Fibre Optic Cables

Firstly, information is sent through fibre optic cables as pulses of light, so the “ones” and “zeros” that make up the information are sent as combinations of the light being on or off. The actual “colour” of the light is in the infrared (this light has a longer wavelength than red light, and red is the longest wavelength of light we can see, so infrared light is invisible to us – which is why it doesn't have a colour).

The wavelength of light is typically measured in nanometers (millionths of a millimeter), which is abbreviated nm, and people can see wavelengths from about 400 nm (which we see as violet light) to 700 nm (which we see as red light). For fibre optic cables, the wavelengths of light used are typically 850, 1,310, or 1,550 nm, because the fibre optic cable is more transparent (that is, has lower attenuation) to these longer wavelengths (this allows longer runs of fibre optic cable). However, the longer wavelength light sources and sensors are much more expensive, so what usually is implemented is as follows:

• 850 nm is used for fibre optic cable runs of up to 2 km (such as within buildings).
• 1,310 nm is used up to about 15 km, such as by telephone companies and other carriers, within a city. For example, the DSL remote concentrators described on this web site use 1,310 nm wavelength light in the fibre optic cables to the central offices.
• 1,550 nm is used for all greater distances, such as 80 km and more, for example, by carriers, between cities.

An important safety note is that because the light is invisible (and can be very bright), it can damage eyes if one looked directly into a lit fibre optic cable. Therefore, you always see warnings about this on fibre optic equipment.

Light typically travels a different speed in different transparent materials, and this is usually expressed as the index of refraction, which is the ratio of the speed of light in the material compared to the speed of light in a vacuum.

When light changes speed, it changes direction, which is why magnifying glasses work – the light rays bend. A strand of fibre optic cable actually has glass with a sharp increase in the index of refraction at the boundary of the cylindrical core of inner glass, and the glass around that is called the cladding. As shown in the diagram above, when light enters the core (from the end), it bounces (which is called total internal reflection – since all of the light reflects) off the boundary between the core and cladding, and therefore ends up at the far end of the fibre optic cable (which is therefore sometimes called a light pipe).

The fibre optic cable is very thin. The diameter is measured in micrometers (thousandths of a millimeter). The cladding has a diameter of 125 µm, which is an eighth of a millimeter (actually, there is usually a primary, and even a secondary coating which may make the diameter up to 250 µm –, but this is still really, really thin. For the single-mode (so called, as there is only one way – or mode – the light travels down the core, so it all arrives at the same time, which is important for longer cable runs) fiber used by carriers, the core has a diameter of about 9 µm or 10 µm.

The strand of fibre optic cable is very fragile, and is protected by a buffer tube. This is a somewhat flexible plastic, and is coloured to so technicians can identify each buffer tube in a cable. For the long-distance fibre optic cables used by carriers, where low loss (also called low attenuation) is most important, but the cable runs don't need to have sharp bends, the buffer tubes are a much larger diameter than the strands of glass, and up to 24 strands will be in each buffer tube. This is called loose tube construction (picture several runs of fishing line, lying in a drinking straw).

Now picture what happens if that drinking straw is bent too much – it kinks, and the fibre optic cables inside will be damaged by being bent so sharply. Therefore, for fibre optic cable runs within buildings, where the cable is short enough that attenuation doesn't matter as much, but where sharper bends are necessary for the confined spaces above ceilings and behind walls, a tight buffer fibre optic cable is used. In this case, the buffer is directly extruded onto each fibre optic strand (similar to plastic insulation on a copper conductor), so the fibre can have a smaller bend radius without kinking.

Note that in all cases there is a minimum bend radius, beyond which the cable will be permanently damaged.



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While glass this thin is flexible (just as a thin copper wire bends easily – but thicker metal won't), if bent too sharply, the glass fibre will crack, and if a small piece breaks and you get a sliver, or some in your eye, that is a problem. Therefore, when technicians are working with bare fibre, they should be wearing safety glasses.

This fibre has a tight buffer – which is the white covering at the lower-left.



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For optical connections, the goal is to have as much of the light from one fibre coupled into the other.

