Optical fiber welding. Part 1: cables and their cutting, optical instrument, couplings and cross-sections, connectors and adapters


Fibers are charged in the welding machine
Hello, readers of Habr! Everyone has heard about optical fibers and cables. There is no need to tell where and why optics are used. Many of you are faced with it at work, someone is developing backbone networks , someone is working with optical multiplexers . However, I did not meet the story about optical cables, sleeves, cross-countries, about the technology of splicing of optical fibers and cables. I am a fiber optic splicer, and in this (my first) post I would like to tell and show you how all this happens, and I will also often be distracted by other things related to this in my story. I will rely mainly on my own experience, so I fully admit that someone will say “this is not entirely correct”, “here it’s not canonical.”
There was a lot of material, so there was a need to break the topic into pieces.
In this first part you will read about the device and cable cutting, about the optical instrument, about the preparation of fibers for welding. In other parts, if the topic turns out to be interesting to you, I will talk about methods and show on video the process of splicing the optical fibers themselves, about the basics and some nuances of measurements on optics, I will touch on the topic of welding machines and reflectometers and other measuring instruments, I will show the jobs of the welder ( roofs, cellars, attics, hatches and other fields with offices), I’ll tell you a little about cable fasteners, wiring diagrams, equipment placement in telecommunication racks and boxes. This is probably useful to those who are going to become a joiner. All this I flavored with a large number of pictures (I apologize in advance for the paint-quality) and photos.
Caution, a lot of pictures and text.

Part 2 is here .

Introduction


First, a few words about me and my work.
I work as an optics splicer. He started with a telephone operator and an installer, then worked in the emergency team to service the main optics. Now I work in an organization that takes general contracts for the construction of facilities and communication lines from various companies. A typical construction project is a cable line connecting several containers of GSM base stations. Or, for example, several FTTB rings. Or what's smaller - for example, laying a cable between two server cables on different floors of a building and unpacking at the ends of the cable cross.
If the tender is won, suitable subcontractors performing the work (design and survey and construction and installation) are sought. In some regions these are our subsidiaries, in some they have their own equipment and resources, in some independent companies are hired. Our main responsibility lies with the control, elimination of jambs of subcontractors and various force majeure, all kinds of approvals with land owners and administrations, sometimes the preparation of executive documentation for the constructed facility (documentation - mainly RD 45.156-2000, here there is a list, plus a section with different licenses is added) and so on. Often, you need to work with optics: weld or digest somewhere an optical coupler or cross, eliminate the consequences of a support knocked down by a street racer or a tree falling on a cable, carry out incoming inspection of the cable drum, take the trace of the section and so on. It is these tasks that I perform. Well, incidentally, when there are no problems in optics, there are other tasks: from loading and installation through courier and delivery to copy and paper work. :)

Optical cable, its types and insides


So what is an optical cable? Cables are different.

By design, from the simplest ones (the sheath, underneath are plastic tubules-modules, the fibers themselves are in them) to the super-heaped (many layers, two-level armor - for example, underwater transoceanic cables).

At the place of use - for outdoor and indoor installation (the latter are rare and usually in high-end data centers, where everything should be perfectly correct and beautiful). According to the laying conditions - for suspension (with Kevlar or cable), for soil (with armor made of iron wires), for laying in cable ducts (with armor made of corrugated metal), underwater (complex, ultra-protective multi-layer construction), for suspension on power transmission towers (in addition to transmitting information, they play the role of lightning protection cable). In my practice, cables for suspension on poles (with Kevlar) and for laying in the ground (with armor) are most often found. Less often come across with a cable and with corrugated armor. Still often there is a cable, which essentially is a thin paired optical patch cord (the yellow sheath in the single mode and the orange in the multimode, a bit of Kevlar and one fiber; two shells are paired). Other optical cables (unprotected, underwater, for indoor installation) - exotic. Almost all the cables I work with are designed as in the picture below.


