The main factors that cause fiber attenuation are: intrinsic, bending, extrusion, impurities, non-uniformity and butt joint.
Intrinsic: It is the inherent loss of the fiber, including: Rayleigh scattering, intrinsic absorption, etc.
Bending: When the fiber is bent, part of the light in the fiber will be lost due to scattering, resulting in loss.
Squeeze: Loss caused by tiny bends in an optical fiber when it is squeezed.
Impurities: Losses caused by impurities in the fiber absorbing and scattering light propagating in the fiber.
Non-uniformity: Loss caused by non-uniform refractive index of the fiber material.
Docking: The loss generated when the optical fiber is docked, such as: different axes (the coaxiality of single-mode fiber is required to be less than 0.8μm), the end face is not perpendicular to the axis, the end face is not flat, the butt core diameter does not match and the welding quality is poor.
When light enters from one end of an optical fiber and exits from the other end, the intensity of the light decreases. This means that after the optical signal propagates through the fiber, the light energy is partially attenuated. This means that there is some substance in the fiber or for some reason that blocks the passage of the light signal. This is the transmission loss of the fiber. Only by reducing the fiber loss can the optical signal be made unimpeded.
Causes of Fiber Attenuation
Classification of Fiber Loss
Optical fiber loss can be roughly divided into the inherent loss of the optical fiber and the additional loss caused by the use conditions after the optical fiber is made. The specific breakdown is as follows:
Optical fiber loss can be divided into inherent loss and additional loss. Inherent loss includes scattering loss, absorption loss and loss caused by imperfect fiber structure, and additional loss includes microbending loss, bending loss and splice 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 optical fibers one by one, and the optical fiber connection will produce loss. The slight bending, extrusion, and tensile force of the optical fiber will also cause loss. These are 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 changes. Additional losses can be avoided as much as possible. Below, we only discuss the inherent loss of fiber.
Among the inherent losses, scattering loss and absorption loss are determined by the characteristics of the fiber material itself, and the inherent loss caused by different operating wavelengths is also different. It is very important to understand the mechanism of loss and quantitatively analyze the loss caused by various factors for the development of low-loss optical fiber and the rational use of optical fiber.
1. Absorption loss of materials
Optical fibers are made of materials that absorb light energy. After the particles in the fiber material absorb the light energy, they vibrate and generate heat, and dissipate the energy, thus resulting in absorption loss. In an optical fiber, when electrons of a certain energy level are irradiated with light of a wavelength corresponding to the energy level difference, electrons located in the orbits of lower energy levels will transition to orbits of higher energy levels. This electron absorbs light energy, resulting in light absorption loss.
2. Scattering loss
In the dark night, you can see a beam of light by shining a flashlight into the air. Large beams of light from searchlights have also been seen in the night sky.
So, why do we see these beams of light? This is because there are many tiny particles such as smoke and dust floating in the atmosphere, and the light irradiates on these particles, which scatters and shoots in all directions. This phenomenon was first discovered by Rayleigh, so people named this kind of scattering "Rayleigh scattering".
How does scattering occur? The tiny particles such as molecules, atoms, and electrons that make up matter vibrate at certain natural frequencies, and can emit light with wavelengths corresponding to the vibrational frequencies. The vibration frequency of a particle is determined by the size of the particle. The larger the particle, the lower the vibrational frequency, and the longer the wavelength of the light emitted; the smaller the particle, the higher the vibrational frequency, and the shorter the wavelength of the emitted light. This vibrational frequency is called the natural vibrational frequency of the particle. But this vibration is not self-generated, it requires a certain amount of energy.
Resonance occurs when a particle is exposed to light of a certain wavelength at the same frequency as the particle's natural vibration. The electrons in the particle start to vibrate at this vibrational 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, and the particle re-emits the energy in the form of light energy. Therefore, for those who observe from the outside, what they see is that the light hits the particles and scatters in all directions.
There is also Rayleigh scattering in the 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 loss is unavoidable. However, since the size of the Rayleigh scattering loss is inversely proportional to the fourth power of the light wavelength, when the optical fiber operates in the long wavelength region, the influence of the Rayleigh scattering loss can be greatly reduced.
3. The fiber structure is not perfect
The fiber structure is not perfect, such as air bubbles, impurities, or uneven thickness in the fiber, especially the core-cladding interface is not smooth. . This loss can be overcome, that is, to improve the fiber manufacturing process. Scattering makes the light radiate in all directions, and a part of the scattered light is reflected back in the opposite direction to the propagation of the fiber, and this part of the scattered light can be 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 intensity of the received light at the transmitting end, we can check the breakpoint, defect and loss of the fiber. In this way, through human ingenuity, bad things are turned into good things.
Optical 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. 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.
4. 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 can also cause scattering and loss.
5. 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.
6. 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.
