More than three hundred years ago in Europe, on a sunny afternoon, Newton laid out such an innings.
Let the sunlight project onto the prism, and after penetrating the prism, the light spreads out into a ribbon of red, orange, yellow, green, blue, and purple, which is projected onto a curtain in the room. In this way, the seemingly transparent sunlight changes into an incredible ribbon under the blessing of the triangular prism.
After that, Newton opened a vertical crack in the middle of the curtain, and placed a second triangular prism and a second curtain behind the curtain.
Only to see him turn the first triangular prism, projecting seven ribbons of red, orange, yellow, green, blue, and purple onto the crack of the first curtain in turn, and then through the second triangular prism onto the second curtain. A miracle was born, only to see that the second curtain was presented in turn with a single color of light. The schematic diagram is as follows:
At this point, the sunlight was separated into a variety of single colors presented on the second curtain, and Lord Niu used the triangular prism to break the heavenly machine: light can be dispersed! The sunlight seems to be sealed, and there is a richly colored core under the ordinary appearance. This is what we now often call the dispersion of light.
How is dispersion generated?
In the triangular prism experiment, sunlight (that is, composite light) enters the glass from the air, and then enters the air from the glass, and refraction occurs twice. You know, everything has profitability, and when refraction occurs, light will naturally choose the shortest path to move forward with minimal energy loss. From Newton's triangular prism experiment above, we know that composite light is essentially composed of many different colors of single light, which have different wavelengths, and the energy size of different wavelengths of light is very different. It is difficult to adjust, and different wavelengths of light have differences on how to choose the path after refraction, so after the triangular prism, they "parted ways".
So, why does the light disperse? Originally, what causes this dispersion is the wavelength of light, the refractive index of different wavelengths of light in the medium is different, and the propagation speed (path) is also different, which will inevitably cause the dispersion of light (we), and dispersion will be formed.
The dispersion phenomenon of light shows that the speed of propagation of light in the medium has a great relationship with the refractive index, the greater the refractive index, the smaller the speed of light, see the following formula:
The effect 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 fact that the refractive index of light causes dispersion, which causes the light pulse to produce inter-code interference, resulting in widening at the output.
What is broadening?
Widening is the expansion of light of different wavelengths in the medium due to different refractive indexes resulting in different propagation speeds, resulting in an increase in the spectral width. In other words, when a beam of light is transmitted in a 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 also move in the established direction.
The discordant phenomenon of light waves causes this beam of light to be wider than before entering the medium, forming a widening.
In the case of dispersion, the farther the optical signal transmission distance, the more serious the widening, the consequence is that the signal is distorted, the bit error rate performance deteriorates, and the transmission quality of the information is seriously affected.
How to avoid the impact of dispersion on communication? 3
How to avoid the effects of dispersion?
There's a good saying in the movie The Lion King: All life in the world survives in a delicate balance.
After a long period of exploration and research, people have found a way to balance the loss of dispersion with compensation. Among the various compensation methods, dispersion compensation optical fiber technology is a relatively recognized dispersion compensation method.
In ordinary single-mode fiber systems, the operating wavelength of the fiber has a high positive dispersion at 1550 nm.
The characteristics of positive dispersion: as the wavelength increases, the refractive index gradually decreases.
According to the idea of compensation, it is necessary to increase negative dispersion in these optical fibers for dispersion compensation to ensure that the total dispersion of the entire optical fiber line is approximately zero. The dispersion compensation fiber (DCF) is a new type of single-mode fiber designed mainly for 1550nm wavelength, which has a high negative dispersion at 1550nm (negative dispersion is the opposite of positive dispersion), which can be used for dispersion compensation in ordinary single-mode fiber systems, as shown in the following figure, the sum of positive and negative dispersions compensated at 1550nm tends to zero.
The following figure shows the formula for applying dispersion compensation fibers to single-mode fibers.
In practical applications, the transmission line adopts DCF and single-mode optical fiber series to compensate for the positive dispersion of single-mode optical fiber at 1550nm optical wavelength, so as to extend the relay distance and reduce the loss, so as to achieve high-speed, large-capacity, long-distance communication. As shown in the following figure:
As 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 fiber to the transmission system.
The amount of dispersion compensation can be controlled on demand, and the amount of compensation actually needed by the transmission system can be adjusted on demand
Friendly reminder: The optical signal runs a long distance on the transmission line, and other losses will be generated, such as line attenuation. In order to avoid line attenuation, it is necessary to consider using an EDFA (Erbium-Doped Fiber Amplifier) bait-doped fiber amplifier.
Well, that's it for the story of dispersion
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Source: ZTE Documentation
Edit: Herding fish