A permanent electrical connection can be made by soldering wires together, and a permanent optical connection can be made by melting the ends of the fibres together, which is called a fusion splice (this requires an expensive machine).

Here is one type of fibre optic connector, called an ST – for “straight tip”, since a predecessor connector (called biconic) used two nesting cones to align the fibres. The locking method used by the ST connector is called bayonet (where you push and turn, so named after the way knives were attached to old rifles). The white tip is a ceramic, which has an extremely low thermal coefficient of expansion (that is, it does not change shape due to temperature changes), so the alignment of the fibres (and therefore the loss through the connector) will not be affected by temperature changes.

There is a small hole running through the length of this ceramic cylinder, into which the bare fibre is placed. The hole is so small, it is barely visible without a microscope.

This type of fibre optic cable is often used for patch cords, where two nearby devices need to be connected (such as in a computer room). Generally, each patch cord will have two fibres (so it is called duplex fibre), one for the transmit data, and the other for the receive data (farther back, these two orange cables are joined, like lamp cord cable).

To identify cables as fibre optic cables (rather than, say, power cables), fibre optic cable jackets are generally orange or yellow. For outdoor cables (which must be black for ultraviolet light resistance), a spiral orange or yellow wrap (which often has a tag identifying the cable's owner) is applied, generally at each utility pole.



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If you look through even a few centimeters (such as a few stacked sheets) of window glass, you'll see a greenish colour. This is due to impurities in the glass, but for typical window pane thicknesses, this attenuation of the light isn't a problem.

A fibre optic cable requires the light to go through many kilometers of glass, so the glass used for fibre optic cables must be extremely pure, resulting in much, much, much (maybe even a few more muches) lower attenuation. However, as a result of contamination with water during the manufacturing process, early fibre optic cables had hydroxyl ions (which have the chemical symbol OH^–) trapped in the glass, and these absorb light in a range of wavelengths centered at 1,383 µm. This is why wavelengths near the 1,383 µm hydroxyl absorption band were not used in early communication systems.

However, for some applications, such as wavelength division multiplexing (WDM, where the fibre's information carrying capacity is increased by sending information simultaneously at different wavelengths), those absorption wavelengths needed to be used. As a result, manufacturing processes were developed to make fibre optic cables (sometimes called “low-water”, or “low water-peak attenuation”) that did not have significantly greater attenuation in the hydroxyl absorption band. These fibres are called enhanced single-mode (ESM) fibre optic cables (most cables manufactured now are of this type, even if that characteristic isn't needed).

The last long number printed on the cable jacket is for product tracability (such as manufacturing batch number and the factory and line identification).



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• It is made by Draka Comteq USA, Inc. (their web site is http://www.drakacomteq.us).
• If the reel is laid flat, the coils of cable can overlap, and if these are not corrected before pulling the cable, too high a pulling tension may need to be used, which can damage the cable, hence the “DO NOT LAY FLAT”.
• From their datasheet here, we can see that this 48-fibre enhanced single-mode (ESM) cable is of loose-tube (LT) construction. The “ezPREP” refers to this manufacturer's combination of features (such as not using a sticky gel to keep water out of the cable) which make this cable easier for a technician to prepare for splicing. The “PSP” is this manufacturer's designation (and the “A2J” is the customer's designation) that this cable has double jacket with a single corrugated steel tape armor layer between. The fibre attenuation is 0.35 dB per km (decibels per kilometer) at 1,310 nm, 0.35 dB/km at 1,383 nm (that is, they completely removed the hydroxyl ion absorption peak), and 0.25 dB/km at 1,550 nm (optical fibres typically have lower attenuation at this longer wavelength, which is why undersea fibre optic cables use 1,550 nm light).
• This reel was shipped with 1,500 meters (4,921 feet) of cable.
  
  
  
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  OK, enough about the fibre. How it is installed.

  Firstly, note the installer's right hand is holding on to a stranded galvanized steel messenger wire (also called a messenger strand or cable). For this aerial installation, it is strung between utility poles (supported by a clamp attached to the bolt with the square washer you can see on this side of the utility pole), and provides support for the fibre optic cable (most communication cables are not strong enough to support themselves over the distance between utility poles).

