Dispersion is so beautiful, why don’t optical fibers like it?

More than three hundred years ago in Europe, on a sunny afternoon, Newton laid out such a plan.

Let the sunlight shine on the prism. After penetrating the prism, the light spreads into colorful ribbons composed of red, orange, yellow, green, blue, and purple, and is projected onto a curtain in the room. In this way, the seemingly transparent sunlight changes into incredible color bands with the help of the prism.

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

I saw him turning the first prism and projecting seven colorful ribbons of red, orange, yellow, green, blue and purple onto the cracks in the first curtain, and then through the second prism onto the second curtain. A miracle happened. What appeared on the second curtain was a single color of light. The schematic diagram is as follows:

At this point, the sunlight is separated into multiple single colors and presented on the second curtain. Lord Niu uses a prism to break the secret : light can be dispersed! The sunlight 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 dispersion produced?

The phenomenon that composite light is decomposed into three colors is called dispersion of light.

In the prism experiment, sunlight (that is, composite light) enters the glass from the air, and then enters the air from the glass, where it is refracted twice. You must know that everything is beneficial. When refraction occurs, light will naturally choose the shortest path and move forward while minimizing energy loss. From Newton's prism experiment above, we know that composite light is essentially composed of many single lights of different colors. These lights have different wavelengths, and the energy of light of different wavelengths is very different. It is difficult to reconcile the opinions, and the light of different wavelengths has different opinions on how to choose the path after refraction. Therefore, after coming out of the prism, they "parted ways".

So, why does light scatter? It turns out that what causes this dispersion is the wavelength of light. Light of different wavelengths has different refractive indexes in the medium and different propagation speeds (paths) , which will inevitably cause the light (s) to spread and spread, and dispersion is formed.

The dispersion phenomenon of light shows that the speed of light propagating 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:

V=C/N

C is the propagation speed of light in vacuum ,

constant 300,000 km/s

N is the refractive index of the medium to light

2. The effect of dispersion

Although dispersion can help us enter a colorful world, in the field of communications, dispersion is really not that beautiful.

During the transmission of optical signals in optical fibers, dispersion is one of the important factors causing loss.

This is because the refractive index of light causes dispersion, which causes inter-code interference in the light pulse, thereby broadening the output end.

What is stretching?

Broadening means that light of different wavelengths travels at different speeds in the medium due to different refractive indices, resulting in an increase in 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 still travel in a predetermined direction.

The dissonance of light waves causes the width of this beam of light to be larger than before entering the medium, forming a broadening.

In the case of dispersion, the longer the optical signal transmission distance, the more serious the broadening will be. The result is signal distortion and the deterioration of bit error rate performance, which seriously affects the quality of information transmission.

How to avoid the impact of dispersion on communication?

3. How to avoid the influence of dispersion?

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

One of the dispersion compensation methods: dispersion compensation fiber DCF

In ordinary single-mode optical fiber systems, the operating wavelength of the optical fiber has high positive dispersion at 1550nm.

Characteristics of positive dispersion: As the wavelength increases, the refractive index gradually decreases.

According to the idea of compensation, negative dispersion needs to be added to these optical fibers for dispersion compensation to ensure that the total dispersion of the entire optical fiber line is approximately zero. The dispersion compensating fiber (DCF) is a new type of single-mode fiber mainly designed for the 1550nm wavelength. It has high negative dispersion at 1550nm (the characteristics of negative dispersion and positive dispersion are opposite) , and can be used in ordinary single-mode fiber optic cables. Dispersion compensation is performed in the optical fiber system. As shown in the figure below, the sum of the compensated positive and negative dispersions approaches zero at 1550nm.

The following is the formula for dispersion compensation fiber applied to single-mode fiber.

D(As)L+Dc(As)Lc=0

D( λ s) is the dispersion coefficient of single-mode fiber at operating wavelength λ s

Dc( λ s) is the dispersion coefficient of DCF at the operating wavelength λ s

L and LC are the lengths of conventional single-mode fiber and D CF respectively.

In practical applications, DCF and single-mode fiber are used in series in the transmission line to compensate the positive dispersion of single-mode fiber at 1550nm optical wavelength, so as to extend the relay distance and reduce loss, so as to realize high-speed, large-capacity, and long-distance communication. As shown below:

As dispersion compensation, DCF has the following advantages:

The compensation effect is remarkable and the system works stably.

The operation is simple and the compensation fiber can be directly connected to the transmission system to realize compensation.

The dispersion compensation amount is controllable as needed and can be adjusted as needed according to the actual compensation amount required by the transmission system.

Notice:

As the optical signal travels farther on the transmission line, other losses will occur, such as line attenuation. In order to avoid line attenuation, it is necessary to consider using EDFA (Erbium-Doped Fiber Amplifier) erbium-doped fiber amplifier.