1 - central power element (in other words, a fiberglass bar, although there may be a cable in a plastic sheath). Serves to center the tube-modules, to give rigidity to the entire cable. A cable is also often fastened to it in the coupling / cross, clamping it under the screw. With a strong bend of the cable, it has a vile property of breaking, breaking along the way and modules with part of the fibers. More advanced cable designs contain this bar, dressed in a polyethylene sheath: then it is harder to break and it will cause less damage in the cable during a fracture. The bar can be the same as in the picture, and very thin. The tip of such a rod is an excellent abrasive tool for delicate work: for example, to clean the relay contacts or a section of a copper part for soldering. If you burn it for a couple of centimeters, you get a good soft brush. :)
2 - optical fibers (in the figure - in varnish insulation). The very finest fiber-optic fibers for which everything is started. The article will focus only on glass fibers, although somewhere in nature there are also plastic fibers, but they are very exotic, they are not cooked by devices for welding optics (only mechanical connection) and are suitable only at very short distances and I personally have not encountered them . Optical fibers are single-mode and multi-mode, I have met only with single-mode, since multi-mode is a less common technology, can be used only for short distances, and in many cases it is perfectly replaced by single-mode. The fiber consists of a glass “shell” of glass with certain impurities (I won’t stop at chemistry and crystallography because I don’t own the topic). Without varnish, the fiber has a thickness of 125 microns (slightly thicker than the hair), and in the center there is a core with a diameter of 9 microns from ultra-pure glass with a different composition and with a refractive index slightly different from the shell. It is in the core that radiation propagates (due to the effect of total reflection at the core – shell boundary). Finally, on top of the 125-micrometer cylinder “shell” is covered with another shell - from a special varnish (transparent or color - for color marking fibers), which EMNIP is also two-layer. It protects the fiber from moderate damage (without lacquer, the fiber bends, but it is bad and easy to break, the fiber will crumble from a mobile phone accidentally placed on it; while in the varnish, you can safely wrap it around the pencil and pull it pretty hard - it will withstand). It happens that the cable span sags on some fibers: it broke (burned, cut) all the shells, Kevlar, the central bar burst, and some 16 or 32 125-micrometer glass fibers can hold the cable span and wind loads for weeks! Nevertheless, even in varnish, the fibers can be easily damaged, therefore, the most important thing in the work of a joiner is meticulousness and accuracy. In one awkward movement, you can spoil the results of an entire day of work or, if you are not particularly lucky and have no redundancy, drop the trunk connection for a long time (if, digging into the “battle” trunk coupling, break the fiber with DWDM under the spine at the cable exit).
Fibers come in many varieties: regular (SMF or just SM), with offset dispersion (DSF or just DS), with non-zero offset dispersion (NZDSF, NZDS or NZ). Externally, they cannot be distinguished, the difference is in the chemical / crystalline composition and, possibly, in the geometry of the central core and in the smoothness of the boundary between it and the shell (unfortunately, this question has not been completely clarified for myself). Dispersion in optical fibers is a harsh and difficult thing to understand, worthy of a separate article, so I’ll explain more simply - fibers with biased dispersion can transmit a signal without distortion further than simple ones. In practice, spikes know two types: simple and “shifted”. In the cable, the first module is often allocated under the "bias", and the rest - under simple fibers. Joining a “shift” and a simple fiber is possible, but undesirable, это вызывает один интересный эффект, о котором я расскажу в другой части, про измерения.
3 - plastic tube modules , in which fibers float in a hydrophobic.
Cable cut to modules

They easily break (more precisely, they suddenly bend) when bent like telescopic antennas in household receivers, breaking fibers inside themselves. Sometimes there is only one module (in the form of a thick tube), and there is a bunch of fibers in it, but in this case you need too many different colors for marking the fibers, so usually several modules are made, in each of which 4 to 12 fibers. There is no single standard for coloring and the number of modules / fibers, each manufacturer does it his own way, displaying everything in the passport on the cable. The passport is attached to the cable drum and is usually stapled to a tree directly inside the drum.
Cable passport

Типичный паспорт на кабель. Извиняюсь за качество.


