There are several reasons for fiber attenuation
1. The main factors causing fiber attenuation are: intrinsic, bending, extrusion, impurities, unevenness and docking.
Intrinsic: is the intrinsic loss of fiber, including: Rayleigh scattering, intrinsic absorption, etc.
Bending: When an optical fiber is bent, light in part of the optical fiber is scattered and lost, resulting in loss.
Extrusion: Loss caused by the slight bending of the optical fiber when it is squeezed.
Impurities: Losses caused by impurities in the fiber that absorb and scatter light propagating in the fiber.
Uneven: loss caused by uneven refractive index of fiber material.
Docking: Loss caused by optical fiber docking, such as: different coaxiality (coaxiality of single-mode optical fiber is required to be less than 0.8μm), end face is not vertical to the axis, end face is uneven, butt core diameter mismatch, and poor welding quality.
As light enters from one end of the fiber and exits from the other, it becomes less intense. This means that after the optical signal travels through the fiber, the light energy is partially attenuated. This means that there is something in the fiber or for some reason, blocking the light signal from passing through. This is the transmission loss of the fiber. Only by reducing the fiber loss can the optical signal be unimpeded.
Causes of optical fiber attenuation
2. Classification of fiber loss
Fiber loss can be roughly divided into the inherent loss of the fiber and the additional loss caused by the service conditions after the fiber is made. The breakdown is as follows:
Fiber loss can be divided into intrinsic loss and additional loss.
The inherent losses include scattering losses, absorption losses and losses caused by the imperfect structure of the fiber.
Additional losses include micro-bending loss, bending loss and connection loss.
Among them, the additional loss is artificially caused in the fiber laying process. In practice, it is inevitable to connect optical fibers one by one, and optical fiber connections will cause losses. Optical fibers may also suffer losses due to slight bending, extrusion, and tensile forces. These are the losses caused by optical fiber service conditions. The main reason is that the transmission mode in the optical fiber core changes under these conditions. Additional loss can be avoided as much as possible. In the following, we will only discuss the inherent losses of optical fibers.
Among the intrinsic losses, the scattering loss and absorption loss are determined by the characteristics of the fiber material itself, and the intrinsic losses caused by different working wavelengths are also different. It is very important to understand the mechanism of loss and quantitatively analyze the loss caused by various factors for developing low loss fiber and rationally using fiber.
3, material absorption loss
Optical fibers are made of materials that absorb light energy. After absorbing light energy, particles in optical fiber materials will vibrate and heat, and the energy will be lost, so the absorption loss.
We know that matter is made up of atoms and molecules, and atoms are made up of nuclei and electrons outside the nucleus, which rotate around the nucleus in certain orbits. Just like our Earth and planets like Venus and Mars revolve around the sun, each electron has a certain energy and is in a certain orbit, or each orbit has a certain energy level. Orbitals closer to the nucleus have lower energy levels, and orbitals farther away from the nucleus have higher energy levels. The magnitude of this energy level difference between orbitals is called the energy level difference. When an electron moves from a low energy level to a high energy level, it absorbs energy of the corresponding energy level difference.
In an optical fiber, when an electron in an energy level is illuminated by a wavelength corresponding to the energy level difference, the electron in the orbit of the lower energy level will transition to the orbit of the higher energy level. This electron absorbs light energy, resulting in an absorption loss of light.
Silicon dioxide (SiO2), the basic material for making optical fibers, absorbs light itself, one called ultraviolet absorption and the other called infrared absorption. At present, optical fiber communication generally only works in the wavelength region of 0.8 ~ 1.6μm, so we only discuss the loss in this working region.
The absorption peak produced by electronic transition in quartz glass is about 0.1 ~ 0.2μm wavelength in the ultraviolet region. With the increase of the wavelength, the absorption effect decreases gradually, but the influence region is very wide, until the wavelength above 1μm. However, the ultraviolet absorption has little effect on the quartz fiber operating in the infrared region. For example, in the visible region of 0.6μm wavelength, the UV absorption can reach 1dB/km, at 0.8μm wavelength down to 0.2-0.3dB /km, and at 1.2μm wavelength, only about 0.ldB/km.
