Shulou(Shulou.com)11/24 Report--

In Europe more than three hundred years ago, Newton set up such a game on a sunny afternoon.

Let the sun project on the prism, through the prism, the light scattered into a ribbon of red, orange, yellow, green, cyan, blue and purple, projected onto a curtain in the house. In this way, the seemingly transparent sunlight changes into an incredible color band under the blessing of the prism.

After that, Newton opened a vertical crack in the middle of the curtain and placed a second prism and a second curtain behind the curtain.

He turned the first prism and projected seven ribbons of red, orange, yellow, green, cyan, blue and purple onto the cracks in the first curtain, and then projected through the second prism onto the second curtain. The miracle was born, and there was a single color of light on the second curtain in turn. The schematic diagram is as follows:

At this point, the sunlight was separated into a variety of single colors on the second curtain, and Lord Niu used a prism to break the sky: light can be dispersed! The light of the sun seems to be sealed, and there is a colorful core under the ordinary appearance. This is what we often call the dispersion of light.

1. How is the dispersion produced?

In the prism experiment, sunlight (that is, compound light) enters the glass from the air, and then into the air from the glass, which refracts twice. You know, everything is profit-oriented, and when refraction occurs, light will naturally choose the shortest path and move forward with as little energy loss as possible. From Newton's prism experiments above, we know that composite light is essentially composed of single light of many different colors, which have different wavelengths, and the energy of different wavelengths is very different. It is difficult to adjust, and different wavelengths of light differ on how to choose the path after refraction, so they "go their separate ways" after coming out of the prism.

So, why is the light scattered? It turns out that it is the wavelength of light that causes this kind of dispersion. The refractive index of light of different wavelengths in the medium is different, and the propagation speed (path) is also different, which will inevitably cause the dispersion of light to spread, and the dispersion will be formed.

The dispersion of light shows that the speed of light propagation in the medium has a great relationship with the refractive index. The higher the refractive index, the smaller the speed of light. See the following formula:

2. The influence of dispersion although dispersion can help us enter a colorful world, in the field of communication, dispersion is really not so beautiful.

In the process of optical signal transmission in optical fiber, dispersion is one of the important factors leading to loss.

This is due to the dispersion caused by the refractive index of the light, which leads to the intersymbol interference of the optical pulse, which leads to broadening at the output end.

What is broadening?

Broadening is the increase of the spectral width of different wavelengths of light in the medium due to different refractive index. In other words, when a beam of light is transmitted in the medium, some light waves have a large refractive index and seriously deviate from the runway.

Some light waves have a small refractive index, and although they are crooked, they can move forward in a given direction.

The disharmony of the light waves causes the width of this beam of light to be larger than it was before entering the medium, resulting in broadening.

In the case of dispersion, the longer the distance of optical signal transmission, the more serious the broadening, the consequence is signal distortion, bit error rate performance deterioration, seriously affecting the quality of information transmission.

In view of the impact of dispersion on communication, how to avoid it?

3. How to avoid the influence of dispersion? There is a good saying in the movie the Lion King: all life in the world lives in a delicate balance.

After a long time of exploration and research, people have found a way to balance the loss of dispersion by compensation. Among a variety of compensation methods, dispersion compensation fiber technology is a highly recognized dispersion compensation method.

In the common single-mode fiber system, the working wavelength of the fiber has a high positive dispersion at 1550nm.

The characteristic of positive dispersion: with the increase of wavelength, the refractive index decreases gradually.

According to the idea of compensation, it is necessary to add negative dispersion to these fibers to compensate for dispersion to ensure that the total dispersion of the whole optical fiber line is approximately zero. Dispersion compensation fiber (DCF) is a new type of single-mode fiber designed mainly for 1550nm wavelength, which has high negative dispersion at 1550nm (negative dispersion is opposite to positive dispersion). It can be used to compensate dispersion in ordinary single-mode fiber system. As shown in the following figure, the sum of positive and negative dispersion compensated at 1550nm approaches zero.

The following figure shows the formula of dispersion compensation fiber applied to single-mode fiber.

In practical application, the transmission line uses DCF and single-mode fiber in series to compensate the positive dispersion of single-mode fiber at 1550nm wavelength, so as to prolong the relay distance and reduce the loss, so as to achieve high-speed, large-capacity and long-distance communication. As shown in the following figure:

As a dispersion compensation, DCF has the following advantages:

The compensation effect is remarkable and the system works stably.

It is easy to operate, and the compensation can be realized by directly connecting the compensation optical fiber to the transmission system.

The amount of dispersion compensation can be controlled on demand, and it can be adjusted according to the actual amount of compensation needed by the transmission system.

Friendly reminder: if the optical signal runs a long distance on the transmission line, it will also cause other losses, such as line attenuation. In order to avoid line attenuation, it is necessary to consider the use of EDFA (Erbium-Doped Fiber Amplifier) bait-doped fiber amplifier.

This article comes from the official account of Wechat: ZTE document (ID:ztedoc), author: ZTE document

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