The main factors causing fiber attenuation are: intrinsic, bending, extrusion, impurities, unevenness and docking. Intrinsic: is the inherent loss of fiber, including: Rayleigh scattering, inherent absorption. Bending: When the fiber is bent, part of the fiber will be lost due to scattering, resulting in loss. Extrusion: Loss caused by tiny bends in the fiber when it is squeezed. Impurities: Impurities in the fiber that absorb and scatter light propagating in the fiber. Unevenness: loss due to uneven refractive index of the fiber material. Docking: Loss caused when the fiber is docked, such as: different axes (single mode fiber coaxiality requirement is less than 0.8μm), the end face is not perpendicular to the axis, the end face is not flat, the butt diameter is not matched, and the welding quality is poor.
When light is incident from one end of the fiber and emitted from the other end, the intensity of the light is weakened. This means that after the optical signal propagates through the fiber, the light energy is attenuated. This means that some substances in the fiber or for some reason block the light signal from passing. This is the transmission loss of the fiber. Only by reducing the fiber loss can the optical signal be unobstructed.
Fiber loss can be roughly divided into the inherent loss of the fiber and the additional loss caused by the use conditions after the fiber is manufactured. The specific breakdown is as follows:
Fiber loss can be divided into intrinsic loss and additional loss. Intrinsic loss includes scattering loss, absorption loss and loss due to imperfect fiber structure. Additional loss includes microbend loss, bending loss and connection loss.
Among them, the additional loss is artificially caused during the laying process of the optical fiber. In practical applications, it is inevitable to connect the fibers one by one, and the fiber connection will cause losses. Micro-bending, squeezing, and tensile forces on the fiber can also cause losses. These are the losses caused by the conditions of use of the fiber. The main reason for this is that under these conditions, the transmission mode in the fiber core has changed. Additional losses can be avoided as much as possible. Below, we only discuss the inherent loss of the fiber.
In the intrinsic loss, the scattering loss and the absorption loss are determined by the characteristics of the fiber material itself, and the inherent losses caused by the different operating wavelengths are also different. To understand the mechanism of loss, and quantitatively analyze the loss caused by various factors, it is of great significance for the rational use of fiber for low loss fiber.
1. Absorption loss of materials
The material from which the fiber is made is capable of absorbing light energy. After the particles in the fiber material absorb the light energy, vibration and heat are generated, and the energy is dissipated, thus generating absorption loss. In an optical fiber, when an electron of a certain energy level is irradiated with light of a wavelength corresponding to the energy level difference, electrons located in a low-level orbital state will transition to an orbit of a high energy level. This electron absorbs light energy and produces absorption loss of light.
2, scattering loss
In the dark, a flashlight is illuminated into the air, and a beam of light can be seen. It has also been seen that the overnight aerial searchlights emit a large beam of light.
So why do we see these beams? This is because there are many tiny particles such as smoke and dust floating in the atmosphere, and the light is irradiated on these particles, causing scattering, and it is shot in all directions. This phenomenon was first discovered by Rayleigh, so people called this scattering "Rayleigh scattering."
How is scattering generated? The tiny particles of molecules, atoms, electrons, and the like that originally constitute the substance vibrate at some natural frequencies, and can emit light having a wavelength corresponding to the vibration frequency. The vibration frequency of a particle is determined by the size of the particle. The larger the particle, the lower the vibration frequency, and the longer the wavelength of the released light; the smaller the particle, the higher the vibration frequency, and the shorter the wavelength of the released light. This vibration frequency is called the natural vibration frequency of the particle. But this vibration is not self-generated, it requires a certain amount of energy. Once the particles are irradiated with light having a certain wavelength, and the frequency of the irradiated light is the same as the natural vibration frequency of the particles, resonance is caused. The electrons in the particle start to vibrate at the vibration frequency. As a result, the particle scatters light in all directions, and the energy of the incident light is absorbed and converted into the energy of the particle, and the particle re-extracts the energy in the form of light energy. Therefore, for those who observe outside, it seems that after the light hits the particle, it flies out in all directions.
There is also Rayleigh scattering in the fiber, and the resulting optical loss is called Rayleigh scattering loss. In view of the current level of fiber manufacturing technology, it can be said that Rayleigh scattering loss is unavoidable. However, since the Rayleigh scattering loss is inversely proportional to the fourth power of the light wavelength, the effect of Rayleigh scattering loss can be greatly reduced when the fiber operates in the long wavelength region.
