Vols single-mode maximum cable length. Fiber optic cable

Some properties of an optical fiber as a light guide directly depend on the diameter of the core. According to this parameter, the fiber is divided into two categories:

multimode(MMF) and single mode(SMF) .

Multimode fibers are divided into stepped and gradient fibers.

Single-mode fibers are classified into stepped single-mode fibers or standard fibers (SF), dispersion-shifted fibers (DSF), and non-zero dispersion-shifted fibers (NZDSF).

Multimode fiber.

This category of fiber has a relatively large core diameter compared to the wavelength of the light emitted by the transmitter. The range of its values ​​is 50--1000 microns at the used wavelengths of about 1 micron. However, the most widely used fibers with diameters of 50 and 62.5 microns. Transmitters for such an optical fiber emit a pulse of light in a certain solid angle, i.e., the rays (modes) enter the core at different angles. As a result, the rays pass from the source to the receiver with unequal paths and, therefore, reach it at different times. This results in a pulse width at the output that is larger than at the input. Such a phenomenon is called intermode dispersion. In stepped optical fiber, which is simpler to manufacture, the refractive index changes stepwise at the core-clad interface. The path of rays in such a fiber is shown in Figure 2.3.

Figure 2.3 - The path of light rays in the fiber

In a gradient OF, the refractive index gradually decreases from the center to the boundary. Rays of light whose paths pass in the peripheral regions with a lower refractive index propagate faster than those that pass near the center, which ultimately compensates for the difference in path lengths. In such fiber, the effect of intermode dispersion is much lower than in stepped fiber (Figure 2.3).

Signal broadening places a limit on the number of pulses transmitted per second that can still be unmistakably recognized at the receiving end of the link. This, in turn, limits the bandwidth of multimode fiber.

Figure 2.4 – Constructions of various fibers

Obviously, the amount of dispersion at the receiving end also depends on the length of the cable. Therefore, the throughput for optical highways is determined per unit length. For stepped optical fiber it is typically 20-30 MHz per kilometer (MHz/km), while for graded optical fibers it is in the range of 100-1000 MHz/km.

Multimode fiber may have a glass core and a plastic jacket. Such fiber has a stepped refractive index profile and a bandwidth of 20-30 MHz/km. single mode fiber

The main difference of such a fiber, which largely determines its properties as a light guide, is the diameter of the core. It is only 7 to 10 microns, which is already comparable to the wavelength of a light signal. A small diameter value allows you to form only one beam (mode), which is reflected in the name (Figure 2.4).

Advantages of multimode optical fibers in comparison with single-mode ones:

Due to the large diameter of the core of a multimode optical fiber, the requirements for radiation sources are reduced, since cheaper and at the same time more powerful semiconductor lasers, and even LEDs, can be used to input radiation. Very simple circuits are used to power the LEDs, which simplifies the device and reduces the cost of FOTS.

In the receiving optical module, photodiodes with a large diameter of the photosensitive area can be used. Such photodiodes are of low cost.

When splicing multimode optical fibers, the required accuracy of matching ends is an order of magnitude lower than in the case of splicing single-mode optical fibers.

Optical connectors for multimode optical fibers for the same reasons have an order of magnitude less stringent requirements than optical connectors for single-mode optical fibers.

They trace their history back to 1960, when the first laser was invented. At the same time, the optical fiber itself appeared only 10 years later, and today it is the physical basis of the modern Internet.

Optical fibers used for data transmission have a fundamentally similar structure. The light transmitting part of the fiber (core, core or core) is in the center, around it is a damper (sometimes called a sheath). The task of the damper is to create an interface between the media and prevent radiation from leaving the core.

Both the core and the damper are made of quartz glass, and the refractive index of the core is somewhat higher than that of the damper to realize the phenomenon of total internal reflection. For this, a difference in hundredths is sufficient - for example, the core may have a refractive index n 1 =1.468, and a damper - the value n 2 =1.453.