However, there is hope that, say, the DPS cable from the manufacturers of Transvok and Beltelekabel will still be the same in configuration. But still, you need to look at the passport on the cable, where the detailed color is always indicated and what type of fiber in which modules are located. The minimum capacity of the "adult" cable that I met is 8 fibers, the maximum is 96. Usually 32, 48, 64. It happens that 1 or 2 modules are taken from the entire cable, then black dummy plugs are inserted instead of the rest of the modules (so that the overall parameters cable has not changed).
4 - film braiding modules. It plays secondary roles - damping, reducing friction inside the cable, additional protection against moisture, keeping the hydrophobic in the space between the modules and, possibly, something else. It is often additionally tied with threads crosswise and on both sides moistened with a hydrophobic gel.
5 - thin inner shell made of polyethylene. Additional moisture protection, protective layer between Kevlar / armor and modules. May be absent.
6 - Kevlar threads or armor . In the figure, the armor is made of rectangular rods, but it is much more common from round wires (in imported cables - steel wires and difficult to bite even with wire ropes, in Russian - usually from nail iron). The armor can also be in the form of fiberglass rods, the same as the central element, but in practice I have not encountered such. Kevlar is needed so that the cable can withstand a lot of tensile strength and not be heavy. It is also often used instead of a cable where there should be no metal in the cable to avoid interference (for example, if the cable hangs along the railway, where there is a contact wire with 27.5 kV nearby). Typical values ​​of permissible tensile force for a cable with Kevlar are 6 ... 9 kilonewtons, this allows you to withstand a large span under wind load. When cutting Kevlar, the cutting tool is terribly stupid. :) Поэтому его лучше резать или специальными ножницами с керамическими лезвиями, или откусывать тросокусами, что я и делаю.
As for the armor - it is designed to protect the underground cable lying directly in the ground, without protection in the form of a plastic pipe, cable duct, etc. However, the armor can only be protected from a shovel, the excavator will still tear any cables. Therefore, the underground cable is laid in the ground at 1 m 20 cm, and above it at a depth of 60 cm a yellow or orange signal tape with the print “Caution! Do not dig! Below the cable ”, as well as along the route are placed columns, warning signs and full houses. But still digging and tearing.
7 - outer thick shell made of polyethylene . The first to accept all the hardships when laying and operating the cable. The polyethylene is soft, so it is easy to cut it when the cable is not properly tightened. It happens that when laying an underground cable, the contractor will tear this sheath several meters to the armor and will not notice it, moisture in the soil gets into the cable despite hydrophobic, and then on delivery, when testing the outer sheath with a megohmmeter, the megaohmmeter shows low resistance (high leakage current) .


If the hanging cable touches a concrete pillar or tree, polyethylene can also quickly rub to fibers.
Between the outer shell and the armor, a plastic film and some hydrophobic gel may be present.

In Russia, unfortunately, optical fibers are no longer produced (here, alas, a joke about polymers would be appropriate). There is a Russian laboratory manufacturing experimental fibers for special purposes, as suggested esvaf .
They are bought from companies such as Corning, OFS, Sumitomo, Fujikura, etc. But here they make cables in Russia and Belarus! Moreover, in my practice, 95% of the cables I worked with are cables from Russia or Belarus. At the same time, imported fiber is laid in the cable. Offhand, I recall such cable manufacturers as Beltelekabel, MosKabel Fujikura (MKF), Eurocable, Transvok, Integra-cable, OFS Svyazstroy-1, Saransk-cable, Incab. There are others. Of the imported cables, only Siemens remained in memory. Subjectively, all cables are similar in design and materials and do not differ much in quality.
Here, in fact, I talked about the device of optical cables. Move on.

Cable cutting: necessary tool and technique


To cut the cable, as well as for welding, a number of specific tools are required. A typical set of a fusion splicer is a suitcase with NIM-25 tools, it contains all the necessary strippers, cable tacks, screwdrivers, side cutters, pliers, a breadboard knife and other tools, as well as a pump or vial for alcohol, a supply of hydrophobic solvent “D- Gel ”, non-woven lint-free napkins, electrical tape, self-adhesive marker numbers for cables and modules and other consumables.

After understaffing with consumables (screeds, worm clamps, etc.) and some auxiliary tools, it is quite enough for working with optics. There are also other sets, richer and poorer in configuration ("NIM-E" and "NIM-K"). The weak point of most sets is the low quality of the “type of aluminum” case, which only looks beautiful, but actually consists of a thin fiberboard glued with textured / corrugated foil, and aluminum thin riveted corners. It does not withstand long in field and city conditions, and it has to be repaired and strengthened. In my case, the case withstood 3 years and, being completely wounded, pulled together with corners and bolts, with a “collective farm” organizer instead of a native one, was replaced with a regular plastic tool box. Some tools and materials from the standard kit may be of poor quality. I personally did not need some tools. Some over 3 years of work have already been replaced. As “branded” consumables are consumed, some are replaced by “improvised” without compromising the quality of work. So, factory non-woven lint-free napkins for wiping fibers are easily replaced with toilet paper of the "throat plus" type. :) The main thing is to be unflavored. Instead of the expensive (about 800 r / liter) D-Gel, if the work is done outdoors, AI-92 gasoline can be used.

When cutting cables, it is important to maintain the length of the cable elements in accordance with the requirements of the instructions for the coupling: so, in one case, you may need to leave a long power element to fix it in the coupling / cross, in another case it is not required; in one case, a pigtail is braided from the Kevlar cable and clamped under the screw, in another case, the Kevlar is cut off. It all depends on the specific coupling and the specific cable.

Consider cutting the most typical cable:

a) Before cutting a cable that has been damp for a long time or without a waterproofing end, cut off about a meter of the cable with a hacksaw (if the supply allows), since prolonged exposure to moisture negatively affects the optical fiber (it may become cloudy) and other cable elements. Kevlar filaments in the cable are an excellent capillary that can “pump” water into itself for tens of meters, which is fraught with consequences if, for example, high-voltage wires run in parallel with the cable: currents can flow along wet Kevlar, water evaporates, crushes from the inside the outer sheath, the cable goes in bubbles and new moisture gets through the bubbles from the rains.

b) If there is a separate cable for the suspension in the cable structure (when the cable in the cross section is in the form of the number “8”, where the cable is in the lower part, in the upper cable), it is bite out with cable taps and cut off with a knife. When cutting the cable, it is important not to damage the cable.

c) An appropriate stripper knife is used to remove the outer sheath of the cable. NIM-25 is usually equipped with a Kabifix knife as in the photo below, but you can also use a knife-stripper for electric cables, which has a long handle.


Such a stripper knife has a rotating blade in all directions, which can be adjusted in length in accordance with the thickness of the outer sheath of the cable, and a clamping element for holding on the cable. Important: if you have to cut cables of different brands, then before cutting a new cable you need to try the knife at the tip and if it cuts too deep and damaged the modules, the blade must be tightened up shorter. Worse than ever, when the clutch is already welded, and suddenly, when laying the fibers, one fiber suddenly “pops out” of the cable, because when cutting, the knife hooked the module and broke this fiber: all the work in vain.
To remove the outer sheath of the cable with a knife-stripper, a circular cut is made on the cable, and then two parallel cuts from opposite sides of the cable to the end of the cable from it, so that the outer sheath splits into two halves.


It is important to correctly set the length of the blade of the stripper knife, since if the blade is too short, the outer shell will not easily split into two halves and it will take a long time to peel off with pliers, and in the case of a long blade, you can damage the modules in the depth of the cable or blunt the rotating blade against the armor.

d) If the cable is self-supporting with Kevlar, then the Kevlar is cut off with cable or scissors with special ceramic blades.

Cable tricks

Kevlar should not be cut with a knife or simple scissors without ceramic overlays on the blades, since Kevlar quickly blunts a metal cutting tool. Depending on the design of the coupling, it may be necessary to leave a part of Kevlar of a certain length for fixing, this will be discussed in the installation instructions for the coupling.
If the cable is designed for laying in the telephone conduit and contains only metal corrugation from the armor (so that the rats are not gnawed), it can be cut longitudinally with a special tool (a reinforced plow knife), or carefully make a circular pipe cutter or even an ordinary knife on the corrugation and stagger to achieve growth of metal fatigue at the place of risk and the appearance of cracks, after which you can remove part of the corrugation, bite the modules and tighten the corrugation. This cutting should be carried out with extreme caution, since it is easy to damage the modules and fibers: the corrugation is not too strong, can be washed in the place where it is picked with tools, and when pulled from the fibers, sharp edges at the point of break can break through the modules and damage the fibers. A cable with a corrugation is not the most convenient for cutting.
If the cable is armored with round wires, they should be bitten off with wire ropes in small batches, 2-4 wires each. Side cutters are longer and harder, especially if the wire is steel. Some couplings require a certain armor length for fixing, and armor (including corrugated) often needs to be grounded.

e) For the inner, thinner sheath present in some cables (for example, self-supporting with Kevlar), you should use a separate, pre-configured knife stripper (you can use the same one as for removing the outer sheath of the cable) so as not to interfere with the knife length settings every time when cutting the cable. In this case, it is especially important to correctly set the length of the blade in the stripper knife, it will be less than in the stripper to remove the outer sheath of the cable, since the inner sheath is much thinner, and immediately below it are the modules with fibers. With a certain skill, you can use a conventional breadboard knife to remove the inner shell, making a longitudinal cut with it, but there is a significant risk of damage to the modules. You can also use a stripper-clothespin to cut the coaxial.

