Multimode optical fibers. Multimode fiber
Fiber optic cables have a similar structure, but can differ in various characteristics. By the number of modules, fibers, thickness, outer shell material, etc. Optical cables are either single-mode or multimode. A single-mode optical cable is designed to transmit one beam of light, while a multimode cable is designed to transmit several beams. As a rule, single-mode optical cable intended for use in telecommunication networks, for creating data transmission highways to long distances.At the same time, multimode ones are used in medium and short-range networks. has a structure different from multimode. IN lately It is said that multimode fiber optic cables have an advantage over single-mode cables, this is essentially true because they are more than a hundred times superior to single-mode in performance. But, despite all this, for long distances it is still preferable to use single-mode optical cables, because they have long proven themselves in this area.
Purpose of single-mode optical cable
A modern single-mode optical cable is a type of fiber optic cable and is intended for transmitting one beam of light (multi-mode beams are transmitted simultaneously through multimode) when used as part of telecommunication networks and when organizing highways that transmit data over long distances.Although the structure is similar, existing fiber optic cables differ in their characteristics, depending on the number of modules, thickness, number of fibers, outer sheath material, etc. A single-mode optical cable, unlike a multimode one, when transmitting a signal, by definition, is devoid of inter-mode dispersion, which arises as a result of different modes simultaneously introduced into the fiber reaching the opposite end of the cable at different times. One of important characteristics The cable is also the SCS diameter of its core; for single-mode it is usually 8-10 microns.
Through practical studies of various optical cables, experts have determined that at distances exceeding 500 meters between objects, it is worth giving preference to single-mode cables, which provide high and reliable transmission speeds over long distances when building large-scale networks. Multimode cable showed lower results.
Features of single-mode optical cable
A single-mode optical cable received its name due to the fact that during operation a small number of modes are formed in the optical fiber, therefore it is conventionally assumed that light propagates along a single path, therefore, such a fiber is called single-mode. And so, modern optical fiber can carry more than two hundred parallel fibers, and, as a rule, it is possible to combine combinations of fibers of different types in one cable.Structurally, a fiber optic cable consists of a single or several optical fibers, which are essentially glass threads. Accordingly, the transmission of information is carried out by the transfer of light within the optical fiber. This uses a process called complete internal reflection. The operating principle is based on the fact that light waves are reflected from the boundary separating two transparent media with different refractive indices.
Most often, single-mode optical cable is used to organize fiber-optic communication systems laid through tunnels, collectors and inside buildings and premises. Its outer shell is usually made of materials that do not support or propagate combustion.
Advantages of single-mode optical cable
Modern single-mode optical cable is characterized by significant advantages over previously used ones copper conductors. These certainly include:- significantly larger bandwidth,
- increased degree of noise immunity (in particular, in the field of immunity to electromagnetic interference and interference),
- relatively small volume and weight,
- light signal with low attenuation,
- galvanic isolation of newly connected equipment,
- reliable protection against unauthorized connections, which further protects transmitted information, etc.
Despite the huge variety of fiber optic cables, the fibers in them are almost the same. Moreover, there are far fewer fiber manufacturers (Corning, Lucent, and Fujikura are the best known) than there are cable manufacturers.
According to the type of construction, or rather according to the size of the core, optical fibers are divided into single-mode (SM) and multimode (MM). Strictly speaking, these concepts should be used in relation to the specific wavelength used, but after considering Figure 8.2, it becomes clear that at the current stage of technology development this can not be taken into account.
Rice. 8.3. Singlemode and multimode optical fibers
In the case of multimode fiber, the core diameter (typically 50 or 62.5 µm) is almost two orders of magnitude larger than the wavelength of the light. This means that light can travel through the fiber along several independent paths (modes). At the same time, it is obvious that different fashions have different lengths, and the signal at the receiver will be noticeably “smeared” over time.
Because of this, the textbook type of stepped fibers (option 1), with a constant refractive index (constant density) over the entire cross-section of the core, has not been used for a long time due to large mode dispersion.