3, congenitally insufficient, can not help
The structure of the optical fiber is imperfect. For example, there are bubbles, impurities, or unevenness in the thickness of the fiber, especially the core-cladding interface is not smooth. When light is transmitted to these places, some light is scattered to all directions, causing loss. . This loss can be overcome by trying to improve the process of fiber manufacturing. Scattering causes light to be directed in all directions, with a portion of the scattered light reflected back in the opposite direction of the fiber propagation, which is received at the incident end of the fiber. The scattering of light causes a portion of the light energy to be lost, which is undesirable. However, this phenomenon can also be used for us, because if we analyze the strength of the received light at the transmitting end, we can check the breakpoint, defect and loss of the fiber. In this way, through the ingenuity of the person, the bad thing becomes a good thing.
Fiber Loss In recent years, fiber optic communication has been widely used in many fields. An important issue in achieving fiber-optic communication is to reduce the loss of the fiber as much as possible. The so-called loss refers to the attenuation per unit length of the fiber, and the unit is dB/km. The fiber loss has a direct impact on the transmission distance or the distance between the relay stations. Therefore, understanding and reducing the loss of the fiber has great practical significance for fiber-optic communication.
4, the scattering loss of the fiber
The scattering inside the fiber reduces the power transmitted and generates losses. The most important of the scattering is Rayleigh scattering, which is caused by changes in density and composition inside the fiber material.
In the heating process of the optical fiber material, due to the thermal turbulence, the compressibility of the atoms is not uniform, the density of the material is not uniform, and the refractive index is not uniform. This unevenness is fixed during the cooling process and its size is smaller than the wavelength of the light wave. When light encounters these light-wavelengths, which are smaller than the wavelength of the light wave, with random fluctuations, the transmission direction is changed, scattering occurs, and loss is caused. In addition, uneven concentration of oxides contained in the optical fiber and uneven doping may cause scattering and loss.
5, waveguide scattering loss
This is due to the random distortion or coarse scattering of the interface, which is actually a mode transition or mode coupling caused by surface distortion or roughness. One mode produces other modes of transmission and radiation patterns due to the fluctuations in the interface. Due to the different modes of attenuation transmitted in the optical fiber, in the long-distance mode conversion process, the mode with small attenuation becomes a mode with large attenuation. After continuous transformation and inverse transformation, although the loss of each mode is balanced, The overall pattern produces additional losses, ie additional losses due to mode switching, which is the waveguide scattering loss. To reduce this loss, it is necessary to improve the fiber manufacturing process. For a well-drawn or high-quality fiber, this loss can basically be ignored.
6. Radiation loss due to fiber bending
The fiber is soft and can be bent, but after bending to a certain extent, although the fiber can guide light, it will change the transmission path of light. The transmission mode is converted into a radiation mode, so that a part of the light energy penetrates into the cladding layer or passes through the cladding layer to become a radiation mode and is leaked outward, thereby causing loss. When the bending radius is larger than 5 to 10 cm, the loss caused by the bending is negligible.
When light is incident from one end of the fiber and emitted from the other end, the intensity of the light is weakened. This means that after the optical signal propagates through the fiber, the light energy is attenuated. This means that some substances in the fiber or for some reason block the light signal from passing. This is the transmission loss of the fiber. Only by reducing the fiber loss can the optical signal be unobstructed.
Fiber loss can be roughly divided into the inherent loss of the fiber and the additional loss caused by the use conditions after the fiber is manufactured. The specific breakdown is as follows:
Fiber loss can be divided into intrinsic loss and additional loss. Intrinsic loss includes scattering loss, absorption loss and loss due to imperfect fiber structure. Additional loss includes microbend loss, bending loss and connection loss.
Among them, the additional loss is artificially caused during the laying process of the optical fiber. In practical applications, it is inevitable to connect the fibers one by one, and the fiber connection will cause losses. Micro-bending, squeezing, and tensile forces on the fiber can also cause losses. These are the losses caused by the conditions of use of the fiber. The main reason for this is that under these conditions, the transmission mode in the fiber core has changed. Additional losses can be avoided as much as possible. Below, we only discuss the inherent loss of the fiber.
In the intrinsic loss, the scattering loss and the absorption loss are determined by the characteristics of the fiber material itself, and the inherent losses caused by the different operating wavelengths are also different. To understand the mechanism of loss, and quantitatively analyze the loss caused by various factors, it is of great significance for the rational use of fiber for low loss fiber.
1. Absorption loss of materials
The material from which the fiber is made is capable of absorbing light energy. After the particles in the fiber material absorb the light energy, vibration and heat are generated, and the energy is dissipated, thus generating absorption loss. In an optical fiber, when an electron of a certain energy level is irradiated with light of a wavelength corresponding to the energy level difference, electrons located in a low-level orbital state will transition to an orbit of a high energy level. This electron absorbs light energy and produces absorption loss of light.
2, scattering loss
In the dark, a flashlight is illuminated into the air, and a beam of light can be seen. It has also been seen that the overnight aerial searchlights emit a large beam of light.
So why do we see these beams? This is because there are many tiny particles such as smoke and dust floating in the atmosphere, and the light is irradiated on these particles, causing scattering, and it is shot in all directions. This phenomenon was first discovered by Rayleigh, so people called this scattering "Rayleigh scattering."
How is scattering generated? The tiny particles of molecules, atoms, electrons, and the like that originally constitute the substance vibrate at some natural frequencies, and can emit light having a wavelength corresponding to the vibration frequency. The vibration frequency of a particle is determined by the size of the particle. The larger the particle, the lower the vibration frequency, and the longer the wavelength of the released light; the smaller the particle, the higher the vibration frequency, and the shorter the wavelength of the released light. This vibration frequency is called the natural vibration frequency of the particle. But this vibration is not self-generated, it requires a certain amount of energy. Once the particles are irradiated with light having a certain wavelength, and the frequency of the irradiated light is the same as the natural vibration frequency of the particles, resonance is caused. The electrons in the particle start to vibrate at the vibration frequency. As a result, the particle scatters light in all directions, and the energy of the incident light is absorbed and converted into the energy of the particle, and the particle re-extracts the energy in the form of light energy. Therefore, for those who observe outside, it seems that after the light hits the particle, it flies out in all directions.
There is also Rayleigh scattering in the fiber, and the resulting optical loss is called Rayleigh scattering loss. In view of the current level of fiber manufacturing technology, it can be said that Rayleigh scattering loss is unavoidable. However, since the Rayleigh scattering loss is inversely proportional to the fourth power of the light wavelength, the effect of Rayleigh scattering loss can be greatly reduced when the fiber operates in the long wavelength region.
3, congenitally insufficient, can not help
The structure of the optical fiber is imperfect. For example, there are bubbles, impurities, or unevenness in the thickness of the fiber, especially the core-cladding interface is not smooth. When light is transmitted to these places, some light is scattered to all directions, causing loss. . This loss can be overcome by trying to improve the process of fiber manufacturing. Scattering causes light to be directed in all directions, with a portion of the scattered light reflected back in the opposite direction of the fiber propagation, which is received at the incident end of the fiber. The scattering of light causes a portion of the light energy to be lost, which is undesirable. However, this phenomenon can also be used for us, because if we analyze the strength of the received light at the transmitting end, we can check the breakpoint, defect and loss of the fiber. In this way, through the ingenuity of the person, the bad thing becomes a good thing.
Fiber Loss In recent years, fiber optic communication has been widely used in many fields. An important issue in achieving fiber-optic communication is to reduce the loss of the fiber as much as possible. The so-called loss refers to the attenuation per unit length of the fiber, and the unit is dB/km. The fiber loss has a direct impact on the transmission distance or the distance between the relay stations. Therefore, understanding and reducing the loss of the fiber has great practical significance for fiber-optic communication.
4, the scattering loss of the fiber
The scattering inside the fiber reduces the power transmitted and generates losses. The most important of the scattering is Rayleigh scattering, which is caused by changes in density and composition inside the fiber material.
In the heating process of the optical fiber material, due to the thermal turbulence, the compressibility of the atoms is not uniform, the density of the material is not uniform, and the refractive index is not uniform. This unevenness is fixed during the cooling process and its size is smaller than the wavelength of the light wave. When light encounters these light-wavelengths, which are smaller than the wavelength of the light wave, with random fluctuations, the transmission direction is changed, scattering occurs, and loss is caused. In addition, uneven concentration of oxides contained in the optical fiber and uneven doping may cause scattering and loss.
5, waveguide scattering loss
This is due to the random distortion or coarse scattering of the interface, which is actually a mode transition or mode coupling caused by surface distortion or roughness. One mode produces other modes of transmission and radiation patterns due to the fluctuations in the interface. Due to the different modes of attenuation transmitted in the optical fiber, in the long-distance mode conversion process, the mode with small attenuation becomes a mode with large attenuation. After continuous transformation and inverse transformation, although the loss of each mode is balanced, The overall pattern produces additional losses, ie additional losses due to mode switching, which is the waveguide scattering loss. To reduce this loss, it is necessary to improve the fiber manufacturing process. For a well-drawn or high-quality fiber, this loss can basically be ignored.
6. Radiation loss due to fiber bending
The fiber is soft and can be bent, but after bending to a certain extent, although the fiber can guide light, it will change the transmission path of light. The transmission mode is converted into a radiation mode, so that a part of the light energy penetrates into the cladding layer or passes through the cladding layer to become a radiation mode and is leaked outward, thereby causing loss. When the bending radius is larger than 5 to 10 cm, the loss caused by the bending is negligible.
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