The core diameter of single-mode fibers is 9 µm, multimode - 50 or 62.5 µm, while the damper diameter for all fibers is the same and is 125 µm. The structure of light guides is shown on a scale in the illustration:

Stepped refractive index profile (step- index fibers) - the simplest for the manufacture of light guides. It is acceptable for single-mode fibers, where it is conditionally considered that there is only one "mode" (the path of light propagation in the core). However, step index multimode fibers are characterized by high dispersion caused by the presence of a large number of modes, which leads to scattering, “spreading” of the signal, and ultimately limits the distance at which applications can work. The gradient refractive index allows minimizing the mode dispersion. Gradient index fibers are highly recommended for multimode systems. (graded- index fibers) , in which the transition from the core to the damper does not have a "step", but occurs gradually.

The main parameter that characterizes the dispersion and, accordingly, the ability of the fiber to support applications over certain distances is the bandwidth factor. Currently, multimode fibers are divided into four classes according to this indicator, from OM1 (which is not recommended for use in new systems) to the most productive class OM4.

Single-mode fibers are divided into classes OS1 (conventional fibers used for transmission at either 1310 nm or 1550 nm) and OS2, which can be used for broadband transmission over the entire range from 1310 nm to 1550 nm, divided into transmission channels, or in even wider spectrum, for example, from 1280 to 1625 nm. At the initial stage of production, OS2 fibers were marked with the designation LWP (Low water peak) to emphasize that they minimize absorption peaks between transparency windows. Broadband transmission in the highest performance single-mode fibers provides transmission rates in excess of 10 Gbps.

Singlemode and multimode fiber optic cable: selection rules

Given the described characteristics of multimode and singlemode fibers, we can give recommendations for choosing the type of fiber depending on the performance of the application and the distance at which it must work:

for speeds over 10 Gb/s choose single-mode fiber regardless of distance

for 10 Gigabit applications and distances over 550 m, also opt for single-mode fiber

for 10 Gigabit applications and distances up to 550 m, OM4 multimode fiber is also available

for 10 Gigabit applications and distances up to 300 m, OM3 multimode fiber is also available

for 1 Gigabit applications and distances up to 600-1100 m, OM4 multimode fiber is possible

for 1 Gigabit applications and distances up to 600-900 m, OM3 multimode fiber is possible

OM2 multimode fiber available for 1 Gigabit applications and distances up to 550 m

The cost of an optical fiber is largely determined by the diameter of the core, so a multimode cable, other things being equal, is more expensive than a single-mode cable. At the same time, active equipment for single-mode systems, due to the use of powerful laser sources in them (for example, a Fabry-Perot laser), is significantly more expensive than active equipment for multimode systems, which use either relatively inexpensive VCSEL surface-emitting lasers or even cheaper LED sources. When evaluating the cost of the system, it is necessary to take into account the costs of both the cable infrastructure and the active equipment, and the latter can be significantly higher.

To date, there is a practice of choosing an optical cable depending on the scope of use. Single mode fiber is used:

in maritime and transoceanic cable communication lines;

in terrestrial long-distance trunk lines;

in provider lines, communication lines between city nodes, in long-distance dedicated optical channels, in trunk lines to the equipment of mobile operators;

in cable television systems (primarily OS2, broadband transmission);

in GPON systems with bringing the fiber to an optical modem located at the end user;

in SCS in highways longer than 550 m (as a rule, between buildings);

in SCS serving data processing centers, regardless of distance.

Multimode fiber is mainly used:

in SCS in trunks inside the building (where, as a rule, distances are within 300 m) and in trunks between buildings, if the distance does not exceed 300-550 m;

in horizontal SCS segments and in FTTD systems ( fibers- to- the- desk), where users are installed workstations with multimode optical network cards;

in data centers in addition to single-mode fiber;

in all cases where the distance allows the use of multimode cables. Although the cables themselves are more expensive, the savings in active equipment offset these costs.

It can be expected that in the coming years, OS2 fiber will gradually replace OS1 (it is being discontinued), and 62.5/125 µm fibers will disappear in multimode systems, since they will be completely replaced by 50 µm fibers, probably of OM3-OM4 classes.

Testing singlemode and multimode optical cables

After installation, all installed optical segments are subject to testing. Only measurements carried out by special equipment can guarantee the characteristics of installed lines and channels. For SCS certification, devices with qualified radiation sources at one end of the line and meters at the other are used. Such equipment is manufactured by Fluke Networks, JDSU, Psiber; all such devices have preset bases of permissible optical losses in accordance with the telecommunications standards TIA/EIA, ISO/IEC and others. Longer optical lines are checked using optical reflectometers having an appropriate dynamic range and resolution.

