The material used to manufacture optical fibers can absorb light energy. After particles in optical fiber materials absorb light energy, they produce vibration and heat, and dissipate the energy, resulting in absorption loss. This article will analyze the absorption loss of optical fiber materials.
We know that matter is composed of atoms and molecules, and atoms are composed of atomic nuclei and extranuclear electrons, which rotate around the atomic nucleus in a certain orbit. This is just like the Earth we live on, as well as planets like Venus and Mars, all revolve around the Sun. Each electron has a certain amount of energy and is in a certain orbit, or in other words, each orbit has a certain energy level.
The orbital energy levels closer to the atomic nucleus are lower, while the orbital energy levels farther away from the atomic nucleus are higher. The magnitude of the energy level difference between orbits is called the energy level difference. When electrons transition from a low energy level to a high energy level, they need to absorb energy at the corresponding energy level difference.
In optical fibers, when electrons at a certain energy level are irradiated with light of a wavelength corresponding to the energy level difference, electrons located on low-energy orbitals will transition to orbitals with higher energy levels. This electron absorbs light energy, resulting in absorption loss of light.
The basic material for manufacturing optical fibers, silicon dioxide (SiO2), itself absorbs light, one called ultraviolet absorption and the other called infrared absorption. At present, fiber optic communication generally only operates in the wavelength range of 0.8-1.6 μ m, so we will only discuss the losses in this working area.
The absorption peak generated by electronic transitions in quartz glass is around 0.1-0.2 μ m wavelength in the ultraviolet region. As the wavelength increases, its absorption gradually decreases, but the affected area is wide, reaching wavelengths above 1 μ m. However, UV absorption has little effect on quartz optical fibers operating in the infrared region. For example, in the visible light region at a wavelength of 0.6 μ m, the ultraviolet absorption can reach 1dB/km, which decreases to 0.2-0.3dB/km at a wavelength of 0.8 μ m, and only about 0.1dB/km at a wavelength of 1.2 μ m.
The infrared absorption loss of quartz fiber is generated by the molecular vibration of the material in the infrared region. There are several vibration absorption peaks in the frequency band above 2 μ m. Due to the influence of various doping elements in optical fibers, it is impossible for quartz fibers to have a low loss window in the frequency band above 2 μ m. The theoretical limit loss at a wavelength of 1.85 μ m is ldB/km. Through research, it was also found that there are some \”destructive molecules\” causing trouble in quartz glass, mainly harmful transition metal impurities such as copper, iron, chromium, manganese, etc. These \’villains\’ greedily absorb light energy under the illumination of light, jumping and jumping around, causing a loss of light energy. Eliminating \’troublemakers\’ and chemically purifying the materials used to manufacture optical fibers can greatly reduce losses.
Another absorption source in quartz optical fibers is the hydroxide (OH -) phase. It has been found that hydroxide has three absorption peaks in the working band of the fiber, which are 0.95 μ m, 1.24 μ m, and 1.38 μ m. Among them, the absorption loss at the wavelength of 1.38 μ m is the most severe and has the greatest impact on the fiber. At a wavelength of 1.38 μ m, the absorption peak loss generated by hydroxide ions with a content of only 0.0001 is as high as 33dB/km.
Where do these hydroxide ions come from? There are many sources of hydroxide ions. Firstly, the materials used to manufacture optical fibers contain moisture and hydroxide compounds, which are difficult to remove during the raw material purification process and ultimately remain in the form of hydroxide ions in the optical fibers; Secondly, the hydrogen and oxygen compounds used in the manufacturing of optical fibers contain a small amount of moisture; Thirdly, water is generated during the manufacturing process of optical fibers due to chemical reactions; The fourth is that the entry of external air brings water vapor. However, the manufacturing process has now developed to a considerable level, and the content of hydroxide ions has been reduced to a sufficiently low level that its impact on optical fibers can be ignored.
Post time: Oct-23-2025