Optical fibers. Classification.
Optical fiber standard de facto in the construction of the main communication networks. The length of fiber-optic communication lines in Russia for large telecom operators reaches> 50 thousand km.
Thanks to fiber, we have all the advantages in connection that weren't there before.
So let's try to consider the hero of the occasion - optical fiber.
In the article I will try to write simply about optical fibers, without mathematical calculations and with simple human explanations.
The article is purely informational, i.e. It does not contain unique knowledge, everything that will be described can be found in a pile of books, however, this is not a copy-paste, and only the essence of the squeeze from the “heap” of information.
')
The most common fibers are divided into 2 common types of fibers
1. Multimode Fibers
2. Singlemode
we will give an explanation at the "everyday" level that there is a single-mode and multimode.
Imagine a hypothetical transmission system with fiber stuck into it.
We need to transfer binary information. The pulses of electricity in the fiber do not spread, because the dielectric, so we wake to transmit the energy of light.
For this we need a source of light energy. These can be LEDs and lasers.
Now we know that we use as a transmitter - it is light.
Think about how light is introduced into the fiber:
1) Light radiation has its own spectrum, so if the fiber core is wide (this is in a multimode fiber), then more spectral components of the light will fall into the core.
For example, we transmit light at a wavelength of 1300nm (for example), the core is multi-mode wide, and the propagation paths of the waves are larger. Each such way is fashion
2) If the core is small (single-mode fiber), then the paths of wave propagation decrease accordingly. And since the additional modes are much smaller, then there will be no modal dispersion (see below).
This is the main difference between multimode and single-mode fibers.
Thank you enjoint, tegger, hazanko for comments.
Multi-mode fibers, in turn, are divided into fibers with a stepped refractive index (step index multi mode fiber) and with a gradient (graded index m / mode fiber).
Single-mode are divided into staggered, standard (standard fiber), with offset dispersion (dispersion-shifted) and non-zero dispersion shifted (non-zero dispersion-shifted)
Each fiber consists of a core and a shell with different refractive indices.
The core (which is the main medium for transmitting the energy of a light signal) is made of an optically more dense material, the shell is made of less.
For example, the 50/125 record says that the core diameter is 50 ÎĽm, the shell is 125 ÎĽm.
Core diameters equal to 50 µm and 62,5 µm are signs of multimode optical fibers, and 8–10 µm, respectively, single-mode.
The shell, as a rule, always has a diameter of 125 µm.
As can be seen, the diameter of the core of a single-mode fiber is much smaller than the diameter of a multimode fiber. The smaller diameter of the core allows to reduce the mode dispersion (about which it will probably be written in a separate article, as well as the issues of light propagation in the fiber), and accordingly increase the transmission distance. However, then single-mode fibers would crowd out multimode, thanks to better “transport” characteristics, if not the need to use expensive lasers with a narrow emission spectrum. In multimode fibers, LEDs with a more spread spectrum are used.
Therefore, for low-cost optical solutions such as local area networks of Internet service providers, multimode happens.
All the dancing with a tambourine at the fiber in order to increase the transmission speed was around the refractive index profile. Since the main limiting factor in increasing the speed is mode dispersion.
Briefly the essence of the following:
when the laser radiation enters the fiber core, the signal is transmitted through it in the form of individual modes (roughly: light rays. But in fact different spectral components of the input signal)
Moreover, the "rays" come in at different angles, so the time of propagation of energy for individual modes varies. This is illustrated in the figure below.
Here are 3 profiles of refraction:
stepped and gradient for multimode fiber and stepped for singlemode.
It can be seen that in multimode fibers the light modes propagate along different paths, but, due to the constant refractive index of the core, with the SAME speed. Those mods that are forced to follow a broken line come later than mods that go in a straight line. Therefore, the original signal is stretched in time.
Another thing with the gradient profile, those modes that used to go in the center - slow down, and the modes that went along the broken path, on the contrary, are accelerating. This happened because the refractive index of the core is now unstable. It increases parabolic from the edges to the center.
This allows you to increase the transmission rate and get a recognizable signal at the reception.
To this we can add that the trunk cables now almost all come with non-zero offset dispersions, which makes it possible to use WDM on these cables without the need to replace the cable.
And when building passive optical networks often use multimode fiber.
Thanks to those who constructively criticized.
PS
if interested, then there may be articles on
- dispersion
- types of fiber optic cables (not fibers)
- transmission systems used for wdm / dwdm seals.
- procedure for welding optical fibers. and types of chips.
Thanks to fiber, we have all the advantages in connection that weren't there before.
So let's try to consider the hero of the occasion - optical fiber.
