The main factors that cause fiber attenuation are: intrinsic, bending, extrusion, impurities, non-uniformity, and docking. Intrinsic: is the inherent loss of optical fiber, including Rayleigh scattering, intrinsic absorption and so on. Bend: When the fiber bends, some of the light in the fiber is lost due to scattering, causing loss. Extrusion: The loss caused by slight bending of an optical fiber when it is squeezed. Impurities: Losses caused by absorption and scattering of light in the fiber by impurities in the fiber. Inhomogeneity: The loss due to the non-uniform refractive index of the optical fiber material. Docking: Loss caused when the optical fiber is docked, such as: different axes (single-mode fiber coaxiality requirements less than 0.8μm), the end surface is not perpendicular to the axis, the end surface is uneven, the mating heart diameter is not matched and the welding quality is poor.
When light enters from one end of the fiber and exits from the other end, the light intensity decreases. This means that after the optical signal propagates through the fiber, the energy of the light decays. This shows that there are certain substances in the optical fiber or for some reason, blocking the passage of light signals. This is the transmission loss of the fiber. Only by reducing the fiber loss can the optical signal be unblocked.
Optical fiber loss can be roughly divided into the inherent loss that the optical fiber has and the additional loss caused by the use conditions after the optical fiber is made. Specific breakdown is as follows:
Optical fiber loss can be divided into inherent loss and additional loss. Intrinsic loss includes scattering loss, absorption loss and loss due to imperfect optical fiber structure. Additional loss includes microbend loss, bend loss and connection loss.
Among them, the additional loss is artificially caused during the laying of the optical fiber. In practical applications, it is unavoidable to connect the optical fibers one by one, and the optical fiber connection will cause loss. Optical fiber bending, extrusion, and tensile stress can also cause losses. These are losses caused by fiber usage conditions. The main reason is that under these conditions, the transmission mode in the fiber core has changed. Additional losses can be avoided as much as possible. In the following, we will only discuss the inherent loss of the fiber.
Intrinsic loss, scattering loss and absorption loss are determined by the characteristics of the optical fiber material itself, and the inherent losses caused by different operating wavelengths are also different. To understand the mechanism of loss, quantitative analysis of the loss caused by various factors, for the development of low-loss optical fiber use of fiber is of great significance.
1, the absorption loss of the material
Fiber-optic materials absorb light energy. After the particles in the optical fiber material absorb light energy, vibrations and heat are generated and the energy is lost, so that absorption loss occurs. In an optical fiber, when a certain level of electrons is irradiated with light of a wavelength corresponding to the energy level difference, the electrons located on the orbit of the low energy level will transition to the orbit having a high energy level. This electron absorbs light energy and produces light absorption loss.
2, scattering loss
In the darkness, a flashlight shines into the air and a beam of light can be seen. People have also seen the overnight searchlights send out large beams of light.
So why do we see these columns of light? This is because there are many tiny particles of smoke, dust, etc. that float in the atmosphere. The light shines on these particles, scattering occurs, and they shoot in all directions. This phenomenon was first discovered by Rayleigh, so people named this scattering as "Rayleigh scattering."
How does scattering occur? The molecules, atoms, electrons, and other minute particles that make up the substance oscillate at certain natural frequencies, and can emit light whose wavelength corresponds to the vibration frequency. The frequency of particle vibration is determined by the size of the particles. The larger the particles, the lower the vibration frequency, and the longer the wavelength of the emitted light; the smaller the particles, the higher the vibration frequency and the shorter the wavelength of the released light. This vibrational frequency is called the natural vibration frequency of the particle. However, this kind of vibration is not generated by itself. It requires a certain amount of energy. Once a particle is irradiated with light having a certain wavelength, and the frequency of the irradiated light is the same as the natural vibration frequency of the particle, resonance occurs. The electrons in the particle begin to vibrate at this vibration frequency. As a result, the particle scatters light in all directions. The energy of the incident light is absorbed and converted into the energy of the particle. The particle then emits the energy again in the form of light energy. Therefore, for those who observe outside, it is as if the light hits the particles and flies out in all directions.
There is also Rayleigh scattering in the optical fiber, and the resulting optical loss is called Rayleigh scattering loss. Given the current level of fiber manufacturing technology, it can be said that Rayleigh scattering losses cannot be avoided. However, since the size of the Rayleigh scattering loss is inversely proportional to the fourth power of the optical wavelength, the influence of Rayleigh scattering loss can be greatly reduced when the optical fiber operates in the long wavelength region.
3, congenitally inadequate, can not help
The structure of the optical fiber is not perfect, for example, there are bubbles, impurities, or uneven thickness in the optical fiber, especially the core-clad interface is not smooth, etc. When the light is transmitted to these places, part of the light will be scattered in all directions, causing loss. . This loss can be overcome by some means, that is, to improve the fiber manufacturing process. Scattering causes the light to be directed in all directions. A part of the scattered light is reflected back in the opposite direction to the propagation of the fiber. This scattered light can be received at the incident end of the fiber. The scattering of light causes some of the light energy to be lost, which is undesirable. However, this phenomenon can also be used by us because if we analyze the strength of the received light at the sending end, we can check the breakpoints, defects and losses of this fiber. In this way, turning bad things into good things is done through human intelligence.
Optical fiber loss In recent years, optical fiber communications have been widely used in many fields. One important problem in realizing optical fiber communication is to reduce the loss of the optical fiber as much as possible. The so-called loss refers to the attenuation per unit length of the fiber in dB/km. The loss of the optical fiber directly affects the distance between the transmission distance or the relay station separation distance. Therefore, it is of great practical significance to understand and reduce the loss of the optical fiber.
4, the optical fiber scattering loss
Scattering inside the fiber will reduce the transmitted power and cause losses. The most important scattering is Rayleigh scattering, which is caused by changes in density and composition inside the optical fiber material.
During the heating process of the optical fiber material, due to thermal turbulence, the compressive properties obtained by the atoms become non-uniform, the density of the material is not uniform, and the refractive index is not uniform. This unevenness is fixed during cooling and its size is smaller than the wavelength of light waves. When the light is encountered in transmission, these wavelengths are smaller than the wavelength of the light wave. When the non-uniform substance with random fluctuations occurs, the transmission direction is changed, scattering occurs, and loss is caused. In addition, the uneven concentration of oxides contained in the optical fiber and uneven doping can also cause scattering, resulting in losses.
5, waveguide scattering loss
This is due to the random distortion of the interface or the scattering produced by the roughness, in fact it is a mode transition or mode coupling caused by surface distortion or roughness. One mode will produce other transmission modes and radiation modes due to the interface fluctuations. Because the attenuation of various modes transmitted in the optical fiber is different, in the long-distance mode conversion process, the mode with small attenuation becomes a mode with large attenuation. After continuous conversion and inverse conversion, although the loss of each mode will be balanced, The overall mode losses produce additional losses, ie, additional losses due to mode transitions are the waveguide scattering losses. To reduce this loss, we must improve the fiber manufacturing process. For a well-pulled or high-quality fiber, this loss can be basically ignored.
6, radiation loss caused by fiber bending
The fiber is flexible and can be bent. However, after being bent to a certain degree, the fiber can guide light, but it will change the light transmission path. 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 to become leaked out of the radiation mold, and losses occur. When the bending radius is greater than 5 to 10 cm, the loss due to bending can be ignored.
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