FOA OSP Construction Guide
Review Of Fiber Optic Technology
To fully understand this material, you need an understanding of basic fiber optic technology. We’ve included a brief review below but recommend you refer to the FOA Guide or take the Basic Fiber Optics course at Fiber U.
Why Fiber Optics?
AT&T photo from mid-1970s comparing fiber with copper cable of equivalent bandwidth (that was then, of course, fiber networks have much more bandwidth today)
Since the introduction of fiber in the 1970s, optical fiber has revolutionized communications, transmitting more information over greater distances than could ever be achieved in copper wires or wireless transmission, including satellites. The greater bandwidth and longer distance capability of fiber optics have made it the most economic method of transmitting information worldwide. On land, buried and aerial cables carry virtually all communications and even under the ocean, fiber connects continents with information capacity unimaginable not so long ago.
Fiber was first adopted by the telecommunications industry but was soon adopted by other users. Cable TV used fiber backbones and cable modems to become the dominant provider of broadband Internet connections while telecom companies struggled to build out fiber networks for DSL connections to the home that could equal them. Cellular mobile communications, following the introduction of the smart phone, needed fiber to provide adequate bandwidth to cell towers and the new small cells.
In addition, fiber has total immunity to electromagnetic radiation, so it can be used even inside high voltage transmission lines, a major advantage for electrical utilities who need reliable communications to control their electric grid. Transportation systems use fiber alongside roads and railways for communications. Cities are learning that to keep up with technologies and become Smart Cities requires a large fiber optic infrastructure that provides services and can also become a source of revenue in the new tech economy.
· It has exceptional bandwidth and is virtually "future proof"
· It has the ability to carry many signals concurrently
· It can transmit signals extremely long distances before regeneration
· It is immune to electromagnetic interference and has no electromagnetic emissions
· It does not corrode like copper cabling and is resistant to tapping
· It is compatible with all current, or proposed communications standards
· It is light weight and easy to handle and it’s pulling strength is higher than that of copper cables
Fiber Optic Components
Optical fiber can be made from glass or plastic but most communications fiber is all glass. The basic construction of fiber looks like this:
Core: The optical core is the light-carrying element at the center of the fiber and is made up of an ultra-pure optical glass in a high temperature process.
Cladding: The cladding surrounding the core is made of pure silica and has a slightly lower index of refraction (i.e. less dense) than the core. This lower refractive index causes the light in the core to reflect when encountering the cladding and remain trapped within the core.
Fiber Manufacture: The core and cladding of the fiber are made first in the form of a large glass rod called a preform that is then heated on the end and pulled into the fiber.
Buffer coating: After pulling the fiber, it is coated with a protective plastic coating to prevent physical damage and protect the fiber from moisture. The buffer is removed during stripping for splicing or termination.
Fiber Geometric Parameters
Fiber can either be single-mode (SM) or multimode (MM) but practically all OSP cable plants are singlemode. Fiber sizes are expressed by using two numbers separated by a slash e.g. 9/125. The first number refers to the core size in microns and the second number refers to the core and cladding size combined in microns. It is impossible to differentiate between SM and MM fiber with the naked eye. There is no difference in the outward appearances; both are 125 microns in size - only the core size differs.
Fiber Optic Cables
A cable protects the fibers from the environment where it is installed. OSP cables may be installed underground in conduit or direct buried, installed aerially on poles or towers or run underwater. The type of cable construction chosen must be compatible with the installation environment and the installation process.
Loose tube OSP cable
Ribbon OSP cable with a central tube
OSP cables are generally loose tube or ribbon designs. The center of the cable is usually a fiberglass strength member to limit the bending and kinking of the cable. The fibers are protected in tubes with water blocking compound for protection against moisture. Surrounding the fibers are aramid fiber strength members which provide for the high pulling tension of these cables. Armor is used on direct buried underground cables only, as protection from crushing loads or rodent penetration.
OSP armored loose tube cable
Aerial cables can be loose tube designs lashed to a messenger cable or another fiber optic cable for support, all dielectric self-supporting (ADSS) or figure-8 cable with a messenger attached to the cable. These cables will be covered more in the section on aerial installation.
The jackets of OSP cables are usually Polyethylene (PE). Since raw PE can degrade rapidly through exposure to sunlight, carbon black is combined with the PE to absorb the UV light and protect the plastic from degradation.
Cable jackets should be marked with manufacturer’s name, month and year of manufacture, sequential meter markings, fiber type and the number of fibers. Cables without these markings will not pass inspections in some areas and should not be installed.
