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Is the optical fiber in the optical cable afraid of water?

2022-08-10
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When the optical fiber is cabled, 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 must 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, and the cable core is glued with ointment and optical fibers.

 

Optical fiber has four waterproof gates, namely: plastic sheath, metal sheath, water blocking layer, and ointment. So the question is, why is optical fiber so afraid of water, why is the fish tank glass and window glass at home not afraid of water but waterproof, why are all glass so different?

 

1. Why is optical fiber afraid of water?

It is generally believed that optical fibers are not afraid of water, because glass has excellent water adhesion. But in fact water is very harmful to optical cables. If water enters the optical cable, it will damage the optical fiber when it freezes and expands in the cold water, so the optical cable needs to be filled with ointment to prevent moisture from entering.

 

Experiments show that the long-term submersion of water into the optical cable will increase the loss of optical fiber, especially at the wavelength of 1.55pm,

fiber optic cable

The reason why the optical fiber is afraid of water is that the optical fiber is composed of glass (SiO4) silicon-oxygen tetrahedra connected with each other. In a Si-O-Si-network, oxygen atoms exist in the form of bridge oxygens.

 

However, in an environment with water, after the surface of the glass adsorbs water vapor, a hydrolysis reaction occurs slowly, resulting in the breakage of the silicon-oxygen bond in the original network—Si—O—Si—, and bridging 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 fiber optic glass, they are all afraid of water. The difference is that fish tank glass and window glass are very thick, with thicknesses of 3mm, 5mm, and 10mm. Even if there is a 0.05mm crack, it will not affect the strength of the glass. There is no change to the naked eye.

 

The optical fiber glass is different. The glass diameter of the optical fiber is only 0.125mm, which is as thick as a hair. If there is a 0.05mm crack, the fiber diameter will be 0.075mm, so the fiber is very easy to break. In addition, the appearance of OH roots will also Increase the absorption loss of the optical fiber to the light. This is why fish tank glass and window glass are not afraid of water, while fiber optic glass is afraid of water.

 

In this case, if the optical cable is damaged, the sealing of the splice box is not good, and the bare fiber is exposed, the life of the optical fiber will be shortened and the fiber will be broken naturally due to water.

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