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Optical Fiber Intro 

In the design of a optical fiber transmission system, the first step is to determine which transmitters and receivers are best suited to the signal type. The best way to find the right system is to compare  data sheets and consult with sales engineers to find which products best  meet the system specifications. Once this is done, the next consideration  is the choice of the fiber optic cable itself, the optical connectors to  be used and the method of attaching  these connectors.
This  portion of the system design is not so straightforward and is shrouded in  a great deal of misunderstandings and fear of complex "glass grinding"  techniques by the inexperienced.  This booklet should clarify several  misconceptions about fiber cable and termination.

Cable  Construction
Like  copper wire, fiber optic cable is available in many physical  variations.  There are single and multiple conductor constructions,  aerial and direct burial styles, plenum and  riser cables and   even ultra-rugged military type tactical cables that will withstand severe  mechanical abuse.  Which cable one chooses is, of course, dependent  upon the application.

Regardless of the  final outer construction however, all fiber optic cable contains one or  more optical fibers.  These fibers are protected by an internal  construction that is unique to fiber optic cable.  The two most  common protection schemes in use today are to enclose the tiny fiber in a  loose fitting tube or to coat the fiber with a  tight fitting buffer  coating.

In the loose tube  method the fiber is enclosed in a plastic buffer-tube that is larger in  inner diameter than the outer diameter of the fiber itself.  This  tube is sometimes filled with a silicone gel to prevent the buildup of  moisture as well.   Since the fiber is basically free to "float"  within the tube, mechanical forces acting on the outside of the cable do  not usually reach the fiber.

Cable containing loose buffer-tube fiber  is generally very tolerant of axial forces of the type encountered when  pulling through conduits or where constant mechanical stress is present  such as cables employed for aerial use.  Since the fiber is not under  any significant strain, loose buffer-tube cables exhibit low optical  attenuation losses.

In the tight buffer  construction, a thick coating of a plastic-type material is applied  directly to the outside of the fiber itself.  This results in a  smaller overall diameter of the entire cable and one that is more  resistant to crushing or overall impact- type forces.  Because the  fiber is not free to "float" however, tensile strength is not as  great.  Tight buffer cable is normally lighter in weight and more  flexible than loose-tube cable and is usually employed for less severe  applications such as within a building or to interconnect individual  pieces of equipment. 

Figure 1 is a diagram of the basic  construction of both loose-tube and tight-buffer fiber optic cable
.

 

Figure 2 is a  drawing of the cross section details of a single and a two conductor fiber  optic cable as well as a more complex multi-fiber cable.  Note that  the two conductor cable is similar to the common AC power line  electrical  cable.

 As  can be seen from the diagram, in all cases the fiber/buffer tube is first  enclosed in a layer of synthetic yarn such as Kevlar for strength.   An outer jacket of PVC or similar material is then extruded over  everything to protect the inside of the cable from the rigors of the  operating environment.  In multi-fiber cables, an additional strength  member is also often added.   While most fiber optic cables are  manufactured of totally non-conductive materials, there are some cable  that employ steel tape-wound outer jackets for rodent resistance (direct  burial types) or metallic strength members such as steel wire for aerial  (telephone pole) use.  There are even fiber optic cables with  imbedded copper electrical conductors for transferring power to remote  electronic packages.  

Optical Fiber
Whether loose-buffer or tight-buffer, the actual glass fiber used  in any fiber optic cable only comes in one of two basic types, multimode  fiber for use over short to moderate transmission distances (up to about  10 Km) and single-mode fiber for use over distances that are generally  greater than 10 Km.  Communications grade multimode fiber normally  comes in two sizes, 50 micron core and 62.5 micron core, the latter being  the size most commonly available.  The outer diameter of both is 125  microns and both use the same connector size.  Single-mode fiber  comes in only one size, 8-10 microns for the core diameter and 125 microns  for the outer diameter.  Connectors for single-mode fiber are not the  same as those designed for multimode fiber but can look the same as we  will soon discuss.

Figure 3 is a  drawing of the construction of two types of optical fiber, step index and  graded index.
 
Step index fiber has a core of ultra-pure  glass surrounded by a cladding layer of standard glass with a higher  refractive index.  This causes light traveling within the fiber to  continually "bounce" between the walls of the core much like a ball  bouncing through a pipe.  Graded index fiber on the other hand  operates by refracting (or bending) light continually toward the center of  the fiber like a long lens.  In a graded index fiber the entire fiber  is made of ultra-pure glass.  In both types of fiber however, the  light is effectively trapped and does not normally exit except at the far  end.

Losses in an optical  fiber are the result of absorption and impurities within the glass as well  as mechanical strains that bend the fiber at an angle that is so sharp  that light is actually able to "leak out" through the cladding  region.  Losses are also dependent on the wavelength of the light  employed in a system since the degree of light absorption by glass varies  for different wavelengths.  At 850 nanometers, the wavelength most  commonly used in short-range transmission systems, typical fiber has a  loss of 4 to 5 dB per kilometer of length.  At 1300 nanometers this  loss drops to under 3 dB per kilometer and at 1550 nanometers, the loss is  a dB or so.  The last two wavelengths are therefore obviously used  for longer transmission distances.

