When the optical cable is formed into a cable, there are two protection requirements for the optical fiber: one is that the optical fiber is less stressed; the other is that the optical fiber should be waterproof.
The outermost layer of the optical cable is a plastic sheath, the inside is a metal sheath, and the inside is a water-blocking layer that swells with water. The cable core is glued with the optical fiber. The optical fiber has four waterproof barriers, namely: plastic sheath, metal sheath, water blocking layer, and grease. So the question is, why are optical fibers so afraid of water? Why are the fish tank glass and window glass in the house not afraid of water but waterproof? Why are all glass so different?
It is generally believed that optical fibers are not afraid of water, because glass has excellent water retention. But in fact, water is very harmful to optical cables. If water enters the fiber optic cable, it will damage the fiber if it freezes and swells in the cold water. Therefore, the fiber optic cable needs to be filled with grease to prevent water from entering.
Experiments have shown that long-term submersion of moisture into the optical cable will increase the fiber loss, especially at the wavelength of 1.55pm.
The reason why the optical fiber is afraid of water is that the optical fiber is composed of glass (Si04) silicon-oxygen tetrahedrons connected to each other, as shown in Figure 1. In a Si—O—Si network, oxygen atoms exist in the form of bridging oxygen. However, in a watery environment, after water vapor is adsorbed on the glass surface, a hydrolysis reaction occurs slowly, which causes the silicon-oxygen bond in the original network-Si-0-Si-1 to be broken, and the bridging oxygen becomes non-bridging oxygen. The glass cracks, and the cracks continue to grow.
Since the optical fiber is glass, why are you afraid of water?
Whether it is fish tank glass, window glass or optical fiber glass, it is afraid of water. The difference is that fish tank glass and window glass are very thick, with thicknesses of 3mm, 5mm, l0mm, and even 0.05mm cracks will not affect the strength of the glass. No changes can be seen with the naked eye. The fiber glass is different. The fiber glass has a diameter of only 0.125mm, which is as thick as a human hair. If there is a 0.05mm crack, the fiber diameter will remain 0.075mm, so the fiber is very easy to break, and OH roots will also appear. Increase the optical fiber’s absorption loss to light. This is why aquarium glass and window glass are not afraid of water, while optical fiber glass is afraid of water.
Even so, if the optical cable is damaged, the connector box is not well sealed, and the exposed fiber is exposed to water, the life of the optical fiber will be shortened and the fiber will be broken naturally.
