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 in contact 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 at home not afraid of water but waterproof? Why are 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 optical cable, it will damage the optical fiber if it freezes and expands in cold water. Therefore, the optical cable needs to be filled with grease to prevent water from entering.
Experiments have shown that long-term immersion 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 slowly occurs, which causes the silicon-oxygen bond in the original network-Si-0-Si-1 to break , and bridge oxygen becomes non-bridge oxygen. Glass cracks, and the cracks continue to grow.
Since optical fiber is glass, why are you afraid of water?
Whether it is fish tank glass, window glass or optical fiber glass, they are afraid of water. The difference is that fish tank glass and window glass are very thick, with thicknesses of 3mm, 5mm, and l0mm. Even 0.05mm cracks do 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 absorption loss of optical fiber to light. This is why fish tank glass and window glass are not afraid of water, while 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.