Resolution: multiple causes of optical fiber attenuation

Resolution: multiple causes of optical fiber attenuation

1. The main factors that cause the attenuation of optical fiber include intrinsic, bending, extrusion, impurity, uneven and docking.

Intrinsic: it is the inherent loss of the optical fiber, including the secondary scattering and the inherent absorption.

Bending: When the optical fiber is bent, the light in some optical fiber will be lost due to scattering, resulting in loss.

Extrusion: the loss caused by slight bending when the optical fiber is squeezed.

Impurity: the loss caused by the absorption and scattering of impurities in the optical fiber.

Uneven: Loss Caused by uneven refractive index of the optical fiber.

Interconnection: The loss produced when the optical fiber is docked, such as: different axes (single-mode fiber coaxial requirements less than 0.8 μm), face and axis are not vertical, face is uneven, mismatch of connection cores and poor weld quality.

When light is injected from one end of the optical fiber and from the other end, the intensity of light decreases. This means that after the optical signal passes through the optical fiber, the optical energy degrades a part. This indicates that there are some substances in the optical fiber or, for some reason, block the optical signal from passing through. This is the optical fiber transmission loss. Only by reducing the optical fiber loss can the optical signal be unobstructed.

2. optical fiber loss Classification

The optical fiber loss can be roughly divided into the inherent loss of the optical fiber and the additional loss caused by the conditions of use after the optical fiber is made. The details are 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.

Additional losses include bending loss, bending loss, and connection loss.

Among them, additional losses are caused by people in the fiber laying process. In practical applications, it is inevitable to connect the optical fiber one by one, resulting in loss of optical fiber connections. Slight bending, extrusion, and stretching of optical fiber may also cause loss. These are the loss caused by the conditions of use of the optical fiber. The main reason is that the transmission mode in the fiber core has changed under these conditions. Additional losses can be avoided as much as possible. Next, we will only discuss the inherent loss of optical fiber.

In the inherent loss, the scattering loss and absorption loss are determined by the characteristics of the optical fiber material, and the inherent loss caused by different working wavelengths are also different. Finding out the mechanism of loss and quantitatively analyzing the loss caused by various factors is of great significance for the development of low-loss optical fibers and the rational use of optical fibers.

3. absorption loss of materials

Optical fiber materials can absorb light energy. After the particles in the optical fiber material absorb light energy, vibration and heat are generated, and the energy is lost, resulting in absorption loss.

We know that a substance is composed of atoms and molecules, and an atom is composed of an atomic nucleus and an external electron. electrons rotate around the nucleus in a certain orbit. This is like the earth we live in and the planet Venus, Mars, and other planets all rotate around the Sun. Every electron has a certain amount of energy in a certain orbit, or each orbit has a definite energy level. The orbital level close to the nucleus is low, and the orbital level farther away from the nucleus is higher. The energy level difference between tracks is called the energy level difference. When electrons transition from low-level to high-level, it is necessary to absorb the corresponding level of energy difference.

In an optical fiber, when an energy-level electron is exposed to light at the wavelength corresponding to the energy-level difference, the electron located in a low-level orbit will jump to a high-level orbit. This electron absorbs light energy, resulting in optical absorption loss.

Silica (SiO2), the basic material used to make optical fibers, absorbs light. One is ultraviolet absorption and the other is infrared absorption. Currently, optical fiber communication only works in 0.8 ~ 1.6 μm wavelength zone, so we only discuss the loss of this work zone.

The absorption peak of electronic transition in the silica glass is 0.1 ~ About 0.2 μm wavelength. As the wavelength increases, the absorption effect gradually decreases, but the affected area is very wide until the wavelength above 1 μm. However, UV absorption has little impact on the work of the Z optical fiber in the infrared area. For example, in a visible zone with a wavelength of 0.6 μm, the UV absorption can reach 1 dB/km, and the wavelength can be reduced to 0.8 ~ 0.2 at a wavelength of μm ~ 1.2/km, and only 0.ldB/ km at μm wavelength.

