Fiber Optic Cable
What Is Fiber Optic Cable?
A fiber optic cable has a core made of a clear inner glass filament thread with a diameter between 9 and 62.5 microns. This is clad by glass with a different refractive index to an overall diameter of 125 microns (about twice the diameter of a human hair). The difference in the refractive index between the core and cladding reflects light, so it travels down the filament. Similar to copper cables, fiber optic cables are used for transmitting data via light pulses across long distances at high speeds. Cable, internet, and other telecommunication signals are most commonly transmitted using fiber optics.
Single Mode and Multimode Fiber Optic Cables
There are two types of fiber optic cables: single mode and multimode.
Single Mode Fiber Optic Cables
Single mode fiber optic cables have an internal diameter of 9 microns (0.35 thousandths of an inch) and an external cladding diameter of 125 microns. This type of fiber optic cable uses specialized laser light sources generating tightly focused light with waves that are parallel to the fiber. While this approach is expensive, the advantage is low signal attenuation and minimal signal interference, allowing their use over long distances.
Multimode Fiber Optics
Multimode fiber optic cables are larger, with the most common diameters being 50 and 62.5 microns. This form of cable originally used an LED light source, but later types use lasers. The larger diameter reflects light along the fiber walls, so individual light rays arrive at different times. This characteristic limits the effective bandwidth of the cable and also restricts their use to shorter distances. Multimode fiber cable is graded according to the Telecommunications Industry Standard TIA-492AAAD:
- OM1: For 62.5-micron fiber suitable for 100 Mbit Ethernet; it has an orange jacket.
- OM2: For 50-micron fiber with an LED light source and suitable for 1 Gbit Ethernet; the cable has an orange jacket.
- OM3: For 50-micron fiber with a laser light source that supports 10 Gbit Ethernet up to 300 meters (984 feet); this cable has an aqua-colored jacket.
- OM4: For 50-micron fiber with a laser light source that supports 10 Gbit Ethernet to 550 meters (1,800 feet) and 100 Gbit to 150 meters (492 feet); this cable also has an aqua-colored jacket
Fiber Optic Cable Construction
Standard single core fiber optic cable consists of a core, cladding, outer coating covered by reinforcement and jacket as follows:
- Core: A 9-, 50- or 62.5-micron glass filament
- Cladding: A 125-micron glass covering with a different refractive index to the core
- Coating: Several layers of plastic to provide protection, the coating may be loose-fitting and filled with a protective gel or tightly fitted with a robust plastic covering
- Reinforcement: An aramid, fiberglass or steel reinforcing that reduces tensile strain on glass fibers.
- Jacket: A Polyvinylchloride (PVC) or high-density polyethylene (HDPE) jacket that covers and protects the cable
Multifiber Cable Construction
Except for patch cables and other limited uses, most fiber optic cables consist of multiple fibers bundled together inside a protective jacket. Cables may contain 2, 12, 24, 72 or even 432 separate fibers. Common types of cable construction include distribution, breakout, flat drop and ribbon cables.
These indoor and outdoor microduct cables comprise multiple individual fiber optic cables arranged within a circular flexible PVC jacket. Depending on the application, the cable may have armoring and larger cables will have a central steel or aramid core. In some instances, the manufacturer groups individual fibers into sub-units.
Breakout cables have multiple fibers, each encased within its own jacket. They are grouped into a larger jacket filled with aramid reinforcing. The advantage of this type of construction is that it's possible to split the cable and terminate each core.
A flat drop cable is self-supporting, and you can suspend it between buildings and structures. It consists of a central buffer tube containing two or more individual fibers with aramid reinforcing. A heavy outer jacket provides external protection for the core and there are engineered dual strength members to carry the suspended load. The outer jacket may have water-resistant material to protect the central buffer tube.
Ribbon cables have a large central tube with individual fiber optic cables laid in rows and secured with Mylar tape. Each fiber cable consists of a core, cladding and covering. Water blocking material and strengthening members fill the space between the central tube and the outer jacket. In some instances, multiple smaller tubes containing bunches of ribbons surround central steel or reinforcing core.