Optical fibers. Classification.

    Optical fiber is the de facto standard when building backbone 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 communication that were not there before.
    So let’s try to consider the hero of the occasion - optical fiber.

    In the article I’ll try to write simply about optical fibers, without mathematical calculations and with simple human explanations.

    The article is purely introductory, i.e. does not contain unique knowledge, everything that will be described can be found in a pile of books, however, this is not copy-paste, but squeezing information from the “pile” of information is just the essence.



    Classification


    Most often, fibers are divided into 2 general types of fibers
    1. Multimode fibers
    2. Single-mode fibers . Let

    us explain at the “everyday” level that there is single-mode and multi-mode.
    Imagine a hypothetical transmission system with fiber stuck in it.
    We need to pass binary information. The pulses of electricity in the fiber do not propagate, because the dielectric, so we will transmit the energy of light.
    For this we need a source of light energy. It can be LEDs and lasers.
    Now we know what we use as a transmitter - this is light.

    Think about how light is injected into a 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 1300 nm (for example), the core of a multimode is wide, then there are more ways of propagation of waves. Each such path is a mode

    2) If the core is small (single-mode fiber), then the wave propagation paths correspondingly decrease. And since there are much fewer additional modes, then there will be no mode dispersion (about it below).

    This is the main difference between multimode and singlemode fibers.
    Thanks enjoint, tegger, hazanko for the comments.

    Multimodein turn, they are divided into fibers with a step index multi mode fiber and with a gradient index (graded index m / mode fiber).

    Single-mode are divided into step, standard (standard fiber), with a dispersion-shifted dispersion (dispersion-shifted) and a non-zero dispersion-shifted dispersion (non-zero)

    Fiber optic design


    Each fiber consists of a core and a sheath with different refractive indices.
    The core (which is the main medium for transmitting the energy of the light signal) is made of an optically denser material, and the shell is made of less.

    For example, a 50/125 record indicates that the core diameter is 50 μm, and the shell is 125 μm.

    Core diameters of 50 μm and 62.5 μm are signs of multimode optical fibers, and 8-10 μm, respectively, are single-mode.
    The shell, as a rule, always has a diameter of 125 microns in size.

    As can be seen, the core diameter of a single-mode fiber has a much smaller size than the diameter of a multimode. Smaller core diameters reduce mode dispersion(which, perhaps, will be written in a separate article, as well as questions of the propagation of light in the fiber), and accordingly increase the transmission range. However, then single-mode fibers would supplant multimode, thanks to better “transport” characteristics, if not for the need to use expensive lasers with a narrow emission spectrum. In multimode fibers, LEDs with a more smeared spectrum are used.

    Therefore, for low-cost optical solutions, such as local area networks of Internet service providers, multimode applications occur.

    Refractive index profile



    The whole dance with a tambourine near the fiber in order to increase the transmission speed was around the profile of the refractive index. Since the main constraint to increasing speed is mode dispersion.
    Briefly, the essence is as follows:
    when the laser radiation enters the fiber core, the signal is transmitted through it in separate modes (roughly: light rays. But in fact, different spectral components of the input signal)
    Moreover, the "rays" enter at different angles, so the propagation time The energy of individual modes varies. This is illustrated in the figure below.


    Three refraction profiles are displayed here:
    stepwise and gradient for multimode fiber and stepwise for single-mode fiber.
    It is seen that in multimode fibers, light modes propagate in different ways, but, due to the constant refractive index of the core with the same speed. Those modes that are forced to go along the broken line come later than the modes that go in a straight line. Therefore, the original signal is stretched over time.
    Another thing is 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 accelerated. This happened because the core refractive index is now inconsistent. It increases parabolic from the edges to the center.
    This allows you to increase the transmission speed and get a recognizable signal at the reception.

    Optical Fiber Applications


    Multimode fiberSingle-mode fiber
    MMF 50 (62.5) / 125
    Gradient
    SF 9/125
    speed
    SF 9/125
    offset dispersion
    (non-zero offset dispersion)
    LAN (GigaEther, FDDI, ATM)Long LAN, SDH TrunkSDH Super Long Trunks


    To this we can add that the trunk cables now almost all come with non-zero biased dispersions, which allows the use of spectral wavelength division multiplexing ( WDM ) on these cables without the need to replace the cable.
    And when building passive optical networks, multimode fiber is often used.

    Thanks to those who criticized constructively.

    PS
    if it is interesting, articles may appear on
    - dispersion
    - types of fiber optic cables (not fibers)
    - transmission systems used for wdm / dwdm compaction.
    - The procedure for welding optical fibers. and types of chips.


    Also popular now: