
Apart from just protection, there are a few more reasons for applying coating over bare silica glass. Bare silica glass is rigid in nature. Though the diameter is extremely small to the size of 125 micrometers, bare silica glass rods are brittle. Try holding it in your hand vertically, it may stand straight for some lengths, say probably less than a meter before it tends to bend. The bend portion will break over a period of time. There is strain at every bending portion which known as bending strain can be calculated by dividing the fiber radius with the radius of curvature of the bend the fiber forms when holding straight. Bending strain can be reduced applying a soft plastic coating over the bare fiber.
Primary coating is done to provide flexibility to the bare fiber. The otherwise brittle silica glass fiber will become flexible after applying acrylate based primary coating material. Optical fibers must be flexible to bends as they pass through different stages in cabling and installation processes. Depending on the design, optical fibers in tubes or tightly coated with plastic material may be bend over strength members to create desire shape to the final product. Hence optical fibers must be capable to withstand bends and stretching. Primary coating provides excellent elongation resistance and flexibility to the bare fiber.
Apart from all the above benefits that primary coatings provides to a bare silica glass fiber, they also act as cladding mode stripper. Refractive index of inner primary coating is higher than that of cladding. This allows the cladding modes to escape to th inner primary coating. Proceeding further….refractive index of outer primary coating is higher than that of inner primary coating. This allows the light that is escaped to inner primary coating to escape further to outer primary coating. Most of them dies out by traveling a few centimeters, but this process is important to reduce unwanted signals traveling along the length of fiber.
As we have seen from the above discussions, the functions of primary coatings, that is inner primary and outer primary coatings are not limited to protection. We want the cladding modes to escape through the coating layers. Refractive index of Inner and Outer primary layers are designed to get the desired escape of cladding modes.
Inner primary coating is a soft acrylate material that has lower index of refraction, tensile strength and youngs modulus compare to the outer primary, which is hard acrylate material. Viscosity and Elongation properties of outer primary hard acrylate are lower compared to inner primary acrylate.