Diferenças em fibras ópticas multimodo e monomodo

Hello, welcome! I'm Marcelo Barboza from Clarity Treinamentos. Today we are going to talk about the differences between multimode and singlemode optical fibers.

The first basic difference between them is the diameter of the core. We know that the optical fiber, its glass part, has two main layers: the core and the shell, which comes around. And it is precisely in the diameter of the core that multimode fiber is differentiated from single-mode.

The shell, which is also made of glass, has a diameter of 125 microns, in both multimode and single-mode fibers. The core of single-mode fibers is approximately 9 microns in diameter. The multimode has two standard core sizes : 50 microns and 62.

5 microns. This dictates all transmission and range and price differences between them. Firstly, how does multimode work in transmission?

As the core is wider than the single-mode, 50 or 62. 5 microns, we have, when we attach light to the core, we have more beams of light, more directions of light paths in there. Some rays of light go to one angle, others to another angle, and then we have several possible transmission paths within the multimode fiber .

Here above I show a diagram showing these transmission paths. Each path is called a mode, so that is why this fiber is called a multimode. Because it has multiple modes of transmission, that is, multiple paths.

The singlemode, on the other hand, has a very thin core, 9 microns, it only fits one direction, one path to the light. Therefore, it is called singlemode. This dictates the differences in the transmission, in the equipment.

Firstly, the transmission equipment is the transmitter's lamp, and for multimode it can be a transmitter, a lamp, a simpler, cheaper light source , such as an LED or VCSEL, which is a kind of a little cheaper, more simplified laser, that can be used for multimode. As for single-mode fibers, as the core is very thin, I must also have a light emitter that concentrates all its energy in those nine microns. So it can only be a laser transmitter.

Laser is more expensive than LED or VCSEL transmitters. So this already determines a big price difference between transmission equipment for multimode and single-mode fibers. Another big difference is precisely because the multimode has several possible light paths.

This somewhat hinders transmission, creating what we call modal dispersion. The modal dispensation causes the digital signal to enter through one end of the fiber, when it exits through the other end, it is scattered, it is diverse, the information spreads along the fiber, the bits can start to scramble, the data symbols start to scramble and the transmission quality is lost. This is proportional to the length of the fiber.

Therefore, the multimode fiber is limited in its final distance because of this modal dispersion. Usually, of course it depends on the application, but normally the maximum range of the multimode fiber is no more than two kilometers. And if we talk about gigabit ethernet, that's a few hundred meters.

The fastest applications today, such as 100 gigabits per second, can have distances or ranges even less than 100 meters on a multimode fiber, mainly because of the modal dispersion. In single-mode fiber, as there is only one mode of transmission, there is no modal dispersion, which hinders transmission, I have a range of several and several kilometers. Depending on the power of the transmitter, I can have 10, 20, 30, 40 kilometers or more in single-mode fiber.

So, due to the fact that it also has a greater distance, single-mode transmission equipment necessarily has more power, this also contributes to the price difference between opto-electronic equipment for multimode fibers and for single-mode fiber. An idea of ​​difference is at least two to three times more expensive, the single-mode equipment than the multimode, but it can be ten times more expensive or more, depending on the range. In terms of ethernet data applications, all of the ethernet speeds that exist today, up to 40 gigabits per second, 100 gigabits per second, are available for transmission in both singlemode and multimode modes.

What will affect my choice of which will be used is the distance really. Longer distances can only be covered by single-mode fibers. Contrary to what happens with equipment prices between singlemode and multimode, where singlemode is much more expensive, singlemode fiber tends to be cheaper than multimode.

So which one to choose? What will really determine is your speed and the transmission distance. For shorter transmissions, with a few hundred or a few tens of meters, generally the most cost-effective will be in multimode fiber, although the fiber is a little more expensive, the transmission equipment is cheaper.

As for longer links, where the multimode is not possible to be used, due to its problem of modal dispersion, then we only have the option of single mode. Then, the solution as a whole is a little more expensive, or perhaps much more expensive, but it is the only possible one. Another difference is in the wavelengths of light that we will use to travel these fibers.

In multimode fibers, transmissions generally occur between 800 and 1300 nanometers of light wavelength. While in the single mode, between 1300 and 1600 nanometers is used. The typical transmission windows for ethernet networks in multimode fibers are 850 nanometers and 1300 nanometers.

And ethernet transmission over single-mode fiber follows in the windows of 1310 nanometers and 1550 nanometers. Another difference between them is the specific attenuation of the fiber. For multimode fibers , we have an attenuation approximately between 3 and 3.

5 dB per kilometer when measured at 850 nm, and from 1 to 1. 5 dB per km when measured at 1300 nanometers. As for the single-mode fiber, type OS1 has approximately 1 dB / km of attenuation at most, and OS2, which is more appropriate, even for outdoor environments, has a maximum attenuation of 0.

4 dB / km. This also determines its greater transmission capacity over longer distances. When testing these fibers, we should test them on these specific wavelengths that I mentioned for multimode and singlemode.

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