f) Threads, plastic film and other auxiliary elements are removed from the modules using napkins and D-Gel / gasoline. The threads can be twisted one at a time, can be peeled off with a special sharp “plow” hook (may be included in the design of some stripper knives to remove the sheath). To remove the hydrophobic, a D-Gel solvent is used (a colorless oily liquid, has an orange smell, is toxic) or gasoline. However, it is neat with gasoline: office workers who have gasoline pouring at their side will not be pleased with the aroma. Yes, and fire.
Work should be done with disposable gloves (surgical, polyethylene or construction gloves), since hydrophobic is a very unpleasant muck (the most unpleasant in the work of the welder!), It is difficult to wash, after gasoline or hydrophobic hands remain greasy for some time, and after cutting the cable, the fibers will be welded, requiring clean hands and a workplace. In winter, hands stained in hydrophobic are very cold. However, having gotten a grip, you can cut cables almost without getting your hands dirty.
After removing the threads and separating the module bundle into separate modules, each module is wiped with napkins or rags with D-Gel solvent / gasoline, and then with alcohol until clean. Although, in order to save time and to get dirty less, you can do the following way - initially, cut the cable to the modules not completely, but in the place where the cutting starts, 30 centimeters, without wiping anything to bite the modules (see point “g”) and pull the entire bundle of modules with winding and threads from the fibers, holding the clean end of the cable like a handle on your hand. Hands remain almost clean, time is saved. However, with this method of cutting, there is a risk of tearing part of the fibers or applying excessive tensile force to the fibers, which will negatively affect the attenuation of the fibers in the future, as well as a greater likelihood of damage to the modules, therefore, this method is not recommended, especially in winter, when the hydrophobic aggregate thickens. First you need to learn how to do it right, and then try different optimizations.

g) At the required length, each module (except for dummy modules, they are bitten out under the root, but first you need to make sure that they really do not have fibers) is bitten by a stripper for modules (suitable for a copper coaxial as well), after which the module can be pulled without much effort with fibers.

Biting the stripper modules is a very crucial moment. It is necessary to choose a notch of the exact diameter, since if the notch is larger than necessary, the module will not bite enough to easily peel off, if less, there is a risk of biting the fibers in the module. In addition, you should carefully monitor the stripper’s dog-clamp: if, at the moment of biting the module, it blocks the stripper’s reverse motion, locking it in the “closed” state, then to undo the stripper and tilt the latch, you will again have to close the tool on the already bitten module, there is a high probability of biting the module, which will lead to the need to re-cut the cable. Remember that when biting one of the modules, other modules actively interfere with us, which must be held with the other hand, and the cable itself must also be held in weight somehow. Поэтому поначалу будет очень неудобно и разделывать кабель следует вдвоём.
There are cable designs where the module is single and has the appearance of a rigid plastic tube in the center of the cable. For a high-quality removal of such a module, it should be cut in a circle with a small pipe cutter (not included in NIM-25), and then carefully break the circular risks in place.
When tightening the modules, make sure that all the fibers are intact and that no fibers are left sticking out of the tightened module.
If the temperature is low, the modules are thin, the hydrophobicity of the modules in the modules is small (= lubrication), or the length of the modules to be removed is significant - the module may not pull off the fibers without effort. In this case, you cannot pull strongly, since stretching can affect the attenuation of the fibers in this place, even if the fibers do not break. You should bite and remove the module in 2-3 doses, in parts and slowly.
When cutting the cable, pay attention to the length of the fibers. It should be no less than indicated in the instructions, usually 1.5-2 meters. In principle, it can be cut into 15 cm and then even welded somehow, but then when laying the fibers in the cassette there will be big problems: a large supply of fibers is needed just so that there is room for “maneuvers” when laying, so that you can play ”along the length and beautifully put all the fibers in the cassette.

Sometimes it becomes necessary to weld into a transit cable without cutting it. In this case, it, just like the usual one, is pulled down to modules, but the requirements for careful cutting are stricter: after all, the cable can already go through the connection. It is cut up to modules and the modules are carefully inserted into the “oval” coupling input (if they don’t enter the regular round, they will break), for this input a special set of heat shrinkage and a metal clip with a hot-melt adhesive block are used. This adhesive, when shrunk from high temperature, melts and fills the space between the two cables, ensuring tightness. Next, the module into which it is necessary to weld is cut, those fibers from it that do not need to be soldered are welded back in transit, and those that we need are welded to the “solder” (branching) cable. Very rarely a situation may arise when we need to take fiber from the module, but you can’t cut the module (an important connection follows). Then apply set for longitudinal cutting of modules : the “chamfer" is longitudinally removed from the module, the fibers are removed from it, wiped from the hydrophobic and sorted. Those that we need are cut and boiled on another cable according to the scheme, and the rest just fit into the cassette. In this case, if a continuous cable is started, the fiber length should be twice as long (2-3 m), this is understandable.

The fibers must be clean (thoroughly wiped from hydrophobic), special care should be taken to ensure that all fibers are intact. Fibers require careful handling, because in the case when the cables are cut and wound up, the welding is almost finished and some fiber breaks at the exit of the cable, you will have to re-cut the cable and weld, which will be time-consuming and extremely undesirable and unprofitable for quick connection recovery on the existing highway.

