Optical Cable



Before any detailed discussion on how to handle optical fiber cable, some brief discussion of fiber and cable design is required. Eliminating confusion between the different terms, and providing an understanding of the cable construction will make handling the products less complicated.

The cable cross-section in figure 1 demonstrates a two fiber cable for interconnect applications. The construction of the glass can be looked at separately from the design of the cable, as the fiber itself is constructed using distinct materials and is shipped by the fiber manufacturer as a finished product. NextGen takes the coated optical fiber and incorporates it into a multitude of finished cable products.

All of the glass fiber used by NextGen is manufactured using the same basic construction. Two layers of glass are covered by a protective coating. As demonstrated in figure 2, the fiber’s core and cladding are both made of silica glass. It is these two layers that propagate the light signal and determine the performance of the fiber. A slight difference in optical characteristics between these layers keeps the signal within the core region. The glass is protected by a dual layer of ultra-violet-cured acrylate material. The coating protects the surface of the glass from abrasion during normal routine handling, thereby ensuring the glass maintains it’s high tensile strength. The acrylate coating, which also functions optically by stripping out any light which might enter the cladding region, is removed for termination and splicing.

Buffer Types
All of NextGen fiber optic cables fall into one of two categories: tight buffered or loose tube buffered. The two cable buffer styles exhibit different optical, mechanical, and cost characteristics. Originally, loose tube cable constructions were developed for long haul telephony applications which required a rugged, low cost, high fiber count outside plant cable solution. In a premises wiring plan this cable type is often used between buildings, although recent developments in cable design have produced loose tube cable for indoor/outdoor applications. The tight buffer cable construction was developed for both indoor and outdoor premises wiring applications. Most of NextGen’s tight buffer cables are rugged enough for many interbuilding applications while offering the tight buffer design advantages of ease of terminations, meeting NEC flammability codes, and cable flexibility (figure 4).

Tight Buffered Fiber
A thermoplastic material is extruded directly over the acrylate coating, increasing the outside diameter of the fiber to  900 micros (0.9 mm), an industry standard. The tight buffer supplies the fiber with added mechanical and environmental protection, increased size for easy handling, and a simple means of adding color coding for fiber identification. During connectorization, the buffer is stripped back to an exact length as required by the connector manufacturer.

Loose Buffered Fiber
In loose tube cables, the coated fiber “floats” within a rugged, abrasion resistant, oversized tube which is filled with optical gel. Since the tube does not have direct contact with the fiber, any cable material expansion or contraction will not cause stress on the fiber. Much of the external stress placed on the tube also will not be transferred to the fiber. The non-hygroscopic gel prevents water from entering the tube. See figure 5 for a diagram of a multi-tube, gel-filled outside plant cable.

Strength Members
NextGen’s optical fiber cable designs utilize aramid yarn as the primary strength member. Some designs also use a fiberglass central strength member. Both of these materials serve as the load bearing members of an optical fiber cable during installation. In many cables the aramid also acts as a strength member during termination. Figure 6 demonstrates a single fiber cable, where the tight buffered fiber is surrounded by aramid and coated with an overall jacket.

Core Wrap and Ripcords
Core wraps and ripcords are designed to make removal of the exterior cable sheath easier, preventing unnecessary stress to the core. The non-hygroscopic core wrap creates a barrier between the core and the jacket, preventing adhesion and facilitating jacket removal. Ripcords provide a means of stripping back the jacket without the use of invasive tools which could harm the cable core and damage fibers.

Outer Jacket
The true cable jacket is usually the outermost element in the cable design. It serves to protect the cable against environmental hazards and gives the installer a means of managing the cable. Without the outer jacket, in many designs the buffered fibers would have only the aramid wrap to cover them. Typical jacket materials include Polyvinylchloride (PVC), Polyethylene (PE) or Polyvinylidene Fluoride (PVDF). Also, without selectively choosing the appropriate jacket material most cables would be entirely incapable of passing a flame test. Outer jackets are always stripped back to expose the fibers at the point of termination or connectorization。