Single-mode fiber and its difference from multimode fiber

When there is a need to transfer data from one remote point to another, increasingly, instead of the traditional copper wire, the customer is offered a laying of the optical cable by contractors . We will talk about this interesting technology today.

Fiber-optic communication lines (FOCL) operate on the principle of transmission of a light wave through a special channel made of extremely pure quartz glass. Electrical pulses of electronic equipment are fed to the signal converter, which generates a stream of light flashes and transfers them to the cable. At the other end, the receiver receives a light flux and transcodes it back into the electromagnetic pulses. Since the whole process is electronically controlled and is a digital transformation, the distortions are minimal.

To build such fiber-optic lines, use a special material - single-mode fiber and multimode fiber.

Optical lines have received such wide distribution not only due to the absence of interference during signal transmission. Among the undeniable advantages of this technology is a wide band, very low signal attenuation, unsurpassed resistance to any electromagnetic interference, a huge transmission range that is many tens of kilometers. A significant advantage is the long service life of communications, laid with fiber-optic communication, which is at least 25 years old.

Types of fiber

When installing communication lines using fiber-optic lines, either multimode or single-mode fiber is selected.

What does this cable consist of? The core of the optical fiber is quartz, ultrapure glass, which transmits the light flux through itself. And it does not sputter, because the refractive index of the shell is lower than that of the core, hence, the light ray is completely reflected from the walls inside the fiber.

Multimode fiber is good because it can launch several hundreds of light modes, which are introduced at different angles. Each such mode has its own trajectory and, as a consequence, a unique propagation time.

The main disadvantage of this type of fiber is the modal dispersion, which narrows the bandwidth and limits the maximum length of the line. Transmitters for multimode communication lines usually have a maximum range of about 5 kilometers.

The problem of reducing the mode dispersion is solved by a cable with a gradient refractive index of the core. In such an optical fiber, unlike standard options, the refractive indices decrease from the center of the core to the shell, which gives a significant improvement in the parameters of the transmitted signal.

Single-mode fiber is designed based on the problem of passing through the communication channel only one mode (main). This approach offers many advantages. Some characteristics of a single-mode cable are an order of magnitude better than that of a multimode cable. This is the decisive factor that influences the choice of engineers in favor of the first when laying new fiber-optic lines. In fact, single-mode fiber attenuates the signal at the level of 0.25db per kilometer, the dispersion value in it is very small, and the wide bandwidth ensures a clear and rapid transmission of large amounts of data without distortion.

But there is a spoon of tar in this barrel of honey. This type is much more expensive than multimode fibers. Since the size of the lightguide core in a single-mode cable is very small, introducing radiation into such a cable is not an easy task and requires very careful monitoring during splicing. The end connectors for these lines also cost much more than the endpoints of multimode lines. In addition, due to the simplicity of introducing a light beam into the wide core, the latter have very simple and cheap radiators, which are also produced by a large number of competing companies.