In the article I will try to write simply about optical fibers, without mathematical calculations and with simple human explanations.
The article is purely informational, i.e. It does not contain unique knowledge, everything that will be described can be found in a pile of books, however, this is not a copy-paste, and only the essence of the squeeze from the “heap” of information.
')
Classification
The most common fibers are divided into 2 common types of fibers
1. Multimode Fibers
2. Singlemode
we will give an explanation at the "everyday" level that there is a single-mode and multimode.
Imagine a hypothetical transmission system with fiber stuck into it.
We need to transfer binary information. The pulses of electricity in the fiber do not spread, because the dielectric, so we wake to transmit the energy of light.
For this we need a source of light energy. These can be LEDs and lasers.
Now we know that we use as a transmitter - it is light.
Think about how light is introduced into the fiber:
1) Light radiation has its own spectrum, so if the fiber core is wide (this is in a multimode fiber), then more spectral components of the light will fall into the core.
For example, we transmit light at a wavelength of 1300nm (for example), the core is multi-mode wide, and the propagation paths of the waves are larger. Each such way is fashion
2) If the core is small (single-mode fiber), then the paths of wave propagation decrease accordingly. And since the additional modes are much smaller, then there will be no modal dispersion (see below).
This is the main difference between multimode and single-mode fibers.
Thank you enjoint, tegger, hazanko for comments.
Multi-mode fibers, in turn, are divided into fibers with a stepped refractive index (step index multi mode fiber) and with a gradient (graded index m / mode fiber).
Single-mode are divided into staggered, standard (standard fiber), with offset dispersion (dispersion-shifted) and non-zero dispersion shifted (non-zero dispersion-shifted)
Optical Fiber Design
Each fiber consists of a core and a shell with different refractive indices.
The core (which is the main medium for transmitting the energy of a light signal) is made of an optically more dense material, the shell is made of less.
For example, the 50/125 record says that the core diameter is 50 ÎĽm, the shell is 125 ÎĽm.
Core diameters equal to 50 µm and 62,5 µm are signs of multimode optical fibers, and 8–10 µm, respectively, single-mode.
The shell, as a rule, always has a diameter of 125 µm.
As can be seen, the diameter of the core of a single-mode fiber is much smaller than the diameter of a multimode fiber. The smaller diameter of the core allows to reduce the mode dispersion (about which it will probably be written in a separate article, as well as the issues of light propagation in the fiber), and accordingly increase the transmission distance. However, then single-mode fibers would crowd out multimode, thanks to better “transport” characteristics, if not the need to use expensive lasers with a narrow emission spectrum. In multimode fibers, LEDs with a more spread spectrum are used.
Therefore, for low-cost optical solutions such as local area networks of Internet service providers, multimode happens.
Refractive index profile
All the dancing with a tambourine at the fiber in order to increase the transmission speed was around the refractive index profile. Since the main limiting factor in increasing the speed is mode dispersion.
Briefly the essence of the following:
when the laser radiation enters the fiber core, the signal is transmitted through it in the form of individual modes (roughly: light rays. But in fact different spectral components of the input signal)
Moreover, the "rays" come in at different angles, so the time of propagation of energy for individual modes varies. This is illustrated in the figure below.
Here are 3 profiles of refraction:
stepped and gradient for multimode fiber and stepped for singlemode.
It can be seen that in multimode fibers the light modes propagate along different paths, but, due to the constant refractive index of the core, with the SAME speed. Those mods that are forced to follow a broken line come later than mods that go in a straight line. Therefore, the original signal is stretched in time.
Another thing with the gradient profile, those modes that used to go in the center - slow down, and the modes that went along the broken path, on the contrary, are accelerating. This happened because the refractive index of the core is now unstable. It increases parabolic from the edges to the center.
This allows you to increase the transmission rate and get a recognizable signal at the reception.
Optical Fiber Applications
| Multimode fiber | Single mode fiber | |
| MMF 50 (62.5) / 125 Gradient | SF 9/125 stepped | SF 9/125 offset dispersion (with non-zero offset disp.) |
| LAN (GigaEther, FDDI, ATM) | Extended LANs, SDH highways | Overstretched SDH highways |
To this we can add that the trunk cables now almost all come with non-zero offset dispersions, which makes it possible to use WDM on these cables without the need to replace the cable.
And when building passive optical networks often use multimode fiber.
Thanks to those who constructively criticized.
PS
if interested, then there may be articles on
- dispersion
- types of fiber optic cables (not fibers)
- transmission systems used for wdm / dwdm seals.
- procedure for welding optical fibers. and types of chips.
Source: https://habr.com/ru/post/46818/
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