Considerations of tensile strength, ruggedness, durability, flexibility, size, resistance to the environment, flammability, temperature range and appearance are important in constructing optical fiber cable.

First Level of Fiber Protection

The optical fiber is a very small waveguide. In an environment free from stress or external forces, this waveguide will transmit the light launched into it with minimal loss, or attenuation.

To isolate the fiber from these external forces, two first level protections of fiber have been developed: loose tube and tight buffer.

In the loose buffer 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.

The loose tube isolates the fiber from the exterior mechanical forces acting on a cable. For multifiber cables, a number of these tubes, each containing single or multiple fibers, are combined with strength members to keep the fibers free of stress, and to minimize elongation and contraction.

By varying the amount of fibers inside the tube during the cabling process, the degree of shrinkage due to temperature variation can be controlled, and therefore the degree of attenuation over a temperature range is minimized.

The other fiber protection technique, tight buffer, uses a direct extrusion of plastic over the basic fiber coating.

Tight buffer constructions are able to withstand much greater crush and impact forces without fiber breakage.

The tight buffer design, however, results in lower isolation for the fiber from the stresses of temperature variation. While relatively more flexible than loose buffer, if the tight buffer is deployed with sharp bends or twists, optical losses are likely to exceed nominal specifications due to microbending.

A refined form of tight buffer construction is breakout cable. In breakout cable, a tightly buffered fiber is surrounded by aramid yarn and a jacket, typically PVC. These single-fiber subunit elements are then covered by a common sheath to form the breakout cable. "This cable within a cable" offers the advantage of direct, simplified connector attachment and installation.

Each construction has inherent advantages. The loose buffer tube offers lower cable attenuation from microbending in any given fiber, plus a high level of isolation from external forces. Under continuous mechanical stress, the loose tube permits more stable transmission characteristics.

The tight buffer construction permits smaller, lighter weight designs for similar fiber configuration, and generally yields a more flexible, crush resistant cable.

Loose and Tight Buffer Tradeoffs

 
Physical Choices

Once a tight or loose buffer construction is selected, the system designer has made some decisions regarding the tradeoffs between microbending loss and flexibility in obtaining optical operation goals.

For installation of a cable, mechanical properties such as tensile strength, impact resistance, flexing, and bending are important. Environmental requirements concern the resistance to moisture, chemicals, and other types of atmospheric or in-ground conditions.

Mechanical Protection

Normal cable loads sustained during installation may ultimately place the fiber in a state of tensile stress.

The levels of stress may cause microbending losses which result in an attenuation increase and possible fatigue effects.

To transfer these stress loads in short term installation and long term application, various internal strength members are added to the optical cable structure.

Such strength members provide the tensile load properties similar to electronic cables, and keep the fibers free from stress by minimizing elongation and contraction. In some cases, they also act as temperature stabilization elements.

Optical fiber stretches very little before breaking, so the strength members must have low elongation at the expected tensile loads.

Strength Member Comparison

Impact resistance, flexing and bending are other mechanical factors affecting choice of strength members.

Strength members which are typically used in fiber optic cable include Aramid yarn, fiberglass epoxy rods (FGE), and steel wire. Pound for pound, Aramid yarn is five times stronger than steel. It and fiberglass epoxy rods are often the choice when all-dielectric construction is required.

Steel or FGE should be chosen when extreme cold temperature performance is required, since they can offer better temperature stability.

Belden fiber optic cables for indoor applications meet the requirements of the plenum, riser and vertical tray cable specifications of the National Electrical Code (NEC/CSA). Belden also offers cables meeting today's specifications for Halogen-free environments.

Fiber Count

Specifying the number of fibers used in the cable plant requires the designer to carefully consider the evolution of future network demands. Depending on the number and type of application in the network and the level of redundancy needed, fiber count can range from 2 to more than 100 in the backbone or to each wiring closet. The following chart summarizes the fiber requirements for various applications.

Currently, due to the expense of multiplexing equipment, separate, dedicated fibers are typically utilized for each application. So, for example, if two buildings were to be networked with a FDDI backbone, four fibers would be required in the cable connecting the buildings - two to transmit, two to receive. Further, it is recommended that at least two times the number of fibers needed are actually placed in the backbone to accommodate expansion requirements.

For purposes of example, assume 3 applications to each floor:

•       1. FDDI Data (4 Fibers)
•       2. Interactive Video (2 Fibers)
•       3. CCTV Security (1 Fiber)

These applications would indicate a need for 7 fibers to each wiring closet. It is recommended that 2 times the number of fibers required are actually run to each wiring closet to allow future network expansion.

Although some systems clearly indicate the number of fibers needed, there are usually no hard and fast rules. Installing the required number of fibers plus others for backup and for the future yields the more flexible, expandable cable plant to service networking requirements into the future.