The losses described  above are independent of the frequency or data rate of the signals being  transmitted.  There is another loss factor however that is frequency  (and wavelength) related and is due to the fact that light can have many  paths through the fiber.  Figure 4 shows the mechanism of this  loss through step-index fiber.

A light path  straighter through a fiber is shorter than a light path with maximum  "bouncing".  This means that for a fast rise-time pulse of light,  some paths will result in light reaching the end of the fiber sooner than  through other paths.  This causes a smearing or spreading effect on  the output rise-time of the light pulse which limits the maximum speed of  light changes that the fiber will allow.  Since data is usually  transmitted by pulses of light, this in essence limits the maximum data  rate of the fiber.  The spreading effect for a fiber is expressed in  terms of MHz per kilometer.  Standard 62.5 micron core multimode  fiber usually has a bandwidth limitation of 160 MHz per kilometer at 850  nanometers and 500 MHz per kilometer at 1300 nanometers due to its large  core size compared to the wavelength of the propagated light.  Single  mode fiber, because of its very small 8 micron core diameter has a  bandwidth of thousands of MHz per kilometer at 1300 nanometers.  For  most low frequency applications however, the loss of light due to  absorption will limit the transmission distance rather than the pulse  spreading effect.

Optical  Fiber Connectors
Since the tiny core of  an optical fiber is what transmits the actual light, it is imperative that  the fiber be properly aligned with emitters in transmitters,  photo-detectors in receivers and adjacent fibers in splices.  This is  the function of the optical connector.  Because of the small sizes of  fibers, the optical connector is  usually a high precision device  with tolerances on the order of fractions of a thousandth of an inch.

Although there are  many different styles available the most common optical cable connector in  current use is the ST type shown in figure 5.  The connector  consists of a precision pin that houses the actual fiber, a spring-loaded  mechanism that presses the pin against a similar pin in a mating connector  (or electro-optic device) and a method of securing and strain-relieving  the outer jacket of the fiber optic cable.  ST connectors are  available for both multimode and single-mode fibers.  The main  difference between the two is the precision of the central pin.   Since this difference is not readily noticeable, care must be taken to use  the correct connector.  While single-mode connectors will work  properly with multimode emitters and detectors, connectors intended for  use with multimode fiber such as the ST type will not work well (or at  all) in a single-mode system.

The traditional  method for attaching optical connectors consists of first stripping the  jacket from the fiber cable with tools that are almost exact equivalents  of those used for electrical cable.  Once this is done the strength  members are trimmed and inserted into various restraining grommets or  sleeves.  For loose-tube fibers, the buffer tube is then removed  exposing the actual fiber.  For tight-buffer fibers, the buffer  coating is removed with a precision stripping tool that looks like a small  wire stripper.  The process, up to this point is still similar to  preparing copper wire.   It is when the bare fiber is exposed  that the differences (compared to copper wire) occur.  The stripped  fiber is now coated with a quick drying epoxy resin and inserted into a  precision hole or groove in the connector pin.  Then the strain  relieving components are assembled and the basic connector is ready for  finishing.  At this point the end of the bare fiber is protruding  from the front of the connector pin.  The pin is placed in a special  tool that is then used to cleave or cut the tiny glass fiber flush with  the end of the pin.  This takes a second or two.  Next the  connector is placed into a small jig and run over two or three grades of  fine lapping film, the equivalent of ultra-fine sandpaper.  This  completes the polishing of the fiber and the optical connector is ready  for use.  The complete task, not including the 5 minutes of epoxy  drying time, takes anywhere from 5 to 10 minutes per connector depending  on the skill level of the person.

Many people have  reservations about "connectorizing" fiber optic cable due to problems they  have heard about concerning the "grinding and polishing of glass".   When one realizes that the "grinding and polishing" takes less than a  minute, and is done within a simple foolproof fixture, the mystery quickly  evaporates.  In fact, assembling an ST style optical connector is, in  reality no more demanding a task than assembling an older style electrical  BNC.  Once one is completely familiar with the process, (which takes  from 30 minutes to an hour to learn) the longest time interval involved in  the finishing process is waiting for the epoxy to cure.   Never -the-less the reservations continue.  As a result, several  connector manufacturers manufacture so-called "quick-crimp" optical  connectors.  These devices are installed with various mechanical  clamp arrangements and hot melt or instant bond adhesives (or, in some  cases no chemical adhesive at all).  Some of these connectors are  even provided with a pre-polished length of optical fiber in the tip  thereby eliminating the finishing step altogether.  Although these  are a bit easier to install, the original "epoxy-polish" method is really  not one that anyone should fear.   Figure 6 shows the various  steps involved in installing conventional ST connectors.

Other optical  connectors that are available such as the SMA, SC and FCPC are similar in  principle in that they position the fiber in a close tolerance tip which  then mates with an equally precise device on the other end.  They  really only differ from each other in the mechanical way that that  connectors mate to each other.  In any event all optical connector  manufacturers provide detailed, easy to follow step-by-step installation  procedures for their respective connectors.

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