Classification of Fiber Loss

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 intrinsic 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 splicing 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 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. Below, we only discuss the inherent loss of optical fiber.

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.

1. Material 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. 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.

2. 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".

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.

3. The optical fiber structure is not perfect

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. Scattering makes the light shoot in all directions, and part of the scattered light is reflected back along the direction opposite 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 part of the light energy to be lost, which is undesirable. However, this phenomenon can also be used by us, because if we analyze the intensity of the received light at the sending end, we can check out the breakpoint, defect and loss of this optical 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 optical fiber, and the unit is dB/km. The level of optical fiber loss directly affects the transmission distance or the distance between relay stations. Therefore, understanding and reducing the loss of optical fiber has great practical significance for optical fiber communication.

4. Scattering loss of optical fiber

Scattering inside the fiber will reduce the transmitted power and cause loss. The most important of the scattering is Rayleigh scattering, which is caused by the density and composition changes 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 substance is not uniform, and the refractive index is not uniform. This inhomogeneity is fixed during cooling, and its size is smaller than the wavelength of the light wave. When light encounters these inhomogeneous substances with random fluctuations that are smaller than the wavelength of the light wave during transmission, the transmission direction is changed, scattering occurs, and loss is caused. In addition, the uneven concentration of oxides contained in the optical fiber and uneven doping will also cause scattering and loss.

5. Waveguide scattering loss

This is due to scattering due to random distortion or roughness of the interface, in fact it is mode conversion or mode coupling caused by surface distortion or roughness. One mode will generate other transmission modes and radiation modes due to the undulation of the interface. Due to the different attenuation of various modes transmitted in the optical fiber, during the long-distance mode conversion process, the mode with small attenuation becomes the mode with large attenuation. After continuous conversion and inverse conversion, although the loss of each mode will be balanced, but The overall mode produces additional loss, that is, the additional loss is generated due to the conversion of the mode, and this additional loss is the waveguide scattering loss. To reduce this loss, it is necessary to improve the optical fiber manufacturing process. For a well-drawn or high-quality fiber, this loss can basically be ignored.