It was replaced by gradient fiber (option 2), which has an uneven density of the core material. The figure clearly shows that the path lengths of the rays are greatly reduced due to smoothing. Although rays traveling further from the axis of the light guide travel greater distances, they also have a higher propagation speed. This happens due to the fact that the density of the material from the center to the outer radius decreases according to a parabolic law. And the light wave propagates faster, the lower the density of the medium.
As a result, longer trajectories are compensated by greater speed. With successful selection of parameters, the difference in propagation time can be minimized. Accordingly, the mode-to-mode dispersion of a graded fiber will be much less than that of a fiber with a constant core density.
However, no matter how balanced gradient multimode fibers are, this problem can be completely eliminated only by using fibers with a sufficiently small core diameter. In which, at the appropriate wavelength, one single beam will propagate.
In reality, fiber with a core diameter of 8 microns is common, which is quite close to the commonly used wavelength of 1.3 microns. Interfrequency dispersion remains with a nonideal radiation source, but its influence on signal transmission is hundreds of times less than intermodal or material dispersion. Accordingly, the throughput of a single-mode cable is much greater than that of a multimode cable.
As is often the case, the higher performance fiber type has its drawbacks. First of all, of course, it is more high cost, due to the cost of components and installation quality requirements.
Tab. 8.1. Comparison of single-mode and multimode technologies.
| Options | Singlemode | Multimode |
| Wavelengths used | 1.3 and 1.5 µm | 0.85 µm, less often 1.3 µm |
| Attenuation, dB/km. | 0,4 - 0,5 | 1,0 - 3,0 |
| Transmitter type | laser, less often LED | LED |
| Core thickness. | 8 µm | 50 or 62.5 µm |
| Cost of fibers and cables. | About 70% of multimode | - |
| Average cost converter to twisted pair Fast Ethernet. | $300 | $100 |
| Fast Ethernet transmission range. | about 20 km | up to 2 km |
| Transmission range of specially designed Fast Ethernet devices. | more than 100 km. | up to 5 km |
| Possible transfer speed. | 10 GB or more. | up to 1 GB. on a limited length |
| Scope of application. | telecommunications | local networks |
Types and types of connectors
Let's consider detachable connections. If the range limit of high-speed electrical lines based on twisted pair depends on the connectors, then in fiber-optic systems the additional losses they introduce are quite small. The attenuation in them is about 0.2-0.3 dB (or several percent).
Therefore, it is quite possible to create complex topology networks without the use of active equipment, by switching fibers on conventional connectors. The advantages of this approach are especially noticeable in short but extensive last mile networks. It is very convenient to divert one pair of fibers for each house from the common backbone, connecting the remaining fibers in a junction box “for passage”.
What is the main thing in a detachable connection? Of course, the connector itself. Its main functions are to fix the fiber in the centering system (connector), and protect the fiber from mechanical and climatic influences.
The basic requirements for connectors are as follows:
· introducing minimal attenuation and back reflection of the signal;
· minimal dimensions and weight with high strength;
· long-term operation without deterioration of parameters;
· ease of installation on cable (fiber);
· Easy to connect and disconnect.
Today, several dozen types of connectors are known, and there is no single one on which the development of the industry as a whole would be strategically oriented. But the main idea of all design options is simple and quite obvious. It is necessary to accurately align the axes of the fibers and press their ends tightly against each other (create contact).
Rice. 8.6. Operating principle of pin-type fiber optic connector
The bulk of connectors are produced according to a symmetrical design, when connectors are connected using special element- coupler (connector). It turns out that first the fiber is fixed and centered in the tip of the connector, and then the tips themselves are centered in the connector.
Thus, it can be seen that the signal is affected the following factors:
· Internal losses - caused by tolerances on the geometric dimensions of the optical fibers. This is the eccentricity and ellipticity of the core, the difference in diameters (especially when connecting fibers different types);
· External losses, which depend on the quality of the connectors. They arise due to radial and angular displacement of the tips, non-parallelism of the end surfaces of the fibers, and the air gap between them (Fresnel losses);
· Reverse reflection. Arises due to the presence of an air gap (Fresnel reflection of the light flux in the opposite direction at the glass-air-glass interface). According to the TIA/EIA-568A standard, the back reflection coefficient is normalized (the ratio of the power of the reflected light flux to the power of the incident light). It should be no worse than -26 dB for single-mode connectors, and no worse than -20 dB for multimode;
· Contamination, which in turn can cause both external loss and back reflection.