The infrared absorption loss of the Z optical fiber is produced by the molecular vibration of the Infrared Zone material. There are several vibration absorption peaks in the band above 2 μm. Due to the influence of various doping elements in the optical fiber, low loss Windows cannot appear in the band above 2 μm. The theoretical ultimate loss at 1.85 μm wavelength is ldB/km.

Through research, it is also found that some "destructive molecules" are in disorder in the silica glass, mainly some harmful transition metal impurities, such as copper, iron, chromium, manganese and so on. Under the light, these "bad guys" greedy to absorb light energy and make a leap, resulting in the loss of light energy. Remove "troublemakers" and perform chemical purification of the materials used to manufacture optical fibers, which can greatly reduce the loss.

Another absorption source in the Z optical fiber is the study of the OH phase. It is found that there are three absorption peaks in the working band of the optical fiber, they are 0.95 μm, 1.24 μm, and 1.38 μm respectively, of which 1.38 μm wavelength has the most serious absorption loss and has the largest impact on optical fibers. At the wavelength of 1.38 μm, the loss of the absorption peaks produced by 0.0001 of the hydrogen and oxygen roots is as high as 33dB/km.

Where did these hydrogen and oxygen come from? There are many sources of hydrogen oxides. First, water and hydrogen oxides are in the materials used to make optical fibers. These hydrogen oxides are not easy to be removed during the purification process of raw materials, at last, water remains in the optical fiber in the form of hydrogen and oxygen. Second, the hydrogen and oxygen in the optical fiber manufacturing process contain a small amount of water. Third, water is generated due to chemical reaction during the optical fiber manufacturing process; fourth, the entry of the outside air brings water vapor. However, the current manufacturing process has developed to a very high level, and the content of hydrogen and oxygen root has been reduced to a sufficiently low level, and its impact on optical fiber is negligible.

4. scattering loss

In the dark, use a flashlight to shine into the air, you can see a beam. People once saw an overnight sky searchlight with a large light column. So why do we see these columns? This is because many small particles, such as smoke and dust, are floating in the atmosphere, and light is shining on these particles to produce scattering, which then emits light in all directions. This phenomenon was first discovered by RUILI, so people name this scattering as "Ruili Scattering ".

How is scattering produced? Small particles, such as molecules, atoms, and electrons, that originally constitute a substance, vibrate at certain natural frequencies and can release the light at the wavelength corresponding to the vibration frequency. The particle vibration frequency is determined by the particle size. The larger the particle, the lower the vibration frequency, the longer the wavelength of the released light. The smaller the particle, the higher the vibration frequency, the shorter the wavelength of the released light. This vibration frequency is called the inherent vibration frequency of particles. However, this vibration is not produced by itself, and it requires a certain amount of energy. Once a particle is illuminated by light with a certain wavelength, and the radiation frequency is the same as the inherent vibration frequency of the particle, it will cause resonance. The electrons in the particle start to vibrate at this vibration frequency. As a result, the particle emits light from all directions, and the energy of the incident light is absorbed and converted to the energy of the particle, the particle emits energy again in the form of light energy. Therefore, for those who observe the outside, it seems that after the light hits the particle, it is scattered in all directions.

The optical fiber also has a Ruili scattering, and the resulting optical loss is called the Ruili scattering loss. In view of the current optical fiber manufacturing technology level, it can be said that the scattering loss is unavoidable. However, due to the inverse relationship between the size of the Ar scattering loss and the four quadrants of the optical wavelength, the influence of the Ar scattering loss can be greatly reduced when the optical fiber operates in the long wavelength zone.

5. inherent deficiency: AI cannot help

The optical fiber structure is incomplete, such as bubbles or impurities in the optical fiber, or uneven thickness, especially when the core-package layer interface is not smooth, when the light is transmitted to these places, there will be a part of light scattering to all directions, resulting in loss. This kind of loss can be overcome, that is, to improve the optical fiber manufacturing process.

Scattering emits light in all directions. Some of the scattered light is reflected back in the opposite direction of the optical fiber propagation, and can be received at the incident end of the optical fiber. The scattering of light causes the loss of some light energy, which is not expected by people. However, this phenomenon can also be used by us, because if we analyze the strength of the split received by the sender, we can check the breakpoint, defect, and loss of the optical fiber. In this way, people turn bad things into good things through their intelligence.