Optical fibers damaged as a result of careless cable cutting (the length of the stripper blade was incorrectly set to remove the inner sheath of the cable, as a result of which the modules were cut and some of the fibers were damaged)

g) The fibers should be thoroughly wiped with lint-free alcohol wipes to completely remove the hydrophobic aggregate. First, the fibers are wiped with a dry cloth, then with cloths dipped in isopropyl or ethyl alcohol. Such an order is named because there is a huge drop of hydrophobic on the first napkin (alcohol is not needed here), but on the 4-5th napkin you can call for help to dissolve the hydrophobic residues. Alcohol from the fibers evaporates quickly.

Used wipes (as well as scraps of cable sheath, chipped fibers and other garbage) must be cleaned up after yourself - take pity on the nature!
The purity of the fibers, especially closer to the ends, is of great importance for high-quality welding. Where micron work is in progress, dirt and dust are not acceptable. The fibers should be inspected for the integrity of the lacquer coating, lack of dirt, broken parts of the fibers. If the varnish on some fiber is damaged, but has not yet broken, it is better not to risk it and re-cut the cable. Spend 10-15 minutes, otherwise you risk spending the whole day.

h) Special adhesive thermal shrinkage is put on the cut cables, which are often included in the coupling kit (if the coupling is equipped with a cable entry pipe). If the coupling provides for clamping the cable in raw rubber with sealant, then heat shrink is not necessary. A very common and very unpleasant mistake of a beginner is to forget to wear shrink! When the sleeve is welded, heat shrink is pushed onto the sleeve of the sleeve and seated with a gas torch, blowtorch or industrial hairdryer, providing a tight cable entry into the sleeve and additional cable fixation. It is most practical to seat with a small burner, worn on a can of tourist gas with a price clamp: one canister is enough for dozens of welded couplings, it just lights up, unlike a blowtorch, weighs little, there is no dependence on electricity, unlike an industrial hair dryer.
Before shrinkage, the sleeve of the coupling and the cable itself must be sanded with rough sandpaper for better adhesion of the glue. If you neglect this, you may get this misunderstanding:


If you still forgot to put on the heat shrink, a heat-shrink sleeve with a lock (known as XAGA) will help. Collective farms cannot be sealed with electrical tape!
Some heat shrinks (for example, by Raychem) are covered with dots of green paint, which blackens when heated, indicating that this place is no longer needed to be heated, but here it is necessary to warm it up again. This is done because the heat shrink can burst if it is overheated in some place.
Seating is best after the clutch is welded. If during welding there is a nuisance (for example, the fiber breaks and you have to re-cut the cable), then you do not have to pick a frozen thick adhesive shrink with a knife, and the heat shrink itself will not be wasted.

i) The cut cables are inserted into the coupling or cross, fixed, and the coupling or cross itself is fixed on the desktop. When fixing the cable in the coupling or in the cross, you should follow the installation instructions - for different couplings everything is different there. In some cases (armored cable and, for example, MTOK A1 coupling with the appropriate input kit) fixing the cable in the coupling is a separate difficult operation with cutting the armor, winding the sealant, etc.
So we brought the cut cable into the sleeve / cross, now we need to measure and strip the fibers, wear KDZS and cook according to the scheme. I’ll talk about this in the next part, since it turns out to be a bit much for one article.

Optical couplers


I’ll tell you a little about optical couplings and crosses. I'll start with the couplings.

An optical coupler is a plastic container into which cables are inserted and connected there. Earlier, in the late 90s and early 2000s, when all the specialized materials for optics were in short supply with sky-high prices, some nimble guys molded sewer fittings or plastic bottles as couplings. Sometimes it even worked for several years. :) Today it is, of course, wildness, normal couplings can be bought in any medium and large city and prices start at 1500-2000 rubles. There are many designs of couplings. The most massive and familiar design for me personally is like the MTOK series of Svyazstroydetalevsky couplings. There is a headband from which pipes for cable entry protrude from the outside. A metal frame is attached to the inside of the headband, to which optical cartridges are attached. A cap is put on top (which for strength can be made with stiffening ribs), sealed with an elastic band. The cap is fixed with a detachable plastic clamp: the coupling can always be opened and closed without spending a repair kit from heat shrinks.


In general, Svyazstroydetal makes generally good couplings for various applications. Of the MTOK series, I personally most like the L6 coupling: universal, inexpensive, easy to install.


