The international standard for the cladding diameter of optical fibers is 125 microns (um). This
compatibility is important in that it allows fibers to fit into standard connectors and splices, and allows standard tools to be used throughout the industry. The differences among fibers lie in their
core sizes the light-carrying region of the fiber. Standard SingleMode fibers are manufactured with the smallest core size, approximately 810 um in diameter. With its greater
information-carrying capacity, singlemode fiber typically is used for longer distance and higherbandwidth applications. MultiMode fibers are available in several core sizes. The most
widely used sizes are 50 um and 62.5 um. Larger core sizes generally have greater bandwidth and are easier to couple and interconnect.
SingleMode Step Index
SingleMode fiber is designed with a "slopindex" profile, which refers to the shape of its refractive
index profile over a cross section of fiber. The refractive index of a material is the ratio of the speed of light in a vacuum (where it is fastest) to the speed of light in the specific material.
In a SingleMode fiber, light is concentrated in the core; however, some light travels in the inner part of the cladding at normal operating wavelengths. The diameter of the spot of light as it
travels through the fiber is called the mode field diameter (MFD). MFD is an important parameter for determining splice loss and the fiber's resistance to bendinduced loss.
Choosing Cable
There are many different types of fiber optic cables. All of the cables are defined by the number of fibers in the cable, the type of fibers (MM or SM), the size of the fiber (50, 62.5 or 125um) and
the type of material used to enclose the fibers. The material used to enclose the fibers have many names. Some of the names are generic and others are names used by the cable
manufacturer. To limit any confusion, you should begin with the following questions.
- What type of fiber do I need?
- How many fibers do I need?
- Do I need indoor or outdoor cable?
- Will the cable be in a hazard environment and require rugged construction?
- Will the cable be underground and require moisture and rodent protection?
- Do you need Plenum, Tempest, NEC, UL, or CSA rated cables?
The remainder of your cable concerns should be finalized by your cable supplier after you have explained your application in detail.
What is and why use PLENUM Cable
Most states and cities have adopted for their building codes the National Electrical Code (NEC) recommendations regarding acceptable wiring methods for cable installed in the air handling or
plenum spaces above suspended ceilings. The NEC states that all cable installed in plenum spaces must be installed in metal conduit unless classified by an approved agency as having fire
resistant, low smoke producing characteristics.
Cables that are not classified by an approved agency, such as Underwriters Laboratories (UL),
as having fireresistant, low smoke characteristics must be installed in conduit. Conduit installation can increase the initial installed cost of a cable system by an average of 100 percent,
and rerouting cables in conduit to accommodate moves, adds and changes is costly and disruptive.
Cables made with several different materials have the UL low smoke, low flame spread
classification. However, plenum cables insulated with TEFLON fluoropolymer resin provide superior electrical performance at a reasonable cost for all computer, voice, data, video, control
and life safety systems.
