Are the optical fibers in optical cables really afraid of water?

Are the optical fibers in optical cables really afraid of water?

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 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 immersion of moisture into the optical cable will increase the loss of the optical fiber, especially at the wavelength of 1.55pm, as shown in the figure:

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-bridging oxygen as shown in Figure 2. , Resulting in glass cracks, and the cracks continue to grow.

Whether it is fish tank glass, window glass or optical fiber glass, it is afraid of water. The difference is that the fish tank glass and window glass are very thick, with thicknesses of 3mm, 5mm, and 10mm. Even if there is an O.05mm crack, it does not affect the strength of the glass. , The naked eye can't see the change. The optical fiber glass is different. The diameter of the optical fiber is only O.125mm, which is as thick as a hair. If there is an O.05mm crack, the optical fiber will have O.075mm in diameter, so the optical fiber is very easy to break. Occurrence will also increase the optical fiber's absorption loss of 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 fiber will be shortened and the fiber will be broken naturally.