Fiber optics relies on a clear path for its signals. Make every effort to install splices, connectors, and terminations as clean as possible, thus reducing their effects on optical data transmission.
This is the last part of a three-part series on the basics of fiber optics. Last month, we discussed total internal reflection, numerical aperture, chromatic, modal dispersion and types of optical fibers. This month, we'll focus on the transmission of data across a network, splicing and connecting, and light loss.
Optical data transmission. Fig. 1 (original article) illustrates an optical data transmission (the method of sending data through optical fiber cables). This drawing shows a telephone conversation traveling over optical fiber cabling. The telephone transforms the voice into an electrical signal. A digital encoder then scans the signal, converting it 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 "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. Finally, the telephone changes the signal back into sound.
Simplex and duplex transmission. Simplex, half-duplex, and full-duplex define the methods of optical transmission. Simplex and half-duplex systems use only one fiber to communicate, therefor e, they are less expensive to build.
The simplex method transmits in one direction, while the half-duplex system can send signals in both directions (but not at the same time). This makes half-duplex similar to a two-way radio.
The full-duplex system uses two fibers to communicate. This allows one fiber to transmit from point A to point B while the other fiber transmits from B to A. Therefore, both ends of a full-duplex system have both transmitters and receivers. Be careful not to transpose the ends of the fibers during installation. Manufacturers include identification methods for fibers used in a full-duplex system, such as color coding or a ridge marking.
Connectors and splices. Connecting and splicing optical fibers is one of the more labor-intensive parts of the installation process. Joining the glass fibers correctly requires time, special tools, and specific skills. All fiber joints must meet two criteria:
• Mechanical strength: Fiber connections must be capable of withstanding moderate to severe pulling and bending tests.
• 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. Generally, fiber connections fall into two categories: the permanent or fixed joint (using a splice), and the non-fixed joint (using a connector).
Splices offer a lower return loss, lower attenuation, and are physically stronger than connectors. They're usually less expensive, require less labor, constitute a smaller joint for inclusion into splice closures, offer a better hermetic seal, 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 heat the fibers to proper temperature (2,000DegrF). The fibers then feed in as semiliquids and meld together. A plastic sleeve or an other protective device replaces the previously removed plastic coating. This process generally requires a controlled environment, such as a splicing van or trailer, to reduce the possibility of dust and other contamination. Don't use fusion splicing in manholes because of the possible presence of explosive gasses and the electric arc generated during this process.
Due to the "welding" process, it's sometimes necessary to modify the fusion parameters to suit particular types of fibers, especially if you have to fuse two different fibers (from two different manufacturers or with different core/cladding structures).
Mechanical splicing. Mechanical splicing is quick and easy. It does not require a controlled environment other than a reasonable level of dust control. The strength of a mechanical splice is better than most fiber-optic connectors, although fusion splices are stronger. Back reflection and loss vary from one type of splice to another.
Equipment investment for specific splicing kits is far less expensive than for fusion splices. Splices are either glued, crimped, or faced. All mechanical splices must use some type of index matching gel or liquid, which is subject to contamination and aging. The splices requiring adhesive glue can become outdated as the glue ages.
Mechanical splices use either a V-groove or tube-type design to maintain fiber alignment. The V-groove is probably the oldest and still most popular method, especially for multifiber 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, while a small tube inside ensures alignment.
Faced-type splices are like miniaturized connectors using ferrules and a polishing process.
Connectors. Many use connectors as canceling fixtures for temporary non-fixed joints allowing them to be "plugged-in" and disconnected many times. Since no connector is ideal for every situation, manufacturers developed this has lead to the development a variety of styles and types.
The Table, below, depicts examples of common connectors (see Fig. 2, original article, for the most commonly used. Connector compatibility exists between manufacturers (one company's ST connector can be used interchangeably with another's). Adapters are available in either sleeve connectors or patch cords to allow coupling of different types of connectors. Although no single connector is best for every application, listed below are the currently popular connectors.
Cable termination and connector installation. Installing a fiber connector onto a pigtail or unbuffered fiber is a widely varied process. The three most common ways to accomplishing this task are:
• Epoxy glue with oven cure, then polish
• Hot melt pre-glued, then polish
• Cleave and crimp, no polish.
Although it's the oldest method, epoxy glue is still the most widely used. This process involves filling the connector with a mixed two-part epoxy. Next, you insert the prepared and cleaned fiber into the connector. After curing the epoxy in an oven for the proper time (10 min to 40 min) you scribe and clean the fiber nearly flush with the end of the connector and polish it 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.
Some techniques use 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 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 it in place.
Each mounting method has advantages and disadvantages: varying from ease of installation to cost per connector to performance qualities.
Terminating single-mode fibers. Terminating single-mode cables generally uses a combination of connector installation and splicing. Since single-mode connectors have fine tolerances, they are generally terminated in a manufacturing lab. Exacting precision for the insertion and contact polishing is better in the controlled environment of the laboratory.
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.
Losses and back reflection. Whether you join fibers using splices or connectors, one negative aspect is always common to both methods: signal loss. We call this loss of light power at fiber connections attenuation, which you measure in decibels.
Another type of loss is back reflection or reflectance, measured as return loss. This happens because as the light travels through the fiber (passing through splices and connections) some of it reflects back.
Typical allowable splice losses for single mode fiber are 0.0 dB to 0.25 dB, with a return loss or back reflectance of less than 150 dB. In multimode fiber splices, typical losses range from 0.0 dB to 0.25dB with an average of 0.20 dB and return loss of less than 150 dB. In the case of fiber connectors, single-mode allowable connector losses range from 0.05 dB to 0.5 dB per connector (0.1 to 1.0 dB per connection) and return loss typically is less than 130 dB. Multimode connectors have a nominal connector loss of 0.06 dB to 0.7 dB per connector (0.12 dB to 1.4 dB per connection) with a return loss less than 125 dB typical.