What are the Design Criteria for Optical Fiber Cables?

Before starting with optical fiber cable design, we must keep in mind some basic requirements and knowledge on where our cable is going to be placed. Optical fiber cables will be exposed to various environmental conditions such as different soil conditions on their way to termination boxes.

It is always better for you to know the composition of the soil, where your cable will be laid. This will help you to select the appropriate sheath type and thereby cable type.

Some of the possible combinations are:

– Sandy soil area – Duct or Armored cable

– Rock or rubble area – Armored cable

– Contaminated soil area – Sheathing with special material

Weather conditions; If you are designing a direct buried cable, keep the following points in mind.

    • Relative humidity –cable must be protected from moisture or with moisture barrier sheath
    • The temperature where your fiber cable will be stored at first at customers location before laying, installation, and operation temperatures thereafter.

Also, bear in mind that the optical fiber cable design should ensure that the optical fiber cable function under the above-mentioned conditions for a period of at least 30 years. Generally, 30 years life period is guaranteed by manufacturers. There are equations related to fiber strength and lifetime, material properties such as oxidation induction time, and hydrogen generation from the cable components that affect the life of fiber cable. We will deal with this subject under a separate topic.

In general, there are two major criteria that should be taken into account.

  1. Safeguarding the Optical properties

When the cable is in operation, the optical properties must not deviate and go out of specification limits. The major parameter of concern is attenuation.

  1. Safeguarding the Mechanical properties

Throughout the lifetime of cable and during installation also, the mechanical properties must remain intact.

Preservation of Optical properties

Attenuation losses are generally caused by two mechanisms in an optical fiber cable. Don’t confuse with the scattering phenomena and absorption losses that cause attenuation in optical fibers. That is at the fiber stage. Once the fiber is made and put into the cable, then the cable design must protect fibers from attenuation increase.

Macro-bending loss occurs when the fiber is bent around a too small a radius. This macro-bending loss happens when the bending radius is less than 40mm.

Micro bending loss is due to small disturbances in the fiber axis, that involves a bending radius less than 1 mm.

A proper cable design can avoid the attenuation increase due to the above reasons by:

  • Not allowing stress points on the fibers. This can be achieved by placing the fibers in loose tubes and thereby ensuring almost mechanical isolation between the fibers and the plastic structure surrounding them.
  • Water blocking in the loose tube is achieved by applying Jelly inside the tube. This will act as a soft cushion for fibers and is generally known as a tube filling compound in the cable industry.

Preservation of mechanical properties

The cable design is based on the assumptions that while processing, transportation, storage, installation, and operation, the fibers should be stressed as little as possible.

If the mechanical protection is effective, the cable is likely to meet the optical parameters as well.

Design engineers may remember the following while designing the cable:

  • maximum allowable cable elongation
  • 5 for Metallic strength members (e.g.: steel wires, aluminum, copper wires)
  • Maximum 1.0% for Non-metallic strength members
  • Maximum allowable tensile force

Customers usually mention in their specification, because they are the best judges on how much pulling force they will lay the cable.

  • The maximum allowable pressure force
  • The Maximum allowable torsion and bending
  • The Maximum allowable temperature range

Related Articles

Design, Construction and Properties of Different Types of Loose Tubes in Fiber Optic Cables

What are the Effects of Bending on Optical Fibers and Cables?

Historical Facts in the Development of Optical Fiber

Calculation of Excess Fiber Length in Loose tube

Characteristics of ITU-T G.656 Non-Zero Dispersion Shifted Fiber for Wideband Optical Transport?

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