  This machine is a cable lashing machine (or lasher). These were developed by GMP Co. (the full company name is General Machine Products, though the company was founded by George M. Pfundt). It is pulled along the cable (by the rope clipped to it), and spiral-wraps two stainless alloy wires (you can see these extending from the machine towards the utility pole, where they are clamped to the messenger wire) around both the messenger wire and the cable being installed. In this case (as is common), there is already one cable installed (and already lashed to the messenger wire), and this new cable (curving down, from the right side of the lasher) is being over-lashed to that.
  
  
  
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  Here is a close-up of the lasher. It has two spools of wire, each with 450 to 2,200 feet (depending on the type and diameter of wire). A brochure on a newer lasher is here, and some information on the lashing wire is here.
  
  
  
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  The lashing wire is clamped to the messenger wire, there's some slack left in the cable at the utility pole, the yellow spiral wrap is installed, the new cable is aligned with the messenger wire, and the pulling rope is attached, so we're ready for some lashing.
  
  
  
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  One installer pulls the new fibre optic cable taut so it doesn't hang down sharply (so it is easier to lash), and the other installer pulls the lasher along. A wheel in lasher rolls along the messenger wire, and this spins the spools of wire around the cable so the lashing wire is wrapped around the existing messenger wire and cable, and the new fibre optic cable.

  An alternative to messenger cable and lashing is to use “figure 8” cable (so-called as the cross-section looks like an “8”). This has a steel cable (the top of the 8) attached to the fibre optic cable (the bottom of the 8).

  Another alternative for aerial installations is All-dielectric self-supporting (ADSS) cable. This has a hard and smooth surface (so ice can't cling to it) over a layer of Kevlar (DuPont's registered trademark for a type of aromatic polyamide fibre they developed in 1965) fibres (or yarn). These provide both strength and rodent protection. Another reason for the Kevlar not being on the surface is that it needs to be protected from the ultraviolet light from the sun, which would degrade the Kevlar.
  
  
  
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  Let's take a break from the fibre optic cable to talk about what's on a utility pole.

  Here's a classic utility pole. This one is wood, and at the very top is the highest voltage (since it is the most dangerous). In this case, two distribution lines at 27,600 volts. One of these is supplying the pole-mounted distribution transformer (through the scary high-voltage insulator at the top). The other input to this transformer is a ground connection, which runs down the pole to a spike driven into the ground.

  The transformer has a two-phase 240/120 volt output. This means that the voltage is 240 volts “phase to phase” (from one line-voltage wire to the other), and 120 volts from either line to ground. The transformer output feeds the heavier wires which are then run directly to the adjacent houses (and also down the pole to adjacent houses that have paid extra to have a buried electrical service entrance).

  Generally these utility poles are owned by the local electricity distribution utility, but may also be owned by the local telephone company or municipality. In any case, whoever the owner is usually rents space on their poles for a charge that is often $20 to $40 per pole per year.

  After a large safety gap, the next lower service on a utility pole is usually the local telephone company (since they have been using the poles for a long time), then below that will be the cable TV company, and below that any fibre optic cable companies. Sometimes, there are messenger wires (and cables) on both sides of a utility pole.

  On this pole, below the 240/120 volt power lines, the next service is the local cable TV company (you can tell by the characteristic coaxial cable slack bend to the left of the pole – also, farther along the cable you can see the line extenders). The cable company is running their fibre optic cables lashed to the same messenger wire (you can tell by the yellow spiral wrap).

  Below that is the telephone company's service (you can tell by the terminal enclosure with telephone line in-line with the 50-pair distribution cable, and the drop cables coming out from the terminal, going directly to people's homes.
  
  
  
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  At every utility pole, a spiral wrap (yellow here) indicates this is fibre optic cable. The tag has the warning about the laser light source, and contact information for the owner of the cable (don't ask why the grass is growing down, and my foot is in the picture, let's just say it was a difficult picture to get).
  
