What are the explanations of fiber loss
In recent years, optical fiber communication has been widely used in many fields. To realize optical fiber communication, an important issue is to reduce the loss of optical fiber as much as possible. So, what are the explanations for fiber loss? Let's go to find out together.
First, the absorption loss of optical fiber
This is caused by the absorption of light energy by fiber materials and impurities. They consume light energy in the form of heat energy in the fiber, which is an important loss in fiber loss. The absorption loss includes the following:
1. Substance intrinsic absorption loss This is the loss caused by the intrinsic absorption of the substance. It has two frequency bands, one is in the 8-12 μm region of the near-infrared, and the intrinsic absorption of this band is due to vibration. The intrinsic absorption band of another substance is in the ultraviolet band. When the absorption is strong, its tail will be dragged to the 0.7-1.1 μm band.
2. Absorption loss caused by dopants and impurity ions Optical fiber materials contain transition metals such as iron, copper, chromium, etc., which have their own absorption peaks and absorption bands and vary with their valence states. The fiber loss caused by the absorption of transition metal ions depends on their concentration. In addition, the presence of OH- also produces absorption loss, the basic absorption peak of OH- is around 2.7 μm, and the absorption band is in the range of 0.5-1.0 μm. For pure silica fiber, the effect of loss caused by impurities can be ignored.
3. Atomic defect absorption loss Optical fiber material will be excited to produce atomic defects due to heat or strong radiation, resulting in light absorption and loss, but generally this effect is small.
2. Optical fiber scattering loss
The scattering inside the fiber will reduce the transmitted power and cause loss. The most important of scattering is Rayleigh scattering, which is caused by changes in density and composition inside the fiber material.
During the heating process of the optical fiber material, due to the thermal disturbance, the compressibility of the atoms is not uniform, the density of the material is not uniform, and the refractive index is not uniform. This inhomogeneity is fixed during the cooling process, and its size is smaller than the wavelength of light. When light encounters these non-uniform substances with random fluctuations smaller than the wavelength of the light wave during transmission, it changes the transmission direction, produces scattering, and causes loss. In addition, the uneven concentration of oxides contained in the optical fiber and uneven doping also cause scattering and loss.
3. Waveguide scattering loss
This is scattering due to random distortion or roughness at the interface, and it is actually mode switching or mode coupling caused by surface distortion or roughness. One mode will give rise to other modes of transmission and radiation due to the fluctuation of the interface. Due to the different attenuation of various modes transmitted in the fiber, in the process of long-distance mode conversion, the mode with small attenuation becomes the mode with large attenuation. After continuous transformation and inverse transformation, although the loss of each mode will be balanced, but The mode generally produces additional loss, that is, additional loss due to mode conversion, and this additional loss is the waveguide scattering loss. To reduce this loss, it is necessary to improve the optical fiber manufacturing process. For well-drawn or high-quality fibers, this loss is essentially negligible.
Fourth, the radiation loss caused by fiber bending
Optical fibers are flexible and can be bent, but after bending to a certain extent, although optical fibers can guide light, the transmission path of light will change. The transmission mode is converted into a radiation mode, so that a part of the light energy penetrates into the cladding or passes through the cladding as a radiation mode that leaks out and is lost, resulting in loss. When the bending radius is greater than 5-10cm, the loss caused by bending can be ignored.
The so-called loss refers to the attenuation per unit length of the fiber, and the unit is dB/km. The level of optical fiber loss directly affects the transmission distance or the distance between repeater stations. Therefore, it is of great practical significance to understand and reduce the loss of optical fiber for optical fiber communication.