The infrared absorption loss of quartz fiber is caused by the molecular vibration of the material in infrared region. There are several vibrational absorption peaks in the band above 2μm. Due to the influence of various doping elements in the fiber, it is impossible to have a low loss window in the band above 2μm, and the theoretical limit loss is ldB/km at 1.85μm wavelength.
Through the study, it was also found that there are some "destructive molecules" in the quartz glass, mainly some harmful transition metal impurities, such as copper, iron, chromium, manganese and so on. These "bad guys" under the light, greedy absorption of light energy, jumping around, causing the loss of light energy. Removing "troublemakers" and chemically purifying the materials used to make the fiber can greatly reduce losses.
Another absorption source in quartz fiber is OHˉ phase. It has been found that there are three absorption peaks in the working band of the fiber, which are 0.95μm, 1.24μm and 1.38μm, respectively. The 1.38μm wavelength has the most serious absorption loss and the biggest effect on the fiber. At the wavelength of 1.38μm, the absorption peak loss generated by the hydroxide content of only 0.0001 is as high as 33dB/km.
Where did all this hydroxide come from? There are many sources of hydroxide. First, there are water and hydroxide compounds in the optical fiber materials, which are not easy to be removed in the raw material purification process, and finally remain in the optical fiber in the form of hydroxide. Second, there is a small amount of water in the hydrogen and oxygen of the optical fiber; Three is the optical fiber manufacturing process due to chemical reactions generated water; Fourth, the entry of outside air brings water vapor. However, the manufacturing process has now advanced to such a high level that the hydroxide content is low enough that its effect on the fiber is negligible.
4. Scattering loss
In the dark, shine a flashlight into the air and you can see a beam of light. Thick columns of light from overnight aerial searchlights have also been seen. So why do we see these pillars of light? This is because there are a lot of smoke, dust and other small particles floating in the atmosphere, light shines on these particles, produced scattering, and shot in all directions. This phenomenon was first discovered by Rayleigh, so the scattering was named "Rayleigh scattering".
How does scattering come about? The molecules, atoms, electrons and other tiny particles that make up matter vibrate at certain natural frequencies and emit light with wavelengths corresponding to those frequencies. The vibrational frequency of a particle is determined by its size. The larger the particle, the lower the vibration frequency and the longer the wavelength of the light emitted. The smaller the particle, the higher the vibration frequency and the shorter the wavelength of the light emitted. This vibration frequency is called the natural vibration frequency of the particle. But this vibration is not spontaneous, it requires a certain amount of energy. Once a particle is exposed to light of a certain wavelength at the same frequency as the natural vibration frequency of the particle, resonance will be caused. The electrons in the particle begin to vibrate at that frequency. As a result, the particle scatterslight in all directions, and the energy of the incident light is absorbed and converted into the energy of the particle, which in turn emits the energy back in the form of light energy. So, to an outside observer, it looks as if the light has hit the particle and gone off in all directions.
Rayleigh scattering is also found in the fiber, and the resulting optical loss is called Rayleigh scattering loss. Given the current state of the art in fiber fabrication, Rayleigh scattering loss is unavoidable. However, because the Rayleigh scattering loss is inversely proportional to the fourth power of the wavelength, the influence of Rayleigh scattering loss can be greatly reduced when the fiber works in the long wavelength region.
5, congenital deficiency, helpless to help
The structure of the fiber is not perfect, such as bubbles, impurities in the fiber, or uneven thickness, especially the core-cladding interface is not smooth, etc., when the light reaches these places, a part of the light will be scattered to all directions, resulting in loss. This loss can be overcome by improving the manufacturing process of the fiber.
Scattering causes light to be emitted in all directions, and a part of the scattered light is reflected back in the direction opposite to the propagation of the fiber. This part of the scattered light can be received at the incident end of the fiber. The scattering of light causes a loss of light energy, which is undesirable. However, this phenomenon can also be used by us, because if we analyze the strength of the received beam at the transmitter, we can detect the breakpoints, defects and losses of the fiber.