Fiber Attenuation Analysis

1 Optical fiber attenuation mechanism

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.

2 Fiber loss factor

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 number of decibels caused by the reduction of optical power caused by the transmission of a unit length (1km) of fiber, generally using α to represent the loss Coefficient in dB/km.

Optical fiber automatic loss tester

Mathematical expression:

Where: 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.

3 Main factors causing fiber attenuation

Optical fiber attenuation is an important factor that hinders the long-distance transmission of digital signals. The level of fiber loss directly affects the transmission distance or the distance between repeaters.

The main factors that cause fiber attenuation are:

Intrinsic, Bending, Extrusion, Impurity, Inhomogeneity and Butt etc.

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.

4 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.

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

●Additional loss includes microbend loss, bending loss and splice loss.

4.1 Additional loss

The additional loss is artificially caused during the laying of the 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.

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

Bending of Fiber

There are two forms of fiber bending:

●Bends with a radius of curvature much larger than the diameter of the fiber, we are used to call them bends or macrobends;

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

4.2 Inherent loss

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.

4.2.1 Absorption loss

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. We know that matter is composed of atoms and molecules, and atoms are composed of nuclei and extranuclear electrons. The electrons revolve around the nucleus in a certain orbit. This is just like the earth where we live and the planets such as Venus and Mars all 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 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 transitions from a low energy level to a high energy level, it absorbs the energy of the corresponding level of energy difference.

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.

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

Optical fiber materials selectively absorb certain wavelengths of light, which can also cause attenuation or signal loss. The mechanism by which light waves are absorbed is similar to the mechanism by which color appears.

UV absorption loss

Ultraviolet absorption loss is the loss caused by the photon flow transmitted in the fiber that excites 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 the loss caused by the interaction of the light wave propagating in the fiber with the lattice, and part of the light wave energy is transferred to the lattice, which intensifies its vibration.

The absorption peak generated by electronic transition in quartz glass is about 0.1-0.2 μm wavelength in the ultraviolet region. As the wavelength increases, its absorption gradually decreases, but the affected area is wide, up to wavelengths 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 0.6μm, the ultraviolet absorption can reach 1dB/km, and at a wavelength of 0.8μm, it drops to 0.2-0.3dB/km, and at a wavelength of 1.2μm, it is only about 0.1dB/km.

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

Impurity absorption loss

Impurity absorption loss refers to the loss caused by the absorption of light by 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 wavelength band above 2 μm, and the theoretical limit loss at the wavelength of 1.85 μm is 1dB/km.

Through research, it is 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. Under the irradiation of light, these "bad guys" greedily absorb light energy and jump around, causing the loss of light energy. Removing "trouble molecules" and chemically purifying the material from which the fiber is made can greatly reduce losses.

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

Where do these hydroxides come from? There are many sources of hydroxides. First, there are moisture and hydroxides in the materials for making optical fibers. These hydroxides are not easy to be removed during the purification process of the raw materials, and they are still used as hydrogen in the end. The form of oxygen radicals remains in the optical fiber; the second is that the hydroxide used to manufacture the optical fiber contains a small amount of moisture; 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 level low enough that its effect on the fiber is negligible.

Atomic defect absorption loss

Usually in the fiber manufacturing process, when the fiber material is subjected to some thermal excitation or optical radiation, a covalent bond will be broken and atomic defects will be generated. At this time, the lattice is easily vibrated under the action of the optical field, thereby absorbing Light energy, causing loss, its peak absorption wavelength is about 630nm.

4.2.2 Scatter 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".

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

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.

4.2.3 Loss due to imperfect fiber structure

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.