During the operation phase, all installed optical segments require careful handling and regular use of special cleaning wipes, sticks and other cleaning products.

It is not uncommon for installed cables to be damaged, for example, when digging trenches or when performing repairs inside buildings. In this case, an OTDR or other diagnostic tool based on the principles of reflectometry and showing the distance to the point of failure is needed to locate the fault (similar models are available from Fluke Networks, EXFO, JDSU, NOYES (FOD), Greenlee Communication and others).

The budget models found on the market are mainly designed to localize damage (bad welds, breaks, macrobends, etc.). Often they are not able to carry out detailed diagnostics of the optical line, identify all its inhomogeneities and professionally create a report. In addition, they are less reliable and durable.

High-quality equipment - on the contrary, it is reliable, capable of diagnosing FOCL in the smallest detail, create a correct table of events, generate an editable report. The latter is extremely important for the certification of optical lines, because sometimes there are welded joints with such low losses that the reflectometer is not able to determine such a joint. But welding is still there, and it must be displayed in the report. In this case, the software allows you to forcibly set an event on the trace and measure the losses on it manually.

Many professional devices also have the ability to expand functionality by adding options: a video microscope for inspecting fiber ends, a laser source and power meter, an optical telephone, etc.

/ Single-mode (SM) and multi-mode (MM) optical cable

Single mode (SM) and multimode (MM) optical cable

Fiber optic fibers can be of two types:

• Single mode (SM, Single Mode)
• Multimode (MM, Multi Mode)

A single-mode optical cable transmits one mode and has a cross-sectional diameter of ≈ 9.5 nm. In turn, a single-mode fiber optic cable can be with unbiased, shifted and non-zero shifted dispersion.

MM fiber optic multimode cable transmits multiple modes and has a diameter of 50 or 62.5 nm.

At first glance, the conclusion seems to be that multimode fiber optic cable is better and more efficient than SM optical cable. Moreover, experts often speak in favor of MM on the grounds that, since a multimode optical cable provides a multiple priority in performance compared to SM, it is better in every respect.

Meanwhile, we would refrain from such unambiguous assessments. Quantity is far from the only basis for comparison, and in many situations single-mode fiber is superior.

The main difference between SM and MM cables is dimensional indicators. The SM optical cable has a fiber with a smaller thickness (8-10 microns). This causes it to be able to transmit a wave of only one length in the central mode. The thickness of the main fiber in the MM cable is much larger, 50-60 microns. Accordingly, such a cable can simultaneously transmit several waves with different lengths in several modes. However, more modes reduce the bandwidth of a fiber optic cable.

Other differences between single and multimode cables relate to the materials from which they are made and the light sources used. A single-mode optical cable has both a core and a sheath made only of glass, and a laser as a light source. The MM cable can have both a glass and a plastic sheath and a rod, and an LED serves as a light source for it.

Single-mode optical cable 9/125 µm

Optical cable single-mode 8 fibers type 9 125, has a single-tube modular design. The light guides are located in the central tube, which is filled with a hydrophobic gel. The filler reliably protects the fibers from various kinds of mechanical influences, in addition, it excludes the effect of temperature changes in the external environment. For protection against rodents and other similar influences, an additional fiberglass braid is used.

In fact, the development and production of fiber optic cable 9 125 comes down to finding the optimal solution to the problem of reducing optical dispersion (down to zero) at all frequencies with which the cable will work. A large number of modes negatively affects signal quality, and a single-mode cable actually has more than one mode, but several. Their number is much less than in multimode, however, it is greater than one. Reducing the effect of optical dispersion leads to a decrease in the number of modes, and, accordingly, to an improvement in signal quality.

In most optical fiber standards used in 9125 cables, zero dispersion is achieved over a narrow frequency range. Thus, in the literal sense, a cable is single-mode only with waves of a specific length. However, existing multiplexing technologies use a set of optical frequencies to receive and transmit several broadband optical communication channels at once.

