Optical fiber Cable design considerations – How to design an Optical fiber cable

With widespread usage of optical fiber cable practically into every field of telecommunication, there are numerous fiber optic cable designs which will confuse the end customer, which one to select. The important point to bear in mind is that each fiber optic cable design is optimized for a given application. There are general application fiber optic cables, but selection of such cables to a specific application needs utmost care.

Mainly the system requirements decide the type of optical fiber cable and its configuration. Some of the physical as well as economical parameters which decide on the configuration of optical fiber cables shall be discussed today.

Optical characteristics and performance: Attenuation and dispersion over the expected life time of the cable if the optical fiber employed is single mode. Attenuation and bandwidth are concerned parameters for a multimode optical fiber cable. The variation of attenuation and dispersion in a fiber optic cable as a function of environmental variables When these parameters change with respect to the climate, it is necessary to make sure that the values will not exceed the dynamic range capability of related accessories employed in the system.

Size and Weight: One of the reasons for installers to love fiber optic cable is its small size with high bandwidth. This low diameter allows fiber optic cables to reach extremely difficult locations where conventional metallic cable installation is not possible. In an already crowded metro area with lot of cables where the existing ducts have no enough space, micro duct cables will pass through the customer destination without much effort. If the installation demands a rugged cable design, tight buffering of optical fibers will give adequate protection and ruggedness to the optical fiber cables that meet severe installation conditions. Tight buffering will reduce the cable size, but it is good for lower count fiber optic cables for special application.

Cable Life time: Optical fiber cable life time shall be more than 20 years as a general. This has many factors including the breaking probability of optical fiber and the oxidation induction time of materials used for the construction of optical fiber cables and if not more the hydrogen evolution from the materials in an O.F cable. The stronger fiber has less probability for a stress induced failure probability. Industry standards call for a 100 kpsi proof tested optical fiber. As we discussed earlier, Peter has agreed to write an article on the life time calculation of optical fiber cable in simple words to make it communicable to common people. There are many technical complex calculations for optical fiber life time calculation. We will do that soon. Similarly the Jacketing materials should posses certain characteristics against failure in environmental conditions.

Optical connector compatibility: Optical fibers link in longer length finally to connect to the equipments to the termination units. As customers, we are connected to optical fibers through the connectors. The connectivity of optical fiber cables with industry standard connectors is an important criterion to perform in the field. The low loss and smallest size connector selected may not be suitable for the optical fiber cable designed. So, network installers while designing for outside plant shall be careful in selecting the compatibility of optical fibers and connectors. Optical fiber cables shall be viewed as a component in the total optical system assembly.

Strength and Durability: The optical fiber cable to be manufactured for the desired system must be capable to withstand the installation stresses, especially the pulling strength requirements. Generally the duct cables are blown through ducts now a days, while before they were pulled through conduits. While pulling, a pulling eye shall be fixed to the end of the cable. Sufficient lengths of optical fiber cable shall be taken out to form a Figure 8 to reduce the high pulling tension, which occurs while directly pulled from the drum. Optical fiber cable installation must be an easier and simple procedure without the need of special handling tools.

Optical fiber cables will run through harsh environments, like immersed in water, laid down in salty soil, or passed near high voltage etc. For aerially installed fiber optic cables, the sag and lashing tension are critical parameters while wind load and ice load will play major roles in day to day operation and performance. Aerial cables are more prone to severe bends and twists compared to duct and direct buried optical fiber cables. Optical fiber cable durability means the resistance to twist, vibration, bend, crush and impact influences. Apart from these mechanical disabilities, environmental durability like resistance to extreme low and high temperatures is also desired for an optical fiber cable to perform for a successful optical network.

That said the general optical fiber cable requirements. There are some special conditions which our optical fiber cable must ensure. Vermin attack resistance, Fire resistance and Anti tracking properties are some of the special system requirements an optical fiber cable in that particular application area must meet. An OSP contractor shall clearly mention the system requirements to the optical fiber cable manufacturing plant at the time of sending inquiry for quotation. Quotations are prepared as per the material cost involved for a particular optical fiber cable in inquiry and will vary according to the application.

Probably the one we mention now must get the top priority – the Cost! This rules the world of fiber optics too. A highly priced cable may always not necessarily be of top quality. The experience says some manufacturers over design the cable due to unwarranted concerns! When asked to the cable design engineer, we can hear “I thought the cable is for XYZ company, so, there should not be any complaint later”. That is over protecting his job. He is concerned about protecting his job than getting orders to his company. Overdesign and providing more strength elements than the required mechanical properties demand will cost more to the buyer. It is better if the cable design engineer rather put his reason as “I thought the optical fiber cable will be pulled by elephants”!!

He is adding up the cost and loosing a possible order to his company. Or in worst case if the OSP contractor doesn’t have any other option but to buy his cable, the total project cost will be going up. Cost must be verified thoroughly for presence unwanted elements. Optical fiber cables don’t need an elephant to pull!