While we have only a few types of optical fiber, we have hundreds of types of fiber optic cable. That’s because cable is designed to protect the fibers in the environment in which it is going to be installed and in the method used for its installation. Of course, the cable must be designed to accommodate the number of fibers needed, which may range from one fiber to thousands and allow easy access for splicing and termination.
In my annual update column last month, I discussed some new developments in cables and their installation, but it occurred to me that I’ve never really discussed the types of cables in detail. Cable selection and installation is extremely important for successful fiber projects, so let’s go back to basics and cover cables thoroughly.
First, some background on fibers. Most glass optical fibers have a 125-micron diameter and are coated with a protective acrylic buffer coating that is 250-microns in diameter. (There are exceptions to this coating size, which I will cover later.) Some fibers that are intended to be installed indoors are then also coated with a large, soft 900-micron diameter buffer. We call these “tight buffer” fibers, and they are made into tight buffer cables such as simplex, duplex and distribution cables.
Simplex cables are used to make patchcords for indoor use. Duplex cables are used as cables to connect equipment with one fiber transmitting in one direction and the other fiber in the reverse direction. Distribution cables are bundles of 900-micron buffered fibers in one jacket with moderate fiber counts, generally up to 144 fibers. The 900-micron buffered fiber is more rugged and easier to handle than the plain 250-micron fiber, so it can be used for direct termination on the fiber with adhesive/polish or prepolished splice connector techniques.
All types of indoor tight buffered cables have a similar construction. Inside are the 900-micron fibers, around them is wrapped a layer of aramid fiber strength members, and it’s all covered by a plastic jacket made of flame-retardant material. The aramid fiber, often called by the trade name Kevlar, provides cushioning protection for the fibers and can be used to pull the cable through conduit or between floors of buildings. The jacket provides protection for the fibers but also must be fire-rated to meet UL requirements and local building codes for indoor cables.
Outside plant cables (OSP) are quite different because of the environmental stress and the desire for large fiber counts. OSP cables are constructed around the smaller 250-micron buffered fibers to allow packing as many fibers into the cable as possible while keeping the cable outside diameter as small as possible.
The most common OSP cable type is called “loose tube” cable because the inner structure of the cable is made of small plastic tubes (“buffer tubes”) with a dozen fibers in each tube. A typical cable will have a central stiffener and strength member made of fiberglass rod with multiple buffer tubes helically wrapped around it. Surrounding that is a layer of aramid fiber strength members and an outer jacket designed to protect the internals of the cable from the environment. Every OSP cable must have some protection from moisture and water—either a gel or dry water- blocking material.
An alternative OSP cable design used ribbons of fibers, 12 fibers barely 1/8-inch wide, to pack more fibers in less space and allow fusion splicing all 12 fibers at once. Splicing ribbons takes much less time than splicing 12 individual fibers, leading to major savings in installation time and cost. Ribbon cables come in several construction types that are generally smaller than loose tube designs.
OSP cables then have major variations of construction depending on the intended installation. Underground cables pulled into conduit are super strong to allow for high-tension forces on long pulls. Direct-buried cables need protection from crushing and rodent penetration, so they have a layer of metal armor over the first jacket followed by a second jacket covering the armor.
Aerial cables come in many varieties: loose tube cables to be lashed to a metal messenger cable, figure-eight cables with a messenger cable attached, all-dielectric self-supporting cables (ADSS) with extra strength and even cables that have fibers inside electrical utility power cables.
The big focus today seems to be on high-fiber count indoor and OSP cables and new, more efficient installation methods needed for applications such as data centers, metro networks and fiber to the home. That has required re-engineering fibers, cables, installation products and processes, which I will discuss over the next few months.