Optical fibers are protected with outer coating material, that is flexible and has good resistance to the outside environments. This makes a bare optical fiber suitable for further processing. Protection of bare silica optical fibers are done by ultra violet curable acrylate material. Protective coating over bare fiber is called primary coating.
I have seen that the term primary coating often confuses some people in the industry. Optical fiber cable specifications that contain requirements of optical fiber characteristics are one of the best example for such confusion. The term “Secondary coating” is used to denote loose tube, tight buffer and such similar second protections. Secondary coating is done after coloring or directly over the natural fiber (uncolored fiber).
At the same time, Primary coating has two layers of UV curable acrylate material; a soft layer and a hard layer. First layer of soft acrylate is applied immediately over the bare fiber that is over the cladding. Second layer of hard acrylate is applied over the first soft layer. This is the point where some people get confused. They think, this second layer is secondary coating. No, it is not. The second layer of acrylate coating over soft layer is called outer primary coating. First layer of soft acrylate is called Inner primary coating.
Making it clear, Primary coating on optical fiber has two layers – Inner Primary Coating and Outer Primary Coating. The scope of primary coating finishes at the fiber manufacturer side. When optical fiber manufacturer draws optical fiber, they make a product with Primary coating that contains two layers. Wet on wet process and Wet on dry process are used for primary coating of optical fibers.
Secondary coating starts with subsequent processing over the optical fibers often in a cable making factory. Optical fiber cable manufacturing generally starts from coloring process. Natural fibers and or colored fibers are placed inside protective tubes/structures in secondary coating line. Loose tube machine is a good example for secondary coating process. Tight buffer is also a secondary coating process. Some cable designs require one more hard acrylate coating over the natural fiber received by fiber supplier. This third layer of ultra violet curable acrylate provides better microbending performances to the fiber when used in indoor cables, pigtails and patch cords.
The confusion finishes here. Inner and Outer Primary coatings are not secondary coating. Optical fiber suppliers provide a product that has primary coating (both inner and outer). Usually secondary coating is done by cable makers.