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As is well known, water seepage is very fatal for optical fibers, as the hydrogen precipitated from water can cause "hydrogen loss" in the fibers, leading to increased attenuation and greatly affecting transmission efficiency and stability.

So what is "hydrogen loss"?

Hydrogen loss can be divided into two types: reversible hydrogen loss and irreversible hydrogen loss.

Reversible hydrogen loss:

Hydrogen molecules diffuse into SiO₂ optical fibers, and the infiltrated free hydrogen is captured by the SiO₂ grid at room temperature. Under the action of a local electric field, a dipole moment is formed in the hydrogen molecules, and additional absorption losses are generated by the vibration of hydrogen. Compared with the connecting bonds between ordinary molecules, the capture force of the grid is relatively weak. If the hydrogen environment of the fiber is removed, hydrogen molecules will escape from the SiO₂ grid, and the fiber loss will recover to its original value. Therefore, this is an intermittent effect, and the hydrogen loss generated is reversible.

Irreversible hydrogen loss:
Hydrogen molecules that diffuse into the human body at high temperatures chemically bond with the defective parts of the fiber mesh to form OH groups, and their vibration increases the peak absorption loss. Due to the strong bonding force, even after removing the hydrogen environment from the optical fiber, the chemical bonds still exist and the absorption loss will not change. Therefore, it is a permanent chemical effect, and the resulting hydrogen loss is irreversible.

So how does optical fiber come into contact with hydrogen molecules? Where do they come from and how should they be suppressed?

Firstly, it is necessary to understand the factors that affect the magnitude of hydrogen loss. It should be noted that whether it is a single-mode optical cable or a multi-mode optical cable, the accumulation of hydrogen gas in the cable structure may deteriorate the optical performance during its service life.

The magnitude of hydrogen loss depends on the following factors:

• The type of optical fiber, its doping composition and concentration, and its inherent sensitivity to hydrogen gas;
• Fiber optic cable design, especially the selection and combination of materials used in its structure;
• Installation environment, including its operating temperature;
• The level of hydrogen gas generated by optical cables during their service life.

Hydrogen gas in optical cables may come from:

• Hydrogen released by optical cable components, including those related to long-term aging effects of materials;
• The hydrogen contained in compressed air pumped into the optical cable;
• Hydrogen evolution caused by corrosion of metal components in the presence of moisture;
• Hydrogen produced by biochemical corrosion.

In order to prevent hydrogen loss, appropriate measures should be considered in the design and manufacturing of fiber optic cables, such as:

• Selecting appropriate dopants when making fiber optic preforms, and using P₂O₅ less or less;
• When making fiber optic preforms, synthetic SiO₂ is used as the cladding instead of natural SiO₂ cladding;
• When drawing optical fibers, choose coating materials with low hydrogen evolution;
• Carefully select the materials used for the optical cable, especially the filling paste that comes into direct contact with the fiber, which is particularly important;
• Take water blocking measures for fiber optic cables to ensure that there is no overall water seepage or hydrogen evolution, and to eliminate external hydrogen environments.