In this video, we will introduce the basic structures of a fiber optic cable.
Fiber optic system installers often refer to fiber cable sizes in the following format: “core/cladding”. For example, a fiber is specified as 50/125, and this means that the core has a diameter of 50 µm and that the cladding diameter is 125 µm. Immediately, we can tell that this is a multimode fiber because of its large core size. If it were single mode fiber, the core diameter would be in the range of 7 ~ 10 µm.
The outer surface of a cladding is covered with a coating that has a diameter of 250 or 500 µm. This table gives the basic parameters of the most common types of optical fiber.
Buffering helps isolate fiber from external forces. Two types of buffering have been developed: loose buffer and tight buffer, as shown in the first illustration here.
In the loose-tube construction, the fiber is contained in a plastic tube that has an inner diameter considerably larger than the fiber itself. The interior of the plastic tube is usually filled with a gel material. For cables containing many fibers, a number of these tubes, each containing one or more fibers, are combined with strength members to keep the fibers free of stress.
Tight buffering is made during the cable manufacturing process using a direct extrusion of plastic over the basic fiber coating. The tight-buffer design results in lower isolation for the fiber from stresses by temperature variation. On the other hand, tight-buffer construction can endure much greater crush and impact forces without damage to the fiber.
Breakout cable is a derivative form of tight buffer construction. In breakout cable, a tightly buffered fiber is surrounded by aramid yarn and a jacket, typically PVC. These single-fiber subunits are then covered by a common sheath. This “cable” within a cable offers the advantage of direct, simplified connector attachment.
Loose tube cables are optimized for outside plant applications. Optical cables are made of silica glass and polymeric plastics. Since these two materials have different coefficient of thermal expansion, the material expansion and contraction will be different for temperature changes.
The loose-tube cable makes a strain-free environment for the optical fiber. This strain-free environment compensates for the movement in the cable structure without inducing mechanical forces on the fiber.
This enhances the operating temperature range of the loose tube design.
Loose tube fiber cables also can withstand ice crush in locations where standing water and freezing temperatures coexist. Water migration inside the cable outer jacket can result in the formation of ice crystals within the optical fiber cable core. This ice will make stresses to the optical fibers and may result in an unacceptable increase in attenuation or even breakage.
Loose tube cables have core water-blocking protection and buffer tube filling compound. Water-blocking protection is done by surrounding the cable core with a gel or dry water-swellable material to stop the entry and migration of should if the cable’s outer jacket is breached.
Filling compound provides a mechanical cushion for the fiber, allowing it to float within the buffer tube, and provides an additional barrier between the optical fiber and water/moisture in the environment.
Ultraviolet (UV) light protection is another matter. In aerial installation, optical fiber cable must be able to withstand direct exposure to UV sunlight. Loose buffered cable incorporates carbon black into its jacket material to provide maximum UV protection.
On the other hand, tight-buffered cables do not isolate the fibers from external forces to the same extent; therefore, temperature-related expansion and contraction effects applied to any component of the tight-buffered cable can be translated directly to the optical fiber. As a result, tight-buffered cables are typically more sensitive to temperature extremes and mechanical disturbances.
Tight-buffered cables are good for indoor applications, however, they are limited in their performance in an outdoor environment. Standard tight-buffered cables do not have filling compounds or water-blocking protection, making them susceptible to damage caused by water penetration and migration.
Tight-buffered cables do not contain carbon black in their outer jacket and thus should not be used in installations exposed to UV-rich sunlight.
Strength members are an important part of a fiber optic cable, especially during the “pulling” process during cable installation. The levels of stress on the cable during pulling and other installation procedures may cause micro-bending losses that can result in an attenuation increase and possible fatigue effects. The internal strength members are added to transfer these stresses in short-term installation and long-term application.
These strength members keep the fiber from stress by minimizing elongation and contraction. It should be kept in mind that optical fiber stretches very little before breaking. Thus, the strength members must have low elongation at the expected tensile loads.
Three types of strength members are commonly used: fiberglass epoxy rod, steel, and aramid. The load to break for the first two is 480 pounds, and that for aramid is 944 pounds. The percentage elongation before break is 3.5 for fiberglass epoxy rod, 0.7 for steel, and 2.4 for aramid.
Impact resistance, flexing, and bending are other mechanical factors affecting the choice of strength members.
Some typical fiber-optic cable cross sections are shown in these two illustrations. The left picture shows four examples of fiber optic cable for long-haul and CATV services.
The right picture shows typical tight-buffered cables for indoor use in risers.