From one remote point to another, increasingly, instead of traditional copper wire, the customer is offered laying by contractors. About this interesting technology we'll talk today.
They work on the principle of transmitting a light wave through a special channel made of especially pure quartz glass. Electrical impulses from electronic equipment arrive at which generates a stream of light flashes and transmits them to the cable. At the other end, the receiver receives the light stream and transcodes it back to Since the entire process is controlled electronically and is a digital conversion, distortion is minimal.
To build such fiber optic lines, special materials are used - single-mode fiber and multimode.
Optical lines have become so widespread not only due to the absence of interference during signal transmission. Among undeniable advantages This technology has a wide band, very low signal attenuation, unsurpassed resistance to any interference of an electromagnetic nature, and a huge transmission range of many tens of kilometers. A significant advantage is the long service life of communications laid using fiber-optic lines, which is at least 25 years.
Types of optical fiber
When installing communication lines using fiber optic links, either multimode or single-mode fiber is selected.
What does this cable consist of? The core of the optical fiber is quartz, ultra-pure glass, which transmits the light flux through it. But sputtering does not occur because the refractive index of the cladding is lower than that of the core, therefore, the light beam is completely reflected from the walls inside the fiber.
Multimode optical fiber is good because it can run several hundred light modes at once, which are introduced under different angles. Each such mode has its own trajectory and, as a consequence, a unique propagation time.
Main disadvantage This type of fiber has mode dispersion, which narrows and limits the maximum line length. Transmitters for multimode communication lines usually have a maximum range of about 5 kilometers.
The problem of reducing mode dispersion is solved by a cable with a gradient refractive profile of the core. In such an optical fiber, unlike standard options, the refractive parameters 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 task of passing through only one mode (the main one). There are many benefits to this approach. Some characteristics of a cable made using single-mode technology are an order of magnitude better than those made using multimode technology. This is exactly what it is decisive factor, which influences the choice of engineers in favor of the first when laying new fiber-optic lines. After all, single-mode fiber provides a signal attenuation of 0.25db per kilometer, the dispersion value in it is very small, and the wide bandwidth ensures clear and fast transmission of large amounts of data without distortion.
But there is a fly in the ointment in this barrel of honey. This type is much more expensive than multimode fibers. Since the size of the fiber core in a single-mode cable is very small, introducing radiation into such a cable is not an easy task and requires very careful control during splicing. The termination connectors for these lines also cost much more than the terminations for multimode lines. In addition, the latter, due to the ease of introducing a light beam into a wide core, have very simple and cheap emitters, which are also produced by a huge number of competing companies.
Translation by Anna Motush
Definition: fibers supporting more than one mode for a particular polarization direction
Multimode fibers are optical fibers that support multiple transverse modes for a given optical frequency and polarization. The number of modes is determined by the wavelength and refractive index of the material. Multimode fibers are divided into step index fibers and gradient fibers.
The values of the core radius and numerical aperture are determined for the fibers, allowing the V-parameter to be determined. For large values In the V-parameter, the number of modes is proportional to V 2 . In particular, for fibers with a large core diameter (right side of Fig. 1), the number of modes can be very large. Such fibers can deliver light from poor quality beam (for example, generated by high-power diodes), but to maintain a quality beam from a high-brightness light source, it will be better to use a fiber with a smaller core and a moderate numerical aperture, although effectively introducing the radiation into the fiber may be more difficult.
Compared to standard single-mode fiber, multimode fiber typically has a larger core as well as a high numerical aperture, such as 0.2-0.3. The latter allows you to work when bending the fiber, but also leads to more intense scattering, which is determined by the violation of the geometric shape of the optical fiber. The consequence of these violations is that some of the rays leave the optical fiber. The intensity of scattering depends not only on the quality of the material from which the core is made, but also on the quality of the cladding, since part of the optical signal also propagates in it. The refractive index profile is mostly rectangular, but sometimes parabolic is also found. (See below).
