First, optical fiber isn't afraid of water because it's protected. When optical fiber is spun into a cable, there are two protection requirements for the optical fiber: one is that the optical fiber is under less stress; the other is that the optical fiber must be waterproof. The outermost layer of the optical fiber cable is a plastic sheath, the inner layer is a metal sheath, and the inner layer is a water-blocking coating that swells when exposed to water. The core of the cable is bonded with an adhesive and optical fibers.
The optical cable has four waterproof doors, namely: plastic cover, metal cover, water-blocking layer, and ointment.
So the question is, is the fiber core afraid of water? Isn't it just glass, what's the big deal about water?
In fact, he's afraid of water.
You might wonder why the glass in fish tanks and windows in your home aren't afraid of water but are waterproof, and why they're all made of glass.
Why is the fiber core afraid of water?
It's generally believed that the fiber core is unaffected by water because the glass has excellent water resistance. However, water is actually very damaging to optical cables. If water gets inside the optical cable, it will damage the fiber as it freezes and expands in the cold water. Therefore, optical cables should be coated with a sealant to prevent moisture from entering.
Experiments show that prolonged moisture ingress into the optical cable will increase optical fiber loss, especially at the wavelength of 1.55 pm
The reason why optical fiber is afraid of water is that optical fiber is composed of silicon-oxygen (SiO4) glass tetrahedra connected together, as shown in Figure 1. In a Si-O-Si network, oxygen atoms exist in the form of oxygen bridges.
However, in an environment with water, after the glass surface adsorbs water vapor, a slow hydrolysis reaction occurs, resulting in the breaking of the silicon-oxygen bond in the original network —Si—O—Si—, and the oxygen bridge becomes unbridged oxygen as shown in Figure 2, resulting in cracks in the glass, and the cracks continue to grow.
Whether it's fish tank glass, window glass, or fiber optic glass, they're all susceptible to water damage. The difference is that fish tank glass and window glass are very thick, with thicknesses of 3 mm, 5 mm, and 10 mm. Even a 0.05 mm crack won't affect the glass's strength. There are no visible changes.
Optical fiber is different. The diameter of the glass in optical fiber is only 0.125 mm, which is as thick as a human hair. If there is a crack of 0.05 mm, the diameter of the optical fiber will be 0.075 mm, making the fiber very easy to break. Furthermore, the appearance of OH roots will also increase the optical fiber's light absorption capacity. This is why fish tank glass and window glass are not affected by water, while optical fiber glass is susceptible to water damage.
In this case, if the optical cable is damaged, the splice box seal is not good and the bare fiber is exposed, the lifespan of the optical fiber will be shortened and the fiber will break naturally due to water.
Therefore, if fiber optic cable is installed in sewers, the splices must be properly secured, and if the fiber optic cable itself is damaged, it must be repaired. Do not allow the inside of the fiber optic cable to come into contact with water.
Post time: Nov-18-2022