There are others. муфты в серии МТОК — малогабаритные, для канализации, для ввода бронированных кабелей, для закапывания под землёй. Each clutch has the opportunity to purchase additional components and kits for cable entry: for example, cast iron armor protection of the underground coupling “MCHZ”, an extra set of optical cassette with consumables or an additional set for inputting another cable.
If it is cheaper, they have a series of MOG couplings, of which the most popular is the MOG-U clutch (Optical City Coupling, Shortened): at a price of less than 2000 rubles, we get a simple and high-quality clutch, which, however, some consider inconvenient for installation.


On a post such a coupling will not look very good, and it is inconvenient to wind up a cable supply with such a coupling while standing on the stairs, so they are usually placed in hatches. This clutch was created to be put in a telephone hatch on special standard consoles. The disadvantage of the “mogushka” is that it does not have a locking collar and you need to cut the heat shrink to open it, and when closing, spend the repair kit from wide heat shrinks (if the cables are wired from one end) or a heat-shrink sleeve (if the cables are on both sides). The MTOs of series A suffer from this. In addition, if you enter cables from two sides, it is important not to forget to put a plastic pipe on one of the "sides" of the cables beforehand, otherwise you can’t put it on without cutting it: beginners also suffer from this.

Also sometimes there are couplings without nozzles in which the cables are sealed by clamping in raw rubber or in sealant. Here, for example, is the “SNR-A" coupling, which my partner and I have welded as part of the construction of the FTTB ring.


This method of sealing the cables requires great care, as otherwise water can get into the sleeve, which is undesirable. Firstly, the water in the coupling can cause cloudiness of the glass fiber and damage to the varnish over time. Secondly, all sorts of metal structural elements will rust, the grounding wire of the armor will rot, if any. Thirdly, Kevlar will pull in water. And most importantly - a clutch full of water in the cold will simply crush along with the fibers.
An optical coupler usually has at least two cables. Of course, you can come up with a wild pin-out scheme when one cable is introduced and welded onto itself, but usually 2-3 cables are introduced. If 4-5 cables are introduced, and all cables are different with different colors and different number of fibers in the modules, then the coupling is difficult to install and then parse what is soldered to where. I cooked my first clutch with my partner for 3 days! :) So it is better to design the network so that no more than 3 cables enter the coupling.

Optical cross


Optical cross is designed to terminate the cable in the place where it was brought: at the base station, in the information center, in the data center, in the server. A typical cross is a metal box of size 19 "for mounting in a standard rack, a terminated cable is inserted into the back, strips with ports are located at the front.
Welded cross with 24 ports type FC / APC, single-unit
Welded cross with 64 ports type LC, 2-inch
A working cross for 96 ports of type FC
There is also a cheaper option - when everything that is possible is thrown out of the cross, then it turns out something like this:

Open cross on 8 ports of type SC / APC, 1 unit. The bad news is that optical pig-tails are not protected by anything and they can be broken by those who dig in a drawer / rack by pulling, say, a new cable.

All of these crosses are mounted in a rack, but there are wall options, and other rare ones.

Wall cross on 16 ports like FC. By the way, it is poorly welded: the yellow shells of the pigtails do not go into the CDS and the fibers can break, and the fibers in the cassette are laid with small bending radii

The cable introduced into the cross is welded with the so-called pigtails: in the photographs these are thin yellow shoelaces inside the crosses. Each fiber has its own pigtail. The other side of the pigtail contains an optical “plug” connector, which is inserted into the optical “socket” adapter from inside the cross-country. Outside of the cross-connect, optical patch cords (thick yellow cords) connect. The patch cord differs from the pig-tail by a more durable connector and the presence of Kevlar inside, so that if someone clings to the patch cord and pulls, it is difficult to pull it out. Well, the patch cord connectors are on both sides, and the pigtails have only one. If necessary, a temporary patch cord can be welded from two pigtails.

In principle, several cables can be made into a cross, part of the fibers from them can be welded together, and some of them can be brought to ports. Then you get something that can be called a "cross-coupling", while we save on materials and welding. This is sometimes done during the installation of FTTB, but it is undesirable to do so, since the complexity of the circuit increases.

Adapters and Connectors


Optical crosses are characterized by the adapters used in them (more simply, by optical sockets). They also have a large number of standards and sub-standards.

In this picture - only a part of the “genera” and “types” of optical sockets

The standard is a complex of an adapter (outlet) and a connector (plug). Of course, there are adapters between different standards, but these are crutches that are suitable only for measurements and which should be avoided in a constantly working communication line. The less in the line all kinds of welded and especially mechanical joints, the better. Of course, if the distance is small, the line will work, even if a couple of decibels are lost on some of the crosses. In the case of short lines, optical attenuators are sometimes specially placed. But for very long lines, where the equipment runs to the limit, the addition of another cross or coupling (that is, some 0.05-0.1 dB loss) can be fatal: the line will not rise.

