How to deal with fiber attenuation

Fiber Loss Coefficient

In order to measure the quality of the loss characteristics of a fiber, the concept of loss coefficient (or attenuation coefficient) is introduced here, that is, the decibel number of optical power reduction caused by the transmission unit length (1km) of fiber, and the loss is generally expressed by α Coefficient, the unit is dB/km.

Optical fiber automatic loss tester

Mathematical expression:

In the formula: L is the length of the fiber, in km; P1 and P2 are the input and output optical power of the fiber, respectively, in mW or μW.

Main factors causing fiber attenuation

Optical fiber attenuation is an important factor hindering the long-distance transmission of digital signals. The level of fiber loss directly affects the transmission distance or the distance between relay stations.

The main factors causing fiber attenuation are:

Intrinsic, bending, extrusion, impurities, unevenness and butt joint, etc.

Intrinsic: It is the inherent loss of the fiber, including: Rayleigh scattering, inherent 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: The loss caused by the slight bending of the optical fiber when it is squeezed.

Impurities: Impurities in the optical fiber absorb and scatter the light propagating in the optical fiber, resulting in losses.

Inhomogeneity: The loss caused by the inhomogeneity of the refractive index of the fiber material.

Butt joint: the loss generated when the fiber is docked, such as: misalignment (the concentricity of single-mode fiber is required to be less than 0.8 μm), the end face is not perpendicular to the axis, the end face is uneven, the butt core diameter does not match, and the welding quality is poor.

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:

●Fiber loss can be divided into inherent loss and additional loss.

●Intrinsic loss includes scattering loss, absorption loss and loss caused by imperfect fiber structure.

●Additional loss includes microbending loss, bending loss and connection loss.

Additional loss

The additional loss is artificially caused during the laying of the optical fiber. In practical applications, it is inevitable to connect the optical fibers one by one, and the optical fiber connection will cause loss. Microbending, extrusion, and stretching of optical fibers will also cause loss. These are the losses caused by the conditions of use of optical fibers. The main reason is that under these conditions, the transmission mode in the fiber core changes. Additional losses can be avoided as much as possible.

Additional losses include microbending losses, bending losses, and splicing losses.

There are two forms of fiber bending:

●The bend whose radius of curvature is much larger than the diameter of the optical fiber is called a bend or a macrobend;

●The optical fiber axis produces micron-level bending, and this high-frequency bending habit is called microbending.

intrinsic loss

Among the intrinsic losses, scattering loss and absorption loss are determined by the characteristics of the fiber material itself, and the intrinsic losses caused by different operating wavelengths are also different. It is of great significance 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.

Absorption loss

Optical fibers are made of materials that absorb light energy. After the particles in the optical fiber material absorb light energy, they vibrate and generate heat, and dissipate the energy, which results in absorption loss. We know that matter is composed of atoms and molecules, and atoms are composed of nuclei and electrons outside the nucleus, and electrons revolve around the nucleus in a certain orbit. This is just like the earth where we live, Venus, Mars and other planets revolve around the sun, each electron has a certain energy and is in a certain orbit, or each orbit has a definite energy level. Orbitals closer to the nucleus have lower energy levels, and orbitals farther away from the nucleus have higher energy levels. The size of this energy level difference between the orbitals is called the energy level difference. When an electron transitions from a low energy level to a high energy level, it must absorb the energy of the corresponding level of energy difference.

In an optical fiber, when electrons of a certain energy level are irradiated by light of a wavelength corresponding to the energy level difference, the electrons in the orbit of the low energy level will jump to the orbit of the high energy level. This electron absorbs the light energy, and light absorption loss occurs.

Silicon dioxide (SiO2), the basic material for making optical fibers, absorbs light itself, one is called ultraviolet absorption, and the other is called infrared absorption. At present, optical fiber communication generally only works in the 0.8-1.6μm wavelength region, so we only discuss the loss in this working region.

Fiber optic materials selectively absorb certain wavelengths of light, which can also cause attenuation or signal loss. The mechanism of absorbing light waves is similar to that of color development.

UV absorption loss

Ultraviolet absorption loss is the loss caused by the photon flow transmitted in the fiber that excites the electrons in the fiber material from a low energy level to a high energy level, and the energy in the photon flow will be absorbed by the electrons.