Multimode fiber consists of a core and a cladding. In common types of fiber optic communication lines (see below) based on 50/125 and 62.5/125 multimode fibers, the core diameter is 50 and 62.5 microns, respectively, and the cladding diameter is 125 microns. Such fibers support hundreds of modes.
Injecting light into multimode fiber is quite simple, because The requirements for maintaining the accuracy of adjusting the angle and position of the beam are not very strict. On the other hand, the spatial coherence at the output of multimode fibers is low, and the output intensity distribution is difficult to control for the reasons explained below.
Figure 2 shows the profiles electric field in modes with refractive pitch of fibers designed for a specific wavelength. This is the main mode (LP 01) with an intensity distribution close to Gaussian, and several higher order modes with more complex spatial profiles. Each mode has a different propagation constant. Any field distribution can be considered as a superposition of modes.
Total electric field, common in multimode fiber, is a superposition of several modes. The intensity depends not only on the optical power in all modes, but also on the relative phase, where a maximum or minimum may arise due to the interference of different modes.
Both power and phase are determined by the initial conditions, and the relative phases vary continuously along the fiber due to the dependence on propagation constants. Thus, the complex pattern of intensity over time changes continuously over a propagation length well under 1 mm.
Figure 3 shows an animated example showing intensity distributions occurring at 2 µm intervals. This interference pattern is highly dependent on any changes in bending or stretching of the fibers, as well as on temperature.
Please note that for light with wide optical throughput(for example, for white light) such complex intensity distributions are not observed because the intensity plot is different for each wavelength, so that the contributions from different wavelengths are averaged out. The longer the fiber, the lower the optical frequency range required for this averaging.
Singlemode and multimode optical cable
A thin transparent vein that carries light is defined fiber optic. The main purpose of an optical cable is the basis of lines capable of transmitting a packet of digital data to over fast speed. The structure of the optics is small: the core, the internal casing and the external casing, which protects the optical fiber from external negative factors. Each of these elements plays a different role in the functioning of fiber optics.
Today there are known types of optical fiber: single-mode And multimode.
Singlemode optical cable
IN single-mode optical cable core size is +/-9mm at standard size sheathing 125 mm. Only one core can fulfill its functional purpose, which is typical for this type of optical fiber. When beams pass through an optical fiber, the trajectory of their movement is constant and simultaneous, so the structure of the supplied signal cannot be distorted. Digital signals can be transmitted over distances of many kilometers without the risk of radiation scattering. To work with monocore optics, a laser is used, which uses light with a specific wavelength. good general characteristics provide reasons for the use of optical fiber of this type everywhere, however, its high cost and relative fragility reduce the evaluation criteria.
In turn, single mode fiber can be:
- with shifted beam dispersion.
Optical fiber of this type has a smaller core diameter, which allows it to be used in the operating range of 1.5 microns on wide-bandwidth lines using optical amplifiers. - with shifted minimum wavelength,
in which an optical fiber can support a single propagated signal. This optical fiber uses big indicator power when transmitting data over long distances, and was developed for use in maritime lines. - with non-zero shifted ray scattering.
When using this type of optical fiber, nonlinear effects will not be able to affect the quality of the supplied signal and its structure, which represents possible use this optical fiber in DWDM technology systems.
Multimode optical cable
IN multimode optical cable(see section) the light rays are significantly scattered, and at the same time a significant distortion of the structure of the transmitted signal occurs. The core has an indicator of +/- 60 microns, the lining is standard - 125 microns. The use of a conventional LED for the operation of a multicore (as opposed to a laser, which is used in a monocore optical fiber) ensures an increase in the operational life of the optical fiber and has a positive effect on its cost. At the same time, the attenuation rate in a multicore is increased compared to a monocore and fluctuates within 15 dB/km.
Multimode fiber differs in stepped And gradient.
The stepped fiber optic cable has large beam dispersion due to the uneven jumping layers of density of the quartz core, so its application is limited to short communication lines. Gradient optical fiber has reduced radiation scattering due to the smooth distribution of the refractive index. The core diameter of the gradient multicore optical fiber is +/- 55 microns, the cladding has standard value(125 µm).
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