  
  
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  A fibre optic cable will start out with some large number or strands, such as the 48-strand cable being installed earlier. Eventually, some strands need to be split off into a smaller branch cables to run along cross-streets, and eventually to, perhaps, a two- or four-strand cable which will be run into an office building. Splitting off smaller cables requires splices, and these are contained in a re-enterable (they can easily be opened-up) closure or terminal.

  This is an aerial type, since it is lightweight and waterproof (but not airtight, so is not used in underground vaults).

  Pictured is the “butt” style, since the cables enter at only one side. If cable entry is required on both sides, then an “in-line” (also called “through”) style is used. A typical closure is the 3M Fiber Aerial Terminal (some details are here, but the pictured one is a Corning Cable Systems (which used to be called Siecor) Splice Closure (read about it here).
  
  
  
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  Here is a fibre optic cable splice closure from Preformed Line Products Company (perhaps so-named as the closure is already formed as needed, unlike the lead closures of old, where the installer had to form and solder it as needed). It is a butt style, and has a stainless steel case. This type is used in underground vaults since it is airtight, so won't let corrosive gases oxidize the conductors. Some details are here.
  
  
  
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  Here is a fibre optic cable splicing truck. This one is nice and roomy inside, and has an opening at the back through which to bring in the cable splice (here there is a 6-strand and a 24-strand fibre cable brought up from the cable vault into the truck).

  Once the fibre to a field device is installed, they install a loopback jumper (they call this a glass-loop) at the end of every pair of fibre optic strands, so a technician in the central office can check the fibre at connector losses using an optical time domain reflectometer (OTDR).

  The truck has a 110v generator, heating and air conditioning (you can see the unit on the roof), and inside there is a long work surface, with fluorescent lighting above it to provide even lighting.

  The guard around the vault opening ensures passers-by don't fall into it.
  
  
  
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  Looking down the entrance to the cable vault, we can see:
  • A Preformed Line Products splice enclosure, with the yellow “80 - 200 volts” warning sticker), bringing the copper pairs to the DSL concentrator above.
  • Slotted supports mounted vertically on the wall, with horizontal brackets inserted to support the heavy cables and splice enclosure.
  • An old, lead-sheathed splice near the bottom.
  • A few inches of rain water on the floor, if there is more then the technician pumps it out. There is some recently-installed air pipe (the curved black tube floating in the water) which will be used to supply pressurized air to the cables, to keep them dry and enable remote monitoring of cable jacket breaks.
  
  
  
  
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  This is a smaller fibre splicing truck parked at a cable vault, with the fibre optic cables brought in the back doors. This pick-up truck has a fibreglass “space pack” on the back, which makes for an inexpensive working environment for the technicans.
  
  
  
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  This truck is very cramped inside (there's another guy farther in too). Here the fibre optic cable splicing technician is working at the fusion splicer (generally all outside plant fibre splices are fusion splices, rather than mechanical splices which can be done without any expensive equipment, but have higher attenuation). Above that is the splicing tray of the closure (the flat white plastic), which holds the splice and the excess bare fibre on either side of the splice.

  Some information on a typical fusion splicer is here.
  
  
  
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  Here's the completed fibre run, started earlier by the crew with the truck and fibre optic cable reel. The new cable is now lashed to the existing cable.

  I'm sure you've noticed the loops of fibre optic cable surrounding splice closures. Here, the splice closure and the cable to the left of it are held up with cable ties. By cutting the cable ties, the splice closure can be brought down to ground level, and worked on in a splicing truck. Often these trucks have a little door in the back, just big enough for the splice closure, so they can work in a well-lit dry environment, even if it is hot, cold, rainy, or windy outside.

  The loop to the right of the splice closure is one of the two cables looping back to the left, as that is where it came from.

  The diagram below shows a simplified view of all this.
  
  
  
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  Let's say the feeder cable comes from the left (and is blue here), and to keep things simple, there is a single distribution cable continuing to the right (this is the smaller orange cable). This configuration, with loops at each side supports both the cables coming from either side, and allowing the splice closure to be brought down to the technician's truck.

  Typically, several distribution cables will exit the splice closure, and after the loop to the left (which allows the splice closure to be brought down to ground) will either continue to the right, or loop (at the right) to continue to the left.

