Fibre Optic, Basic Cable Design

Two basic cables design are loose-tube cable, used in the majority of outside-plant installations in North America, and tight-buffered cable, primarily used inside buildings.

The modular design of loose-tube cables typically holds up to 12 fibres per buffer tube with a maximum per cable fibre count of more than 200 fibres Loose-tube cables can be all-dielectric or optionally armoured The modular buffer-tube design permits easy drop-off of groups of fibres at intermediate points, without interfering with other protected buffer tubes being routed to other locations. The loose-tube design also helps in the identification and administration of fibres in the system.

Single fibre tight-buffered cables are used as pigtails, patch cords and jumpers to terminate loose-tube cables directly into opto-electronic transmitters, receivers and other active and passive components.

Multi-fibre tight-buffered cables also are available and are used primarily for alternative routing and handling flexibility and ease within buildings.

Loose-Tube Cable

In a loose-tube cable design, colour coded plastic buffer tubes house and protect optical fibres A gel filling compound impedes water penetration. Excess fibre length (relative to buffer tube length) insulates fibres from stresses of installation and environmental loading. Buffer tubes are stranded around a dielectric or steel central member, which serves as an anti-buckling element.

The cable core, typically uses aramid yarn, as the primary tensile strength member. The outer polyethylene jacket is extruded over the core. If armouring is required, a corrugated steel tape is formed around a single jacketed cable with an additional jacket extruded over the armour.

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

Tight-Buffered Cable

With tight-buffered cable designs, the buffering material is in direct contact with the fibre This design is suited for "jumper cables" which connect outside plant cables to terminal equipment, and also for linking various devices in a premises network.

Multi-fibre, tight-buffered cables often are used for intra-building, risers, general building and plenum applications.

The tight-buffered design provides a rugged cable structure to protect individual fibres during handling, routing and connecting Yarn strength members keep the tensile load away from the fibre

As with loose-tube cables, optical specifications for tight-buffered cables also should include the maximum performance of all fibres over the operating temperature range and life of the cable. Averages should not be acceptable.

What is the best way to terminate fibre optic cable? That depends on the application, cost considerations and your own personal preferences. The following connector comparisons can make the decision easier.

Epoxy & Polish

Epoxy & polish style connectors were the original fibre optic connectors. They still represent the largest segment of connectors, in both quantity used and variety available. Practically every style of connector is available including ST, SC, FC, LC, D4, SMA, MU, and MTRJ. Advantages include:

• Very robust. This connector style is based on tried and true technology, and can withstand the greatest environmental and mechanical stress when compared to the other connector technologies.
• This style of connector accepts the widest assortment of cable jacket diameters. Most connectors of this group have versions to fit onto 900um buffered fibre, and up to 3.0mm jacketed fibre
• Versions are available that hold from 1 to 24 fibres in a single connector.

Installation Time: There is an initial set-up time for the field technician who must prepare a workstation with polishing equipment and an epoxy-curing oven. The termination time for one connector is about 25 minutes due to the time needed to heat cure the epoxy. Average time per connector in a large batch can be as low as 5 or 6 minutes. Faster curing epoxies such as anaerobic epoxy can reduce the installation time, but fast cure epoxies are not suitable for all connectors.

Skill Level: These connectors, while not difficult to install, do require the most supervised skills training, especially for polishing. They are best suited for the high-volume installer or assembly house with a trained and stable work force.

Costs: Least expensive connectors to purchase, in many cases being 30 to 50 percent cheaper than other termination style connectors. However, factor in the cost of epoxy curing and ferrule polishing equipment, and their associated consumable.

Pre-Loaded Epoxy or No-Epoxy & Polish

There are two main categories of no-epoxy & polish connectors. The first are connectors that are pre-loaded with a measured amount of epoxy. These connectors reduce the skill level needed to install a connector but they do not significantly reduce the time or equipment needed. The second category of connectors uses no epoxy at all. Usually they use an internal crimp mechanism to stabilise the fibre These connectors reduce both the skill level needed and installation time. ST, SC, and FC connector styles are available. Advantages include:

• Epoxy injection is not required.
• No scraped connectors due to epoxy overfill.
• Reduced equipment requirements for some versions.

Installation Time: Both versions have short set-up time, with pre-loaded epoxy connectors having a slightly longer set-up Due to curing time, the pre-loaded epoxy connectors require the same amount of installation time as standard connectors, 25 minutes for 1 connector, 5-6 minutes average for a batch. Connectors that use the internal crimp method install in 2 minutes or less.

Skill Level: Skill requirements are reduced because the crimp mechanism is easier to master than using epoxy. They provide maximum flexibility with one technology and a balance between skill and cost.

Costs: Moderately more expensive to purchase than a standard connector. Equipment cost is equal to or less than that of standard connectors. Consumable cost is reduced to polish film and cleaning supplies. Cost benefits derive from reduced training requirements and fast installation time.

No-Epoxy & No-Polish

Easiest and fastest connectors to install; well suited for contractors who cannot cost-justify the training and supervision required for standard connectors. Good solution for fast field restorations. ST, SC, FC, LC, and MTRJ connector styles are available. Advantages include:
• No set-up time required.
• Lowest installation time per connector.
• Limited training required.
• Little or no consumable costs.

Installation Time: Almost zero. Its less than 1 minute regardless of number of connectors.

Skill level: Requires minimal training, making this type of connector ideal for installation companies with a high turnover rate of installers and/or that do limited amounts of optical fibre termination's.

Costs: Generally the most expensive style connector to purchase, since some of the labour (polishing) is done in the factory. Also, one or two fairly expensive installation tools may be required. However, it may still be less expensive on a cost-per-installed-connector basis due to lower labour cost.