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Optické vlákno

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ABC práce s optickými vláknami pre začiatočníkov
Introduction on working with optical fibers
The transition from copper to fiber-optic technology means the need for new tools and skills. Aside from acquiring some theoretical knowledge, installers must improve their manual skills as the work with optical fibers requires a different, more careful approach.
Similarly to other types of cable systems, the process of installation of a fiber optic system can be divided into three main stages:
  • laying cables
  • installing connectors and passive components
  • installing and configuring active devices
The way of laying cables is essential for attenuation of the transmitted optical signals. Each cable contains one or more fibers. The fiber type determines the minimum bending radius, however the installer should not confuse the minimum bending radius of a single fiber and of the cable itself.
The minimum bending radius of a cable is usually equal to 20x its diameter. The minimum bending radius of a single fiber depends on its standard, e.g.:
  • 30 mm for G.652D fibers
  • 10 mm for G.657A1 fibers
  • 7.5 mm for G.657A2 fibers
In the case of FTTH (Fiber To The Home) systems, the recommended standard is the G.657A2, with the smallest minimum bending radius. This greatly facilitates the work of the installer and minimizes problems associated with the increase of signal attenuation caused by sharp bends (right angles etc).
During installation of cables it is not allowed to exceed maximum pulling force. Unlike in the case of coaxial or twisted pair cables, the fault location in a fiber is not easy unless the installer has an expensive device - reflectometer.
The connection and configuration of active optical devices is not difficult, in general the procedures are similar to those having place in LAN networks based on UTP cabling. However, the installer should remember about two important issues:
1. IR laser light is invisible to the human eye, but can be very harmful - all connections should be done before any devices are powered on.
2. Optical devices and connectors require high standards of cleanliness - any dirt will increase the attenuation of the optical link.

Most problems experienced by novice installers are encountered during connecting optical fibers. Here are several practical tips that are common for all splicing techniques (fusion splicing, mechanical splicing, gluing and polishing). We hope they will help some people start the adventure with fiber optic technology.
1. Fiber cable reserve and free fibers
It may be part of the installer to think ahead and inform the investor/customer that some spare cable lengths and several additional fibers in each cable can solve many problems in the future.
One point-to-point link requires two fibers, or with the use of WDM technology - a single fiber. When, for example, one needs four fibers, it is reasonable to use an 8-fiber cable than the "minimum" four-fiber one (popular optical cables usually have 2, 4, 8, 16, 24 ... fibers). Fiber optic cables are cheap, but the re-installation cost can be high and should be avoided, so it is much better to have spare fibers for the future needs or simply as a backup in the case of a failure of one fiber.
Fiber cable reserve left by the cable input to the building. The additional length of the cable is stored on a spare cable storage rack mounted on a wall.
It is highly recommended to leave an additional length of the cable on a spare cable storage rack - usually about 10 meters or so. The additional attenuation is negligible in comparison with that caused by splices and connections. In the future, the reserve of the cable can solve problems caused by unexpected situations (new location of a server, a need to change the cable path etc). Having a cable reserve, the installer can make changes without additional costs that would be necessary in the case of laying new cable.
Multimode cables
Single-mode cables
Universal cables
Outdoor cables
2. Distribution frames and boxes
As mentioned earlier, cables and fibers feature respective minimum bending radii. The easiest and fastest solution for securing cable/fiber interconnections and terminations is to use suitable distribution frames and boxes with trays for the cables and fibers. They force correct positioning of the cables and fibers and prevent accidental damages to the fibers at the interconnections and terminations.
View of the TB-02H box with L3551 pigtail, L4211 adapter, and L5550 mechanical splice
Installation components:
  • trays – components of cabinets and distribution frames
  • distribution frames – usually used in 19'' rack systems
  • boxes – used in smaller systems or for line terminations, branching points, at posts etc.