Single mode fiber optic cable 9 125 is used both inside buildings and on external highways. It can be buried in the ground or used as an overhead cable.

Multimode optical cable 50/125 µm

Fiber-optic cable 50/125(OM2) multimode, used in optical networks with 10-gigabyte speeds, built on multimode fiber. In accordance with changes to the ISO/IEC 11801 specification, it is recommended to use a new type of OMZ class patch cord with a size of 50 125 in such networks.

Optical cable 50 125 OMZ, according to 10 Gigabit Ethernet network applications, is intended for data transmission at 850 nm or 1300 nm wavelengths, which differ in the maximum allowable attenuation values. It is used to provide communication in the frequency range of 1013-1015 Hz.

Multimode optical cable 50 125 is intended for patch cords and wiring to the workplace, and is used only indoors.

The cable supports short distance data transmission and is suitable for direct termination. The structure of a standard multimode optical fiber G 50/125 (G 62.5/125) µm complies with the following standards: EN 188200; VDE 0888 part 105; IEC "IEC 60793-2"; ITU-T Recommendation (ITU-T) G.651.

MM 50/125 has an important advantage, which is low losses and absolute immunity to various kinds of interference. This allows you to build systems with hundreds of thousands of telephone channels.

Types of fibers used

In the production of SM and MM cables, single-mode and multi-mode fibers of the following types are used:

• single-mode, ITU-T G.652.B recommendation (type “E” in marking);
• single-mode, ITU-T recommendation G.652.C, D (type “A” in marking);
• single-mode, ITU-T G.655 recommendation (type “H” in marking);
• single-mode, ITU-T G.656 recommendation (type “C” in marking);
• multimode, with a core diameter of 50 microns, ITU-T G.651 recommendation (in the marking type “M”);
• multimode, with a core diameter of 62.5 microns (in the marking type “B”)

The optical parameters of the fibers in the buffer coating must comply with the specifications of the supplier companies.

Optical fiber parameters:

Where can I buy?

You can buy a multimode and single-mode optical cable (the price and delivery terms are specified separately, depending on the specific features of the product and the wishes of the customer) directly on our website. To do this, please fill out the appropriate form in the on-line order. There is always a 4-fiber multi-mode optical cable, a single-mode self-supporting optical cable, a single-mode 4-fiber and 8-fiber optical cable, and other types of OK (see Catalog).

By agreement between the customer and the manufacturer, it is allowed to supply a cable with parameters that differ from those given in the table.

Fiber optic cable(aka fiber optic cable) is a fundamentally different type of cable compared to the two types of electrical or copper cable. Information from it is transmitted not by an electrical signal, but by light. Its main element is transparent fiberglass, through which light passes over long distances (up to tens of kilometers) with little attenuation.

Rice. 1. Optical fiber. Structure

The structure of a fiber optic cable is very simple and similar to that of a coaxial electrical cable (Figure 1). Only instead of a central copper conductor, thin (about 1 - 10 semi-dark in diameter) fiberglass (3) is used here, and instead of internal insulation, a glass or plastic sheath (2) is used, which does not allow light to go beyond the fiberglass. In this case, we are talking about the regime of the so-called total internal reflection of light from the interface of two substances with different breaking coefficients (the glass shell has a much lower breaking coefficient than that of the central fiber). The metal sheath of the cable is usually omitted, because shielding from external electromagnetic obstructions is not needed here. However, sometimes it is still used for mechanical protection from the environment (such a cable is sometimes called armored, it can combine several fiber optic cables under one sheath).

Fiber optic cable has exceptional characteristics in terms of security and secrecy of the transmitted information. No external electromagnetic obstacles, in principle, are capable of disfiguring the light signal, and the signal itself does not generate external electromagnetic radiation. Connecting to this type of cable for unauthorized listening to the network is almost impossible, because the integrity of the cable is violated. Theoretically, the bandwidth of such a cable reaches 10 12 Hz, that is, 1000 GHz, which is incomparably higher than that of electric cables. The cost of fiber optic cable is constantly decreasing and is currently approximately equal to the cost of a thin coaxial cable.