The tip of the “plug” is, roughly speaking, a cylinder with a thin through hole in the center of the fiber. The end face of this cylinder is not flat, but slightly convex. The tip consists of awesomely hard and resistant to destructive scratches cermets, although metal ones are very rare. Rumor has it that people broke side cutters, trying to bite this tip. :) I myself easily scratched steel and glass with these tips. Nevertheless, they must be handled with care, to prevent dust from entering, not to touch the end of the connectors with your finger, and if touched, wipe with a cloth moistened with alcohol. Ideally, a special microscope (optical or with a camera) is used to monitor the condition of patch cords. Dirty - cleaned, scratched, if a scratch crosses the center with glued fiber - for cancellation or polishing. Грязные и исцарапанные розетки и патч-корды — частая причина затуханий в линии.
The optical fiber is fixed in the tip by gluing it with epoxy (or some other) glue and then polishing it on a special machine, although this is only done if you need to make long non-standard patch cords: it is easier and cheaper to buy ready-made ones. The price of a conventional optical patch cord 2 meters long is about 200-400 rubles.

Making patch cords. Emilink
In practice, such standards as FC, SC, LC are most often used. Less common are FC / APC, SC / APC, ST. LC can be both duplex and single.

FC





Pluses - excellent quality of connection, therefore it is suitable for responsible highways. Old proven standard. Metal (hard to break). If you move a well-screwed connector with your hand, this will not affect the connection.
Cons - unscrew / twist for a long time when switching. If the cross is located closely - it is very inconvenient to crawl to unscrew one of the connectors in the crowd of others.
The connector itself is fixed motionless thanks to a groove on it and a recess on the adapter, and only a notched nut rotates with your fingers.

The contact side of the tip is not flat, but slightly convex (this also applies to other standards), so that two fibers of two tips on opposite sides of the outlet (pigtail and patch cord) are guaranteed to combine without air and dust between them.
The socket contains a hollow thin-walled ceramic cylinder having a longitudinal section. When the plug is inserted into the outlet, the cut is heard in some microns, springing and centering the plug. Thus, a precise alignment of the two connectors in the socket is achieved (remember that the signal is transmitted along the fiber core with a diameter of 9 μm and a shift of even 1 μm causes a loss of signal power at the socket and spurious back reflection). Therefore, dust and dirt are detrimental to optical crosses, patch cords and pigtails should be regularly cleaned with a lint-free cloth with alcohol, and sockets should be blown with compressed air or cleaned with special cleaning sticks. A common reason for the loss of connection is a broken ceramic insert in the outlet.
In order for the connectors to be pressed tightly against each other in the socket, in each FC and FC / APC connector (whether it is a patch cord or pigtail connector), the cermet tip is spring-loaded and can “push” into the plug for about a millimeter and a half. In the SC, LC, ST standards, the entire plug is spring-loaded, and in the case of ST, the locking element is very similar to the one used in local networks on a thin coaxial.

SC





Everything is the same as in FC, only the adapter and connector are square, plastic and the connector is fixed by clicking, not screwing. Pros - cheaper than FC, more convenient and faster to switch, cons - plastic is easier to break, less connection-disconnection resource. Sometimes it happens that the magnitude of reflection and attenuation at the connection noticeably changes after touching the connected connector, which is undesirable for critical lines. The color of the connectors is usually blue.

LC and LC Duplex





The properties are similar to SC, but they have much smaller dimensions: a two-unit cross on LC accommodates as many as 64 ports, and on SC only 32. Due to their small dimensions, they are often mounted directly on optical multiplexer boards.

FC / APC, SC / APC, LC / APC

The same as FC, SC and LC, but with oblique (A - angle, angle) polished tip.

The difference between ceramic tips with regular and oblique polishes. The image is a little inaccurate: in fact, in the case of both polishing, the ends are not flat, but slightly convex, respectively, when connecting, only the tip centers where the fiber will touch.

Such adapters and connectors are made green and when compared with regular polishing UPC (or just PC), the difference is visible with the eye. This is necessary to reduce the back reflection at the junction of the two connectors. As far as I know, this type of polishing was developed for the transmission of analog television through optics, so that there is no double image on the screen, but I could be wrong.
It is possible to join “normal” and “oblique” polishing between each other, but only if it is necessary to remove the trace according to the principle “if only the length of the track was visible”: a large air gap will give strong losses and strong back reflection.

Today my story is over. Ask questions, try to answer. If you find this topic interesting, I will write a sequel.