  Note that the loops must be large as outdoor fibre optic cable has a relatively large minimum bend radius (the 48-strand fibre here has a minimum bend radius of 11"). Sometimes you see a spoked frame inside the loops to ensure the bend radius isn't too small.
  
  
  
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  The aerial splices are often owned by cable TV companies, since their cable is aerial, as their cables have often been installed after houses were built. The local telephone company typically buried their fibre optic cables before the houses and buildings were built, so their splices are either in enclosures in underground cable vaults, or enclosures such as this (the near one, the far one is for cable TV).
  
  
  
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  This is a Fiber Distribution Interface (FDI) from Corning Cable Systems (previously Siecor, which was jointly owned by Siemens AG and Corning Incorporated), and some technical detail is here. The concrete pad in front is so the technician doesn't get muddy feet if it is raining. In fact, splicing the fibre optic cables in this enclosure requires that it be done at the cabinet (there isn't enough slack in the cabinet to bring the cables into a truck), so the technician usually sets up a tent to ensure a dry work environment.
  
  
  
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  This enclosure is of similar construction as the JWI used for outside plant copper pairs, but is slightly narrower. A clipboard on the left door holds papers, and latches at the top of the doors ensure they don't swing shut. On the right door is a chart in which the technician can record the cable splices made.

  The white panel across the top is a heavy sheet metal shelf which is inserted into the slots along the sides at many heights (similar to adjusting the height of your refrigerator shelves).
  
  
  
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  Here's the chart on the side door. It shows that there is room for 20 splicing trays, each for a feeder cable of up to 24 fibres.
  
  
  
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  This close-up of the top of the chart shows that this FDI is designated 101 (as also shown by the stickers on the front of the cabinet), that it is fed (from the central office, which in this case is about 1 km south) by feeder cable F 25, and the distribution cable is FR 101.
  
  
  
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  Here is the bottom of the chart. It shows that the only splice tray with terminated fibres is the bottom one, and that the first 22 fibre strands of feeder cable F 25 are spliced to strands in the FR 101 distribution cable.
  
  
  
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  Here's a direct buried fibre optic cable coming up from underground, with the jacket and armor removed, and the loose buffer tubes exposed.
  
  
  
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  Here the working shelf is removed. Running through the white plastic guides down the left side are the fibre strands from the feeder cable. There are a total of six flexible plastic tubes, each with 24 fibre strands (for a total of 144 fibre strands), each tube running to a hinged splice tray. At the top are the coloured buffer tubes from the distribution fibre optic cable, these are wrapped around the two mandrels to take up the extra slack. The fibrs run to a point on the right side where the strands are inserted into thin plastic tubes, which are all somewhat clear. Since the buffer tubes typically have four or more strands of fibre each, this allows any distribution fibre (to customers) to be spliced to any feeder fibre strand (from the central office). The extra distribution fibres are run to the top, where they are each stored in cassettes. These cassettes each hold 12 fibres, and they are labelled FR 101 1-12, FR 101, 13-24, and so on, up to FR 101 205-216.
  
  
  
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  At the bottom are six splice trays, each with a clear plastic cover. The feeder fibres enter at the top-left.
  
  
  
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  Only the bottom splice tray has splices, each covered by a bright yellow cover, and inserted into the blue holder. The 24 feeder fibres enter through the single plastic tube at the top-left, which wraps clockwise around the outside track at the right, and terminates at the bottom-right. The bare fibre strands then continue into the centre section, where they wrapped around a few times to leave extra slack so that splices can be redone a few times (the splices much be cut off to redo them).

  The distribution fibres enter at the top-right, in the six clear plastic tubes, the first five tubes each have four fibres, and the sixth tube has two fibres, for a total of 22. The distribution fibres wrap counter-clockwise around the outside track to the bottom-left where the plastic tubes end, and the bare fibres enter the middle, where they wrap around a few times for slack, and are fusion spliced to the feeder fibres.
  
  
  
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  Here's a close-up of the end of the plastic tubes, and the entry of the fibres into the middle section of the splice tray.

  A page from the Superior Essex Inc. catalogue showing the ANSI/TIA/EIA-598-B colour code used for fibre strands is here.