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Physical-Design Considerations

Many factors must be considered when designing a fiber-optic cable plant. First and foremost, the designer must determine whether the cable is to be installed for an inside-plant (ISP) or outside-plant (OSP) application. The answer to this question usually determines whether a loose buffer or a tight buffer cable will be used. An important factor in fiber-optic cable design and implementation is consideration of the cable's minimum bend radius and tensile loading. There are two kinds of submarine cable systems: shallow-water and deep-water systems.

Tight Buffer Versus Loose Buffer Cable Plants

Tight buffer or tight tube cable designs are typically used for ISP applications. Each fiber is coated with a buffer coating, usually with an outside diameter of 900 m. Tight buffer cables have the following cable ratings:

  • OFNR—Optical fiber, nonconductive riser rated

  • OFNP—Optical fiber, nonconductive plenum rated

  • LSZH—Low smoke, zero halogen rated

The type of ISP tight buffer cable selected usually depends on the application, environment, and building code. Loose-buffer or loose-tube cables mean that the fibers are placed loosely within a larger plastic tube. Usually 6 to 12 fibers are placed within a single tube. These tubes are filled with a gel compound that protects the fibers from moisture and physical stresses that may be experienced by the overall cable. Loose buffer designs are used for OSP applications such as underground installations, lashed or self-supporting aerial installations, and other OSP applications. These cables require additional cleaning, including the removal of the protective compounds when the fibers are to be terminated. Loose-tube cable designs include multifiber armored and non-armored cable systems.

Bend Radius and Tensile Loading

An important consideration in fiber-optic cable installation is the cable's minimum bend radius. Bending the cable farther than its minimum bend radius might result in increased attenuation or even broken fibers. Cable manufacturers specify the minimum bend radius for cables under tension and long-term installation. The ANSI TIA/EIA-568B.3 standard specifies a bend radius of 1.0 inch under no pull load and 2.0 inches when subject to tensile loading up to the rated limit.

For ISP cable other than two-fiber and four-fiber, the standard specifies 10 ∴ the cable's outside diameter under no pull load and 15∴ the cable's outside diameter when subject to tensile load. Cable tensile load ratings, also called cable pulling tensions or pulling forces, are specified under short-term and long-term conditions. The short-term condition represents a cable during installation and it is not recommended that this tension be exceeded. The long-term condition represents an installed cable subjected to a permanent load for the life of the cable. Typical loose-tube cable designs have a short-term (during installation) tensile rating of 600 pounds (2700 N) and a long-term (post installation) tensile rating of 200 pounds (890 N).

Submarine Cable Systems

Shallow-water systems are similar to their armored loose-buffered terrestrial counterparts, whereas deep-water submarine cables use a special hermetically sealed copper tube to protect the fiber from the effects of deep-water environments. Deep-water and submarine cables also have dual armor and an asphalt compound that is used to fill interstitial spaces and add negative buoyancy. In addition to the significant external physical forces that might be encountered in a submarine environment, the other major concern is the effect of hydrogen on the performance of the optical fiber in cables used in such applications.

The effect of hydrogen on fiber performance depends on specific system characteristics. System attributes include fiber type, system operating wavelength, and cable design and installation method. Hydrogen can chemically react with dopants, such as phosphorus, to produce irreversible absorption peaks, resulting in a significant increase in the attenuation coefficient across various wavelength ranges. This phenomenon, also known as the Type 1 hydrogen effect, occurred primarily in early optical-fiber designs that used a phosphorus dopant. Unlike early phosphorus fibers, current fibers using germania dopants are not susceptible to Type 1 hydrogen effects.

The second hydrogen effect arises from the propensity for molecular hydrogen to diffuse readily through most other materials. When diffused into glass optical fiber, hydrogen creates distinct absorption peaks at certain wavelengths. The most predominant of these occurs at 1240 nm and 1380 nm. The tails of these peaks can extend out, depending on the hydrogen concentration, affecting the optical performance at 1310 nm and 1550 nm. Unlike the Type 1 effect, the effect created by molecular hydrogen is reversible and is known as the Type 2 hydrogen effect. The major sources are typically understood to be the corrosion of the metal armoring and the presence of bacteria. Proper span design must take into consideration hydrogen safety margins for submarine applications. The attenuation coefficient is proportional to the water depth role because as depth increases, the partial pressure of hydrogen increases, resulting in an increase in the amount of interstitial hydrogen that can be present in the fiber.

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