Basic Knowledge about Fiber And Fiber Cables

Accompanied by the continuous advancement of network technology, fiber optic cable products constantly in the construction of telecommunications networks, the construction of the national information highway, FTTH fiber to the desktop and other occasions for large-scale use. But how to really get a good performance, state-of-the-art products needs to understand some basic to identify the types fiber optic cable.

Fiber Structures

Fiber optic cables are available in a wide variety of physical constructions. Fiber cables can be anything from simple simplex or duplex (zipcord) cables used for jumpers to 144-fiber cable for intercity transmission. However most of the fibers used in these cables come down to two basic configurations – 900um tight buffered fibers or 250um bare fibers (also called coated fibers). Actually tight buffered fibers cover a coated fiber (the coating is soft plastic) with a thick layer of harder plastic, making it easier to handle and providing physical protection. 

 

²  The structure of a 900um tight buffered fiber *Core (9um for standard single mode fibers, 50um or 62.5um for multimode fibers) *Cladding (125um) *Coating (soft plastic, 250um) *Tight buffer (hard plastic, 900um)

 

²  The structure of a 250um bare fiber (coated fiber) *Core (9um for standard single mode fibers, 50um or 62.5um for multimode fibers) *Cladding (125um) *Coating (soft plastic, 250um is the most popular, sometimes 400um is also used)

Fiber Cable Constructions

Based on 900um tight buffered fiber and 250um coated fiber there are two basic types of fiber optic cable constructions – Tight Buffered Cable and Loose Tube Cable.

>>Tight Buffered Cable 
Multiple color coded 900um tight buffered fibers can be packed tightly together in a compact cable structure, an approach widely used indoors, these cables are called tight buffered cables. Tight buffered cables are used to connect outside plant cables to terminal equipment, and also for linking various devices in a premises network.

Multi-fiber, tight buffered cables often are used for intra-building, risers, general building and plenum applications. Tight buffered cables are mostly built for indoor applications, although some tight buffered cables have been built for outdoor applications too.

>>Structure of a Tight Buffered Cable

Elements in a tight buffered fiber optic cable

l   Multiple 900um tight buffered fibers (stranded around the central strength member)

l   Central strength member (in the center of the cable)

l   Aramid Yarn (trade name Kevlar, Kevlar was developed by Dupont) (wrapped around the fibers, for physical protection and cable pulling)

l   Ripcord (for easy removal of outer jacket)

l   Jacket (also called sheath,  PVC is most common for indoor cables because of its flexible, fire-retardant and easy extrusion characteristics. 

 

Tight Buffered Fiber Optic Cables (BOC multi purpose break-out cable)

>>Loose Tube Cable

On the other hand multiple (up to 12) 250um coated fibers (bare fibers) can be put  inside a color coded, flexible plastic tube, which usually is filled with a gel compound that prevents moisture from seeping through the hollow tube. Buffer tubes are stranded around a dielectric or steel central member. Then an outer polyethylene jacket is extruded over the core. These cables are called loose tube cables.

Loose tube structure isolates the fibers from the cable structure. This is a big advantage in handling thermal and other stresses encountered outdoors, which is why most loose tube fiber optic cables are built for outdoor applications.

Loose-tube cables typically are used for outside-plant installation in aerial, duct and direct-buried applications.

3 Types Loose Tube Cable

 

>>Structure of a Loose Tube Cable

 

Elements in a loose tube fiber optic cable:

 

l   Multiple 250um coated bare fibers (in loose tube)

l   One or more loose tubes holding 250um bare fibers. Loose tubes strand around the central strength member.

l   Moisture blocking gel in each loose tube for water blocking and protection of 250um fibers

l   Central strength member (in the center of the cable and is stranded around by loose tubes)

l   Aramid Yarn as strength member

l   Ripcord (for easy removal of outer jacket)

l   Outer jacket (Polyethylene is most common for outdoor cables because of its moisture resistant, abrasion resistant and stable over wide temperature range characteristics. 

Loose Tube Cable(GYFTA/Y53)
 

Terms you must understand:

Aramid Yarn	yellow color, fiber looking material. It is strong and is used to bundle and protect the loose tubes or fibers in the cable.  It is the strength member to provide tensile strength along the length of the cable during and after installation. When a cable is pulled into a duct, the tension is applied to the aramid yarn instead of the fibers.
Central Strength Member	Many fiber optic cables has a central strength member, made of steel, fiberglass or aramid yarn. Central strength members are needed to provide the rigidity to keep the cable from buckling. Central strength members are common in outdoor cables and some high fiber counts indoor cables.
Gel Compound	Gel compound fills buffer tubes and cable  interiors, making the cable impervious to water. It needs to be completely cleaned off when the cable end is stripped for termination.
Ripcord	Ripcord is a thin but very strong thread embedded just below the cable jacket.  Its role is to split the cable easily without harming cable interiors.
Plenum	Plenum is an air-handling, air flowing and air distribution system space such as that found above drop ceiling tiles or heating and ventilation ducts.  Plenum rated cables must meet  UL-910 specification and their outer jacket are made of materials that retard the spread of flame, produce little smoke and protect electronic equipment from damage in fires. Plenum cables can be run through plenum spaces without special conduits.