We explored the principles behind how light travels through optical fiber last month. In the first installment two months ago, we took a look at reasons for installing optical fiber in the first place-namely fiber's high transmission rates and immunity. We also explained the three main types of optical fibers and explained the manufacturing process. We will now wrap up the series with installation details covering different types of fiber-optic cables, connectors, and splices.
Optical data transmission Optical data transmission (the method of sending data through optical fiber cables) is illustrated in Fig. 1. This drawing shows a telephone conversation traveling over optical fiber cabling. The voice is first transformed by the telephone into an electrical signal. This signal is then scanned by a digital encoder, which reduces the signal into binary code (a series of "offs" and "ons"). The driver, which activates the LED or laser light source, transmits the "ons" as bursts of light and the "offs" as the absence of a light pulse. The light travels through the optical fiber cable until it is received at its destination, amplified, and fed into a digital decoder. The decoder translates the digital signal back into the original electrical signal. And finally, the telephone changes the electrical signal back into sound.
Transmitters and decoders are the devices which change electricity into light pulses and change light pulses back into electrical pulses.
Simplex and duplex transmission Optical transmissions are called simplex, half-duplex, or full-duplex in terms of their method of transmission. Both simplex and half-duplex systems use one fiber to communicate, and are less expensive to build.
The simplex method transmits in only one direction, while the half-duplex system can send signals in both directions, but not both directions at the same time. Half-duplex is similar to a two-way radio in its operation. The terminal electronics must act as both a receiver and transmitter. They are appropriately called transceivers.
The full-duplex system, on the other hand, uses two fibers to communicate. This fiber always transmits from point A to point B while the other fiber is transmitting from B to A. Therefore, both ends of a full-duplex system have both transmitters and receivers. Because of this, you must be careful not to mix them up during installation by reversing the transmit and receive fibers. Sometimes the fibers are color coded, or they may have some other identifying feature such as a ridge marking one fiber on duplex cables. In any case, be careful to not reverse the two fibers when installing connectors on the ends.
Optical cables Because of the variety of conditions to which they are exposed, optical fibers have to be encased in many layers of protection.
The coating mentioned above is a buffer that is typically extruded over this coating to further increase the strength of the single fibers. This buffer can be either a loose or tight tube. Most datacom cables are made using either one of these two constructions. A third type, the ribbon cable, is frequently used in the telecommunications work and may be used for datacom applications in the future.
Loose-tube (loose buffer) cable is used mostly for long-distance applications and outside plant installations where low attenuation and high cable pulling strength are required. Loose tubes are similar to thin drinking straws, into which optical fibers are inserted. (Picture a thread inside of a straw). Several fibers can be incorporated into the same tube, providing a small-size, high-fiber density construction. The cost per fiber is also lower than for tight buffered cables. The tubes are filled with a gel, which prevents water from entering the cable, and offers additional protection to the fibers. Because these cables are terminated either by fusion splicing to pre-connectorized pigtails or by using breakout kits, they are more cost effective for longer distance applications than they are for short-distance applications. The fibers are completely separated from the outside environment so the loose tube cables can be installed with higher pulling tensions than tight-buffered cables.
A tight-buffer cable design is better when cable flexibility and ease of termination are a priority. In the tight-buffer design, a layer of thermoplastic is extruded directly over the fiber in the same way as electrical wires are covered with a layer of plastic insulation. Most indoor cables have a tight-buffer design because of the relatively short distances between devices and distribution racks.
A ribbon cable consists of up to 12 coated fibers bonded to form a ribbon. Several ribbons can be packed into the same cable to form an ultra-high-density, low cost, small-size design. (More than 100 fibers can be put into a 1/2-inch-square space with ribbon cables.) Ribbon fibers can be either mass fusion spliced or mass terminated into array connectors, saving up to 80% of the time it takes to terminate conventional loose- or tight-buffer cables.
Simplex and zip cord cables contain one or two fibers, tight-buffered, Kevlar reinforced and jacketed. They are used for patch cord and backplane applications.
Tightpack cables are also known as distribution cables and contain several tight-buffered fibers bundled under the same jacket with Kevlar reinforcement. They are used for short, dry conduit runs, riser and plenum applications. These cables are small in size, but because their fibers are not individually reinforced, these cables need to be terminated inside a patch panel or junction box.
Breakout cables are made of several simplex units, cabled together. This is a strong, rugged design, and is larger and more expensive than the tightpack cables. It is suitable for conduit runs, riser and plenum applications. Because each fiber is individually reinforced, this design allows for a strong termination to connectors and can be brought directly to a computer backplane.
Loose tube cables are composed of several fibers cabled together, providing a small, high fiber count cable. This type of cable is ideal for outside plant trunking applications. Depending on the actual construction, it can be used in conduits, strung overhead or buried directly into the ground.
Hybrid or composite cables: These terms have caused confusion, especially since the 1993 United States National Electrical Code switched their terminology from "hybrid" to "composite."
Under the new terminology, a composite cable is one that contains a number of copper conductors properly jacketed and sheathed depending on the application, in the same cable assembly as the optical fibers.
This situation is made all the more confusing since another type of cable, which was formerly called composite, contains only optical fibers but have two different types of fibers. These fibers are multi-mode and single-mode.
Remember that there is confusion over these terms; with some people using them interchangeably. At this point the proper terminology is the following: A composite cable is a fiber/copper cable. A hybrid cable is a fiber/fiber cable.
Connectors and splices Connecting and splicing optical fibers is one of the more labor-intensive part of the installation process. Pulling the cables into place is relatively easy, and other parts of the installation are comparably simple. But connecting these glass fibers correctly requires time, special tools, and specific skills.
All fiber joints must meet two criteria. They must be:
1. Mechanically strong: Fiber connections must be capable of withstanding moderate to severe pulling and bending tests.
2. Optically sound with low loss: Since the purpose of fiber is to transmit light, the fiber joint must transmit as much light power as possible with as little loss and back reflection as can be designed into the joint.
Fiber connections fall generally into two categories: the permanent or fixed joint which uses a fiber splice, and the non-fixed joint which uses a fiber optic connector.
Splices are used as a permanent connection. Typical uses include reel ends, for pigtail vault splices and at distribution b reakouts. The criteria for good fiber splices are low loss and high mechanical strength. Additional considerations are expense per splice and possible reusability of the splice itself.
Fiber optic connectors are used as a termination for inside cables, outside cables as they terminate in a central office, for interfaces between terminals on LANs, for patch panels, and for terminations into transmitters and receivers. Whether one joins fibers using splices or connectors, one negative aspect is always common to both methods-signal loss. This loss of light power at fiber connections is called attenuation, measured in decibels.
Another type of loss is back reflection or reflectance and is measured as return loss. Light travels through the fiber, passing through splices and connections. When it finally arrives at the end point, some of that light is reflected back by fiber end faces at those points.
Typical allowable splice losses for single mode fiber are 0.0 to 0.25 dB with a return loss or back reflectance of less than - 50 dB. In multimode fiber splices typical losses are 0.0 to 0.25dB with an average of 0.20dB and return loss of less than -50 dB. In the case of fiber connectors, single mode allowable connector losses range from 0.05 to 0.5 dB per connector (0.1 to 1.0 dB per connection) and return loss typically is less than -30 dB. Multimode connectors have a nominal connector loss of 0.06 to 0.7 dB per connector (0.12 to 1.4 dB per connection) with a return loss less than -25 dB typical.
Connectors Remember that connectors are used as terminating fixtures for temporary non-fixed joints. As such, they are made to be "plugged-in" and disconnected several or many times. Since no one connector is ideal for every possible situation, a wide variety of connector styles and types have been developed over the short life of fiber communications.
Examples of common of connectors can be seen in Fig. 4. Connector compatibility exists between same types from different manufacturers (for example, AT&T's ST connector can be used interchangeably with AMP's ST connector). Adapters are generally available in either sleeve connectors or patch cords to allow coupling of different types of connectors. No single connector is best for every application. The following are the currently popular connectors found in many different types for various applications.
Datacommunications (mostly multimode) Telecommunications (mostly singlemode)
SMA (decreasing) FC/PC (widely used)
ST (most commonly used) ST (singlemode version)
SC (specified for newer systems) SC (growing)
FDDI (duplex) D4 (decreasing)
ESCON (duplex) Biconic (decreasing)
Cable termination and connector installation Before the installation of connectors onto a fiber cable, a breakout kit may have to be installed. This procedure will not be necessary on breakout cables having 2 mm buffered fibers, but will be required on 250, 500 or 900 micron tight buffer cables. The break out kit consists of 2 mm buffer tubing over 900 micron inner tubing. The bare fibers are inserted into these buffer tubes to provide handling protection and strength when mounted onto connectors.
