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.
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
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.
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
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.