Why is the fiber optic cable afraid of moisture?

In the manufacturing process of optical cables, there are two key requirements for the protection of optical fibers: one is to minimize the physical stress on the fiber; the other is to ensure that the fiber has good waterproof performance.

The structure of the fiber optic cable from the outside to the inside is a plastic sheath, a metal sheath, a water-blocking layer that expands in contact with water, and a cable core containing ointment, which is closely integrated with the fiber optic cable. The waterproof design of the fiber optic fiber includes four lines of defense: plastic sheath, metal sheath, water-blocking layer and ointment. However, people may wonder why the glass is also made of glass, why the fiber optic fiber is so afraid of water, but the fish tank glass and window glass in the home are waterproof?

It is generally believed that optical fibers are not afraid of water because glass itself has good water adhesion. But in fact, water is extremely harmful to optical cables. If water enters the cable, in a low temperature environment, the water freezes and expands to damage the fiber. Therefore, the inside of the cable needs to be filled with ointment to prevent moisture from invading.

Experiments have shown that long-term moisture intrusion results in increased attenuation of the fiber, especially at the 1.55-micron wavelength, as depicted in the figure.

The reason why an optical fiber is afraid of water lies in its structure. The optical fiber is composed of silicon-oxygen tetrahedra of glass (SiO ²) interconnected, as shown in Figure 1. In the Si-O-Si network, oxygen atoms exist in the form of bridge oxygen. However, in an environment with water, after the glass surface absorbs water vapor, a hydrolysis reaction will gradually occur, resulting in the fracture of the silicon-oxygen bond in the Si-O-Si network, and the bridge oxygen will transform into non-bridge oxygen, as shown in Figure 2. This will cause cracks in the glass and cause it to continue to expand.

Whether it is fish tank glass, window glass or fiber optic glass, they are actually afraid of water. The difference is that the thickness of fish tank glass and window glass is usually 3 mm, 5 mm or even 10 mm. Even if a crack of 0.05 mm appears, it will not significantly affect the strength of the glass, and it is difficult for the naked eye to detect. The diameter of fiber optic glass is only 0.125 mm, which is as thin as a hair. If there is a crack of 0.05 mm, the diameter of the fiber will be reduced to 0.075 mm, which is very easy to break. In addition, the presence of OH roots will also increase the absorption and loss of light by the fiber optic fiber. This is why fish tank glass and window glass are not afraid of water, while fiber optic glass is afraid of water.

Therefore, cable damage, poor sealing of the splice box, or exposure to bare fibers can shorten the service life of the fiber due to water intrusion, and even lead to natural fiber breakage.