Typical signal attenuation in fiber optic cables at frequencies used in local area networks is from 5 to 20 dB/km, which approximately corresponds to the performance of electric cables at low frequencies. But in the case of a fiber optic cable, with an increase in the frequency of the transmitted signal, the attenuation increases very slightly, and at high frequencies (especially over 200 MHz), its advantage over an electric cable is irrefutable, it simply has no competitors.

Disadvantages of fiber optic cable

The most important of them is the high complexity of installation (with installation of fiber optic cable separation requires micron accuracy, the attenuation in separation strongly depends on the accuracy of the glass fiber and the degree of its polishing). To install the separation, welding or gluing is used using a special gel, which has the same light breaking coefficient as fiberglass. In any case, this requires highly qualified personnel and special tools. Therefore, most often, fiber optic cable is sold in the form of pre-cut pieces of different lengths, at both ends of which the required type of separation is already installed. It is worth remembering that a poor-quality separation setting drastically reduces the allowable cable length due to attenuation.

You also need to remember that the use of fiber optic cable requires special optical receivers and transmitters that will turn light signals into electrical signals and vice versa, which at times significantly increases the cost of the network as a whole.

Fiber optic cables allow signal branching (special passive distributors are produced for this ( couplers) for 2-8 channels), but as a rule they are used for data transmission in only one direction between one transmitter and one receiver. After all, any branching inevitably greatly weakens the light signal, and if there are many branches, that light may simply not reach the end of the network. In addition, there are internal losses in the distributors, so the total signal power at the output is less than the input power.

Fiber optic cable is less strong and flexible than electrical cable. A typical allowable bend radius is about 10 - 20 cm, with smaller bend radii the central fiber may break. Poorly tolerates cable and mechanical stretching, as well as crushing influences.

Sensitive fiber optic cable and to ionizing radiation, through which the transparency of fiberglass decreases, that is, signal attenuation increases . Sudden changes in temperature also negatively affect it, fiberglass can crack.

Use fiber optic cable only in networks with a star and ring topology. There are no problems of matching and grounding in this case. The cable provides ideal galvanic isolation of network computers. In the future, this type of cable is likely to crowd out electrical cables, or at least greatly crowd them out. Copper reserves on the planet are depleted, and there is enough raw material for glass production.

Types of fiber optic cables

1. multimode or multimode cable, cheaper, but of lower quality;
2. single mode cable, more expensive, but has better performance compared to the first.

The essence of the discrepancy between the two types is reduced to different modes of passage of light rays in the cable.

Rice. 2. Propagation of light in a single-mode cable

In a single-mode cable, almost all beams travel the same path, as a result of which they reach the receiver at the same time, and the signal shape is almost not distorted (Fig. 2). A single-mode cable has a center fiber diameter of about 1.3 µm and only transmits light at the same wavelength (1.3 µm). Dispersion and signal loss are very small, which allows you to transmit signals over a much greater distance than in the case of using a multimode cable. For a single-mode cable, laser transceivers are used, which use light only with the required wavelength. Such transceivers are still relatively expensive and not durable. However, in the future, single-mode cable should become the main type due to its excellent performance. In addition, lasers are faster than conventional LEDs. Signal attenuation in a single-mode cable is about 5 dB/km and can even be reduced to 1 dB/km.

Rice. 3. Propagation of light in a multimode cable

In a multimode cable, the trajectories of light rays have a noticeable spread, as a result of which the signal shape at the receiving end of the cable is distorted (Fig. 3). The central fiber has a diameter of 62.5 µm and the outer sheath diameter is 125 µm (this is sometimes reported as 62.5/125). A conventional (non-laser) LED is used for transmission, which reduces the cost and increases the life of the transceivers compared to single-mode cable. The wavelength of light in a multimode cable is 0.85 µm, while there is a spread in wavelengths of about 30 - 50 nm. The permissible cable length is 2 - 5 km.

Multimode cable- This is the main type of fiber optic cable at this time, because it is cheaper and more available. The attenuation in a multimode cable is greater than in a single mode cable and is 5 - 20 dB/km.

The typical delay for the most common cables is around 4-5 ns/m, which is close to the delay in electrical cables.
Fiber optic cables, like electrical cables, are available in plenum and non-plenum.