Infrared absorption loss

Infrared absorption loss is due to the interaction between the light wave propagating in the fiber and the lattice, a part of the energy of the light wave is transferred to the lattice, which intensifies its vibration, thus causing the loss

The absorption peak generated by electronic transition in quartz glass is around the wavelength of 0.1-0.2 μm in the ultraviolet region. As the wavelength increases, its absorption gradually decreases, but the influence area is very wide, until the wavelength above 1 μm. However, UV absorption has little effect on silica fibers operating in the infrared region. For example, in the visible light region with a wavelength of 0.6 μm, the ultraviolet absorption can reach 1dB/km, and it drops to 0.2-0.3dB/km at a wavelength of 0.8 μm, while at a wavelength of 1.2 μm, it is only about 0.1dB/km.

The infrared absorption loss of the quartz fiber is produced by the molecular vibration of the material in the infrared region. There are several vibration absorption peaks in the band above 2 μm.

Impurity absorption loss

Impurity absorption loss refers to the loss caused by the absorption of light by the harmful impurities in the optical fiber mainly including transition metal ions, such as iron, cobalt, nickel, copper, manganese, chromium, etc. and OH-.

Due to the influence of various doping elements in the fiber, it is impossible for the silica fiber to have a low-loss window in the band above 2 μm, and the theoretical limit loss at the wavelength of 1.85 μm is ldB/km.

Through research, it is also found that there are some "destructive molecules" in the quartz glass making troubles, mainly some harmful transition metal impurities, such as copper, iron, chromium, manganese and so on. Under the light irradiation, these "bad guys" greedily absorb light energy and jump around, causing the loss of light energy. Eliminating "troublemakers" and performing a high-level chemical purification of the materials used to make optical fibers can greatly reduce losses.

Another source of absorption in the silica fiber is the study of the hydroxide radical (OHˉ) period. It was found that the hydroxide radical has three absorption peaks in the optical fiber working band, which are 0.95 μm, 1.24 μm and 1.38 μm, of which the wavelength of 1.38 μm The absorption loss is the most serious and has the greatest impact on the fiber. At a wavelength of 1.38μm, the absorption peak loss caused by the hydroxyl radical with a content of only 0.0001 is as high as 33dB/km.

Where do these hydroxides come from? There are many sources of hydroxides. One is that there are moisture and hydroxides in the materials used to make optical fibers. The form of oxygen remains in the optical fiber; the second is that the hydroxides used to make the optical fiber contain a small amount of water; the third is that water is generated due to chemical reactions during the manufacturing process of the optical fiber; the fourth is that the entry of outside air brings water vapor. However, the manufacturing process has now advanced to such a high level that the hydroxide content has been reduced to a low enough level that its effect on the fiber is negligible.

atomic defect absorption loss

Usually in the manufacturing process of optical fiber, when the optical fiber material is subjected to some kind of thermal excitation or light radiation, a certain covalent bond will be broken to generate atomic defects. Light energy causes loss, and its peak absorption wavelength is about 630nm.

scattering loss

In the dark night, shine a flashlight into the sky, and you can see a beam of light. People have also seen thick beams of light from searchlights 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. This phenomenon was first discovered by Rayleigh, so people named this scattering "Rayleigh scattering".

Because of the total reflection of light, the light can be transmitted in the fiber core. Rough, irregular surfaces, even at the molecular level, can reflect light in random directions, a phenomenon known as diffuse reflection or light scattering. Characteristic is usually a variety of different reflection angles.

How does scattering occur? It turns out that tiny particles such as molecules, atoms, and electrons that make up matter vibrate at certain natural frequencies, and can emit light with 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 emitted light; the smaller the particle, the higher the vibration frequency, and the shorter the wavelength of the emitted light. This vibration frequency is called the natural vibration frequency of the particle. But this kind of vibration is not generated by itself, it needs a certain amount of energy. Once a particle is irradiated with light of a certain wavelength, and the frequency of the irradiated light is the same as the natural vibration frequency of the particle, resonance will be induced. The electrons in the particle start 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, and the particle emits the energy again in the form of light energy. Therefore, to a person observing from the outside, it appears as if the light hits the particle and is scattered 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 optical 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, the influence of the Rayleigh scattering loss can be greatly reduced when the fiber works in the long wavelength region.

Loss due to imperfect fiber structure

The optical fiber structure is not perfect, such as bubbles, impurities, or uneven thickness in the optical fiber, especially the core-cladding interface is not smooth, etc. When the light passes to these places, a part of the light will be scattered in all directions, causing loss . This kind of loss can be overcome by thinking of ways, that is to improve the process of optical fiber manufacturing.