Cable Strength members
Aramid fibers (trade name Kevlar - a very strong, very light, synthetic compound developed by DuPont) – is used when a cable is pulled into a duct, with the tension being applied to the Kevlar. The Kevlar is used as a drawstring to pull the cable into the duct so as not to put stress on the fibers.
The term is sometimes also used for the fiberglass or steel rod in some cables used to stiffen it. Impact resistance, flexing and bending are other mechanical factors affecting the choice of strength members.
In a loose tube cable design, a filling compound, either dry water swell-able yarns or powder or gel are commonly incorporated in the cable. This minimizes the chance of water or moisture penetrating the length of the tube in the event that the tube is damaged. Moisture can cause fiber degradation and when water freezes it expands by approximately 9% causing damage to the cable.
Micro Cable Technology
Since SM fiber was first introduced in the early 1980s, not too much has changed in its basic geometric parameters. The SM core size has remained somewhere between 8 and 10 μm depending on the particular fiber type while the core / cladding diameter has remained at 125 μm. The outside plant (OSP) coating has typically been 250 μm. Standardizing these dimensions has greatly improved interoperability and consistency across optical networks.
A typical standard 144 fiber loose cable would be 15-20 mm diameter (0.6-0.8 inches) and a ribbon cable 12-15 mm diameter (0.5-0.6 inches). Smaller cables are often desirable since more can be placed in standard ducts or smaller ducts which are easier to bury can be utilized.
A major fiber development has made smaller cables possible – bend insensitive (BI) fibers. BI fibers reduce fiber sensitivity to bending losses so more fibers can be packed into a single fiber tube. Plus, the buffer coating of BI fibers can be reduced to 200 microns to pack more fibers into the cable.
Microcables with 288 and 144 fibers compared to a pencil
Microcables offers a great deal more density. Only a few years ago, a cable diameter of ± 12 mm was required for a 48-fiber cable design. Today, a microcable only 8 mm diameter has a capacity of 144 fibers. This is achieved by using 200 μm coated fibers and doubling the number of fibers in smaller buffer tubes. The 200 μm coated fiber’s cross-sectional area is ~46% smaller than that of the conventional 250 μm coated fiber.
Microcables are designed for installation in microducts, small fiber ducts made of high-density polyethylene (HDPE) material. They are typically installed as bundles in larger ducts or direct buried by microtrenching or sawing a shallow trench in roadways or sidewalks. It’s generally accepted that deploying multiple microducts is an economically sensible option, helping to reduce the costs of subsequent installation by allowing more microcables to be installed in the future.
Micro cables are designed for installation in microducts by blowing or jetting. Blown cables are floated on a high-pressure jet of air while being pushed into the microduct. The high-density polyethylene (HDPE) outer sheaths minimize friction with the inner surface of micro ducts. Perhaps more significantly, these cables also have optimal stiffness properties to help prevent buckling and easily negotiate modest changes in direction of the micro duct along the jetting route.
The central tube microduct design provides the highest fiber density, yielding a relatively small cable OD. The individual fibers are bundled into groups of twelve within the cable’s central tube, and the bundles are easily identifiable with colored binders in accordance with EIA/TIA-598B, "Optical Fiber Cable Color Coding”.
Color codes for ducts may differ from supplier-to-supplier or as supplied to customer order.
EIA/TIA-598B, "Optical Fiber Cable Color Coding”
Splicing And Termination
OSP cables are often spliced to provide very long link lengths. Cables can be purchased in lengths of 4-12 km but there are limits to how long a length of cable can be pulled in ducts, even with lubrication. And in many cases the cables need splicing for dropping cables at locations or splitting cables for diverse fiber connections.
Most splicing is done by fusion splicing, essentially welding fibers together in an electric arc in an automatic fiber splicing machine. Splicing is usually done in a protected environment like an air-conditioned splicing trailer or van to prevent problems with dirt or other contamination.
Splices will be organized in trays which are stacked in splice closures. Closures are sealed to protect the fibers from the environment. Many different types of closures are available so designs suitable to the location should be chosen.
Termination of OSP singlemode cable is generally done by fusion splicing a terminated pigtail or a splice-on connector (SOC) on the fiber. Requirements for low loss and reflectance prevent direct termination methods like adhesive/polish connectors from being used in OSP installations.
Hardware to protect the cable plant and splice points includes pedestals, handholes and manholes. Huts and offices are provided for termination of the cable plant at electronics sites.
(C) 2018 The Fiber Optic Association Inc.