Fiber Optic Distribution Box ULTIMODE TB-04B (wall-mounted)
Fiber Optic Box: ULTIMODE MT-548
Fiber Optic Splice Tray: ULTIMODE P-6
Distribution Box
Splice Tray
3. Fiber optic cable stripping
The protecting layers of optical cables can be stripped off with typical tools such as knives, cutters, strippers. Of course, the installer must take great care of the exposed fibers. The whole cable with the jacket and protective coatings has to be inserted into the frame or box and attached to it - it is not allowed to used for this purpose the extracted fibers.
1. Indoor cable - LSZH jacket, fibers in 0.9 mm buffers.
2. Universal cable - gel-filled central tube, LSZH jacket, fibers in 0.25 mm buffers.
3. Outdoor cable - tugel-filled central tube, LLDPE jacket, fibers in 0.25 mm buffers.
If it is an gel-filled outdoor cable, it is necessary to clean the fibers with isopropyl alcohol from the protective gel before inserting the cable end into a box.
In the case of cables reinforced with aramid yarn, it is necessary to remove it. This operation requires special shears for cutting Kevlar. These specially built tools with blades made of hardened material ensure good results and are durable. The use of ordinary knives or scissors in most cases will not provide the desired effect and may be risky for the optical fibers (mainly due to extra force needed to separate the yarn).
The construction of Kevlar shears blades - special cloves allow for cutting hard and tough aramid fibers
4. Preparation for splicing
This phase of work is the most demanding. The workplace should be kept clean and the installer, equipped with necessary tools, has to use them precisely and effectively.
The first step – preparing the fiber end
The extracted fibers usually have diameters of 0.9 mm (900 μm) or 0.25 mm (250 μm). They consist of multiple layers which are (from the center): core, cladding, buffer, protective coating.
Depending on the type of the fiber, the core has diameter of 62.5/ 50 micrometers (multimode) or 9 micrometers (single-mode). The cladding has diameter of 125 micrometers, and the buffer of 250 micrometers. It is the latest coating in gel-filled cables, colored in accordance with the GB13993.3-2001 standard. In the case of other cables, the protective coating is usually 0.9 mm thick and suitably colored. The fibers of this type are used in the distribution cables including easy access cables with loose tube.
Stripper is used to take off buffer and protective layers before cleaving
Regardless of the splicing/connection method, the final thickness of the fiber/s should be 125 micrometers, both in the case of single-mode and multimode fibers. So, the best choice is a universal tool capable of taking excessive layers from fibers with different diameters.
Three-Hole Fiber Optic Stripper
Three-Hole Fiber Optic Stripper
The stripper has three openings. The first strips 250 µm buffer coating from 125 µm optical fiber. The second hole strips 900 µm buffer coating down to the 250 micron coating. The third hole is used to strip 2-3 mm jackets down to the 900 µm buffer coating. The length of the stripped-off 125 µm fiber should be chosen depending on the splicing method and kind of cleaver.
The second step – fiber cleaning
After preparation of the fiber end, it is necessary to clean it from the dirt and debris left after the removed coatings. It is highly recommended to use for that purpose isopropyl alcohol (IPA). Ethyl alcohol would leave some deposits. The same applies to the wipes - it should be dust-free ones.
IPA Solvent Cleaner (for fiber optics)
Dust-free Kim-Wipes
IPA Solvent Cleaner
Dust-free Kim-Wipes
IPA solvent cleaner and dust-free wipes are also used to clean the face of the ferrule after polishing.
NOTICE! The fiber should be cleaned before cleaving, never in the reverse order!
The dirt from the side of the fiber is moved to its end, that's why the first step is to clean the fiber, then to cut it. The reverse order would effect in leaving some dirt at the fiber tip and in significant deterioration of splice parameters.
Cleaning optical fiber - good and bad practices
The third step – fiber cleaving
Properly prepared and cleaned fiber can be cleaved. For this purpose there have been engineered special, precision cleavers. Cleaving is the most important step for the splice quality and low attenuation of the connection.
