Since the optical fiber is made of glass, why should it be protected from water?

Jul 2, 2024

When the optical cable is laid, there are two protection requirements for the fiber: less stress and waterproof.

Usually there are 4 waterproof barriers: Outer sheath, metal tape, water blocking rope and filling compound. So, the question is, why is optical fiber so afraid of water? Why are the fish glass tank glass and glass window at home not afraid of water but waterproof? Why are they so different?

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

Experiments have shown that long-term immersion of water in optical cables will increase the loss of optical fibers, especially at the 1.55pm wavelength.

The reason why optical fiber is afraid of water is that the optical fiber is composed of glass (Si04) silicon-oxygen tetrahedrons connected to each other. In a Si-O-Si network, oxygen atoms exist in the form of bridge oxygen. However, in an environment with water, after the water vapor is absorbed on the glass surface, a hydrolysis reaction slowly occurs, which causes the silicon-oxygen bonds in the original network - Si-0-Si- to break, and the bridging oxygen becomes non-bridging oxygen, resulting in the glass cracks and causes the cracks to grow.

Whether it is fish tank glass, window glass or fiber optical 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 are 0.05mm cracks, the strength of the glass will not be affected. No changes are visible to the naked eye. The fiber glass is different. The diameter of the fiber glass is only 0.125mm, which is as thin 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 optical fiber's light absorption loss. This is why fish tank glass and window glass are not afraid of water, but fiber optic glass is afraid of water.

In this case, if the optical cable is damaged, the connector box is not properly sealed, or the bare fiber is exposed to water, the life of the fiber will be shortened and the fiber will break naturally.