Plastics in the Mix
Glass fibers are generally not field-serviceable, as they require special skills and tools. However, plastic cable can be cut with scissors and not fracture, and will continue to function if the alignment is "close." This makes it much more field-service-friendly. Manufacturers sell kits to finish plastic fiber, although plastic fiber lengths can be restricted to as little as 50 m if finished this way. Despite the limitations of plastic, Kurt Wadowick, safety and I/O specialist at Beckhoff Automation indicates that the company sells more plastic than glass fiber.
SEE ALSO: Shed Some Fiberoptic Light
A typical fiberoptic cable construction from the inside or core to the outside consists of the following layers: fiber(s), buffer tube, rip cord, strength member, steel armor and outer jacket. Let's look a little closer at the role each of these plays in a successful installation.
Multimode glass fiber typically has a 50-µm or 62.5-µm core with 125-µm cladding outside diameter, while plastic fiber is thicker at 200 µm or 230 µm for hard-clad silica (HCS) and plastic silica (PCS) and 1 mm for all-plastic fiber.
Attenuation in fiberoptic cable is the loss of optical power as the signal travels through the cable. In general, attenuation decreases as wavelength increases, but certain wavelengths are more easily absorbed in plastic and silica fibers than others. One of the reasons for establishing standard operating wavelengths of 850 nm, 1300 nm and 1550 nm in silica fiber and 650 nm for plastic is to avoid the high-attenuation regions. The typical attenuation of a 62.5-µm to 125-µm fiber at 850 nm is 4 dB/km, and the attenuation at 1300 nm is 1.5 dB/km. Plastic fiber is low-bandwidth, high-attenuation (19 dB/m), and due to these higher losses, best suited to short-run applications, such as those found in the manufacturing sector, where there is a high degree of repetitive motion.
When fiberoptic cable becomes kinked, several things can happen. Glass can fracture, resulting in splinter(s) that can't be repaired, while plastic will crinkle like a garden hose. In both cases, the fiber will cease to operate properly. However, the effect in a glass bundle is obviously more catastrophic, and that's why plastic fiber tends to be used in robotics and similar applications.
Because multimode is able to transmit to approximately 2 km, it is what is most commonly used for IT projects. It can therefore be sourced from the same suppliers, and often purchased with prefabricated ends, making it more easily available than single-mode or industrial fiber. As indicated earlier, single-mode fiber can handle 20-km distances before requiring amplification, and is typically installed with one fiber transmitting and the other receiving, so when purchasing a cable bundle, it's recommended to always install it in pairs or an even-numbered set of fibers.
Dispersion affects bandwidth by limiting how close together the individual light pulses are from each other to prevent overlapping. To minimize the chance of dispersion affecting communications, the signal rise-and-fall times generally must be no more than 20% of the width of the shortest period. The amount of time between pulses directly affects overall bandwidth.
Signal gain, expressed in dB, determines distance between light source and receiver. Both the passive and active components of the circuit have to be included in the loss-budget calculation. Passive loss is made up of fiber loss, connector loss and splice loss. Active component specifications of interest are system wavelength, transmitter power, receiver sensitivity and dynamic range — the difference between transmitter power and receiver sensitivity.
There are two types of buffers: tight and loose. A tight buffer is applied directly over the fiber. This protects the fiber, but introduces a potential problem if the temperature drops below freezing. At low temperatures, the buffer material shrinks more than the glass fiber, which puts stress on the fiber and causes the glass to develop "micro-bends" or spots where light escapes from the fiber, thus increasing the attenuation of the fiber.
Loose buffers do not protect the glass fiber as well as tight buffers, but they hold the glass strand in a tube filled with a compound to keep moisture from getting onto the fiber. Because they are loose at low temperatures, the buffer can shrink without the fiber developing micro-bends. As a result, loose-tube cable is primarily used for outside plant installations where low attenuation and high cable-pulling strength are required.
Until recently, most installations chose a gel-filled cable, but now dry-water-blocked cables are widely available and preferred by many users. Dry-water cables use water-absorbing tape that expands and seals the cable if any water enters it. Installers prefer dry cable, as it does not require the messy, tedious removal of the gel used in many cables and greatly reduces cable preparation for splicing or termination.
The strength member in a cable is either fiberglass or aramid, and the cable should be pulled only by these strength members. Any other method can put stress on the fibers and harm them. Similarly, cables should not be pulled by the jacket unless specifically approved by the cable manufacturer and an approved cable grip is used. In addition, twisting the cable can also stress the fibers.
Remember when installing cable that the typical minimum bend radius for fiberoptic cable is 20 times the outside diameter of the jacketed cable for installation and 15 times this same diameter over the long term. Bearing in mind that installation of an outside plant (OSP) cable could cost 100 times the cost of the cable itself, the recommendation is that you do not bend your fiber to less than 20 times the outside radius of the jacketed cable.
As it is the fiber sheath that provides strain relief and strength for pull-through conduit, more sheath means more rigidity and, therefore, a requirement for increased bend radius. In addition, you should always pull cable down to avoid stress from the weight of cable on the fibers in the core.
Knowing the limitations of the physical layer will help you prepare a better design, and the references at the end of this article and in the sidebar offer sources for additional information about the network layout as well as the specification and execution of the project in the field.
Beckhoff Automation's Wadowick reinforced the importance of terminations, saying, "Technical support calls are almost always about fiber terminations, the connections and ends, or splicing."
Fiberoptics will continue to grow in the industrial setting. Fiberoptic cable is different than copper and requires special skills and tools to install. Terminations continue to play a critical role in the success of an installation. And, yes, the constant remains that the physical layer, which is the underpinning of any industrial network, continues be critical to the success of any project.
Reference: "Guide to Industrial Fiberoptics," from Relcom.