Precision Fiber Cleaver F1-6000
High-precision Fiber Cleaver: KeyCleave
Precision Fiber Cleaver F1-6000
High-precision Fiber Cleaver: KeyCleave
In fact, the fiber cleaver is used to scratch the fiber and bend it in order to break it along the scratching. It similar to glass cutting - the final cleaving is the result of the internal stress generated during bending. A mentioned above, after cleaving the fiber should not be cleaned again. A common mistake made by users of F1-6000 cleaver is applying excessive force onto the blade. It leads to micro-cracks and opaque surface.
Correct and incorrect use of the F1-6000 cleaver
4. Fiber splicing
Properly prepared fiber ends can be spliced using various methods, i.e. fusion splicing, mechanical splicing, gluing and polishing.
Arc fusion splicing is performed with special machines which melt the fiber ends together with an electric arc. This method ensures best results, but the equipment may be still too expensive for small installation companies and individual installers.
Mechanical splices are made with the use of special plastic cases that position the fiber ends. The optical gel that is also used in the devices reduces the effects of imprecise cuts and a possible gap between the end faces of the fibers.
Fiber Optic Mechanical Splice: ULTIMODE FAST-MS1 [5 pcs.]
Universal Mechanical Splice: KeyQuick C-MS100-NT
Fiber Optic Mechanical Splice: ULTIMODE FAST-MS1
Universal Mechanical Splice KeyQuick C-MS100-NT
In each method, the length of the bare fiber must meet the specifications of the fusion splicing machine or mechanical splice. Otherwise, the connection may cause significant attenuation that will reduce the range of the link or even will make data transmission impossible.
Fiber splices are delicate, so they must be adequately protected. The best way is to place them in dedicated boxes. Majority of popular installation boxes does not have special compartments for splices, but it is easy to install inside them additional trays intended for this purpose.
Visual Fault Locator: VFL650-5
Visual Fault Locator VFL650-5
Using mechanical splices, the installer can verify the quality of the connections on the basis of "leakages" of light. The VFL650-5 L5934 locator is designed to test single-mode and multimode fiber-optic cables and connections with 2.5 mm ferrules. It emits visible light (650 nm) and its output power is sufficient to check even 5 km long links.
Using visual fault locator, the installer is able to detect typical problems that are normally invisible. Mechanical defects are often found at the ends of fibers, and are the result of poor handling of the cable during installation.
5. Attenuation measurements in fiber optics
On the completion of an optical system the installer should measure the attenuation of optical connections. Measurements of optical power loss in fiber networks performed with optical power meters are possible only when the source of light is calibrated and stable. It is unacceptable to measure the attenuation of an optical path with the use of active devices, such as SFP modules, media converters, etc. Inadequate precision and stability of the light sources embedded in these devices does not allow for reliable measurements, and the uncertainty can reach 1 dB or so.
A measurement of an optical path : x km long cable, two splices and two pigtails with connectors -
total attenuation of 0.4 dB
The diagram above shows typical measurement configuration with GRANDWAY optical laser source and optical power meter. Any measurements should be performed in the same transmission window that will be used in the optical system.
Attenuation measurements of optical paths primarily allow for verification of the quality of the installation work. But it is not the only reason for taking measurements. From a practical point of view, the knowledge of attenuation of optical paths may be necessary to determine the need for using optical attenuators.
Active fiber-optic devices are characterized by two main parameters, output power of the transmitter and sensitivity of the receiver (in the case of WDM transceivers one device is both the transmitter and receiver).
In the case of short links (e.g. in buildings), devices with high output power could overdrive or even destroy the receiving equipment. Accurate measurement of optical path attenuation enables the installer to predict whether and what type of attenuator should be used in a given case.
Optical Attenuator ULTIMODE T-12 (SC, 2 dB)
Optical Attenuator ULTIMODE T-15 (SC, 5 dB)
Optical attenuator ULTIMODE T-12 (SC, 2 dB)
Optical attenuator ULTIMODE T-15 (SC, 5 dB)