Installing a fiber connector onto a pigtail or unbuffered fiber is a widely varied process. The three most common ways of accomplishing this task are:
1. Epoxy glue with oven cure, then polish
2. Hot melt pre-glued, then polish
3. Cleave and crimp, no polish. The epoxy glue method is the oldest and is still widely used today. This process involves filling the connector with a mixed two-part epoxy. The prepared and cleaned fiber is then inserted into the connector. After curing the epoxy in an oven for the proper time (10 to 40 minutes) the fiber is scribed and cleaved nearly flush with the end of the connector and polished with a succession of finer and finer lapping papers. Typical polish papers start at 3 microns and go as fine as 0.3 micron grit
The Hot Melt (trademark of 3M Co.,) uses a preloaded connector. The connector is placed into an oven to soften the glue and allow insertion of the prepared fiber. After cooling, the scribe and polish process is the same as the previously described polish process.
Cleave and crimp connectors, on the other hand, do not need a polish procedure. The connector already has a polished ferrule tip and requires only the insertion of a properly cleaved fiber to butt against the internal fiber "stub." Once in place, the fiber connector is crimped to hold down the fiber.
Each mounting method has its advantages and disadvantages, varying from ease of installation to cost per connector to performance qualities.
Terminating singlemode fibers Terminating singlemode cables generally uses a combination of connector installation and splicing. Since the singlemode connector has very fine tolerances, they are generally terminated in a manufacturing lab. The proper fiber insertion and physical contact polishing can then be controlled precisely. Complete cable assemblies with connectors on both ends are made and tested,
Because testing a cable with two ends is easier than with bare fiber on one end. In the field, the assemblies are cut in half and spliced onto the installed backbone cables. Although the splice adds some additional loss and cost, the overall method provides a higher yield and better connection at lower cost than trying to control the termination process in the field.
Splices Splices are a fixed type of normally permanent joint. The two basic categories of splices are fusion and mechanical. Generally speaking, splices offer a lower return loss, lower attenuation, and are physically stronger than connectors. Also, splices are usually less expensive than connectors (two connectors being required to equal one splice), they require less labor, constitute a smaller joint for inclusion into splice closures, offer a better hermetic seal than connectors, and allow either individual or mass splicing.
Fusion splicing Fusion splicing uses an electric arc to ionize the space between prepared fibers to eliminate air and to heat the fibers to proper temperature (20007 F). The fibers are then fed in as semi-liquids, and meld together. The previously removed plastic coating is replaced with a plastic sleeve or other protective device. The perfect fusion splice results in a single fiber rather than two fibers having been joined. One drawback to fusion splicing is that it most generally must be performed in a controlled environment, such as a splicing van or trailer, and should not be done in open spaces because of dust and other contamination.
Fusion splicing in manholes is prohibited because of the explosive gases, which are frequently found in such locations, and because of the electric arc generated during this process.
Due to the welding process, it is sometimes necessary to modify the fusion parameters to suit particular types of fibers, especially if it is necessary to fuse two different fibers (fibers from two different manufacturers or fibers with different core/cladding structures).
Mechanical splicing Mechanical splicing is quick and easy for restoration, its major use. It is also sometimes used for new construction. It does not require a controlled environment other than a reasonable level of dust control. A mechanical splice is stronger than most connectors, although fusion splices are stronger. Back reflection and loss vary dramatically from one type of splice to another.
Equipment investment for specific splicing kits are far less expensive than for fusion splicers. Splices are either glued, crimped or faced. Mechanical splices all must use some type of index matching gel or liquid which is subject to contamination and aging. Those splices which require adhesive glue can become outdated as the glue ages.
Mechanical splices use either a V groove or tube-type design to obtain fiber alignment. The V groove is probably the oldest and still most popular method especially for multi-fiber splicing of ribbon cable. This type of splice is either crimped or snapped to hold the fibers in place.
Tubular splices on the other hand may rely on glue or crimping to hold the fibers together. The fibers are inserted into a small tube, which causes alignment to occur.
Faced type of splices are very much like miniaturized connectors using ferrules and a polishing process.
Completed splices, whether fusion or mechanical, are placed into splicing trays that are designed to accommodate the particular type of splice in use. Splicing trays then fit into splice organizers and in turn into a splice closure.